Understanding what Drives Plant Diversity in Alaska

Infographic summarizing our framework outlining the hypothetical relationships among microclimate, vegetation structure, and species group richness (biogeographic affinities and growth forms) in our study region.

Our study reveals that strong variation in vascular plant richness across elevation gradients in interior Alaska is structured by the pronounced, frequently contrasting patterns of richness demonstrated by different growth form and biogeographic species groups in relation to microclimate, soils, and topography. We found that while the direct effects of microclimate and solar radiation on plant species richness are clearly important, the direct effects of a suite of soil attributes and the indirect effects of microclimate and solar radiation via influence on vegetation structure are equally, if not more important. Our approach demonstrates the inter‐relationships among the ecological dynamics affecting richness patterns and the historical factors that have likely influenced the composition of the regional species pool. In particular, the relative contributions of plot‐level variation in richness among these groups parallel their overall richness in Alaska, linking ecological factors driving local richness patterns with historical dynamics that have shaped the composition of the regional species pool. Our analyses thus also provide a useful paradigm for considering the possible ramifications of future climate warming for vegetation structure and plant diversity patterns across the region.

A structural equation model linking past and present plant diversity in Alaska: a framework for evaluating future change

Abstract

Recent findings of peak plant species diversity occurring in alpine tundra in Alaska may reflect the filtering of the regional species pool during Pleistocene Epoch cold periods. Specifically, herbaceous plant groups and those with far northern geographic distributions centered in Beringia may have persisted coincident with repeated diminution of tall‐statured and southern continental plant groups during cold intervals, thus potentially structuring current diversity patterns in relation to elevation. Numerous biotic and abiotic drivers of plant species distributions vary with elevation, raising questions concerning which of these drivers are responsible for these surprising diversity patterns. We used structural equation modeling to investigate relationships among a suite of biotic and abiotic covariates and the richness of species groups based on growth form (GF) and biogeographic affinity across interior Alaska. Our results confirm the primary importance of microclimate in controlling the distribution, abundance, and richness of woody plants, which increased in warm, south‐exposed plots. In contrast, the suite of other site factors had considerably stronger combined direct influences on richness of forbs, graminoids, and dwarf shrubs as compared to microclimate. Thus, species groups with negative richness responses to increasing temperature and potential solar radiation may have a competitive disadvantage to woody species, suggesting competitive displacement as one mechanism for these patterns. Our findings reveal that conditions associated with the high plant diversity (including cooler air temperatures, ground disturbance, and weakly acidic soil pH) are spatially restricted as compared to widespread conditions associated with low species diversity (warm lowlands with acidic soil reaction). Our results confirm a strong correspondence between localized richness patterns and variation in the overall fractions of the GF and biogeographic groups within the regional species pool. Our work suggests important links between current and historical drivers of plant diversity that help explain the apparent contradictions suggested by the relatively limited spatial extent of species‐rich habitats in Alaska. By elucidating the relationships among biotic and abiotic site characteristics and between these characteristics and richness, our findings provide a useful framework within which to evaluate possible changes to vegetation structure and species richness patterns caused by rapid warming.

Roland, Carl A., Giancarlo Sadoti, E. Fleur Nicklen, Stephanie A. McAfee, and Sarah E. Stehn. 2019. A structural equation model linking past and present plant diversity in Alaska: A framework for evaluating future change. Ecosphere 10(8): e02832.