Research

Climate change | Fire disturbance | Sea level rise | Vegetation shift | Carbon Cycling

1. Sea level rise & Coastal landscape reorganization

Climate-driven sea-level rise (SLR) has triggered massive landward marsh migration and forest retreat, with potentially large impacts to the delivery of essential ecosystem services to coastal populations. A refined understanding of SLR-driven coastal transgression is imperative to allow informed decisions for sustainable development. However, current information on the spatiotemporal patterns of coastal transgression remains limited. In this ongoing project, I investigate the spectral, temporal, and phenological characteristics of coastal vegetation using the full archive of Landsat images to, (1) develop a robust classification algorithm focusing on the transition forest (the ecotone between upland forest and marsh where tree mortality due to seawater intrusion begins) as means to consistently track the evolving coastal treeline, and (2) develop a spatial analysis approach for automatic and spatially-explicit quantification of forest retreat rate. As SLR continues to accelerate, I expect the results to prompt new scientific understanding of complex ecological processes that better prepare coastal ecosystems and societies for future climate change.

2. interactive impacts of Climate change and sea level rise on Coastal biomass carbon pool

Coastal ecosystems represent a disproportionately large but vulnerable global carbon sink. Sea-level driven wetland degradation and forest mortality threaten coastal carbon pools, but responses of the broader coastal landscape to interacting facets of climate change remain poorly understood. In this study, I used multi-decadal Landsat observations across the mid-Atlantic sea-level rise hotspot to show that climate change has actually increased the amount of carbon stored in the biomass of coastal ecosystems despite substantial aerial loss. We find that sea-level driven reductions in wetland and low-lying forest biomass were largely confined to elevations less than 2 m above sea level, whereas a warmer and wetter climate led to an increase in the biomass of adjacent upland forests. These results point to an intriguing decoupling between upland and wetland carbon trends that can only be understood by integrating observations across traditional ecosystem boundaries.

3. Linkage between Arctic warming and Permafrost degradation

Climate warming is projected to continue to intensify across high latitudes, with profound implications for permafrost thaw. A major uncertainty is how intensified warming will interact with fire disturbance to exacerbate thermokarst (ground-surface collapse resulting from permafrost thaw) and glacial lake drainage. In this project, I integrated time-series remote sensing analysis with machine learning algorithm to show (1) that thermokarst rates increased by 60% with warming climate and wildfire from 1950 to 2015 in Arctic Alaska, and wildfire was disproportionately responsible for 10.5% of the thermokarst by burning merely 3.4% of the tundra landscape; and (2) that recent warming hastens both catastrophic and gradual lake drainage across northern Alaska, and total lake area is predicted to continue to decline across northern Alaska by 15%–21% by 2050. These results combined suggest that climate change and wildfire will synergistically accelerate permafrost degradation with potentially large consequences on high-latitude carbon stocks as the Arctic transitions in this century.

4. Response of Arctic tundra ecosystem to wildfire activation

Fire regime shifts, driven by amplified climate change in the northern high latitudes, pose growing threat to key properties and functions of tundra ecosystems, including soil carbon stocks and tundra vegetation types. However, it remains poorly understood how tundra ecosystems will feedback to the intensified force of climate-driven fire activation. In this project, I used paleoecology, process-based ecosystem modeling and remote sensing observation (1) to address the resilience and sensitivity of tundra carbon stocks to shifting fire regimes, (2) to improve the characterization of wildfire disturbance and its consequences in tundra ecosystems, and (3) to explore the patterns and drivers of shrub expansion in tundra landscape with accelerated warming and intensified burning over past 60 years. Overall, the finding of this project yields new insights into the complex responses of tundra ecosystems to fire-regime changes, which allows an improved understanding of the land-atmosphere feedbacks in high latitudes.

xxxxxxxxxxxxxxxxxxxx

xxxxxxxxxxxxxxx

5. Mangrove forest restoration and saltmarsh invasion

Mangrove forests provide essential ecosystem services to human populations. However, the persistence of these ecosystems is increasingly threatened by global changes and human activities, such as vegetation invasion, sea-level rise, and nutrient enrichment. To improve our understanding of mangrove forest in response to key environmental drivers, I conducted a series of greenhouse and field experiments in coastal wetlands of China, which (1) explored the growth and physiological response of different mangrove forests to increasing sea level, (2) investigated the interactive effects of increasing salinity and nutrient enrichment on mangrove forests during various developmental stages (i.e. gemination, seedling and sapling), and (3) quantified the altered patterns of greenhouse gas emissions (CO₂, CH₄ and N₂O) in native mangrove forests encroached by invasive marsh and exotic mangrove forests. Results of these studies helped guide restoration of native mangrove forests, and facilitated a better understanding of the ecological impacts associated with species invasion in coastal wetlands.