Riparian ecosystems of the southwestern United States have been invaded by the weedy species, Tamarix ramosissima, which outcompetes native plains cottonwood (Populus deltoides) and sandbar willow (Salix exigua). Various management methods have been used in attempts to eradicate Tamarix populations, including biological control agents, herbicides, and mechanical removal, none of which are fully effective strategies. Based on observations of Tamarix fitness and abundance in Colorado, we hypothesized that Tamarix spp. may be shade-intolerant, and that functional traits of Tamarix may vary depending on their degree of exposure to sunlight (open, canopy-edge, and full canopy). Physiological consequences of shading might have implications for how riparian corridors are managed in parts of the Southwest U.S.
Results/Conclusions
Hypotheses about physiological and fitness consequences of varying light levels were tested during the 2015 field season at three distinct sites along Fountain Creek, CO. We randomly selected 150 plants across these 3 sites, which varied only in light intensity (similar soil substrates and soil moisture availability). For all 150 tamarisk trees, we measured the following traits: leaf gas exchange with a porometer, chlorophyll content index, photosystem efficiency/leaf fluorescence, leaf temperature, and flower number (estimated). The number of flowering branches (a proxy for fitness) varied significantly between exposure categories, with the most flowers found on plants in the open, and the least under a canopy. Photosynthetic efficiency also varied significantly between exposure categories, with the plants in the canopy exhibiting the highest efficiency and producing more chlorophyll than in open conditions. Plants under a full canopy did seem to have significantly lower gas exchange rates, though. Therefore, while Tamarix may have compensatory mechanisms to deal with decreased light, plants in full shade were less fit than in open areas.