The role of precocious reproduction in the mangrove range expansion
Climate change-driven shifts in species ranges are ongoing and expected to increase. However, biotic effects may interact with climate to influence species ranges, potentially accelerating or slowing range shifts in response to climate change. Tropical mangroves have expanded their ranges poleward in the last three decades, which has previously been explained by a decrease in freezing temperatures. In this study, we examined an additional, biotic, component underlying the poleward expansion of mangroves: precocious reproduction and a shift in propagule traits at the range edge. We used a common garden experiment and field observations to examine age of first reproduction in Rhizophora mangle and Avicennia germinans. We established a common garden in 2012 at the northern range limit of mangroves in St. Augustine, Florida with seedlings from five populations spanning the Atlantic coast of Florida. We also sampled naturally occurring plants in populations along the mangrove-salt marsh ecotone for age of first reproduction. Additionally, we examined dispersal traits in populations approaching the range limit by comparing propagule size among populations.
At the range edge, mangroves have developed precocious reproduction. Precocious reproduction increased with latitude, with only 3% of R. mangle seedlings from the southernmost population at 25.5°N flowering in the common garden, and 38% of R. mangle seedlings from the northernmost population at 29.9°N flowering. Seedling survival in the common garden was not significantly different among source populations (log-rank test, R. mangle: X2= 8.6, df = 4, p= 0.073; A. germinans: X2=5.8, df= 4, p = 0.217), with 45-60% of R. mangle seedlings and 20% of A. germinans seedlings surviving to 2 years. In naturally occurring populations, R. mangle individuals reach reproductive maturity at a younger age in the northernmost populations compared to populations in the middle and south of the ecotone (log-rank test, X2 = 9.5, df = 2, p = 0.008). Seedling size increased with latitude in R. mangle and decreased with latitude for A. germinans. We show that individuals from northern populations reproduce at a younger age than those from southern populations. We also demonstrate a shift in propagule size in populations at the leading range edge. Together these phenomena may accelerate the expansion of mangroves northward.