Stella Lab

Research

Floodplain Ecology and Restoration in Semi-arid River Basins

  Riparian zones in Mediterranean and other semi-arid regions are important ecosystems that are sustained by flooding regimes and other physical factors that affect water availability to organisms. In many dry regions, riparian ecosystems have been negatively affected by dam construction, water diversions, and floodplain development. In addressing these impacts, we must distinguish between restoring ecosystem processes that will be sustainable and resilient over time and under variable conditions, versus creating ecosystem structure that is usable immediately but which may need periodic intervention to be maintained over time. My research on floodplain ecology seeks to provide basic information useful to both restoration approaches. Rehabilitating ecosystem processes requires us to understand the physical drivers and life history phases that limit populations of interest, and the critical interactions that influence community dynamics. Rigorously quantifying these relationships allows us to identify where ecological processes are still functioning (and therefore can be co-opted for restoration) versus areas where processes are so impaired that more intensive and frequent interventions would be required to create and maintain restored habitats. In the case of stream and river ecosystems, the distinction between process and structure is critical in designing interventions with acceptable levels of resource and economic cost, whether through land conservation, site revegetation, or corridor-scale flow releases.

Limiting factors to riparian tree recruitment on regulated rivers

A major stressor to riparian communities in Mediterranean and other aridland regions is the lack of tree recruitment on regulated rivers as a result of modifications to the flow regime. Willows and poplars are disturbance-dependent species that require flooding to regenerate. Since 2002, I have been conducting ecological studies and experiments on mechanisms of seedling establishment, including demography and competition, ecophysiology and isotope biogeochemistry (Stella et al., in review), and environmental controls on life history timing (Stella et al. 2006).

I integrate analytical methods from many disciplines, including hydrology, geomorphology and plant biology to develop recommendations for ecologically-sustainable flow regimes on rivers in California Central Valley. In addition, we are studying the response of vegetation to flow regulation on rivers in southeastern France (Ain River, Drome River) and Central Portugal (Sado River). Through this research we hope to develop a framework for river managers to slow the decline of riparian forests along regulated rivers by optimizing natural recruitment with the highest degree of certainty and lowest water cost.

Collaborators:  John Battles (University of California, Berkeley), Joe McBride (University of California, Berkeley), Hervé Piégay (French National Centre for Scientific Research), Simon Dufour (Université de Rennes)

Ecological constraints to re-establishing native trees on severely-degraded floodplains

In cases where ecosystem degradation is severe, restoring natural processes such as flow regimes may not achieve restoration goals quickly, or ever, and habitat must be created by direct intervention (e.g., re-grading floodplains, constructing wetlands, planting trees). Dredger spoils from floodplain surface gold mining are particularly difficult to revegetate in arid regions because of the near-total loss of fine sediment and moisture-holding soils. Tree establishment is often constrained by both abiotic factors (e.g., drought, soil salinity) and biotic interactions (e.g., competition, herbivory), and the relative strength of these limiting factors can shift within the first few years following restoration. I am using survival time analysis to determine the shift in factors limiting survival of native riparian trees planted within a dredged floodplain on the Merced River in California’s Central Valley. Initial results indicate that temporal shifts in mortality drivers (e.g., from initial planting size to groundwater depth) interact strongly with plant traits and may play a strong role in community assembly following restoration of highly disturbed sites.

Collaborators:  John Battles (University of California, Berkeley), Bruce Orr (Stillwater Sciences)

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Modeling Riparian Forest Dynamics on Meandering Rivers

In high-order alluvial river systems, physical factors of flooding and channel migration are particularly important drivers of riparian forest dynamics because they regulate habitat creation, mortality from fluvial disturbance, and resource fluxes of water, nutrients and light that are critical for growth. Predicting riparian forest dynamics throughout a river corridor requires coupling a mechanistic understanding of patch-scale processes with river ecosystem drivers operating at a landscape-scale. Non-equilibrium conditions, namely long-term changes in the flood regime due to flow regulation and climate change, further complicate our ability to predict riparian forest trajectories.

In an effort to help conserve critical riparian forest habitat along the middle Sacramento River, CA, we are modeling linkages between fluvial and biotic processes for Fremont cottonwood (Populus fremontii), a keystone pioneer tree in dryland river ecosystems of the U.S. southwest. With funding by the CALFED Bay-Delta Program, we have coupled a physical model of channel meandering with a patch-based population model that incorporates the climatic, hydrologic, and topographic factors critical for tree recruitment and survival. We are using this integrated model to predict corridor-wide trends in forest composition, areal extent and age structure based on likely scenarios of flow regulation, regional climate change, and floodplain development.

We are also using this patch modeling approach to predict trends in forested woodlands on other semi-arid rivers, including the Missouri River (South Dakota), which experiences high human impacts from flow regulation, and the Virgin River (Arizona and Nevada), which is recovering from widespread invasion of tamarisk, a non-native shrub.

Collaborators: Elizabeth Harper (SUNY-ESF), Alexander Fremier, (University of Idaho),
Hervé Piégay (French National Centre for Scientific Research), Simon Dufour (Université de Rennes), G. Matthias Kondolf (University of California, Berkeley), Tom Dudley(University of California, Santa Barbara), Mark Dixon (University of South Dakota)

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Forested Wetland Ecology

Landscape-scale influence of an ecosystem engineer on forest composition and structure (Adirondacks, New York)

The widespread reforestation of the Northeastern United States has been accompanied by a resurgence of beaver (Castor candensis) throughout the region, resulting in massive changes to stream and meadow ecosystems. A dynamic relationship exists between habitat site selection by beavers and the effects of beaver occupancy on landscapes over time.  Beaver site selection and occupancy are driven by the topography, hydrology and vegetation present in the landscape.  In return, these landscape features are altered by beaver presence. 

In this research, we developed and are testing a conceptual model of landscape-scale beaver impacts in the Adirondacks. We are using a 30-year monitoring record of beaver lodge occupancy in ESF’s Huntington Wildlife Forest to determine (1) landscape controls such as slope and stream length on beaver occupancy and density; and (2) the effects of beaver residence time on forest tree composition and structure. This research will help to better understand non-equilibrium effects on forest composition from a biotic ecosystem engineer that is increasing in influence throughout North American and Europe.

Collaborators: Stacy McNulty (SUNY-ESF), Ralph Nyland (SUNY-ESF),  Jake Bendix (Syracuse University)

Vernal pool construction and colonization in a Northern forest (Central New York)

Vernal pools are small, isolated, seasonal wetlands that contribute to biodiversity and provide critical habitat for amphibian and invertebrate species. They are highly vulnerable to destruction and are not protected under federal law, but their ecological role necessitates their conservation.  In areas of the Northeast where reforestation is occurring on former agricultural fields, creation of new pools may be used to complement preservation efforts.  However, for creation of vernal pools to be successful, the relationships between a number of physical and biological variables must be understood. 

The goal of this collaborative research project is to determine how the creation of a vernal pool network (~70 small pools within a 10,000-Ha research forest) affects the composition and dynamics of forest amphibian populations, aquatic invertebrates and plants. Our lab is studying the process of colonization of small gaps in the forest by understory plants, as well as effects on overstory trees of changes in local hydrology. The results of this study will inform the creation and management of vernal pools for specific conservation goals. 

Collaborators: Don Leopold (SUNY-ESF), James Gibbs (SUNY-ESF), Kim Schulz (SUNY-ESF)

Tree structure and growth in wetland forests along a hydrological gradient in southern Europe

In forested wetlands, hydrology exerts complex and sometimes compensatory influences on tree growth, particularly in semi-arid ecosystems, where water can be both a limiting resource and a stressor. We are studying the effects of site waterlogging and edaphic controls (soil nutrient content and texture) on the density, growth patterns and overall productivity of forested wetlands dominated by alder (Alnus glutinosa) and willow (Salix atrocinerea) in coastal Portugal and Spain.  We are using dendrochronology to compare radial growth in trees between sites with varying levels of waterlogging, and between trees showing various degrees of ‘shrubbiness,’ or multi-stemmed architecture. Understanding whether flooding is a subsidy or stress to wetland trees in this arid region is important in considering the conservation value of these rare forests and the impacts of human modifications to natural hydrology.

Collaborators: Patricia Rodríguez-González (Universidade Técnica de Lisboa, Portugal), Teresa Ferreira (Universidade Técnica de Lisboa, Portugal)

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Stable isotope chemistry of poplars and willows as indicators of physiological function and ecosystem change

Willows and poplars are hardy, fast-growing trees and shrubs, but they need perennial water sources in order to survive and are poorly adapted to rapid fluctuations in resource conditions. As a result, they are good indicators in wet sites of rapid changes in groundwater availability and streamflow. Our work is motivated by the high potential of stable isotopes in wood tissue of fast-growing poplars and willow to serve as (1) indicators of hydrologic change due to both natural causes and human manipulation, and (2) a tool for improving the functional response of plants used in human-constructed systems such as woody biomass production and phytoremediation. In conjunction with Dr. Mark Teece’s stable isotope laboratory at ESF, we are using stable isotopes in leaf and stemwood tissue to study the following questions:

An ecological signal of groundwater decline from instream gravel mining along the Drôme River, France

Along the Drôme River, a tributary of the Rhône River flowing through Provence, France, riparian poplar (Populus nigra) stands along the river have recently declined, with high rates of adult mortality and crown dieback. It is suspected that instream gravel mining beginning in the 1970s has lowered the baseflow of the river, potentially disconnecting floodplain trees from their perennial groundwater source.

For over a decade Dr. Hervé Piégay of Centre National de la Recherche Scientifique (CNRS) has been studying the geomorphic alteration of the river. In 2008 I began a collaboration with him to investigate whether growth patterns of the poplars declined following gravel mining, and whether changes in ring-wood chemistry indicate increased drought stress during this period. A systematic trend in carbon isotope ratio (δ13C) independent of climate would indicate that trees increase water use efficiency as a consequence of water limitation. Though these patterns have been demonstrated for riparian trees in natural systems, this approach has not been previously used to document impacts on native plant communities to human manipulation of river geomorphology and hydrology.

Collaborators: Hervé Piégay (French National Centre for Scientific Research), Mark Teece (SUNY-ESF)

 

Water relations of hybrid willows used for commercial biomass and groundwater remediation (Central New York)

Willow biomass is an environmentally sound, locally produced, renewable source of energy and bioproducts. For ~20 years, researchers at SUNY-ESF have developed willow hybrids, cultivation and harvesting methods to facilitate the commercialization of willow crops for bioenergy in the Northeast and Midwest US. In 2008 I began a collaboration with Dr. Tim Volk to investigate water relations of commercial willow varieties in order to evaluate (1) water use efficiency across a range of sites and (2) effectiveness in transpiring groundwater for phytoremediation of a severely-degraded industrial waste site. We are interested in understanding whether growth patterns and ecophysiological function are collinear across all sites, or whether there is a range of environmental conditions in which increased water use efficiency confers other benefits such as increased survival.

Collaborators: Tim Volk (SUNY-ESF, Woody Biomass Program), Mark Teece (SUNY-ESF)

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Watershed Nutrients and Stoichiometry in Multi-use Catchments

Like many catchments with complex land use patterns and development history, the Onondaga Creek watershed in Central New York (home to Syracuse and SUNY-ESF) faces unique management challenges which make it compelling for studying cumulative impacts on stream ecosystems. Dr. Karin Limburg, an aquatic ecologist at ESF, and I are investigating the patterns of instream nutrients at local and synoptic (catchment-wide) scales throughout this catchment. Nutrient concentrations are monitored and loads calculated at 40 sites seasonally; in addition, experimental nutrient additions are being conducted at a subset of sites to determine if the ratios of limiting nutrients change at point sources and tributary junctures. 

Collaborators: Karin Limburg (SUNY-ESF), Kim Schulz (SUNY-ESF)