Ecotype assessment for guiding arid land restoration in the desert southwest of the U.S.
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The need for ecological restoration of disturbed ecosystems around the world is increasing as environmental disturbances occur more frequently, with greater intensity, and spanning larger areas. Introducing genetically appropriate plant materials when local ecotypes are not available is of utmost importance for long-term restoration success. In the course of a multiple common garden experiment to determine which ecotypes perform best along an environmental gradient, I asked how the practice of raising seedlings in the greenhouse before outplanting could affect subsequent performance traits such as survivorship and growth. The focal species was the Mojave Desert shrub Ambrosia dumosa. Nineteen populations (ecotypes) of this species, raised from seeds that were collected across the Mojave Desert bioregion, were grown in the greenhouse for 13 months before planted into three common gardens. Ecotypes from winter-warmer, summer-wetter environments had significantly higher growth rates in the greenhouse and individuals that were larger at the time of transplanting tended to maintain their size advantage over several years. In addition, initial transplant size had a significantly positive effect on survivorship over two years. This means that ecotype differences in performance traits measured post transplanting are in part caused by ecotype responses to the greenhouse environment. I suggest that this potential bias should be accounted for by using initial transplant size as a covariate in the analysis of performance traits.