Contrasting effects of water and nutrient additions on grassland communities: A global meta‐analysis

Abstract

Aim Changes in global climate and land use are expected to alter water and nutrient availability. Various meta‐analyses and large‐scale experiments show that increasing nutrient availability is expected to decrease the diversity of ecological communities, but so far, no study has attempted to provide a global‐scale perspective of diversity responses to water manipulation. Location Global. Methods We conducted a meta‐analysis focusing on the effects of water and nutrient additions both on species richness and on biomass of herbaceous plant communities. We identified 41 water addition experiments, of which 19 experiments manipulated both water and nutrients. Results Although both water and nutrient additions increased biomass (by c. 15 and 34%, respectively), only the latter consistently decreased richness (by c. 23%). Biomass responses to water addition were mainly derived from an increase in forb biomass (by c. 37%), whereas corresponding responses to nutrient addition were derived from an increase in graminoid biomass (by c. 56%). Addition of both water and nutrients led to larger biomass responses compared with the addition of each resource alone (by c. 69%), but the negative effect on species richness was similar to nitrogen addition alone. None of these responses could be explained by general (resource‐independent) theories, such as the productivity–diversity hypothesis or the niche dimension hypothesis. Main conclusions While highlighting overlooked patterns, this meta‐analysis reveals a fundamental knowledge gap in our ability to predict biodiversity responses to global change and demonstrates that future theories attempting to explain and predict such changes must take into account the potential implications of resource‐specific and functional group‐specific responses.

Nitrogen fertilization, not water addition, alters plant phylogenetic community structure in a semi‐arid steppe

“…In our experiment, water was applied in the middle of the growing season (July and August); future experiments should investigate whether earlier water addition reduces plant diversity in the semi‐arid steppe we studied, as suggested by the phenology hypothesis. Note that adding N and water together removed the positive effect of water addition on species richness in our experiment, which is consistent with the finding of a recent meta‐analysis (DeMalach et al., ). One possible explanation for this result is that increased photosynthetic rate after N addition led to greater transpiration rate and more rapid depletion of water from the rooting zone and therefore reduced soil moisture (Harpole et al., ; Zavaleta et al., ), counteracting the effect of direct water addition.…”

“…The generality of our findings, nevertheless, remains to be assessed by future studies. We hope our work will stimulate the rapid emergence of these studies, given a number of field experiments similar to ours have already been conducted (reviewed by DeMalach et al., ).…”

“…Fertilization studies in N‐limiting systems have found that increases in N availability tend to have positive effects on primary productivity, but negative effects on plant species richness (Bobbink, Hornung, Roelofs, Hornungt, & Roelofst, ; DeMalach, Zaady, & Kadmon, ; Gough, Osenberg, Gross, & Collins, ; Harpole et al., ; Suding et al., ). Water addition also often results in increases in primary production, whereas its effects on plant species richness are varied (DeMalach, Zaady, & Kadmon, ; Knapp & Smith, ; Yang et al., ). Notably, much research on N and water addition effects has focused on ecosystem‐level responses (e.g.…”

Summer precipitation limits plant species richness but not overall productivity in a temperate mesic old‐field meadow

“…In contrast to the rainout shelter‐induced decrease in species richness, our water addition treatment did not alter productivity, yet increased species richness. A recent meta‐analysis of 41 grassland experiments (involving a wide range of climatic and soil environments) reported no statistically significant effect of water addition on species richness (DeMalach et al., ). However, two other experiments in temperate‐mesic old‐field vegetation also found that species richness was increased by water addition (Sternberg, Brown, Masters, & Clarke, ; Stevens, Shirk, & Steiner, ).…”

“…These studies used a wide variety of experimental designs (e.g., some with just added water, just reduced water, or both, and some also included nutrient fertilizer treatments), variable durations of treatment applications (one year vs. multi‐year), and sites from a wide range of climatic zones, soil types, and vegetation successional stages — including forest, shrubland, grassland, tundra, desert and agricultural fields. Not surprisingly, therefore, it is difficult to make meaningful comparisons of results between studies, and to draw overall conclusions (Beier et al., ; DeMalach et al., ; Hoover, Wilcox, & Young, ; Knapp et al., ; Wu et al., ).…”