Effects of variation in shade level, shade duration and light quality on perennial pastures

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1 New Zealand Journal of Agricultural Research ISSN: (Print) (Online) Journal homepage: Effects of variation in shade level, shade duration and light quality on perennial pastures M. B. Dodd, A. W. McGowan, I. L. Power & B. S. Thorrold To cite this article: M. B. Dodd, A. W. McGowan, I. L. Power & B. S. Thorrold (25) Effects of variation in shade level, shade duration and light quality on perennial pastures, New Zealand Journal of Agricultural Research, 48:4, , DOI: 1.18/ To link to this article: Published online: 17 Mar 21. Submit your article to this journal Article views: 455 View related articles Citing articles: 11 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Agricultural Research, 25, Vol. 48: /5/ The Royal Society of New Zealand Effects of variation in shade level, shade duration and light quality on perennial pastures M. B. DODD A. W. MCGOWAN I. L. POWER AgResearch Ltd Ruakura Research Centre Private Bag 3123 Hamilton, New Zealand B. S. THORROLD Dexcel Ltd Private Bag 3221 Hamilton, New Zealand Abstract Two experiments examining the effect of artificial shade on pasture net herbage accumulation (NHA), botanical composition and soil characteristics were conducted between 1994 and 1999 at Whatawhata Research Centre. Both experiments included a range of shade levels (-94%) and shade durations (3-12 months per year). Experiment 1 also included a light quality treatment, incorporating a range in the red:far red ratio (.49-1.). All three shading factors decreased annual NHA, with the most influential being the level of shade, which accounted for 68% of the variation and reduced NHA by 2-8% compared with open pasture. The second most influential factor was shade duration, which accounted for only 6% of the variation in NHA. Shading also led to changes in pasture botanical composition, most notably a decline in legume content in both experiments. There was no evidence that hairy pasture species (grasses or legumes) had any advantage over glabrous species under shade. At shade levels >6%, herbage nitrogen concentrations were elevated by.2 percentage points on a per unit dry weight basis, although reduced pasture A414; Online publication date 18 October 25 Received 9 December 24; accepted 1 August 25 NHA under higher shading meant lower demand for soil nitrogen. Potential nitrogen mineralisation measurements also indicated that nitrogen cycling is likely to be reduced under shading. Keywords light quality; nitrogen; pasture net herbage accumulation; shade level; shade duration INTRODUCTION Competition between trees and pasture plants for light, moisture, and nutrients are key factors which influence the pasture yield in silvo-pastoral systems. The shading effect of trees is the one factor where that competition occurs without reciprocal interaction, that is, the pasture has only the light available to it which the tree canopy allows through. Measurements of light levels under closed tree canopies indicate that very low levels of light are available for understorey growth (Knowles et al. 1999). Different tree species and functional types (e.g., deciduous versus evergreen) will have differing effects on the amount of light available to pasture understorey and the timing of that light availability (Power et al. 1999, 21). Previous field studies in silvo-pastoral systems have demonstrated that pasture yield, pasture quality, and pasture legume content are all reduced under tree canopies (Hawke 1991; Gilchrist et al. 1993; Guevara-Escobar et al. 1997; Devkota et al. 1998; Power et al. 1999). In such studies it is not clear what component of the reduction in pasture productivity is attributable to shade alone. The extent to which these effects are caused by shading (rather than competition for water and nutrients) is important in assessing the impact that tree species with different light transmission patterns might have on pasture yield and composition. A few recent studies have examined individual pasture species' responses to shade (Devkota et al. 1997), and only recently has the form of a shade response for a pasture sward been quantified (Power et al. 21). Development of models of tree-pasture interactions (e.g., Knowles et al. 1999) requires that

3 532 New Zealand Journal of Agricultural Research, 25, Vol. 48 the main effects and interactions of component factors (e.g., shading, root competition, allelopathy) be determined. This study aimed to contribute information on the main effect of shading to the process of decision support model development (Dodd et al. 24). In addition to quantity of light transmission, research has shown that the quality of light (often measured by the red to far red ratio, R:FR) can influence clover growth and stolon branching (Thompson & Harper 1988) and the tillering of perennial ryegrass plants (Deregibus et al. 1983). Different tree species have a range of understorey R:FR light properties (Smith 1982). Devkota et al. (2) found that the impacts of decreased R:FR light on the morphology of selected pasture species were small relative to those attributable to decreased photosynthetic flux density. An understanding of the extent to which spectral composition of light (as distinct from light quantity) causes pasture production and composition changes in the field would give direction as to the potential to manipulate tree-pasture interactions through the selection of tree species with differing R:FR understorey light. Given the apparent susceptibility of legume growth to shading (Hawke & Gillingham 1997; Power et al. 1999), it is of interest to determine whether this has flow-on effects to nutrient cycling. In particular, a reduction of legume growth under shade is likely to reduce nitrogen (N) fixation (Goh et al. 1996), resulting in a lower overall N status of the pasture-soil system. Likely shading effects on soil temperature and moisture content may also alter mineralisation rates and thus affect N cycling. This paper reports on two pasture shading experiments. The first examined the effect of shade level, seasonal duration and spectral composition on pasture yield and botanical composition. The main objectives were to determine which of these three effects was most influential, and to establish pasture response curves for variation in shade levels. Of additional interest was testing the hypothesis that hairy pasture species are more shade tolerant than glabrous pasture species (Hawke & Gillingham 1997). Some of the results of Experiment 1 have been reported in Power et al. (21). Those results did not include any effect that shading might have on altering the water balance under trees to improve the growth of pastures in dry periods during the summer-autumn. In Experiment 2 we aimed to determine the effect of shade level and seasonality in an experiment which allowed shade effects on soil moisture content to be expressed. A further objective of Experiment 2 was to determine the effects of shading on soil chemical properties, in particular nitrogen cycling. Soil nitrogen mineralisation measurements also required that soil moisture and temperature data be collected for interpretive purposes. This information was intended to allow us to build up a better understanding of nutrient cycling under shade conditions, again to distinguish shade-induced effects as distinct from tree effects. METHODS AND MATERIALS Site The work described in this paper was conducted at the Whatawhata Research Centre (Lat S, Long E, 22 m altitude), located in the western hill country of the Waikato region in New Zealand. The experimental site was near the top of a ridge, and had a westerly aspect with a slope of The soil type is a Kaawa hill soil, a sedimentary Brown Soil (fulvic) over argillite and greywacke (Bruce 1978). The existing pasture was variable in species composition across the site, hence it was removed by spraying with glyphosate (36 g a.i. ha 1 ) and cultivation with a rotary hoe to a depth of 2 mm. Following a 4-week fallow, regrowth was sprayed again and the site was levelled and consolidated with sheep treading. In May 1994, a new pasture seed mix was broadcast sown, which comprised 5 kg ha 1 'Ellett' perennial ryegrass (Loliumperenne L.), 4.2 kg ha 1 'Massey Basyn' Yorkshire fog (Holcus lanatus L.), 3 kg ha 1 'Grasslands Huia' white clover (Trifolium repens L.), and 7.5 kg ha 1 'Grasslands Maku' lotus major (Lotuspedunculatus Cav.). These species were chosen to include both glabrous and hairy grasses and legumes. Three days after sowing, the site was mob-stocked with ewes to tread and bury the seed for improved establishment. Experimental design Two consecutive shading experiments were conducted on the site, the first over a 13-month period from May 1994 to December 1995, the second over a 3-month period from September 1996 to March Experiment 1 incorporated variation in shade level, shade duration, and light quality, and focused on the effects of these factors on pasture net herbage accumulation (NHA) and botanical composition. Experiment 2 repeated the examination of variable shade level and shade duration (applying different

4 Dodd et al. Shade and pastures 533 levels to increase the range of these factors compared to Experiment 1) without the light quality treatments. This experiment also examined the effects of these two shading treatments on soil and plant nitrogen parameters. Experiment 1 Forty 1.8 m 2. m plots were marked on the site, with a 2 m separation between plots. On each plot to be shaded, wooden frames supported by four short posts were constructed. The height of the frame above the pasture varied between.25 and.8 m due to the slight slope of the ground. The frames were covered with horticultural shade cloth and blue filters (see Table 1 for specifications) to provide a range of shading levels and light quality. Side light was reduced by hanging 8% shade cloth around all four sides of every frame between the frame edge and the ground. A microjet sprinkler system was established to maintain soil moisture content at a level not limiting to pasture growth (>4% v/v). Soil moisture content to 15 mm depth was measured by time domain reflectometry (TDR, Topp et al. 198) at 4- to 6-weekly intervals. A Trase 65X1 TDR meter (accuracy ±2%; Soil Moisture Equipment Corp., Santa Barbara, CA) equipped with 15 mm portable probes was used for these measurements. Five levels of shading and four levels of light quality were applied to the plots in a randomised partial factorial design (Table 1). The design also included three levels of shade duration: 12 months, 8 months (November through July), and 4 months (December through April). Individual treatments were not replicated. Experiment 2 The shade frames were realigned to establish 32 new pasture plots (of the same dimensions) that were previously between the plots in Experiment 1. These pasture areas had been mown since the beginning of Experiment 1 at the same times as the plot harvests in that experiment. For Experiment 2, the frames were only covered with porous horticultural shade cloth, allowing natural rainfall to reach the plots and eliminating the need for supplementary irrigation. Eight levels of shading were applied to the plots for four different durations in a randomised factorial arrangement. The levels of shading based on the nominal specifications of the shade cloth were 9, 8, 7, 5, 32, 25, 15, and % (open pasture). The four time periods of shade imposed were: 12 months, 9 months (September through May), 7 months (October-April), and 3 months (December-February). As in Experiment 1, the individual treatments were not replicated. Table 1 Shade cloth and light filter specifications, and measured light levels, for Experiment 1. Treatment Shade cloth 1 specification (%) None Filter type 2 None Clear '/ 4 CT Blue»/2 CT Blue Full CT Blue Clear % CT Blue l A CT Blue Full CT Blue Clear Full CT Blue Vi CT Blue»/2 CT Blue Clear Measured shade level (%) Measured red:far red index 3 1 Green/black knitted. Manufacturer: Donaghys Sarlon Pty Ltd, P.O. Box 15 7, Christchurch. Specifications as per the manufacturer: Chris James & Co Ltd, 74 Broadfield Lane, York Way, London. 3 Red:far red ratio below shade cloth or filter divided by ratio above shade cloth or filter

5 534 New Zealand Journal of Agricultural Research, 25, Vol. 48 Measurements Experiment 1 Measurements of light transmission and R:FR ratio were made to check the alignment of nominal and actual shade and light quality levels incident on the pasture (Table 1). Actual shading levels on the plots (photosynthetically active radiation, PAR) were measured with a LI-COR LI-191SA line quantum sensor (LI-COR Inc., Lincoln, NE). Percent shading was calculated as the PAR measurement below the shade cloth divided by the PAR measurement above the shade cloth ( 1). Actual light quality on the plots was measured with a Skye 1 logger and Skye 11 sensor, 66/73 nm range (Skye Instruments Ltd, Powys, UK) and calculated as the R:FR ratio below the shade cloth divided by the R:FR ratio above the shade cloth. Pasture production was determined by harvesting the NHA within a.25 m 2 quadrat from the centre of each plot at monthly intervals using an electric hand-piece fitted with a 5 mm skid. Following the harvests, the whole area was mown with a rotary mower to a similar level. The harvested pasture samples were washed and dried at 7 C for 24 h to determine dry matter accumulation. Annual relative pasture yield was calculated as the sum of NHA from the individual harvests on the shade treatments from January to December 1995 divided by the sum of NHA from the individual harvests on the unshaded plots ( 1). From each cut, a subsample of c. 4 pieces was dissected to determine pasture species composition. The subsample was separated into the required species groups: L. perenne, other glabrous grasses, H. lanatus, other hairy grasses, T. repens, other glabrous legumes, Lotus spp., other hairy legumes, weeds, mosses, and dead matter. Each fraction was dried at 7 C for 24 h for measurement of dry mass. Experiment 2 Measurements of shading level and pasture NHA were as described for Experiment 1. For Experiment 2, the pasture dissection categories were L. perenne, Agrostis capillaris (browntop), Poa spp., other glabrous grasses, H. lanatus, Anthoxanthum odoratum (sweet vernal), other hairy grasses, T. repens, Lotus spp., Trifolium subterraneum (subterranean clover), other hairy legumes, weeds, mosses, and dead matter. Soil moisture content to 15 mm was measured on a monthly basis during summer and autumn only, using the Trase TDR meter. Soil temperatures at 5 mm were measured on a daily basis under the 12-month shading treatments (all shade levels) with a CR21 data-logger (Campbell Scientific Inc., Loughborough, UK) for 6 months from September 1997 to March The foliage nitrogen concentration of the harvested pasture was determined by near infrared reflectance (NIR) analysis (Corson et al. 1999) of dried herbage samples from each of the 12 harvests in Soil nitrate (NO 3 ) and ammonium (NH 4 + ) levels were measured by soil sampling in March 1998 and April From each plot, five 25 mm diameter soil cores to a depth of 75 mm were taken. The soil samples were analysed for NO 3 and NH 4 + using the 2M KCl method of Blakemore et al. (1987). Aliquots were extracted using a Tecator 527 sampler and analysis was completed by flow-injection with a Tecator FIAstar 51 analyser, read by a Tecator 523 spectrophotometer at 54 nm (NO 3 --N) and 59 nm (NH4+-N) and recorded onto computer where the quantities of NO 3 -N and NH 4 + -N in ppm were calculated with the manufacturer's software. Potential N mineralisation was measured using 5 g soil from each core, anaerobically incubated in 2 ml water for 7 days at 4 C. The amount of N mineralised (μg g 1 of dry soil) was calculated as the difference between NH 4 +-N after incubation and the sum of NO 3 -N and NH 4 +-N before incubation, which assumes immobilisation of the initial NO 3 -N during the anaerobic incubation. Olsen-extractable phosphate (P), sulphate-sulphur (S), Ca, K, Mg, Na, and ph (1:2.5 soil:water suspension) were measured on all plots at the end of Experiment 2. Five 25 mm diameter soil cores to a depth of 75 mm were taken from each plot and analysed using the Quick Test method (Cornforth & Sinclair 1984). Statistical analyses The nominal and measured shade levels, and the nominal and measured light quality levels, were compared by Pearson correlation analysis, using the CORR procedure of SAS (SAS Institute 1999). Annual relative pasture yield for each treatment was calculated from the NHA data, assuming the mean NHA of the unshaded plots represented 1% of potential yield at the site. The annual NHA data from Experiment 1 were initially analysed separately, as this experiment incorporated the additional light quality factor. The annual NHA data were analysed using the GLM procedure of SAS (SAS Institute 1999), followed by regression analysis using

6 Dodd et al. Shade and pastures 535 stepwise selection (significance levels for entry and exit were set at.5) to obtain partial R-squares for the selected terms. Use of the term significant denotes P <.5 unless otherwise noted. The pasture NHA data from both experiments were also analysed using a multidimensional mixmodel smoother ('Flexi'; Upsdell 1994), using measured shade level as the independent variable. The repeated measures nature of these data was accommodated by including a plot time term to account for the time correlations within a plot. An independent identically distributed Day term was included to allow for effects occurring only on the day of measurement. The 83% confidence bands for the different effects are shown so that the curves are significantly different at the 5% level where the bands do not overlap, and any point on a curve is significantly different from another where their associated bands do not overlap. The pasture species dissection data were combined into six groups for analysis: glabrous and hairy grasses, glabrous and hairy legumes, weeds, and dead matter. Changes in botanical composition were analysed by using the GLM procedure in SAS (SAS Institute 1999) in both a repeated measures analysis and an analysis of differences in percentage content between spring 1994 and 1995 (Experiment 1) and between spring 1996 and 1998 Table 2 A General linear model and regression analysis of pasture net herbage accumulation data in Experiment 1. Model Factors Significance R-square Relative yield Nominal shade level Shade duration Filter specification Relative yield Measured shade level Shade duration Measured R:FR index NS Table 2B General linear model and regression analysis of pasture botanical composition data in Experiment 1. Model Factors Significance R-square Grass (%) Nominal shade level Shade duration Filter specification Legume (%) Measured shade level Shade duration Measured R:FR index NS < (Experiment 2). The soil moisture and temperature data, herbage nitrogen data, and soil nutrient data (the latter measured at the end of the two experiments) were also analysed with the GLM procedure and least squares means calculated in SAS (SAS Institute 1999) using nominal treatment level as the independent variable. RESULTS Nominal versus actual light quantity and quality Although there was variation between the nominal shade levels of the cloth and the actual degree of shading (Table 1), this variation was systematic. There was a high positive correlation between nominal and actual shading levels (R =.87, P <.1). Similarly, there was a high positive correlation between the nominal filter specifications and the measured R:FR index (R =.85, P <.1). Pasture production In Experiment 1, annual NHA decreased significantly under all three main factors: increased shade level, longer shade duration, and increased light filtering (Table 2A). No interactions were significant in the general linear model. Regression analysis indicated that shade levels accounted for the most variation in NHA, with shade duration and light filtering accounting for a relatively small proportion of the variation. However, where measured shade level and R:FR level were used as the independent variables, only the shade level and duration were significant terms in the model (Table 2A). In the analysis of the data combined from both experiments, NHA again decreased significantly under both increased shade level and longer shade duration. However, the interaction between shade level and duration was significant, indicating that the decrease due to shade level was more marked under the longer shade duration (Fig. 1). Use of the nominal or measured shade level as the independent variables gave similar results. Pasture botanical composition The contributions of the various pasture botanical fractions to herbage accumulation were variously influenced by the level of shade, shade duration, and light quality. In Experiment 1, the proportions of both sown grass species (perennial ryegrass and Yorkshire fog) decreased significantly under

7 536 New Zealand Journal of Agricultural Research, 25, Vol. 48 A) 3 month shade period B) 4 month shade period C) 7 month shade period Q) D) 8 month shade period 12 -: E) 9 month shade period F) 12 month shade period 1 SO - ~~~~ \ o \ s Shade level (%) Fig. 1 Pasture relative yield response to measured shade level for six shade durations. A, 3 months, ; B, 4 months, ; C, 7 months, ; D, 8 months, ; E, 9 months, ; F, 12 months, and ). increased shade levels, but were not significantly affected by shade duration or light quality. When the proportion of all grass species was considered in the regression model, shade level accounted for only 1% of the variation, and light quality became a significant factor (Table 2B). In addition, the proportions of both sown legume species (white clover and lotus) decreased significantly under increased shade levels and increased R:FR conditions. When the proportion of all legume species was considered in the regression model, shade level accounted for only 13% of the variation, and shade duration and light quality also became significant factors (Table 2B). In Experiment 2 (Fig. 2A D), while increasing shade levels again resulted in significantly greater losses of legume content (from -1 up to -25 percentage points), total grass content was not significantly affected, and increasing shade led to significantly greater gains in weed (from +5 up to +2 percentage points) and losses in dead matter (from +6 down to percentage points) content. Clearly the most consistent effect of shading across both experiments was a decreasing proportion of total legume content, and this component of the sward was the only one to be significantly affected by shade duration in both experiments. For example, in Experiment 2 legume percentage was near zero when shading was extreme either from level or duration, e.g., 9% for 7 months or 7% for 12 months. The shade level effect on legume content was similar for hairy and glabrous legumes (Fig. 3). Contrary to the original hypothesis, the proportion of hairy grasses decreased while the proportion of glabrous grasses increased with greater shade levels (Fig. 3). This effect was consistent across both experiments. The exception within the glabrous grass category was perennial ryegrass, which decreased in proportion with greater shade levels. Soil moisture and temperature In Experiment 2, soil moisture content was significantly greater at higher shade levels (Fig. 4) and for the intermediate shade duration treatments (7 and 9 months). The interaction between shade level and shade duration was not significant. The seasonal pattern of soil moisture content showed a typical annual

8 Dodd et al. Shade and pastures 537 S i (A) Grasses 6. -i (B) Legumes u Nominal shade level (%) Nominal shade level (%) (C) Weeds 6. (D)Dead 4. 1 Q. O Nominal shade level (%) -6. Nominal shade level (%) Fig. 2 Change in botanical composition (percentage points gain or loss) between spring 1996 and spring 1998 in Experiment 2 for A, grasses; B, legumes; C, weeds; and D, dead matter (lines are linear least squares fit, with slope coefficients significant at P <.5 for legumes, weeds, and dead matter). Fig. 3 Effect of shade level on the content of glabrous and hairy pasture species as a percentage of dry matter in Experiment 2 (bars represent SEM). f-^ glabrous grass hairy grass glabrous legume hairy legume Nominal shade level (%) 1

9 538 New Zealand Journal of Agricultural Research, 25, Vol U o 2 Fig. 4 Effect of shade level on mean soil moisture content (-15 mm) and soil temperature (at 5 mm) in Experiment 2 (bars represent SEM). 15 g. E Soil moisture Soil temperature 1 = a>? I Nominal shade (%) 8 1 Table 3 Soil nitrate (μg N g 1 ), ammonium (μg N g 1 ), and potential N mineralisation (μg N g 1 ) in Experiment 2 (March 1998 and April 1999). Shade level (%) SEM Soil NO SoilNH Potential N mineralisation pattern of lowest levels in summer and autumn, but there was no significant interaction between measurement date and shade level. Soil temperatures were significantly lower under all shaded conditions, compared to open pasture (Fig. 4). However, there were no significant differences in average soil temperature amongst the higher shade levels (32-9%). Soil chemistry Soil nitrate levels were consistently low and not significantly different between shade level treatments in March 1998, but significantly increased with greater shade level in April 1999 (Table 3). Soil ammonium levels were highly variable in 1998 with no clear pattern across shade treatments. They were much lower and less variable in 1999, with the highest levels occurring at intermediate shade levels. Anaerobic incubation measurements showed significant differences in potential N mineralisation between shade levels in 1999 but not in In 1999, potential mineralisation rates were not significantly different for nominal shade levels between and 32%, but decreased with increasing shade levels above 32% (Table 3). At the end of Experiment 2, there were significant differences between shade level treatments for P, S, and ph (Table 4). Soil P, S, and ph levels were 1.5, 8.3, and 5.25 in the no shade treatment compared to 21., 19.3, and 5., respectively in the 9% shade

10 Dodd et al. Shade and pastures 539 Fig. 5 Effect of shade level on average herbage nitrogen concentration (% of DM) and nitrogen yield (kg N ha 1 ) in Experiment 2 (bars represent SEM). A Herbage N% N yield f l '. V To z NHA (kg en yield ftorn ) 1 > Nominal shade level (%) Table 4 Soil phosphate (μg ml 1 ), sulphate (μg ml 1 ), and ph levels after Experiment 2. Shade level (%) SoilP SoilS SoilpH SEM treatment. There were no significant differences between shade level treatments for the other soil nutrients tested, with means for these of 3.4 (Ca), 3.7 (K), 12.6 (Mg), and 8.6 (Na). There were also no significant differences between shade duration treatments for any of the soil test parameters. Herbage nitrogen In Experiment 2, the average nitrogen concentration of the herbage increased significantly at shading levels >6% (Fig. 5). A significant interaction with measurement date indicated that this effect was greatest during the summer-autumn period. There was no significant effect of shade duration on herbage nitrogen concentration. When these N tissue concentration data were combined with NHA data, an overall significant decline in nitrogen yield was evident at higher shade levels (Fig. 5) and under longer shade durations. DISCUSSION Pasture production declines under tree canopies have been commonly observed for a number of tree species (Hawke 1991; Gilchrist et al. 1993; Guevara- Escobar et al. 1997; Power et al. 1999; Devkota et al. 2). In most field studies, the causal factors are confounded, and hence this effect could be due to shading, alterations in light quality, competition for soil moisture or nutrients, or biotic interactions resulting from the presence of trees (e.g., smothering by leaf litter, proliferation of pasture pest or diseases that impact on net pasture herbage accumulation). In some cases, there is evidence to support particular factors being ruled out. For example, Power et al. (1999) measured no significant changes in the moisture regime along a tree stocking rate gradient at a high rainfall site. However, the large number of potentially influential factors and their likely interactions make it difficult to identify the most important ones for inclusion in models (e.g., Knowles et al. 1999; Dodd et al. 24) and manipulation in the field (by choice of species and management regimes). The

11 54 New Zealand Journal of Agricultural Research, 25, Vol. 48 known strong effects of shading on pasture species (Devkota et al. 1997) suggest that canopy shading may be the main cause of understorey pasture production declines with increasing tree stocking rate. Experiment 1 was designed to compare the relative magnitude of light quantity and quality effects on pasture NHA. This experiment showed that light quantity had a much greater effect than light quality, as also noted in the study of Devkota et al. (2) with a range of pasture species in the glasshouse. The observation of a significant light quality effect was dependant on the type of data analysed, i.e., the analysis with nominal filter specification data compared with actual R:FR index data gave a significant and non-significant result, respectively. The range in R:FR index values observed in this experiment was similar to that observed in the field under tree canopies of various species (.4-.9, I. Power unpubl. data). Thus, light quality appears to be of minimal importance to the NHA of these mixed pastures in the field. The interaction of shade level with shade duration demonstrated that short periods (i.e., 3-4 months) of low-moderate shade levels (<6%) had minimal effects on overall annual NHA (Fig. 1A,B). There was some evidence of compensatory growth in these pastures, in that the March harvests (following removal of the cloth under the 3-month shade treatment) of 1998 and 1999 gave mean relative yields of 124 and 118%, respectively. However, high levels of shade (>6%), and/or periods of shade longer than 4 months will have a significant impact on annual NHA. Using the data from Experiment 1, Power et al. (21) have demonstrated that seasonal NHA is unable to recover following shade removal where the duration of shade is longer than 4 months and the level of shade is greater than 6%. There are few deciduous trees with such a short leaf period, limiting the potential for selection of tree species to minimise impacts on pasture production. Therefore, manipulation of shade levels through tree stocking rate, thinning, and pruning management may offer more scope for minimising pasture production impacts. Pasture species composition was markedly affected by shading, and the strongest influence was again shade level, as opposed to duration or light quality. As in other studies, legume content declined with shading (Percival et al. 1984; Hawke & Gillingham 1997). This effect was consistent across both experiments, for both hairy and glabrous legumes, and for individual species. Hence, there did not appear to be an advantage in selecting a hairy legume (specifically L. pedunculatus) over white clover for sowing under tree canopies. Lotus major is a valuable understorey plant in Pinus radiata plantations for the purpose of nitrogen fixation and cattle grazing (West et al. 1991) but its success in this context may be less to do with shade tolerance than lack of competition on infertile soils. The sown grasses (perennial ryegrass and Yorkshire fog) also declined under shade and were replaced by species such as Poa annua, Hydrocotyle spp., and rosette forbs. Thus, there also appeared to be no advantage for selecting a hairy grass (specifically H. lanatus) over ryegrass for sowing under tree canopies. Yorkshire fog has been observed to be a greater component of swards under P. radiata than in the open, but it was suggested that this was a residual effect of grazing exclusion in the first 2-3 years after pine planting (Hawke & Gillingham 1997). When introduced under pine trees, Yorkshire fog did not perform well in the experiment of Percival et al. (1984). Pasture NHA response curves have been developed for variation in tree stocking rate and shade levels under Acacia melanoxylon and Eucalyptus nitens (Power et al. 1999). Those curves have also been compared with the data from the 12-month shade duration treatment in Experiment 1 reported here (Power et al. 21). The relationships were shown to be very similar, indicating that shade accounts for most of the effect of trees on understorey pasture growth. There were, however, two differences of note at low shade levels (2-4%), NHA under then-fixinga. melanoxylon was 5-1% greater than under 1% shade; and at moderate shade levels (5-7%), NHA under both A. melanoxylon and E. nitens, was 5-1% less than under 1% shade. This led the authors to suggest that competition for nitrogen (N) may be influencing pasture growth under the trees. At low shade levels, increased nitrogen availability in soils under N-fixing trees may be of benefit to pasture production, but at high shade levels, shade becomes the overriding factor, with associated factors such as smothering by leaf litter further depressing pasture production below that expected under 1% shade. The availability of nitrogen also appears to be an important factor in tree-pasture interactions, supplementary to shading effects. Steele & Percival (1984) reported increased N responsiveness of pasture under P. radiata compared to open pasture in the spring, but the reverse in winter. Sparling et al. (1989) have measured a decline in mineralisable N with increasing tree stocking rate of P. radiata. Anaerobic incubation measurements in this study

12 Dodd et al. Shade and pastures 541 also indicated reduced potential N mineralisation rates with greater shading intensity. This indicates that a smaller pool of organic N is available to be mineralised, probably a function of reduced plant decomposition as herbage yield decreases. While herbage N concentration increased as shade level increased, total N content (i.e., herbage N concentration herbage DM accumulation) decreased, pointing to both reduced uptake and return of mineral nitrogen. Nitrogen availability for plant uptake is controlled by moisture and temperature effects on decomposition and mineralisation (Meentemeyer 1978). Wilson (1996) attributed increased tropical grass growth under artificial shade to enhanced N mineralisation, whereby shade increased soil moisture and reduced extreme soil temperatures. In the present experiment, greater shading intensity also led to increases in soil moisture content, probably due to reduced evapo-transpiration from pasture, and decreases in soil temperatures. There was some evidence of increased soil mineral N (nitrate) in the second autumn of Experiment 2 under greater shade levels, suggesting that under conditions of reduced N cycling, N uptake was still lower than N input/return. The last factor, combined with the observation of greater soil moisture contents with greater shade levels, points to the possibility of increased nitrate leaching potential under shade. The further observation of lowered soil ph under greater shade levels supports the hypothesis of greater leaching, since N leaching is known to be an important causal factor in soil ph reductions (de Klein et al. 1997). However, under trees there will be further competitive demand for mineral N in the soil. The trees therefore represent another N sink, and given their deeper root structure, they may mitigate increased N leaching under the conditions of increased shade provided by their canopy. The influence of tree leaf litter quality is another consideration in this respect. Higher C: N ratios and lignin content are likely to retard net mineralisation (Wedderburn & Carter 1999). Under shade alone, three effects appear likely to influence N dynamics: 1) reduced legume content and therefore N fixation, given that fixation is usually strongly linked to legume production (Ledgard et al. 1987); 2) reduced N cycling, as indicated by the reduction in potential mineralisation of soil N, probably through lower levels of plant tissue decay; and 3) reduced overall demand for N by pasture as a result of reduced growth under low light conditions. This reduced demand was also reflected in increased levels of P and S in the shaded soils at the end of the experiment, an effect also observed under Pinus radiata (Hawke & O'Connor 1993). In attempting to quantify how much of the effects of trees on understorey pasture and soils is attributable to shade alone, we can compare the results of this single factor study with those under trees where all factors and their interactions are expressed. The similarity between the relative yield curve versus shade level under 12 months shading and the equivalent curve under two evergreen trees (Acacia melanoxylon and Eucalyptus nitens), indicating that shade accounts for most of the pasture yield reduction (Power et al. 21), has already been noted. In this study, legume content was reduced by 5% at shade levels of c. 35%, similar to the shade levels corresponding to a 5% legume content reduction under Acacia (c. 45%) and Eucalyptus (c. 35%) noted in Power et al. (1999). Such observations would indicate that shade is the dominant factor by which trees influence changes in understorey pasture characteristics. However, shade does not always account for the total magnitude of effects, which appear to be tree-species dependant. For example, the empirical canopy closure model of Knowles et al. (1999) predicts a much greater reduction in NHA under P. radiata than is accounted for by the relative yield versus shade level in Fig. 1F up to three times greater across the range of shade levels found under pines. The greater discrepancy in NHA (from the shade-only curve in Fig. 1F) under non-n-fixing trees (Eucalyptus and Pinus) compared with the N- fixing Acacia may be an important consideration (see Power et al. 1999). Nitrogen fertilisation experiments under P. radiata show significant responses (Steele & Percival 1984). It should be recognised that shade may only be observed to be the most important constraint on understorey pastures in the context of the temperate, mesic, relatively high fertility environments where these studies were conducted. In other environments, tree and pasture root competition for moisture and nutrients becomes more prominent (e.g., Anderson et al. 21). In some environments facilitation, rather than competition, effects have been observed, through the enhancement of soil nitrogen dynamics by shade (e.g., Wilson 1996). CONCLUSIONS Pasture net herbage accumulation was significantly reduced with increasing shade levels, longer shade durations, and lower proportions of incident red light wavelengths. Of these three factors, the manipula-

13 542 New Zealand Journal of Agricultural Research, 25, Vol. 48 tion of shade levels, as opposed to light quality, is likely to have the most impact on understorey pasture production. Shade level also influenced pasture species composition, with legume content markedly reduced under increasing shade levels. Soil temperature decreased in shaded compared to unshaded conditions, and soil moisture increased under increased shading. These patterns affected soil nutrient dynamics. Increased shade levels led to reduced N cycling, a function of reduced legume N inputs, decomposition rates, and plant uptake with lower pasture production. Shade appears to play a dominant role in constraining pasture growth in moist temperate perennial pastures. ACKNOWLEDGMENTS We thank Martin Upsdell for statistical analyses and Grant Douglas for his thorough comments on the original manuscript. The comments of the journal reviewers are also appreciated. This study was funded by the New Zealand Foundation for Research, Science and Technology under PGSF grant C1633. REFERENCES Anderson LJ, Brumbaugh MS, Jackson RB 21. Water and tree-understory interactions: a natural experiment in a savanna with oak wilt. Ecology 82(1): Blakemore LC, Searle PL, Daly BK Methods for chemical analysis of soils. New Zealand Soil Bureau Scientific Report No. 8. Bruce JG Soils of part Raglan County, South Auckland, New Zealand. Soil Bureau Bulletin 41. Wellington, Department of Scientific and Industrial Research. Cornforth IS, Sinclair A, Fertiliser and lime recommendations for pasture and crops in New Zealand (2nd rev. ed.). Wellington, New Zealand, Ministry of Agriculture and Fisheries. 76 p. Corson DC, Waghorn GC, Ulyatt MJ, Lee J NIRS: forage analysis and livestock feeding. Proceedings of the New Zealand Grassland Association 61: de Klein CAM, Monaghan RM, Sinclair AG Soil acidification, a provisional model for New Zealand pastoral systems. New Zealand Journal of Agricultural Research 4: Deregibus VA, Sánchez RA, Casal JJ Effects of light quality on tiller production in Lolium spp. Plant Physiology 72: Devkota NR, Kemp PD, Hodgson J Screening pasture species for shade tolerance. Proceedings of the Agronomy Society of New Zealand 27: Devkota NR, Kemp PD, Valentine I, Hodgson J Performance of perennial ryegrass and cocksfoot cultivars under tree shade. Proceedings of the Agronomy Society of New Zealand 28: Devkota NR, Kemp PD, Valentine I, Hodgson J 2. Shade tolerance of pasture species in relation to deciduous tree, temperate silvopastoral systems. Proceedings of the Agronomy Society of New Zealand 3: Dodd MB, Walcroft AS, Mackay AD, Luckman PG 24. Predicting tree canopy growth and shading impacts on pasture production. Primary Industry Management 7(2): Gilchrist AN, Dez Hall JR, Foote AG, Bulloch BT Pasture growth around broad leaved trees planted for grassland stability. Proceedings of the XVII International Grassland Congress Vol. III: Goh KM, Mansur I, Mead DJ, Sweet GB Biological nitrogen fixing capacity and biomass production of different understorey pastures in a Pinus radiata-pasture agroforestry system in New Zealand. Agroforestry Systems 34(1): Guevara-Escobar A, Kemp PD, Hodgson J, Mackay AD, Edwards WRN Case study of a mature Populus deltoides-pasture system in a hill environment. Proceedings of the New Zealand Grassland Association 59: Hawke MF Pasture production and animal performance under pine agroforestry in New Zealand. Forest Ecology and Management 45: Hawke MF, Gillingham AG Changes in understorey pasture composition in agroforestry regimes in New Zealand. Proceedings of the XVIII International Grassland Congress Vol. 1: Hawke MF, O'Connor MB Soil ph and nutrient levels at Tikitere agroforestry research area. New Zealand Journal of Forestry Science 23(1): Knowles RL, Horvarth GC, Carter MA, Hawke MF Developing a canopy closure model to predict overstorey/understorey relationships in Pinus radiata silvopastoral systems. Agroforestry Systems 43: Ledgard SF, Brier GJ, Littler RA Legume production and nitrogen fixation in hill pasture communities. New Zealand Journal of Agricultural Research 3: Meentemeyer V Macroclimate and lignin control of litter decomposition rates. Ecology 59:

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