Mitigating drought mortality by incorporating topography into variable forest thinning strategies

Authors: THOMAS, A. - New Jersey Institute Of Technology; KOLB, T. - Northern Arizona University; Biederman, Joel; VENTURAS, M. - Universidad Politécnica De Madrid; MA, Q. - Nanjing University; YANG, D. - University Of Wyoming; DORE, S - Northern Arizona University; TAI, X. - New Jersey Institute Of Technology

Submitted to: Environmental Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2024
Publication Date: 2/27/2024
Citation: Thomas, A., Kolb, T., Biederman, J.A., Venturas, M., Ma, Q., Yang, D., Dore, S., Tai, X. 2024. Mitigating drought mortality by incorporating topography into variable forest thinning strategies. Environmental Research Letters. 19(3). Article 034035. https://doi.org/10.1088/1748-9326/ad29aa.
DOI: https://doi.org/10.1088/1748-9326/ad29aa

Interpretive Summary: Globally, forests are suffering increased frequency and severity of disturbances including wildfire, insect infestation, and other drought-related occurrences. Thinning the forest by removing some trees has been explored as a means of improving the water available to remaining trees. However, computer models used to predict how thinning will improve water availability for the forest often fail to account for the spatial arrangement of trees within a forest stand and/or how sloped terrain moves water laterally. Here we trained a computer model of tree water use and soil water movement using 5 years of sensor measurements. We then conducted a model simulation experiment to test the impacts of tree arrangement and topography. We found that model simulations accounting for space between trees produced more conservative thinning benefits because they accounted for the fact that water saved by thinning a tree in one place must move in the soil to the location of another tree for it to be beneficial. We also found that thinning can have maximal benefits when more trees are left alive in low, valley positions and fewer trees are left in high, hilltop positions. These results illustrate the importance of accounting for spatial arrangement of trees and topography within models and suggest that variable thinning strategies should be employed in non-level terrain.

Technical Abstract: Anthropogenic climate changes are threatening forests globally. Developing adaptation strategies hinges on an adequate understanding of the mechanisms governing the drought vulnerability of forest stands. Prescribed reduction in stand density has been used as a management tool to reduce water stress and wildfire risk, but the processes that modulate fine-scale variations in plant water supply and water demand are largely missing in ecosystem models. We used an ecohydrological model that couples plant hydraulics with groundwater hydrology to examine how within-stand variations in tree spatial arrangements and topography might influence forest vulnerability to drought at individual-tree and stand scales. Our results demonstrated increasing inter-tree spacing generally ameliorated drought stress and improved ecosystem functions, although the effectiveness varied by climate and topography. Variable thinning that adjusted tree spacing based on topography-mediated water availability achieved higher productivity and lower mortality risk, compared to evenly-spaced thinning. The results from numerical experiments provided mechanistic evidence that topography mediates the effectiveness of thinning and highlighted the need for an explicit consideration of within-stand heterogeneity in trees and abiotic environments when designing forest thinning to mitigate drought impacts.