The Sensitivity of Western US Forest Carbon Budgets to Topography, Climate, and Disturbance Regimes

Abstract

To properly assess terrestrial carbon budgets in the forests of the Western Rockies, it is vital that we are able to accurately predict tree growth and mortality in a variety of deciduous and evergreen species. Many modern forest models, including the Colorado Variant of the Forest Vegetation Simulator (FVS) used by the United States Forest Service, neglect topographic variables such as slope, aspect, and elevation when considering tree growth. This omission may lead to errors in model outputs. Using a database from the Forest Inventory and Analysis (FIA) archives containing more than 120,000 trees surveyed between 2002 and 2011 in Western Colorado, the impact of topographic variation on tree growth was assessed. Each of these trees was measured using FIA methodology and organized into distinct plots and subplots with their own topographic specifications. Using slope, slope aspect, elevation data for each tree, along with physical characteristics like diameter, height, and age, the growth patterns of Populus tremuloides (Quaking Aspen), Picea engelmannii (Engelmann Spruce), Pseudotsuga menziesii (Douglas Fir), Pinus contorta (Lodgepole Pine), and Abies lasiocarpa (Subalpine Fir) were compared. Tree species and particularly foliage type governed the major growth trends that were observed. The hardwood Quaking Aspen dominated relatively sunnier, drier southfacing slopes (>30% basal area) while conifers like Engelmann Spruce and Subalpine Fir accounted for more of the basal area on north-facing slopes (>30% and 15% basal area, respectively). Quaking Aspen were typically concentrated on shallower slopes (<40% gradient) at lower elevations as compared to conifers, which flourished at higher elevations and on steeper slopes (>50% gradient). These differences in plot condition give rise to variations in soil pH, temperature, nutrient abundance and frequency of disturbance regimes. In assessing the relative importance of these various effects, it appears that the influence of topography on disturbance regimes is significant and must be taken into account in any accurate growth model. Accounting for disturbances like forest fires and droughts are vital for forest growth models like FVS becomes increasingly important with a warming, drying climate.