Great efforts by ecologists have
been made to predict plant community dynamics with ongoing climate
change (Grabherr et al., 1994; Lenoir et al., 2008; Gottfried et al.,
2012; Pecl et al., 2017; Steinbauer et al., 2018; Briscoe et al., 2019;
Oldfather et al., 2021). However, up to date, there is not a single
model to perfectly forecast biodiversity changes, especially those
associated with foundation species. Generally, plant population
demographic rates are determined by diverse ecological factors at
different life history stages, including the recruitment of new
individuals (e.g. , reproduction, seed germination and seedling
survival) and the mortality of old individuals. For example, the seed
germination and seedling establishment rates, which can dictate the
potential magnitude of future populations and scale of associated
recruitment dynamics (Gimenez-Benavides et al., 2008), are strongly
influenced by diverse abiotic and biotic factors (Butterfield, 2009;
Klanderud, 2010; Klanderud et al., 2017; Chen et al., 2020a).
Additionally, those factors may also cause, or contribute to, the
mortality of individuals at any life stage, together with competitive
exclusion (Hardin, 1960). Taking alpine cushion plants as examples, once
established, they grow very slowly but may persist for centuries (Molau,
1997). During these long periods of life history, cushion plants accrue
increasing facilitative effects on beneficiary species through
modifications of the micro-environments. However, as numbers of
beneficiary species/individuals grow, they may exert constraints on
plants’ long-term survival, growth and reproductive output (Schöb et
al., 2014a, b).
Continually upslope shifting of lowland species has induced various
challenges in alpine ecosystems (Lenoir et al., 2008; Chen et al., 2011;
Gottfried et al., 2012). However, how that affect the foundational
cushion plants and how the biodiversity sustained by cushion plants will
change remain largely unclear. Here, we delve into the complex drivers
of these biodiversity changes by combining
laboratory experiments with
demographic data along two “space-for-time” (Pickett, 1989) gradients
in the Himalaya-Hengduan Mountains, SW China. One gradient is along a
spatial elevation gradient where the air and soil temperatures gradually
decrease with increasing elevation (Wang, 2006; Chen et al. 2019), thus
serving as a surrogate for temporal climate warming. Another gradient is
along a microsite gradient from cushion-dominated to cushion-free
microsites and from high to low vegetation cover, thus serving as a
surrogate for different community successional stages. Under these
circumstances, we can explicitly assess the community dynamics of
foundational cushion plants under anticipated climate warming and the
associated biodiversity changes. Specifically, we selected eight
populations of the cushion plant Arenaria polytrichoidesdistributed along an elevational gradient to, firstly, reveal the
current and historical individual distributions within communities using
a landscape ecological conception and approach (Pickett & Cadenasso,
1995); after this, we determined the current population age structure,
density and productivity. We predict that cushion populations that
experience climate warming (lower elevation) will become gradually
fragmented (degenerated) because i) they are particularly
cold-adapted (Körner, 2003; Aubert et al., 2014; Boucher et al., 2016),
thus sensitive to climate warming (Cranston et al., 2015), ii)cushion plants’ reproductive output are constrained by increasing
beneficiary species (Schöb et al., 2014a) thus reducing population
recruitment probability and iii) seedling establishment is
difficult due to their low competitiveness (Chen et al., 2020a).
Secondly, we elucidated the successional processes of cushion-dominated
communities and the associated plant diversity changes. We predict that
plant diversity will significantly decrease when cushion plants
completely degenerate, because those species exclusively sustained by
cushion plant may lose safe microsites and hence go secondly extinct
(Losapio & Schöb, 2017). Finally, we determined the ecological factors
that drive cushion population dynamics in consecutive life history
stages, including flower and fruit production, seed germination and
seedling establishment both in simulated and field climates, and the
mortality of aged individuals. We predict that both climate change and
the associated changes in interspecific interactions simultaneously
regulate the cushion population dynamics hence plant diversity.