Variable species establishment in response to microhabitat indicates
different likelihoods of climate-driven range shifts
Abstract
Climate change is causing geographic range shifts globally, and
understanding the factors that influence species’ range expansions is
crucial for predicting future changes in biodiversity. A common, yet
untested, assumption in forecasting approaches is that species will
shift beyond current range edges into new habitats as they become
macroclimatically suitable, even though microhabitat variability could
have overriding effects on local population dynamics. We aim to better
understand the role of microhabitat in range shifts through its impacts
on establishment by i) examining microhabitat variability along large
macroclimatic gradients, ii) testing which of these microhabitat
variables explain plant recruitment and seedling survival, and iii)
predicting microhabitat suitability beyond species range limits. We
transplanted seeds of 25 common tree, shrub, forb, and graminoid species
across and beyond their current elevational ranges in the Washington
Cascade Range, USA, along a large elevational gradient spanning a broad
range of macroclimates. Over five years, we recorded recruitment,
survival, and microhabitat characteristics rarely measured in
biogeographic studies. We asked whether microhabitat variables correlate
with elevation, which variables drive species establishment, and whether
microhabitat variables important for establishment are already suitable
beyond leading range limits. We found that only 30% of microhabitat
parameters covaried in the expected way with elevation. We further
observed extremely low recruitment and moderate seedling survival in our
study system, and these were generally only weakly explained by
microhabitat. Moreover, species and life stages responded in contrasting
ways to soil biota, soil moisture, temperature, and snow duration.
Microhabitat suitability predictions suggest that distribution shifts
are likely to be species-specific, as different species have different
suitabilities, and availabilities, of microhabitat beyond their present
ranges, thus calling into question large-scale macroclimatic projections
that will miss such complexities. We encourage further research on
species responses to microhabitat and the inclusion of microhabitat in
range shift forecasts.