Effects of cold water and aridity on Baja California mangrove survival and ecophysiological traits

Abstract

Abstract Determining the factors that set species' range limits is of critical importance in an era of rapid climate and biotic change. Mangroves dominate global tropical coastlines, but their subtropical range limits are thought to be mainly constrained by cold air. However, several tropical mangrove range limits are conspicuously warm, arid, and dominated by cold water countercurrents. Despite speculation that the combination of cold water and aridity controls these range limits, investigations of this phenomenon are rare. Here, we conducted an experimental study examining the separate and interactive effects of cold water and aridity on survival, growth, and ecophysiological traits of three mangrove species from the arid, countercurrent range limit in Baja California, Mexico. It is one of the few range edges worldwide where red and white mangroves (Rhizophora mangle and Laguncularia racemosa, respectively), not the putatively stress‐tolerant black mangroves (Avicennia germinans), are the most poleward distributed mangroves, providing further opportunity to examine how mangrove ecophysiological traits affect species‐specific range limits. Black mangroves were surprisingly intolerant of cold water regardless of aridity, with no seedlings surviving in the lowest water temperature (13°C) after 8 months. In contrast, both white and red mangrove seedlings were able to survive at 13°C when humidity was 65% (but not 40%). In cold water and arid conditions, leaf stomatal conductance was consistently lowest for white mangroves, intermediate for red mangroves, and highest for black mangroves, suggesting that white mangroves were the most resistant to transpiration water loss in the arid conditions typical of Baja. Similarly, when exposed to drought after the eight‐month experiment, white mangroves grown in cold water survived longer than red and black mangroves. Synthesis: Ultimately, our results suggest that the species‐specific range limits of mangroves in Baja California reflects their species‐specific adaptations (or lack thereof) to cold water and aridity, with white and red mangroves surprisingly outperforming black mangroves. Further, the difference in our results compared to the same species in other ranges underscores the need to incorporate species and range‐specific traits when conducting distribution modelling to predict the effects of climate change on organism range limits.