Variable species establishment in response to microhabitat indicates different likelihoods of climate-driven range shifts

Climate change is causing geographic range shifts globally, and
understanding the factors that influence species’ range expansions is
crucial for predicting future biodiversity changes. 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 in plants through its
impacts on establishment by Q1) examining microhabitat variability along
large macroclimatic (i.e., elevational) gradients, Q2) testing which of
these microhabitat variables explain plant recruitment and seedling
survival, and Q3) 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 (i.e., high resolution soil, air,
and light) 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 with
elevation. We further observed extremely low recruitment and moderate
seedling survival, 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
suitability and availability of microhabitat beyond their present ranges,
thus calling into question low-resolution macroclimatic projections that
will miss such complexities. We encourage further research on species
responses to microhabitat and including microhabitat in range shift
forecasts.

气候变化正在全球范围内引发物种地理分布范围的迁移，明晰影响物种分布范围扩张的驱动因素，对于预测未来生物多样性变化至关重要。当前预测模型中存在一项普遍却未被验证的假设：即便微生境（microhabitat）的异质性可能对局域种群动态产生决定性影响，物种仍会突破当前分布范围边界，迁入宏观气候（macroclimatic）适宜的新生境中。本研究旨在通过探讨微生境对植物定植过程的影响，深化对微生境在物种分布范围迁移中作用的认知，具体通过三个研究问题展开：Q1）探究沿大尺度宏观气候（即海拔）梯度的微生境异质性；Q2）检验哪些微生境变量能够解释植物的招募与幼苗存活情况；Q3）预测物种分布范围边界以外区域的微生境适宜性。我们在美国华盛顿州喀斯喀特山脉（Washington Cascade Range）沿覆盖宽泛宏观气候区间的大尺度海拔梯度，对25种常见乔木、灌木、非禾本草本植物（forb）以及禾草类植物（graminoid）的种子开展了跨其当前海拔分布范围乃至范围以外的移植实验。在为期五年的监测周期内，我们记录了植物招募、存活情况，以及生物地理学研究中极少被测量的微生境特征——包括高分辨率的土壤、空气与光照参数。我们旨在解答三个核心问题：微生境变量是否与海拔存在协同变化关系？哪些变量主导了物种的定植过程？对物种定植至关重要的微生境变量，在物种分布范围边界以外的区域是否已经具备适宜性？研究结果显示，仅有30%的微生境参数随海拔发生协同变化。我们还观测到极低的植物招募率与中等水平的幼苗存活率，而微生境对这两项指标的解释力整体偏弱。此外，不同物种及其发育阶段对土壤生物群落、土壤湿度、温度以及积雪持续时长的响应模式存在显著差异。微生境适宜性预测结果表明，物种分布范围的迁移可能具有物种特异性：不同物种在其当前分布范围以外区域的微生境适宜性与可获得性均存在差异，这使得依赖低分辨率的宏观气候预测难以捕捉这类复杂的生态过程，进而对这类预测的可靠性提出了质疑。我们呼吁未来开展更多针对物种微生境响应的研究，并将微生境因子纳入物种分布范围迁移的预测模型中。