Saturday, 15 July 2006

Pine Straw Harvesting Effects on Water Content of the Soil Vadose Zone.

Daniel H. Pote and David M. Burner. USDA-ARS, 6883 South State Highway 23, Booneville, AR 72927

Pine needles that accumulate on the forest floor help to conserve soil moisture, protect the soil surface against erosion, moderate soil temperature, inhibit weed growth, and provide soil nutrients and organic matter. Pine needles tend to interlock and form a straw layer that is not easily dislodged by high winds or water flows, but retains a loose structure that allows air, water, and nutrients to easily infiltrate the soil surface. These qualities have made pine straw a valuable landscaping mulch, and a multi-million dollar business in several southeastern states. However, some forest managers are concerned that the loss of those mulching benefits from forests may hinder timber productivity in areas where pine straw has been harvested. The primary concern is that removal of the protective pine straw layer allows water to be lost more quickly from the soil surface through runoff or evaporation. Therefore, we hypothesized that pine straw harvesting decreases water content of the soil vadose zone, which could potentially increase drought stress on the trees. To test this hypothesis, three pine-straw harvesting practices (schedules) and a control treatment (no straw harvest) were compared to determine pine straw harvesting effects on water content of the soil vadose zone in an established (16 yrs) loblolly (Pinus taeda L.) plantation (3.0 x 1.5 m tree spacing). There were 24 plots (0.18 ha each) to provide six replications of each harvesting treatment, and a time domain reflectrometry (TDR) tube was installed in each plot. For 25 weeks (from May to November), a TDR probe was used to monitor volumetric soil water content (%) at 20-, 50-, and 80-cm depths in each tube. Results showed that pine straw harvesting tended to decrease soil water content, but the effect was significant (p<0.05) only at the 50-cm depth in weeks 3 and 4 (late June) of the study, when water content at this depth averaged 20.9% for soils where straw was harvested annually, and 30.2% for soils where the straw was never harvested (controls). In soils where pine straw had been allowed to accumulate for at least a year after being harvested, average water content was not significantly different than in the control plots.

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