Effect of water and nutrient supply on mean canopy stomatal conductance of Pinus taeda and Picea abies

To investigate the partitioning of mass and energy in forests, a three year study of mean canopy stomatal conductance (GS) was performed in 11-year-old P. taeda (L.) with elevated supplies of water (I) and nutrients (F) and the combination of both (IF) in comparison to a control (C). The first stage of the study found that errors in GS were minimized when vapor pressure deficit (D) ≥0.6 kPa.; The next phase of the study investigated whether leaf specific hydraulic conductance (K_L) and root hydraulic conductance (k_s) could explain the lower G_S that was found in F. Because L and root-to-leaf area ratio were similar in F and IF, the 50% reduction in maximum root k_s in F is the most compelling explanation for a similar reduction in G _S. This may reflect morphological differences (not caused by mean hydraulic tracheid diameter) resulting from development under higher water stress which prevent high G_S in F even when soil moisture is high. These morphological differences conferred greater resistance to cavitation in F roots which were half as vulnerable to cavitation as roots of other treatments.; Another portion of the study was performed to determine whether the G_S response to the same treatments in shade tolerant Pidea abies would be the same as in shade intolerant Pinus taeda. G_S of Picea abies was approximately half that of Pinus taeda and, as predicted, the sensitivity of G_S to D was proportionally lower. Even though L increased three-fold in F, G _S in Picea abies remained nearly the same in contrast to Pinus taeda. Despite these differences, stem growth among species reflected absorbed radiation, and stem growth response to treatment reflected mostly the changes in L.; The last stage of the study estimated maximum G _S in Pinus taeda in several ways and assessed which was best when used in a simple growth model that also predicts transpiration. Model predictions of transpiration based on maximum G _S estimated under highest native soil moisture (0.07 m3m –3) and D = 0.6 kPa, were nearly identical to measured transpiration in C and I stands, and were under- and over-predicting in F and IF, respectively, by ∼15%.