Photosynthesis and Biomass Allocation in Oak Seedlings Grown under Shade

Northern’ red oak (Quercus rubra L.) (NRO) and white oak (0. alba L.) (WO) acorns were sown into wooden plots and grown under 30 percent shade screen (30 percent S) or 70 percent shade screen (70 percent S). Seedlings grown under full sun were the controls (C). At the end of the first year, the 30 percent S NRO had 30 percent greater seedling dry weight (DW) than C seedlings. No growth differences existed between these two treatments after 2 years. Compared to C, 70 percent S NRO had a 40 percent lower net photosynthetic rate (A), twofold less seedling DW and leaf number, and fourfold less lateral root DW after 2 years. Dry weight biomass allocation to lateral roots increased from 17 percent at the end of the first growing season to 22 percent after 2 years for both the C and 30 percent S NRO. The 70 percent S seedlings, however, allocated only 8 percent DW to lateral roots for both years. White oak seedlings responded similarly to shade as NRO seedlings. The 70 percent S WO had 30 percent lower A and fourfold less lateral root DW than the controls after 2 years. At the end of the second year, DW biomass allocation to lateral roots was 12, 7, and 5 percent, respectively, for C, 30 percent S, and 70 percent S WO seedlings. The impact of shade (reduced light intensity) on seedling growth of both oak species was discussed in terms of A, photoprotection, and DW biomass allocation. INTRODUCTION Fcr the last three decades VariOUS SilViCUltUral praCtiCeS /lava been suggested to improve the success of natural and ,,orficiaj regeneration of northern red oak (Quercus rubra L.) (NRO) on high-quality mesic sites. Several factors have r)ccn implicated in the less-than-satisfactory results of NRO rcganeration. Competition for light between NRO seedlings & other hardwood species (such as Acer saccharum Marsh., A. rubrum L., and Liriodendron tulipifera L.) is the one most commonly mentioned in the literature (Barton and Gleeson 1996, Loftis and McGee 1993). Indeed, full-suncjrown NRO seedlings have higher net photosynthetic rates IA) than shaded seedlings (Crunkilton and others 1992, Ktrbiske and Pregitzer 1996, McGraw and others 1990). Yet, there still exist controversial results on the effects of ‘,hade on NRO growth. For example, Gottschalk (1965, 1987) reported that NRO seedlings receiving 70 percent of lull sunlight grew better than seedlings receiving 6 to 57 i)crcent or 94 percent of light. Similar contradictions exist for other Quercus species. Jarvis (1964) concluded that sessile oak [Cl petraea (Matt.) Liebl.] is intolerant to light intensity (Ircater than 56 percent, whereas Gross and others (1996) reported that root collar diameters were smaller in sessile find pedunculate oak (Q. robur L.) seedlings grown under 58 Percent shade for 3 years. Even when oak seedlings are outplanted on clearcut sites, their poor initial growth results in their becoming overtopped by herbaceous vegetation and other faster growing hardwood species. Use of large size nursery stocks in the artificial regeneration practice has been suggested as a method of Improving slow initial growth (Farmer 1975, Foster and Farmer 1270). Kormanik and others (1994) reported a nursery Prctocol that produced large-size oak seedlings as compared 10 seedlings used in various studies (Farmer 1979, Go&chalk ‘885, Teclaw and lsebrands 1993). Nevertheless, shelterwood Planting has been recommended as an alternative to cutplanting oak seedlings on clearcut sites for various reasons Research Plant Physiologist and Principal Silviculturist, respectively, USDA Forest Service Southern Research Station, 320 Green Street, 4thens, CA 80602-2044; and Biometrician, USDA Forest Service, Southern Research Stat&n, P.O. Box 2680, Asheville, NC 28804. (Loftis and McGee 1993, Teclaw and lsebrands 1993). However, these shaded oaks did not have fast growth even several years after release (Loftis and McGee 1993). Chlorophyll bleaching has been reported to occur in shade leaves as well as in sun leaves formed from shade buds in the released understory plants. In other words, the sudden improvement of light intensity and quality resulting from overstory canopy removal actually imposes damage to the released plants. During the last decade, it has been well documented that under conditions when absorbed light energy cannot be fully utilized for photochemical reactions in photosynthesis, the xanthophyll cycle-dependent and pHdependent dissipation of excessive energy prevents photooxidative damage to chlorophyll, chloroplasts, and cells. Of the three carotenoid pigments in the xanthophyll cycle, zeaxanthin (2) and antheraxanthin (An) can dissipate excess energy but violaxanthin (V) cannot. Under light conditions, the de-epoxidation of V to Z via the intermediate An is catalyzed by de-epoxidase in the presence of ascorbate and low thylakoid lumen pH. Thus, the xanthophyll pool size and the ratio between Z+An and Z+An+V have been used to describe a leafs photoprotection capacity (Demig-Adams and Adams 1996). Indeed, several reports showed that shaded leaves have lower levels of xanthophyll cycle pigments and smaller ratio of Z+An to Z+An+V as compared to sun leaves (Demig-Adams and Adams 1996, Faria and others 1996). The objectives of this study are to use the protocol of Kormanik and others (1994) to grow NRO and white oak (Q. alba L.) (WO) seedlings under different shade conditions and to evaluate the effects of shade on photosynthesis and biomass allocation. MATERIALS AND METHODS Seedling Growth and Harvest In January 1993, 16 acorns of NRO or WO were sown into 1 meter by 1 meter by 0.6 meter wooden plots at a depth of 1 centimeter below soil surface. Seedlings began to emerge by mid-March. The study was a complete random design with three treatments and two replications. A total of 12 plots were used for each species. In early April, a layer of neutral density screen was draped over a wooden frame 3.3 meters in height. Four plots were enclosed in each frame with a 0.6-meter distance between plots and a 0.8meter distance between plots and the screen. The extent of shade was created by screens of different densities. Maximal photosynthetic active radiation, measured by a LiCor quantum sensor, for 30 percent shade (30 percent S) and 70 percent shade (70 percent S) treatments was 1100 $/m2.s and 550 pE/m2.s, respectively. The full sun control (C) had greater than 1800 $/m2.s. A gap of 0.3 meters was left between the ground and the lower edge of the screen to help air circulation. Seedlings were watered and fertilized according to the protocol of Kormanik and others (1994). The screen was lifted after leaf abscission in December 1993 and placed back in early April 1994. In early December of 1993, seedlings were harvested from two of four plots for each species in each treatment. Care was taken to minimize root loss during harvest. Seedlings were separated into stems (with branches), leaves, taproots, and lateral roots. Each seedling component was oven dried at 95 “C until constant weight was obtained. Leaf area was measured with a portable CID Leaf Area Meter. The rest of the seedlings were grown for another year and harvested in December 1994. Dry weight for each seedling component was obtained. Photosynthesis and Leaf Pigments A portable LiCor 6200 Infrared gas analyzer was used to measure net photosynthetic rate (A) from recently mature leaves still attached to seedlings. In early September 1994, developing leaves of elongating flushes and mature leaves of the latest mature flushes from NRO seedlings grown under full sun were harvested throughout a day and immediately frozen in liquid N,. Procedures for leaf pigment extraction and analysis were modified from the method by Gilmore and Yamamoto (1991). Ethanol (95 percent) and CaHCO, were used to extract pigments. A Dionex Al450 High Performance Liquid Chromatograph with a 4.5 millimeter by 25 centimeter Zorbax non-endcapped Cl 8 column was used. The gradient system used was as followed: 0 to 6 minutes, eluent A (acetonitrile : methanol, 80 percent : 20 percent); 6 to 9 minutes, eluent A to eluent I3 (methanol : ethyl acetate, 68 percent : 32 percent); 9 to 15 minutes, eluent B. Flow rate was 2 milliliters per minute for both eluents. Pigments were detected at 445 nanometers. Retention times (in minutes) are: neoxanthin 2.4, violaxanthin 2.8, antheraxanthin 3.7, lutein 4.7, zeaxanthin 5.0, chlorophyll b 9.4, chlorophyll a 11 .O, a-carotene 13.9, and p-carotene 14.1. RESULTS AND DISCUSSION Seedling Growth Growth parameters of NRO and WO seedlings grown under different shades were presented in tables 1 to 4. Regardless of treatments, mean seedling size and weight for both species in this study are much greater than those reported in the literature (Fam-rer 1975, 1979; Gottschalk 1985, 1987; Teclaw and lsebrands 1993). The NRO seedlings were similar in size to the “1-O large” seedlings classified by Johnson and others (1984). Because this study was designed for 2 years, the planting density used was about one-fourth of the prescribed density by Kormanik and others (1994). The screen used in this study only decreased light intensity but did not change light quality. The 30 percent S grown NRO were significantly greater in total leaf area per seedling, total seedling dry weight (DW), and stem plus branch DW than the controls in 1993 (table 1). Although not statistically Table l-Effects of shade on first-year growth of northern red oak seedlings Variable measured Full sun 30% shade 70% shade p-Value Height (cm) Groundline diameter (mm) FOLR numbelb Taproot DWC (g) Lateral root DW (g) Leaf DW (g) Leaf number Leaf area (cm2) Average leaf area (cm2) Stem and branch DW (g) Seedling DW (g) 1 00.7ae 14.5a 16.4a 62.6a 28.5a 34.9a 53.2a 3237.0b 66.3a 48.lb 174.lb 139.5a 16.4a 17.8a 70.la 34.2a 38.5a 50.la 4590.0a 93.2a 85.2a 228.0a 100.8a .0513 11.7b .0063 13.8a .4209 38.9b .0077 9.4a .0871 23.2a .0403 34.5a .1925 3381 .Ob .0016 98.8a .0795 34.5b .0009 105.9c .0029 B Least-square means for a given variable are not significantly different at the 0.05 experimentwise level using the Bonferroni approach when each pairwise contrast is tested at the 0.05/3 = 0.0167 level. b First-order lateral root. c Dry weight. I