Discussion
While many species ranges are on average shifting in the direction
predicted by SDMs, there is large variation among species that is
difficult to explain. Microhabitat variability is increasingly
recognized as influencing population dynamics across species ranges
(Oldfather & Ackerly, 2019) and may be a key factor in understanding
and predicting range shifts (Lembrechts, Nijs, et al., 2019; Maclean &
Early, 2023; Stickley & Fraterrigo, 2023). However, how microhabitat
suitability, and variability therein, is distributed along species
ranges is vastly understudied, impeding an understanding of how
microhabitat can facilitate or constrain range shifts. We found that
microhabitat is highly variable across species ranges and is largely
decoupled from the macro-scale at which SDMs are typically constructed.
We further found variable, albeit weak, effects of microhabitat for
different species and life stages, suggesting that the drivers of
establishment are complex and difficult to detect. Our microhabitat
suitability predictions show microhabitat will either constrain range
expansions, or have no effect, for most species, with the range shifts
of just one species (L. latifolius ) likely facilitated by
microhabitat.
Complex microhabitat patterns across elevation
gradients
Surprisingly, most of our microhabitat variables, even ones related to
climate (summer soil moisture, summer soil temperature, summer plant
height temperature) did not follow an elevational pattern representative
of the macroclimate (Fig. S8) commonly used in SDMs. This is consistent
with many studies finding that microhabitat is often decoupled from
elevation and regional climate (Ford et al., 2013; Lembrechts, Lenoir,
et al., 2019). We further found that none of the soil composition
parameters we measured (fungus:bacteria, carbon:nitrogen, water holding
capacity) followed an elevational pattern, even though other alpine
studies have found soil microbial community (Hiiesalu et al., 2023) and
soil carbon:nitrogen (Weintraub et al., 2016) to covary with elevation.
We further found that only canopy openness, winter minimum soil
temperature, and spring snow days covary with elevation. Despite higher
elevation areas characterized by more open forest canopies, the high
variability at each site (Fig. 2) is in line with the large body of work
showing that forest canopies can provide climate refugia (De Frenne et
al., 2019; Haesen et al., 2021). As species light requirements can be
variable, light availability and canopy cover can shape species ranges
(Muñoz Mazon et al., 2023) and therefore can be an important component
of range shifts (Tourville et al., 2022).
Low and variable effects of microhabitat on
establishment
Our results agree with other studies, which find overall low recruitment
in seed addition sites (Clark et al., 2007) and decreasing fitness of
transplants beyond the range (Lee-Yaw et al., 2016; Stanton-Geddes et
al., 2012). Plant establishment beyond current range edges is necessary
for plant species to shift their ranges upward, however this is
complicated by plants usually being recruitment limited. This means that
they are limited by some aspect of their environment (e.g., microhabitat
unsuitability, seed predation) and not by dispersal (Clark et al.,
2007). While we did not control for seed predation, herbivory, or fungal
infections, we captured mortality with our methods. Even though seed
predation rates decline in higher elevation areas (Hargreaves et al.,
2019), we still removed fleshy fruits around seeds to deter predation.
We also did not collect data to test for effects of community
composition on establishment, but with low recruitment rates, plants in
our system would still be considered recruitment limited.
We did not find that establishment responses to any particular
microhabitat were restricted to certain growth forms, with
species-specific relationships to microhabitat. This is not surprising,
as species have unique environmental requirements (Table S1) and species
characteristics even impact predictive power in SDMs (Guisan et al.,
2007). We also did not find differences in effects of the microhabitat
aspects that are expected to be directly versus indirectly affected by
climate change. Interestingly, we found that some parameters, such as
winter soil temperature, increased likelihood of recruitment but these
effects switched to decreasing likelihood of seedling survival. One
exception was spring days with snow, which generally had positive
effects, and this is similar to findings by Davis & Gedalof (2018), who
found positive effects of winter snow cover on recruitment. The
importance of microbial community in plant dynamics is also increasingly
acknowledged (Castro et al., 2022), although soil fungus:bacteria ratio
was not selected more than other variables in our system. Microbial
communities could even be an important factor in determining species
range shifts, such as favorable soil microbial composition mediating
climate tolerance to promote tree seedling survival (Allsup et al.,
2023).
Predicting microhabitat
suitability
We found that microhabitat suitability beyond the leading range edge was
species- and life stage-specific, with most species showing either
decreased suitability or no pattern with elevation. We posit that the
role that microhabitat may play in facilitating or constraining range
shifts for any given species is closely tied to how microhabitat itself
varies with elevation. For example, if a favorable microhabitat
parameter for establishment decreases with elevation, establishment will
overall not be favored and a range shift could be constrained. We found
this in our system, with increasing likelihood of L. latifoliusrecruit counts and seedling survival with warmer winter minimum soil
temperatures, but soil temperature decreases with elevation. If a
favorable microhabitat parameter increases with elevation, however, this
could lead to a facilitated range shift. This is the case for canopy
openness, which increases with elevation and also increases likelihood
of L. latifolius establishment. Since microhabitat suitability
increases beyond the range for this species, these cases highlight the
complex ways in which microhabitat parameters may interact to mediate
range shifts. The alternative scenarios, where an unfavorable
microhabitat parameter either declines with elevation to inversely favor
establishment or increases with elevation to inhibit establishment, can
also facilitate or inhibit range shifts, respectively.
Macroclimatic parameters vary predictably with elevation (Fig. S8) in
our study system, however only a third of our microhabitat variables
show an elevational pattern. Since species distributions can be driven
by microhabitat, understanding how microhabitat parameters vary across
species’ ranges can give insights into how microhabitat may facilitate
or inhibit range expansion (Lembrechts et al., 2017; Tourville et al.,
2022; Kemppinen et al., 2023). Incorporating microhabitat suitability
into ecological studies is not new, including quantifying habitat
suitability in the field (Jabis & Ayers, 2014), with remote sensing
(Falco et al., 2019), and at the leading range edge (Mamet & Kershaw,
2013). Transplant studies often find decreasing macroclimate suitability
beyond the range (Lee-Yaw et al., 2016), but to our knowledge no studies
exist that assess microhabitat suitability beyond species range edges.
Despite increasing work showing the necessity of including microhabitats
in SDMs (Lembrechts, Nijs, et al., 2019), the vast majority of SDMs
still focus on macroclimatic variables and this may lead to the
mismatches between predicted and observed range shifts, or lack thereof.
In our system, we found that suitable microhabitat is unchanged or
reduced beyond the leading edge for many species, yet we also find that
macroclimatic conditions enable an upwards shift in species’ recruitment
optima at the community level (unpublished data ). However, this
pattern is only evident at the community level, with high species-level
variability and no effect of canopy cover. This matches the large
variability, and low explanatory power, seen in our microhabitat
suitability predictions, and suggests that species can find pockets of
suitable microhabitat to recruit beyond their macroclimatic cold range
edge. Microhabitat suitability may be just high enough to allow for
successful establishment, but is overall very low and the complicated
ways in which microhabitat acts to affect species establishment makes it
difficult to detect patterns. We also found continued species
recruitment at the warm edge of the range (unpublished data ) and
this buffering of contractions at the warm edge could be due to
microhabitat refugia (De Lombaerde et al., 2022) created by the range of
microhabitat found at the lower elevation sites (Fig. 2)
Limitations
Our experimental design is not without limitations. Seed provenance
might cause different recruitment responses across microhabitat
gradients, however variation in germination rates was not biologically
meaningful (Fig. S1). Since we sowed all seeds in the same year, we also
cannot test if we sowed in favorable versus unfavorable years and aimed
to capture a large germination window by recording recruitment over
three years. Finally, we collected some of our data on species responses
and microhabitat measurements asynchronously and therefore emphasize
that only differences between sites, not absolute values, should be
interpreted for their effects. Finally, we strongly urge future studies
to incorporate increased replication at each microhabitat value for
higher statistical power in explaining results.
Conclusion
To our knowledge, our work is one of the first studies to measure key
demographic life stages of a large group of species along a microhabitat
gradient both within and beyond their current range limit. As such, this
work yields a more comprehensive understanding of the mechanisms that
set species range limits and indicates ways in which species shift their
distributions with climate change. Our results suggest that complex ways
in which microhabitat parameters influence early life stages are complex
and difficult to detect, which complicates range shift predictions. A
greater understanding of the role of macrohabitat in shaping species’
distribution limits will ultimately improve predictions of how species
distributions will shift with climate change. We emphasize that
predictions accounting for complex microhabitat drivers are necessary to
create tailored conservation and management decisions in order to
mitigate ongoing biodiversity loss.