4.1 Overall biomass
While we found differences in community biomass across our climates characterized by different soils, the species-specific contributions to community biomass within these climates were consistent: as expected,D. glomerata , the graminoid species, typically produced the most biomass with P. lanceolata and L. corniculatus being less productive. Species-specific biomass scaled with the community biomass across climates. While this may suggest that species do have consistent contributions to community biomass, comparing individual biomasses across species and climates offers a different perspective.
Individual biomass analysis revealed a similar overall relationship as species biomass between climates, showing community hierarchies were largely maintained when accounting for survivorship. However, when looking at individual biomass, different intra-climate hierarchies emerge than when looking at species biomass. While D. glomerataconsistently dominated species biomass, either P. lanceolata orL. corniculatus demonstrated higher individual biomass at all climates except one. This in part is due to high survival rates ofD. glomerata , contrasting with fewer surviving individuals ofP. lanceolata and L. corniculatus . Thus, low survivorship led to higher individual competitiveness, and survival is not solely determinant of biomass dominance.
Species generally maintained their biomass hierarchies across communities while having different individual biomass hierarchies. This relationship reveals that survivorship is differentially affecting the study species within the same communities. While warming is known to influence the stability of biomass production because species respond asynchronously (Z. Ma et al. 2017), our findings do not support this. Instead, we find that species contributions to community productivity is relatively stable across climates. Mortality was closely associated with reference soil in our study – this trend may therefore be an experimental artifact. The reference soil treatment led to higher community biomass, root biomass, and mortality compared to local soils. High root biomass can indicate stronger belowground competition, with increases in belowground biomass typically being symmetric for neighboring individuals (Cahill and Casper 2000; Broadbent et al. 2018), but this effect was not quantified here. With limited resources available in each pot, intensive root competition may have resulted in decreased species abundance, however overall higher productivity (Tilman 1990; Rajaniemi, Allison, and Goldberg 2003), explaining how species with lower survivorship were able to increase their individual biomass to maintain overall community hierarchies. This demonstrates how competition for space strongly affects community productivity (Schmid, Huth, and Taubert 2021), while also agreeing with past studies finding that net primary production can be maintained even with shifts in community composition (Liu et al. 2018). Nevertheless, this observation underscores the role of mortality and individual species dynamics in contributing to overall community biomass.
Individual biomass experienced the strongest positive relationship with species biomass in cases where survival was low. This reveals a mortality influenced trade-off, where high total species biomass is achieved with the loss of individual biomass. Thus, an individual-rich community leads to higher net productivity in plant-model communities. While this trade-off has been documented in grassland monocultures (Heisse et al. 2007; Chalmandrier, Albouy, and Pellissier 2017), this is the first documentation in plant-model communities. High species evenness is considered critical in maintaining community biomass (Rohr et al. 2016). In our study, community biomass was maintained across communities of varying species evenness, due to the limited ability of species experiencing high relative mortality to produce larger individuals.