Grantee Research Project Results

2009 Progress Report: Elevated Temperature and Land Use Flood Frequency Alteration Effects on Rates of Invasive and Native Species Interactions in Freshwater Floodplain Wetlands

EPA Grant Number: R833837
Title: Elevated Temperature and Land Use Flood Frequency Alteration Effects on Rates of Invasive and Native Species Interactions in Freshwater Floodplain Wetlands
Investigators: Richardson, Curtis J. , Qian, Song S. , Flanagan, Neal
Current Investigators: Richardson, Curtis J. , Flanagan, Neal , Ho, Mengchi
Institution: Duke University , Nicholas School of the Environment and Earth Sciences
EPA Project Officer: Packard, Benjamin H
Project Period: April 1, 2008 through March 31, 2011 (Extended to March 31, 2012)
Project Period Covered by this Report: April 1, 2009 through March 31,2010
Project Amount: $598,107
RFA: Ecological Impacts from the Interactions of Climate Change, Land Use Change and Invasive Species: A Joint Research Solicitation - EPA, USDA (2007) RFA Text | Recipients Lists
Research Category: Aquatic Ecosystems , Ecological Indicators/Assessment/Restoration , Climate Change

Objective:

The primary objective is to assess how predicted climate and land use driven changes in hydrologic flux and temperature regimes of floodplain ecosystems affect plant communities in terms of their vulnerability to the establishment and spread of invasive species, and in turn ecosystem functions and services. Future climate scenarios for the southeastern U.S. predict that surface water temperatures will warm (in concert with air temperature) and that stream flows will likely decrease, with a greater proportion of annual watershed hydrologic yield occurring during major storm events. Land use changes (urban vs. forested etc.) have been shown to also raise water temperature and increased pulsed water releases during storms. Specifically, we focus on the relationships between native species composition, diversity, productivity, and invasibility of floodplain ecosystems affected by alterations of water temperature and annual hydrographs driven by climate change and land use change (urban, forested and agricultural). We will use a combination of varying scale experimental studies and one novel large-scale regional study to verify our experimental and threshold modeling results.

Progress Summary:

Experimental Level 1 (elevated temperature and pulsed water). The runoff from an urban watershed (58 ha) provides conditions for examining the effects of temperature and hydrographs on changes in plant community composition, species invasions, and productivity and nutrient dynamics not possible in more unregulated settings. To address climate change effects we have constructed a symmetrical stream and restored a riparian wetland treatment ecosystem that is divided into 3 connected wetland cells down each side of the restored stream. One side of this uniquely designed treatment floodplain is heated, and comparisons in the behavior of the two halves (heated and unheated water) will be used to test for the hypothesized shifts in species composition, diversity and ecosystem thresholds. Water is drawn from the adjacent stream, heated and then circulated via pipes lying at the ground wetland surface, but beneath at least 15 cm of water. Water levels are maintained in each cell with flashboard risers. The system is analogous to hot water under floor heating technologies where a temperature controller relays for more warm water when the set temperature is not maintained. All heated water is re-circulated, in a closed system such that heat will dissipate into the treated systems without the addition of any actual water.

The experiment design consists of matched cells, each approximately 20m x 5m, one heated and one unheated (Figure 1). The heated cell has a grid of 4.8 m – 4.5m x 1.95 cm diameter copper pipes that re-circulate heated water pumped from an 1892 liter (500 gallon)

Figure 1. A depiction of the experimental cells in the Duke Forest SWAMP site used for the wetland cell water heating treatments

holding tank to the grid by selective valves. Each valve controls 4 heated pipes that are 4 cm above the soil/sediment layer within a naturally flooded zone. Two temperature monitors in the reference cell control the valves to keep the temperature 3-5 ºC higher in the heated cell. There is a gradual slope in each cell; regulating the outlet standpipe can control the water depth in each cell. Water for the storage tank is heated automatically by solar panels (Silicon Solar Evacuated Tube collectors) and the heated water is re-circulated as needed. A photovoltaic cell powered pump (Swiftech #MCP655) circulates water from the tank to the solar tube collectors’ and back to the tank. The heated tank water is pumped into the grid from the tank by a solar recharged battery-powered pump (Shurflow #2088) and is controlled by four latched valves (Alcon series #985A-243N). The valves are controlled by a Campbell Scientific controller (CR1000) that monitors the temperature of the reference and heated cells and controls the heating grid valves to maintain a temperature difference of 3-5 ºC on the heated wetland cell compared to the ambient cells. The batteries, Shurflow pump; Campbell controller and latched valves are contained in a large construction site locked storage bin to provide security and weather protection.

The temperature elevation to be utilized above background in our initial tests is 3-5 °C; a predicted range of stream change from global climate effects (IPCC 2001), but the actual elevated temperature range that can be held is currently being determined with test trials. One test run has been completed and initial trials indicate that warmer water (5° ± 1) above background can be maintained in the spring and summer months. Over the next year we will elevate stream and wetland surface water temperatures and model the rates of species invasions, plant community shifts, determine threshold conditions as well as assess differences in nutrient inputs, outputs, retention rates and transformation throughout a warmed wetland compared to a non-warmed wetland within the SWAMP site in Duke Forest. The water quality and flow dynamics of this stream have been monitored for four years prior to restoration thus providing a long-term basis for assessing upstream watershed inputs, temperatures, and flood frequencies.

Level 2 Experimental Level 2 (regional floodplain hydrology and temperature shifts). We have identified nine synoptic sites located on river flood plains throughout the North Carolina and southern Virginia (Roanoke River Basin). These sites are in two categories: a) three sites are located downstream of reservoirs used as flood storage and cooling water supplies for coal-fired or nuclear power plants, and where outflow is drawn from warmer surface water layers of the reservoir [Cowan’s Ford, Kerr and Gaston Dams], and b) three sites are located downstream of hydroelectric plants where outflow is drawn from deeper (cooler) strata nearer the lake bottom [Catawba, Smith Mountain, and Philpott Dam (VA)] (Figure 2).

Figure 2. A map of the riparian study site locations on cold, warm and reference streams in Virginia and North Carolina.

Floodwaters in the first groups of sites are significantly warmer than those from the second group by > 5° C degrees. For example, during most summer periods, the water temperatures released from the sites in the cold treatments (Smith Mountain Lake, Philpot Dam, Catawba Dam) plant are less than the 20ºC, while summertime water temperatures at warm treatment sites exceed 25o (Figure 3). We use differences in temperature and hydroperiod [flood storage dams versus hydroelectric dams] to examine effects of temperature and flood frequency on floodplain riparian plant communities and ecosystem functions, with an emphasis on the effects of warmer water on invasive species and plant community shifts.

Figure 3. Mean +/- SD June surface water temperatures measured during site visits.

Our sites were divided into two vegetation zones, emergent and riparian. The emergent zone begins at the lower boundary of persistent emergent vegetation and is dominated by sedges, reeds, and mudflat species. The riparian zone occurs at a slightly higher elevation were the plant community is dominated by grasses, clonal dominant species, and occasional shrubs. The operational boundary of the emergent and riparian plant zones was an elevation of 20 centimeters above the lower boundary of the emergent zone. The upper boundary of the emergent zone is a typically abrupt line where woody species become dominant (Figure 4).

Figure 4. Site layout showing location of temperature probes and water level recorders within the vegetation zones occurring at our riparian sites.

Soil Temperature

Soil temperature is being measured at two different elevations at each project site.

Temperature loggers have been installed at the lower limit of emergent herbaceous vegetation at each site (marsh zone). A second temperature logger has installed at an elevation approximately 20 centimeters higher (riparian zone). Soil probes are located in areas that are not under a woody canopy and placed at a probe depth of 10 centimeters with temperature logged every 15 minutes.

Soil temperatures are being used to calculate accumulated soil growing degree days

(SGDD) that are being used to compare the thermal balances between warm and cold riparian sites. As in agricultural systems, soil temperature is often used in ecology as an indicator of the effects of climate on plant productivity and competition. Because of highly variable seasonal and diurnal patterns of temperature fluctuation we used soil-growing degree-days to produce a measure of accumulated heat to be used for comparing the temperature regimes of sites within our three temperature treatments (warm, cold and reference). These results are summarized in Figure 5 where clear differences in the heat accumulation within the sites belonging to our three treatments. Figure 6 presents both time series of soil temperature and accumulated SGDDs from three representative sites used in our study. This figure shows substantial differences in the heat accumulation at our cold, warm, and reference sites (640, 1050, and 1380 respectively).

Figure 5. Mean +/- SD soil temperature expressed as soil growing degree days (SGDD) by treatment.

Figure 6. Typical time series of soil temperature and accumulation of soil growing degree days from sites representative of the Warm, Cold and Reference treatments.

Hydrology

We have installed a water-level recorder at each site and are evaluating flood depth and return intervals using leveling survey techniques to estimate to relative elevation of plant survey points and the zero datum of the water level recorder so that the water level recorder data can be used to predict when plant survey plots are submerged or exposed (Figure 7). Table 1 presents a summary of average hydrologic parameters by treatment. These include the number of flood events, average duration of individual flood events, cumulative annual flood duration, average return period, and the parameter “pulse" we use to describe flood pulse or the average rate of increase in water depth during rising stage (mm/hour). Table 1 shows sites in the warm and cold treatments have what could be characterized as more “pulsing” hydrology with more frequent flood events but shorter flood duration, and smaller cumulative flood duration. The parameter pulse also suggests greater flood energy at the cold and warm treatment sites than at the reference sites. These differences could be explained by the absence of dams upstream of the Reference sites.

Figure 7. Representative hydrographs from sites in the a) Warm, b) Cold and c)

Reference treatments showing water level fluctuations relative to a fixed local datum (mounting post) during the 2009 water year.

Table 1. Summary of hydrologic parameters

Plant Communities and Environmental Factors

Plant surveys were performed in the fall of 2008 and twice during the 2009 field season on all regional wetland sites. Surveys were performed on 10 meter transects using line intercept methods. The elevation of the beginning, middle and end points of each transect was recorded using survey equipment and related to a fixed datum on the site water-level recorder.

Three replicate soil samples were collected from each vegetation zone in the fall of 2008. Soil samples were stored at 4o C between sampling and analysis (less than 1 week). Half of each core was dried at 105oC for 24 hours to determine the moisture content. The remaining half was held at field moisture and passed through a sieve (2 mm). Soil and sediment pH were measured with a glass electrode in a 1:2 soil: water slurry. The field-moist sieved soil was analyzed for 2M KCl extractable nitrate+nitrite –N (NO3-N) and ammonium (NH4-N) on a Bran and Lubbe TRAACS autoanalyzer and for soluble organic carbon and nitrogen with a Shimadzu TOC 5000 solution C analyzer equipped with a TN module. Total C and N content were measured (Carlo-Erba NA 1500 CNS analyzer) and results expressed on a per gram dry soil/sediment basis.

Figures 8ab summarizes mean species richness by treatment and vegetation zone.

Species richness was highest at the Warm treatment sites, intermediate at the Reference sites and lowest at the Cold sites. The vegetation zones displayed substantial differences in mean species richness (Figure 8a) with higher species richness observed in the higher elevation riparian zone than in the emergent zone. We also used the portion of individual plant stems classified as nonnative as an indicator of the “degree of invasion” at our sites (Figure 8b). The degree of invasion was highest in our Cold treatment, moderate in the Reference treatment, and lowest in Warm treatment. There was little difference in the degree of invasion between the Riparian and Emergent zones. The results presented Figures 8 and 9 suggest plant communities at our Warm sites are have the highest species richness and lowest degree of invasion, while the inverse is true for the Cold site, and Reference sites are somewhere between the Warm and Cold treatments.

Figure 8ab. Mean +/- SD by temperature treatment and hydrologic zone of a) species diversity and b) percent of counted stems classified as invasive species.

To explain these trends, we used several statistical approaches to evaluate relationships between plant community data and environmental variables monitored at our sites. Random Forest models were used to examine the strength of relationship between a single site-specific measure of alpha diversity and a suite of environmental (independent) variables to identify those with the greatest predictor importance at influencing model accuracy. The Random Forest model explained 51% of variance in species richness and 36% of variance in non-native stem density. The importance values for our independent variables are presented in figure 9.

Figure 9. The importance of independent variables in explaining measures of site alpha diversity including: a) species richness and b) degree of invasion.

Figure 9a shows that variables related to hydrology (flood duration, Standard Deviation of depth, flood frequency) were the most important factors explaining species richness, followed by SGDDs. Temperature (SGDD) was by far the most important predictor of degree of invasion followed by hydrology and exchangeable nitrogen (Figure 9b). The most important hydrologic variable explaining degree of invasion was “pulse” (see Hydrology section)

To compare community composition between sites, we examine patterns of beta diversity and their relationship to environmental factors we used non-metric Multi Dimensional Scaling (NMDS). Figure 10 shows plot of plant community dissimilarity from both emergent and riparian communities from our Warm, Cold and Reference sites. Points that are plotted close together have similar plant community composition. The sizes of the plot symbols are proportional to species richness. Vectors representing environmental variables are overlain on this plot. The length and orientation of these vectors are proportional to the strength of correlation with the X and Y-axes. The plot suggests two primary gradients associated with plant community composition. The X-axis is associated with a gradient between sites with high SGDD and low pulse on the left side of the figure and sites with low SGDD and high pulse on the right hand of the figure. On the vertical axis is a gradient between sites with high nutrient content (N, extractible P, and carbon) and low sand content at the top of the diagram and sites with high sand content and low nutrient content at the bottom of the figure. There is a general trend of high species richness at warm sites without pulsing hydrology, low nutrient availability and high sand content. Species richness was relatively low at cool sites with pulsing hydrology, high nutrient availability and fine soil texture. The lowest species richness is in the upper right sector of the plot where sites have pulsing hydrology, lower SGDD accumulation, higher nutrient availability, and lower sand content in soils.

Figure 10. Plot of site community similarity by treatment (Warm, Cold and Reference) and zone (Emergent and Riparian) using non-metric multidimensional scaling. Degrees of correlation of environmental variables with the axes are shown as vectors whose relative R2 values are expressed by length.

Expected Results. At the end of this study we will have an extensive dataset across multiple years and several spatial scales that can be used to explore the feedbacks occurring in floodplains where invasive species have become established, and how these feedback processes enhance the ability of invasive species to persist and spread? We propose several powerful statistical techniques to relate changes in abiotic variables to shifts in community composition and ecosystem functions. Bayesian multilevel change point analysis a statistical modeling methodology for detecting abrupt changes in variables of interest will be used to explore factors influencing the variability in threshold for species shifts in each community.

Future Activities:

We will continue data collection using our established protocols and applying adaptive management approaches as required. In year three we will present preliminary results at national meetings and prepare manuscripts for submission to peer-reviewed scientific journals.

Journal Articles:

No journal articles submitted with this report: View all 9 publications for this project

Supplemental Keywords:

wetlands, watershed, land use, climate change, invasive species, temperature shifts, pulsed water, water quality, altered stable states, nonlinear thresholds., RFA, Ecosystem Protection/Environmental Exposure & Risk, Air, Scientific Discipline, Ecological Risk Assessment, Atmosphere, Regional/Scaling, Monitoring/Modeling, Air Pollution Effects, Atmospheric Sciences, Hydrology, climate change, Environmental Monitoring, invasive species, biodiversity, Global Climate Change, ecosystem assessment, climate model, water quality, coastal ecosystem, global change, atmospheric chemistry, ecological models, coastal ecosystems, climate models, environmental measurement, climate variability, habitat preservation, environmental stress, UV radiation, habitat diversity, anthropogenic, meteorology, land use, regional anthropogenic stresses, greenhouse gases

Relevant Websites:

Duke University Wetland Center website www.env.duke.edu/wetland Exit

Progress and Final Reports:

Original Abstract

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.