Thinning and Fertilizing Young Coastal Plain Hardwoods

Abstract

A thinning and fertilization study was installed on a 7-year-old naturally regenerated hardwood stand on somewhat poorly drained soils in the Atlantic Coastal Plain of North Carolina (Northampton County). Dominant species were sweetgum (<i>Liquidambar styraciflua</i> L.) and red maple (<i>Acer rubrum</i> L.). Treatments were thinning (to 3000 stems per hectare in February 1997) and fertilization (238 kg/ha N and 58 kg/ha P hand broadcast in May 1998 as urea and diamonium phosphate), applied separately and in combination. Trees were measured for diameter and height in 1997 and again during the dormant season in 2000/2001. Other measured variables included canopy, specific leaf area, foliar nutrients, competition (woody and herbaceous), soil properties and nutrients, and depth to the water table. Fertilization had a significant positive impact on diameter, height, basal area and volume growth, stem densities, foliar N, leaf area, and leaf area duration. In addition, fertilization appeared to limit the growth and development of woody shrubs and coppiced stems, and accelerate natural self-thinning. Thinning had little to no significant impact on tree growth or nutrient levels, and appeared to allow for increased levels of competing vegetation. The canopy on the thinned-only treatment was less dense than any other treatment and there were more shrubs (predominantly wax myrtle [<i>Myrica cerifera</i> L.]) and vines (predominantly poison ivy [<i>Toxicodendron radicans</i> L.]) on the thinned-only treatment. The combined thinning and fertilization treatment resulted in the highest levels of incremental growth between 1997 and 2001, but only the fertilization effect was significant. Based on the results from foliar and soil analyses, the site does not appear to be phosphorus-deficient, and the strong fertilization effect may be the result of ameliorating a nitrogen deficiency.

Both sweetgum and red maple responded well to fertilization. Given the lack of significant block effect noted for red maple throughout the study, red maple appears to be the more elastic of the two species. Trees observed in this study would most likely benefit from another application of nitrogen, as would most forest stands on similar sites. This study has shown that it is highly unlikely that stand development at this age can be accelerated through thinning alone, although growth benefits gained from thinning may become more evident as the stand ages. Fertilization alone, or in combination with thinning, not only accelerates stand development, but may also increase long-term productivity.