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.