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published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

The effects of deforestation and biomass burning in tropical dry forests (TDF) remain a little studied phenomenon. We quantified total aboveground biomass (TAGB), carbon and nutrient (N,S,Ca,P,K) loss under two separate fire severity scenarios; one early when the fuels were higher in moisture content, one later when the slash fuels were drier and then compared the loss and the regrowth of the sites. The TAGB and nutrients were measured (1993-1995) after the forest was cut, after a forest slash fire, one year after pasture establishment and, two years after the slash fire, biomass was quantified before and after a pasture fire. The treatments were based upon time from slash to burn. The low severity fires (Baja) were burned 65 days and the higher severity fires (Alta) were burned 95 days after the initial slash of TDF on ��� 3.5 ha near the Chamela Biological Research Station on the Ejido San Mateo, Jalisco, Mexico. As a result of the 1993 slash fire, TAGB declined from 118.2 to 43.6 Mg ha����� (62%) in the Baja treatment and from 134.9 to 26.8 Mg ha����� (80%) in the Alta treatment. Nutrients pools declined 57-88% with ��� 10% higher combustion of the Alta pools. In 1995, after the pasture fires, TAGB declined from 40.3 to 14.8 Mg ha����� (63%) and from 29.0 to 7.6 Mg ha����� (75%) in the Alta treatment and nutrient pools declined 57-88%. Total aboveground biomass loss from 1993-1995 was 103.4 Mg ha����� (87%) in the Baja treatment and 127.3 Mg ha����� (94%) in the Alta treatment. Carbon and nutrient losses ranged from 87-96% over the three-year study. We found little ash retention after fire, no increase in nutrient soil concentrations and, highly volatized nutrients (i.e. Ca and P) were essentially lost due to wind and water erosion on 40 to 60% slopes. Wood decomposition between fires reduced TAGB and nutrient pools by 15% in the Baja treatment and 3% in the Alta treatment. / Graduation date: 2000

Soil properties and their spatial variability was measured to provide a strong database to assess the modification in soil properties associated with future changes in land use. Surface (0--10 cm) soil samples were collected from a 9 ha, 46-year-old pasture being converted to a native tree plantation and a neighboring control pasture near Sardinilla, Panama. A small-scale nested grid of surface soil samples was replicated in the future plantation and a primary forest in the region to evaluate the spatial variability of soil properties. Seven 1 m profiles were sampled in the future plantation and litter samples were collected at the forest and the future plantation. / Assuming the three sites were identical before the conversion to pasture, the difference in surface soil organic carbon (SOC) was 0.75 kg m-2 or equivalent to a loss of 0.017 kg m-2 yr -1 since the original land-use change. The control pasture is higher in bulk density but lower in percent SOC than the future plantation, which is critical to future comparisons. / The pasture soils showed signs of soil compaction and of the homogenization of soil properties. For the 7 profiles in the future plantation, total SOC mass ranged from 13.45 to 23.80 kg m-2, and stable isotopes revealed that 82% of the SOC in the top 10 cm is derived from the pasture vegetation, down to 23% at 1 m depth. / Spatially, the full scale of spatial variability was not determined from the nested grids alone. In addition, the 15 x 15 m grid in the future plantation generally over-estimated the semivariance at the scales below 3 m. The nested grids assessed the minimum measurable semivariance below 5 m. / The precision of surface estimations from point observations can be improved by adding a 5 x 5 m grid to any large-scale sampling scheme.

Although deforestation remains widespread in the tropics, many places are now experiencing significant forest recovery, offering an optimistic outlook for natural ecosystem recovery and carbon sequestration. Natural forest regeneration is a key component of global ecosystem restoration scenarios. Regenerated forests, however, may not persist so a more nuanced understanding of the drivers of forest persistence in the tropics is critical to ensure the success reforestation efforts and carbon sequestration targets. Furthermore, the maintenance of native forests, including young second-growth ones, is essential for the continuous provision of a myriad of ecosystem services that we, as a society, rely on. More specifically, native forests play a crucial role in watershed protection and forest cover loss via changes in land use can lead to deterioration of water quality. Ensuring a sufficient and adequate supply of water for humans and ecosystems is a pressing environmental challenge and land use decisions can severely degrade stream water quality and compromise water supply. This dissertation focusses on two pressing current issues, the dynamics of tropical forest regeneration and the effects of land use on water resources. First, I use a long-term series of detailed land cover data to study forest cover trajectories and persistence of regenerated forest in the Brazilian Atlantic Forest (AF), a restoration hotspot. Secondly, I use 20 years of stream water quality data combined with land cover information to investigate the effects of land cover composition on water resources in the state of São Paulo, Brazil. In Chapter 1, I investigate forest cover trajectories in 3,014 municipalities and quantify the carbon sequestration potential of forest regeneration in the AF. I found that deforestation reversals were the prevalent trend in the region (38%) but concomittant reforestation reversals (13%) suggest that these short-term increases in native forest cover do not necessarily translate into persistent trends, which limited carbon sequestration from reforestation to less than one third of its potential. In Chapter 2, I quantify forest regeneration in the AF and study its persistence. I mapped over 4.47 Mha of native forest regeneration in the region between 1985 and 2019, of which, two thirds persisted until 2019 (3.1 Mha). The relatively low persistence of second-growth forests suggests a rapid turnover of regrowing forests under certain conditions. In Chapter 3, I combine stream water quality data with detailed and land cover information to investigate the effects of landscape composition on the quality of water resources in the state of São Paulo, Brazil. I found that human dominated watersheds had lower overall water quality when compared to conserved ones, with urban cover showing the most detrimental impacts on water quality, while forest cover was associated with a better overall water quality across the studied watersheds. Finally, in Chapter 4, I examine temporal changes in water quality and their association with land use and sewage treatment also in the state of São Paulo. I show that a large proportion of stream water samples failed to meet legal thresholds for at least one water quality metric and that urbanization and agricultural activity led to deterioration of water quality over time, while sewage treatment infrastructure was an important factor in improving water quality. Overall, my dissertation underscores the importance of developing policies that promote second-growth forest persistence to ensure the success of future restoration efforts. It also highlights the need to need to plan and manage landscapes to improve water quality and reduce the growing costs of water treatment, including restoring native forest cover, which is a cost-effective intervention to sustain adequate water quality.

Forests are crucial for mitigating the effects of climate change and providing ecosystem services. Global deforestation trends coupled with changes in climate and disturbance regimes threaten the ability of forests to sequester carbon, provide timber, and regulate other ecosystem processes. My dissertation draws from principles in ecology, hydrology, and geography to identify the effects of forest disturbance from extreme events (i.e., hurricanes and droughts) on carbon sequestration and streamflow in Puerto Rico. In Chapter 1, I utilized hydroclimatic time series, geospatial analysis, and statistical techniques to evaluate the relationships between forest cover and rainfall variability on streamflow regimes. I determined that (i) watersheds with high forest cover had consistently greater streamflow than deforested ones and (ii) during dry antecedent rainfall conditions, highly forested watersheds had higher streamflow than deforested ones, suggesting that increased hillslope infiltration and storage afforded by well-developed forest soils outweigh higher vegetation water demand in forests compared to other land cover types. In Chapter 2, I combined field data from permanent forest plots with geospatial attributes, satellite imagery, and lidar point clouds to estimate forest damage extent and a loss in carbon sequestration across Puerto Rican forests after Hurricane María (2017). I also developed a random forest model to identify the biophysical and climatic drivers of spatial variation in forest damage from the hurricane. I found (i) a strong relationship between field-based estimates of aboveground biomass (AGB) loss after Hurricane María and the remotely sensed estimate of damage, (ii) 23% (10.44 ±2.33 Tg) of pre-hurricane forest AGB was lost, and (iii) storm-related rainfall was a strong predictor of forest damage, in addition to canopy height, maximum wind speeds, and soil moisture and water storage variables. Finally, in Chapter 3, I quantified changes to streamflow in Puerto Rico in the months following Hurricane María and tested whether estimates of storm-related forest damage account for these differences. I found that (i) “fast” (i.e. quickflow) and “slow” (i.e., baseflow) streamflow components reacted differently to Hurricane María, (ii) magnitude of change in quickflow was positively correlated to the magnitude of forest damage from the storm, while changes to baseflow were unrelated to forest cover or damage, and (iii) under the same level of relative damage, watersheds with low forest cover had greater quickflow increases than deforested ones, suggesting that interactions between forest cover and forest damage modulate the effects of disturbance on quickflow in highly forested watersheds and exacerbate effects of disturbance in less forested watersheds. Overall, the results from my dissertation inform management practices by quantifying vulnerability to changing climate and disturbance regimes, and highlight the importance of forests for regulating water provision and other ecosystem services.

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Economic and ecological processes interact with one another over both spatial and temporal dimensions.This dissertation explores four socio-ecological systems where space crucially matters for both economic and ecological outcomes. In the first chapter, a windborne chemical dictates the diffusion in space of a new agricultural technology. The second chapter dissects the notion of landscape complexity to find which of its components matter for the intensity of insect pressure in agriculture, and thus the use of insecticides. In the third chapter, the location of participants in an environmental program seeking to curb deforestation points to additionality problems and anticipates the lack of measurable effects of the program. Knowing where crops are grown and temperatures less well-suited for their thriving is key to identifying in chapter four the effects of weather fluctuations on asylum applications into the European Union. The spatial dimension tends to be hard to apprehend and overlooked, but those four pieces together stress that space matters in the study of sustainable development.