TURNING UP THE HEAT ON THE LITTLE THINGS THAT RUN THE WORLD: EVALUATING THE IMPACTS OF CLIMATE CHANGE ON ANT BIODIVERSITY IN THE TEMPERATE FOREST COMMUNITIES OF THE NORTHEASTERN UNITED STATES

Climatic change threatens biodiversity worldwide. In the forests of the northeastern United States, climate change is expected to increase mean annual temperatures by up to 4.5˚C and change precipitation seasonality. These changes in climate are likely to have impacts on the biodiversity of the region. In order to better understand the impacts of climate change on biodiversity, I used ants, an indicator taxonomic group, to predict how ant communities and ant-mediated ecosystem processes change as the climate warms. In the first chapter of this dissertation, I review the major ecosystem processes and services mediated by ants using the Millennium Ecosystem Assessment framework. In chapter two of this dissertation I present the results of a major ant sampling effort along environmental gradients of the Appalachian region of the northeastern United States. In 2010 I sampled ant communities in forested and open habitats at 67 localities from Virginia north to Maine and developed macroecological models which show that ant community composition in forested habitats can be explained by the region’s climatic properties. In chapter three, I intensively sampled open and forested plots at Harvard Forest LTER and Myles Standish State Forest in eastern Massachusetts. In chapter four, I present the results of a warming mesocosm experiment using the ant species Formica subsericea. I found that as warming increases, soil movement and soil respiration increases but decomposition and nitrogen availability decreases. In the final chapter of this dissertation, I use different functional diversity and species distribution models to classify the ant communities of the region into different functional groups and explore how their distributions will change in future climates. In this dissertation, I show that ant diversity and ant-mediated ecosystem processes are likely to change under future environmental and climatic conditions. I used observational, experimental and modeling approaches to evaluate and predict the consequences of climatic change on the biodiversity of ants in the northeastern U.S. Ants are considered to be amongst the little things that run the world, and the impacts of climatic change on their communities, abundances, distributions are likely to have major impacts on the forests of the region

Modeling the forest phosphorus nutrition in a southwestern Swedish forest site

In this study, a phosphorus (P) module containing the biogeochemical P cycle has been developed and integrated into the forest ecosystem model ForSAFE. The model was able to adequately reproduce the measured soil water chemistry, tree biomass (wood and foliage), and the biomass nutrient concentrations at a spruce site in southern Sweden. Both model and measurements indicated that the site showed signs of P limitation at the time of the study, but the model predicted that it may return to an N-limited state in the future if N deposition declines strongly. It is implied by the model that at present time, the plant takes up 0.50 g P m−2 y−1, of which 80% comes from mineralization and the remainder comes from net inputs, i.e. deposition and weathering. The sorption/desorption equilibrium of P contributed marginally to the supply of bioavailable P, but acted as a buffer, particularly during disturbances

An Ecogeomorphic Model to Assess the Response of Padilla Bay\u27s Eelgrass Habitat to Sea Level Rise

Estuaries worldwide are facing the possibility of conversion to open water if accretion cannot keep pace with increasing rates of sea level rise. Recent research into sediment elevation dynamics in Padilla Bay, a National Estuarine Research Reserve in Puget Sound, has revealed a mean bay-wide elevation deficit of -0.37 cm yr-1 since 2002. However, a more mechanistic prediction of the estuary’s response to future sea level rise should also incorporate non-linear feedback mechanisms between water depth, plant growth, and sediment deposition. Therefore, I used measurements of sediment accretion rates, suspended sediment concentrations, eelgrass stem density, and above- and belowground eelgrass biomass to build and calibrate a marsh equilibrium model (MEM), developed elsewhere but applied here for the first time to this eelgrass-dominated intertidal habitat. I then coupled the MEM with a relative elevation model (REM), which has previously been applied here, to create a hybrid that combines each model’s strengths in mechanistically simulating above- and belowground processes, respectively. The model predicts elevation change under various scenarios of sea level rise and suspended sediment concentrations. I used a 12-year elevation change dataset obtained from an extensive surface elevation table (SET) network in Padilla Bay for model validation. Field measurements indicated sediment accretion rates to be primarily determined by eelgrass stem density instead of biomass or relative elevation. I modified the hybrid model to reflect this relationship, which differentiates it from its predecessors. The model validation exercise revealed the need for an erosion parameter, without which projected relative elevation gain was substantially overestimated. Model projections without erosion showed an increase in relative elevation over much of the bay’s elevation gradient over a 100-year timeframe, reaching an equilibrium at an elevation where Zostera japonica stem density is maximized. These scenarios would involve an increase in Z. japonica cover in Padilla Bay, and a decrease in Z. marina cover. In contrast, model projections with erosion revealed a loss in relative elevation along the entire elevation gradient for all but the most conservative sea level rise scenario. The magnitude of loss was predicted to be greater at higher elevations. The suspended sediment concentrations required for the bay to maintain a stable relative elevation were higher than the current concentration of 3.93 mg L-1 for all sea level rise scenarios, with up to 15 mg L-1 being required for the most extreme scenario

Dynamic modelling of weathering rates – the benefit over steady-state modelling

Weathering rates are of considerable importance in estimating the acidification sensitivity and recovery capacity of soil and are thus important in the assessment of the sustainability of forestry in a time of changing climate and growing demands for forestry products. In this study, we modelled rates of weathering in mineral soil at two forested sites in southern Sweden included in a monitoring network, using two models. The aims were to determine whether the dynamic model ForSAFE gives comparable weathering rates to the steady-state model PROFILE and whether the ForSAFE model provided believable and useful extra information on the response of weathering to changes in acidification load, climate change and land use. The average weathering rates calculated with ForSAFE were very similar to those calculated with PROFILE for the two modelled sites. The differences between the models regarding the weathering of certain soil layers seemed to be due mainly to differences in calculated soil moisture. The weathering rates provided by ForSAFE vary seasonally with temperature and soil moisture, as well as on longer timescales, depending on environmental changes. Long-term variations due to environmental changes can be seen in the ForSAFE results, for example, the weathering of silicate minerals is suppressed under acidified conditions due to elevated aluminium concentration in the soil, whereas the weathering of apatite is accelerated by acidification. The weathering of both silicates and apatite is predicted to be enhanced by increasing temperature during the 21st century. In this part of southern Sweden, yearly precipitation is assumed to be similar to today's level during the next forest rotation, but with more precipitation in winter and spring and less in summer, which leads to somewhat drier soils in summer but still with increased weathering. In parts of Sweden with a bigger projected decrease in soil moisture, weathering might not increase despite increasing temperature. These results show that the dynamic ForSAFE model can be used for weathering rate calculations and that it gives average results comparable to those from the PROFILE model. However, dynamic modelling provides extra information on the variation in weathering rates with time and offers much better possibilities for scenario modelling.</p

Direct and indirect pressures of climate change on nutrient and carbon cycling in northern forest ecosystems : Dynamic modelling for policy support

Northern forest ecosystems play an important role in mitigating climate change by sequestrating carbon (C), while additionally providing and regulating other ecosystem services. A majority of the Swedish environmental quality objects (EQOs) that guide Swedish environmental policy and management are associated with the forest, and they have proven difficult to achieve. Several of them relate to the biogeochemical cycling of C and nutrients. Climate change increases the direct pressure on the forest ecosystems, and affects the C and nutrient cycling through direct pressures such as increasing temperatures and the risk of droughts, and through indirect pressures caused by an increasing demand of renewable energy from forests. Dynamic forest ecosystem models can be a useable tool for holistically studying, understanding and predicting the effects of increasing pressures, as a basis for policy support.This thesis aimed to compare, quantify and analyze the effects of direct and indirect pressures of climate change on forest soil and vegetation processes, and indicators related to nutrient and C cycling in forests, focusing on three of the EQOs that relate to forests, Natural Acidification Only, Zero Eutrophication and Reduced Climate Impact. The dynamic ecosystem model ForSAFE was applied on sites in different climate regions in Sweden, with different exposure to deposition. First, the effect of historical land use change on nitrogen (N) leaching and the risk of eutrophication was studied. Then, effects of intensified forest management on tree growth and concentrations of base cations (BC) and N in the soil solution were investigated. Finally, ForSAFE was used to study the effect of climate change on weathering of BC, which is an important process for providing vegetation with nutrients and for buffering against acidification.Using a combined approach with empirical data and the ForSAFE model, we could conclude that present environmental conditions alone are not enough to predict the risk of N leaching from two geographically close and comparable forest sites. Information about previous land use and moisture conditions was required to be able to correctly model the current dynamics of the soil organic matter. The effect of N fertilization on tree growth and N leaching was studied at three sites in areas with high, intermediate and low nitrogen deposition. The tree growth was the largest at the low deposition site, whereas the effect on N leaching was more pronounced at the high deposition site. These results support the Swedish Forest Agency´s current recommendations for N fertilization, which differ between regions depending on historical and present N deposition. Whole-tree harvesting (WTH), i.e. harvesting of not only stems but also branches and tops at final felling, led to a temporary reduction (20-30 years) of BC concentrations in the soil solution compared to stem only harvesting, in a study on six sites all over Sweden. This could not be explained by higher weathering rates after WTH, which has been suggested in earlier studies. Instead, it could be explained by higher BC leaching and BC uptake in trees during a period after stem only harvesting. Direct effects of climate change led to an increase in weathering rates in all of Sweden, with increased weathering rates year around in southern Sweden but not in winters in Northern Sweden. Future droughts may reduce weathering due to reduced soil moisture, and the risk is the highest in southern Sweden, where low soil moisture during summers already inhibits weathering. The study also highlighted the importance of soil texture and mineralogy for predicting weathering throughout Sweden, moderating the strong effect of temperature on weathering.The results highlighted the potential of using process-based models with high temporal resolution on well-investigated sites, for increasing the process knowledge and providing results useful for policy makers. An important message to policy makers is that site history and soil properties should be taken into account when planning for future forest management recommendations to reach the EQOs

A spatially explicit relative elevation model for Padilla Bay, Washington

The dynamics that govern the elevation of a coastal wetland relative to sea level are complex, involving non-linear feedbacks among opposing processes. Changes in the balance between these processes can result in significant alterations to vegetation communities that are adapted to a specific range of water levels. Given that the accretion rate in Padilla Bay, Washington, is suspected to be considerably lower than historical levels and that eustatic sea level rise continues to accelerate, the long term sustainability of the Zostera marina (eelgrass) meadows in the bay may be at risk to eventual submergence. I extended an existing Relative Elevation Model that incorporates many of the non-linear feedbacks that govern estuarine sediment dynamics by adding a spatial component. I used the model to project changes in Padilla Bay bathymetry and Z. marina distribution and productivity over the next century given various sea level rise scenarios. Analysis of field data collected in Padilla Bay for calibration and validation of the model indicated a net accretion deficit of 0.463 ± 0.173 cm/yr. Model projections for 100 years showed an increase in depths within the bay over time under all scenarios. Total annual Z. marina productivity and spatial coverage was greater at the end of the simulations than at the initial state for most scenarios due to shoreward expansion. In the most extreme scenarios, it reached a peak between 2052 and 2077 and began to decline as it was pushed beyond the existing shoreline. These results suggest that Padilla Bay is not stable with respect to rising sea level. It is possible that the increase in Z. marina productivity may be beneficial to marine organisms in the near term, but the projections indicate that this is only a temporary state until a peak and subsequent decline in total Z. marina productivity is reached

Modeling Carbon and Water Fluxes of Managed Grasslands: Comparing Flux Variability and Net Carbon Budgets between Grazed and Mowed Systems

The CenW ecosystem model simulates carbon, water, and nitrogen cycles following ecophysiological processes and management practices on a daily basis. We tested and evaluated the model using five years eddy covariance measurements from two adjacent but differently managed grasslands in France. The data were used to independently parameterize CenW for the two grassland sites. Very good agreements, i.e., high model efficiencies and correlations, between observed and modeled fluxes were achieved. We showed that the CenW model captured day-to-day, seasonal, and interannual variability observed in measured CO2 and water fluxes. We also showed that following typical management practices (i.e., mowing and grazing), carbon gain was severely curtailed through a sharp and severe reduction in photosynthesizing biomass. We also identified large model/data discrepancies for carbon fluxes during grazing events caused by the noncapture by the eddy covariance system of large respiratory losses of C from dairy cows when they were present in the paddocks. The missing component of grazing animal respiration in the net carbon budget of the grazed grassland can be quantitatively important and can turn sites from being C sinks to being neutral or C sources. It means that extra care is needed in the processing of eddy covariance data from grazed pastures to correctly calculate their annual CO2 balances and carbon budgets

Functional ecology of tropical forest recovery

Electronic abstract of the thesis for the library for the acquisitions department of Wageningen UR library (published as a html file so hyperlinks may be included) In English, one or 2 pages. Functional ecology of tropical forest recovery Currently in the tropics, the area of secondary forest exceeds that of mature forest, and the importance of secondary forest will probably continue to increase in the future. Understanding secondary forests’ potential for maintaining biodiversity and critical ecosystem functions is thereby vital. The aim of this study was to mechanistically link tropical forest succession with the recovery of ecosystem functioning after agricultural field abandonment using a trait-based approach. Such an approach makes use of functional traits; components of an organism’s phenotype that are key to assess ecosystem responses to global change drivers, and are at the same time indicators of how organisms drive changes in ecosystem functioning. Trait-based approaches could therefore provide a mechanistic way to scale up from organisms to ecosystems and thereby contribute towards a more predictive biodiversity and ecosystem functioning science. For this study, I made use of secondary forest data from a wet forest region in Chiapas (main study site), that cover the first 3 decades of succession, complemented with data from a dry forest region in Oaxaca, that cover the first 6 decades of succession. Both are tropical regions in Mexico, characterized by high biodiversity levels and rapid forest loss for agricultural expansion. In this study I found that functional diversity (the range of different functional traits) increases rapidly and functional composition (the weighted average functional trait value) changes directionally with succession (chapter 2 and 3). These reflect changing habitat filters (changing environmental gradients that underlie succession), and also a gradual shift from habitat filtering towards an increasing effect of competitively driven limiting trait similarity (chapter 4 and 5). Such successional changes in community functional properties suggest strong changes in ecosystem functions, however in situ ecosystem function rates were primarily explained by the total amount of biomass present rather than by biodiversity or functional trait properties of secondary forests (chapter 6). Only the more controlled ex situ decomposition rates were additionally significantly influenced by functional diversity and functional composition. When evaluating the identity of species that drive most of the ecosystem functions I found that different functions were largely driven by the same (dominant) species, implying a limited effect of biodiversity for multifunctionality at a given moment in time. This suggests that biodiversity is mainly important for maintaining multifunctional ecosystems across temporal and spatial scales (chapter 7). Deforestation is a major threat to natural forests and biodiversity, and I recognize that secondary forests are generally a poor substitute of mature forest. Nevertheless, I show that unassisted recovery through natural succession can be rapid, and contribute considerably to maintenance of biodiversity, functional strategies and ecosystem functions. So while protecting the remaining tracts of mature forests is vital, secondary forests are key components of multifunctional human-modified landscapes where synergies between biodiversity, ecosystem functions and human wellbeing can be optimized.</p

Adapting an Ecosystem Process Model to Estimate Ecosystem Services in Exurban Ecosystems

Ecosystem services (ES) are the physical goods and associated benefits that are provided to humans by ecological systems. Assessment of ES requires knowledge of ecology and ecosystem processes, and ES estimates can be improved when they include knowledge of nonlinearities, feedbacks, and interactions within ecosystems. A variety of assessment tools have been proposed to estimate the provision of ES. However, they fail to acknowledge interconnectedness of services or connections between ecosystem processes and services. This dissertation examines connections of ecosystem processes and ES with the assumption that knowledge of ecosystem ecology and ecosystem processes can be applied to improve estimates of ES capacity over time and under a variety of management scenarios. To investigate this connection, I modified the ecosystem process model Biome-BGC to simulate the provision of ES in exurban Southeastern Michigan. The modification resulted in a new version of the model, Biome-BGC-Ex, and involved detailed changes to the source code. The modified model included the ability to model competition between turfgrass and open grown trees in a single grid cell, to incorporate residential management practices, and to translate model outputs into well-defined, quantitative estimates of ES. My research was conducted as part of a larger collaboration, the SLUCE (Spatial Land Use Change and Ecological Effects) project and addresses the exurban residential landscape as a coupled human-natural system. It references and builds on previous elements of the SLUCE project including an empirical ecological field study, developer and homeowner interviews, web-based surveys, and modeling in a coupled human-natural system framework. My contributions to the project, specifically modifying Biome-BGC and linking it to ES, can be applied to future research on coupled human-natural systems in exurban residential landscapes. Chapter two describes how Biome-BGC was modified for the exurban landscape and then calibrated and parameterized for Southeastern Michigan. It examined which yard management practices have the greatest effect on carbon sequestration and model results suggested N fertilization was the strongest driver across three major vegetation types. Chapter three describes how Biome-BGC-Ex was modified to estimate ES capacity of ten services and evaluated the impact of yard management practices on ES capacity. Model simulations showed trade-offs between ES relating to high amounts of carbon or biomass and freshwater recharge. Chapter four took a broader approach and evaluated ecosystem process models as a potential tool for ES assessment and discussed how the integration of Biome-BGC-Ex with other tools could improve ES assessment. I found that while process models can improve understanding of interconnected ecosystem processes and biophysical feedbacks that affect the production of ES, they require more detailed data and complex knowledge to run. These chapters also discuss limitations of Biome-BGC-Ex and its ability to adequately address ecological complexities of exurban landscapes. One major limitation was accurately modelling N dynamics of exurban tree cover and model simulations likely overestimating C sequestration under high levels of fertilization. My dissertation research is the first to modify Biome-BGC to measure ES in a residential ecosystem. It is also novel because the work focuses on how human management of the landscape affects ES production as opposed to land use or land cover change. My dissertation research can likely be replicated in similar ecosystems to inform more complex ES modelling frameworks that rely on ES production modelling grounded in the understanding of ecosystem processes and their feedbacks.PHDResource Ecology & Mgt PhDUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169753/1/sekiger_1.pd