Browsing by Subject "Forest"

Rising temperatures with increased drought pose three challenges for management of future biodiversity. First, are the species expected to be vulnerable concentrated in specific regions and habitats? Second, are the impacts of drought and warming varying across regions? Third, could recent advances in remote sensing techniques help us in monitoring the impacts in real-time? This dissertation is an effort to address the above questions in the three chapters.

First, I used foliar chemistry as a proxy for drought vulnerability. I used soil and moisture gradients to quantify habitat variation that could be critical for alleviating drought. I used a large dataset of forest plots covering the eastern united states to understand how community weighted mean foliar nitrogen and phosphorus vary across climate and soil gradients. I exploited trends in these variables between species, traits, and habitats to evaluate sensitivity. Critical to our approach is the capacity to jointly model trait responses. Our data showed that nutrient demanding species strongly respond to environmental gradients. I identified a wide range of sites across low to high latitudes threatened by drought. The sensitivity of species to high temperatures is largely explained by soil variations. Drought vulnerability of nutrient and moisture demanding species could be amplified depending on local soil and moisture gradients. Although local soil moisture may dampen drought-induced stress for species with large leaves and high water use, nutrient demanding species remain vulnerable in wet regions during droughts. Phosphorus demanding species adapted to dry sites are drought resilient compared to communities in wet sites. This research is consistent with the studies that supports declining nutrient demanding species with increasing temperature and decreasing moisture. I also detected strong soil effects on shaping community weighted traits across a large geographical and environmental range. Our data showed that soil effects on controlling foliar traits strongly vary across different climates. The findings are critical for conservations and maintaining the biodiversity.

Next, I used space-borne remotely sensed vegetation indices to monitor the process of leaf development across climate gradients and ecoregions in the southeastern United States. A hierarchical state-space Bayesian model was developed to quantify how air temperature, drought severity, and canopy thermal stress contribute to changes in leaf opening from mountainous to coastal regions. I synthesized daily field climate data with daily remotely sensed vegetation indices and canopy surface temperature during spring green-up season. The study was focused on observation of leaf phenology at 59 sites in the southeast United States between 2001 to 2012. Our results suggest strong interaction effects between ecosystem properties and climate variables across ecoregions. The findings showed that despite the much faster spring green-up in the mountains, coastal forests express a larger sensitivity to inter-annual anomaly in temperature than mountain sites. In spite of the decreasing trend in sensitivity to warming with temperature in all regions, there is an ecosystem interaction: Deciduous-dominated forests are less sensitive to warming than are those with few deciduous trees, possibly due to the presence of developed leaves in evergreen species throughout the season. The findings revealed mountainous forests are more susceptible to intensifying drought and moisture deficit, while coastal areas are relatively resilient. I found that increasing canopy thermal stress, defined as canopy-air temperature difference, slows the leaf-development following a dry year, accelerates it after a wet year.

Finally, I demonstrate how space-borne canopy “thermal stress”, i.e. surface-air temperature difference, could be used as a surrogate for drought-induced stress to estimate forest transpiration. Using physics-based relationships that accommodates uncertainties, I showed how changes in canopy water flux may be reflected in surface energy balance and in remotely-sensed thermal stress. Validating with field measurements of canopy transpiration in the southeastern US, I quantified sensitivity of transpiration to thermal stress in a range of atmospheric and climate conditions. I found that a 1 mm change in daily transpiration may cause 3 to 4 °C of thermal stress, depending on site conditions. The cooling effect is large when solar radiation is high or wind speed is low. The effect has the highest control on water-use during warm and dry seasons, when monitoring drought is essential. I applied our model to available satellite and metrological data to detect patterns of drought. Using only air and surface temperatures, I predicted anomaly in water-use across the contiguous United States over the past 15 years, and then compared with anomaly in soil water content and conventional drought indices. Our simple model showed a reliable accuracy in compare to the state-of-the-art general circulation models. The technique can be used in varying time-scales to monitor surface water-use and drought in large scales.

An increasing number of experimental studies attempt to maximize biomass production of trees in plantations by removing nutrient and water limitations. The results from these studies begin to inform operational managers. We investigated a Populus trichocarpa Torr. x P. deltoides Bartr. & Marsh plantation with a combined irrigation and nutrient supply system designed to optimize biomass production. Sap flux density was measured continuously over four of the six growing season months, supplemented with periodic measurements of leaf gas exchange and water potential. Measurements of tree diameter and height were used to estimate leaf area and biomass production using allometric relations. Sap flux was converted to canopy conductance, and analyzed based on an empirical model to isolate the effects of water limitation. Actual and soil water-unlimited potential CO2 uptakes were estimated using a Canopy Conductance Constrained Carbon Assimilation (4C-A) scheme, which couples actual or potential canopy conductance with vertical gradients of light distribution, leaf-level conductance, maximum Rubisco capacity (Vcmax) and maximum electron transport (Jmax). Net primary production (NPP) was ~0.43 of gross primary production (GPP); when estimated for individual trees, this ratio was independent of tree size. Based on the same ratio, we found that current irrigation reduced growth by ~18 % compare to growth with no water limitation. To achieve this maximum growth, however, would require 70% more water for transpiration, and would reduce water use efficiency by 27 %, from 1.57 to 1.15 g stem wood C kg-1 water. Given the economic and social values of water, plantation managers appear to have optimized water use.

Forests change with changes in their environment based on the physiological responses of individual trees. These short-term reactions have cumulative impacts on long-term demographic performance. For a tree in a forest community, success depends on biomass growth to capture above- and belowground resources and reproductive output to establish future generations. Here we examine aspects of how forests respond to changes in moisture and light availability and how these responses are related to tree demography and physiology.

First we address the long-term pattern of tree decline before death and its connection with drought. Increasing drought stress and chronic morbidity could have pervasive impacts on forest composition in many regions. We use long-term, whole-stand inventory data from southeastern U.S. forests to show that trees exposed to drought experience multiyear declines in growth prior to mortality. Following a severe, multiyear drought, 72% of trees that did not recover their pre-drought growth rates died within 10 years. This pattern was mediated by local moisture availability. As an index of morbidity prior to death, we calculated the difference in cumulative growth after drought relative to surviving conspecifics. The strength of drought-induced morbidity varied among species and was correlated with species drought tolerance.

Next, we investigate differences among tree species in reproductive output relative to biomass growth with changes in light availability. Previous studies reach conflicting conclusions about the constraints on reproductive allocation relative to growth and how they vary through time, across species, and between environments. We test the hypothesis that canopy exposure to light, a critical resource, limits reproductive allocation by comparing long-term relationships between reproduction and growth for trees from 21 species in forests throughout the southeastern U.S. We found that species had divergent responses to light availability, with shade-intolerant species experiencing an alleviation of trade-offs between growth and reproduction at high light. Shade-tolerant species showed no changes in reproductive output across light environments.

Given that the above patterns depend on the maintenance of transpiration, we next developed an approach for predicting whole-tree water use from sap flux observations. Accurately scaling these observations to tree- or stand-levels requires accounting for variation in sap flux between wood types and with depth into the tree. We compared different models with sap flux data to test the hypotheses that radial sap flux profiles differ by wood type and tree size. We show that radial variation in sap flux is dependent on wood type but independent of tree size for a range of temperate trees. The best-fitting model predicted out-of-sample sap flux observations and independent estimates of sapwood area with small errors, suggesting robustness in new settings. We outline a method for predicting whole-tree water use with this model and include computer code for simple implementation in other studies.

Finally, we estimated tree water balances during drought with a statistical time-series analysis. Moisture limitation in forest stands comes predominantly from water use by the trees themselves, a drought-stand feedback. We show that drought impacts on tree fitness and forest composition can be predicted by tracking the moisture reservoir available to each tree in a mass balance. We apply this model to multiple seasonal droughts in a temperate forest with measurements of tree water use to demonstrate how species and size differences modulate moisture availability across landscapes. As trees deplete their soil moisture reservoir during droughts, a transpiration deficit develops, leading to reduced biomass growth and reproductive output.

This dissertation draws connections between the physiological condition of individual trees and their behavior in crowded, diverse, and continually-changing forest stands. The analyses take advantage of growing data sets on both the physiology and demography of trees as well as novel statistical techniques that allow us to link these observations to realistic quantitative models. The results can be used to scale up tree measurements to entire stands and address questions about the future composition of forests and the land’s balance of water and carbon.

“Spaces of Order” argues that the African novel should be studied as a revolutionary form characterized by aesthetic innovations that are not comprehensible in terms of the novel’s European archive of forms. It does this by mapping an African spatial order that undermines the spatial problematic at the formal and ideological core of the novel—the split between a private, subjective interior, and an abstract, impersonal outside. The project opens with an examination of spatial fragmentation as figured in the “endless forest” of Amos Tutuola’s The Palmwine Drinkard (1952). The second chapter studies Chinua Achebe’s Things Fall Apart (1958) as a fictional world built around a peculiar category of space, the “evil forest,” which constitutes an African principle of order and modality of power. Chapter three returns to Tutuola via Ben Okri’s The Famished Road (1991) and shows how the dispersal of fragmentary spaces of exclusion and terror within the colonial African city helps us conceive of political imaginaries outside the nation and other forms of liberal political communities. The fourth chapter shows Nnedi Okorafor—in her 2014 science-fiction novel Lagoon—rewriting Things Fall Apart as an alien-encounter narrative in which Africa is center-stage of a planetary, multi-species drama. Spaces of Order is a study of the African novel as a new logic of world making altogether.

There is no consensus on climatic warming's effect on phenology, photosynthesis, and leaf nitrogen content in temperate deciduous tree species. A major question is whether or how these trees will utilize a longer growing season. Data on leaf photosynthetic rates, leaf nitrogen content, and leaf phenological status were collected weekly or biweekly in 2012 for Acer rubrum, Liquidambar styraciflua, Liriodendron tulipifera, Quercus alba, Quercus rubra, and Fraxinus americana at Duke Forest in Orange County, North Carolina. Seedlings were grown in open-topped chambers established in 2009 that were maintained at either ambient or 5 degrees Celsius above ambient temperatures. Half the chambers were shaded, half were under gap conditions.

Four of six species had advanced spring phenology with warming treatment, and four of six species delayed leaf senescence with warming treatment. Shade delayed spring phenology later more than earlier in the season, and gap conditions delayed fall phenology. Warming had an inconsistent effect on photosynthetic rate. Two species increased photosynthetic rate with chambered +5C conditions, another decreased photosynthetic rate in unchambered plots. Half of the species studied had no significant correlation between temperature and photosynthetic rate. The timing of photosynthetic decline was simultaneous within species across warming treatments, even in species which delayed visible senescence with warming.

There were major differences between years in terms of the effect of warming on nitrogen content and resorption. In 2011, percent leaf nitrogen was lower in warmed chambers, and nitrogen resorption efficiency was not correlated with warming treatment. In 2012, warmed chambers had higher nitrogen concentration and a wider range of nitrogen resorption values than ambient, and resorption efficiency was positively correlated with warming in 2012. It is possible that these differences between years are driven by differences in availability of soil moisture. 2011 was much drier during the senescent period than 2012. The direction and magnitude of warming on nitrogen was consistent within years, but varied by year.

Warming often causes visible change in the timing of phenology, but the timing of photosynthetic decline was unaffected and changes in nitrogen content and resorption were interannually variable. The benefits of delaying visible senescence are unclear. The effect of interactions between warming and soil moisture on leaf processes must be explored further. It is possible that the delay in visible phenology without a concurrent delay in leaf decline is due to a mismatch in environmental cues under a warmer climate. Because of the lack of obvious benefit to plant carbon uptake with warming, it is possible that there is no true extension of the growing season in fall.

Deforestation and forest degradation in developing countries are leading causes of environmental problems such as soil erosion, biodiversity loss, and climate change. As a result, policies aimed at slowing down or reversing the trend of deforestation and forest degradation have attracted considerable attention. This dissertation consists of three essays on evaluating forest conservation programs in developing countries. Although the focus of each essay differs, they all use rigorous econometric methods to provide insights on impacts of historical forest conservation programs and assist stakeholders in modifying existing policies and making future ones more efficient and effective.

I begin by assessing the nutritional impact of payments for ecosystem services (PES) in the context of rural China (i.e., Chapter 2). PES is a special type of conditional cash transfer (CCT) in which the conditionality is explicitly attached with conservation practices. In this chapter I develop a stylized household-farm model to show that when households participate in a land-diversion PES program, they would settle for lower levels of food consumption if they lack market access. Exploiting panel data from the China Health and Nutrition Survey (CHNS), I use a triple difference (TD) model to examine the impact of China’s Sloping Land Conversion Program (SLCP), one of the largest PES programs in the world, on the nutrient intake of farming households. My findings suggest that the SLCP had a significant negative impact, though small, on calorie intake and this effect was likely driven by missing market in areas that implemented the SLCP. This essay demonstrates that land-diversion PES, which is a dual conservation and development tool, could affect food consumption and nutrition in ways very different from other conservation programs such as protected areas (PAs) as well as regular CCT programs that only aim for poverty reduction.

I then shift the focus from PES to PAs by implementing innovative evaluation methods to assess the effectiveness of mangrove protection in Southeast and South Asia (i.e., Chapter 3 and Chapter 4). Economists typically estimate the average treatment effect on the treated (ATT) when evaluating government programs. The welfare interpretation of the ATT can be ambiguous when program outcomes are measured in purely physical terms, as they often are in evaluations of environmental programs (e.g., avoided deforestation). In Chapter 3, my co-authors and I present an approach for inferring welfare impacts from physical outcomes when the ATT is estimated using propensity-score matching. We employ the discrete-choice Roy model of selection into treatment to show that the ex post net social value of a forest conservation program can be proxied as a weighted ATT, with the weights being utility measures derived from the propensity of being treated. We apply this new metric to mangrove forest conservation in Thailand during 1987–2000. Wefind that the Thai government’s conservation program protected approximately 30% of the social welfare that would have been lost if all the protected mangrove area had been deforested. This magnitude is very similar to the magnitude of a conventional ATT that measures avoided deforestation, but we show that the potential range of the welfare-based ATT extends from barely a quarter of the conventional physical measure to nearly twice as large as it.

While Chapter 3 adopts an indirect approach to infer the welfare impact of PAs, Chapter 4 exploits the same idea in a direct approach. In Chapter 4, my co-authors and I exploit rich data on carbon stock and land values in India toestimate and predict spatial heterogeneity in the benefit (i.e., carbon sequestration) and cost (i.e., forgone land value) of mangrove conservation. We combine this information with satellite-based data on India’s mangrove coverage in 1990– 2010 to construct a net land value, and then estimate the causal impact of PAs on the net land value. This new approach allows us to account for spatial heterogeneity in the net economic benefit of conservation. Our results show that incorporating the economics of conservation into evaluation could detect impact of PAs that would not be detected under the conventional approach that focuses only on avoided deforestation. Estimates from our heterogeneity treatment effect model suggest that the level and direction of PA’s impact is associated with the road proximity of mangrove sites and differs between the short run and the long run.

The three essays in my dissertation examine the heterogeneity in effects of forest conservation programs in one way or another. They highlight that the efficiency and effectiveness of conservation programs depend on local contexts. When designing and implementing future conservation programs, policymakers should assess local contexts and adjust program features accordingly.