Introduction
Global change is altering plant community dynamics, yet impacts are often difficult to predict and can vary across multiple, interacting drivers (Valladares et al. 2015). Understanding the net outcomes of global change on local plant community structure is challenging because it requires integrating both direct effects of changing environmental conditions on individual species as well as shifts in the magnitude and types of biotic interactions (Götzenberger et al.2012; Kraft & Ackerly 2014; Vandvik et al. 2020). Global change can cause complete restructuring of plant communities via species turnover and/or reshuffling of competitive hierarchies (Brown et al. 1997; Smith et al. 2009; Dovrat et al. 2020). Alternatively, global change may further favor already dominant species within a community, reducing species diversity via competitive exclusion or decreased evenness (Sheil 2016; Regina et al. 2018). These dynamics can take years to play out, especially in long-lived and slow-growing systems, as short term responses may not fully encompass both environmental effects and shifts in biotic interactions (Komatsuet al. 2019). To meet these challenges, approaches that assess both density-independent and density-dependent mechanisms over long time periods are essential.
Adding to this complexity, both the type (e.g. climate change, nutrient pollution, land use change) and the number of drivers can have differential effects on plant communities (Komatsu et al. 2019). Warming temperatures and altered precipitation regimes, can shift species hierarchies through changes in competitive interactions under novel climate conditions (Hoover et al. 2014; Valladares et al. 2015). This has been shown to reshuffle species dominance in field studies (Evans et al. 2011; Cavin et al. 2013; Mariotteet al. 2013), particularly in response to drought, given the well-established trade-off between dominance and stress tolerance (Gilman et al. 2010). On the other hand, nutrient pollution, such as atmospheric nitrogen deposition, is likely to reduce niche differentiation by homogenizing habitats and may lead to competitive exclusion by dominant species (McKinney & Lockwood 1999; Smart et al. 2006). Reduced species richness and increased production of one or a few species under nitrogen deposition is common, particularly in grassland ecosystems (Zavaleta et al. 2003; Borge et al.2004). In most natural systems, these different global change drivers occur simultaneously, and thus their net outcomes on community structure are often unclear.
While global change is altering plant community dynamics worldwide, alpine tundra ecosystems are particularly vulnerable, as elevation dependent warming often amplifies the rate of temperature increase in high versus low elevation systems (Pepin et al. 2015). Additionally, shifts in winter precipitation and snow pack and atmospheric nutrient pollution from nearby urban and agricultural areas also pose a serious threat to the stability and diversity of alpine plant communities often finely adapted to local gradients of soil moisture and nutrients (Roth et al. 2013; Gobiet et al.2014; Little et al. 2016). However, while there is high confidence that alpine regions will continue to warm at a rate faster than the global average (IPCC 2018), predictions for changes in snow and nutrient pollution are much more uncertain, and vary considerably by region, latitude, and land use history (Hock et al. 2019). Thus, correctly attributing changes in alpine tundra plant communities to warming temperatures, versus co-occurring changes in snow and nutrient dynamics, is an ongoing challenge. What’s more, how these interacting global change drivers influence both density-independent and density-dependent processes is an important knowledge gap in our understanding of rapidly shifting tundra plant communities.
Recent emphasis has been placed on understanding how dominant species within a community respond to global change, given their high abundances, and disproportionate influence on ecosystem functions (Winfree et al. 2015; Wohlgemuth et al. 2016; Hillebrandet al. 2018; Avolio et al. 2019). Determining the mechanisms that allow species to dominate under novel environmental conditions can serve as proxies for whole community and ecosystem responses to global change (Avolio et al. 2019). In fact, the idea that “super-dominants,” or overabundant populations of native species, may have similar impacts as non-native invasive species on community and ecosystem function has begun to gain traction (Reginaet al. 2018; Zhao et al. 2021). Conversely, deciphering pathways by which dominant and subordinate species become more evenly distributed is critical for predicting long term maintenance of biodiversity and the preservation of rare species (Csergo et al.2013; Felton & Smith 2017). Broadly, viewing changes in plant community structure from an abundance-based rather than species or trait lens, has shown to be a powerful way to make general predictions across systems (Suding et al. 2005).
Here, we present a 15-year fully factorial warming, snow manipulation, and nitrogen (N) addition experiment with corresponding shifts in alpine plant community composition at Niwot Ridge, Colorado, USA. We estimate the influence of multiple global change drivers on the density-independent growth responses and density-dependent interactions of groups of dominant, subdominant, moderate and rare plant species over time using gjamTime, a dynamic, biophysical competition model (Clarket al. 2020). We use these model estimates to inform changes in relative abundance of each species group observed in experimental field plots. Furthermore, we estimate the net effects of density-independent and dependent factors on steady-state (ie. equilibrium) abundances of each species group across both ambient and experimentally manipulated environmental gradients. We asked: 1) What global change scenarios lead to further favoring dominant species versus reordering species hierarchies? 2) How do density-independent and dependent mechanisms influence the net outcomes of changes in plant community structure over time? 3) How do density-dependent shifts influence community stability under global change?