Secrets in the CTRN: Causal factors of thinning response and transfer to adaptive management regimes in Maine spruce-fir forests
Funded by the Cooperative Forestry Research Unit at the University of Maine

Stand density management through thinning is a common silvicultural method that has traditionally been utilized for production of commodity timber goods, tree regeneration, and quality improvement. Recently, there has been renewed interest in thinning regimes to mitigate forest health risks, enhance C sequestration and structural complexity, and control competing vegetation. While a rich volume of work from the Commercial Thinning Research Network (CTRN) has examined thinning practices under a variety of treatment timing and intensities, questions remain regarding the influence of site conditions on thinning response (i.e., site productivity). To date, little is known about the edaphic, climatic, and physiologic mechanisms of the duration and magnitude of tree response to thinning in the Northeast region. Therefore, quantifying the site-specific limiting factors to growth is imperative to adaptative management strategies and effective silviculture.

 

In other regions, tree ring stable isotopes have been used to reconstruct tree water use efficiency and disentangle the influential factors of silvicultural treatments, including thinning, and site conditions on stem growth response. However, most efforts have been limited to a single study area and inadequate replication across a landscape or region. In parallel, advances in remote sensing derived estimates of potential evapotranspiration can now provide unbiased, high spatial resolution predictions of thinning treatment response, yet lack a descriptive biological mechanism. Therefore, this project aims to quantify the causal mechanisms of operational thinning response through the integration of tree-ring stable isotopes (δ13C and δ18O) with high spatiotemporal resolution remote sensing estimates of evapotranspiration (20 meter at 1-month intervals) across the CTRN study sites in Maine. Findings from this work can offer insight to the limiting factors to site carrying capacity (e.g., Stand Density Index) and thinning response, leading to new site-specific density management guidelines, and may serve as a framework for growth model calibration and silvicultural planning.