MODELING THE PHENOLOGY AND SPREAD OF TAMARISK BEETLE INFESTATION AND IMPACT ON WATER SAVINGS

Tamarisk is an introduced shrub that is widespread in western U.S. riparian corridors. There is concern that it displaces native vegetation and consumes large amounts of water from riparian aquifers. Consequently, the saltcedar leaf beetle (Diorhabda carinulata) has been introduced into the western US to control the spread of tamarisk. We summarize preliminary findings of our assessment of phenology and water use (sap flux and satellite-derived). In two adjacent stands of beetle-infested tamarisk stands on the Dolores River, UT, a 10 m tower was erected prior to 2008. Beetle damage was measured using canopy cover from images taken from tower-mounted visible and infrared cameras (“phenocams”).Time-lapse image sets from the cameras were compared with fine-scale estimates of water use using stem sap flow measurements conducted over three growing seasons (2007-2009). Fractional cover from tower phenocams was comparable to cover from MODIS Enhanced Vegetation Index (EVI). Also, EVI was combined with meteorological data to estimate evapotranspiration (ET) at 15 release sites in UT, CO, NV and WY and in adjacent sites to which the beetle might have spread. ET was estimated at 16-day intervals from 2000-2009, encompassing pre- and post-release periods at each site. Ground data collected at four saltcedar-dominated sites on the Dolores River include vegetation structure, composition and phenology, as well as bird monitoring and productivity. For the last 3 years, monthly monitoring of 100 trees at each site were observed for percent flower and leaf, coupled with ratios of green-to-brown needle observations, done from spring green-up to senescence. Bird census data were collected at 100 m radius circular plot stations (n=20) and birds were captured in mist-nets, in which the type and number of birds were reported as birds per net hour. Preliminary results from the sap flux, phenocams and imagery show that both cover and plant transpiration fell dramatically during or shortly after the defoliated period, but recovered when new leaves were produced each year. Baseline ET rates were low, 2-6 mm d^-1 in summer (<0.5 potential ET). At 4 of 15 sites, estimated ET by MODIS decreased markedly after release. At other sites, no decrease in ET was detected, and ET tended to recover to pre-release levels at affected sites. At each location, the results support our past 3 years of findings for the ET seasonally and annually time-series curves as the beetle came into the area and defoliated saltcedar. Potential water salvage was constrained to the relatively brief period of defoliation. These preliminary findings support both satellite and phenological observations showing that beetle damage is transient and spotty and localized at most sites, and reduction in ET is confined mainly to July when beetles are actively feeding. Because defoliation by the beetle is new to the ecosystem (~ 5 years), relative to the presence of tamarisk (>100 years), the long-term effect of the defoliation on water salvage is unknown. Prospects for water salvage over large areas of river so far appear to be limited as beetle – tamarisk interactions have not yet stabilized on western rivers.