Warming influences growth of salt marsh but not mangrove species near the northern range limit of mangroves
Increasing temperatures and a reduction in the frequency and severity of freezing events has been linked to shifts in species distributions, growth patterns and interactions. Differences in responses to both minimum temperatures and average temperatures may alter species interactions leading to enhancements or reductions in predicted range shifts of plant communities. The distributional ranges of mangrove species are primarily limited by freezing events which limit growth and survival. In recent years, a northern range expansion of mangrove species has been observed, likely due to the diminishing occurrences of fatal freezing events associated with climate change. As these mangrove communities move to higher latitudes they are interacting with salt marsh communities that inhabit the same intertidal area in temperate zones.
In this study we conducted an in situ warming experiment to test the effects of temperature increases (maximum 7°C, mean 2°C) on co-occurring mangrove and salt marsh species. Warming was achieved using passive warming with three treatment levels (ambient, shade control, warmed). The chambers were installed at a mixed salt marsh-mangrove site, located at Merritt Island, Florida, which is a transitional zone for the two communities. Experimental plots consisted of the salt marsh species of Distichlis spicata and Salicornia virginica and the mangrove seedling Avicennia germinans.
Results indicated that mangroves showed little to no response to increase average temperatures whereas salt marsh showed varied responses. It is important to note that no freezing temperatures occurred at the site during the study likely reducing the effect of minimum temperature differences on mangroves. Mangroves, primarily A. germinans, showed poor survival overtime regardless of the warming treatment and no significant differences in growth between treatments except for a small reduction in maximum leaf area with warming. Salt marsh response varied by species. Distichlis spicata, the dominant salt marsh species in the experiment, showed a significant increase in growth both in density and height with warming during summer months. Salicornia virginica showed an opposite response with a reduction of summer growth with warming. In the second year of the experiment, a decline in the growth and survival of both mangrove and salt marsh species was noted. Distichlis spicata appeared to be partially protected from this decline in the warming treatment however A. germinans and S. virginicia showed significant decline regardless of treatment. The next phase of experiments will determine if average temperature increases have a similar effect on adult mangrove trees.