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Avicennia germinans, the black mangrove, has shifted its range limit northward, and increased in abundance in the Southeastern United States. In January 2018, a 3-day freeze event caused substantial defoliation of green leaves in A. germinans near its northernmost range limit in St. Augustine, FL. During their recovery, plants that lost their leaves grew similarly to plants fertilized with nitrogen, leading to the hypothesis that freeze-killed green leaves (frozen green) 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 frozen green leaves were at least 5 cm taller and had 0.5 cm longer internodes than those treated with senescent leaves. Seedlings grown in the green leaf treatment were 8 cm taller, on average, and internodes were 0.12 cm longer than the senescent treatment. Biomass was 0.6–0.8 g greater in seedlings treated with green or frozen green litter than in the control treatments. We also conducted a leaching experiment to compare mass loss rates. Frozen and green leaves lost more mass in the leaching experiment than senescent leaves or controls. These results support the hypothesis that green litter that freezes can act as a nutrient source within the ecosystem and spur rapid recovery from freeze damage. [ABSTRACT FROM AUTHOR]

Nature-based solutions that use a counterfactual scenario depend heavily on the methodology used to determine the business as usual (BAU) case, i.e., the "baseline." Reducing emissions from deforestation and forest degradation (REDD+) projects traditionally set baselines using a "reference area" as a control for estimating BAU deforestation and emissions in the treatment (project) area. While the REDD+ market is shifting from project-based to nested approaches as countries increase their efforts to meet nationally determined contributions (NDCs) to the Paris agreement's global climate target, methodologies for allocating national baselines are not yet formalized and tested, despite an urgent need to scale the market. We present a novel method for mapping deforestation risk and allocating national forest reference emission levels (FREL) to projects: baseline allocation for assessed risk (BAAR). This approach provides a spatial predictor of future deforestation using a dynamic vector, and a method for allocating a FREL to differentiated risk areas at the project level. Here, we present BAAR using 34 REDD+ projects in the Democratic Republic of the Congo (DRC). We demonstrate the importance of risk-based FREL allocations to balance fitness for purpose and scientific rigor. We show how BAAR can be used by governments to focus voluntary carbon market finance in areas at highest risk of imminent deforestation, while maintaining alignment with nationally determined contribution (NDC) goals. [ABSTRACT FROM AUTHOR]

Low temperature stress is the primary factor determining the latitudinal limits of tropical plants. As the climate warms, tropical species are migrating poleward, displacing native species and modifying ecosystem structure and function. Changes are particularly evident along latitudinal gradients with the highest velocity of change occurring in wetlands. In coastal wetlands, saltmarshes dominate at latitudes above 30°, whereas mangroves occur mostly in the tropics because most species are intolerant of freezing temperatures, but others, like Avicennia germinans (black mangrove), do tolerate freezing temperatures. In response to a warmer climate and fewer killing freezes, mangroves are currently expanding into saltmarshes. However, the speed of the transition from saltmarsh to mangrove can also be modified by extreme events and nutrient subsidies. In a fertilization experiment along the Atlantic coast of North America, we found that nitrogen addition altered plant traits in Avicennia, which increased their resistance to freezing temperatures. This trait shift resulted in negligible freeze effects during a January 2018 extreme freeze event compared to unfertilized plants, which lost more than 80% of the leaves and more than 40% of the wood in their canopies. The freeze-killed litter from unfertilized plants provided a nutrient pulse that influenced recovery, growth and mangrove cover for three years following the freeze. Nutrient enrichment and recovery from the freeze effects led to increased growth and structural complexity of the mangrove canopy, which further enhanced freeze tolerance, shrub growth form and the ability of Avicennia to displace the saltmarsh in the temperate–tropical ecotone. [ABSTRACT FROM AUTHOR]

In boreal species, populations at a southern range edge are at risk of being extirpated due to rising temperatures from climate change and potentially from demographic factors that affect small populations. The ability to track climate through phenological responsiveness can affect survival and reproductive success, with implications for long-term population persistence. This study examines phenological responsiveness and fecundity in populations of an understory plant species, Trientalis borealis, in the Midwestern USA using herbarium specimens and field observations. Date of flowering was well predicted by the mean May temperature. We detected a shift in the mean flowering date for populations north of latitude 45° N of an average of 1.4 days earlier per decade. We did not find shifts in mean flowering date for populations south of 45° N. Trientalis borealis exhibits phenological responsiveness differently in different parts of its range. In populations north of latitude 45° N, T. borealis advances its date of flowering, on average, by approximately 3 days for every 1 °C increase in mean May temperature. In populations south of 45° N, T. borealis advances its flowering date less than 1 day for every 1 °C increase in mean May temperature. Flowering and seed set increased along a latitudinal gradient away from the range edge. If population growth is limited by seed production in this species, this suggests that populations near the southern range limit are not performing well, perhaps due to direct effects of warming, a lack of phenological responsiveness that exacerbates exposure to warming, or a lack of genetic diversity. [ABSTRACT FROM AUTHOR]

Climate change and nutrient enrichment are two phenomena impacting coastal ecosystems. In coastal wetlands, mangroves in temperate–tropical ecotones are encroaching on adjacent saltmarshes, a pattern that is primarily attributed to warmer winter temperatures. Climate change is also expected to increase the vulnerability of coastal wetlands to eutrophication, and increases in nutrient availability may further mediate the rate of mangrove expansion. We investigated the consequences of nutrient enrichment on coastal wetlands in the mangrove–saltmarsh ecotone near the temperate edge of mangrove distribution along the northeast coast of Florida. We tested the hypothesis that nutrient enrichment enhances the ongoing, climate-driven expansion of mangroves into areas historically dominated by saltmarshes by increasing mangrove growth and cover, allowing them to outcompete and overgrow adjacent saltmarsh plants. We manipulated nitrogen (N) and phosphorus (P) availability and measured the effects on growth, cover, diversity, leaf traits, and nutrient dynamics of Avicennia germinans. We found that A. germinans shrubs growing in the saltmarsh–mangrove ecotone in northern Florida grew taller, increased their canopies, and had higher reproductive output when enriched with N compared to control plants and P-enriched plants. Nutrient enrichment did not alter Sarcocornia perennis growth and increased Batis maritima height but did not alter density or biomass. Nitrogen addition caused an increase in A. germinans cover and decrease in B. maritima cover and Simpson's index of diversity, suggesting that N enrichment, an ongoing phenomenon, can hasten the invasion of mangroves into saltmarshes by favoring mangrove growth and reproduction without significantly enhancing saltmarsh plant growth. [ABSTRACT FROM AUTHOR]