COS 204-5 - Disentangling drought tolerance and avoidance, two key yet non-mutually exclusive ecological drought strategies: implications for drought resilience

Abstract: Drought tolerance and avoidance are often described as contrasting extremes on an ecological drought strategy continuum, with trait trade-offs implying species cannot excel at both. However, distinguishing among these two strategies in field contexts is challenging because individual rooting depth and zone of accessible soil moisture are difficult to measure directly. Here we circumvent some of these problems by using seedlings as a model system and present an integrated mathematical and experimental framework to distinguish among these strategies, while teasing apart the role of initial soil moisture. We subjected seedlings of five woody semi-arid species of the Tamaulipan biotic province to a point-of-no-return drought experiment and measured gas exchange, soil moisture content, leaf phenology (wilting), plant water status (relative water content and pre-dawn and mid-day water potential), in addition to time to incipient mortality and stem, leaf, and root morphological traits. Our results show that, after controlling for variation in initial soil moisture, drought tolerance explained most of the interspecific variation in drought performance. However, species still exhibited aspects of both tolerance and avoidance, suggesting that these two ecological strategies are not mutually exclusive. Root architecture had surprising relationships with drought strategy: the sensitivity of plant water status to soil drying increased with investment in fine roots, while initial soil water content, a key predictor of time to wilting, decreased with increasing investment in coarse roots. Our integrated framework can inform both field and mesocosm studies using larger individuals to better understand spatiotemporal variation in drought-induced mortality, while also informing strategies for enhancing drought resilience in reforestation.