An empirical model for predicting diurnal air-temperature gradients from edge into old-growth Douglas-fir forest

Edge — the boundary line between clearcut and adjacent old-growth forest — is one ofthe critical landscape elements in the highly fragmented forest landscapes of NorthAmerica's Pacific Northwest. Ecological phenomena at edges may be better understood byexamining the physical environments near the edge. To further this objective diurnal airtemperature gradients were measured along 16 gradients from the edge into the interiorold-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forest over the 1989 and 1990growing seasons and analyzed effects of edge orientation (relation of edge-facing to theazimuth) and macroclimate (local weather conditions of the clearcut) on these gradientswere also explored through regression analysis. The air temperature gradient was expressedwith a simple exponential equation involving three intermediate variables of interest: airtemperature in the interior forest (TEMPIF), difference in air temperature between theedge and inside the forest (A AT), and changing ratio of temperature along the gradient(SLOPE). Linear or nonlinear regression equations were developed to predict TEMPIF,1AT, and SLOPE. Correlation analysis always preceded regression analysis, in which therelationships between the regression parameters and independent variables representingedge orientation and macroclimate were further explored. A computer model developedfrom the final empirical relationships successfully predicted air temperature gradients.circumventing the need for time-consuming field measurements with expensive meteorologi-cal instruments, and generated new information about the influences of edge orientationand macroclimate on air temperatures. TEMPIF, AT, and SLOPE were shown to behighly sensitive to the dependent variables. Although model application should be limited to edges created by recent (10- to 15-year-old) clearcuts adjoining old-growth Douglas-firforest, the modeling approach could be applied to edges with different characteristics bymodifying the relationships.