Modelling the impacts of the foliar pathogen, Phaeocryptopus gaeumannii, on Douglas-fir physiology: net canopy carbon assimilation, needle abscission and growth

This paper describes the parameterisation, testing and implementation of needle-level stomatal conductance ( g s) and net CO 2 assimilation ( A net) models that include the physiological impacts of the Douglas-fir pathogen, Phaeocryptopus gaeumannii. Hourly estimates of g s were modelled by assuming that stomata regulate water flux such that plant water potential is maintained above a critical threshold, and A net was modelled based on the kinetics of photochemistry. The model was tested using summer field measurements from trees at three western Oregon Douglas-fir ( Pseudotsuga menziesii) plantations with varying levels of P. gaeumannii, and showed a high degree of accuracy: r 2=0.777 and 0.792 for g s and A net, respectively. Instantaneous needle-level estimates of g s and A net were also scaled-up to a whole-canopy estimate for a 10-month period (July 1998–April 1999). At all three sites, a significant seasonality in A net was observed, with the highest rates occurring during the summer months (up to 400 g CO 2 m −2 LA) declining to near or below zero during the winter. The presence of P. gaeumannii had a significant impact on needle- and whole-canopy A net, and for the needle age classes where colonisation levels reached 25% pseudothecia density (i.e. percent of stomata with visible fruiting bodies), estimated total carbon budgets were negative. However, at the whole-canopy level all trees maintained a positive carbon budget due to the large contribution from current year needles that remain unaffected by the fungus for the first 6 months of development, or until the emergence of pseudothecia. Furthermore, the abscission of the older, more-heavily diseased foliage, shortly after becoming a carbon sink, has a significant mitigating effect on whole-canopy A net. For example, at the high-disease site, P. gaeumannii-associated reductions in A net per unit leaf were estimated to reduce whole-canopy A net by ca. 110% without needle abscission, but this was reduced to 85% when older, more-heavily diseased needles were abscised.