Hydraulic regulation and control of photosynthesis in Pinus ponderosa

Foresters and ecologists have long recognized that all trees eventually stop growing significantly taller, and forests decline in net primary productivity after canopy closure. The hydraulic limitation hypothesis suggests that these patterns can be explained by the fact that as trees grow larger and older, whole tree leaf specific hydraulic conductance (K_L) declines. Studies in this dissertation test several physiological aspects of this hypothesis and examine how hydraulics affect stomatal behavior and leaf gas exchange in ponderosa pine. Our study site was an open grown stand of ponderosa pine in the Oregon Cascades. K_L at the end of the summer of 1995 (calculated from leaf water potential and leaf gas exchange measurements on one-year old needles) was 44% lower in old compared to young trees. In old trees, stomatal conductance (g _s) and assimilation (A) exhibited a steeper decline with air saturation deficit (D) for the entire summer.; If stomatal control acts to maintain constant leaf water status during transpiration, and if lower stomatal conductance reduces assimilation, then reducing K_L will result in a proportional decline in g_s and assimilation. To vary K _L, we systematically reduced stem hydraulic conductivity (k ) of ponderosa pine seedlings using air injection to induce cavitation. We found that g_s, transpiration, A, and K_L all declined with decreasing k . This response was independent of bulk leaf water potential which remained constant near −1.5 MPa, except at the extreme 99% reduction in k when it fell to −2.1 MPa. Stomatal conductance and assimilation were also directly proportional to K _L.; To assess the effects of path length within individual tree crowns, we measured leaf gas exchange, foliar δ13C, branch sap flux, and K_L on branches at two different heights (10 and 25m) within four tall (∼30m) ponderosa pine. We found no difference in leaf gas exchange, branch sap flux or K_L between the upper and lower canopy of our study trees. However, we did find significantly lower leaf area to sapwood ratios (A_l:A_s) in branches from the upper compared to the lower canopy. A_l:A_s averaged 0.17 (m2 cm−2) and 0.27 in the upper and lower canopy respectively.