Effects of Light Regimes on 1-Year-Old Sweetgum and Water Oak Seedlings

Light regimes vary significantly within small forest openings, ranging from full sunlight to total shade. This may affect establishment, early growth, and competitive status of hardwood seedlings. We used modified shadehouses to simulate light conditions within forest openings and to test the effects of daily photosynthetically active radiation and time of direct light exposure on growth of sweetgum (Liquidambar sfyraciflua L.) and water oak (Quercus nigra L.) seedlings. The study was a split-plot design in a completely randomized block layout with four replicates. The five light regime treatments representing the time of exposure to direct sunlight were NO, NOON, MORNING, AFTERNOON, and FULL. Greenhouse-raised sweetgum and water oak seedlings were planted in the treatment plots at a 0.3 x 0.3 meter spacing in early May 2000. Height, groundline diameter, and leaf surface area were determined at the end of the first growing season. Growth for both species generally increased with the amount of direct sunlight received. For treatments receiving some direct sunlight, sweetgum and water oak were the same height at the end of the growing season. However, sweetgum was 35 percent taller than water oak in the fully shaded treatment. For sweetgum, surface area of the average leaf was significantly larger in the fully shaded treatment than in other treatments, but no treatment differences occurred for surface area in water oak. Results suggest that sweetgum seedlings are more adaptive to low light levels than water oak seedlings during the first year of development. INTRODUCTION Oak seedlings are shade intolerant to intermediately intolerant and do not grow well under a closed forest canopy (Smi th 1992) . Once advanced oak reproduc t ion i s es tablished, seedlings need adequate light to grow faster than compet ing vegeta t ion (Minck le r 1957, Bey 1964, Sander 1972, Johnson 1979). Light conditions under a canopy can be complex; direct and partial sunlight may reach seedlings during certain times of a day, but seedlings may be fully shaded at other times. A complex light regime with fluctuating periods of direct and indirect sunlight is difficult to mimic but may strongly affect seedling establishment and growth. Gardiner and Hodges (1998) used shadehouses to study the effect of various light conditions on cherrybark oak (Quercus pagoda Raf.) seedlings and found that height of 2-year-old seedlings was greatest with 27 and 53 percent of full sun l igh t . Ground l ine d iameter showed a s imi la r pa t te rn , except that it was greater with 53 percent of full sunlight than with 27 percent. Similar results have been reported by others (Kolb and Steiner 1990a, 1990b; Gottschalk 1994). Sweetgum (Liquidambar styraciflua L.) and water oak (Q. nigra L.) are widely distributed in the southeastern United States and have a very similar range (Kormanik 1990; Vozzo 1990). Both species are commercially important within the region. Sweetgum is a rapidly growing, pioneer species while water oak is a medium-sized rapidly growing species. Sweetgum and water oak are potentially major competitors because o f the i r common occur rence . We hypo thes ized that timing and amount of photosynthetically active radiation (PAR) would affect the growth and characteristics of sweetgum and water oak seedlings. To test this hypothesis, we designed a non-traditional type of shadehouse to simulate the light conditions occurring within small forest open ings . Each shadehouse had sec t ions tha t had no shade cloth on top, which allowed direct sunlight to reach seedlings during different times of day. Applying the methods of Marquis (1965) and Satterlund (1983), we calculated the length of time seedlings were exposed to direct sunlight and tested the hypothesis that the timing and amount of direct sunlight and daily PAR affected seedling growth. METHODS The study site was located in Drew County, AR in the West Gulf Coastal Plain. The soil is an Amy silt loam (Typic Ochraquults). Site index for sweetgum and water oak is about 26 meters at 50 years. Before the study was established, the area was an open field, but native vegetation is classified as mixed pines and hardwoods (Larance and others 1976) . Annua l p rec ip i ta t ion averages 134 cent imeters, with most occurring in winter and early spring. The study was a split-plot design in a completely randomized block layout with four replicates. The main plot was exposure to direct sunlight, and subplot was tree species. With the shadehouses oriented toward north, five light regimes were created based on when direct sunlight ‘Assistant Professor, School of Forest Resources, University of Arkansas-Monticello, Monticello, AR 71656; Research Forester, Southern Research Station, Monticello, AR 71656; and former Graduate Research Assistant, School of Forest Resources, University of ArkansasMonticello, Monticello, AR 71656, respectively. Citation for proceedings: Outcalt, Kenneth W. , ed. 2002. Proceedings of the eleventh biennial southern silvicultural research conference. Gen. Tech. Rep. SRS-48. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 622 p.