Ecosystem impacts of tamarisk (Tamarix spp.) management in the Arkansas River watershed, Colorado: effects of disturbance and herbicide residues on passive plant community restoration

Abstract

2013 Spring.Includes bibliographical references.Tamarisk (Tamarix L.) is now one of the most common species of woody plants along waterways in arid and semi-arid areas of the western United States. Tamarisk was intentionally introduced over a century ago for ornamental purposes and erosion control projects, but its expansion since has been influenced by altered hydrologic regimes and global climate change. Approximately sixty years ago the species started to be perceived by federal scientists as noxious and was targeted for control. As the first chapter in this dissertation outlines, management of tamarisk has occurred by many methods, but primarily combinations of herbicides and mechanical tree removal. Successive chapters detail laboratory, greenhouse and field experiments that determined the ecological impacts of currently used tamarisk control strategies, with a particular emphasis on the effects of herbicide residues on plant community restoration patterns following management. First, an in vitro study and high-performance liquid chromatography (HPLC) analysis were used to quantify soil degradation rates for imazapyr and triclopyr from six sites in Colorado. A dose response study was then conducted at two of these sites to determine the relative sensitivity of important restoration plant species to the two herbicides. Exponential decay models estimated imazapyr half-lives (t50) for two soils at 51 and 76 days, and triclopyr half-lives (t50) for all soils averaged 7 days. Glycyrrhiza lepidota was the only species to demonstrate sensitivity to triclopyr. Atriplex canescens, Elymus canadensis and Sporobolus airoides were the most sensitive to typical imazapyr residues. Fecundity in S. airoides and Bouteloua curtipendula were also negatively impacted by the highest rate of both triclopyr (3.92 kg ai ha-1) and imazapyr (0.28 kg ai ha-1). Microbially-mediated degradation of triclopyr was estimated to occur 6.5 times more rapidly than imazapyr. Second, at three field sites in southeastern Colorado a study was conducted that used three dimensional artificial trees and repeated soil sampling to determine whether tamarisk tree canopies retained aerially-applied imazapyr, and if this retention affected soil residues and degradation. Tamarisk mortality was also quantified using repeated stand and individual tree measurements. The average tree canopy captured 75% of aerially-released imazapyr, resulting in significantly lower soil residues beneath the tree canopy. Although initial imazapyr soil residue levels outside the tree canopy were almost four time greater than those inside, soil degradation occurred more than twice as rapidly in outside soils and resulted in lower residue levels. Helicopter imazapyr applications resulted in 98% tamarisk mortality within two years, but the consistency of treatment effectiveness was reduced by non-linear stand boundaries and tall site obstructions. The same factors also increased variability in the actual quantity of herbicide applied to sites, increasing the probability of substantial non-target ecosystem impacts. Last, field plots were established at four sites in southeastern Colorado where tamarisk stands were treated with either an aerial imazapyr application or mechanical biomass removal followed by secondary herbicide (imazapyr and triclopyr) or biological control treatments. In the fourth chapter a study conducted at these sites is detailed in which the tamarisk control and cost effectiveness of the different treatments was quantified over a three year period. Whole plant extraction caused 20% higher tamarisk mortality than aerial imazapyr applications or biomass mulching. Of the secondary treatments evaluated, individual plant treatments (IPTs) of imazapyr caused higher mortality than either triclopyr IPTs or releases of tamarisk leaf beetles (Diorhabda carinulata). Aerial imazapyr applications alone were very cost effective, but when the subsequent removal of tree biomass was accounted for, this strategy was less cost effective than primary mechanical treatments followed by biological control releases. In the final chapter a second study carried out at the same sites is described in which the validity of ecologically based integrated pest management (EBIPM) models for tamarisk management are tested by measuring plant community and ecosystem responses to the different tamarisk control strategies. Plant community dynamics in response to the adjacent treatments were evaluated over three years. Helicopter imazapyr applications severely reduced plant community richness, diversity and abundance and appeared to facilitate invasion by resistant populations of Bassia scoparia. Plant communities did not show a strong response to integrated tamarisk management, which in itself was notable because mechanical tree removal caused soil disturbances that in theory would have promoted secondary invasions of existing ruderal species. Ultimately data suggested that plant community re-vegetation patterns following tamarisk removal were more strongly affected by drought and longer term shifts towards community assemblages dominated by upland plant species. These results provide evidence for the need to integrate state and transition models of ecosystem structure and function into the EBIPM framework in order for this tool to be valuable in managing tamarisk and other woody invaders