Ecosystem Service Analysis of Duke Forest

Abstract

Our team was tasked with evaluating the quantitative and monetary value of ecosystem services offered by the Duke Forest. Our client, the Duke Forest, manages and actively harvests 7,100 acres of timberlands used for research, education, and recreation by Duke University and the broader community. The overall purpose of assessing these services is to communicate the importance of the Duke Forest and offer implications for resource management. The term “ecosystem service” refers to benefits humans obtain from nature, and it is categorized into four different services; provisioning service; regulating service; supporting service; cultural service. Based on the client’s requests, we analyzed a subset of ecosystem services provided by the Duke Forest – carbon storage and sequestration, which have an important implication for climate change mitigation, and nutrient and sediment retention, which contribute to downstream water quality improvement. For spatial analysis of the focal ecosystem services, we used the InVEST suite of models, developed by the Natural Capital Project at Stanford University. We used the InVEST Carbon Storage & Sequestration model to spatially assess carbon storage and sequestration in the Duke Forest. For the land cover/ land use data input, we used spatial forest class and age data provided by the client. We referred to a USDA study to estimate carbon storage for the different forest types and age classes in the spatial data input and to populate the carbon pool table, another input of the InVEST carbon model. The monetary values of carbon storage and sequestration were estimated with the average carbon credit value for forestry projects from the World Bank, as well as with two domestic markets: the California Cap and Trade (CaT) and Regional Greenhouse Gas Initiative (RGGI), a regional northeastern US market. For assessment of water quality improvement, we ran the InVEST Nutrient Delivery Ratio (NDR) and Sediment Delivery Ratio (SDR) models to estimate phosphorus, nitrogen, and sediment export across four 10-digit HUC watersheds which Duke Forest occupies. Model calculations are determined by hydrological modelling, as well as biophysical statistics on a variety of land use/land cover classes. SDR results were used to produce a monetary estimation of Duke Forest’s contribution to sediment retention using estimates of Neuse River water treatment facility cost savings from reductions in turbidity. InVEST Carbon modelling estimated a total of 543,000 tons of carbon being stored across all Duke Forest divisions at an average of 80 tons per acre. The highest storage rates were observed in the Oosting Natural Area at 94 tons per acre and the lowest storage rates were seen in the Hillsboro division at 71 tons per acre. Using the value of carbon offset projects from terrestrial forests globally, this total storage is estimated to be worth over $15 million in value. In terms of domestic carbon offset markets across all projects, this value is estimated to be even greater: ranging from $17.3 to 35.8 million. Our future projections of carbon for the next 50 years revealed an estimate of 2,000 tons being stored yearly, equaling about $56,000 in monetary value using the global estimate for forestry offset projects. Results from NDR and SDR indicated Duke Forest’s contribution to downstream water quality protection and improvement. NDR estimated nutrient export rate in the Duke Forest is significantly lower than the watershed average. Average nitrogen export values in the Duke Forest in each of the four watersheds were lower than the average value in the watersheds by 25.7% - 44.7%. Mean phosphorus export values in the Duke Forest were lower than the watersheds by 67.3% - 83.1%. Similarly, SDR estimated sediment export rate in the Duke Forest significantly lower than the watersheds, by 78.8% ~ 98.4%. The monetary value of sediment retention based on turbidity reduction was estimated to be worth $43,000 and $113,000 annually in two different alternative land use scenarios. The greatest annual value was found in the B Everett Jordan Lake – New Hope River basin, where Duke Forest’s sediment buffering was valued at $26,000 and $50,000 in the two scenarios. For communication of significance and key results of this project to a broader audience, we developed a StoryMap on ArcGIS Oline. This StoryMap includes a brief description of the Duke Forest, an introductory explanation of ecosystem services, and key results from our analysis. It uses plain language and visual materials so audiences without a strong background can become interested in and grasp the benefits the Duke Forest provides the larger region. Future work on ecosystem service analysis in Duke Forest should focus on collecting accurate field data to refine the biophysical statistics which drive all the models we ran, rather than using values found in the literature. In addition, assessment of other ecosystem services offered by the Duke Forest would complement the results of this analysis. Final recommendations for the client include conservatively managing older stands with high carbon stocks, tracking opportunities to become involved in carbon offsets, and mitigating erosion during timber harvests.