Frozen green leaves as potential nutrient subsidies in North American mangrove ecosystems

Abstract

Abstract Avicennia germinans, the black mangrove, has shifted its range limit northward, and increased in abundance, in the southeastern United States. In January 2018, a three-day freeze event caused substantial defoliation of green leaves in A. germinans near its northernmost range limit in St. Augustine, Florida. During their recovery, plants that lost their leaves grew similarly to plants fertilized with nitrogen, leading to the hypothesis that freeze-killed green leaves may have acted as a fertilizer. To assess the value of frozen-green litter as a nutrient subsidy, we performed an experiment in which A. germinans seedlings were grown in sand with green, frozen-green, senescent, and control leaf litter. We measured growth response in seedlings using the following criteria: total plant height, internode elongation, and plant biomass. All litter treatments stimulated growth in seedlings to a greater extent than that of controls. Seedlings treated with green or frozen-green leaves grew taller and had longer internodes, than those treated with senescent leaves. Biomass was greater in seedlings treated with green or frozen-green litter, than in the control treatments. Frozen and green leaves lost more mass in a leaching experiment than senescent leaves or controls. These results support the hypothesis that green leaves that freeze can act as a nutrient source within the ecosystem.