Temperature thresholds for black mangrove (Avicennia germinans) freeze damage, mortality and recovery in North America: Refining tipping points for range expansion in a warming climate
Near the tropical-temperate transition zone, warming winter temperatures are expected to facilitate the poleward range expansion of freeze-sensitive tropical organisms. In coastal wetlands of eastern and central North America, freeze-sensitive woody plants (mangroves) are expected to expand northward into regions currently dominated by freeze-tolerant herbaceous salt marsh plants. To advance understanding of mangrove range expansion, there is a need to refine temperature thresholds for mangrove freeze damage, mortality and recovery. We integrated data from 38 sites spread across the mangrove range edge in the Gulf of Mexico and Atlantic coasts of North America, including data from a regional collaborative network – the Mangrove Migration Network. In 2018, an extreme freeze event affected 60% of these sites, with minimum temperatures ranging from 0 to −7°C. We used temperature and vegetation data from before and after the freeze to quantify temperature thresholds for leaf damage, mortality and biomass recovery of the black mangrove (Avicennia germinans) – the most freeze-tolerant mangrove species in North America. For A. germinans individuals near their northern range limit, our results indicate that temperature thresholds for leaf damage are close to −4°C, but temperature thresholds for mortality are closer to −7°C. Thresholds are expected to be warmer for more southern A. germinans individuals and for the other two common mangrove species in the region (Laguncularia racemosa and Rhizophora mangle). Regenerative buds allowed A. germinans to resprout and recover quickly from above-ground freeze damage. Hence, biomass recovery levels during the first post-freeze growing season were 90%, 78%, 62% and 45% for temperatures of −4, −5, −6 and −7°C, respectively. Due to a combination of vigorous resprouting and new recruitment from propagules, we expect full recovery at most sites within 1–3 years, assuming no further freeze events. Synthesis. To improve predictions of tropical range expansion in response to climate change, there is a need to better understand tropical species' responses to winter temperature extremes. Collectively, our results refine temperature thresholds for A. germinans freeze damage, mortality and recovery, which can improve predictions of mangrove range expansion and coastal wetland ecological transformations in a warming climate.
