Mangrove growth response to experimental warming is greatest near the range limit in northeast Florida

Abstract

Shrubs are invading into grasslands around the world, but we don’t yet know how these shrubs will fare in a warmer future. In ecotonal coastal wetland ecosystems, woody mangroves are encroaching into herbaceous salt marshes owing to changes in temperature, precipitation, and sediment dynamics. Increasing mangrove biomass in wetlands often increases carbon storage, which is high in these productive ecosystems, but little is known about how mangrove growth will change in response to warming. To address this knowledge gap, we deployed warming experiments at three coastal wetland sites along a latitudinal gradient in northeast Florida where Avicennia germinans, black mangroves, are encroaching into salt marshes. We achieved air temperature warming (+1.6°C during the day) at all three sites and measured stem elongation, canopy height and area changes, and leaf and node number. After 2 yr of warming, we found that mangrove growth rate in height increased due to warming. Warming increased stem elongation by 130% over unwarmed control plots after 1 yr at the northern site. Mangrove growth in canopy area did not respond to warming. Site differences in growth rate were pronounced, and mangrove growth in both height and area were lowest at the northern site, despite greater impacts of warming at that site. We also found that area‐based relative growth rate was five times higher across all treatments than height‐based relative growth rate, indicating that mangroves are growing wider rather than taller in these ecotonal environments. Our findings indicate that the growth effect of experimental warming depends on site characteristics and growth parameter measured. We also propose that differential mangrove growth across the three sites may be driven by biotic factors such as the identity of the salt marsh species into which mangroves are encroaching. Our results suggest that, as seen in other ecosystems, wetland plants may respond most strongly to warming at their poleward range edge.

Temperature acclimation of leaf respiration differs between marsh and mangrove vegetation in a coastal wetland ecotone

“…In our experiment, the southern site (SS) was near an inlet and thus closer to the open ocean while the northern site (NS) was further from the ocean and closer to freshwater inputs. As a result, salinity tends to be higher at SS than at NS (Chapman et al, 2021). Previous work at SS has also indicated that salinity can be very high during summer (up to 60 ppt) and lower during winter (48 ppt) (Dangremond et al, 2020).…”

“…The south site (SS) was located roughly 20 km south of St. Augustine (34 km to the south of NS) and just north of the Matanzas Inlet (29°43'38.3"N, 81°14'25.0"W). NS and SS have similar elevations, and average annual precipitation at the GTMNERR reserve is 1317 mm (Chapman et al, 2021). Mean annual temperature (2001–2018) at St. Augustine is 20.8°C.…”

“…On the Atlantic coast, its distribution stretches from North Florida to southern Brazil. All mangroves selected for this study ranged from 4 to 11 years old in 2018 with an average age of 7.4 and 7.2 years at SS and NS, respectively (Chapman et al, 2021).…”

“…The study included six replicates of four treatment plots at each site. The four treatments were: Avicennia ambient, Avicennia warmed, Spartina ambient, and Spartina warmed (Chapman et al, 2021). Treatment plots were randomly positioned within a 1‐ha area at each site.…”

Salt Marsh Plant Community Structure Influences Success of Avicennia germinans During Poleward Encroachment

“…Site characteristics of Guana Tolomato Matanzas ResearchReserve along the northeastern coast of Florida. Shown with permission fromChapman et al 2021 …”