Growing-season temperature and soil moisture along a 10 km transect across a forested landscape

This study characterizes a set of growing-season microclimate variables at the land- scape level and examines the relationships between these variables and landscape structure along a transect in the SE Missouri Ozarks. Temperature and soil moisture and their spatial variations at the landscape level were also compared with those at the stand level, a 200 m segment of the transect. We measured air temperature (Ta; at a height of 1 m), soil temperature (Ts; 5 cm in depth), soil-surface temperature (Tsf), and soil moisture (0 to 10 cm in depth) every 10 m along a 10 050 m transect using mobile and permanent weather stations during the growing season, June to September, 1996. Topo- graphic features, overstory and understory coverage, and landscape patch types were also recorded at each point. Elevation at each point was measured using a submeter-resolution GPS (global posi- tioning system) in November 1996. We describe the spatial variation of microclimate with standard deviation. We found that the spatial variations of Ta, Ts and Tsf and soil moisture were large along the transect, ranging from 19.6 to 22.7°C for seasonal mean Ta and from 3.5 to 28.6% for gravimetric soil moisture. We found that seasonal means at the landscape level were not significantly different from those at the stand level. However, the spatial variations at the landscape level were significantly dif- ferent from the variation at the stand level. In addition, the diurnal patterns of the spatial variation at the 2 scales were also different, with high spatial variation observed during the daytime and low vari- ation during the nighttime. The spatial variations of Ts and Tsf had typical 'bell-shaped' diurnal pat- terns, while the diurnal pattern of the spatial variation of Ta was relatively 'flat'. In general, the peaks of spatial variation at the stand level occurred earlier in the day than those at the landscape level, most noticeably for Ts. No apparent seasonal trend was identified for the spatial variations of the microclimatic variables examined. Based on the data collected, topography (such as aspect, slope position, and elevation), patch type, and overstory canopy coverage explain 22 to 52% of the varia- tion in the microclimatic variables examined.