Non-freezing cold event stresses can cause significant damage to mangrove seedlings: Assessing the role of warming and nitrogen enrichment in a mesocosm study

Abstract

Mangroves are expanding poleward along coastlines globally as a response to rising temperatures and reduced incidence of freezing under climate change. Yet, knowledge of mangrove responses to infrequent cold events in the context of climate warming and regional nitrogen (N)-enriched eutrophication is limited. We conducted a mesocosm experiment in which the seedlings of two mangrove species (A. marina and B. gymnorrhiza) were grown either at ambient temperature or under warming with and without nitrogen (N) loading. During a short winter period, an unusually severe cold event occurred with the lowest temperature of 2 °C in the experimental region. We took this unique opportunity to assess possible response of these mangrove species to the cold stress under various environmental conditions. The cold event caused various degrees of damage to the seedlings of both mangrove species, with the warming treatment seemingly protecting leaves and branches from the cold damage. However, the warming treatment did not buffer mangroves to mortality from the low temperature stress in either species. The cold event resulted in significant decreases in seedling growth rates and net ecosystem CO2 uptake in the post-cold period relative to the pre-cold period, although the cold event did not alter the effects of warming treatment on these parameters of both mangrove species. The cold event differentially altered physiological responses of the two species growing under N loading, with the seedlings of A. marina growing in higher N concentrations having a reduced growth response after the cold event, whereas those of B. gymnorrhiza showed no change in post-cold period versus pre-cold period growth. Our results suggest that cold events may play a pivotal role in regulating mangrove survival and growth even under future warming scenarios. Two mangrove species exhibited differential survival and growth responses to the cold event at different N concentrations, which may have implications for how we can restore and conserve mangroves among the world’s eutrophied sub-tropical estuaries and with future warming.