Results
Microhabitat variability
Microhabitat variation within sites was higher than among site variation
for half of the microhabitat parameters for which we could fit nested
LMMs. Out of these 9 microhabitat parameters that we measured, only 3
varied predictably with elevation (canopy openness, winter minimum soil
temperature, and spring days of snow). The parameters that followed an
elevational pattern are both expected to be directly (winter minimum
temperature, spring days of snow) as well as indirectly (canopy
openness) affected by climate (Fig. 2).
Our principal component analysis plot shows that the microhabitat
variables we measured cluster broadly by soil characteristics
(carbon:nitrogen, fungus:bacteria, water holding capacity), soil
moisture (summer minimum soil moisture, spring days of snow), light
availability (canopy openness), soil temperature (summer maximum soil
temperature), and above-ground temperature (summer minimum plant-height
temperature) (Fig. S3). An additional parameter describing soil
temperature (winter minimum soil temperature) clusters in the middle of
the microhabitat space.
Species establishment
Out of the 25 species that we sowed, 84% (21/25) recruited in at least
1 plot and 56% (14/25) recruited in 8 plots or more (Table S2). Of the
species that recruited, seedlings (i.e. at least one seedling) survived
for one year for 57% (8/14) of species and survived for two years for
43% (6/14) of species. Almost all species that had sites beyond their
range edge recruited and survived beyond their range edge, a pattern
seen for species with either high- or low-elevation range edges (Table
S2). Almost all of our models met the GLM assumption of independence of
residuals. However, many of our models did not meet homogeneity of
variance, variance did not equal the mean, or the link function was
sometimes inappropriate (> 0.5 difference from 1 for slope
of link function). Together with the low proportion of variance (Table
S3) from each set of candidate models, we are therefore cautious in
interpreting our results.
Overall, we found microhabitat parameters both related and not related
to climate identified as the most important parameters in model
selection of the effects of microhabitat on establishment, but models
had low explanatory power (Tables 1, S3). At the community level, the
main patterns we found were that certain microhabitat variables had
largely negative or positive effects, but the direction of these effects
changed with different life stages (Fig. 3). At the species level, we
found that some species only had few microhabitat parameters chosen in
model selection, whereas others had many (Fig. S4). Within the same
species, parameters usually had the same directionality of effect on
likelihood of recruitment, recruit counts, and seedling survival except
for T. grandiflora. For two species, A. lasiocarpa andE. lanatum , the directionality of effects was consistent across
life stages.
Microhabitat suitability
We generated predictions of microhabitat suitability to better
understand if the distribution of microhabitats along climatic gradients
suggests range shifts will likely be facilitated (i.e. where suitability
increases with elevation), constrained (i.e. decreases), or unaffected
(i.e. is constant). We note that due to poor model fit in the original
models, our predictions cannot be used to make definitive forecasts of
which species are likely to shift their ranges with climate change, but
rather use these predictions to assess how microhabitat suitability
changes across a large elevational gradient. Our predictions indicate
that only range shifts for L. latifolius are likely to be
facilitated with increasing microhabitat suitability with elevation
(Fig. 4), although our experimental sites extended only just beyond the
range limit for the species. For all other species, microhabitat
suitability either declines or has no pattern with elevation and thus
range shifts are likely to either be constrained or unaffected,
respectively (Figs. S5, S6, S7). We also found that suitability patterns
were modified by transect and life stage.
Together with these suitability predictions, the microhabitat parameters
that significantly vary with elevation can give further insight on what
aspects of microhabitat can facilitate or constrain range shifts. For
example, the increasing microhabitat suitability for L.
latifolius (Fig. 4a) together with the positive effects of canopy
openness on the species (Table 1) and canopy openness increasing with
elevation (Fig. 2a) point to a likely facilitated range expansion for
the species. Spring snow cover shows the same pattern, and thus is a
further microhabitat variable that will likely facilitate range
expansion for this species. However, winter minimum soil temperature
decreases with elevation in the Okanagan transect but positively affectsL. latifolius recruit counts and seedling survival, so this
microhabitat variable could constrain range expansion.