1. Introduction
Grasslands cover a quarter of Earth’s terrestrial surface, having both agricultural and conservational importance (White, Murray, and Rohweder 2000). High-elevation grasslands are particularly known for being both highly endemic and having high species richness at regional scales, making them communities of high interest for biodiversity conservation (Körner 2003; 2004; Gillet et al. 2016). Grassland communities sequester carbon, protect soil against erosion and supply nutrient rich feed for agriculture (Zhao, Liu, and Wu 2020). These alpine regions however are at an especially high risk of disturbance from climate change (Schirpke et al. 2017). European alpine temperatures are expected to increase at above-average rates due to climate change (Gobiet et al. 2014; Pepin et al. 2015; Li et al. 2019). Surface air temperature in the European Alps is rising at 0.3 ± 0.2 °C per decade, exceeding global warming trends (Hock et al. 2019). Rising temperatures have implications for plant functional trait (hereafter ‘trait’) responses (Wipf, Rixen, and Mulder 2006; Alexander, Diez, and Levine 2015; Debouk, Bello, and Sebastià 2015), and can lead to community instability by increasing species synchrony (more synchronic responses of species composing the community) (Z. Ma et al. 2017). No consensus exists regarding generalized plant community responses to climate change due to complex interactions between climate and soil compositions (Yang et al. 2018). Measures such as trait responses offer an improved understanding of ecological responses that are comparable across regions and experimental approaches (Sporbert et al. 2021; Vandvik et al. 2020).
High-elevation grassland communities are vulnerable to climate change, in part, because these specialists perform poorly when faced with increased competition from invading lowland species (Alexander, Diez, and Levine 2015; Giejsztowt, Classen, and Deslippe 2020; Hansen et al. 2021; Smithers, Alongi, and North 2021). Community responses can vary because temperature affects both competitive and facilitative processes within semi-natural grassland ecosystems (Olsen et al. 2016). While some studies have correlated rising temperature to increases in aboveground community biomass (Berauer et al. 2019; Halbritter et al. 2018; Y. Niu et al. 2019), others have identified no such trend (Fu et al. 2013; Liu et al. 2018), demonstrating the sensitive nature of biomass and other trait responses to climatic variation. While community biomass is a coarse way to compare productivity across communities, a more nuanced understanding of community dynamics is enabled by investigating species-specific or functional group responses. Elevated temperatures can lower community biomass stability if composing species have asynchronous responses (X. Ma et al. 2017). Dominating species stability has also been identified as a stronger driver of biomass production stability than species richness (Valencia et al. 2020). The stability of biomass production has immediate consequences for human activities such as agriculture as well as implications for long-term ecosystem function and resistance to stressors like drought (Muraina et al. 2021). Consequently, examining biomass and other intra-specific trait responses is critical to understanding climate change effects on community level productivity.
Although responses to soil characteristics can be species-specific, studies measuring community responses to soil variation are nonetheless able to draw general trends (Zas and Alonso 2002). For example, nutrient addition can destabilize grassland primary production (Bharath et al. 2020). While this could be explained by asynchronous species responses to fertilization; unfortunately, species’ trait differences are often omitted from community-level studies investigating soil effects, which could reveal differential species responses within a community. In contrast, the effects of climate change on both above- and belowground traits is well documented, with effects typically mediated by changes in soil chemistry, fauna, and the microbial community (Briones et al. 2009; Hagedorn, Gavazov, and Alexander 2019). Traits changes can in turn affect soil microbiota, resulting in interdependency of species within a community (Wang et al. 2017). Puissant et al. (2017) projected that climate warming would lead to reduced soil organic carbon content, thus decreasing soil microbial activity, and ultimately lowering plant biomass, while Chen et al. (2020) predicted increases in soil organic carbon as a result of warming. These contrasting findings highlight the dependence of community responses on climate and local soil. Field experiments that manipulate climate while incorporating natural soil variation will therefore more accurately predict trait responses in plant grassland communities than observational studies that cannot partition the effects of these drivers.
Plant communities will experience changes in several abiotic parameters due to climate change, such as precipitation, seasonality, and temperature regimes, resulting in altered biotic conditions. For species to cope with climatic changes, interspecific trait variation, phenotypic plasticity and local adaptations are essential (Gonzalo-Turpin and Hazard 2009; Frei et al. 2014; Midolo and Wellstein 2020). Grassland species generally respond plastically to changing environmental conditions (Valladares et al. 2014; Cui et al. 2018; Kreyling et al. 2019), however co-occurring grassland species exhibit differences in trait responses to climatic stress (Hamdani, Krichen, and Chaieb 2019). While it can be expected that species respond differently under stress, how these species-dependent responses affect overall community trends remains unclear.
Here, we monitored model-grassland communities in the European Alps for one year. By experimentally manipulating both soil and climate, we identified the independent effects of each driver on species and community-level traits. We measured a variety of traits related to productivity and fitness. Specifically, we hypothesized that 1) the relative contribution of individuals and species to total community biomass would remain consistent irrespective of community productivity, 2) climate and soil differences would lead to trait variation across species and locations, and 3) our community-based approach would identify separable effects of climate and soil on plant trait dynamics.