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 (gs) and net CO2 assimilation (Anet) models that include the physiological impacts of the Douglas-fir pathogen, Phaeocryptopus gaeumannii. Hourly estimates of gs were modelled by assuming that stomata regulate water flux such that plant water potential is maintained above a critical threshold, and Anet 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: r2=0.777 and 0.792 for gs and Anet, respectively. Instantaneous needle-level estimates of gs and Anet 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 Anet was observed, with the highest rates occurring during the summer months (up to 400 g CO2 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 Anet, 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 Anet. For example, at the high-disease site, P. gaeumannii-associated reductions in Anet per unit leaf were estimated to reduce whole-canopy Anet by ca. 110% without needle abscission, but this was reduced to 85% when older, more-heavily diseased needles were abscised.