Forest floor mass, litterfall and nutrient return in Central Himalayan high altitude forests

Transcription

1 Vegetatio 12: , (~) 1995 Kluwer Academic Publishers. Printed in Belgium. Forest floor mass, litterfall and nutrient return in Central Himalayan high altitude forests S. C. Garkoti & S. E Singh Department of Botany, Kumaun University, Naini Tal , India Accepted 5 April 1995 Key words: High elevation, Litter production, Turnover rate, Nutrient content, Nutrient use efficiency Abstract Dynamics of forest floor biomass, pattern of litter fall and nutrient return in three central Himalayan high elevation forests are described. Fresh and partially decomposed litter layer occur throughout the year. In maple and birch the highest leaf litter value was found in ctober and in low-rhododendron in August. The relative contribution of partially and more decomposed litter to the total forest floor remains greatest the year round. The total calculated input of litter was g m -2 yr -1 for maple, g m -2 yr -1 for birch and g m -2 yr -1 for lowrhododendron forests % of the forest floor was replaced per year with a subsequent turnover time of yr. The annual nutrient return through litter fall amounted to (kg ha -1 yr -l) N, P and K. The tree litter showed an annual replacement of 26-54% for different nutrients and it decreased towards higher elevation. The nutrient use efficiency in terms of litter produced per unit of nutrient was higher in present study compared to certain mid- and high-elevation forests of the central Himalaya. Introduction Litter fall represents an essential flow in organic production-decomposition cycles and is, in many ways a fundamental ecosystem process (Meentemeyer et al. 1982). It is one of the most important pathways through which nutrients are returned to the forest floor. Litter on the forest floor affects the moisture status, run-off pattern, and nutritional characters of the land. A considerable amount of data on aboveground litter production exists for various parts of the world (Bray and Gorham 1964; Bonger et al. 1985; Klinge and Herrera 1983; Meentemeyer et al. 1982; Proctor 1983). The studies carried out in forests occurring at relatively low elevation of central Himalaya (3-22 m) indicated faster turnover of litter nutrients than in temperate forests (Singh and Singh 1992). Moderate winters in tropical latitudes, and high relative humidity and moist conditions are suggested to be some of the main reasons of rapid decomposition in these forests. We have studied three representative forest types (maple at 275 m, birch at 315 m and lowrhododendron at 33 m) occurring in high elevations of central Himalaya with the main objectives of examining how these forests differ from their counterparts in temperate latitudes and forests in low elevations in central Himalaya in relation to the dynamics of the forest floor, litter fall and nutrient return. n the basis of existing information we expect that though decline in temperature towards higher elevation would retard the litter turnover, the more favourable water balance due to higher precipitation effectiveness throughout the growing season (see MUller 1982) would compensate for the adverse effect of temperature. Material and methods Description of study area Three high elevation forests (viz.; maple, birch and low-rhododendron) were selected for litter fall and nutrient return estimation. The study sites are located in the northeastern part of the Central Himalaya between 79 41E and 8 51E long. and 3 17'N and

2 34 Table 1. Seasonal variation in forest floor components (g m -2) in different forests of Central Himalaya Litter category Maple Forest Birch Forest Low-rhododendron Forest Winter Summer Rainy Winter Summer Rainy Winter Summer Rainy Fresh leaf litter Partially and more decomposed litter Wood litter Miscellaneous litter Herbaceous dead Total litter 'N lat. These sites are located between 275 m and 33 m elevation. The climate of the entire area is characterized by a summer monsoon and the year is divisible into three distinct seasons viz., rainy (mid une - September) winter (ctober - April) and summer (mid April - une). The snowfree rainfall is reported to be 217 mm (Sundariyal and oshi 1992). Mean monthly temperatures varied from 7 C (in ctober) to 13 C (in une). The area has a high relative humidity (7%). Geologically, the present study area lies in Pindari formation, which is made up of sillimanite rich Kyanite-garnet bearing psammite schists and gneisses interbedded with calc silicate rocks and amphibole bearing calc gneisses. The soil was sandy loam with ph The water holding capacity of the soil ranged from 4%-96%. The deciduous maple (Acer cappadocicum) and birch (Betula utilis) forests occur on relatively gentler slopes, while the evergreen low rhododendron (Rhododendron campanulatum ) occurs on relatively steeper slopes. Tree density was 55 trees ha -1 in maple forest, 7 trees ha -1 in birch forest and 118 trees ha -1 in low-rhododendron forest. (Garkoti and Singh 1995). Me~o~ Following earlier studies undertaken in central Himalaya (Pandey and Singh 198 la and 198 lb; Rawat and Singh 1989) forest floor mass was collected from 9, 5 x 5 cm quadrats at monthly intervals from May 1989 to November 1989 and thereafter in April 199. From December to February sampling was not possible due to heavy snowfall and persistent ground snow. In each forest the study plot was divided into three parts of approximately similar areas, and from each of them litter was collected from three randomly distributed quadrats. In each quadrat all herbaceous dead shoots were clipped and were measured to calculate herb litter. The fallen herb litter was considered a part of miscellaneous litter category, which also included debris from reproductive parts, bark, shrubs, mosses and ferns. The material on the forest floor was sorted into fresh leaf litter, partially and more decomposed litter, wood litter (fine wood, i.e., 1 mm diameter) and miscellaneous litter, while avoiding the contamination with soil and placed in polyethylene bags. The fresh leaf litter includes the leaves with no apparent tissue disintegration. Litter (leaf, wood and miscellaneous litter together) with disintegrated tissues, but still large enough to hold with finger and forceps was considered as partially decomposed, and the decomposed material smaller than that could be held with the finger or forceps were considered as more decomposed litter. The litter input was measured monthly again by using 9 litter traps, consisting of wooden slates of 5 cm length, 5 cm width and 15 cm depth with nylon mesh in bottom. The sampling design was same as described for litter mass collection. Litter from each trap was collected separately and separated into leaf, wood, reproductive and miscellaneous litter components. The samples were ovendried at 8 C to constant weight. The turnover rate (K) of litter was calculated indirectly according to lson (1963): K = A/A + F where 'A' is the annual increment of litter, i.e., annual litter fall and 'F' is the mean standing crop of litter

3 was estimated colorimetrically. Potassium content was determined by flame photometry (ackson 1958). The nutrient concentration was multiplied by the weight of annual litter fall to compute the amounts of nutrients transferred to the forest floor. The nutrient use efficiency (NUE, Vitousek 1984; Lugo 1992) was calculated as the ratio of mass fall to nutrient in mass fall of a forest. 35 r-~c5 ~ c5 Results Forest floor litter mass t~ o~ 6 7 o N ~.~.~ o. e-. ',1 o6d ~ t rq o.~.~ o. ~. t'q - ~ = (annual averaged across months). Turnover time (t) is the reciprocal of the turnover rate t = 1/k. For each of the litter components (i.e., leaf, wood, etc.) nutrient concentration of litter was determined separately. For each of the components material collected in different months was mixed thoroughly to prepare a composite sample, which was divided into three parts, and analysed separately. Nitrogen concentration was determined with N-auto analyzer. Phosphorus -d I z The maximum monthly fresh leaf litter mass declined uniformly and steeply with rise in elevation from 147. g m -2 in maple forest (in ctober) to 1.3 g m -2 in birch forest (in ctober) and 5.9 g m -2 in lowrhododendron forest (in August), whereas the minimum monthly values were similar and very small ( g m -2, Fig. la--c). Seasonal biomass of fresh leaf litter varied between g m -2 in maple, g m -2 in birch and g m -2 in lowrhododendron forests (Table 1). Across the months in the standing crop of partially and more decomposed litter ranged from to g m -2 in maple forest to g m -2 in birch forest and from 335. to g m -2 in low-rhododendron scrub (Fig. lac). The woody litter on all sites peaked in November and was similar ( g m-2). The miscellaneous litter occurred with similar ranges ( ; and g m -2, respectively in maple, birch and low-rhododendron forests). The herbaceous dead shoot mass was relatively small (Fig. la-c). Litter fall The total annual litter fall declined uniformly and markedly with elevational rise from 275 to 33 m (Table 2). The trend in relation to elevation was also found similar for leaf as well as wood litter fall. The contribution of leaf fall to the total litter fall was about 59% in maple forest and declined with increasing elevation to about 48% in low-rhododendron forest. The proportion of wood litter in total litter fall ( %) increased towards higher elevation. n the maple forest the monthly leaf fall ranged from g m -2 (Fig. 2) and the cumulative leaf fall increased until the end of the annual cycle. Monthly wood litter fall ranged from 9.7 to 38.8 g m -2. The

4 36 7 (a) 6-7 I I 'E ~6 4-8,, I t I M A S N A M A S N A MNTHS MNTHS,,,,.,, M A S MNTHS Fig. 1. (a-c) Variation in forest floor litter mass (4-1 SE, n = 9) in maple (a), birch, and low-rhododendron (c) forests. Hollow circles, solid circles with solid lines, solid circles with dotted lined, traingles and X's, respectively represent biomass of fresh leaf litter, partially and more decomposed litter, herbaceous dead, miscellaneous litter and wood litter. Standard error indicated. peak wood litter fall occurred in ctober (Fig. 3). The monthly reproductive litter ranged between 1.2 and 9.7 g m -2 (Fig. 4). The monthly miscellaneous litter was highest in ctober (11.6 g m -2) and minimum in August 3. g m -2 (Fig. 5). n the birch forest the monthly leaf fall ranged from 4.1 (in May) to 73.2 g m -2 (in November) (Fig. 6). The wood fall among different months ranged between 2.3 (May) and 3.2 (August) g m -2 (Fig. 7). The monthly reproductive litter fall ranged from g m -2 (Fig. 8) and the monthly miscellaneous litter ranged from g m -2 (Fig. 9). n the low rhododendron forest the monthly leaf fall ranged from 4.1 (May) to 37.7 (August) g m -2 (Fig. 1). The fall of wood litter among different months ranged between 5. and 29. g m -2, with peak in August (Fig. 11). The reproductive litter fall ranged between 1.7 and 1.5 g m -2 with a peak in November (Fig. 12), and the miscellaneous litter fall ranged between 2.2 and 6.5 g m -2 with a peak in ctober (Fig. 13). The seasonal pattern of leaf fall in maple and birch forests was similar. In these forests 58.6 and 55.7% of the annual leaf fall occurred during autumn/winter seasons; 7.6 and 6.3% during rainy season and 33.8 and 37.8% during summer season, respectively. In lowrhododendron forest the highest leaf fall occurred during rainy season (6.4%). The wood fall in the two deciduous forests was approximately evenly distributed between winter and rainy seasons. In the low-rhododendron scrub the more wood fall occurred during rainy season (about 5%) than during the other seasons (Table 2). The reproductive and miscellaneous litter fall was concentrated in winters. Litter nutrients In general, the concentration of N, P and K in litter fall ranked in this order: reproductive > leaves > miscellaneous > wood (Table 3). It is evident from the data on nutrient concentration that there exists considerable variation between the nutrient contents

5 37 E.._1 kl "3 i, k~ I" ' 2" 4- - 'E 36- C~ 32- " 28- tz L,I 24- ~ 2- I.,i_,,, 16- '" 12- _> I ~: 4-23 (.. i i i (bl I I I 1 Fig. 2. (a) The monthly variation in leaf fall (4-1 SE, n ), and cummulative leaf fall in maple forest. (a) 2- {b) v _._1 Ii r.r' t,t I,- F-.- -q r~ q~e t, 14' i 5 12o' -i l- i 8-,,, 6- > 4- _ i i i i i 1 i! M j, A ~ b. ~, M A S b Fig. 3. (a) The monthly variation in wood fall (4-1 SE, rz ), and cumulative wood fall in maple forest. E3 2- i N i A 199

6 tl- (a) 1- fit E C7' 9.-. h r i,i I.-- I.--.._1 i,i > o 'E o', 35-._,t 3. LI..I >52s. 7: ~ "3 2" G hi 3 2 I I I I D > 15- (o D 1- (:3 c,, ch 5- i..d "" t i i i i i i Fig. 4. (a) The monthly variation in reproductive fall (-4-1 SE, n = 9), and cmulative reproductive fall in maple forest. E ,I,,. ~ 12.o. m I' "" C D 6.. o ILl z 4-"._. "' o to m 2.' (a) '~E 32. v " 28",, u. 24. t.u I~ > ~ 2. --I 16. ~ :~ ua 12 o z 8... o 4 V) i M l l A ; 6. ~ M,989,99o,989 l i i i i A S N A 199 Fig. 5. (a) The monthly variation in miscellaneous fall (4-1 SE, rt = 9), and cumulative miscellaneous fall in maple forest.

7 39 (a) ~ 27- E :...]... ~ 21- L,t. c~ d t, Q:: I..u I " u_ 12- U.I - 9- I11... LI_ t.~ d 2" 1- ~_ 6o....I ~ ,, ~ ~ i I i i i i, Fig. 6. (a) The monthly variation in leaf fall (+ 1 SE, n = 9), and cumulative leaf fall in brich forest. I'M E v -i kl_ " 24' 2' Ca) t"nl 'E t~!8' v,,_i 16., l.,l. r, 14. lad F o 8. _..I ,,, 6. >_,,,., 4' ~; 2. o i i i! M A S N A ,.) i i i i i t l! M A S N A Fig. 7. (a) The monthly variation in wood fall (4-1 SE, n = 9), and cumulative wood fall in birch forest.

8 4 (o] E 4-.,,., ii ~" 3- e'x" LU I- l-._ I- 8 >._ ILl > %.., C3 U I I I I l" I! b- ~ I- n lad Fig. 8. (a) The monthly variation in reproductive fall (+1 SE, n = 9), and cumulative reproductive fall in birch forest. 1- (a] ea v 8-7-._, u.. 6- r,- u 5" m.,,. 4- U z o 3- u. z,..j 2,. u ( u ~ 24-.,,1. ~2- m 16- m ~ 12- ~. ua,.- 8-,.-I,,- 3r 4- :D i M 1989 i i"' i ' ~ i ~' / A S N A 199 T! I I I I M d A S N A Fig. 9. (a) The monthly variation in miscellaneous fall (4-1 SE, n = 9), and cumulative miscellaneous fall in birch forest.

9 j 36 ~ (o) 18- 'IE, ,N E I:7", v 16-14" ii 24-,--I 12- pr W I,-.5 iii _ rr I,,,I I,,- I,,- hi >,, 1-8" , :D 2" i :3 U t i i l l i Fig. I. (a) The monthly variation in leaf fall (4-1 SE, n = 9), and cumulative leaf fall in low-rhododendron forest (o) 'E t35- ~ 12- ~4 i E 28- ~ 15- v 24- " 2" ii :'q cr 16- i,i ~ 6- _ 12-..d C3 8 N 4 ~ 45 v- 3 ~ 15- M 3 k s o. k ) M A ~ 6 ~,k" Fig. 11. (a) The monthly variation in wood fall (+1 SE, n = 9), and cumulative wood fall in low-rhododendron forest.

10 42 Table 3. Nutrient concentration (% DW 4-1 SE, n = 3) in litter fall of different components Litter category N P K Leaf litter fall: A. cappadocicum R. arboreum Wood litter fall: A. cappadoc&um R. arboreum Reproductive litter fall: A. cappadocium R. arboreum Miscellaneous litter fall Leaf litter fall: B. utilis R. campanulatum Wood litter fall: B. utilis R. campanulatum Reproductive litter fall: B. utilis R. campanulatum Miscellaneous litter fall Maple forest Birch and Low-rhododendron Forests of various species. The amounts of nutrients returned were usually in proportion to the quantity of litter fall in various species. In this respect A. cappadocicum (in maple), B. utilis (in birch) and R. campanulatum (in low-rhododendron) were the dominant tree species which produced maximum litter annually. The nutrient return through these species were also maximum. Thus the chemical composition and litter fall of the dominant tree species largely determined the amount of various nutrients in the total litter fall of the community. The amount of nutrients returned in the annual litter fall was highest in the maple forest with 56.1 kg ha -1 yr -t N, 5.4 kg ha -1 yr -1 P and 23.3 kg ha -l yr -1 K (Table 4). Across the three forests leaf litter contained 58-75% of the total nutrient return through litter fall. Discussion Madge (1965) has reviewed accumulation data of forests and mentioned values from 1.7 to 14.7 t ha -t within tropical zone and from 3.6 to 39.9 t ha -1 in temperate zone. ur values are close to mid point of the range for tropical zone and towards the lower side of the range for temperate forests (Table 5, values are given in t ha- 1 for convenience in comparison). Since there is also an absence of amorphous humus layer

11 43 'E..d LL 14"- 12"- c~ I'" I..IA L 8'. "' 6,- > U 4.- :D 2.- rr" Q.. LI,-,r (a) Lu 'E " 35-7._ u_ 3- rr >~ ~ 2- D 2~u ~>- t5- (-3 m 1- Q rr 5" Q. ry i i i 1! I I Fig. 12. (a) The monthly variation in reproductive fall (:El SE, n = 9), and cumulative reproductive fall in low-rhododendron forest. ~-" 8.- E (ca) % 24-7'-,, 6.- U... r'~ '" 5-- F- 15- _a 4.- (,,9 D 3.- iii z 2.- _,,_.,,, I'- t.) c _ M d A S N M A S Fig. 13. (a) The monthly variation in miscellaneous fall (4-1 SE, n = 9), and cumulative miscellaneous fall in low-rhododendron forest. above mineral soil in these forests (also see Singh and Singh 1992) they are close to tropical forests than the forests of temperate latitudes in forest floor characteristics. Values of turnover rate for litter in the three forests studied indicated that 49-61% of the forest floor is replaced each year and the turnover rate of litter decreased with increase in elevation. Decomposition rates vary as a function of temperature, moisture

12 44 Table 4. Amount of nutrients return through tree litter fall (kg ha- 1 yr- 1) Litter components N P K Mapple Forest Leaf: A. cappadocicum R. arboreum Wood: A. cappadocicum R. arboreum Reproductive A. cappadocicum R. arboreum Miscellaneous Total Leaf: B. utilis R. campanulatum Wood: B. utilis R. campanulatum Reproductive: B. utilis R. campanulatum Miscellaneous Total Leaf Wood Reproductive Miscellaneous Total Birch Forest Low-rhododendron Forest and quality of litter material, as indicated by nutrient concentration and lignin content, and that of other structural tissues. Doubling in microbial activity is suggested per 1 C increase in temperature (Singh and Gupta 1977). There is a decline of 1 C temperature from the lowest elevation (3 m) where Shorea robusta forest occurs to low-rhododendron forest of the present study at 33 m. According to relationship between temperature and microbial activity at the highest elevation of low-rhododendron site it should be two times lower than at the lowest elevation of S. robusta site. However, the litter turnover rate is only 35% slower at the highest elevation site compared to the S. robusta site. Evidently several environmental factors compensate for the adverse effect of low temperature at low rhododendron site. f the several possible factors more favourable water balance due to higher precipitation effectiveness and longer period of wet season seems to be most important. In the present study while annual precipitation was invariant (see Singh et al 1994), the decline in mean annual temperature was 2 C from maple forest site to lowrhododendron forest site, which indicates about 22% decline in microbial activity, if other conditions were to remain the same. However, in the present elevational transect litter quality was not the same. For example, the specific leaf mass (SLM), which roughly indicates the degree of leaf schlerophylly, toughness of tissues and dilution of nutrients by the presence of mechanical tissues (Daalen 1984), increased by almost 65% from 17 g m -2 in maple to 176 g m -2 in rhododendron leaves (Garkoti 1992). We suggest that decline in temperature along with increased schlerophylly account for most of the increase in residence time of floor litter and nutrients in the forest floor from maple to low-rhododendron forest. The litter of poor quality is not only resistant to physical factors but also supports smaller populations decomposer microbes (Singh et al. 199). The mean residence time for organic matter and nutrients in the forest floor litter of broad-leaved forsests of the world are reported to range as following: organic matter.4 to 4. yr, N 2. to 5. yr, P 1.6 to 5.8 yr, K.7 to 1.3 yr (Cole and Rapp 1981; Edwards and Grubb 1982; Edwards 1977, 1982; Gray and Schlesinger 1981). In these all the lowest values are for tropical rain forests and all the highest values are for temperate deciduous forests to mediterranean forests. Most of our values, including those of the subalpine forests (low-rhododendron and birch) fall between the mean values for tropical rain forests and mediterranean/temperate deciduous forests. It seems that severe subalpine conditions in our above forests is moderated by high precipitation and relatively high relative humidity, which seldom falls below 5% even during dry seasons (Sakai and Malla 1981) and moderate winter temperatures (MUller 1982).

13 45 Table 5. Mean litter biomass (t ha- 1) in some temperate and Himalayan forests Vegetation Forest floor Reference accumulation Quercus Woodland 4. enny et al. (1949) Mixed Woodland 3.6 Witkamp & Vander Drift (1971) Betula 1.8 Van Cleve and Noonan (1971) Betula 4.1 Van Cleve and Noonan (1975) Acer 9.8 Vitousek (1982) Mature ak forest 5.3 Duvigneaud and Denaeyer de Smet (197) ak forest 12.6 Reiners & Reiners (197) ak Conifer forest 4.7 Pandey and Singh ( 1981 a) ak forest Rawat and Singh (1989) Maple forest 6.4 Present study Birch forest 5.5 Present study Low-rhododendron forest 5.6 Present study Despite continental location, the climate in the south of the main range in the central Himalaya resemble more maritime condition than the continental one (Sakai and Malla 1981). These factors, by keeping microbial activities relatively high and litter not so recalcitrant (litter quality is also a product of climate and other environmental factors, such as, soil nutrients, Vitousek 1984), make possible a turnover higher than those generally found for subalpine forests occurring in temperate latitudes. Since precipitation effectiveness increases with decline in temperature, these sites of high altitudes are more moist than lower elevation sites of the central Himalaya (Singh et al. 1994). Though the maple and birch forests exhibited year-round litter fall, the leaf fall is concentrated in the autumn (ctober- November). Such a pattern with a peak in ctober or November has also been observed in the cool temperate zone of Northern hemisphere (Witkamp and Vander Drift 1971; Carlisle et al. 1966; Duvigneaud et al. 1969). The annual wood litter fall t ha-1 yr-l estimated in this study was higher than the world mean of.9 t ha -~ yr -1 for cool temperate forests (Bray and Gorham 1964) and close to the values (1.3 t ha-t yr-l) reported for an oak-hickory forest in Central Missouri (Rochow 1974). Mean contribution of wood fall to total litter fall ranged between % in the present study, which is higher than the values reported for a mixed oak-conifer forest of Central Himalaya (2%; Pandey and Singh 198 lb), and for an oak-hickory forest (21.3%; Rochow 1974). The proportion of wood Table 6. Turnover rate (K) of litter mass and nutrients of the forest floor Forest Mass N P K Maple Birch Low-rhododendron fall increased towards the higher side of the elevational range, possibly mainly as a result of greater snowfall. A greater proportional wood fall may indicate smaller biomass accumulation, resulting in shorter vegetation in high elevations. Some of the litter fall data for certain temperate broad-leaved forests are given in Table 7. The present values ( t ha -l yr -1) are well within the range of values reported for temperate broad-leaved forests. The values of the present diciduous forests are close to that ofacer, Fagus and Betula forest (4.78 t ha -1 yr -l) of New England, U.S.A. and of an Indiana, U.S.A. forest (5.23 t ha -1 yr -1) (Vitousek et al. 1982). For a Quercus-Betula forest litter fall was reported 5.7 t ha -t yr -l for North Hampshire (Gosz et al. 1972) and 3.7 t ha -t yr -l for Arnhem, Netherlands (Witkamp and Vander Drift 1971) forests. f the total annual nutrient input through litter fall, leaf fall generally accounts for 75-85% and wood fall (including flower and fruits) for 1-35% (Klinge and Rodrigues 1968; Bernhard-Reversat 1972). In the

14 46 Tab/e 7. Annual litter fall (t ha -1 yr -l) for certain temperate broad leaf forests Forest Location Litter fall Reference Alnus rubra USA Zavitkovski & Newton (1971) Fagus sylvatica Sweden 5.7 Nihlgard (1972) Acer sac. & Mix Conn., USA 2.1 Scott (1955) Acer, Fagus, Betula New England, USA 4.78 Vitousek et al. (1982) Acer, Fagus, Quercus Indiana, USA 5.23 Vitousek et al (1982) Quercus, Acer New England, USA 4.89 Vitousek (1982) Quercus, Betula Arnhem, Netherlands 3.7 Witkamp & Vander Drift (1971) ak conifer India 5.5 Pandey and Singh (1981b) ak India Rawat and Singh (1989) Maple India 6.28 Present study Birch India 4.78 Present study Low-rhododendron India 3.46 Present study Table 8. Nutrient use efficiency (litter fall mass/litter nutrient) in different central Himalayan forests Forest Elevation NUE Reference (m) N P K Quercus leucotrichophora Singh & Singh (1992) Quercus lanuginosa Singh & Singh (1992) Quercusfloribunda Singh & Singh (1992) Maple Present study Birch Present study Low-rhododendron Present study present forests leaf litter fall accounted for % and wood litter fall (excluding reproductive and miscellaneous litter fall) 12.l-29.9%. Gosz et al. (1972) reported % nutrient input through leaf litter fall and % through wood fall (excluding flower and fruits), which are comparable with the present estimates, with the exception of the value of low-rhododendron in which wood fall accounts for about 3% in nutrient return through litter fall. The nutrient return obtained in maple forest (56.1 kg ha -t yr -t) and birch forest (34.1 kg ha -I yr -1) approximates the values reported for a spruce forest (54. kg ha -t yr -t) of USSR (vington 1965) and birch forest (33.5 kg ha -t yr -1) of Denmark (Thamdrup 1973), respectively, but both deciduous forests showed lower N return compared to a birch forest (66. kg ha -l yr -t) of USSR (vington 1965). The N and P return obtained for low-rhododendron community of the present study are comparable with those of sub- alpine coniferous forest (23.6 N and 2. kg P ha -l yr -1) of apan (Kitazawa 1973). Vitousek (1982) found that the litter dry matter: N ratio varied beteen 6-2 among a large number of forests. In the present study, the ratio of dry mass: N in the litter fall in maple, birch and low rhododendron forests was, 112, 14 and 135, respectively. It is apparent that the nutrients examined in this study show an annual replacement rate of 38-54% in maple forest, 33-39% in birch forest and % in low-rhododendron forest (Table 6). In conformity with the pattern of forest floor litter mass along the elevational gradient the turnover rate for nutrients decreased with increase in elevation. In a temperate oak forest of Central Missouri the nutrients showed an annual replacement rate of 18-23% (Rochow 1974) as against % in the present study. Thus for complete nutrient turnover of the litter in the temperate oak forest approximately 5 years are needed as compared

15 47 to years in present study. However, the nutrient replacement rates in present study are lower than the values reported for a mixed oak-conifer forests (54-65%, Pandey and Singh 1981b), and for oak forest (64-84%, Rawat and Singh 1989). In this study birch forest produced more litter per unit N and K compared to low-rhododendron and maple forests. Rhododendron was particularly efficient with P, and had high nutrient use efficiency (NUE) compared to birch and maple forests. However, all the forests of the present study were more efficient in nutrient use than certain high- and mid-elevation forests of central Himalaya (Table 8). In conclusion, the rate of litter fall decreased with the increase in elevation of the forests studied. However, the forest floor mass was relatively invariant, thus indicating that the litter disappearance was faster towards lower elevation. The amount of litter production, nutrient input to the forest floor and turnover rate of litter and nutrients set these forests in between the tropical and the true temperate forests. It appears, therefore that though the region of the present study lies in the tropical belt experiencing monsoon effects, it represents temperate conditions (low temperature, light, heavy snowfall, etc.) altitudinally, positioning it midway between tropical and temperate forests. Compared to forests of lower elevations of the central Himalaya (3(22 m) these forests, however, show greater nutrient-use efficiencies with respect to litter production, both because of lower nutrient concentrations in tissues and withdrawal of nutrients from them during senescence. 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