Modeling early responses of loblolly pine growth to thinning in the western gulf coastal plain region

Abstract

Growth response to thinning has long been a research topic of interest in forest science. This study presents the first 3-4 years of response of loblolly pine (Pinus taeda L.) growth to thinning at different intensities. Data were collected from the East Texas Pine Research Project's region-wide loblolly pine thinning study, which covers a wide variety of stand conditions. Four treatments, light, moderate, and heavy thinning, respectively having 370, 555, and 740 residual trees per hectare after thinning, and an unthinned control, were included. Individual tree diameter at breast height (dbh) and total height were recorded annually for the first 3-4 years after thinning. Results indicate significant differences between treatments in dbh growth in each year after thinning, as well as for all years combined. Each thinning treatment had significantly greater dbh growth than the control in the first growing season with this positive response being more evident in the case of the heavier thinning or at the later years post-thinning. Conversely, the thinning effect on tree height growth was initially negligibly negative, then becoming positive after 2-4 years, with the heavier thinning becoming positive sooner. Tree size class, assigned based on prethinning dbh, had a significant effect on both dbh and height growth responses. Compared to the control, small trees had a greater response both in dbh and in height growth than the medium and large trees over the measurement period. At the stand level, the heavier thinning had significantly less stand basal area per hectare, but the difference in stand basal area per hectare between the thinned and the unthinned plots decreased with years post-thinning. Results from this study can improve our understanding in thinning effects and help forest managers make accurate decisions on silvicultural regimes. Study Implications: Loblolly pine plantations are the most economically important forests in the West Gulf Coastal Plain, and thinning is the most common midrotation silvicultural treatment used in their management. Thinning is an effective practice to improve timber value and reduce rotation lengths, thus increasing the economic return of the stand. Our results suggest that the common thinning regime in the region, having 555 residual trees ha-1 after thinning, seems optimal, as it greatly enhances diameter at breast height (dbh) growth yet maintains comparable BA growth to unthinned stands, whereas the other regimes, having 370 and 740 residual trees ha-1 after thinning, are suboptimal in terms of balancing dbh and BA growth. Thinning redistributes site resources to growing fewer, higher-value residual trees, making them less stressed from competition and potentially more resilient to changing climatic conditions, pest attacks, and other environmental stresses. Therefore, thinning practices may be modified to address economic, timber production, and environmental goals. Applying the findings to thinning regimes that are not reflected in the data from this study is not recommended. Such applications are extrapolations beyond the range of the data in this study, and predictions of response may not be reliable.