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Nevins, L. McKinley [1], Smith, Robert J. [2], Zambrano, Jenny [1].

Tree demographic and neighborhood responses to complex regional gradients of the northwestern United States.

It is essential to understand the drivers of variation in tree demography to anticipate how forests will be impacted by future climate conditions. Studies frequently include only abiotic factors, failing to account for plant-plant biotic interactions that can have significant impacts on demographic rates. We combined data on two prominent environmental gradients: climatic moisture deficit and fire probability, with modelled tree functional neighborhoods, to assess variation in survival and growth in tree species in the northwestern United States. This region is one of the most biotically diverse in the country, and is experiencing shifts in the frequency and intensity of droughts and wildfires, which are only expected to increase in severity in the future. We utilized continental-scale tree demographic data from the US Forest Service’s Forest Inventory and Analysis (FIA) program on 336,428 individual trees of 56 tree species in 37,739 unique plots. Trait data from the TRY and USDA PLANTS databases for eight physiological and morphological traits related to drought and fire tolerance were used to model tree functional neighborhoods, which calculated trait differences between each focal tree and its neighbors. We hypothesized that 1) functional neighborhood diversity would increase with increasing environmental stress, and 2) tree survival and growth would vary in response to the interaction of environmental variation and functional neighborhood, with high variation between tree species. We found evidence for a niche differentiation in functional neighborhoods along the environmental stress gradients, with a shift from less trait-diverse to more trait-diverse neighborhoods as stress increased. Furthermore, our results suggest substantial species variation in survival and growth responses to the combined effects of environmental variability and functional neighborhood. We targeted our considerations at a broad scale to assess interspecific variation across large environmental gradients, intentionally excluding measurements of intraspecific variation; even still, we observed strong trends in demographic responses, suggesting that intraspecific variation may be amplifying the effects, and the variation we observed between species. This observation supports the need for more local-scale assessments in the future, which can consider variation within species, and the biotic and abiotic mechanisms underlying their demographic responses. In sum, our findings support that environmental variation and biotic interactions jointly shape forest tree demography. Results of this study offer a different perspective of tree species and community responses to variable environmental conditions necessary to better anticipate responses to future climate and disturbance changes.

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1 - Washington State University , School of Biological Sciences, Pullman, WA, 99164, USA
2 - USDA Forest Service, Air Resource Management Program, Washington, DC, 20250, USA

functional traits
Trait Convergence/Divergence
Climatic Moisture Deficit
Forest Inventory Analysis.

Presentation Type: Oral Paper
Session: ECO9, Ecology: Species Ranges and Distributions
Location: /
Date: Friday, July 23rd, 2021
Time: 4:45 PM(EDT)
Number: ECO9008
Abstract ID:413
Candidate for Awards:Ecological Section Best Graduate Student Paper

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