Effects of Restoration Thinning on Dry Season Water Status And Drought Vulnerability in a Coniferous Temperate Rainforest

Garrett Price

doi:10.7273/000005300

Restoration thinning has been shown to be an important management tool for increasing the resistance of dry forest ecosystems to climate change-exacerbated droughts. However, few studies have explored if such treatments also improve drought resistance in historically wet temperate rainforests. In addition, many studies utilize growth-based metrics and drought-related mortality to evaluate effectiveness of restoration treatments. However, these only allow understanding of drought resistance in retrospect (i.e., are not predictive before major drought events). Growth metrics, such as basal area increment, also do not directly measure the impacts of thinning on reducing competition for water resources, which is most relevant for drought resistance and resilience. Here, I present a study that addresses both of these research gaps by utilizing direct measurements of canopy water status (leaf water potential) and drought vulnerability (hydraulic vulnerability to cavitation) of 3 common Pacific Northwest coniferous rainforest species (Picea sitchensis, Thuja plicata, and Tsuga heterophylla) in a case study comparing second-growth coastal rainforest when unmanaged (unthinned) versus 13 years after thinning treatment that was conducted with the aim of ecological restoration (old-growth-like forest structure, biodiversity, etc.). I found evidence that thinning increased water availability for P.sitchensis and T.heterophylla but less so for T.plicata. Treatments did not significantly impact cavitation resistance in any of the species. These data were combined to calculate hydraulic safety margins. Safety margins (margins for drought stress, HSM50 and margins for drought mortality, HSM88) were consistently larger for trees in treated stands compared to untreated controls, suggesting that restoration thinning increases drought resistance of surviving canopy trees in temperate rainforests and demonstrating that plant hydraulic traits are an informative tool for evaluating management effects, providing a clear prediction of how trees will respond to regional warming, drying conditions. Another important factor in the use of restoration thinning is its effect on drought refugium formation. Drought refugia, areas of the landscape where more moisture is available or drought has a smaller ecophysiological impact, may play an important role in protecting larger tree populations or ecosystems from drought. Drought refugia can be detected via remote sensing, allowing for the evaluation of restoration thinning treatments across broad spatiotemporal scales. Drought refugia in a larger area of managed Pacific Northwest rainforest surrounding the case-study field sites were detected using changes in Normalized Difference Moisture Index (NDMI) and net primary production (NPP) between a non-drought baseline period and a drought year (2015 and 2018). The NDMI anomaly was more negative in thinned stands than unthinned stands, suggesting that thinning may reduce upper canopy vegetation moisture content, which might reduce the likelihood of a thinned area being a drought refugium. However, lowered NDMI anomalies could be partially explained by the fact that the baseline period included information from years prior to treatment and thus is potentially confounded with canopy cover differences driven by the management interventions. NPP anomaly was unchanged between thinned and unthinned stands, suggesting that thinning did not negatively impact total canopy productivity. By this metric, thinned and unthinned stands would be equally likely to be drought refugia. Boosted Regression Tree modeling was also used to model the relationship between possible drought refugia and landscape variables such as elevation. Modeling results were inconclusive, but model results suggested that refugium formation may be more likely in stands aged 25-50 years or older than 75 years, and in intermediate elevation stands. Together, this case-study and related remote sensing research demonstrate that thinning may be capable of protecting stands against drought but may not alter occurrence of drought refugia.

Effects of Restoration Thinning on Dry Season Water Status And Drought Vulnerability in a Coniferous Temperate Rainforest

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