Modeling the impacts of climate variability and hurricane on carbon sequestration in a coastal forested wetland in South Carolina

The impacts of hurricane disturbance and climate variability on carbon dynamics in a coastal forested wetland in South Carolina of USA were simulated using the Forest-DNDC model with a spatially explicit approach. The model was validated using the measured biomass before and after Hurricane Hugo and the biomass inventories in 2006 and 2007, showed that the Forest- DNDC model was applicable for estimating carbon dynamics with hurricane disturbance. The simulated results indicated that Hurricane Hugo in 1989 substantially influenced carbon storage immediately after the disturbance event. The simulated net ecosystem exchange (NEE) for the 58-year period (1950-2007) indicated that the hurricane reduced CO2 sequestration due primarily to the increased decomposition of a large amount of litter and woody debris, including fallen trees (over 80% of pre-hurricane trees), debris and branches, and dead roots. The inter-annual fluctuation of soil CO2 flux showed that the climate variability interfered substantially soil carbon dynamics in the forest. The results showed that there were substantial spatial and temporal differences in CO2 flux (3.2 - 4.8 Mg·C·ha–1) and wood biomass due to the differences in physical and biogeochemical characteristics in the forest.

Resilience of Bottomland Hardwood Stands Following Agricultural Use on the Santee Experimental Forest

Resilience is a key function that affects an ecosystem’s ability to recover from disturbance. Understanding the extent to which forest communities recover after a long period of disturbance without direct intervention is important to provide context for considering ecosystem response to disturbance regimes. Species composition and structure were recorded on bottomland hardwood stands that were once inland and freshwater tidal rice fields. We sampled 17 old-field sites and 7 reference sites across three geomorphic settings. The old-field sites ranged from 30 to 120 years since agricultural abandonment. A total of 89 species were found across the old field sites and reference sample areas. Of that total, trees comprised 33 species, shrubs—5 species, and vine/herbs/forbs—51 species. Using field data, combined with stand inventory records, aerial photography (1936-2010), and high-resolution LiDAR imagery, we chronicled the evolution of the forest since the cessation of agriculture. Our findings demonstrate how Pinus taeda seeded directly after the rice fields went to fallow;and this conversion of bottomland swamp to rice to pine was a direct result of water management embankments constructed across the landscape to aid in crop irrigation. The remnant water management features may still alter flooding patterns thereby affecting development of Taxodium distichum and Nyssa aquatica in the old-fields. These results suggest, that over the 100+ years forest stands on the Santee Experimental Forest have developed to represent bottomland hardwoods characteristic of the southeastern United States coastal plain.

Turkey Creek—A Case Study of Ecohydrology and Integrated Watershed Management in the Low-Gradient Atlantic Coastal Plain, USA

Water yield, water supply and quality, wildlife habitat, and ecosystem productivity and services are important societal concerns for natural resource management in the 21st century. Watershed-scale ecohydrologic studies can provide needed context for addressing complex spatial and temporal dynamics of these functions and services. This study was conducted on the 5240 ha Turkey Creek watershed (WS 78) draining a 3rd order stream on the Santee Experimental Forest within the South Carolina Atlantic Coastal Plain, USA. The study objectives were to present the hydrologic characteristics of this relatively undisturbed, except by a hurricane (Hugo, 1989), forested water-shed and to discuss key elements for watershed management, including water resource assessment (WRM), modeling integrated water resources management, environmental assessment, land use planning, social impact assessment, and information management. Runoff coefficients, flow duration curves, flood and low flow frequency curves, surface and ground water yields were assessed as elements of the WRM. Results from the last 10 years of interdisciplinary studies have also advanced the understanding of coastal ecohydrologic characteristics and processes, water balance, and their modeling including the need of high resolution LiDAR data. For example, surface water dynamics were shown to be regulated primarily by the water table, dependent upon pre- cipitation and evapotranspiration (ET). Analysis of pre- and post-Hugo streamflow data showed somewhat lower but insignificant (α = 0.05) mean annual flow but increased frequency of larger flows for the post-Hugo compared with the pre-Hugo level. However, there was no significant difference in mean annual ET, potentially indicating the resiliency of this coastal forest. Although the information from this study may be useful for comparison of coastal ecohydrologic issues, it is becoming increasingly clear that multi-site studies may be warranted to understand these complex systems in the face of climate change, sea level rise, and increasing development in coastal regions.

Consistency of Hydrologic Relationships of a Paired Watershed Approach

Paired watershed studies are used around the world to evaluate and quantify effects of forest and water management practices on hydrology and water quality. The basic concept uses two neighboring watersheds (one as a control and another as a treatment), which are concurrently monitored during calibration (pre-treatment) and post-treatment periods. A statistically significant relationship between the control and treatment watersheds is established during calibration period such that any significant shift detected in the relationship during treatment is attributed to the treatment effects. The approach assumes that there is a consistent, quantifiable, and predictable relationship between watershed response variables. This study tests the hypothesis that the hydrologic relationships between control and treatment watersheds for daily water table elevation (WTE) and daily flow data were similar without any statistically significant difference during two different calibration (1988-1989 and 2007-2008) and treatment periods (1995-1996 and 2009), when the control and treatment watersheds were interchanged. The watersheds are two artificially drained loblolly pine forests (D1: 24.7 haand D2: 23.6 ha) located in coastal North Carolina. Results depicted significantly similar WTE regression relationships during the two calibration periods but significantly different WTE relationships during the two treatment periods with reversed control and treatment watersheds. Calibration and treatment flow relationships, and the mean treatment effects on WTE and flow, before and after treatment reversal were significantly different (α = 0.05). The study also discusses causes of differences in hydrologic relationships and treatment effects for such reversal of treatments during a 21-year span of the study on these two similar and adjacent watersheds. The observed differences in the hydrologic relationships between control and treatment watersheds before and after treatment reversal may be attributed to climate or hydrologic non-stationarity which may affect the reliability of paired watershed approach especially when the calibration periods are short.

SWAT Model Prediction of Phosphorus Loading in a South Carolina Karst Watershed with a Downstream Embayment

The SWAT model was used to predict total phosphorus (TP) loadings for a 1555-ha karst watershed—Chapel Branch Creek (CBC)—which drains to a lake via a reservoir-like embayment (R-E). The model was first tested for monthly streamflow predictions from tributaries draining three potential source areas as well as the downstream R-E, followed by TP loadings using data collected March 2007-October 2009. Source areas included 1) a golf course that received applied wastewater, 2) urban areas, highway, and some agricultural lands, and 3) a cave spring draining a second golf course along with agricultural and forested areas, including a substantial contribution of subsurface water via karst connectivity. SWAT predictions of mean monthly TP loadings at the first two source outlets were deemed reasonable. However, the predictions at the cave spring outlet were somewhat poorer, likely due to diffuse variable groundwater flow from an unknown drainage area larger than the actual surface watershed, for which monthly subsurface flow was represented as a point source during simulations. Further testing of the SWAT model to predict monthly TP loadings at the R-E, modeled as a completely mixed system, resulted in their over-predictions most of the months, except when high lake water levels occurred. The mean monthly and annual flows were calibrated to acceptable limits with the exception of flow over-prediction when lake levels were low and surface water from tributaries disappeared into karst connections. The discrepancy in TP load predictions was attributed primarily to the use of limited monthly TP data collected during baseflow in the embayment. However, for the 22-month period, over-prediction of mean monthly TP load (34.6 kg/mo) by 13% compared to measured load (30.6 kg/mo) in the embayment was deemed acceptable. Simulated results showed a 42% reduction in TP load due to settling in the embayment.