The effect of restoration treatments on the spatial variability of soil processes under longleaf pine trees

The objectives of this study were to (1) characterize tree-based spatial patterning of soil properties and understory vegetation in frequently burned ("reference state" ) and fire-suppressed longleaf pine forests; and (2) determine how restoration treatments affected patterning. To attain these objectives, we used an experimental manipulation of management types implemented 15 years ago in Florida. We randomly located six mature longleaf pine trees in one reference and four restoration treatments (i.e., burn, control, herbicide, and mechanical), for a total of 36 trees. In addition to the original treatments and as part of a monitoring program, all plots were subjected to several prescribed fires during these 15 years. Under each tree, we sampled mineral soil and understory vegetation at 1 m, 2 m, 3 m and 4 m (vegetation only) away from the tree. At these sites, soil carbon and nitrogen were higher near the trunk while graminoids, forbs and saw palmetto covers showed an opposite trend. Our results confirmed that longleaf pine trees affect the spatial patterning of soil and understory vegetation, and this patterning was mostly limited to the restoration sites. We suggest frequent burning as a probable cause for a lack of spatial structure in the "reference state" . We attribute the presence of spatial patterning in the restoration sites to accumulation of organic materials near the base of mature trees.