The response of soil temperatures to vegetation cover changes in the southern Appalachians, North Carolina, USA

Land cover changes in agricultural areas frequently decrease vegetation canopy and ground litter, reducing shading and subsequently increasing the diurnal and seasonal variability of soil temperatures. In regions of seasonally frozen ground, such as the highland region of the southern Appalachian Mountains, USA, increases in the amplitude of diurnal and seasonal soil temperatures directly affect local hydrology by increasing ephemeral runoff and decreasing seasonal groundwater-surface water interactions. As the southern Appalachians of North Carolina serve as a water tower for downstream population centers, it is important to quantify the impact of vegetation cover on soil temperatures for common land use types.

In this study we use field data and a 1D vegetation-soil temperature model to elucidate the influence of ground litter versus canopy cover on soil temperatures. The model is developed for two neighboring 25 m² plots, one pasture site and one forest site, representative of the varying land-use types observed throughout the southern Appalachians. We calibrate the model with field data from February-April 2018 including: air temperature, soil temperature at 2, 5, 10, and 20 cm depths, snow depth, ground litter depth, and canopy openness. We quantify canopy openness with hemispherical photography and a gap light analyzer.

Field data analysis indicates that from February to April 2018, the average 5 cm daily soil temperature at the pasture site was 8.1 ºC with a range of 1.4-18.3 ºC, while the average 5 cm soil temperatures at the forest site were more muted and colder, ranging from 1.7-14.8 ºC with a mean temperature of 6.6 ºC. We hypothesize that the colder, muted soil temperatures at the forest site are likely due to differences in ground liter versus canopy cover. Preliminary model analysis suggests that the 2 cm layer of ground litter, which is observed at both sites, has a marginal effect on heat transfer between the air and soil. Instead, colder soil temperatures in the forest appear to be due to the additional shading provided by the 44-54% deciduous canopy cover observed during the period of record. This study may improve our understanding of the effects of land use and vegetation cover on soil temperatures with implications for the regional hydrology of the southern Appalachians.