Disturbance

Ecological resilience is difficult to define and monitor, yet the concept is critical for conservation and landscape management. When monitored from space, undesirable vegetational trends can be obscured by normal disturbance, climate variability and seasonal phenology. The cumulative effects of multiple drivers of change adds further complexity, as does the need to monitor broad areas and long time periods to gain context. We explore this need and these challenges using an 11-year 8-day time series of the Normalized Difference Vegetation Index (NDVI) for the conterminous United States based on MODIS satellite technology. We document responses to a range of rapid to gradual and severe to mild disturbances including wildland fire, insect defoliation, development and wind. This demonstrates the value and limits of using high frequency, coarse spatial resolution datasets for defining and monitoring landscape resilience.

The National Early Warning System (EWS) provides an 8-day coast-to-coast snapshot of potentially disturbed forests across the U.S. The EWS has produced national maps of potential forest disturbances every eight days since January 2010, identifying locations that may require further investigation. Through phenology, the system detects all types of unexpected forest disturbances, including insects, disease, wildfires, frost and ice damage, tornadoes, hurricanes, blowdowns, harvest, urbanization, landslides, drought, flood, and climate change. The EWS uses differences in phenological responses between an expectation based on historical data and a current view to strategically identify potential forest disturbances and direct attention to locations where forest behavior seems unusual. Disturbance maps are available via the Forest Change Assessment Viewer (FCAV) (http://ews.forestthreats.org/gis), which allows resource managers and other users to see the most current national disturbance maps as soon as they are available. Unsupervised statistical multivariate clustering of smoothed phenology data every 8 days over an 11-year period produces a detailed map of national vegetation types, including major disturbances. Using spectral unmixing methods, national maps of evergreen decline can be produced which are a composite of insect, disease, and anthropogenic factors causing chronic decline in these forests, including hemlock wooly adelgid, mountain pine beetle, wildfire, tree harvest, and urbanization.

After causing massive mortality in lodgepole pine forests west of the Continental Divide, epidemic populations of mountain pine beetle (MPB) are threatening additional forest types on the eastern slope of the Rockies. In Colorado's Front Range, we investigated the resilience of mixed-conifer and ponderosa pine ecosystems to MPB in a collaborative study across lands managed by 9 federal, state, county, and city agencies. Throughout a 200-mile area in 2009-11, we measured MPB activity and tree/stand/site characteristics on 57 half-acre transects in mixed-conifer stands, and 36 ten-acre plots in ponderosa-dominated stands with diverse previous management histories (thinned, burned, or no treatment). Our results revealed considerable spatial and temporal variation in MPB impacts across the landscape, but two major trends: 1) a striking north-south gradient of MPB-caused mortality in mixed-conifer forest (50.9 n 7.5 % basal area (BA; mean n SE) of all pines killed in northern sites, but only 14.1n 1.5 % BA killed in southern sites); 2) less MPB activity in ponderosa stands treated by thinning (1.19 n 0.76% trees per acre (TPA) killed) vs. prescribed fire (14.00 n 7.6% TPA) vs. no treatment (4.12 n 3.99% TPA). Our findings will inform adaptive management by agencies and communities responding to MPB outbreaks in many western states.

Although disturbance change receives less attention than climate change, disturbance change has caused widespread declines in disturbance-dependent oak and pine forests. Drought due to climate warming may cause stress similar to fire and thus, examination of disturbance change may help predict how species will respond and ultimately how ecosystems will alter under novel climate conditions. We compared changes in composition, distribution, site factors, and density of 21 tree species or species groups in the Missouri Ozarks. In upland forests, dominant oak species have been replaced by hickories, elms, eastern redcedar, maples and a variety of other shade-tolerant species, with specific changes by subsection. Distribution maps and ordinations showed homogenization of distribution, due to expansion of less common species from ecological subsections along the Missouri and Mississippi Rivers. Important site factors for species have changed over time, as mesic species increased in drier areas. Density has doubled in mesic forests compared to oak and pine-dominated forests. Recognition of the on-going regime shift to an alternative stable state is important for all predictive models. Mesic forests are composed of numerous species, unlike oak- and pine- dominated forests, which will make mesic forests stable and resilient. Disturbance change influenced the effectiveness of life history strategies, by shifting relevance to aboveground growth in competition for light rather than root reserves to survive stress. Mesic species share similar shade-tolerant and fire- and drought-intolerant traits, which may indicate that composition will be determined primarily by competitive dynamics in the future rather than drought disturbance, except in areas that intensify drought. Based on the effects of disturbance change, climate warming should influence future forest composition more than drought.

Multiple interacting stressors are generating unprecedented challenges to ecosystem resilience, necessitating efforts to understand how ecosystems will respond to concurrent biotic and abiotic changes. To address this need, we examined the effects of Eastern hemlock (Tsuga canadensis) loss due to an exotic insect on nitrogen retention at three elevations (low, mid, high) subject to increasing atmospheric nitrogen deposition in mixed hardwood stands in western North Carolina. We found that nitrogen pools and fluxes varied substantially with elevation: total forest floor and mineral soil nitrogen increased and forest floor and soil carbon to nitrogen ratio decreased with elevation, suggesting that these high elevation pools are accumulating available nitrogen. Contrary to expectations, subsurface leaching of inorganic nitrogen was minimal overall and was not higher in stands with hemlock mortality. Moreover, although nitrogen loss increased with nitrogen availability in reference stands, there was no relationship between nitrogen availability and loss in stands experiencing hemlock decline. Higher foliar nitrogen and observed increases in the growth of hardwood species in high elevation stands suggest that hemlock decline has stimulated nitrogen uptake by healthy vegetation within this mixed forest, and may thereby contribute to decoupling the relationship between nitrogen deposition and ecosystem nitrogen loss.

Temperate forests sequester a significant amount of atmospheric carbon (C), storing much of it in aboveground woody stems of living and dead trees. Biological disturbances, such as insect defoliation of forest canopies, have the potential to shift forest ecosystems from net carbon sinks to sources over interannual time-scales, yet their effects on C sequestration remain poorly characterized over decades or centuries. We simulated changes in temperate forest composition and C storage over 400 years under novel disturbance regimes that included wind, harvesting, a native defoliator, and the introduced and destructive European gypsy moth using LANDIS-II. Simulations predict that the forest mosaic in the eastern U.S. study area will maintain a net increase in woody increment over the next 70-80 years, likely providing a sink for atmospheric carbon, over a range of potential harvest and insect disturbance regimes. Beyond that time range, the potential C sink for the landscape is much weaker, providing less of a buffer for increases in atmospheric CO2. Insect defoliation significantly reduced aboveground biomass and changed trends in forest composition, unexpectedly by slowing the expansion of shade tolerant species such as P. strobus and A. saccharum, even though they are less favored by gypsy moth.

Disturbance and succession in forest ecosystems have been highly altered by human land-use change and associated fire frequency changes due to fire suppression efforts. We assessed the effects of human development patterns on fire and forest composition in the pinelands of New Jersey. These assessments showed lower fire frequency and higher transitions from pine to oak forest cover closer to human altered land; an area we describe as the ecological wildland urban interface. We used a spatially-explicit forest disturbance and succession model LANDIS-II to investigate how scenarios of altered land and modern fire regimes, including the area of reduced fire in the ecological wildland urban interface, may affect forest composition in the future. We used a process based ecosystem model PnET-II for LANDIS-II to incorporate climate change as an additional disturbance forcing. All scenarios showed an overwhelming trend toward oak dominated forest within 100 years due to a continued decline in wildfire. The potential of this type of dramatic shift from pine to oak cover represents a radical departure from current forest composition. Managers of the Pinelands National Reserve are beginning to consider the possibility of this shift in canopy cover and the difficulties involved in maintaining the essential pinelands landscape.

Climate change in the Greater Yellowstone Ecosystem is predicted to increase fire frequency dramatically, from 100-300 years to <30 years. More frequent fires will alter carbon (C) stocks by reducing the amount of C stored in biomass and soil and, potentially, by shifting vegetation distribution. However, the thresholds of fire frequency that could shift heterogeneous landscapes from C sinks to C sources are not known. Using downscaled climate projections and a dynamic ecosystem process model, we simulated that fire intervals <90 years will cause forests to shift from a net C sink to C source because the time between fires would be less than the time required to recover the C lost to fire. The capacity for post-fire regeneration of lodgepole pine and the projected increase in lodgepole pine productivity under warmer climate would not counter the consequences of reduced fire-return intervals. The magnitude of this shift depends on the future distribution of forest and non-forest ecosystems, fuels, ignition factors, and the accuracy of fire-climate relationships as future climate diverges increasingly from the past. Science-management partnerships should be encouraged to foster understanding of vegetation recovery capacity, patterns, and variability; early warning signals and indicators; and no-regrets strategies.

The Energy Independence and Security Act of 2007 (EISA) mandated the U.S. Department of Interior to assess carbon storage, carbon sequestration, and fluxes of other greenhouse gases (GHG) for ecosystems of the United States. The U.S. Geological Survey developed a methodology to quantify baseline (current) carbon sequestration and GHG fluxes, and to evaluate carbon sequestration potential and GHG fluxes for multiple future scenarios. The baseline component relied on existing inventory and land-use data to analyze spatial distributions of carbon stocks and GHG fluxes. Potential future conditions were analyzed using IPCC scenarios. Qualitative storylines and quantitative proportions of land use and land cover (LULC) were downscaled to individual ecoregions for each scenario. The FORE-SCE model was then used to produce spatially explicit LULC projections based on each IPCC SRES scenario. A separate disturbance model was used to model fire occurrence and distribution for each scenario. Integrated LULC and disturbance projections were used by the General Ensemble Modeling System (GEMS) to analyze future carbon stocks and GHG fluxes for each scenario. Our scenario-based LULC and disturbance projections have the potential to support analyses of other ecosystem processes, including impacts of LULC change on hydrology, biodiversity, and socially or economically important ecosystem services.

Understanding landscape disturbance patterns is critical for the conservation and management of fire-adapted ecosystems. To that end, we explored metrics that quantify landscape heterogeneity in topography as it shapes patterns of burning. First, we assessed the utility of metrics commonly used to quantify topographic roughness including the ratio of surface area to planimetric area, and found inherent difficulties due to confounding of variations in slope length and steepness. Past studies have established the potential value of fractal dimension to characterize surface roughness, its scale invariance providing advantage when comparing across ecosystems. Thus, our second goal was to evaluate several estimators of fractals for spatial data using open source software (R fractaldim package). We ultimately adopted the transect increment algorithm, a variance method based on second differences. To demonstrate its utility, we analyzed the relationship between fire size (75th quantile of the distribution) and topographic roughness (i.e., fractal dimension) along a North-South gradient spanning the US Rocky Mountains. Fire size was limited by topographic roughness at the lower range of fire sizes, consistent with the idea that the largest fires burn in extreme conditions unrestrained by rugged topography.