Physiological performance of an Alaskan shrub (Alnus fruticosa) in response to disease (Valsa melanodiscus) and water stress

Authors: ROHRS-RICHEY, JENNIFER - University Of Alaska; MULDER, CHRISTA - University Of Alaska; Winton, Loretta; STANOSZ, GLEN - University Of Wisconsin

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2010
Publication Date: 1/20/2011
Citation: Rohrs-Richey, J.K., Mulder, C.P., Winton, L.M., Stanosz, G. 2011. Physiological performance of an Alaskan shrub (Alnus fruticosa) in response to disease (Valsa melanodiscus) and water stress. New Phytologist. 189(1):295-307.

Interpretive Summary: At northern latitudes, plants are being exposed to multiple climate-related stresses as warming temperatures push plants beyond the physiological limits of their current range. Our study focused on two stresses related to the warming and drying of the Alaskan boreal forest: drought and disease. We examined the effects of these stressors on the physiology of green alder, a symbiotic, nitrogen-fixing shrub that fuels the nitrogen economy of the boreal forest. The susceptibility of alders to drought and disease was recently observed during the hottest, driest summer on record, which coincided with rapid development of Cytospora canker disease. In an inoculation experiment, we evaluated two types of disease-water stress relationships for green alder: 1) the effect of water stress on disease predisposition; and 2) the effects of water stress and disease on host physiology. Although water stress was not required for disease predisposition, severity of the canker disease increased with a longer duration of water stress. Our experiment suggests that green alders under water and disease stress have less physiological capacity to meet the high photochemical and water transport demands expected with continued warming and drying of Alaska’s boreal forest. Warming may also contribute to higher disease severity and continue to reinforce the positive feedbacks between stressed hosts and disease damage.

Technical Abstract: At northern latitudes, plants are being exposed to multiple climate-related stresses as warming temperatures push plants beyond the physiological limits of their current range. Our study focused on two stresses related to the warming and drying of the Alaskan boreal forest: drought and disease. We examined the effects of these stressors on the physiology of green alder (Alnus viridis subsp. fruticosa), a symbiotic, nitrogen-fixing shrub that fuels the nitrogen economy of the boreal forest. The susceptibility of alders to drought and disease was recently observed during the hottest, driest summer on record, which coincided with rapid development of Cytospora canker disease (associated with fungal pathogen Valsa melanodiscus anamorph =Cytospora umbrina). In an inoculation experiment, we evaluated two types of disease-water stress relationships for green alder: 1) the effect of water stress on disease predisposition; and 2) the effects of water stress and disease on host physiology. Although water stress was not required for disease predisposition, severity of the canker disease increased with a longer duration of water stress (45-60 days). Under high light and temperatures, water-stressed, diseased hosts had lower light-saturated photosynthesis (Amax) ( 6.33 ± 0.74 µmol CO2 m-2s-1) and lower light-saturation point LSP (667 ± 190 µmol m-2s-1) than well-watered, controls ( Amax: 9.13 ±0.69 µmol CO2 m-2s-1; LSP: 1643 ± 178 µmol m-2s-1 ). During the midday hours of high vapor pressure deficit, total water transport was reduced by an average of 23.8 ± 6.7 g H2O in diseased hosts. Stressed alders minimized water loss by operating over a low, narrow range of stomatal conductance (60-80 vs. 67-137 mmol H2O m-2s-1 for well-watered alders). Our experiment suggests that green alders under water and disease stress have less physiological capacity to meet the high photochemical and water transport demands expected with continued warming and drying of Alaska’s boreal forest. Warming may also contribute to higher disease severity and continue to reinforce the positive feedbacks between stressed hosts and disease damage.