Seasonal Fine-root Carbohydrate and Growth Relations of Plantation Loblolly Pine after Thinning and Fertilization’

In 1989, lwo levels each of stand density and fertilization were established in an 8-year-old loblolly pine (Pinus faeda L.) plantation. In March 1995, treatments were reapplied, and root elongation and carbohydrate concentrations were monitored for 2 years. Our objective was to evaluate relationships between seasonal root growth and carbohydrate concentration in response to thinning and fertilization. Peak root elongation occurred between May and July. Root elongation was greater in response to thinning throughout 1995 and, although not always significant, was consistently greater in thinned plots in 1996. Root growth was reduced in the fertilized plots throughout 1996. Positive effects of thinning on fine-root starch concentrations were observed. Starch levels were consistently lower in response to fertilization for most of 1995 but were greater in fertilized plots during winter 1996. Glucose levels tended to be greater in response to thinning both years and less in response to fertilization in 1995. We conclude that fine-root carbohydrate concentration and net root elongation are characterized by distinct seasonal patterns, and that the magnitude of seasonal root elongation and carbohydrate concentrations is influenced by silvicultural treatments. INTRODUCTION Loblolly pine (Pinus taeda L.) is of significant economic importance in the southern United States (Schultz 1997). Many abiotic factors, such as moisture, fertility, and light, limit the growth of this species (Allen and others 1990, Teskey and others 1994a). Roots supply the essential water and mineral nutrients needed for growth. Thus, the ability of tree root systems to supply these resources affects stand productivity (Cropper and Gholz 1994, Eissenstat and Van Rees 1994). The production of new roots in forest stands may increase or decrease in response to silvicultural treatments such as thinning and fertilization (Sword and others 1998a, 1998b, Albaugh and others 1998). Root-growth responses to silvicultural treatments have been linked to changes in leaf area, carbon fixation, and photosynthate allocation to the root system (Albaugh and others 1998; Gower and others 1992). Since new root growth is regulated, in part, by carbohydrate availability in the root system (Kozlowski and Keller 1966, Noland and others 1997), knowledge of how silvicultural treatments affect root carbohydrate relations is needed to understand how root growth is manipulated by these treatments. Our objective was to evaluate relationships between seasonal root growth and carbohydrate relations of plantation loblolly pine in response to thinning and fertilization. We hypothesized that: (1) seasonal patterns of fine-root starch and glucose concentrations are closely related to new root growth, and (2) manipulation of stand density and soil fertility affects the relationship between fineroot starch and glucose concentrations and root growth. MATERIALS AND METHODS This study was conducted in a 14-year-old loblolly pine plantation on the Palustris Experimental Forest in Rapides Parish, LA. The soil is a Beauregard silt loam that is low in available phosphorus (Kerr and others 1980). Genetically unimproved, container-gyown loblolly pin& seedlings were planted in 1981 at 1.8x 1.8-m spacing. In 1988, 12 treatment plots, 13 rows of 13 trees each (0.06 ha), were established. Two levels of fertilization (none; 744 kg ha-’ diammonium phosphate) in April 1989 and two levels of thinning (none: 2990 trees ha-‘; row thinned: 731 trees ha-‘) in November 1988 were randomly applied in a two-by-two factorial design with three replications. In March 1995, fertilization (none; 444 kg urea + 248 kg triple super phosphate + 100 kg potash ha-‘) and thinning (none: 42 m2 ha-‘; 15.6 mz ha-‘) were reapplied. Two of the three replications were blocked by topography. Precipitation was quantified electronically in a clearing approximately 25 m from the study. Volumetric soil water content of the 15-cm depth was measured biweekly at three locations per plot of each replication using time domain reflectometry. Using previously described methods, net root elongation (mm dms2) in five Plexiglas rhizotrons per plot of two replications was quantified on a biweekly basis between April 1995 and March 1997 (Sword and others 1996, Sword and others 1998b). Root elongation from April 1995 through February 1996 is also reported elsewhere (Sword and others 19986). At 2to 4-week intervals, 10 soil cores (6.5 cm x 15 cm) were extracted from random locations in the periphery of each plot of two replications using a metal coring device (Ruark 1985). Branched fine-roots (s 1 .O-mm diameter) were elutriated from soil cores (Smucker and others 1982). Roots from each plot were pooled, washed, frozen (-80 “C), lyophilized, and ground (40-mesh). Fine-root starch and glucose concentrations were determined using a modification of the procedure described by Jones and others (1977). Starch and soluble sugars were extracted from 25 mg ground root tissue and enzymatically converted to glucose. Glucose was quantified by the glycolytic production of reduced nicotinamide adenine dinucleotide phosphate (NADPH). Spectrophotometrically, NADPH was measured at 320 nm. Carbohydrate concentrations are expressed as mg g” ash-free dry weight. ’ Paper presented at the Tenth Biennial Southern Silvicultural Research Conference, Shreveport, LA, February 16-18, 1999. ’ Plint Physiologist, Research Plant Physiologist, and Forestry Technician, USDA Forest Service, Southern Research Station, Pineville, LA 71360, respectively.