Sarah Gergel

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
Linking landscape indicators to groundwater nitrate concentrations in a transboundary aquifer (2018)

Groundwater aquifers provide nearly half the freshwater used in drinking and cooking. However, in the last century, massive transformations of landscapes have produced enduring impacts on natural resources such as groundwater. Excess nitrate contamination of groundwater is a growing health concern, particularly in agricultural regions. Despite its importance, very few studies have quantitatively linked land use land cover (LULC) and groundwater nitrate concentrations. Furthermore, understanding the impacts of LULC on transnational water resources is especially challenging as multi-jurisdictional data disparities and inconsistencies can complicate monitoring efforts.Here, I developed a suite of innovative long-term monitoring approaches and evaluated their utility in a well-studied transnational aquifer where elevated groundwater nitrate concentrations are of concern. My overall objective was to develop approaches for examining LULC impacts to groundwater via two primary components. First, I used remote sensing to examine two decades of LULC change surrounding 11 groundwater dependent cities. Second, I created more localized landscape indicators and evaluated their correspondence to long-term trends in groundwater nitrate concentrations. I examined two nested spatial extents spanning the US-Canada border including: small cities throughout the Greater Abbotsford-Sumas Aquifer region as well as the confined extent of the Abbotsford-Sumas Aquifer (ASA) proper. I integrated a unique combination of historical photography, transnational satellite imagery, and groundwater monitoring wells spanning four decades. Throughout the larger region, I found that landscape evenness increased over time driven by greater forest losses in Canada and greater losses of agricultural land in the USA. Within the localized ASA, I determined that groundwater nitrate concentrations could be explained using landscape features measured within the vicinity of wells. Landscape indicators such as the proportional area of berries, raspberry fields undergoing renovations, as well as forage/pasture were particularly useful. I further determined that long-term trends in nitrate were best explained by historical landscape indicators from two decades prior (as opposed to contemporaneous indicators). Very few studies have examined LULC emphasizing transboundary aquifers and even fewer have quantitatively linked groundwater nitrate concentrations to land use practices. Thus, this work demonstrates a valuable, consistent monitoring approach that is transportable to other regions facing similar challenges.

View record

Quantifying the development of small-scale fisheries on coral reefs, and their impact on habitats (2017)

Growing human populations place multiple pressures on social-ecological systems, including coastal oceans. However, the effects of long-term and/or overlapping stressors remain poorly understood, particularly over large spatial scales. My dissertation evaluated how pressures from fishing and co-occurring stressors correspond to current ecological conditions in the Danajon Bank, a coral reef ecosystem in the central Philippines. I used long-term local ecological knowledge (LEK) to map fishing practices (1950-2010) and high spatial resolution satellite imagery to map coastal habitats. This innovative suite of methods enabled me to examine patterns over broader spatial scales and longer time periods than those usually assessed. I met five primary objectives: (1) quantify the spatio-temporal dynamics of fishing effort and gear use; (2) examine the influence of fisheries governance; (3) map the spatial distribution of benthic (seafloor) habitats; (4) model the spatial distribution of living corals in relation to co-occurring stressors and biophysical conditions; and (5) explore the conservation implications of these relationships. While individuals’ fishing practices were fairly consistent over time, this small-scale fishery has changed dramatically. First, total fishing effort (days per year fished by all fishers) accelerated between 1960 and 2010 because of rapid growth in the number of fishers. Aggregate fishing effort increased almost 2.5-fold and spatially-explicit fishing effort increased over 20-fold. Second, the areal extent of fishing grounds expanded greatly. Third, use of fishing gears changed over time. Diversity of fishing gears increased, as did fishing effort with destructive, active, and non-selective gears. Considering the timing of these changes, I found a lasting influence of fishing policies, and small improvements in the sustainability of fishing gears following implementation of co-management. Finally, I found that the probability that an area supported living corals was affected by fishing through both long- & short-term mechanisms, and I documented strong coral-landscape relationships. My research demonstrates that to strengthen ocean conservation, it will be essential to reduce the frequency and intensity of stressors, remove some areas from exploitation, foster resilience traits of ecosystems, gather data to better understand systems, and strengthen the institutions that can support these endeavors.

View record

Long-term mapping of ecosystem services in a river-floodplain system (2016)

Humans derive a wide range of benefits from ecosystems, known as ecosystem services (ES). At the nexus of land and water, floodplains are particularly important for providing ES. Recently, problematic declines in ES have motivated research to better understand their spatial distributions. However, the temporal dynamics of critically important floodplain-specific ES remain poorly understood. These spatial and temporal dynamics as well as trade-offs that occur when management enhances one ES at the expense of others are particularly germane as a warming climate alters river flows. Landscape history is foundational to elucidating these dynamics. Here, I explore the importance of landscape history for understanding the historical, contemporary, and future distributions of ES in the Wenatchee watershed, central Washington State. Using several widely-used datasets in novel ways, my dissertation has five primary objectives (1) quantify the relative importance of different landscape positions for frontier settlers, (2) map change in ES from 1949-2006 using high-resolution imagery, (3) enhance understanding of ES interactions by incorporating change in ES over time, (4) explore the spatial distribution of floodplain-specific ES, and (5) conceptualize shifts in ES under future climates. I found riparian zones and floodplains were disproportionately important for frontier settlement, setting the stage to explore floodplain-specific ES in more detail. ES were dynamic from 1949-2006, largely driven by increasing urbanization and forest densification. Next, I showed how history can provide important insights into ES interactions. Finally, I also found floodplain ES varied considerably with floodplain position. Analyses over broad time frames and at fine spatial scales greatly enhance our understanding of ES dynamics, highlighting the need for long-term monitoring for ES, especially as ES continue to interact under future climates.

View record

Relative influence of cultural identity and market access on agricultural biodiversity in swidden-fallow landscapes of eastern Panama (2012)

Agricultural biodiversity is essential to local and global food security, yet is being rapidly eroded world-wide. The increasing reach of global transportation and trade networks is predicted to homogenize agriculture at regional scales. However, relatively little is known about how cultural values and norms, as reflected in local farmer decision-making, will interact with market forces to sustain or erode agricultural biodiversity. Working with farmers from three ethnic backgrounds — Black, Emberá or Kuna — in a region of Panama undergoing rapid landscape change, I determined the relative influence of farmer cultural identity and market access on several indicators of agrobiodiversity. Twelve villages were chosen to minimize environmental differences while maximizing differences in access. Villages were classified as “highway” or “remote” based on time and cost of travel to Panama City markets, with each ethnic group represented by two highway and two remote villages. From 2007-2009, a combination of crop inventories and land-use mapping (for 645 fields) as well as interviews with > 130 farmers were conducted. Diversity of staple food crop varieties, agroforest trees and shrubs, and the spatial and temporal dynamics of shifting cultivation were compared among villages. Farmer cultural identity had a stronger impact on agrobiodiversity indicators than did access. For staple food crops (e.g., maize, rice, yam, cassava, bananas, and taro), ethnicity explained 2.5 to 8.5 times more variation in assemblages than access. Distinct assemblages of staple crop landraces (varieties) and agroforest trees and shrubs were associated with different ethnic groups, even where access was high, reflecting culturally patterned dietary preferences, ceremonial or customary uses, and culturally-bounded seed-exchange networks. Mean number, size, and types of fields maintained (homegardens, outfield agroforests, annual fields, pastures), as well as their management (e.g., forest felling, herbicide use), also varied among ethnic groups. These differences reflected culturally-based crop preferences, values of land, traditional settlement patterns, and contemporary relationships to other actors, including the Panamanian government. Together, the distinct agricultural practices of individual ethnic groups combined to create diversity across many levels of biotic organization: from landrace, to species, to patch, to landscape. These findings strongly suggest that new approaches to conservation that support and respect heterogeneous socio-cultural systems will be critical to global efforts to maintain agrobiodiversity.

View record

Sustainable Forest Management in the Context of Integrated Watershed Management in Southern China (2009)

No abstract available.

Relative influence of temperature and disturbance on vegetation dynamics in the Low Arctic : an investigation at multiple scales. (2008)

Climate change will affect Arctic plant communities directly, by altering growth and recruitment, and indirectly, by increasing the frequency of natural disturbance. Since the structure of northern vegetation influences global climate, understanding both temperature and disturbance effects on vegetation is critical. Here, I investigate the influence of temperature and disturbance on Low Arctic vegetation at several spatio-temporal scales in the Mackenzie Delta Region, N.W.T. To disentangle the relative impact of temperature and disturbance on forest-tundra and tundra ecosystems, I sampled microenvironmental variability, plant community composition, and green alder abundance, growth, and reproduction on disturbed (burns and thaw slumps) and undisturbed sites across a regional temperature gradient. Disturbed areas showed increases in alder productivity, catkin production, and seed viability, as well as differences in plant community composition and microenvironment. The magnitude of plot-level responses to disturbance compared to variation across the temperature gradient suggests that in the short-term, increasing the frequency of disturbance may exert a stronger influence on tundra ecosystems than changes in temperature. At the plot level, increases in alder seed viability and recruitment at warmer sites point to the fine-scale mechanisms by which shrub abundance will change. To examine the relative influence of temperature and biophysical variables on landscape-level patterns of shrub dominance, I mapped Low Arctic vegetation using aerial photos. At this broader scale, correlations between temperature and the areal extent of shrub tundra suggest that warming will increase the dominance of shrub tundra. To assess the magnitude of changes in temperature and thaw slump activity, I analyzed climate records and mapped retrogressive thaw slumps using aerial photographs. An increase in thaw slump activity in recent decades, coincident with higher temperatures, suggests that continued warming will change the area affected by thermokarst disturbances like slumps. Taken together, my research indicates that the effects climate change will be magnified by shifts in the frequency of disturbance, initiating changes to Arctic vegetation with significant implications for global climate. My work also shows that to fully understand the influence of patch-landscape feedbacks on Arctic vegetation dynamics, the effects of disturbance must be examined across longer temporal and broader spatial scales.

View record

Master's Student Supervision (2010-2017)
Towards improved understanding and management of mixed-severity fire regimes in mountain forests (2013)

Understanding spatial and temporal patterns of fire regimes is critically important forsustainable forest management and fire hazard mitigation. Mixed-severity fire regimes, inparticular, are poorly understood, yet increasingly recognized as important drivers of stand and landscape-heterogeneity. I address knowledge gaps pertaining to the management andunderstanding of mixed-severity regimes including: (1) classification and mapping, (2)prevalence in mountain forests, (3) underlying topographical drivers, and (4) stand dynamics. Research questions were addressed using dendrochronological field data (fire scars, tree establishment dates, stand composition and structure) from 20 randomly selected research sites in southeastern British Columbia,I examined whether mixed-severity regimes, as currently represented in fire-regimeclassification schemes, led to erroneous landscape-level fire regime mapping. I used my field data to evaluate the accuracy of two classification systems (Natural Disturbance Type (NDT)and Historical Natural Fire Regime (HNFR)) used by managers to map fire regimes in BritishColumbia (Chapter 2). Each classification system made considerable and contrasting errors in identifying mixed-severity regimes relative to the field data and these misrepresentations were tied to elevation. I attributed these errors to assumptions about disturbances underlying each classification system, as well as limitations of the research methods used to estimate firefrequency (i.e., using either stand-age or fire-scar data in isolation). I explored the prevalence of mixed-severity fire regimes, importance of underlying topographic drivers, as well as the influence of mixed- versus high-severity fires on forest composition and structure (Chapter 3). I found evidence of mixed-severity fires at 55%. At these sites, most reconstructed fires (73%) were documented solely by fire scars, indicating many were of low-to-moderate severity. The remaining 27% of fires were severe enough to create conditions suitable for even-aged cohort to establish. Spatial patterns of fire severity were primarily controlled by elevation (i.e., severity increased with elevation). Composition varied with disturbance history; however, structural differences (e.g., tree size classes) were subtle, with the exception of snag densities, which were much greater in old, high-severity forests (where time-since-last-fire >250 years). Understanding the ecological heterogeneity created by mixed-severity regimes potentially influences decisions related to conservation, silviculture, wildfire and fuel mitigation.

View record

 

Membership Status

Member of G+PS

Program Affiliations

 

If this is your researcher profile you can log in to the Faculty & Staff portal to update your details and provide recruitment preferences.