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?