Evaluating different soil and plant hydraulic constraints on tree functionusing a model and sap flow data from ponderosa pine

Relationships between tree size and physiological processes such as transpi ration mag have important implications for plant and ecosystem function, bu t as yet are poorly understood. We used a process-based model of the soil-p lant-atmosphere continuum to investigate patterns of whole-tree sap flow in ponderosa pine trees of different size and age (36 m and similar to 250 ye ars versus 13 m and 10-50 years) over a developing summer drought, We exami ned three different hypothetical controls on hydraulic resistance, and foun d that size-related differences in sap flow could be best explained by abso lute differences in plant resistance related to path length (hypothesis 1) rather than through different dynamic relationships between plant resistanc e and leaf water potential (hypothesis 2), or alterations in rates of cumul ative inducement and repair of cavitation (hypothesis 3). Reductions in sap flow over time could be best explained by rising soil-root resistance (hyp othesis 1), rather than by a combination of rising plant and soil-root resi stance (hypothesis 2), or by rising plant resistance alone (hypothesis 3), Comparing hourly predictions with observed sap dow we found that a direct r elationship between plant resistance and leaf water potential (hypothesis 2 ) led to unrealistic bimodal patterns of sap flow within a day, Explaining seasonal reduction in sap flow purely through rising plant resistance (hypo thesis 3) was effective but failed to explain the observed decline in pre-d awn leaf water potential for small trees. Thus, hypothesis 1 was best corro borated. A sensitivity analysis revealed a significant difference in the re sponse to drought-relieving rains; precipitation induced a strong recovery in sap flow in the hypothetical case of limiting soil-root resistance (hypo thesis 1), and an insignificant response in the case of limiting plant resi stance (hypothesis 3), Longer term monitoring and manipulation experiments are thus likely to resolve the uncertainties in hydraulic constraints on pl ant function.