Evaluating factors influencing forest growth across climatic and silvicultural gradients in Northern Forests of the United States

Long-term silvicultural experiments are used to study growth and yield at multiple temporal and spatial scales in forestry. Differences in forest type, stand characteristics, site conditions, and silvicultural system implemented affect outcomes across studies. Due to their varied implementation and methodologies, it is difficult to compare results across experiments at different locations, i.e. on different experimental forests. This study investigates the required effort and potential conclusions that can be garnered using previously collected data from independent long-term U.S. Forest Service silvicultural studies across a subset of Northern Forest types.

Results from long-term studies are utilized for site-specific conclusions, with results pertinent to similar forested areas. While site-specific conclusions have furthered understanding of growth and yield, cross-site comparisons would provide new perspectives on regional variation in growth response. Large-scale comparisons across long-term silvicultural experiments could provide multiple comparison metrics to further understanding of growth and yield within and between stand types. This project presents a start-to-finish description of how to utilize historical forest growth records to quantify regional variation in growth responses attributed to factors at multiple spatial scales.

Data used were collected across the northern United States from 1927 to 2010. Multiple gradients of Northern Forest complexity are realized in these data, i.e. forest type, stand structure, and silvicultural system. The relative influence of climate, stand attributes, soil, and silvicultural variables were identified at a regional and site- specific level. Influential factors, or the relationship between independent variables and the dependent variable of growth models provide large-scale trends and quantification of  influential factors across multiple stand and landscape scales. Periodic annual increment of basal area (PAI) in trees greater than 11.7 cm (0.48±0.25 m²ha^-1yr^-1) was relatively similar across all sites, from Minnesota south to Missouri and north-east to Maine. Within-site influential factors varied, with different factors driving growth response (PAI). While rank and relative importance varied,  density- and diameter-and climate- related variables were the most common influential factors on site-level PAI.