Thirty years of forest productivity in a mountainous landscape: The Yin and Yang of topography

We measured light-related patterns of primary productivity within a topographically complex Oregon watershed over a 30-year period. Second-growth conifer densities were experimentally altered in 1981. Plots receiving at least 3434?MJ?m?2 over a 6-month growing season averaged 40% greater aboveground net primary productivity (ANPP) than those receiving less light (p?=?0.000). Unthinned stands potentially built enough LAI to compensate for low light, but risked mortality that exceeded resilience. The two light levels acted as basins of attraction for other physiological and ecological processes, including size?density relationships and limiting foliar nutrients. Initial (1981) LAI and the irradiation step (above or below 3434?MJ?m?2) explained 60% of variation in a 30-year ANPP. Irradiation within each light group did not affect ANPP. At high irradiation, foliar N/Ca and slope steepness (both negative) explained 58% of the variation in residuals from the initial models, while at low irradiation on north, east, and west aspects, 83% of residual variation was explained by foliar Mg (+), understory cover (+), and 30-year mortality (?). Light use efficiency (LUE) of fully stocked stands correlated with LAI and foliar N/K. Results suggest that understory influence on tree foliar N (+ or ?) enhances ANPP by regulating critical nutrient ratios. Mortality reduced or eliminated differences among thinning levels, which did not vary at low light and only between unthinned and heavily thinned at high light. Values associated with relatively open forests (biodiversity, resilience) may be attained without large sacrifice of long-term carbon sinks. In our study, light interacts with topography to produce nonlinear dynamics in which small changes in irradiation can have large consequences. Reduced sunlight has been suggested as a geoengineering option to combat global warming. Ecological changes out of proportion to lowered irradiation are a distinct possibility, including sharp reductions in terrestrial carbon sinks.
Keywords: carbon, irradiation, light-use efficiency, LTER, mortality, mountains, plantations, thinning, thresholds, topography