Key words: Disturbance, Net Ecosystem Productivity (NEP), Biometric NEP, Eddy Co-variance.

Changes in climate caused by increased concentrations of carbon dioxide (CO2) in the Earth’s atmosphere have led land and ocean surface temperatures to increase by 0.85°C and sea level to increase by 19 cm relative to preindustrial times. Global climate change will lead to further alterations in mean temperature and precipitation, as well as their extremes that are likely to influence disturbance regimes. Disturbance play an important role in forest dynamics and succession, by influencing forest ecosystems structure and function, reorganizing forests by reducing live and increasing dead matter, and thus affecting ecosystem carbon (C) balances. Under a changing climate disturbances are likely to cause widespread tree mortality across forested landscapes, creating vast amounts of coarse woody debris (CWD) that will emit C to the atmosphere to a degree that regional C balances and future C dynamics are likely to change. C balance of forested regions depends on inputs in form of C sequestered by live components during growth and outputs in form of C emitted from dead components through decomposition and combustion. Live trees in many forest ecosystems represent the largest aboveground C pool and the dynamics of this pool, as controlled by growth and mortality, have been extensively studied. In contrast, few have examined either the post-disturbance fate of CWD C or assessed C storage potential of salvaged biomass despite the occurrence of multiple recent large-scale disturbance events. Biomass and C stores and their uncertainty were estimated in the Temperate and the Boreal ecoregions of Coastal Alaska using the empirical data from the Forest Inventory and Analysis (FIA) program, literature data, and modeling using standard methods employed by the FIA program. The average aboveground woody live (218.9±4.6 Mg/ha) and log (28.1±1.8 Mg/ha) biomass in the Temperate ecoregion were among the lowest in the Pacific Northwest, whereas snag biomass (30.5±1.0 Mg/ha) was among the highest. In the Boreal ecoregion, CWD biomass comprised almost 50% of the regional aboveground woody store (76.7±3.8 Mg/ha) with bark beetle damaged stands containing 82% of the total CWD biomass. In contrast, in the Temperate ecoregion, CWD comprised 20% of the regional aboveground woody store (277.5 ±5.4 Mg/ha) with 76% of total CWD biomass in undisturbed stands. Total C stores estimates in Coastal Alaska ranged between 1523.6 and 1892.8 Tg with the highest contribution from soils and the largest potential reductions in uncertainty related to the tree and soils C pools. The impact of a large-scale spruce bark beetle (SBB) outbreak on aboveground dead wood C dynamics on the Kenai Peninsula was modeled utilizing data from the FIA program and CWD decomposition rate-constants from a chronosequence and decomposition-vectors analysis. Decomposition rate-constants from the chronosequence ranged between -0.015 yr−1 and -0.022 yr−1 for logs and -0.003 yr−1 and +0.002 yr−1 for snags. Decomposition rate-constants from the decomposition-vectors ranged between -0.045 yr−1 and +0.003 yr−1 among decomposition phases and -0.048 yr−1 and +0.006 yr−1 among decay classes. Relative to log generating disturbances those creating snags delayed C flux from CWD to the atmosphere, produced a smaller magnitude C flux, and had the potential to store 10% to 66% more C in a disturbed system over time. The effect of several management strategies ranging from "leave-as-is" to "salvage-and-utilization" on C stores and emissions following SBB outbreak on Kenai Peninsula, Alaska was evaluated. A forest with immediate post-disturbance regeneration reached pre-disturbance C stores faster than one with delayed regeneration. Lack of regeneration, representing a loss of tree cover on the disturbed portion of the landscape, caused a permanent decrease in wood C stores. Among the "salvage-and-utilization" scenarios considered, biomass fuel production with substitution for fossil fuels created the largest long-term C storage assuming the substitution was permanent. Given that reduction in near-term emissions may be a more robust strategy than long-term ones, the "leave-as-is" scenarios may represent the most feasible way to mitigate global climate change following disturbance.

Understanding the differences in carbon and nitrogen distribution and cycling both spatially and temporally using various approaches is essential in forest ecosystems. In addition, the influence of biotic and abiotic factors as well as natural and artificial disturbances on carbon and nitrogen cycling need to first be understood before drawing their implications to forest management practices. This Special Issue aims to understand carbon and nitrogen distribution and cycling in forest ecosystems for ecosystem-based forest management under different natural and artificial disturbances.

This assessment provides input to the reauthorized National Integrated Drought Information System (NIDIS) and the National Climate Assessment (NCA), and it establishes the scientific foundation needed to manage for drought resilience and adaptation. Focal areas include drought characterization; drought impacts on forest processes and disturbances such as insect outbreaks and wildfire; and consequences for forest and rangeland values. Drought can be a severe natural disaster with substantial social and economic consequences. Drought becomes most obvious when large-scale changes are observed; however, even moderate drought can have long-lasting impacts on the structure and function of forests and rangelands without these obvious large-scale changes. Large, stand-level impacts of drought are already underway in the West, but all U.S. forests are vulnerable to drought. Drought-associated forest disturbances are expected to increase with climatic change. Management actions can either mitigate or exacerbate the effects of drought. A first principal for increasing resilience and adaptation is to avoid management actions that exacerbate the effects of current or future drought. Options to mitigate drought include altering structural or functional components of vegetation, minimizing drought-mediated disturbance such as wildfire or insect outbreaks, and managing for reliable flow of water.

"Boreal forests contain large amounts of soil carbon and are susceptible to periodic wildfires. Predicting the response of soil carbon dynamics to fire disturbance requires understanding: (1) the environmental factors governing CO2 efflux; (2) the extent to which fire alters these factors; and, (3) the length of time over which these perturbations persist. In interior Alaska seasonal patterns of CO2 efflux, soil temperature. and soil moisture potential were measured in burned and control pairs of aspen, white spruce, and black spruce stands. Averaged over the growing season, mean CO2 efflux from burned stands (0.51 ± 0.26 g CO2 m−2 hr−1) was two-thirds that of control stands (0.77 ± 0.44 g CO2 M−2 hr1). Soil temperature explained 85 to 90% of the seasonal variability in the control, whereas moisture was a more important determinant in burned stands. Laboratory incubations of recently burned and control humic material indicate that changes in substrate chemistry and increased temperature may enhance rates of decomposition by a factor of 2.2 to 2.8 in the first decade after fire, resulting in a release of 6.3 to 13.4 Mg C ha−1 to the atmosphere. Under saturated moisture conditions, respiration from mosses may contribute 16 to 50% of total soil CO2 emissions. In a 140-year age-sequence of burned black spruce stands, CO2 efflux increased at an average rate of 8.3 kg C ha−1 yr1 up to a maximum of 1.83 Mg C ha−1 yr1. During this same time, accumulation of carbon in organic horizons ranges from 0.34 to 0.50 Mg C ha−1 yr1 and the ratio of microbial to root respiration decreased from 76:24 to 13:87. Numerical modeling of carbon accumulation suggests that these soils functioned as a net source of carbon for the first 7 to 15 years after fire and released 1.8 to 11.0 Mg C ha−1 to the atmosphere. Although conservative, these estimates of post-fire biogenic emissions are on the same order of magnitude as carbon losses during combustion itself, suggesting that current models may underestimate the impact of fire in northern latitudes by as much as a factor of two"--Leaves iv-v.