Downed Coarse Woody Debris Dynamics in Ash (Fraxinus spp.) Stands Invaded by Emerald Ash Borer (Agrilus planipennis Fairmaire)

Emerald Ash Borer Management and Research: Decades of Damage and Still Expanding

Since the discovery of the ash tree (Fraxinus spp.) killer emerald ash borer (EAB; Agrilus planipennis) in the United States in 2002 and Moscow, Russia in 2003, substantial detection and management efforts have been applied to contain and monitor its spread and mitigate impacts. Despite these efforts, the pest continues to spread within North America. It has spread to European Russia and Ukraine and is causing sporadic outbreaks in its native range in China. The dynamics of EAB's range expansion events appear to be linked to the lack of resistant ash trees in invaded ranges, facilitated by the abundance of native or planted North American susceptible ash species. We review recently gained knowledge of the range expansion of EAB; its ecological, economic, and social impacts; and past management efforts with their successes and limitations. We also highlight advances in biological control, mechanisms of ash resistance, and new detection and management approaches under development, with the aim of guiding more effective management.

Potential impacts of emerald ash borer and adaptation strategies on wildlife communities in black ash wetlands

Black ash wetlands cover approximately 1.2 million ha of wetland forest in the western Great Lakes region, providing critical habitat for wildlife. The future of these wetlands is critically threatened by a variety of factors, including emerald ash borer (Agrilus planipennis; emerald ash borer [EAB]), which has been eliminating native populations of otherwise healthy ash throughout the Great Lakes region since it was discovered in 2002. To quantify the potential impacts of tree mortality from EAB on wildlife communities, we measured seasonal bird, mammal, and amphibian diversity in black ash wetlands using a dual approach: (1) documenting bird and amphibian species across 27 mature reference black ash wetlands in northern Minnesota, USA and (2) assessing how bird, mammal, and amphibian communities respond to experimental manipulations of black ash forests that emulate mortality and management strategies related to the potential impact of EAB. In total, 85 wildlife species were recorded for the entire study including 57 bird species, 5 amphibian species, and 23 mammal species. Results from the reference sites show that hydrologic regime, percentage of ash canopy cover, and understory cover were important habitat characteristics for bird and amphibian communities. Results from the experimental sites show there may be short-term increases in species richness for mammal and bird communities associated with changes in forest structure due to ash mortality; however, anticipated changes resulting from EAB-caused mortality, particularly the conversion of these sites to non-forested wetlands, will lead to significant shifts in bird and mammal community composition. Loss of ash may cause declines in forest-dependent species and increases in open-canopy and wetland-associated species. Additionally, whereas increased ponding extent and longer hydroperiods may be beneficial for some amphibian species, the loss of the forest canopy will result in an overall decrease in bird diversity and reduce forest connectivity for all species. Our results indicate the potential for significant large-scale impacts of black ash mortality on forest-associated wildlife. Management strategies that focus on establishing alternative trees species to maintain long-term forest cover and structural complexity in these wetlands will help to maintain and conserve wildlife diversity.

Mortality in Forested Ecosystems: Suggested Conceptual Advances

Mortality of trees is an important ecological process altering forest structure and function as well as influencing forest management decisions. Recent observations suggest that the overall rate of tree mortality is increasing at local to global scales. While more data on mortality is needed to document these changes, key concepts are also needed to guide the collection, interpretation, and use of this information. Mortality can be considered as a general process that includes all forms of tree-related death ranging from parts of trees to large-scale disturbances. Viewing mortality as a continuum allows one to examine how the lifespan of trees and their parts (e.g., branches), as well as multiple disturbances, influence ecosystem structure and function. Statistically, mortality does not follow the law of large numbers because, regardless of the scale analyzed, consequential, infrequent episodes can occur. This causes mortality to occur in irregular pulses. While the causes of mortality are indeed complex, this stems from the fact many processes, each with its own set of controls, can lead to mortality. By analyzing and predicting mortality using a chain of events influenced by specific mechanisms, a clearer understanding of this process should develop, leading to a more science-based and less reactive forest management.

Release of coarse woody detritus-related carbon: a synthesis across forest biomes

BACKGROUND

Recent increases in forest tree mortality should increase the abundance coarse woody detritus (CWD) and ultimately lead to increased atmospheric carbon dioxide. However, the time course of carbon release from CWD is not well understood. We compiled CWD decomposition rate-constants (i.e., k) to examine how tree species, piece diameter, position (i.e., standing versus downed), canopy openness, and macroclimate influenced k. To illustrate their implications we modeled the effect of species and position on estimates of decomposition-related carbon flux. We examined a subset of currently used models to determine if their structure accounted for these factors.

RESULTS

Globally k of downed CWD varied at least 244-fold with interspecies variation at individual sites up to 76-fold. While k generally decreased with increasing piece diameter, under open canopies the opposite occurred. Standing CWD sometimes exhibited little decomposition, but sometimes had k values up to 3 times faster than downed CWD. There was a clear response of k to mean annual temperature of ≈ 2.6 times per 10 ℃; however, there was considerable variation for a given mean annual temperature related to species, diameter, and position. A key feature of carbon release from CWD after disturbance was the "evolution" of the ecosystem-level k value as positions and species mixtures of the remaining CWD changed. Variations in decomposition caused by disturbance (e.g., changes in species, positions, sizes, and microclimate) had the potential to cause net carbon fluxes to the atmosphere to be highly nonlinear. While several models currently being used for carbon accounting and assessing land-use/climate change would potentially capture some of these post disturbance changes in fluxes and carbon balances, many would not.

CONCLUSIONS

While much has been learned in the last 5 decades about CWD decomposition, to fully understand the time course of carbon release from increased mortality and other aspects of global change a new phase of global CWD research that is more systematic, experimental, and replicated needs to be initiated. If our findings are to be fully applied in modeling, an approach acknowledging how the rate of carbon release evolves over time should be implemented.

Dynamic Responses of Ground-Dwelling Invertebrate Communities to Disturbance in Forest Ecosystems

In forest ecosystems, natural and anthropogenic disturbances alter canopy structure, understory vegetation, amount of woody debris, and the properties of litter and soil layers. The magnitude of these environmental changes is context-dependent and determined by the properties of the disturbance, such as the frequency, intensity, duration, and extent. Therefore, disturbances can dynamically impact forest communities over time, including populations of ground-dwelling invertebrates that regulate key ecosystem processes. We propose conceptual models that describe the dynamic temporal effects of canopy gap formation and coarse woody debris accumulation following disturbances caused by invasive insects, wind, and salvage logging, and their impacts on ground-dwelling invertebrate communities. Within this framework, predictions are generated, literature on ground-dwelling invertebrate communities is synthesized, and pertinent knowledge gaps identified.