Effects of drainage for silviculture on wetland hydrology
Wetland silviculture on the Southeastern Coastal Plain attempts to balance competing objectives of draining topsoils sufficiently to provide access and promote tree growth while maintaining hydric soil characteristics in compliance with the Clean Water Act. This balancing act is dynamic as soil hydraulic properties, and thus water table regimes, change over the stand rotation cycle. Previous studies have defined threshold ditch spacings that would sustain wetland hydrology on a wide range of poorly drained coastal plain soils in US southeast. There is strong evidence that the hydraulic conductivity (K) and transmissivity (T) of the soil profile increase as trees mature. Such changes increase drainage rates, lower water tables, and potentially remove wetland hydrology from drained sites. Results from a long term forested watershed study showed that T of the profile decreased from 50 m2/d under a mature loblolly pine plantation to 5.5 m2/d after harvest and site preparation for regeneration, and then increased to 34 m2/d by 8 years after replanting. Based on these results, a modeling case study was conducted to determine the effects of changes in soil properties on wetland hydrology for a pine plantation. Results showed that 90 cm deep drainage ditches would have to be more than 62 m apart in a young plantation (YP) to sustain wetland hydrology on a site with 15 cm of surface storage. Increases in K and T as the plantation matures increased the predicted threshold spacing (LT) for wetland hydrology from 62 m for YP to 290 m for a mature plantation (MP). For ditch spacings greater than LT, wetland hydrology will be sustained in a broad center section midway between ditches, with the width of the wetland section dependent on ditch depth, spacing and soil properties. Methods developed to estimate the lateral effect of a single ditch on wetland hydrology were used to determine the width of a strip adjacent to the ditch where wetland hydrology is not sustained, and, thereby the percentage of wetland loss from the plantation. Depending on ditch depth, wetland hydrology will not be sustained on 14 to 27% of the land area for a 200 m ditch spacing on the young plantation of the case study. As the plantation matures (to MP), wetland hydrology will not be sustained for normal ditch spacings (100 to 200 m) on these soils. More research is needed to determine effects of stand age and production practices on hydraulic conductivity for a wide range of drained soils. Wetland hydrology can be enhanced by limiting depth and increasing spacing of drainage ditches, by use of control structures in some cases, and by allowing field ditches to fill naturally as plantations mature.