4.2 Soil effects
Our study design allowed us to isolate soil effects from climatic
effects on community and species-specific trait dynamics. Surprisingly,
we found that despite each study location having a unique combination of
soil and climate, all communities experienced similar changes in plant
traits. This community-level finding contrasts with findings at the
species level, which revealed stark variation in trait responses.
Interactions between climate and species were prevalent across most
response variables. A lack of distinguishable patterns in the magnitude
or direction of soil effects demonstrates high trait stochasticity
within our plant communities, which has previously been attributed
largely to environmental variation (Davison et al. 2010; Riginos et al.
2018). Random forest analysis supported this finding, with no increases
in predictive power being found when examining soil effects. Nutrient
availability has also been shown to influence community assembly within
grassland communities (Guo et al. 2014), with community dynamics
shifting away from niche-based determination towards stochasticity and
species asynchrony in the short term under high nutrient availability
(Zhang et al. 2016; Conradi, Temperton, and Kollmann 2017). While our
study only examined these responses following one year of treatment, our
findings demonstrate the short-term effects of nutrient variation
leading to high grassland community interspecific trait stochasticity.
We accept our hypothesis that soil effects would lead to high trait
variation across both species and climate. No species consistently had
the largest changes for any measured trait. For example, L.
corniculatus had greater maximum height at Stubai (1850 m) relative to
other species but grew less than others at Graswang (850 m). Further,
soil effects resulted in unique species trait variation across climates.
Surprisingly, species with high values for one trait that is
traditionally linked to fitness did not concurrently increase in other
fitness-linked traits. For example, if a species had high survival rates
at a given climate, this did not necessitate high values for traits such
as leaf dry weight, leaf area, and biomass. This offers another
trade-off example between community-level survivorship and individual
fitness.
The combination of local soil and climate yields stochastic trait
responses in our study for all species. Past experiments have documented
shifts in dominance hierarchies depending on interactions between
nutrient and climate treatments (Alatalo et al. 2014; S. Niu and Wan
2008). For example, Klanderud and Totland (2005) found that climate
change and nutrient addition in grassland ecosystems caused changes in
dominance hierarchies, community structure, and diversity. While
nutrient addition alone increased the competitiveness of graminoid and
forb species, the climatic treatment did not have this effect. This
aligns with our results, underscoring the role of changing species
interactions resulting from variation in soil nutrient composition on
the trait responses of individuals. It is difficult to simulate natural
soil conditions. Naturally occurring soil minerals extend well beyond
standard nitrogen, phosphorous, and potassium. Many other minerals are
known to have interactions with shifts in climate change-relevant plant
traits such as transpiration or root acquisition of soil minerals (Lynch
and St.Clair 2004). For this reason, natural system experiments remain
the most complete look into the future of plant grassland communities.
Given the importance of soil composition on community trait dynamics, we
suggest further work investigating grassland community responses to
climate change incorporate natural soil systems.