Drought-driven shifts in relationships between plant biodiversity and productivity in temperate steppes

1. Drought events induced by global climate change strongly modify
ecosystem structure and function in grasslands. Higher plant biodiversity
can enhance ecosystem resistance to drought, however the potential effects
of drought on the relationships between aboveground productivity (ANPP)
and biodiversity along a natural aridity gradient remain poorly
understood. 2. Here, we studied the effects of experimental drought on
plant community structure and ANPP during the two different dry years
(2015 and 2017) spanning three temperate steppe sites with low, medium and
high aridity levels. We also examined the effects of drought and aridity
on the associations of plant species (richness and abundance), functional
(community-weighted trait means and functional dispersion) and
phylogenetic diversity (evolutionary lineages) with ANPP. A structural
equation modeling (SEM) was used to elucidate how the drought affected
ANPP by altering biodiversity attributes. 3. We found that experimental
drought shaped the biodiversity-ANPP relationships, and the
biodiversity-ANPP relationships differed among three sites. At the low
aridity site, ANPP was correlated with plant functional traits, whereas
ANPP was correlated with functional dispersion, species and phylogenetic
diversity at the medium aridity site. At the high aridity site, ANPP was
correlated with species richness, functional traits and phylogenetic
diversity. As compared with 2015, natural drought in 2017 strengthened the
associations of species diversity, functional dispersion, leaf dry matter
content and leaf nitrogen content with ANPP across three sites. SEM
further showed that both experimental and natural drought directly
decreased ANPP, and indirectly decreased ANPP by reducing
community-weighted mean of plant height. 4. Synthesis. These results
suggest that the aridity increase or drought occurrence is likely to
induce the shifts in the associations of ANPP with species and functional
diversity, highlighting the important role of water availability or
precipitation conditions in regulating the biodiversity-ANPP relationships
in temperate steppes. Overall, functional trait related to plant size
plays a crucial role in modulating the response of productivity to drought
across three steppe sites. Thus, to buffer the negative effects of drought
on steppe ANPP should pay attention to protect the species with large
plant size.