Soil moisture response to seasonal drought conditions and post-thinning forest structure

Authors: BELMONTE, A. - Northern Arizona University; SANKEY, T. - Northern Arizona University; Biederman, Joel; BRADFORD, J. - Us Geological Survey (USGS); KOLB, T. - Northern Arizona University

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2022
Publication Date: 2/28/2022
Citation: Belmonte, A., Sankey, T., Biederman, J.A., Bradford, J., Kolb, T. 2022. Soil moisture response to seasonal drought conditions and post-thinning forest structure. Ecohydrology. 15(5). Article e2406. https://doi.org/10.1002/eco.2406.
DOI: https://doi.org/10.1002/eco.2406

Interpretive Summary: Western forests are a critical source of water supply, and they face increasing pressure from climate change, which increases their susceptibility to die-off from drought, fire, and insect infestation. Tree stress depends largely on the depth, duration and intensity of soil moisture stress. Here we measured soil moisture stress in the root zone of a ponderosa pine forest for two years following a range of thinning treatments and compared this against an unthinned control stand. Forest thinning was effective at reducing soil moisture stress for remaining trees. Post-thinning stands with taller trees experienced less stress than stands with shorter trees, possibly, due to different water use or to the effects of short trees in reducing snow accumulation. Collectively, our results demonstrate new ways to assess objectively the impacts of forest thinning on the health of remaining trees.

Technical Abstract: Prolonged drought conditions in semi-arid forests can lead to widespread vegetation stress and mortality. However, the distribution of these effects is not spatially uniform. We use high spatial and temporal resolution soil water potential time-series data to assess the effects of fore-summer drought period on the timing, magnitude, and extent of drying throughout the top 100 cm of the soil profile. These measurements were made during two abnormally dry years. Additionally, we use high-resolution terrestrial lidar measurements of forest structure to develop relationships between soil drying and fine-scale forest structure. In these dry years, we found that at all depths (25, 50, and 100 cm) soil drying onset occurs statistically significantly earlier and significantly more days are observed below a critical drying threshold for ponderosa pine trees. Additionally, we show that significantly drier soils are found in areas with higher stand-level basal area, canopy cover, and tree density conditions. We also show that overall drier soils were found in areas with shorter trees. Taken together, our results suggest that prolonged and seasonal drought conditions can compound to create significant soil moisture deficits, and that tailored restoration thinning can be used to increase and prolong the availability of deep soil water to trees during drought using specific tree density and size parameters.