Long-Term Ecosystem Fluxes of Carbon Dioxide and Water in a Tamarisk Invaded Riparian System: A 10-Year Perspective on Biological Control by the Northern Tamarisk Beetle
 
Keirith Snyder1* and Russell Scott2
 
1 USDA ARS, Great Basin Rangelands Research Unit, Reno, NV, USA; keirith.snyder@ars.usda.gov
2USDA ARS, Southwest Watershed Research Unit, Tucson, AZ, USA; russ.scott@ars.usda.gov
 
 
Biological control of Tamarix spp. (saltcedar) with Diorhabda spp. (tamarisk beetle) was initiated in several western states in 2001. We analyzed a long-term data set from a site in western Nevada along the Truckee River, where Tamarix ramosissima forms a dense stand. Diorhabda carinulata (the northern tamarisk beetle) migrated to this section of river in 2007. Outbreak conditions were observed in 2008 and 2009, since then the beetle population has fluctuated, but overall declined through time. Time-series of Normalized Difference Vegetation Index (NDVI) obtained from the Landsat satellite platform, showed periodic beetle-induced defoliation events during the growing season from 2008 to 2014.  There was no evidence of defoliation events in 2015 and 2016. In 2017, beetles were present and there was limited evidence of beetle-induced defoliation. Since 2015, NDVI has increased during the successive growing seasons to the present day, likely due to release from drought and declining beetle densities.  The Landsat-derived NDVI interpretation of defoliation events was supported by field observations and on-the-ground measures of Leaf Area Index (LAI). However, total evapotranspiration (ET) only noticeably declined in 2008, 2009 and 2010. But the decline in ET was transient as trees regrew leaves, consequently, there were only small effects on total ET losses. Total ET losses were primarily driven by annual precipitation amounts with the greatest in “wet” years and reduced ET when precipitation fell below 100 mm. Due to wildlife concerns, no further releases of the beetles occurred. Declines in beetle populations eliminated the “outbreak” conditions apparently required to impair Tamarix physiological function. This suggests that additional actions are needed, such as maintaining greater beetle populations, additional biological control agents that attack other parts of Tamarix anatomy, or mechanical clearing, for reducing Tamarix cover.