Growth response of trees to fuel reduction treatments along a topographic moisture gradient in the mixed oak forests of southeastern Ohio
Soil moisture availability is an important factor that influences forest ecosystem structure and functioning. In unglaciated oak forests of eastern North America, soil moisture availability and demand vary at fine scales along topographic and edaphic gradients and greatly influence tree growth and productivity. Here, we examine how these topographically controlled microclimates affect tree growth following prescribed fire and thinning treatments in the mixed-oak forests of Ohio. We employed a water balance approach, a GIS-derived model that incorporates topographic and edaphic gradients to assess potential evapotranspiration (PET), actual evopotranspiration (AET), solar radiation and deficit for each tree (averaged for a 5-m buffer around each tree point). A long-term moisture status of the trees was modeled using the integrated moisture index (IMI), also based on GIS. Increment cores from 348 trees (DBH ≥ 25 cm), comprising five species collected from 80 0.1-ha plots distributed across four treatments (control, thin, burn, thin+burn) were obtained and prepared for measurement using standard dendrochronological procedures. Five-year periodic annual basal area increment (BAI) was computed from the ring-width series as an indicator of growth for each tree. The relationships between the moisture variables and BAI were evaluated using simple linear regression.
Water balance varied considerably across the landscape, with ridges, southeasterly and southwesterly slopes exhibiting higher moisture demand or stress than northeasterly and northwesterly slopes and valleys. In general, BAI corresponded strongly with seasonal (April-September) moisture deficit and PET, but was only marginally influenced by seasonal radiation. The IMI exhibited a significant positive effect on BAI of surviving trees. Results indicated strong mechanistic link between moisture stress and BAI of yellow-poplar in the control plot. However, this link weakens in the three treatment units as trees respond more to the treatment manipulations and/or release from competition. Our results indicate that the water balance approach and the integrated moisture index (IMI) are useful for predicting tree growth and productivity in topographically heterogeneous landscapes even in managed stands where the importance of soil moisture availability for tree growth is much less.