Browsing by Subject "Landscape ecology"

Climate change is driving vegetation community shifts in coastal regions of the world, where low topographic relief makes ecosystems particularly vulnerable to sea level rise, salinization, storm surge, and other effects of global climate change. Salinization has clear effects on vegetation, as few plant species can survive in brackish water, and these shifts in vegetation lead to declines in biomass carbon stocks, as well as significant changes in habitat structure and biodiversity. The rate and extent of these impacts on other wetland ecosystem properties and function is far less certain. This dissertation investigates the ecosystem consequences of saltwater intrusion in coastal wetlands, from shifting vegetation at the landscape scale, to soil biogeochemistry and wetland carbon cycling.Coastal plant communities globally are highly vulnerable to future sea-level rise and storm damage, but the extent to which these habitats are affected by the various environmental perturbations associated with chronic salinization remains unclear. In 2016, a series of vegetation plots across the Albemarle-Pamlico Peninsula that had been surveyed 7-13 years earlier were revisited in order to measure changes in tree basal area and community composition over time. I found reduced tree basal area in plots at lower elevations and with higher current soil salt content, while these factors explained only a small fraction of the measured changes in tree community composition. While tree basal area increased in the majority of plots, I measured declines in basal area in multiple sites with high soil salt content or low elevation. This decadal comparison provides convincing evidence that increases in soil salinity and saturation can explain recent changes in tree biomass, and potential shifts in community composition in low-elevation sites along the North Carolina coast. In Chapter 3, I quantified land and land cover change in the Alligator River National Wildlife Refuge (ARNWR), North Carolina’s largest coastal wildlife preserve, from 1985 to 2019 using classification algorithms applied to a long-term record of satellite imagery. Despite ARNWR’s protected status, and in the absence of any active forest management, 32 % (31,600 hectares) of the refuge area has changed land cover classification during the study period. A total of 1151 hectares of land was lost to the sea and ~19,300 hectares of coastal forest habitat were converted to shrubland or marsh habitat. As much as 11 % of all forested cover in the refuge transitioned to ghost forest, a unique land cover class that is characterized by standing dead trees and fallen tree trunks. This is the first attempt to map and quantity coastal ghost forests using remote sensing. These unprecedented rates of deforestation and land cover change due to climate change may become the status quo for coastal regions worldwide, with implications for wetland function, wildlife habitat and global carbon cycling. Salinization of freshwater wetlands is a symptom of climate change induced sea level rise. The ecosystem consequences of increasing salinity are poorly constrained and highly variable within prior observational and experimental studies. Chapter 4 presents the results of the first attempt to conduct a salinization experiment in a coastal forested wetland. Over four years, marine salts were applied to experimental plots several times annually with the goal of raising soil salinity to brackish levels while soil porewater in control plots remained fresh. Each year I measured aboveground and belowground vegetation biomass along with soil carbon stocks and fluxes. Despite adding more than 1.5 kg of salt per m2 to our experimental plots over four years, the ecosystem responses to salt treatments were subtle and varied over the multi-year experiment. In the final year of the experiment, soil respiration was suppressed, and bulk and aromatic soil carbon became less soluble as a result of salt treatments. The more stable carbon pools—soil organic carbon and vegetation associated carbon—remained unaffected by the salt treatment. This experiment demonstrates substantial ecosystem resistance to low dose salinity manipulations. The inconsistent soil carbon responses to experimental salinization I observed in the field led me to question how differences in soil pH and base saturation might alter the impacts of salinity of soil microbial activity. To test this, I performed a salt addition experiment on two series of wetland soils with independently manipulated salt concentrations and solution pH to tease apart the effect of these seawater components on soil carbon cycling (Chapter 5). Microbial respiration and dissolved organic carbon solubility were depressed by marine salts in both soils, while pH manipulation alone had no effect. Salinity treatments had a far greater effect on soil pH than did our intentional pH manipulation and there was a strong interaction between salt treatments and soil type that affected the magnitude of soil carbon responses. Site soils varied significantly in pH and base saturation, suggesting that the interaction between salinity and edaphic factors is mediating soil carbon processes. The degree of salinization and the effective pH shift following seawater exposure may vary widely based on initial soil conditions and may explain much of the variation in reports of salt effects on soil carbon dynamics. I suggest that these edaphic factors may help explain the heretofore inconsistent reports of carbon cycle responses to experimental salinization reported in the literature to date.

Globally, trees are increasingly dying from extreme droughts and heatwaves, a trend that is expected to increase with climate change. Loss of trees has significant ecological, biophysical, and biogeochemical consequences. In this thesis, I explored how forests will respond to increases in droughts and heatwaves projected under climate change, by studying the impacts of the 2011 drought on the forests and woodlands of Texas. I first developed new methods using remote sensing that improved monitoring of forest disturbances from droughts and heatwaves at regional scales. I then explored multiple modeling approaches, to improve forecasts of forest vulnerability to future droughts and heatwaves.

Drought-induced tree mortality is unique because it often is limited to only subtle and diffuse changes in forest cover. Therefore, I first developed a new approach towards quantifying drought-driven canopy loss in open canopy woodland systems using remotely sensed imagery, across a Landsat scene in central Texas (>30,000 km2). I used classifications of 1-m orthophotos to calibrate and validate 30-m Landsat imagery. In scaling up to create regional canopy loss maps, I assembled a Landsat time-series and separated mortality pixels experiencing persistent canopy loss from pixels with only background noise by applying the Landtrendr algorithm. I then estimated percent tree canopy loss within each of these mortality pixels by comparing two models capable of handling zero-inflated continuous proportions: random forest and a zero-or-one inflated beta (ZOIB) regression model. I found that the ZOIB regression model had the highest accuracy in predicting canopy loss (mean absolute error = 5.16%, root mean square error = 8.01%).

Again using remotely sensed imagery, I developed a second method to quantify canopy loss due to the 2011 drought across the many diverse systems of Texas, from the eastern pine/hardwood forests to the western shrublands. I then used these maps to better understand which ecological systems were most impacted and climatic trends that could explain spatial patterns of canopy loss. Canopy loss observations in ~200 multi-temporal fine-scale orthophotos (1-m) were used to train coarser Landsat imagery (30-m) to create 30-m binary statewide canopy loss maps. I found that canopy loss occurred across all major ecoregions of Texas, with an average loss of 9.5%. The drought had the highest impact in post-oak woodlands, pinyon-juniper shrublands, and Ashe juniper woodlands. Focusing on a 100-km by ~1000-km transect spanning the State’s 5-fold east-west precipitation gradient (~1500 to ~300 mm), I compared spatially explicit 2011 climatic anomalies to our canopy loss maps. Much of the canopy loss occurred in areas that passed specific climatic thresholds: warm season anomalies in mean temperature (+1.6 °C) and vapor pressure deficit (VPD, +0.66 kPa), annual percent deviation in precipitation (-38%), and 2011 difference between precipitation and potential evapotranspiration (-1206 mm). Although similarly low precipitation occurred during the landmark 1950s drought, the VPD and temperature anomalies observed in 2011 were even greater. Furthermore, future climate data under the representative concentration pathway 8.5 trajectory project that average values will surpass the 2011 VPD anomaly during the 2070-2099 period and the temperature anomaly during the 2040-2099 period. Identifying vulnerable ecological systems to drought stress and climate thresholds associated with canopy loss will aid in predicting how forests will respond to a changing climate and how ecological landscapes will change in the near term.

As climate change continues, forest vulnerability to droughts and heatwaves is increasing, but vulnerability varies regionally and locally through landscape position. However, most models used in forecasting forest responses to heat and drought do not incorporate relevant spatial processes. Therefore, thirdly, to improve spatial predictions of tree vulnerability, I employed a non-linear stochastic model of soil moisture dynamics accounting for landscape differences in aspect, topography, and soils. Across a watershed in central Texas I modeled dynamic water stress for a dominant tree species, Juniperus ashei and projected future dynamic water stress through the 21st century. Modeled dynamic water stress tracked spatial patterns of drought-impacted area derived using remote sensing. Accuracy in predicting drought-impacted stands increased from 60%, accounting for only soil variability, to 72% when also including lateral redistribution of water and radiation/temperature effects attributable to aspect. Our analysis also suggests dynamic water stress will increase through the 21st century, with minimal buffering from the landscape. Favorable microsites/refugia may exist across a landscape where trees can persist; however, if future droughts are too severe, the buffering capacity of a heterogenous landscape could be overwhelmed. Incorporating spatial data will improve projections of future tree water stress and identification of potential resilient refugia.

Lastly, droughts impact tree species unequally and therefore have the potential to alter the species composition of forests. As droughts intensify under climate change, models that can account for both changing environmental conditions as well as joint species-specific responses are needed to forecast shifts in species ranges, composition, abundance, and mortality. Therefore, lastly, I used a Generalized Joint Attribute Model, GJAM, to simultaneously model live and dead basal area by species, across Texas, using Forest Inventory and Analysis plots from 2001-2015 and covariates related to climate, topography and soils. I then projected shifts in species abundance into the 21st century under multiple climate warming scenarios. Model projections show that many of the eastern hardwood species will likely shift eastward throughout the 21st century. Many of the western woodland species are also projected to shift eastward and become increasingly dominant throughout Texas. By modeling species abundance and mortality simultaneously and by using covariates related to climate variability, we can improve projections of forest responses to continuing climate change.

One of the major challenges in understanding aquatic ecosystems is teasing apart the interrelated influences of multiple stressors on ecosystem function to determine their relative importance. Urban areas are expanding across the globe at unprecedented rates, and as low-lying areas within landscapes, streams and ponds are particularly hard hit by the multiple stressors associated with the urban stream syndrome. This dissertation investigates the effects of stressors as fundamental drivers of urban freshwater ecosystem function in lentic and lotic systems.

Stormwater ponds and retention basins are ubiquitous lentic features throughout urban landscapes that potentially serve as important control points for nitrogen (N) removal from surface water bodies via denitrification. However, there are possible tradeoffs to this water quality benefit if high N and contaminant concentrations in stormwater pond sediments decrease the complete reduction of nitrous oxide (N₂O), a potent greenhouse gas, to dinitrogen (N₂) during denitrification. Here, I evaluated whether urban stormwater pond sediments from 64 ponds across eight major US cities had elevated potential emissions of N₂O (Chapter 2). I found surprisingly little correlation between surrounding land cover urbanization intensity and pond sediment chemistry. I measured highly variable potential rates of denitrification, but generally low proportions of N₂O relative to total denitrification, allaying the concerns that motivated the study. However, the lack of a relationship between land cover and sediment chemistry within urban ponds calls into question our commonly held assumptions about the relationships between development intensity and the loading, routing, and retention of nutrient and contaminants within urban landscapes.

The typically elevated loading of chemical constituents into urban freshwater lotic ecosystems is due to the efficient routing of water over surfaces heavily modified by human activities (i.e. stormwater and wastewater infrastructure). In this study, I investigated the dominant controls on ion routing and loading within 24 urban watersheds that fell within a narrow range of development intensity but spanned the widest possible range in spatial configuration and connectivity metrics in the Triangle region of the North Carolina Piedmont (Chapter 3). By pairing analysis of land cover attributes with temporal trends in baseflow chemistry and high-frequency data of specific conductance and discharge, I found that increases in watershed road and pipe density lead to increasingly chemically distinct stormflows and baseflows. This enhanced bimodality of both flow and chemistry results in more variable chemical regimes in watersheds where linear urban infrastructure (roads and pipes) connects impervious surfaces directly to streams.

In addition to altered chemical loading, headwater streams draining urbanized catchments are subject to frequent and intense flooding. Here, I investigated how altered hydrologic regimes in urban landscapes affect headwater stream form and function (Chapter 4). I found a surprisingly wide range in dissolved oxygen regimes, ranging from frequent hypoxia to near constant saturation, both of which resulted in net heterotrophic streams with low rates of productivity. This work, paired with work presented in Chapter 3, advances understanding of carbon cycling within streams by documenting a shift in ecosystem dynamics in urban streams towards bimodality between the fast dynamics of advective transport from pavements and hillslopes during storms, and the slow dynamics of redox active zones in eutrophic and organically enriched stream pools at base flow.

Stream ecosystems draining highly urbanized areas are often some of the most extreme cases of altered physical and chemical stressor regimes and provide a testbed for examining the influence of these drivers on ecosystem function. The findings presented improve our understanding of how nutrients and contaminants move through landscapes, undergo biogeochemical transformations, and their ultimate effect on aquatic ecosystem processes.

Currently, cities comprise 52% of the Earth's land surface, with this number expected to continue to grow, as most of the predicted 2.3 billion increase in population over the next 40 years is expected to occur in urban areas (United Nations Population Division 2012). Urban areas necessarily concentrate food, energy, and construction materials, and as a result tend to be hotter and more polluted than the surrounding landscape. All urban ecosystems are thus quite altered from their pre-urban state, but urban streams are particularly impacted. As low lying points on the landscape, streams are subject to the degradation caused by urban stormwaters, which are transmitted rapidly from the surfaces of pavements, roofs, and lawns through stormwater infrastructure to streams.

The systematic changes seen in many urban streams have been described as the "Urban Stream Syndrome" (USS) and serve as an organizing conceptual framework for urban stream research (Walsh et al. 2005b). A primary symptom of USS is increased flashiness in hydrographs, as stormwater in urban areas is routed efficiently into streams (Booth and Jackson 1997, Konrad and Booth 2005). With this stormwater runoff comes intense scour leading to deeply incised channels, large amounts of contaminants and nutrients, and, as will be discussed in this thesis, heat surges (Booth 1990, Tsihrintzis and Hamid 1997, Walsh et al. 2005a, Nelson and Palmer 2007, Bernhardt et al. 2008). At baseflow, urban streams are contaminated by sanitary sewage leakages, are unable to exchange water with their floodplains due to incision and with groundwater due to lower water tables, and are warmer due to canopy loss and urban heat island effects (Paul and Meyer 2001, Pickett et al. 2001, Groffman et al. 2002, 2003). These baseflow and stormflow changes lead to the loss of sensitive taxa and increase in tolerant biota, as well as changes in ecosystem function, including carbon and nitrogen processing (Paul and Meyer 2001, Meyer et al. 2005, Imberger et al. 2008, Cuffney et al. 2010).

The urban heat island effect can increase air temperatures up to 10°C above those in surrounding, non-urban areas, while impervious surfaces can reach temperatures up to 60°C (Asaeda et al. 1996, Pickett et al. 2001, Kalnay and Cai 2003, Diefenderfer 2006). These changes are particularly troublesome, as research has shown that temperature is a controlling factor in aquatic systems for both stream biota and ecosystem processes (Allen 1995, Kingsolver and Huey 2008). Thermal changes control and can alter basic morphological features of biota, such as size and growth rates (Gibbons 1970, Kingsolver and Huey 2008). USS synthesis reports have called for further research into the processes by which urban areas influence the temperature of streams and the resulting effects on the ecosystems, but until recently have largely been ignored (Paul and Meyer 2001, Wenger et al. 2009). This dissertation explores the timing, magnitude, and pattern of thermal pollution for streams within urban heat islands, with the goal of understanding what aspects of watershed development most strongly influence the thermal regimes of streams. In order to explore thermal pollution in urban streams, I asked three overarching questions:

1) How much hotter are highly urban streams than streams in less developed watersheds?

2) How far do urban heat pulses propagate downstream of urban inputs?

3) How can development configuration mitigate or exacerbate development amount in mediating urban thermal pulses?

In Chapter 2, I explore the differences in baseflow and stormflow temperatures in 60 watersheds across the North Carolina Piedmont that ranged across a gradient of urbanization. I asked:

1) How do maximum temperatures at baseflow and maximum temperature surges at stormflow differ across watersheds with varying development intensity?

2) What reach- and watershed-scale variables are most correlated with these 2 aspects of stream thermal regimes?

3) Do stream management approaches (riparian buffers, channel restoration) address the links between these variables and stream temperature?

I found that the 5 most urban streams were on average 0.6°C hotter at baseflow than the 4 most forested streams. During a single storm event, urban streams showed an increase over five minutes of up to 4°C, while forested streams showed little or no thermal increase. Reach-scale characteristics, specifically canopy closure and width, primarily controlled baseflow temperatures. These local factors were not important drivers of stormflow temperature changes, which were best explained by watershed-scale development and road density. Management that focuses on baseflow temperatures, such as riparian buffers and reach-scale restoration, ignores the intense urban impacts that occur regularly during storm events.

Next, in Chapter 3, I explore longitudinal temperature patterns in a single stream, Mud Creek, in Durham, North Carolina. Mud Creek's headwaters are suburban, and the stream travels through a number of housing developments before entering a 100-year-old forest. I placed 62 temperature loggers over a 1.5 km reach of this stream. To explore the mechanisms by which stormflow heat pulses dissipate along this stream reach, I asked:

1) What is the range of heat pulse magnitudes that occur over a year?

2) What is the maximum distance that a heat pulse travels downstream of urban inputs?

3) How do the magnitude and distance vary with storm characteristics, including antecedent air temperature and amount and intensity of precipitation?

I found that heat pulses with amplitude of greater than 1°C traveled more than 1 km downstream of urban inputs in 11 storm events over one year. This long dissipation distance, even in a best-case management scenario of mature and protected forest, implies that urban impacts across a developing landscape travel far downstream of the impacts themselves and into protected areas. Heat pulses greater than 1°C occurred in storms with greater intensity of and total precipitation and greater time of elevated storm flow. Air temperature, flow intensity, maximum flow, and total precipitation controlled the magnitude of the heat pulse, while the distance of dissipation was controlled by the magnitude of the heat pulses and total precipitation. The importance of air temperature, flow, and precipitation metrics imply that both magnitude and distance of dissipation of heat pulses are likely to increase with climate change, as air temperatures increase and sudden, intense storms become more frequent. This translates to even greater ecological impacts in urban landscapes like Durham municipality, where the 98.9% of streams less than 1 km downstream of a stormwater outfall will become even more likely to be impacted by urban stormwaters.

In Chapter 4, I examine which aspects about development best explain thermal differences observed at baseflow and stormflow. To do this, I selected 15 similarly sized watersheds in the North Carolina Piedmont region within 45 to 55% development that varied in other development characteristics, specifically density of stormwater infrastructure and aggregation of development patches. I asked two questions:

1) How does the configuration and connectivity of development within a watershed influence baseflow and stormflow temperatures in receiving streams?

2) How do baseflow and stormflow temperatures vary with development characteristics?

I found that aspects of development varied greatly within this urban intensity subset, with ranges for some metrics nearly equal to the variation observed across all watersheds in the landscape. Longer pipe lengths, shading from incised channels, and shaded impervious surfaces resulted in cooler baseflow temperatures. As in Mud Creek, stormflow metrics were influenced through two physical pathways: air temperature and either flow intensity, to explain overall thermal change, or antecedent flow, to explain intensity of thermal change. Greater sub-surface connectivity of development to the stream network increased thermal responsiveness to storms through faster delivery and greater amount of heated runoff. Greater proportions of forest in a watershed decreased the amount and temperature of runoff delivered to the stream, while development within the riparian zone throughout a watershed led to warm baseflow temperatures and lack of response to stormflow heat surges. By decreasing the connectivity of development to the stream network, thermal regimes of streams can be less impacted even in relatively urban watersheds.

Thermal pollution in urban streams is a problem that will only be exacerbated by predicted climate change and urban expansion. These findings imply that thermal pollution is a problem throughout urban landscapes, even far downstream of urban inputs and within protected areas, and must be managed as an important component of the USS. Future research should focus on the transferability of these findings to regions outside of the southeastern United States and to the movement of other urban pollutants, and on exploring the potential to manage these systems by decreasing sub-surface connectivity.

An ecosystem is a community of organisms interacting with its environment, and landscapes are spatially interactive ecosystems. Earth's burgeoning human population demands ever more from finite ecosystems; but if managed well, landscapes can sustain their provision of resources and services and adapt to fulfill the changing human appetite. Management relies on sound information, and managing landscape change requires reliable spatio-temporal databases of ecologically relevant information. Remote sensing technologies fill this niche, providing increasingly large and diverse datasets, but the algorithms to extract information from the data must be developed. I developed and compared three remotely sensed measurements of forest canopy height to one another and to in situ field measurements. Both the precision and the accuracy (as well as the cost) of the measurements sorted along an axis of spatial scale, with Light Detection and Ranging (lidar) measurements proving most reliable at fine scales but prohibitively expensive over large areas and various radar technologies more appropriate for larger areas, especially when calibrated to the more accurate and precise lidar measurements. I also adapted traditional, single-time landcover classification algorithms to extract dense time series of categorical landcover maps from archival multi-spectral satellite images. These measurements greatly expand the potential spatio-temporal scope of landscape ecology and management, facilitating a shift away from data-imposed reliance on "space-for-time substitution" and loosely connected case studies toward robust, statistical analysis based on consistent information.