Research Article

Plant Diversity and Productivity Experiments in European Grasslands

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Science  05 Nov 1999:
Vol. 286, Issue 5442, pp. 1123-1127
DOI: 10.1126/science.286.5442.1123

Abstract

At eight European field sites, the impact of loss of plant diversity on primary productivity was simulated by synthesizing grassland communities with different numbers of plant species. Results differed in detail at each location, but there was an overall log-linear reduction of average aboveground biomass with loss of species. For a given number of species, communities with fewer functional groups were less productive. These diversity effects occurred along with differences associated with species composition and geographic location. Niche complementarity and positive species interactions appear to play a role in generating diversity-productivity relationships within sites in addition to sampling from the species pool.

Because species differ in their ecological attributes, the loss of biodiversity from local communities may be detrimental to the ecosystem goods and services on which humans ultimately depend (1). This issue has been the subject of major recent research efforts using experimental plant assemblages (2–6). However, differences in aims and approaches, and the fact that experimental manipulations of diversity have been restricted to single localities, limit the ability of ecologists to make generalizations and predictions. The design, analysis, and interpretation of these experiments are also complex (7), and the view that the loss of plant species can be detrimental to ecosystem functioning remains contentious (8–11). In particular, the mechanisms underlying the relationship between species richness and ecosystem functioning are still the subject of debate because of the difficulty in identifying and interpreting the importance of niche complementarity versus “sampling effects” (8, 12, 13). Here we report patterns of aboveground plant biomass from the most extensive experiment to date in terrestrial ecosystems, and we examine the underlying mechanisms.

We used standardized protocols to establish experimental assemblages of grassland species (grasses and forbs) that varied in species richness, and we measured above-ground plant biomass production at two localities in the United Kingdom and at single sites in Germany, Ireland, Greece, Portugal, Sweden, and Switzerland (14, 15). Sites differed widely in climate and other major environmental factors (Table 1). We simulated the loss of plant species by removing the existing vegetation and seedbank and reestablishing plant communities from seed (16). At each site, we established five levels of species richness, ranging from monocultures of grasses or forbs to higher-diversity assemblages that approximately matched background levels of diversity in comparable unmanipulated semi-natural grasslands at each site (Table 2). If reducing the number of species reduces productivity because of a decrease in functional diversity and therefore the amount of niche space occupied in the resulting depauperate community (2, 4, 6,17), then we expect, for a given number of species, that productivity will also be lower in communities with fewer functional groups. To test this, we categorized species into three functional groups: graminoids (grasses), nitrogen-fixing legumes, and other herbaceous species (herbs) and established communities containing one, two, or three of these groups. To replicate plant diversity, each level of species richness and functional group richness was represented by several different plant assemblages at each site (18). Each assemblage contained a different species or mixture of species. We used constrained random selection from the local pool of grassland species (14, 15) to form experimental plant assemblages where all polycultures contained at least one grass. To investigate the effects of species composition, each assemblage was replicated in a minimum of two plots including monocultures of many of the species involved. In total, the experiment comprised 480 plots and 200 different plant assemblages (19).

Table 1

Details of the eight field sites, including location [site, country, degrees of latitude and longitude, and altitude above sea level (asl)]; climate (mean January and July temperatures and annual precipitation); previous land use (arable crops, horse grazing, fallow land, or none); method of site preparation (methyl bromide fumigation or steam sterilization of the soil, hand weeding only, or use of a sterile sand substrate); number of biomass harvests; and mean aboveground biomass of the plant assemblages with eight species. Aboveground biomass comprised all living and standing dead plant material above 5 cm, harvested in two quadrats 20 cm by 50 cm once or twice each season around the times of peak biomass (where two harvests were taken, the values reported are the sum totals per plot). For brevity, we refer to accumulated net annual aboveground biomass as productivity but note that it provides only an estimate of the aboveground component of this process. All vegetation was cut to a height of 5 cm at the times of harvest and the clippings were removed. Plots received no fertilizer during the first 2 years of the experiment.

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Table 2

The experimental design at each location, showing numbers of plots per species richness level and for each level of functional group richness. Plant assemblages (where an assemblage is a particular species or mixture of species) were replicated in two plots at each site, with the same assemblage sometimes occurring at more than one site.

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Aboveground biomass patterns. Above-ground plant biomass in the second year of the experiment (an estimate of net annual aboveground primary production) differed significantly between sites [F7,185 = 24.73, P < 0.001 (Table 3)]. The productivity of plots with eight species (the highest richness common to all sites) ranged from 337 g m−2 in Greece to 802 g m−2 in Germany (Table 1) and was driven by environmental differences among sites. Extreme northern and southern locations in Sweden, Portugal, and Greece, where growing seasons are short and productivity is often limited by temperature and water (20,21), had the lowest biomass.

Table 3

Summary of the analysis of second-year aboveground biomass. We present the combined effect of the two richness terms and partition the separate species and functional group richness effects from initial analysis of variance (ANOVA) into a linear contrast (regression) and a deviation from linearity; that is, the quadratic and higher order polynomial terms (shown indented). Our experiment has multiple error terms: Diversity terms are tested against the plant assemblage term, the site differences and the assemblage term against the assemblage-by-location interaction, and the assemblage-by-location interaction against the overall residual. Nonsignificant block effects and locality-by-diversity interactions are omitted.

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Species richness and functional group richness had highly significant effects on aboveground biomass; overall, assemblages with lower diversity were less productive on average [combined effect of species richness and functional group richness:F 12,185 = 7.01, P < 0.001 (Table 3)] (22). Because there was no location-by-species richness interaction, differences in slopes between sites were not significant [F 22,29 = 1.25, P = 0.287 (Table 3)]. The overall effect of decreasing species richness was best described by a linear relation between productivity and the natural logarithm of the number of plant species [F 1,185 = 55.13, P < 0.001 (Fig. 1A)], which is similar to patterns reported from previous single-location experiments (4, 5) and predicted by theory (17,23). The log-linear relation corresponds to an initially weak but increasing reduction of productivity with decreasing species richness. Each halving of the number of plant species reduced productivity by approximately 80 g m−2 on average.

Figure 1

Productivity declines with the loss of plant diversity. (A) Overall log-linear reduction of above-ground biomass with the simulated loss of plant species richness. (B) Linear reduction with the loss of functional group richness within species richness levels. Points in (A) are total aboveground biomass for individual plots; lines are slopes from the multiple regression model using species richness on a log2 scale. Silwood and Sheffield are labeled together as UK. In (B), assemblages with 11 species occurred only at Silwood, whereas assemblages with 2, 4, and 8 species are represented at all sites, including the more productive, and therefore have a higher average biomass.

Plant cover was reduced by loss of plant species richness (F 1,185 = 3.84, P < 0.001). Cover and aboveground biomass are likely to be correlated, and biomass patterns may not occur after controlling for differences in cover (8, 24). However, highly significant reductions in aboveground biomass with declining plant species richness remained in multiple regressions which included cover as a covariate, and when plots with less than 80% cover were excluded.

For a given number of species, assemblages with fewer functional groups were less productive [F 2,185 = 6.34,P < 0.01 (Fig. 1B)]. A multiple regression using the (untransformed) number of functional groups, after accounting for species richness (Table 3), revealed that the omission of a single functional group reduced productivity by approximately 100 g m−2 on average.

Importance of scale. When all sites were analyzed together, the lack of a significant location-by-species richness interaction determined that the log-linear regression with parallel slopes provided the best overall model (Table 3 and Fig. 1A). However, when the data for individual sites are plotted separately, they look different, and when analyzed alone, produce a variety of different models (Fig. 2) corresponding to alternative qualitative relationships between species richness and ecosystem processes (25). There are two explanations for this result: (i) all sites conform to the same underlying model, and apparent differences between sites are due to the lower sample sizes and statistical power at each site; (ii) sites differ in their responses, but the analysis is not powerful enough to reveal a significant location-by-species richness interaction when sites are analyzed together. Much of the individual site deviation from the overall log-linear model may be due to lower within-site replication. There may also be transient effects at this early stage of the experiment that largely disappear by the following year (26). For these reasons, and for parsimony, we favor the more general and powerful combined analysis, which shows that differences between locations are not significant and suggests that there may be a single general relationship between species richness and diversity across all sites.

Figure 2

Biomass patterns at each site (displayed with species richness on a log2 scale for comparison with Fig. 1A). Best-fit models from individual sites based on adjustedR 2 are as follows: log-linear in Switzerland and Portugal; linear (untransformed species richness) in Germany and Sweden; quadratic in Sheffield; ANOVA with five species richness levels (significant treatment effects with no simple trend) in Ireland and Silwood; and no significant effect in Greece.

Our results highlight the importance of considering scale when studying relationships between diversity and productivity (14), as predicted by theory (23). Despite large differences in productivity between locations and no clear relationship between productivity and maximum within-site species richness (Fig. 1A), within a site, productivity generally declines as species are lost, reconciling apparent contradictions in the literature (27).

Multiple influences on productivity. Our experiment reveals the relative roles of richness, location, and species composition as determinants of productivity; these key variables explained approximately 18, 28, and 39% of the total sums of squares, respectively (Table 3). Although it accounted for a large amount of the total variation, species composition was not statistically significant [F185,29 = 1.29, P = 0.21 (Table 3)] (28). However, when we tested the presence in an assemblage of a particular plant species or functional group (29), of the 71 more commonly occurring species, 29 had significant (P < 0.05) effects on productivity, although virtually all these effects were small (Fig. 3). Only one species, the nitrogen-fixingTrifolium pratense, had particularly marked effects. On average, the omission of this species reduced productivity by approximately 360 g m−2. We also found highly significant effects from the presence of legumes and herbs when considered collectively as functional groups.

Figure 3

Percentages of the total sums of squares explained by the effects of individual species and functional groups. Twenty-nine species had significant effects (P < 0.05); the 15 most highly significant species (P < 0.001) are shown.

Evidence for niche complementarity and positive species interactions. There are three processes through which the loss of plant species richness could decrease productivity: (i) the “sampling effect” (17) or “selection probability effect” (8), in which more diverse synthesized plant communities have a higher probability of containing, and becoming dominated by, a highly productive species (10,23); (ii) niche complementarity, where ecological differences between species lead to more complete utilization of resources in intact communities relative to depauperate versions (2, 4, 6, 17,23); (iii) a reduction in positive mutualistic interactions between species in simplified assemblages (6). Distinguishing (ii) from (iii) will require detailed local experiments, nor is separating the sampling effect from complementary and positive interactions straightforward (6, 12,13, 30). However, the sampling effect predicts that the dominance of some species in high-diversity mixtures should be compensated for by reductions in the biomass of subordinate species. In contrast, fewer species declined in performance in polycultures than increased, which is consistent with a reduction of competition in mixtures of species relative to monocultures due to niche complementary or positive species interactions or both (Table 4) (31). Similar results have been reported elsewhere (5).

Table 4

Summary of regression analyses of the aboveground biomass of individual species across the species richness gradients. Slopes are from simple regressions analyzing change in estimated biomass per individual sown of a species with increasing log2number of species, after adjusting for differences between blocks and sites. “Plots” gives the sample size for each regression, and “sites” gives the number of locations from which they were derived.

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Only niche complementarity and positive species interactions can generate “overyielding,” where the total biomass of a mixture of species exceeds the monoculture biomass achieved by the highest yielding of the component species (32). We adapted (13) well-established techniques from agricultural and plant competition experiments (33) and used data from our replicated monocultures to test for overyielding in individual plant assemblages. As in the productivity analysis, overyielding differed significantly between sites (F 7,136 = 6.59; P < 0.001), but there was a consistent average decrease in overyielding with the simulated loss of species richness (slope = −0.021, SE = 0.0075,F 1,126 = 6.09, P < 0.05) and with declining number of functional groups within species richness levels (slope = −0.143, SE = 0.0544,F 1,126 = 5.08, P < 0.05). These results are again consistent with the occurrence of complementary and positive interactions within our mixtures of plant species and provide a second line of evidence indicating that our productivity patterns cannot be explained solely by the sampling effect.

Our results demonstrate multiple control of the productivity of experimental plant communities by geographic location and by the richness and composition of plant species and functional groups. Biomass patterns predict a log-linear decline in productivity with the loss of plant species richness, in which reductions of niche complementary or positive species interactions or both appear to play a role.

  • * To whom correspondence should be addressed. E-mail: a.hector01{at}ic.ac.uk

  • Present address: Department of Agriculture, The University of Reading, Earley Gate, Post Office Box 236, Reading, Berkshire, UK, GB-RG6 6AT.

  • Present address: Departamento de Botânica, Universidade de Coimbra, 3000 Coimbra, Portugal.

  • § Present address: Ecologie des Populations et Communautés, Université Paris Sud XI, URA CNRS 2154, Bâtiment 326, Orsay Cedex, France, FR-91405.

  • || Present address: Victoria University of Wellington, School of Biological Sciences, Post Office Box 600, Wellington, New Zealand.

  • Present address: Department of Environmental Resource Management, University College of Dublin, Belfield, Dublin, Ireland.

  • # Present address: Centre for Ecological Research, Kyoto University, Kyoto 606-01, Japan.

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