Short-Term Photosynthesis, Root Exudate, and Fine Root Growth Responses to Fertilization and Drought in Clonal Pinus taeda.

A range of growth responses to fertilization have been observed among clones of Pinus taeda in field conditions. Increased leaf area is widely accepted as the long-term mechanism leading to increased carbon (C) fixation and thus volume response as a result of fertilization. However, clonal variability with respect to short-term changes in C allocation and photosynthetic rates (A_sat) as a result of fertilization is less well-understood. Increased C allocation to above-ground portions of the tree may be due to short-term increases in A_sat or reductions in C allocation below-ground. The objective of this research was to determine if short-term fine root and root exudate C allocation or A_sat may be altered by fertilizer application. Our hypotheses were (1) fertilization and water stress decrease C allocation to root exudates and fine roots, (2) fertilization increases and drought decreases A_sat, and (3) specific C allocation patterns and changes in A_sat vary between clones. To test these hypotheses a 2 x 2 x 2 factorial experiment with clone, fertilizer, and drought treatments was replicated in five large growth rooms. Seedlings were planted in pots containing 120,000 cm³ of coarse, nutrient and organic matter deficient sand. Root exudates were quantified with XAD-7 resin capsules placed in situ sequentially throughout the experiment. Capsules were extracted with deionized water and a 50 % methanol solution to partition anthrone reactive C and phenols. Fine roots were quantified with root cores and the destructive harvest of all trees at the conclusion of the experiment. A_sat was quantified with a LI-6400 portable photosynthesis system. Our results will quantify changes in carbon allocation patterns between the clones due to changes in nutrient and water availability.