Geology topics

Aaron Macy

Publications and source records attributed to Aaron Macy.

3 recordsLinked to original sources

USGS2021

Effects of chronic and acute stressors on transplanted black mangrove (Avicennia germinans) seedlings along an eroding Louisiana shoreline

Coastal wetland restoration can be used to offset past wetland losses and/or reduce future losses due to land-use changes, rising sea levels, and accelerating climate change. However, there is a need for information regarding the restoration-relevant performance of foundation species like mangrove and marsh plants, including their responses to acute and chronic stressors that can affect restoration outcomes. Mangrove encroachment and poleward range expansion into marsh, facilitated by warming winters, has provided restoration practitioners in the northern Gulf of Mexico with a new foundation plant species to consider using during restoration. To evaluate the performance of transplanted mangroves and characterize restoration-relevant marsh–mangrove interactions, we planted nursery-raised black mangrove ( Avicennia germinans ) seedlings within different marsh cover treatments along an eroding marsh-dominated shoreline in Louisiana. Mangrove seedling survival increased with greater densities of marsh cover, indicating that marsh grass ( Spartina alterniflora ) may facilitate mangrove establishment. However, only 35% of transplanted mangrove seedlings established after 10 weeks, suggesting a low return on resources expended in raising seedlings for 1–3 years in greenhouse conditions. Moreover, a 2018 freeze event killed 100% of transplanted mangrove seedlings, while nearby naturally established mangroves suffered minor damage. Our results, along with those in the mangrove restoration literature, indicate that planting mangroves in the northern Gulf of Mexico may not be the most efficient use of limited resources. Rather, restoration efforts may benefit from focusing initially on the restoration of abiotic conditions (e.g. elevation and hydrologic regimes), followed by using marsh plants (rather than transplanted mangroves) to jump-start ecosystem development.

Louisiana↗

USGS2020

Changes in ecosystem nitrogen and carbon allocation with black mangrove (Avicennia germinans) encroachment into Spartina alterniflora salt marsh

Increases in temperature are expected to facilitate encroachment of tropical mangrove forests into temperate salt marshes, yet the effects on ecosystem services are understudied. Our work was conducted along a mangrove expansion front in Louisiana (USA), an area where coastal wetlands are in rapid decline due to compounding factors, including reduced sediment supply, rising sea level, and subsidence. Marsh and mangrove ecosystems are each known for their ability to adjust to sea-level rise and support numerous ecosystem services, but there are some differences in the societal benefits they provide. Here, we compare carbon and nitrogen stocks and relate these findings to the expected effects of mangrove encroachment on nitrogen filtration and carbon sequestration in coastal wetlands. We specifically evaluate the implications of black mangrove ( Avicennia germinans ) encroachment into Spartina alterniflora -dominated salt marsh. Our results indicate that black mangrove encroachment will lead to increased aboveground carbon and nitrogen stocks. However, we found no differences in belowground (that is, root and sediment) nitrogen or carbon stocks between marshes and mangroves. Thus, the shift from marsh to mangrove may provide decadal-scale increases in aboveground nitrogen and carbon sequestration, but belowground nitrogen and carbon sequestration (that is, carbon burial) may not be affected. We measured lower pore water nitrogen content beneath growing mangroves, which we postulate may be due to greater nitrogen uptake and storage in mangrove aboveground compartments compared to marshes. However, further studies are needed to better characterize the implications of mangrove encroachment on nitrogen cycling, storage, and export to the coastal ocean.

Louisiana↗

USGS2019

Temperature thresholds for black mangrove (Avicennia germinans) freeze damage, mortality, and recovery in North America: Refining tipping points for range expansion in a warming climate

Near the tropical‐temperate transition zone, warming winter temperatures are expected to facilitate the poleward range expansion of freeze‐sensitive tropical organisms. In coastal wetlands of eastern and central North America, freeze‐sensitive woody plants (mangroves) are expected to expand northward into regions currently dominated by freeze‐tolerant herbaceous salt marsh plants. To advance understanding of mangrove range expansion, there is a need to refine temperature thresholds for mangrove freeze damage, mortality, and recovery. We integrated data from 38 sites spread across the mangrove range edge in the Gulf of Mexico and Atlantic coasts of North America, including data from a regional collaborative network — the Mangrove Migration Network. In 2018, an extreme freeze event affected 60% of these sites, with minimum temperatures ranging from 0 to ‐7°C. We used temperature and vegetation data from before and after the freeze to quantify temperature thresholds for leaf damage, mortality, and biomass recovery of the black mangrove (Avicennia germinans) — the most freeze‐tolerant mangrove species in North America. For A. germinans individuals near their northern range limit, our results indicate that temperature thresholds for leaf damage are close to ‐4°C, but temperature thresholds for mortality are closer to ‐7°C. Thresholds are expected to be warmer for more southern A. germinans individuals and for the other two common mangrove species in the region (Laguncularia racemosa and Rhizophora mangle). Regenerative buds allowed A. germinans to resprout and recover quickly from aboveground freeze damage. Hence, biomass recovery levels during the first post‐freeze growing season were 90, 78, 62, and 45% for temperatures of ‐4, ‐5, ‐6, and ‐7°C, respectively. Due to a combination of vigorous resprouting and new recruitment from propagules, we expect full recovery at most sites within 1‐3 years, assuming no further freeze events. Synthesis To improve predictions of tropical range expansion in response to climate change, there is a need to better understand tropical species’ responses to winter temperature extremes. Collectively, our results refine temperature thresholds for A. germinans freeze damage, mortality, and recovery, which can improve predictions of mangrove range expansion and coastal wetland ecological transformations in a warming climate.

Florida, Mississippi, Louisiana, Texas↗