The effects of insect biological control on a Tamarix invaded ecosystem: ecosystem water and carbon dioxide fluxes and plant-level responses

Background/Question/Methods

Tamarix spp. (saltcedar) has invaded many river systems in the western United States with detrimental impacts to flora and fauna. Traditional methods of invasive plant control have been ineffective or costly.  Therefore, insect biological control of Tamarix with Diorhabda spp. (tamarisk leaf beetles) was initiated in five states. Assessments of the efficacy of biological control efforts have rarely considered ecosystem level effects such as carbon and water cycling. Given the successful dispersal of this insect and its many unknown consequences, it is important to understand the ecosystem effects of this widespread insect biocontrol.  This study examines the ecohydrological effects of the beetle on a Tamarix invaded ecosystem in the Great Basin Desert, Nevada. We predicted beetle herbivory would decrease evapotranspiration (ET) due to reduced canopy leaf area and as ET decreased, net carbon uptake would decrease making this ecosystem more of a carbon source. Furthermore, these effects should increase in magnitude with successive years of herbivory. We used a variety of techniques including eddy-covariance, stable isotopes, leaf gas-exchange and remotely-sensed NDVI to determine the effects of biocontrol on carbon dioxide (CO₂) and ET fluxes and the ability of remaining Tamarix trees to use groundwater for photosynthetic assimilation.

Results/Conclusions

We examined four years of ecosystem CO₂ and ET fluxes in relation to NDVI and ground-based measures of LAI. We found that four years of herbivory and the resulting defoliation events produced short-term decreases in ET and C uptake.  However, trees were able to regrow leaves generally within a few weeks after defoliation. Additionally, total ET and C fluxes over the four growing seasons were not affected in a clear directional trajectory of reduced ET loss and reduced CO₂ uptake. LAI and NDVI were fairly well correlated with each other and NDVI was correlated with ET during the summer months, indicating that these measures are useful for detecting beetle damage. Preliminary analyses indicate remaining live trees were still connected to groundwater after multiple years of herbivory. Differences in gas exchange parameters were largely non-existent, but occasionally increased in foliage that re-grew after seasonal herbivory versus initial foliage that grew before herbivory. In sum, after four years of herbivory there was no clear trend in ET and CO₂ fluxes. ET and CO₂ responses were transient as Tamarix was able to refoliate, likely due to its access to groundwater and because beetle herbivory is a relatively recent phenomenon at this site.