Early impacts of biological control on canopy cover and water use of the invasive saltcedar tree (Tamarix spp.) in western Nevada, USA

Abstract

The success of biological control programs is rarely assessed beyond population level impacts on the target organism. The question of whether a biological control agent can either partially or completely restore ecosystem services independent of population level control is therefore still open to discussion. Using observational and experimental approaches, we investigated the ability of the saltcedar leaf beetle [Diorhabda carinulata (Brullé) (Coleoptera: Chrysomelidae)] to reduce the water use of saltcedar trees (Tamarix ramosissima Ledeb.) in two sites (Humboldt and Walker Rivers) in Nevada, USA. At these sites D. carinulata defoliated the majority of trees within 25 and 9 km, respectively, of the release location within 3 years. At the Humboldt site, D. carinulata reduced the canopy cover of trees adjacent to the release location by >90%. At a location 4 km away during the first year of defoliation, D. carinulata reduced peak (August) stem water use by 50-70% and stand transpiration (July to late September) by 75% (P = 0.052). There was, however, no reduction in stem water use and stand transpiration during the second year of defoliation due to reduced beetle abundances at that location. At the Walker site, we measured stand evapotranspiration (ET) in the center of a large saltcedar stand and found that ET was highest immediately prior to D. carinulata arrival, dropped dramatically with defoliation, and remained low through the subsequent 2 years of the study. In contrast, near the perimeter of the stand, D. carinulata did not reduce sap flow, partly because of low rates of defoliation but also because of increased water use per unit leaf area in response to defoliation. Taken together, our results provide evidence that in the early stages of population expansion D. carinulata can lead to substantial declines in saltcedar water use. The extent of these declines varies spatially and temporally and is dependent on saltcedar compensatory responses along with D. carinulata population dynamics and patterns of dispersal.

Tamarisk biocontrol, endangered species risk and resolution of conflict through riparian restoration

“…At those sites establishment and defoliation by D. carinulata was dramatic (Pattison et al 2010), leading to substantial target mortality over the course of three or more years (Fig. 2;Hudgeons et al 2007).…”

“…; also known as saltcedar) has been one of the most visually dramatic weed biocontrol programs ever developed Hudgeons et al 2007;. Following one or two years of initial establishment at several release sites in western North America, insect dispersal and subsequent feeding by larval and adult insects numbering in excess of 1,000 per tree, near-complete defoliation of plants over large areas has been observed (Tracy and Robbins 2009;Pattison et al 2010). In particular the establishment of D. carinulata Desbrochers in Nevada, USA and several other western states, now including dispersal into Arizona, has led to thousands of hectares defoliated, often two or three times over the course of a single season with multiple generations of beetles (Bean et al 2007;Pattison et al 2010;Dalin et al 2010).…”

“…Following one or two years of initial establishment at several release sites in western North America, insect dispersal and subsequent feeding by larval and adult insects numbering in excess of 1,000 per tree, near-complete defoliation of plants over large areas has been observed (Tracy and Robbins 2009;Pattison et al 2010). In particular the establishment of D. carinulata Desbrochers in Nevada, USA and several other western states, now including dispersal into Arizona, has led to thousands of hectares defoliated, often two or three times over the course of a single season with multiple generations of beetles (Bean et al 2007;Pattison et al 2010;Dalin et al 2010). This success has met not with cheers of approval, but instead has exacerbated a long-running conflict between weed control measures and protection of endangered species that not only has greatly hindered the expansion and evaluation of this biocontrol program, but has repercussions for biocontrol development programs targeting many environmental weeds.…”

Field TestingDiorhabda elongata(Coleoptera: Chrysomelidae) From Crete, Greece, to Assess Potential Impact on Nontarget Native California Plants in the GenusFrankenia

“…Saltcedars are highly invasive Eurasian trees that infest vast areas of riparian ecosystems in the western United States (DeLoach et al 2004, Tracy and Robbins 2009. D. carinulata established quickly at many of the release sites, dispersed rapidly, and defoliated extensive stands of saltcedar, resulting in sustained reductions in canopy cover and water use (Pattison et al 2011) as well as short-term increases in ecosystem nutrient cycling through leaf litter decomposition (Uselman et al 2011). However, D. carinulata failed to establish in California, apparently owing to the combination of intense predation by ants (Herrera 2003) and diapause characteristics incompatible with environmental conditions at the release sites (Bean et al 2007, however see Dalin et al (2010) for indications that adaptations are now allowing reproduction at more southern latitudes).…”

Effects of habitat extent and forest disturbance on bird communities in lowland nepal

“…Using four randomly-located 20 m x 20 m quadrats, the percentage of tree canopy cover was estimated, the number of trees counted, and their diameters measured within the 20 m x 20 m quadrat. Tree cover was estimated visually (Pattison et al 2011). We divided the quadrat into quarters, and assessment of tree canopy cover was determined by two observers for each quarter.…”