Physiological attributes associated with drought resistance and productivity in loblolly pine

Abstract

Clonal forestry allows for selection of trees and traits that may increase productivity in different environmental conditions. The first part of this research project utilized ramets from three half-sibling loblolly pine (Pinus taeda L.) clones to evaluate intraspecific responses to moisture stress. The second part used ramets from a single clone to better understand how four needle fascicles differed physiologically and morphologically from three needle fascicles. In the first set of experiments, we subjected the clones to three soil moisture treatments: low water (-1.5 MPa), high water (-0.3 MPa), and an alternating water treatment of cyclical low and high water conditions. Traits associated with moisture stress and productivity such as needle level gas exchange, carbon allocation and carbon isotope discrimination were measured. A rank order change in biomass accumulation was observed among the clones between the low and high soil moisture treatments. One clone, relative to the other two clones, had more biomass accumulation in the low soil moisture treatment, but less biomass accumulation in the high soil moisture treatments. The two clones that had the most biomass accumulation under high soil moisture conditions had greater below ground carbon allocation and higher rates of morning gas exchange under the high soil moisture conditions. Carbon isotope discrimination, an index for water use efficiency, was higher in the clone that had the most biomass accumulation under low water conditions. The clone that had the most biomass accumulation under the low water conditions also had an abnormally high abundance of four-needle fascicles, which was the subject of the second set of experiments. Four-needle fascicles, compared to three needle fascicles, had equal diameter, volume and specific leaf area as well as similar anatomy, yet needle tissue density and fascicle surface area were greater. The changes in fascicle anatomy did not affect most of the physiological gas exchange parameters measured with A/Ci and light response curves. The maximum rate of RuBisCo-mediated carboxylation was, however, significantly higher in the four-needle fascicles. This higher rate of carboxylation may have accounted for the greater amount of biomass accumulation observed in this clone in the low water treatment.