Regional scale impacts of Tamarix leaf beetles (Diorhabda carinulata) on the water availability of western U.S. rivers as determined by multi-scale remote sensing methods

Abstract

Tamarix leaf beetles (Diorhabda carinulata) have been widely released on western U.S. rivers to control introduced shrubs in the genus Tamarix. Part of the motivation to control Tamarix is to salvage water for human use. Information is needed on the impact of beetles on Tamarix seasonal leaf production and subsequent water use over wide areas and multiple cycles of annual defoliation. Here we combine ground data with high resolution phenocam imagery and moderate resolution (Landsat) and coarser resolution (MODIS) satellite imagery to test the effects of beetles on Tamarix evapotranspiration (ET) and leaf phenology at sites on six western rivers. Satellite imagery covered the period 2000 to 2010 which encompassed years before and after beetle release at each study site. Phenocam images showed that beetles reduced green leaf cover of individual canopies by about 30% during a 6–8week period in summer, but plants produced new leaves after beetles became dormant in August, and over three years no net reduction in peak summer leaf production was noted. ET was estimated by vegetation index methods, and both Landsat and MODIS analyses showed that beetles reduced ET markedly in the first year of defoliation, but ET recovered in subsequent years. Over all six sites, ET decreased by 14% to 15% by Landsat and MODIS estimates, respectively. However, results were variable among sites, ranging from no apparent effect on ET to substantial reduction in ET. Baseline ET rates before defoliation were low, 394mmyr−1 by Landsat and 314mmyr−1 by MODIS estimates (20–25% of potential ET), further constraining the amount of water that could be salvaged. Beetle–Tamarix interactions are in their early stage of development on this continent and it is too soon to predict the eventual extent to which Tamarix populations will be reduced. The utility of remote sensing methods for monitoring defoliation was constrained by the small area covered by each phenocam image, the low temporal resolution of Landsat, and the low spatial resolution of MODIS imagery. Even combined image sets did not adequately reveal the details of the defoliation process, and remote sensing data should be combined with ground observations to develop operational monitoring protocols.