Temporal controls on crown nonstructural carbohydrates in southwestern US tree species.

In trees, large uncertainties remain in how nonstructural carbohydrates (NSCs) respond to variation in water availability in natural, intact ecosystems. Variation in NSC pools reflects temporal fluctuations in supply and demand, as well as physiological coordination across tree organs in ways that differ across species and NSC fractions (e.g., soluble sugars vs starch). Using landscape-scale crown (leaves and twigs) NSC concentration measurements in three foundation tree species (Populus tremuloides, Pinus edulis, Juniperus osteosperma), we evaluated in situ, seasonal variation in NSC responses to moisture stress on three timescales: short-term (via predawn water potential), seasonal (via leaf δ13C) and annual (via current year's ring width index). Crown NSC responses to moisture stress appeared to depend on hydraulic strategy, where J. osteosperma appears to regulate osmotic potentials (via higher sugar concentrations), P. edulis NSC responses suggest respiratory depletion and P. tremuloides responses were consistent with direct sink limitations. We also show that overly simplistic models can mask seasonal and tissue variation in NSC responses, as well as strong interactions among moisture stress at different timescales. In general, our results suggest large seasonal variation in crown NSC concentrations reflecting the multiple cofunctions of NSCs in plant tissues, including storage, growth and osmotic regulation of hydraulically vulnerable leaves. We emphasize that crown NSC pool size cannot be viewed as a simple physiological metric of stress; in situ NSC dynamics are complex, varying temporally, across species, among NSC fractions and among tissue types.

Female-biased sex allocation in relation to growth rate of fruit in Erythronium japonicum.

Using four populations of the liliaceous perennial Erythronium japonicum, I examined the hypothesis that sex allocation will be female-biased if the duration of sink-limited growth of fruits, during which fruits grow exponentially, is long. I found that all marked fruits in each population had a period of sink-limited growth. Among the four populations, the mean length of sink-limited growth increased, and the mean dry mass ratio of the sum of the corolla and androecium/fruit decreased, in a consistent order. Thus, plants in populations where the duration of sink-limited growth was long allocated relatively more of their resources to their female functions. This result was consistent with the above hypothesis.