Regeneration patterns in southern rata (Metrosideros umbellata) kamahi (Weinmannia racemosa) forest in central Westland, New Zealand

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1 New Zealand Journal of Botany ISSN: X (Print) (Online) Journal homepage: Regeneration patterns in southern rata (Metrosideros umbellata) kamahi (Weinmannia racemosa) forest in central Westland, New Zealand Glenn H. Stewart & Thomas T. Veblen To cite this article: Glenn H. Stewart & Thomas T. Veblen (1982) Regeneration patterns in southern rata (Metrosideros umbellata) kamahi (Weinmannia racemosa) forest in central Westland, New Zealand, New Zealand Journal of Botany, 20:1, 55-72, DOI: / X To link to this article: Published online: 10 Feb Submit your article to this journal Article views: 252 View related articles Citing articles: 39 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 20 November 2017, At: 11:00

2 New Zealand Journal of Botany, 1982, Vol. 20: Regeneration patterns in southern rata (Metrosideros umbejlata) - kamahi (Weinmannia racemosa) forest in central Westland, New Zealand GLENN H. STEWART I THOMAS T. VEBLEN2 Protection Forestry Division Forest Research Institute P.O. Box , Christchurch, New Zealand Abstract The regeneration modes of the dominant tree species, in the highland conifer-broadleaved forests of central Westland, South Island, New Zealand especially southern rata (Metrosideros umbellata) and kamahi (Weinmannia racemosa), are inferred from detailed analyses of stand structures in the Taramakau area. Both species regenerate intermittently and sparsely beneath canopy gaps in old-growth stands as well as abundantly on sites devastated by extensive natural disturbances such as landslides. The numerous evenaged rata-kamahi stands in Westland reflect the importance of the catastrophic regeneration mode for these 2 species. The eventual synchronous senescence of many of the trees which established at approximately the same time on sites bared by natural disturbances may be an important contributory factor in the canopy tree mortality which is widespread in the region. Keywords Metrosideros umbel/ata; Weinmannia racemosa; tree regeneration; spatial pattern; plant succession; natural disturbances; ecology; Westland, New Zealand INTRODUCTION Extensive areas of dead standing trees in the highland conifer-broadleaved forests of central Westland, South Island, New Zealand, have caused concern about the persistence of the forest cover. The main canopy tree species affected by this Ipresent address: Department of Botany and Plant Pathology, Oregon State University Corvallis Oregon 97331, USA. "!National Research Advisory Council Post-Doctoral Fellow ( ); present address: Department of Geography, University of Colorado Boulder Colorado 80309, USA. " Received 17 July 1981 apparently excessive mortality include southern rata (Metrosideros umbellata) and kamahi (Weinmannia racemosa) and, towards higher elevations, Hall's totara (Podocarpus hal/it) and mountain cedar (Libocedrus bidwillii)*. Although the cause of the mortality has been debated (Chavasse 1955, Hoy 1958), it is widely believed (at least for rata and kamahi) to result principally from browsing by the introduced, Australian brush-tailed possum (Trichosurus vulpecula). Consequently, in recent years there have been numerous studies of possum diet and densities in relation to vegetation condition. In contrast, there has been little research on the processes by which the dominant tree species perpetuate themselves. For example, knowledge of forest dynamics in the rata-kamahi forests is insufficient to fully answer the following basic questions (cf. Webb et al. 1972): 1. Are the regenerating species reproducing a forest similar in structure and composition to the surrounding forest? 2. Under what circumstances do the dominant tree species establish and attain their positions in the main canopy? 3. ~o th~ main canopy tree species regenerate mtenmttently or continuously? 4. Does ea~h. main canopy tree species have a charactenstlc age- or size-class distribution? 5. What ro~e does chance play in the tree regeneration processes which determine the species composition of the future forests? 6. What is the role of natural disturbance i~ the dynamics of these forests? P. Wardle (1966, 1971, 1977) gives information on the reproductive biology and preferred sites for se~dling esta~lishment of rata and kamahi in the mld- and high-elevation forests of Westland. However, for. the rata-kamahi forest type in general, there IS no comprehensive understanding of how re~enerative processes account for the present vegetation patterns. At least tentative answers to the above 6 questions must be provided before the roles of introduced animals in influencing vegetation patterns can be assessed. 'Nomenclature follows Allan (1961). Moore & Edgar (1970), and Edgar (1973).

3 56 New Zealand Journal of Botany, 1982, Vol. 20 APPROACH The essence of the pattern and process approach to the study of vegetation dynamics is summarised by Watt (1947) as being"... based on the unit pattern, which is the present expression of the continuous process of development and decline in a population broadly interpreted. The qualitative and quantitative assessment, therefore, of the plant community may be expressed in terms of the temporal changes in such a population". Close consideration of present vegetation patterns in relatively small areas (e.g., 0.4 ha) can reveal the dynamic processes by which these patterns have developed. So, in attempting to answer the above 6 questions about vegetation dynamics, we carried out detailed analyses of stand structure and spatial patterns of trees (d. Veblen et al. 1980). A few sites were selected for intensive analysis to relate present vegetation patterns to tree regeneration processes. Stand structure (i.e., the frequency distribution of variously sized trees in a given stand), commonly reflects the modes of regeneration of the constituent tree species. In interpreting modes of regeneration, knowledge of the spatial patterns of the dominant tree species has proven useful (Williamson 1975, Forman & Hahn 1980) and, consequently, spatial patterns receive considerable attention in the present study. The pattern and process approach used here differs fundamentally from the survey approach of J. Wardle & Hayward 1970, James et al. 1973, J. Wardle 1974, and Coleman et al which has been used over the past 2 decades to assess the adequacy of regeneration of the major tree species in rata-kamahi forests. Some comment is required on the important differences in objectives bu.t complementary nature of the pattern and process approach as opposed to the survey approach. The latter has been applied to large areas of forest from a few thousand to hundreds of thousands of hectares (usually catchments) with the objective of assessing the regeneration status of the major tree species. It involves the collection of vegetation data from many sample sites covering a wide range of forest composition and structure. To date, in reports and publications of surveys, data from many sample sites have been pooled on the assumption that if enough objective samples are included, pooled data should be representative of all the forest in the entire survey area or of an extensive association within the survey area. However, the area covered by plots or plotless samples in relation to the total catchment or survey area is extremely small (usually < 0.1 %). The survey approach has provided useful information on the floristic composition of vegetation over wide areas and on the short-term influences of ungulate browsing on understories as reflected by frequencies in height classes of some species of shrubs, herbs, and tree seedlings (i.e., < 1.4 m tall). We propose, however, that the survey approach alone is unlikely to achieve the objective of assessing the potential long-term major changes in the forest cover (c!. J. Wardle & James 1973) because it does not provide the information required to answer the 6 basic questions relating to forest dynamics (see Introduction). The regeneration status of a tree species over a large area cannot be adequately evaluated until it is, at least generally, understood how that species regenerates. The main difficulties inherent in investigating tree regeneration processes by pooling data from extensive sampling sites, as in a survey, include the following: 1. A composite picture of stand structure is derived from data pooled from many stands of often widely differing structure, ranging from dense, young, even-aged stands to stands consisting of only large, senescent trees. 2. The history of disturbances affecting the vegetation at each sample site is likely to vary greatly. Pooling data from stands with differing histories of disturbance by windthrow, landslides, mass movements, etc, obscures the mechanisms involved in forest dynamics. 3. The inference of regeneration status from stand structure is based on the assumption that there is at least a general relationship between tree diameter and tree age. However, when data are pooled from sites at different altitudes and of widely varying soil characteristics, the consequent variation in tree growth on such sites often invalidates this assumption (cf. Ogden 1978). To understand how a stand with a given species composition and structure has developed requires detailed analyses of individual stands rather than a composite vegetation picture obtained by pooling data from stands of radically differing structures. The information about regeneration modes derived from this study is intended to complement the information gained from surveys of the same type of forests. STUDY AREAS Forest structure in central Westland was studied in the Taramakau area at Kellys Creek, Aickens, and Goat Creek. Tree species establishment on bare surfaces was investigated at Rolleston River and Camp Creek. (For detailed locations see NZMS 1 S52, Harper Pass, and S59, Otira.) AilS sites are in areas of moderate to steep topography at altitudes of approx m a.s.1. on the eastern side of the Alpine Fault. Soils are derived from schists at Camp

4 Stewart & Veblen-Regeneration in rata-kamihi forest 57 Creek, semi-schistose greywackes at KelIys Creek and Aickens, and strongly indurated greywackes and argi11ites at Rolleston River and Goat Creek (Gregg 1975). The soils in the region are typically skeletal but in areas of greater slope stability are highly leached, podzolised yeliow-brown earths and gley podzols (Luke 1968). The climate is characterised by high annual rainfall and high humidity. Records from the nearest meteorological station (Otira at 383 m a.s.1.) for indicate an annual precipitation of mm, a mean monthly humidity of 83-87%, and mean monthly temperatures from 3.5 C in winter to 18.8 C in summer (New Zealand Meteorological Service ). Although the prevailing winds are westerlies, strong gusty south-easterlies occasionally occur, along with heavy frosts and torrential rainfalls (up to approx. 200 mm in 24 h) which are also characteristic of the local climate. All 5 study sites are within the West Coast "beech gap". At this latitude (approx 'S) on the western side of the South Island, beech species become less frequent and are almost absent from just south of the Taramakau River southwards to the Karanagarua River in south Westland (approx 'S). At mid-elevations (approx. 50~OO m a.s.1.) the rata-kamahi forests are characterised by the dominant hardwoods, Metrosideros umbellata, Weinmannia racemosa, and Quintinia acutifolia with the occasional to locally frequent Podocarpus hallii. Emergent miro (Podocarpus ferrugineus) and rimu (Dacrydium cupressinum) may occur below approx. 600 m a.s.l., and above approx. 700 m a.s.1. Libocedrus bidwillii is a conspicuous tree (Franklin & Nicholls 1974). Above approx. 800 m a.s.1. the forest is typically of lower stature and is dominated by species such as broad leaf (Griselinia littoralis), mountain ribbon wood (Hoheria glabrata), O/earia spp., and Dracophyllum spp., with scattered to locally abundant P. hallii, L. bidwillii, mountain toatoa (Phylloc/audus a/pinus), and pink pine (Dacrydium biforme). On poorly drained sites at similar altitudes small L. bidwilli, P. a/pinus, and D. biforme may dominate over a dense canopy of poorly developed small hardwoods. Study sites were selected where there was no evidence of direct human disturbance, such as logging or fire. Nevertheless, some indirect human influence via introduced browsing animals (principally the brush-tailed possum and red deer (Cervus e/aphus» is likely at all sites. Red deer have been present in the Taramakau area since (Pekelharing 1979) and possums, liberated as early as 1926 at Otira (Pracy 1974), were common by ; red deer were at their highest densities and possums were widespread in 19~1969. For the main canopy tree species only the smallest diameterclasses could have been affected by deer browsing, since on most sites at least 50 years are necessary for the trees to reach 10 cm dbh (P. Wardle 1966,1971, Veblen & Stewart 1982). Also, all the main canopy tree species are rarely or only moderately browsed by deer (J. Wardle & Hayward 1970, J. Wardle et al. 1973, J. Wardle 1974). Stands with a high proportion of dead standing trees, as well as those with no apparent excessive mortality, were included in this study. METHODS Old-growth forest Stand structure For the analysis of stand structure in tall forest, three ha plots and two ha plots at Kellys Creek and Aickens, respectively, were used. In each plot tree diameters at breast height (dbh at 1.4 m) of all trees ;;.5 cm dbh were measured and numbers of saplings (i.e., stems of tree species <5 cm dbh and > 1.4 m tall) were counted. If a tree or sapling showed obvious signs of having established on a log, trunk buttress, or root plate this was recorded. Stumps and dead standing trees ;;.5 cm dbh and logs ;;.20 cm dbh were also measured and, if possible, identified. The degree to which dbh measurements reflect age was not assessed here because previous studies indicate a generally positive relationship between diameter and age for all the major tree species in this forest type (P. Wardle 1966, 1971; Wells 1972; Veblen & Stewart 1982). In depicting size-class structures, and by implication age-class structures, relatively large diameter-classes (Le., 10 cm) were used to allow for considerable variation in the relationship of tree age and diameter. To characterise the vertical component of stand structure, trees> 1.4 m tall were classified into the foliowing relative canopy-height categories: emerge~t, upper main canopy,lower main canopy, below maio canopy, and overtopped. Tree species limited to the latter 2 categories are referred to as subcanopy species. Stand structure was also depicted by vegetation profiles obtained from along transects of 50 x 3 m. ~patia/ pattern Spatial distributions of tree populations w~re assessed by mapping all individuals> 1.4 m tall In 3 x 3 m contiguous quadrats forming blocks of up to 432 quadrats in the largest plots. This mapp~ng permitted a variation of the nested-quadrat techmque analogous to that described by Greig Smith (1964) and Kershaw (1973) to be used to detect scales of pattern. The measure selected for determining the departure from a random distributio~ is Morisita's (1959) index, the application of which has been described by Williamson (1975).

5 58 New Zealand Journal of Botany, 1982, Vol. 20 Morisita's index is given by: fa = q ~~-l n, (n,-i)1 N (N-I) where q = number of quadrats, n, = number of individuals of the species in the ith quadrat, and N = total number of individuals in all q quadrats. The index, la' = 1.0 when the population is randomly dispersed, where random implies an independent distribution of individuals into quadrats with an equal probability of each individual occurring in any quadrat. If the individuals are clustered, la is > 1.0, and if evenly distributed or hyperdispersed, la is <1.0. Where the number of individuals is small, Ia tends to vary erratically; thus la was computed only for the more abundant tree species populations. la was computed for different species and different size-classes (0;;10 cm and >10 em dbh) for quadrats of increasing size from I x 1 unit, where each quadrat unit was 3 x 3 m. For quadrat sizes 0;;9 x 9 units the original unit quadrats were grouped into perfect squares, thus avoiding fluctuations in Ia which result from change in the shape of the blocks of quadrats (Pielou 1977). Larger blocks of quadrats were rectangles, the size of which is reported as the number of 3 m units on one side of a square quadrat of equivalent area. The tendency for some species to occur together at various quadrat sizes was also investigated by a chi-square association test (Mueller-Dombois & Ellenberg 1974). The test was applied only for species sufficiently abundant to provide adequate cell frequencies in 2 x 2 contingency tables, but not so abundant as to be present in all quadrats. Tree canopy density Each of the contiguous unit quadrats was assessed as occurring beneath a canopy gap (i.e., an opening at least 3 m in diameter), beneath closed canopy, or beneath an intermediate canopy condition. In each stand canopy density was also assessed photographically, vertical hemispherical canopy photographs were taken from a height of 1.9 m (i.e., above the level of most understorey shrubs) at 8 evenly spaced points along a transect bisecting each plot longitudinally. The amount of light blockage by vegetation on the photographs was assessed following the procedure described by Anderson (1964). The percentage of total diffuse and direct sunlight beneath the canopy of a given stand, compared with total light in the open, was assumed to be proportional to the percentage of visible sky in the photographs; this value corresponds to the "total site factor" of Anderson (1964). ' Understorey sampling In each large plot the understorey vegetation was sampled with one hundred 1 x 1 m plots distributed in a restrictedrandom lay-out. Each 1 x 1 m plot was assessed as a predominantly log plot (i.e., > 50% of the total area occupied by 1 or more logs) or non-log plot to obtain information about the abundance of log sites for seedling establishment. Presence of all vascular species was recorded and the numbers of tree seedlings (Le., stems of tree species <1.4 m tall) were counted according to the substrate types of (1) forest floor as opposed to (2) logs, prostrate trunks, stumps, upturned root plates, or trunk buttresses. In each unit quadrat the total cover of understorey species (i.e., excluding tree species) was estimated in percentage cover classes of <1, 1-5,6-25,26-50, 51-75, and Recently devastated sites To document regeneration on recently massively disturbed sites, rectangular plots of 2 x 10 m and 20 x 40 m were systematically located on slips at Camp Creek and Rolleston River, respectively. In these plots tree seedlings and saplings were counted and diameters at breast height of trees were recorded. At Goat Creek in a 30 x 50 m plot in a young, evensized rata-kamahi stand, the diameters at breast height of all major canopy trees were measured and saplings counted. RESULTS AND INTERPRETATION Old-growth forest Kellys Creek Adjacent stands KC - 1 and KC - 2 are on a 10 slope at approx. 600 m a.s.l. on the north-east face of the Kelly Range in the Kellys Creek catchment. The podzolised soils have an approx. 20 cm deep, organic-rich 0 horizon overlying an approx. 10 cm greyish clay A2 horizon. These stands are dominated by m tall Metrosideros umbellata and Podocarpus hallii and shorter 3-6 m tall Weinmannia racemosa, Quintinia acuti!olia, and Phyllocladus alpinus (Table 1). Griselinia littoralis and Pseudopanax simplex are common subcanopy trees and the shrub Neomyrtus pedunculata is locally abundant (Fig. la). Many of the M. umbellata have established on large boulders or logs or epiphytically on live trees, and some of them are reproducing vegetatively by layering. Vegetative reproduction of M. umbel/ata has also Fig. 1.. Profiles of (a) sta~d KC-2, (b) stand KC-3, and (c) stand A-I at Kellys Creek and Aickens. At, Archeria tral)e~sl/; ~d,.blechnum d/scol~r; Bm, Blechnum minus; GI, Griselinia littoralis; Lb, Libocedrus bidwillii; L, log; Md, Myrsme ~'va"cata; Mu, Metroslderos umbella~~; Np, Neomyrtus pedunculata; P, root plate; Pa, Phyllocladus alpinus; Pc, Pseudowmtera colorata; Ph, Podocarpus halll/; Ps, Pseudopanax simplex' Qa Quintinia acut;4"olia' Wr Weinmann,'a racemosa. ', J',,

6 Stewart & Ve bl en -Regeneration In. rata-kamihi forest 59 Ph

7 60 New Zealand Journal of Botany, 1982, Vol. 20 Table I Number of trees> 1.4 m tall at Kellys Creek and Aickens in crown-classes: I, overtopped (including saplings); II, below main canopy; III, lower main canopy; IV, upper main canopy; and, V, emergent. II III IV V Kellys Creek STANDS KC - 1 & KC - 2 (5508 m 2 ) Metrosideros umbellata 6 Podocarpus hallii 24 Quintinia acutifolia 730 Weinmannia racemosa 81 Phylloclal1us a/pinus 211 Elaeocarpus hookerianus 12 Libocedrus bidwillii 0 Podocarpus ferrugineus 1 Pseudopanax simp/ex 76 Griselinia littoralis 25 STAND KC - 3 (3888 m 2 ) Metrosideros umbellata 92 Podocarpus hal/ii 336 Quintinia acutifolia 1982 Weinmannia racemosa 341 Phyllocladus a/pinus 982 Elaeocarpus hookerianus 220 Libocedrus bidwillii 32 Pseudopanax simplex 142 Griselinia littoralis 49 Alckens STANDS A-I & A-2 (7776 m 2 ) Metrosideros umbellata 63 Podocarpus hawi 123 Quinrinia acutifolia 3418 Weinmannia racemosa 391 Phyllocladus alpinus 933 Pseudopanax simplex 310 Griselinia littoralis 103 been reported for Westland, by P. Wardle (1971), as well as for Auckland Island and Stewart Island (Cockayne 1909 a, b; Veblen & Stewart 1980). In these humid localities, after reaching a diameter of em, M. umbellata often produce aerial roots which hang freely or grow down the trunk, increasing in diameter only after their tips enter the ground. M. umbellata of epiphytic origin establish near the base of the crown of the host (often Libocedrus bidwillii or P. hall;') and become heavily branched above the point of establishment. The descending roots coalesce to form a single irregular trunk, enclosing the host trunk which eventually decays (Fig. 2). The size-class structures and composition of stands KC - 1 and KC - 2 are very similar and are thus combined for a more economical presentation. In these stands Q. aculi/olia is the only species with a size-class distribution clearly reflecting continuous regeneration (Fig. 3d), with tree-size individuals being steadily recruited from an abundant population.of saplings. Although the other main canopy species (M. umbellata, P. hallii, and W. racemosa) have much smaller sapling populations, they have moderate numbers of stems in a wide range of sizeclasses, including saplings and stems <15 cm dbh (Fig. 3a-c); the regeneration of these 3 species appears to have been intermittent. Of the subcanopy tree species P. alpinus is represented by abundant individuals in the sapling size-class, many of which have arisen from layering stems, but a few attain diameters >5 cm dbh (Fig. 3g). In contrast, G. littoralis is represented by only a few saplings but has abundant individuals in the 5-35 cm size-classes (Fig. 3f); it is a preferred food of red deer (J. Wardle 1974), so its regeneration has probably been impeded by deer. Pseudopanax simplex is similarly a highly palatable species (J. Wardle 1974) and browsing by deer may also have reduced the number of saplings of this species (Fig. 3e). In contrast to stands KC -1 and KC - 2, on Bench Island, where no browsing mammals are found and where G. Iittoralis and P. simplex occur beneath closed forest canopies of similar composition, more than 75% of their total populations consist of saplings (Veblen & Stewart 1980).

8 Stewart & Veblen-Regeneration in rata-kamihi (orest 61 Fla. l Metrosideros umbel/ata of epiphytic origin. host trunk of Podocarpus hal/ii on left. Stand KC-2. KeJlys Creek. 60 a b 1.0 % 20 JIIa." Size c1ass frequency diagfams tor abundant main canopy and lubcanopy tree species in stands KC-l and KC-2 at Kel\ys Creek. (I) MetrosideTos umbertata (n - 65). (b) Podocarpus hallii (n - 131), (c) Weinmannia racemosa (n.. 189), (d) Quinlinia acutifolia (n - 854), (e) Pseudopanax simplex (n - 84), (f) Griselinia lirtoralis (n '" 32). and (s) PfsylloclaJus alpinus (n == 219). The tize-classes used are s (sapltap) for trecs <S em dbh but at lcast 1.4 m tau, and 1-17 for trees in 10 an dbh classes from 5 to 174 em. n is the total number of stems >2.4 m tau for elch spedes % c S 2 L 6 d Size S 2 Size & 5 2 I. 6 & classes e 9 I. S 2 S 2 I. S 2 I. classes

9 62 New Zealand Journal of Botany, 1982, Vol. 20 Table 2 Number of seedlings «1.4 m tall) of dominant tree species per ha. (:t: SE) at Kellys Creek and Aickens (100 1 m 2 sample plots were assessed in each stand). Only species occurring in at least four 1 m 2 plots in a stand are included. The numbers in parentheses refer to percentages of seedlings rooted on stumps. fallen logs, prostrate trunks. upturned root plates, and tree buttresses. Kellys Creek Aickens Stand KC-l KC-2 KC-3 A-I A-2 Percentage log plots Metrosideros umbellata 1900:t: :t: ± :t: :t:1429 (36.8) (94.4) (95.5) (73.3) (78.1) Weinmannia racemosa 16400:t: :t: ± ± ±1218 (42.1) (54.5) (80.2) (60.3) (55.6) Podocarpus hallii 7000:t:l :t: :t: :t: :t:1774 (10.0) (13.9) (38.6) (8.8) (27.3) Quintinia acutifolia 47900± :t:5829 IS100± ± ±10267 (24.0) (3S.3) (26.5) (28.8) (20.2) Griselinia littora/is 700± :t: ±S ± ±903 (42.9) (36.0) (63.0) (27.5) (56.5) Phy/loc/adus a/pinus 17300:t: :r ± :t: ±5183 (27.2) (8.8) (19.4) (8.3) (33.7) Podocarpus /e"ugineus 1200:t:383 looo:t:333 (0) (0) Libocedrus bidwillii 6OO:t: :t:419 (16.7) (12.5) Pseudopanax simplex 12900:t: :t: ±12S ± :t:3193 (23.3) (23.8) (59.3) (34.9) (50.0) Table 3 Percentage of saplings and stems ~ 5 cm dbh of tree species established on elevated surfaces (logs. stumps. prostrate trunks. trunk buttresses, or upturned root plates) at KeUys Creek and Aickens. Kellys Creek Aickens Stands KC-l KC-2 KC-3 A-I A-2 Main canopy species Metrosideros umbellata Weinmannia racemosa Podocarpus ha/lii Quintinia acutifolia Phyllocladus alpinus Libocedrus bidwillii Sub-canopy species Griselinia littoralis Pseudopanax simplex In stands KC - 1 and KC - 2 the mean percentage understorey covers are relatively high (22.0 and 22.5%, respectively). By establishing on logs, stumps, prostrate trunks, trunk buttresses, and upturned root plates, seedlings may avoid competition with understorey species. Some species. especially M. umbellata, W. racemosa, and G. liltoralis, have very high percentages of seedlings established on such elevated sites (Table 2), and this is genera\ly characteristic of the old-growth rataka~a.hi forests in Westland (P, Wardle 1966, 1971). ThIs IS clearly a successful mode of regeneration as reflected by the abundance of saplings and small stems >5 em dbh of these species which also are established on elevated surfaces (Table 3). Stand KC - 3 is located 30 m downslope frorn stands KC - 1 and KC - 2 on 'the same type of soil. Numerous logs and stumps of P. hallii, M, umbellata, and L. bidwilliiindicate extensive canopy mortality, and provide a basal area of dead standing trees and stumps as great as that of live trees, Following this mortality of the dominant trees, a dense 8-10 m tal1 thicket of saplings and small trees has developed with all the main canopy species represented. Q. acutifolia is the most numerous species in the subcanopy stratum (Table 1). M,

10 Stewart & Veblen-Regeneration in rata-kamihi forest 63 FIg. 4 Size-class frequency diagrams for abundant main canopy and subcanopy tree species in stand KC-3 at Kellys Creek. (a) Metrosideros umbel/ata (n = 196), (b) Podocarpus hallii (n = 414), (c) Weinmannia racemosa (n = 495), (d) Quintinia acuti/olia (n = 3063), (e) Phyl/ocladus a/pinus (n = 1098), (f) Pseudopanax simplex (n = 202), (g) Griselinia Iittoralis (n 59), (h) Elaeocarpus hookerianus (n = 235), and (i) Libocedrus bidwillii (n = 44). The size-classes used are s (saplings) for trees < 5 em dbh but at least 1.4 m tall, and 1-9 for trees in 10 em dbh classes from 5 to 94 em. n is the total number of stems> 1.4 m tall for each species. 8 % o a. e. b. c. d. S 2 4 S Size classes f. g. h. i. % Size classes S 2 S umbellata is the most abundant survlvmg main canopy tree in association with fewer Q. acuti/olia, W. racemosa, and the coniferous species (Fig. 1 b). All tree species are abundant as saplings and small stems; 97% of Podocarpus halli;, 92% of W. racemosa, 99% of Q. acuti/olia, 99% of Phylloc /adus a/pinus, 100% of Pseudopanax simp/ex, 86% of G. littoralis, and 99% of Elaeocarpus hookerianus are < 15 cm dbh (Fig. 4b-h). Although there has been some recruitment of M. umbel/ata via saplings, the abundant cm dbh -stems appear to be mostly stems that have established by layering of large fallen trees (Fig. 4a). Thus, after canopy breakdown rata responds both by seedling establishment and by vegetative reproduction. Although many of the stumps and dead trees are L. bidwi/lii, several cm dbh stems of this species survived the canopy collapse and numerous saplings have subsequently established (Fig. 4i); regeneration in this manner has been documented previously for the Aickens area (Veblen & Stewart 1982). The abundance of G. littoralis and Pseudopanax simplex saplings (in contract to KC -1 and KC - 2) may be caused by logs and the dense thicket of saplings impeding the access of red deer. As expected from the great abundance of subcanopy stems in stand KC-3 (Table 1), light levels are relatively low despite the open nature of the main canopy (Table 4). Under these conditions the development of the understorey vegetation is markedly less than in the other stands; the mean understorey cover value is only 1.1 %. Gaps in the Table 4 Mean light conditions (± SE) determined from the analysis of hemispherical photographs taken in each stand from a height of 1.9 m; n is the number of photographs analysed. Only within-area comparisons were made; the values for all Kellys Creek stands are significantly different at P<O.OS level (Mann-Whitney U test).. Area Stand n Total sunlight (%) Kellys Creek KC-l ±2.4 KC ±1.2 KC ±t.8 Aickens A-I ±1.8 A ±1.S main canopy are common (Table 5) and tree seedling establishment on elevated sites such as logs may be an important advantage in occupying a gap. The high percentages of seedlings (Table 2), as well as of saplings and trees (Table 3), established on logs reflect the greater abundance of logs in this stand compared with KC -1 and KC - 2. The percentage of seedlings on elevated sites is higher than for saplings and trees because the decay of'the logs makes recognition of the site of establishment less obvious for the latter. Tree spatial patterns (Fig. 5), also reflect modes of regeneration. M. umbel/ata Ei)lO em dbh were not sufficiently abundant in KC - 1 and KC - 2 for analysis of spatial patterns. In KC - 3, however, M. umbellata em dbh show intense clustering at the smallest quadrat size and remain significantly

11 64 New Zealand Journal of Botany, 1982, Vol. 20 Table 5 Percentage frequency of unit (3 x 3 m) quadrats assessed as occurring beneath canopy gaps at Kellys Creek and Aickens. Within-stand differences were subjected to a Chi-square test using Yates' correction (Greig-Smith 1964). Stand KC-l KC-2 KC-3 A-I A-2 Number of quadrats assessed Percent frequency gaps Stands compared Chi-square P<. KC-l, KC-2 KC-l, KC-3 KC-2, KC-3 A-I, A om clustered up to a quadrat size of 7 (441 m 2 ; Fig. Sa). Clustering at the smallest quadrat size is probably because of to establishment on logs, and clustering at the larger quadrat size probably reflects their greater abundance in relatively large canopy gaps (approx.21 x 21 m;tables3, 5). M. umbella ta >10 cm dbh are slightly clustered at small quadrat sizes in KC - 3 (Fig. Sa), possibly indicating groups of stems arising from the layering of a fallen tree or by seedling establishment on logs. The large difference in I values for stems EOlO cm dbh compared with ste~s >10 cm dbh at the smallest quadrat sizes indicates drastic thinning of the thickets as they age. For example, in KC - 1 where more time has elapsed during which thinning would have occurred, M. umbellata stems > 10 cm dbh are evenly distributed (Ia = 0) at quadrat size 1 (9 m 2 ). In oldgrowth stands such as KC - 1 and KC - 2, w. racemosa, Q. acuti/olia, and P. alpinus EO 10 cm dbh are all intensely clustered at quadrat size 1 mainly because of vegetative reproduction (e.g., in KC -1; Fig. 5b, c, e). Generally in Westland, many seedlings of W. racemosa in forest undergrowth have procumbent, adventitiously rooting stems which through branching give rise to clumped saplings (P. Wardle 1966); young plants of P. a/pinus and Q. acuti/olia have weak branches which root adventitiously, producing clumps of saplings and small trees (P. Wardle 1963, 1977). To avoid measuring the obvious clustering caused by the multi-stemmed habit of many trees, only the largest stem of a multi-stemmed tree was recorded. Thus, clustering resulting from vegetative reproduction is only included where stems now appear to be rooted separately, even though subsurface connections may occur. Clustering of small stems of W. racemosa, Q. acuti/olia, and P. alpinus at larger quadrat sizes probably reflects establishment beneath canopy gaps of various sizes. For example, in KC-3 small stems of W. racemosa and Q. acuti/olia remain clustered at quadrat sizes up to 12 (1296 m 2 ) probably because of discontinuity of the main canopy over much of this stand. Rapid thinning of the same species in equivalent young thickets has caused their stems > 10 cm dbh to be evenly or radomly dispersed at quadrat size 1 as in stand KC -1 (Fig. 5b, c). In stand KC - 3 stems of W. racemosa > 10 cm dbh are still clustered at small quadrat sizes because self-thinning has not yet reached the stage represented in the other 2 stands; the intensity of clustering of the large stems (I = 2.24 at quadrat size 1) is similar to that of the s~ali stems (la = 2.34 at quadrat size 1). The thinning of W. racemosa stems as regeneration thickets age is also reflected by the drastic reduction in total numbers of stems <15 cm dbh in stands KC - 1 and KC-2 compared with stand KC-3 (Fig. 3c,4c). Stems of P. haw; EO 10 em dbh in stand KC - 3 are slightly clustered at quadrat sizes <7, reflecting establishment in large patches (Fig. 5d). In stands KC - 1 and KC - 2, however, P. ha/lii stems> 10 em dbh tend to be evenly or randomly distributed at quadrat sizes 1 and 2 (Ia = 0.76 and 0.94, respectively, at quadrat size 2) which implies that although several small stems may establish in clusters beneath small canopy gaps (approx 36 m 2 ) only 1 or 2 trees from each cluster attain main canopy stature. In stand KC - 3, the spatial pattern of L. bidwillii reflects establishment of saplings in small to relatively large patches (Fig. 5e). In general, stands KC-l and KC-2 illustrate that in relatively undisturbed old-growth stands the regeneration of species such as M. umbellata, W. racemosa, and P. hallii is only sporadic. Their FIll: 5 Values of Morisita's index, I, at different quadrat sizes for abundant main canopy tree species in stands KC-l.a~d.KC-~ at.keii~ Cre~k. (a) Metroslaeros umbellata, KC-3; (b) Weinmann;a racemosa, KC-l; (c) o.u~'ftmla aculil0.lla, K~ -.1; (d) Podocarpus halliihkc - 3; (e) Phyllocladus alpinus. KC - 1. Libocedrus ~1i:1~llllI. ~C-3. Cltc}es.m.~lcate stems EOIQ cm db and squares indicate stems >10 cm dbh, triangles mdlcat~ Llbocedrus bldwlllli.~10 em dbh (e); fiiied symbols are I values significantly >1.0 (P<0.05) a~o~dll~g to an F test of M~nslta (1959). The dashed lines at I = l.d'represent random patterns. Quadrat size mdlcates length of 1 Side of a square quadrat of equiviient area, in 3-m units.

12 Stewart & Veblen-Regeneration in rata-kamihi forest 65 g o a. b. Metrosideros umbellata g o Weinmannia racemosa lit lo a Quadrat size Quadrat size 10' lct c. Quintinia acutifolia ~u., 1 0 ~ Quadrat size d. Podocaq~us hallii Icr 190 e. 1 ~ e.bylloclad 1301 \ !.IS g!p'inus Llbocedrus bidwillii 0 0 I I I I I I Quadrat size Quadrat size

13 66 a b. 60 J, i ~ ~ i rsi l"iiinin Size classes % ~: % i i i c. d. e. f. New Zealand Journal of Botany, 1982, Vol. 20 i i i i 6 8 i i 10 g. in 12 Fig. 6 Size-class frequency diagrams for abundant main canopy and subeanopy tree species in stands A-I and A-2 at Aiekens and stand GC 1 at Goat Creek. Aick ens: (a) Metrosideros umbellata (n = 151), (b) Podocarpus hallii (n = 218). (c) Weinmannia racemosa (n = 681), (d) Quintinia acutifolia (n = 4117), (e) Phyllocladus a/pinus (n = 957). (f) Pseudopanax simplex (n = 373), (g) Griselinia littoralis (n = 116). Goat Creek: (h) Metrosideros umbellata (n = 214). (i) Weinmannia racemosa (n = 336), (j) Quintinia acutifolia (n = 305). The size-classes used are s (saplings) for trees <5 em dbh but at least 1.4 m tall, and 1-17 for trees in 10 em dbh classes from 5 to 174 em. n is the total number of stems> 1.4 m tall for each species S 2 4 Size ciao;ses h i. S 2 4 S 2 Size classes regeneration depends on favourable circumstances such as the availability of a log for seedling establishment beneath a small canopy gap or fortuitous establishment as an epiphyte. In contrast, Q. acutifolia and P. a/pinus regenerate much more vigorously under a closed forest canopy, and if such stands are not opened up by some type of disturbance their relative abundances are likely to increase. However, where massive mortality occurs and results in a breakdown of the main canopy, as in stand KC-3, all the main canopy tree species regenerate vigorously. Aickens Adjacent stands A -1 and A - 2 are at approx. 650 m a.s.l. on the north-facing slopes of the Kelly Range above Aickens. The soils are podzolised yellow-brown earths with a' thin dark brown 0 horizon. As with stands KC-l and KC - 2. similar structure and composition permitted combination of the size-structure data. Numerous M. umbellata and P. haw; emerge approx. 5 m above the m tall main canopy of predominantly the same 2 species and Q. acutifolia and W. racemosa (Table 1. Fig. tc). G. littoralis, Pseudopanax simplex and Phylloc1adus alpinus are common as subcanopy trees. Saplings of Q. acutifolia dominate the forest understorey and form particularly dense thickets beneath canopy gaps. On the 25 slope large boulders are common. indicating slope instability which may have disturbed these stands. The most abundant main canopy tree species (P. hallii, W. racemosa, and Q. acutifolia) have sizeclass distributions indicating continuous regeneration (Fig. 6b-d). M. umbel/ata is represented by numerous saplings and at least a few stems in all size-classes up to Class 17 ( cm dbh) reflecting successful periodic establishment (Fig. 6a); prostrate, layering trunks of this species are also common in these stands. As at Kellys Creek, P. a/pinus is represented by abundant saplings and only scatt~red larger stems (Fig. 6e), and Pseudopanax simp7ex and G. littoralis have a tendency, although not as marked, toward an under-representation of saplings (Fig. 6f, g) which may be induced by deer browsing. Establishment on logs is important for the regeneration of some tree species (Tables 2, 3) and follows a pattern similar to that previously described

14 Stewart & Veblen-Regeneration in rata-kamihi forest 67 Table 6 Numbers of seedlings «1.4 m tall) and small stems (>1.4 m tall and <10 em dbh) of tree species on slip sites at Rolleston River; recently bared surface (RC-l), young forest of variable height (RC-2A,B,C). Plot size is 800 m 2 Metrosideros umbel/ata Weinmannia racemosa Quintinia acuti/olia Podocarpus hallii Griselinia littoralis Pseudopanax simp/ex RC-l Numbers of seedlings o Plot RC-2A RC-2B RC-2C Numbers of small stems o for Kellys Creek. Podocarpus fe"ugineus seedlings, however, which are abundant at Aickens, do not appear to establish on logs. Chi-square association tests indicate that W. racemosa and M. umbellata E; 10 em occur together more often than would be expected by chance. For example, in stand A-I they are positively associated at quadrat sizes 1 (X 2 = 5.6; P<0.025) and 3 (X2 = 4.0; P<O.05), and in stand A - 2 they are positively associated at quadrat size 1 (X2 6.7; P<O.01). These positive associations may reflect their joint establishment on logs. Spatial patterns of trees in A -1 and A - 2 are similar to those described for Kellys Creek. The spatial patterns of M. umbel/ata, W. racemosa, Q. acutifolia, P. hallii (Fig. 7a~) and Phyllocladus a/pinus all suggest a gap-phase mode of replacement, involving establishment in dense thickets beneath canopy gaps of various sizes and subsequent thinning. Stands A -1 and A - 2, therefore, provide further evidence for the tree regeneration modes described for old-growth stands at Kellys Creek (KC- 1 and KC - 2). The main difference between the 2 pairs of stands is the apparently more continuous regeneration mode suggested by the size-class distribution of W. racemosa and P. hallii at Aickens. This difference is probably caused by more frequent, small-scale disturbances on the steeper slopes at Aickens as reflected by the greater abundance of large logs and boulders. In both areas in the oldgrowth stands with relatively intact main canopies, regeneration of the main canopy tree species follows a gap-phase mode. Recently devastated sites Rolleston River On the steep slopes in this catchment, sites recently bared by mass movements are common; at lower elevations periodic flooding and debris deposition have also provided new sites for colonisation. The important shrub and small tree species colonising these sites are Olea ria avicenniae/olia, o. arborescens, Griselinia littoralis, numerous Coprosma species, Pseudopanax colensoi, Hebe salid/olia, Aristotelia serrata, and Pseudowintera colorata; the main canopy trees, M. umbel/ata and W. racemosa, may also be abundant. For example, in an area of 800 m 2, 68 M. umbellata and 293 W. racemosa seedlings up to 1.4 m tall were counted on the rocky rubble of a recent debris fall (Table 6); they were mainly restricted to the more stable part of the rubble. The previous occupation of this site by tall forest was indicated by dead partly buried trunks of M. umbel/ata, W. racemosa, and Podocarpus hallii. On sites at various stages of regrowth, M. umbel/ata and W. racemosa are common, with their relative abundance depending on factors such as the availability and viability of seed at the time of disturbance and competition from fast growing shrubs. Seedfall of M. umbellata, W. racemosa, and Q. acutifolia is highly variable from year to year. For example, near stands A - I and A-2 at Aickens in , M. umbellata produced a significant amount of seed during only 1 year and W. racemosa and Q. a curi/olia, although producing some seed every year, produced large amounts during only 2 years (Forest Research Institute, unpublished data). Absence of some species such as M. umbellata from some recently colonised sites may reflect the unavailability of seed at the time the site became available for colonisation. On the other hand, Q. aculi/olia and P. si"!plex ~re consi~tently.absent or rare on recently colomsed sites despite thelt abundance in surrounding tall forest, which suggests that they are not well suited to grow on such sites. Camp Creek Compared with the Rolleston River slips, th~ slips sampled at Camp Creek were charactensed by more fragmented and finer debris. Consequently, the plant cover on these slopes was much more complete than on the fractured greywacke and argillite slips at Rolleston River. Shrub and small tree species establishing in abundance on these sites include Oltaria aviunniae!ol.ia, Hebe salicif~lia, Carmichaelia sp., Conarza sp., Gaulthena rupestris, Carpodetus

15 68 New Zealand Journal of Botany, 1982, Vol. 20 a. 7 0 umbellata lit b. Weinmannia racemosa o 2 L Quadrat size O~~--~~--~-r--ro Quadrat size 3 0 c. Quintinia acutifolia 3 0 d ~~1It 'ii 1 0 o 2 L Quadrat size o 2 L Quadrat size Fig, 7 Values of Morisita's index, I, at different quadrat sizes for abundant main canopy' t~e«? A -1 at Aickens. {a) Metrosid~ros umbel/ata, (b) Weinmannia racemosa, (c) Qumtmla a~utifoila, (d) Podocarpus hallii. CIrcles indicate stems,.;;10 em dbh and squares indicate stems> 10 cm dbh; flll.ed symbols are I values significantly >1.0 (P<O.05) according to an F test of Morisita (1959). The dashed lines at I == 1.0 lepresent ra!,dom patterns. Quadrat size indicates length of 1 side of a square quadrat of equivafent area, 10 3-m umts. speci~s i~ stand serratus, Pittosporum coiensoi, Griselinia littoralis, and several Coprosma species, in association with numerous ferns, herbs, and grasses; tree seedling densities are also high (Table 7). M. umbellata and W. racemosa account for large percentages of the total number of main canopy tree species establish_ ing on the slip sites; Q. acutifolia is much less abundant than it is in the surrounding forest. The relative abundance of these species may depend greatly on the degree to which site was devastated

16 Stewart & Veblen-Regeneration in rata-kamihi forest 69 Table 7 Numbers of seedlings «1.4 m tall) of main canopy tree species on slip sites at Camp Creek; plot size is 20 m 2 CC-l A B C Metrosideros umbellata Weinmannia racemosa Quintinia acuti/olia Total seedlings Plot CC-2 D E A B C D E FlI_ 8 Even-aged rata-kamahi stand (in centre) on steep slope at Goat Creek. and the availability of viable seed at the time of disturbance. Goal Creek At Goat Creek near Otira a stand on a 50" slope was sampled to illustrate the development of an even-aged stand which would be expected to arise after site devastation by a mass movement (Fig. 8). In this stand M. umbel/ala is even-sized with 87% of all stems 5-24 em dbh (Fig. 6h). Nineteen stumps or dead stems of M. umbel/ata < 10 em dbh indicate that the thinning of this even-sized patch has already begun. With 58% of its stems also in the 5-24 em dbh size range, most of the W. racemosa probably established contemporaneously with the M. umbellata; however, survival of many sapling size suckers (as well as a few saplings beneath small gaps) results in a much higher percentage of stems in the sapling size-class (Fig. 6i). Q. aculifolia, which in the Old-growth stands is many times more abundant with M. umbellata and W. racemosa, is not significantly more abu dant than these 2 species (Fig. 6j). However, its habit of reproducing vegetatively has already resulted in a large percentage of stems in the sapling size-class, and as this young stand (all stems <4.5 em dbh) thins the relative abundance of Q. acutilolia is likely to increase. Presently, the conditions necessary for the regeneration of M. umbellata do not exist but as the

17 70 New Zealand Journal of Botany, 1982, Vol. 20 stand ages sporadic regeneration of this species is likely. Already, abundant saplings of Pseud~I?anax crassifolius, P. simp/ex. and Podocarpus hal/ii have established in smajl canopy gaps. The plots in ajl 3 recently devastated areas illustrate that M. umbellata and W. racemosa are capable of successful establishment on su.ch surfaces. The relative abundance of each species may be a fortuitious consequence of the amount of viable seed available at the time of disturbance. In contrast to the situation in old-growth forest, regeneration of M. umbellata and W. racemosa on these sites may be prolific. CONCLUSIONS The interpretations of tree regeneration modes based on the structural data from the stands analysed in this study provide a t:relimina.ry model of stand dynamics for rata-kamahl forests In central Westland. Whether or not the species composition and structures of the stands analysed are statistically representative of similar forests over extensive areas is irrelevant to the study's basic objectives. Our own reconnaissance as well as the observations of others (J. Wardle & Hayward 1970, P. Wardle 1977) indicate that the types of stands analysed in this study are widespread. The proportion of a large catchment which is covered by even-aged ratakamahi stands such as that described at Goat Creek or old-growth stands such as tho.se described ~t Aickens could be estimated by aenal photographic methods and from forest surveys (but with levels of sampling intensity much greater than previously attempted in Westland protection forests). The regeneration modes inferred from these stands should be generally characteristic of the same tree species in forests of similar species composition occuring in similar habitats. If other modes of tree regeneration not described in the present study are important for the components of the rata-kamahi forest, they can be documented in future studies and the information can be added to the present interpretation. However, the data presented here have provided at least a preliminary basis for attempting to answer the questions regarding the dynamics of rata-kamahi forests stated in the Introduction. For example: 1. Conditions were described under which virtually all of the main canopy tree species regenerate. Thus, there is no basis for inferring a radical shift in forest type, even though fluctuations in the relative abundance of the dominant tree species may be expected. Also see Veblen & Stewart (1982). 2. The various circumstances under which the dominant tree species establish as seedlings and attain main canopy stature have been documented quantitatively. Our quantitative results largely confirm the observations of P. Wardle (1966, 1971, 1977, 1978) made mostly in south Westland. 3. All the main canopy tree species except Quintinia acutifolia regenerate intermittently after both gapphase and catastrophic modes. 4. Given the predominance of intermittent regeneration, no characteristic (or "normal") size- or ageclass distribution should be expected for the dominant tree species. The exception is Q. acutifolia which, because of its capacity to tolerate low light levels and its vigorous vegetative reproduction, usually has a size-class distribution in old-growth stands that is weli described by either a negative exponential or power function (Hett & LoUcks 1976). 5. Fortuitous circumstances, such as the availability of abundant viable seed at the time a bare surface becomes available for colonisation by plants, playa major role in determining the species composition at any given site. 6. Natural disturbances have a predominant influence on the dynamics of the rata-kamahi forests of central Westland. Previous studies have attempted to relate the structure of similar forests to browsing by possums and/or wild ungulates (e.g., J. Wardle & Hayward 1970, James et al. 1973, Coleman et al. 1980). However, if our interpretations of stand dynamics are correct, the scale and recency of natural disturbances are likely to be much more important in determining size-class' distributions of the main canopy tree species. The role of mass movements in initiating development of even-aged rata-kamahi stands has long been recognised (Chavasse 1955, Charles E. Douglas in Holloway 1957), and it has recently been suggested that many such stands in Westland established after a major earthquake in about AD (P. Wardle 1980). Furthermore, extensive wind throw, a well known feature of the lowland forests (Hutchinson 1928, 1932, Roche 1929), also affects the highland rata-kamahi forests of Westland (Coleman et al. 1980). In evaluating the effects of introduced wild animals on these forests, greater attention should be paid to stand dynamic processes in relation to natural disturbances. Where a high proportion of the tree population has established more or less synchronously on slips triggered by an earthquake, after several hundred years many of these trees may reach a stage of senescence at approximately the same time. In such a senescent state, species such as M. umbel/ata and

18 Stewart & Veblen-Regeneration in rata-kamihi forest 71 W. racemosa would be much more likely to die as a consequence of some detrimental influence - such as possum browsing. Thus, in any given area the susceptibility of the forest to lethal effects of possum browsing has probably been importantly affected by the long-term history of natural disturbances. Attempts to explain the present patterns of tree mortality in central Westland should, in addition to considering feeding habits and short-term history of possum densities, give more attention to the influences of natural disturbances on the dynamics of rata-kamahi forests. ACKNOWLEDGMENTS We thank M. Kennett. R. Morison, and D. Calder for assistance in the field and with data analysis. We are also grateful to other technic~1 staff at the Forest Resea~ch Institute and to M. McBnde and A. Veblen for drawmg the figures. REFERENCES Allan, H. H. 1961: Flora of New Zealand, Vol. 1. Wellington, Government Printer. Anderson, M. 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