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.