COS 33-1
Soil physical response to vegetation management and consequent effects on Pinus vigor in the West Gulf Coastal Plain
On the West Gulf Coastal Plain, access to water may rely on root elongation through macropores created by old roots. Degradation of these macropores also contributes to development of large micropores that supply plant-available water. Herbicide application and prescribed fire manipulate vegetation above the soil surface; it is likely that parallel changes in rooting occur. Vegetation management causing a departure from normal rooting could affect Pinus water acquisition and therefore, tree vigor. We hypothesize that herbicide application and prescribed fire affect soil porosities by changing non-pine rooting, and these soil changes have the potential to influence Pinus vigor. The study is on the Kisatchie National Forest in central Louisiana, USA. Using a randomized complete block design with five blocks, three levels of vegetation management were established: Control (C)—no post-planting management, Burn (B)—prescribed burning in spring 1998, 2000, 2003, 2005, and 2007, and Herbicide (H)—post-planting herbicide application. Two blocks are on a Ruston fine sandy loam complex (RST), and three blocks are on a Beauregard silt loam complex (BRG). Pinus palustrisMill. seedlings were planted in 1997. Soil cores were extracted within 1 m of three sample trees per plot in 2002, 2004, 2006, and 2009. For the A, and upper and lower Bt1 horizons, micro-, (MIP) and macro- (MAP) porosity fractions, and plant-available water holding capacity (PAWHC) were estimated. Pine vigor variables (predawn water potential and net photosynthesis) were measured eight times between May 2003 and October 2005 on sample trees.
Results/Conclusions
A natural recovery of soil physical properties was observed on both soils. For the A horizon, this was seen only on the BSL. For the subsoil horizons, both the BSL and RST exhibited improved physical properties over time. On the RST, H and B did not affect porosity fractions. On the BRG, H and B led to decreases in A MIP and PAWHC, and decreases in lower Bt1 PAWHC. Decreases in A PAWHC are likely due to the loss of MIP. In the absence of a similar effect on lower Bt1 MIP, the loss of lower Bt1 PAWHC may have been due to an increase and a decrease in fractions of MIP allotted to hygroscopic and plant-available water, respectively. Relationships between pine vigor and soil properties differed between soils. Dissimilar vigor-soil relationships suggest that factors such as clay illuviation, cutan formation, and sand content play a role in pine vigor on the West Gulf Coastal Plain.