Forest carbon management in mangroves and monoculture plantations

Restoring forest cover is a prominent strategy for offsetting emissions of greenhouse gases. When done appropriately, restoring forest cover can remove atmospheric carbon dioxide and confer non-carbon benefits such as biodiversity conservation, ecosystem service provisioning, or direct economic returns to local communities. Although a range of pathways exist for restoring forest cover, two prominent avenues have caught the attention of decision-makers and investors across the globe: blue carbon and plantations. The three chapters of my dissertation consequently examine the degree to which mangroves, a prominent blue carbon ecosystem, and monoculture plantations, the dominant approach for restoring forest cover, can meet our climate change goals./nIn the first chapter of my dissertation, I compare and critically assess estimates of carbon stocks in mangroves using i) empirical field data, and ii) outputs of predictive models at global scales. Global maps of carbon stocks are increasingly used to inform environmental management, policy, and decision-making, but may produce biased estimates of mangrove carbon stocks at local scales. My findings suggest that although global maps of mangrove carbon perform well at regional scales, comparison with empirical field data at local scales identifies statistically significant differences in carbon stocks. The results suggest that despite recent advances in computing power, compilation of global datasets, and remote sensing, empirical measurements of mangrove carbon stocks will remain key to informed decision-making at local scales./nMy second chapter examines a critical question for mangrove conservation and restoration policy-making: are the carbon stocks of deforested mangroves recoverable over time scales meaningful for mitigating climate change? Deforestation of mangroves induces massive carbon emissions from the soil organic carbon pool; however, the pace at which these soil organic carbon stocks are recovered following reforestation is unclear. Using high resolution spatial datasets of land cover and land use change over the last 20 years and models of carbon stock loss and gain in mangroves, I use scenario analysis to examine the net balance of carbon stocks associated with mangrove gain and loss within Thailand. My findings indicate that in areas where regenerating mangroves are commonly re-cleared, carbon stock losses may be four-fold greater than what net change in mangrove extent statistics would otherwise suggest./nFinally, my third chapter turns to monoculture plantations, the dominant approach for restoring forest cover globally. Approximately two thirds (66%) of high-level commitments in the tropics for forest restoration involve planting and almost half (45%) involve the establishment of monoculture plantations. Despite the prominence of this approach, the science and policy communities lack systematic understanding of the degree to which monoculture plantations can meet climate change mitigation goals. Thus, my colleagues and I compiled a database of approximately 3,500 empirical estimates of aboveground carbon in monoculture plantations across the globe. I then used this database to examine variation in growth patterns as well as the key drivers that mediate this growth. I used nonlinear growth curves to more accurately model the accumulation of aboveground carbon stocks with time, which I found to vary substantially by genus of tree and plant functional type. Furthermore, I found factors such as genus, endemism, prior land use, and plant traits to strongly mediate carbon accumulation in monoculture plantations, whereas factors such as biome were found to have little effect./nTaken comprehensively, the chapters of my dissertation critically assess the scientific datasets and approaches that are guiding actions and commitments within the reforestation space. The chapters employ data that have been collected through in situ forest inventories, predictive modeling, and government programs, as well as methodological approaches such as statistical modeling and data synthesis. The findings provide key insights such as i) improved understanding of a critical uncertainty (i.e., the recoverability of mangrove carbon stocks) for investors in reforestation of blue carbon ecosystems, as well as ii) improved understanding of carbon accumulation patterns in global monoculture plantations. I anticipate that these findings will facilitate improved environmental outcomes from reforestation efforts, which are rapidly scaling across the globe.

Advisor: Potts, Matthew D