The nudge trees need: disturbance accelerates climate-driven range shifts in trees of the western US

Background/Question/Methods

Climate change is projected to cause shifts in tree species distributions, but the ability of trees to track current changes in climate is uncertain. Although climate conditions may become suitable, biotic competition and lack of suitable microsite conditions for seed germination may still prevent migration of tree species. Disturbances may provide a catalyst for tree species range shifts by reducing competition and creating needed microsite conditions. In this talk, we investigate how fire vs. insect outbreaks impact range shifts of tree species in the western US through analysis of a large dataset and a complementary field study. We used Forest Inventory and Analysis data to compare adult vs. seedling climatic niches of thirteen widespread tree species affected by different recent disturbances using multivariate analyses. We identified whether shifts in climatic niche had occurred for each species and whether the magnitude or direction of shift was altered by disturbance history or species traits. Our field study focused on trembling aspen and involved seedling surveys in recently burned and adjacent unburned areas in beetle-killed forests upward of the adult elevational limit. We assessed the extent that aspen upward expansion was facilitated by these disturbances and the conditions associated with seedling establishment.

Results/Conclusions

Across the western US, larger shifts occurred in disturbed areas with most tree species shifting away from hot climates and shifts in precipitation space differing by disturbance type. Overall, range contractions were more common than expansions and cold-adapted species are experiencing more contraction than warm-adapted species, making them more at risk of local extirpation. Trembling aspen was one of three species that showed greater expansion than contraction in burned areas. Our field surveys also only documented aspen seedling upslope expansion in burned, but not beetle-killed forests, further suggesting that fire is catalyzing aspen range expansion. However, aspen expansion was highly localized in the study area, only occurring on certain slopes. Our analysis across the western US reveals that tree species are primarily responding to climate warming by reduced regeneration in hotter climate zones and that disturbances are accelerating this response. Trembling aspen is relatively unique in its ability to expand its range in burned areas, likely due to its long-distance seed dispersal and dependence on bare mineral soils. The increasing frequency and extent of disturbances across the western US may largely accelerate range contractions for trees, whereas evidence for expansion is scarce, species-specific, and highly localized.