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Patterns of herbivory-induced mortality of a dominant non-native tree/shrub (Tamarix spp.) in a southwestern US watershed

Hultine, Kevin R., Dudley, Tom L., Koepke, Dan F., Bean, Daniel W., Glenn, Ed P., Lambert, Adam M.
Biological invasions 2015 v.17 no.6 pp. 1729-1742
Diorhabda carinulata, Tamarix, biological control agents, canopy, carbon, defoliation, dieback, ecological invasion, growing season, habitats, herbivores, isotopes, leaves, life history, moderate resolution imaging spectroradiometer, mortality, photosynthesis, plant stress, rivers, shrubs, soil salinity, soil texture, spring, surveys, tree mortality, trees, water stress, watersheds, Southwestern United States
The capacity for plant species or populations to cope with herbivory depends in large part on the complex interactions between resource availability, life history and adaptive strategies to maximize defense and/or tolerance to herbivory. Given these complex interactions, the impacts of repeated herbivory on plant stress and subsequent mortality is often difficult to predict. To better understand relationships between herbivory and environmental condition, we studied the relationship between the non-native shrub/tree tamarisk (Tamarix spp.) and a specialist herbivore, the northern tamarisk leaf beetle (Diorhabda carinulata) released as a biological control agent of Tamarix in the Virgin River watershed in the southwestern United States. The beetle feeds exclusively on Tamarix foliage resulting in complete stand foliage desiccation (i.e. defoliation) that lasts several weeks. Approximately 900 Tamarix plants were surveyed over three consecutive growing seasons for canopy dieback and mortality across 10 sites varying in the number of defoliation events, tree height, soil salinity, soil texture and bulk leaf carbon isotope ratios (δ¹³C). Canopy dieback increased from 27 % by volume in the spring of 2012 to 41 % and 54 % in 2013 and 2014, respectively. Tree mortality increased from 0 % in 2012 to 6 % and 10 % in 2013 and 2014, respectively. Surprisingly, percent canopy dieback was not related to the number of defoliation events that ranged from 2 to 7 across the 10 sites prior to the 2013 growing season. On the other hand, canopy dieback increased with soil salinity in both 2013 (R² = 0.39, F = 5.07, P = 0.055) and 2014 (R² = 0.56, F = 10.26, P = 0.015). Canopy dieback in 2013 increased with bulk leaf δ¹³C (R² = 0.38, F = 4.08, P = 0.078), although δ¹³C also decreased with the number of defoliation events (R² = 0.64, F = 14.17, P = 0.0055), suggesting that photosynthetic rate or drought stress (as indicated by leaf δ¹³C) may serve as a poor predictor for Tamarix canopy dieback in response to defoliation. Percent canopy dieback was correlated with shifts in NDVI measured from annual MODIS imagery (R² = 0.61, F = 12.32, P = 0.008), demonstrating that the tree surveys reflect site-scale changes in canopy cover. Results show that patterns of Tamarix canopy dieback and subsequent mortality following episodic defoliation by D. carinulata are likely to vary across broad gradients in soil salinity and other abiotic and biotic factors. Documented impacts of this biocontrol agent reported here will aid management efforts aimed at preserving riparian habitat in the short-term with conservation efforts targeting the removal and control of Tamarix over the long term.