Large-scale shifts in transpiration dynamics following bark beetle infestation: Stomatal conductance responses

Tree mortality triggered by bark beetle infestation can significantly affect terrestrial carbon and water balances. However, how to improve the parameterization of the stomatal conductance to express the dynamics of ecosystem disturbance remains unclear. A subalpine forest located in the Rocky Mountains experienced a severe bark beetle outbreak in 2008, which provided a unique opportunity to investigate carbon and water flux changes covering the periods of pre-infestation (2005-2007), infestation (2008-2009), and post-infestation (2010-2014). Affected by bark beetle infestation, the stomatal conductance during the summer season (July and August) significantly reduced from 0.0018 m/s in the pre-infestation period to 0.0011 m/s in the infestation period. The decrease in stomatal conductance was not solely caused by the decrease of LAI, but also related to variation in parameter g1 in three commonly-used models of Ball-Barry, Leuning, and Medlyn. The parameter g1 was related to water use efficiency (WUE), and WUE values increased in the infestation period and decreased in the post-infestation period providing evidence that the physiological behavior was significantly changed due to bark beetle infestation. As for the simulation of transpiration, the fitted parameter significantly improved the accuracy in comparison with recommended parameterization. With the inclusion of temporally varied stomatal conductance, estimated transpiration during the infestation period and post-infestation period was improved by 4.3%~13.6% in comparison with the unvaried parameterization fitted in the pre-infestation period. Accounting for the temporally varied stomatal conductance parameters in response to disturbed environments may improve the description of stomatal conductance leading to better model performance in estimated water and carbon balances.