Non-tree competing vegetation can hamper artificial and natural regeneration. Notable problem species are Rubus spp. in the coastal areas of Oregon and Washington, manzanita and Ceanothus in the valley region of California and mountain Mahogany and western juniper in the east-side of Oregon and Washington states, and bracken fern and sweetfern in the Northeastern US. This is especially a concern on droughty sites- either coarse textured and/or shallow soils, and southern aspects where summer available moisture content is a limiting factor for tree survival. Established shrubs coppice well after a stand-replacement disturbance using stored belowground carbohydrate resources and the newly available light for growth. With their extensive, deep root systems, they are able to out-complete seed-origin natural or planted seedlings, accessing deeper soil moisture and putting on greater height growth. It has become routine for site preparation prior to planting to utilize chemical, manual or mechanical methods to reduce shrub extent or vigor to ensure adequate survival of planted tree seedlings.
In an effort to ensure adequate survival and successful stocking on marginal sites, high planting densities are often utilized to promote the pace and likelihood of canopy closure. These excess trees soon prove in excess of site carrying capacity and the stand soon moves into the self-thinning stage of stand development. If not thinned at this stage with hazardous natural dead and activity-created fuels abated, the young stand is at high risk to high-severity wildfire or insect/disease due to the presence of dead trees and lowered individual tree vigor (sap flow/moisture content) in the competitive environment of high stocking. Intervention such as weed and release or precommercial thinning helps accelerate stand development though this phase and mitigate the risk of stand-loss. Though at high risk to disturbance, this period of stand development is beneficial to the long-term health of the stand by encouraging self-pruning and resource allocation to high-growth, ensuring expression of the best-performing and vigorous trees best adapted to growth conditions of the site (or microsite). They form a cohort of the canopy dominant trees and are likely to be retained in further intermediate stand management activities such as commercial thinning.
Ecologists and Foresters noted poor performance of plantations on sites otherwise indicated by individual site tree performance as having high site index. The darling of early foresters, site index (an individual tree measure) as a methodology for basing planting and thinning densities (which should relate to SITE carrying capacities) was found to be lacking. The average of all trees on the site- stand growth- was encumbered by limiting factors such as non-tree competition, available soil moisture, and epidemiological factors such as insects/disease.
Alternative methods to quantitatively assess and relate sites on a spectrum of growth and stocking potential were subsequently investigated and developed. Stand density index (SDI - Reineke 1933) describes a stand by the number of trees per acre if the woody volume of the stand was entirely represented by 10" diameter trees. Growth Basal Area (GBA - Hall 1983) measures a stand's potential as the basal area of the dominant trees at which they grow 1 inch per year at age 100. Tables, growth/yield models were developed to assist field users to assess a stand and where it fell on the scale of site productivity and thereby base their planting/thinning level decisions to limit self-thinning mortality (and substitute natural density reduction with harvest or noncommercial thinning), and mitigate risk or reduce the period of susceptibility to catastrophic disturbance and promote maximum growth.
Again, however, in practice these established tables and methodologies worked best well within the "normal" range of forest ecosystems, but on the fringe of the forest, the borderlands of ecological niches - the forest-shrubland divide- further refinement was needed to correct management mistakes, silviculture method failures that were based on these guides. From empirical observations of the Forest Service's wide-ranging Forest Inventory and Analysis program as well as quantitative measurements of Level of Growing Stock studies, progeny tests, and many other evaluation plantations that were controlled for varying levels of stand density, variables were identified such as indicator plants and physical geographic or geologic factors to adjust established "normal" stocking guides and recommended planting and thinning densities. In 1973 and 1974, MacLean and Bolsinger published guides for Oregon and California "Estimating Productivity on Sites with a Low Stocking Capacity" and "Stockability equations for California forest land" wherein they recommended incorporating other factor indices as a negative weight. In 1989, Hall advocated his Growth Basal Area index technique as a method to site potential and management guides in a paper entitled "The concept and application of growth basal area: a forestland stockability index."
Looking back to gain historical perspective, I find it interesting to see the convergence of forest biometricists and quantitative ecologists using their various tools, mindsets and training to approach the concept of site characterization - for the foresters the aspect of growth and yield, stockability and tree performance of a characteristic site under an active forest management regime, from the ecologists- the "typical" plant community range of plant species, dominant tree species, stand structure, wildlife habitat values, and stand development trend under the typical disturbance regime typified by a characteristic plant association. Some ecologically-curious foresters and management-minded ecologists have attempted to have both the peanut butter and the chocolate in one cookie by cross-referencing forest types, site index, SDI/basal area information with ecological site information and this has ultimately made the most valuable type of resource to a forester making a management decision- that either way to approach a site classification/characterization you can see the forest for the trees or the trees with the numbers- either ecologically classify or quantitatively assess - a right brain or left brain person.
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