Enough space in a warmer world? Microhabitat diversity and small‐scale distribution of alpine plants on mountain summits

Abstract

Aim Global warming is predicted to shift distributions of mountain species upwards, driven by a release from climatic restrictions at their upper distribution limit and increased biotic pressure at their lower distribution limit. In alpine ecosystems, which are characterized by large microclimatic diversity and sparse vegetation cover, the relative importance of abiotic and biotic drivers for species distribution is poorly understood. To disentangle abiotic and biotic mechanisms affecting distributions of alpine species, we investigated how alpine plant species with differing elevational ranges and frequency trends over the past century differ in their microhabitat distribution, and how they respond to neighbouring vegetation. Location A total of 11 summits (2635—3410 m a.s.l.) in SE‐Switzerland. Methods We quantified the microscale abundance of 12 species in relation to biogeographic (frequency trend, i.e., change in occurrences over the past century, and elevational range on summits) and local microhabitat characteristics (temperature, substrate type). We assessed species size traits in relation to neighbouring vegetation characteristics to investigate possible neighbour interactions. Results Species with increasing frequency on summits over the past century were most abundant on scree and warmer slopes. Species with negative or stable frequency trends on summits were more abundant on organic soil and colder slopes. The preferred microhabitats of the latter were rarest overall, decreased with increasing elevation, and had the most competitive neighbours. Size of one high‐alpine specialist, Ranunculus glacialis was negatively related to cover of neighbouring vegetation, whereas other species showed no response to neighbours. Main conclusions Long‐term frequency trends of species correlate with their microhabitat association. Species with most negative frequency trends show preferences for the rarest microhabitat conditions, where they likely experience higher competitive pressure in a warming climate. This finding emphasizes the importance of characterizing microhabitat associations and microclimatic diversity to assess present and future distributions of alpine plant species.

Increases in thermophilus plants in an arid alpine community in response to experimental warming

“…In this temperature-and soil moisture-limited alpine system, we show that minimally warmer conditions may drive a community shift toward relatively hotter, drier adapted species within a short time frame. This work is in agreement with others who found an increase in hotadapted species, but a lesser subsequent decrease in cooladapted species Alexander et al 2018;Kulonen et al 2018). We observed no response to the experimental heating in the mean CCN or for the coolest, wettest adapted species (twenty-fifth percentile CCN; Figure 4c, d).…”

“…This process is shaped by the loss of or decrease in the abundance of cryophilic species, and a gain or increase in abundance of thermophilic species . Although fine-scale microclimatic gradients driven by topography may buffer species loss in response to warmer conditions (Scherrer and Körner 2011;Lenoir et al 2013;Patsiou, Conti, and Zimmermann 2014), community shifts are likely to occur as competition and facilitation dynamics play out across fine-scale gradients of climate and substrate type (Alexander et al 2018;Kulonen et al 2018). Further, competitive exclusion by low elevationassociated species invading alpine vegetation may more strongly shape community responses to a warming climate than the change in climate itself (Alexander, Diez, and Levine 2015).…”

Community turnover by composition and climatic affinity across scales in an alpine system

“…As dispersal limitation at this scale is unlikely, two major potential factors that may be instead be shaping community distributions are habitat type (e.g., presence of moveable scree) and species interactions. In addition to microclimate, substrate type (rock, scree, organic soil) likely heavily influence species distributions, as well as species’ responses to climate change (Kulonen et al., ). Species interactions directly, or in conjunction with habitat type and microclimate, can also influence species distributions at fine spatial scales (Alexander et al., ; Blonder et al., ; Graae et al., ).…”

“…Species interactions directly, or in conjunction with habitat type and microclimate, can also influence species distributions at fine spatial scales (Alexander et al., ; Blonder et al., ; Graae et al., ). For example, lower‐elevation species may be more competitive on organic soils than on rock or scree, whereas higher elevation obligate alpine plants potentially have more adaptive traits (e.g., contractile roots) for persisting on the dynamic scree habitat (Kulonen et al., ). As we build our temporal data set, future studies will examine whether interactions between habitat segregation and elevation influence species and community responses to climate change over time.…”

Incorporating microclimate into species distribution models

“…In-situ measurements of temperature and other microhabitat characteristics -without interpolation -have additionally been shown to be valuable for descriptive distribution modelling at the local scale (Opedal et al 2015, Frey et al 2016. For example, strong correlations have been observed between changes in the frequency of plant species over time and the in-situ temperature of their preferred microhabitat on mountain summits in Switzerland (Kulonen et al 2018). Sometimes, topoclimatic variables derived directly from DEMs (like elevation, solar radiation or cold-air pooling) are also used independently in SDMs, thus using an indirect topoclimatic derivative instead of actually downscaled climate to improve the spatial resolution of SDMs (Roslin et al 2009, Maclean et al 2015, Shinneman et al 2016, Patsiou et al 2017.…”