An examination of shade intolerance in Tamarix ramosissima

Background/Question/Methods: The invasive species Tamarix ramosissima or tamarisk pervades riparian habitats throughout the Southwest U.S. Tamarix poses a threat to native plant community structure due to its fast growth rate, high water use, and stress tolerance. Therefore, it is of great importance to find long-term, viable strategies to mitigate its invasion. Here, we test the hypothesis that T. ramosissima is a shade intolerant plant, as understanding the plant’s eco-physiology is helpful in employing an effective conservation strategy. We measured functional traits, morphological traits, and flower number of T. ramosissima within open and canopy habitats to detect physiological responses to shade. Hypotheses about physiological and fitness consequences of varying light levels were tested during the 2015 & 2017 field seasons at three distinct sites along Fountain Creek, CO. We randomly selected 231 plants (2015) and 174 plants (2017) across these 3 sites, which varied only in light intensity (similar soil substrates and soil moisture availability). For all the studied tamarisk trees, we measured the following traits: stomatal conductance, stomatal density, chlorophyll content index, photosystem efficiency/leaf fluorescence, leaf temperature, and flower number (estimated).

Results/Conclusions: The data indicate significant differences in photosystem efficiency, chlorophyll content, stomatal density, stomatal aperture, and flower number between open and canopy plants. 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 plants in the canopy habitat exhibiting the highest photosystem efficiency and producing more chlorophyll than in open conditions. Plants under a full canopy did seem to have significantly lower gas exchange rates and decreased stomatal density, though. Therefore, while Tamarix may have compensatory mechanisms to deal with decreased light, plants in full shade were less fit than in open areas. Overall, these data indicate shade intolerance in T. ramosissima, potentially suggesting that promoting native plant canopy cover could be an effective conservation strategy.