| Description |
The metabolic scaling theory identifies network architecture as a major predictor of whole plant metabolism via hydraulic conductance of the xylem and the shared stomatal pathway for water loss and carbon gain. To predict hydraulic properties, this theory utilizes the West, Brown, Enquist (WBE) architectural model, which is based on principles of space-filling, biomechanical stability, and optimality of hydraulic transport and it is meant to be generally representative of plants. However, plants are highly diverse in their network architecture. Does this diversity matter or does it represent different ways of accomplishing the same task? This dissertation addresses that question by extending WBE to include architectural variation and by testing model predictions and assumptions. The model predicts the scaling exponent between hydraulic conductance and plant size. This exponent depends on the ""bottleneck"" effect, where greater hydraulic resistance in leaves and twigs steepens the exponent. The bottleneck effect was greater when xylem conduits were much larger or more abundant in the trunk than in the twigs. Observed diversity in xylem properties predicted that different functional groups had substantial overlap in hydraulic transport and its scaling. Branching architecture did not influence the bottleneck effect. However, deviating from WBE increased hydraulic conductance and biomechanical stability while requiring less tissue but reducing light interception. Branching could alter hydraulic scaling if architecture changed ontogenetically, which data suggested. MST assumes direct proportionality between sap flow and growth. This was supported in five of six tested species. However, tree species grew more per water use than shrubs, likely reflecting differential allocation. Differences between species were partially attributable to xylem anatomy and plant size. Among this variation in xylem anatomy, branching architecture, and plant stature, the dimensions of leaves and twigs also vary with thicker twigs curiously tending to support few large leaves instead of many small leaves (Corner's rule). Why do plants coordinate leaf and twig size? Corner's rule was recast as the prediction that larger twig leaf areas are composed of larger leaves. Species supported this prediction and had highly convergent scaling. A model predicted that Corner's rule exists to optimize the return on investment in leaves. |
