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Thinning is expected to improve both individual tree- and stand-level growth efficiency since more environmental resources (growing space, light, water, and nutrients) become available to fewer trees. However, thinning can also promote asymmetric competition among residual trees, and potentially increase mortality and growth stagnation by creating an immediate and often dramatic change in stand micro-environments. To strengthen our understanding of stand-level responses to thinning, we used long-term and replicated experiments for the three most commercially important softwood forest types in the North America, including: shade-intolerant loblolly pine ( Pinus taeda L.), moderately shade-tolerant Douglas-fir ( Pseudotsuga menziesii Mirbel), and shade-tolerant red spruce ( Picea rubens Sarg.) and balsam fir ( Abies balsamea L.). The primary objectives of this study were to quantify the stand-level and dominant tree (100 largest trees ha −1 ) response to various thinning treatments in terms of relative ( RVG ) and cumulative volume growth ( CVG ), while evaluating the effects of stand conditions, timing/intensity of thinning, and key stand-level biotic factors. Our results showed relatively limited differences between the stand-level and dominant-tree responses. Thinning generally increased the RVG of all three forest types, but the effect was dependent on time since treatment for spruce-fir. When compared to unthinned stands, thinning increased the overall RVG , but the pattern of change over time since thinning was negative for both loblolly pine and Douglas-fir. CVG was generally higher in unthinned than thinned stands across the three forest types. Thinning intensity had a positive effect on the RVG of Douglas-fir and loblolly pine, but negative on RVG of spruce-fir forest type. Relative age (based on age of maximum periodic annual increment) and stand density at time of thinning had negative effects on the RVG of dominant trees for all species examined, except spruce-fir. Overall, our results highlight a range of responses to thinning among three distinct softwood forest types, and revealed that thinning does not always improve the growth efficiency of dominant trees, and that the responses are governed largely by a variety of stand characteristics at the time of thinning. [ABSTRACT FROM AUTHOR]

Herbicide application and precommercial thinning (PCT) are common silvicultural treatments used across North America and Europe. Despite this widespread use, long-term growth and yield responses from controlled experiments that include both of these treatments are relatively rare. We used 40-year growth and yield responses of spruce-fir stands to various combinations of early herbicide and PCT in a long-term silvicultural experiment in central Maine, USA, to calibrate the Forest Vegetation Simulator (Northeast Variant). Using the calibrated model, we projected rotation-length outcomes for stand development, merchantable wood volumes, and stumpage-based financial returns. Projections indicated gains in total yield (17%-31%) from herbicide treatments at the end of the rotation (∼60 years postharvest) relative to untreated stands. Substantial increases in merchantable wood volume also were achieved with PCT. Twenty-four years after PCT, stand stumpage value averaged $907 USD·ha−1 higher than that for unthinned stands. Total yield and stumpage gains from PCT were projected to continue through the end of rotation. Highest stumpage values resulted from combined herbicide and PCT treatments, followed by PCT-only and then herbicide-only. At end of rotation, highest net present value (NPV) resulted from PCT, whether alone or in combination with herbicides. PCT and herbicide investments substantially improved the NPV relative to untreated stands when using discount rates of 2% and 4%, but not when using a 6% rate. Our results documented that good financial returns are possible over the long-term from early investments in herbicide and PCT treatments in Maine spruce-fir stands. [ABSTRACT FROM AUTHOR]

Herbicide technology has evolved with forest management in North America over the past 60 years and has become an integral part of modern forestry practice. Forest managers have prescribed herbicides to increase reforestation success and long-term timber yields. Wildlife managers and others interested in conserving biodiversity, however, have often viewed herbicide use as conflicting with their objectives. Do herbicides increase forest productivity, and are they compatible with the objectives of wildlife management and biodiversity conservation? Results from the longest-term studies (10-30 years) in North America suggest that the range of wood volume yield gains from effectively managing forest vegetation (primarily using herbicides) is 30-450% in Pacific Northwest forests, 10-150% in the southeastern forests, and 50-450% in northern forests. Most of the 23 studies examined indicated 30-300% increases in wood volume yield for major commercial tree species and that gains were relatively consistent for a wide range of site conditions. Meeting future demands for wildlife habitat and biodiversity conservation will require that society's growing demand for wood be satisfied on a shrinking forestland base. Increased fiber yields from intensively managed plantations, which include the use of herbicides, will be a crucial part of the solution. If herbicides are properly used, current research indicates that the negative effects on wildlife usually are short-term and that herbicides can be used to meet wildlife habitat objectives. [ABSTRACT FROM AUTHOR]