Light, temperature, and soil moisture responses to elevation, evergreen understory, and small canopy gaps in the southern Appalachians
Small canopy openings often alter understory microclimate, leading to changes in forest structure and composition. It is generally accepted that physical changes in the understory (i.e., microclimatic) due to canopy removal drive changes in basic forest processes, particularly seedling recruitment which is intrinsically linked to soil moisture availability, light and, to a lesser extent, temperature. We examined the impact of small canopy gaps of the type (snags) and size (~300 m2) most frequently observed in the southern Appalachians on the understory microclimate. We created artificial canopy gaps at two elevations (a.m.s.l.) by girdling trees in areas with and without a Rhododendron maximum L. (rosebay rhododendron) understory. Soil and air temperature (°C), photosynthetically active radiation (PAR; µmol m-2 s-1), and volumetric soil water content (%WC) in the upper 15 cm of soil were measured along transects generally running north to south through each gap. Overall, PAR was substantially less in rhododendron gaps than in non-rhododendron gaps. We found a slight increase in PAR in non-rhododendron gaps during spring and summer compared to controls. Very little seasonal variation in PAR was observed in rhododendron gaps compared with non-rhododendron gaps. In general, %WC in rhododendron gaps was lower than in non-rhododendron gaps and less variable. We also found a gap response in incident PAR during the spring and summer seasons. There was no significant effect of gap creation on %WC, nor were there site (elevation) differences or effects due to the presence of rhododendron. Further, there was no significant gap effect on mean, maximum, or minimum soil and air temperature; however, there were significant effects from the presence of rhododendron and between sites for PAR and soil and air temperature during certain seasons of the year. Although there is some certainty about measurable responses in key microclimatic variables in rhododendron and non-rhododendron gaps found in this study, we could not be certain if responses represent a significant enough departure from values in undisturbed conditions to elicit a tree recruitment response. Our results indicate that for openings of the size examined here, topography and evergreen understory are the primary determinants of spatial and temporal heterogeneity in understory microclimate.