Technical Bulletins

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Technical Bulletin No. 1046: Model-Based Forecasts of North American Forest Growth: A Review

PDF , Source: Technical Bulletins Published: 01/2018 View Abstract

Abstract: In constantly changing environmental conditions, planning for forest management goals and projecting wood supplies becomes complicated. Possible changes in precipitation, temperature, and CO2 can affect tree growth substantially and potential effects differ by species and region. However, integration of potential forest growth responses to these factors can be achieved using models. Because of the need to understand the range of forest growth forecasts and the strengths and limitations of different modeling approaches, results from 18 studies of forecasted forest responses over coming decades were summarized. Some models used statistical relationships between tree rings and climate to forecast growth responses to potential future climate, some simulated net photosynthesis of a standard forest canopy, and many used tree or stand growth models at various levels of mechanistic detail. In general, models that included CO2 responses predicted enhanced forest growth by 2100 across most of the commercial timberland areas of the US and Canada. For modest warming, most models showed growth enhancement in most regions. For hotter scenarios, many models and regions showed even more growth enhancement, but some regions such as the Southwest, mountain West, and southwestern Canada were predicted to experience drought stress, although projections in these regions were variable. Young stands, angiosperms, and early-successional species were predicted to exhibit the most positive responses. As a result, commercial harvest ages might be accelerated by several years, depending on species. Some simulations for the Midwest and Northeast US predicted a doubling or more of net primary productivity; other studies show a lesser response. Models that did not include mechanisms of CO2 fertilization showed positive growth responses in limited cases and generally showed growth declines. There also was evidence indicating potential spread of forest into woodland at shrub or prairie ecotones.

Technical Bulletin No. 1045: Forest Growth Trends in the United States and Canada

PDF , Source: Technical Bulletins Published: 01/2018 View Abstract

Abstract: Reports of changes over the past century in factors such as temperature, precipitation, fire regimes, ozone, atmospheric CO2, and nitrogen deposition have led to questions about forest growth over this same time period. Determining changes in forest growth over long intervals is complicated by constantly changing growth conditions due to tree maturation, stand self-thinning, disturbance, fires, and other factors. Because a comprehensive review is lacking, results from publications examining forest growth trends in the United States and Canada over the past 100 years were evaluated. Reviewed papers used multiple sources of data, including remote sensing, permanent plots, growth models, tree ring analysis, and historical photography, to evaluate forest growth trends. In the Pacific Northwest (including British Columbia), the entire eastern US, and eastern Canada, reviewed publications report medium to strong growth enhancement based on a variety of data types over periods exceeding 100 years in some cases. For the inland West, historical photography shows clear densification and expansion of Ponderosa pine (Pinus ponderosa) forest across the region. However, a recent drought, probably linked to ocean cycles, has caused a growth setback, especially in the Southwest. In western Canada and Alaska, results are mixed. Studies have found forest expansion both upslope and down into grassland in many areas of the boreal zone. On the other hand, aspen (Populus tremuloides) dieback has been noted due to recent drought in these same forests. Studies using remote sensing and inventory data present mixed results in this region, with disagreement between studies and methods as well as probable heterogeneous responses. Factors identified as the cause of enhanced growth varied by region, but included reduced fire incidence, rising CO2 concentrations, N deposition, increased precipitation, and warming temperatures.

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