About Bonanza Creek LTER

The Bonanza Creek Long-Term Ecological Research program (BNZ LTER) is located in the boreal forest of interior Alaska, USA. BNZ LTER was established as part of the National Science Foundation's Long Term Ecological Research Network in 1987. The program is jointly managed by the University of Alaska Fairbanks and the Boreal Ecology Cooperative Research Unit, the northernmost outpost of the USDA Forest Service Pacific Northwest Research Station. BNZ LTER research is concentrated in two sites near Fairbanks, Alaska: the Bonanza Creek Experimental Forest and the Caribou Poker Creeks Research Watershed, with additional research infrastructure in boreal tundra near Healy, Alaska, just north of Denali National Park and Preserve. However, the BNZ LTER has recently established a Extensive Site Network to address questions of regional resilience and change in response to changing climate and changing disturbance regimes.

The Boreal Forest of Interior Alaska

The boreal forest is the largest forested biome on Earth, covering 17 million km2 of the Northern Hemisphere, and accounting for approximately one third of Earth's total forest area. Globally, the northern and southern boundaries of the boreal forest are associated with the Arctic air mass, which extends southerly in wintertime to define the southern most extent of the boreal forest, and shrinks northward during summer to delineate the boundary between boreal forest and tundra.

In Alaska, boreal forest occupies some 60-70% of the land area, with tundra and coastal coniferous forest making up the remainder. Approximately 32%, or 42.8 million ha, of the total 137 million ha that make up the interior Alaska boreal ecoregion is forested, of which >60% is dominated by black spruce (Picea mariana). The remainder is complex mosaic of grasslands, shrublands, bog and fen meadows, shrub and sedge tundra, and open water, with rock outcrops, snow and ice at high elevations.

Climate is ultimately the factor that differentiates the boreal forest from other biomes. Within interior Alaska, climate is a consequence of the northerly latitude of the region and the presence of the Alaska and Brooks Ranges, which block maritime and polar influences, yielding a dry continental climate with cold winters and warm summers.

The cold climate slows most soil-forming and biogeochemical processes and leads to the formation of permafrost, soil which remains frozen year round. The interior Alaskan boreal forest is underlain by discontinuous permafrost, which serves as a barrier to roots and water percolation, promoting anaerobic conditions.

The potential biota of the boreal forest is also limited by the constraints of climate. The vegetation and animals that dominate here are well adapted to the harsh environment. The mosaic of vegetation found within the boreal forest results from variation in environmental conditions and from past disturbances, such as fire, insect and pathogen outbreaks, and flooding.

Alaska’s Changing Climate and Disturbance Regimes

The boreal forest is also a land undergoing rapid environmental change. During the last third of the twentieth century, interior Alaska’s boreal forest has warmed at a rate approximately twice that of the global average. These patterns are consistent with projections of general circulation models, which predict that human-induced increases in greenhouse gases will continue to cause the global climate to warm, with warming occurring most rapidly at high latitudes. Summer precipitation is expected to increase as air temperatures warm, but the environment will most likely be drier for plant growth due to increased evaporation from the land surface.

Another dramatic consequence of changing climate throughout interior Alaska is a shortening of the period during which the landscape is covered with snow. This is likely to create mismatches between native plants and their environment, including the disruption of plant-pollinator connections, or offer new opportunities for invasive plants and pests to gain a foothold. Less snow on the landscape also reduces surface albedo and translates to a positive feedback to climate warming , in a manner similar to the loss of sea ice.

In addition to the direct effects of climate change on the structure and function of the boreal landscape, climate warming has radically changed the dynamics of and interaction among disturbance regimes. This includes notable changes in fire size and severity, surface hydrology and the rates of permafrost thaw, and the outbreak behavior of insects and pathogens, resulting in apparent threshold shifts in biogeochemical cycling, successional trajectories, and ecosystem and landscape function.

Changes in the boreal forest are also affecting its rural and urban human residents, many of whom live off the road system and rely heavily on harvested foods. Modifications to the biota, permafrost distribution, soil stability, hydrologic regime, forest productivity, and disturbance regimes are affecting the services that the boreal forest provides and influencing the livelihoods of rural and urban communities in complex ways. Finding ways to adapt to and manage for changes in ecosystem services are challenges facing these communities.

• see Summary of Findings from Prior Research

Research Overview

The guiding research question of the Bonanza Creek LTER is: How is the boreal biome responding to climate change and what are the local, regional and global impacts of those responses?

This question is of timely relevance for several reasons. (1) The boreal forest is experiencing among the fastest rates of warming on Earth, leading to significant climate feedbacks resulting from landform changes and associated atmospheric C, water and energy exchanges. These feedbacks are of global significance because the boreal forest covers 12 million km2 of the Northern Hemisphere and contains a massive pool of soil C which is vulnerable to atmospheric exchange. (2) Climate warming has radically changed the dynamics of and interaction among disturbance regimes, notably fire size and severity, surface hydrology and the rates of permafrost thaw, and the outbreak behavior of insects and pathogens, resulting in apparent threshold shifts in biogeochemical cycling, successional trajectories, and ecosystem and landscape function. (3) Subsistence hunting and gathering traditions of interior Alaskan Native communities are historically tied to interactions between the availability of subsistence resources and regional gradients in climate and disturbance regimes. Rural and urban human populations alike rely heavily on ecosystem services provided by the boreal forest. However, economic, social, and ecological changes are affecting human-ecological interactions, cultural traditions, and the provisioning and use of ecosystem services by Alaskans.

The BNZ LTER program is designed to understand the interactive effects of changing climate and disturbance regimes on the Alaska boreal forest, and study associated consequences for regional feedbacks to the climate system, and sustainability of subsistence Alaskan communities.

1. Direct effects of climate change on ecosystems and disturbance regimes. Interior Alaska is experiencing increases in mean annual temperatures and shifts in the seasonal patterns of precipitation, but also changes in modes of long-term oscillations in global atmospheric pressure and sea surface temperatures that control seasonal climate and establish modes of climate variability at decadal to multi-decadal scales. These complex climate dynamics are having dramatic direct effects on the structure and function of the boreal landscape. The BNZ LTER is studies the direct effects of climate variability on ecosystems and disturbance regimes by 1) characterizing current and historical responses of key species and disturbances to these modes of climate variability, 2) studying climate controls over the distribution and patterns of change in landscape functional types, and 3) experimentally manipulating climate variables in order to understand the underlying mechanisms for threshold responses of key ecosystem components and disturbance agents to change in climate drivers. Results from these efforts are being integrated with studies on mechanisms and consequences of change in order to predict climate sensitivity of ecological communities (plant and animal species, plant functional types, community structure), ecosystem processes (NPP, C and N storage), landscape structure and heterogeneity, and the severity and distribution of disturbance regimes (fire, permafrost thaw, insect/pathogen outbreaks).

2. Climate-disturbance interactions as drivers of ecosystem and landscape change. Vegetation composition and other structural aspects of the ecosystem and landscape modulate climate sensitivity of disturbances and associated ecosystem responses to those disturbances. For example, the distribution of conifers and hardwoods on the landscape influences both fire severity and the outbreak behavior of insects and pathogens, which in turn influence the likelihood that forest communities will shift to a new stability domain. However, changing disturbance regimes also influence the climate sensitivity of species, plant communities and landscape functional types. The impacts of permafrost thaw on soil water content, for example, strongly affect vegetation responses to climate warming. Moreover, important thresholds may be manifested through interactions between changing disturbance regimes. For example, increased fire severity may increase the vulnerability of permafrost to thaw. Similarly, interactions between insect and pathogen outbreaks and fire regimes may produce novel responses by overwhelming the resilience of ecosystems to either disturbance in isolation. Understanding the underlying mechanisms for these interaction pathways is essential for predicting whether climate-driven changes will contribute to ecosystem and landscape resilience or cause abrupt shifts to a new landscape mosaic with fundamentally different dynamics.

3. Regional ecosystem dynamics and climate feedbacks. Interactions among changes in climate, ecosystem structure and function, and disturbance regimes affect exchanges of trace gases, water, and energy between the boreal forest and the atmosphere, and result in feedbacks to regional and global climate. Important positive feedbacks to climate warming include decreases in surface albedo due to changes in snow cover, and respiratory release of permafrost C. Negative feedbacks include increases in surface albedo due to a greater proportion of the landscape occupied by deciduous forest accompanying a shorter fire-return interval, and greater vegetation C uptake resulting from an extended growing season. Our study of the net effects of these feedbacks uses retrospective and prospective modeling to integrate field-based assessments of disturbance and climate-change effects on net ecosystem C balance with a regional assessment of the mechanisms and patterns of change in landscape structure and function. A suite of models is being developed and continuously refined by the BNZ LTER to address how changing climate-disturbance interactions are influencing regional ecosystem dynamics and climate feedbacks:

4. Coupled social-ecological dynamics of interior Alaska. Social, economic and climatic drivers of change are dramatically modifying the ecosystem services on which communities throughout interior Alaska depend, and influencing the social-ecological interactions that link people to the land. For example, climate-driven changes in the physical system and fire regimes are influencing the distribution and abundances of plants and animals harvested by Native peoples of rural communities. Access to subsistence resources is increasingly constrained by changes in snow cover, extent and timing of river and lake freeze-up and thaw, escalating fuel costs, and increased travel times associated with changes in landscapes. Strong reliance and cultural ties to the harvesting and sharing of subsistence foods are fundamental to the sustainability of interior Alaskan Native communities. However, the high costs of energy, combined with limited cash employment opportunities, increasing conflicts between rural and urban subsistence hunters, and a complex governance structure for natural resource management dominated by urban interests are posing serious threats to the rural subsistence lifestyle. BNZ LTER is working with rural villages to a) identify important ecosystem services, and past trajectories, rates of change and likely future changes in those services; b) model social-ecological interactions at the household level; c) conduct institutional analysis to understand if and how local knowledge affects regional policy making and d) through partnerships with communities and agency resource managers identify conditions that facilitate management strategies and innovation in future human adaptation and transformation.

Study Design

Our study design recognizes three landscape units that differ in their environmental controls and likely response to climatic change:

• Uplands
• Floodplains
• Wetlands

The uplands and floodplains have been the focus of previous BNZ LTER research while wetlands, which are widespread in the boreal region, have not been intensively studied in Alaska. Alaska contains more than half the wetlands in the U.S., but the response of permafrost to complex interactions among topography, surface and ground water, soil properties, vegetation, and precipitation is changing the distribution and functioning of boreal wetlands and tundra.

Our research is concentrated at two intensive study areas that contain representative sites of each landscape unit:

• At Bonanza Creek Experimental Forest (BCEF), we maintain permanent plots in primary floodplain succession and secondary post-fire succession (3-5 successional stages x 3 replicate sites/stage). BCEF also includes a forest-wetland gradient study where we are experimentally manipulating water-table height and summer air temperature.
• At the Caribou-Poker Creek Research Watersheds (CPCRW), we maintain four intensive watersheds, two of which are unmanipulated (low vs. high % permafrost) and two of which have burned recently (a low-severity experimental burn in 1999 and a high-severity natural wildfire in 2004).

For selected variables, we extend observations from intensive studies at BCEF and CPCRW to the Tanana Valley, the watershed in which our intensive sites are situated and which drains the northern flanks of the Alaska Range. The Tanana Valley contains both pristine glacial and non-glacial watersheds and the major areas of agricultural and forestry development in interior Alaska, providing opportunities to examine social-ecological interactions. Where available, we maintain databases of climatic and ecological variables for all of interior Alaska as a basis for modeling and synthesis. Our research design thus gives us a hierarchical study design from plots/watersheds to all of interior Alaska.

Long-Term Monitoring

The BNZ research program has two intensive research sites: The Bonanza Creek Experimental Forest (BCEF) is within the Tanana Valley State Forest and is managed by the Boreal Ecology Research Unit (i.e., the FS component of the LTER) through a renewable 50- year lease to the US Forest Service (renewable in 2018). The Caribou-Poker Creek Research Watersheds (CPCRW) includes lands under the jurisdiction of the University of Alaska and the Alaska Department of Natural Resources. The BNZ LTER manages BCEF and CPCRW, with the Interagency Hydrology Committee, which represents agency interests in Alaskan hydrology, acting as an external advisory committee. We have close working relationships with both the Tanana State Forest and the Alaska Division of Natural Resources. The Alaska Legislature recently passed legislation to transfer the BCEF and CPCRW from the State of Alaska to the University of Alaska to be managed by the BNZ LTER, enhancing the long-term site security. The BNZ site manager (Jamie Hollingsworth) is responsible for managing LTER research in the two research sites, including permitting, transportation, and the planning and implementation of the core research program. Recent improvements in site management include automation and wireless radio communication with climate and microclimate stations, improved coordination of field work, improved boat communication and safety, and improved methods and assessment of statistical power and required sample sizes for long-term vegetation measurements. These efforts have substantially improved the quality, continuity, efficiency, and safety of data collection, releasing time to undertake new activities.

Our core research includes monitoring of climate, hydrology, vegetation, and other essential long-term site measurements:

Parameters measured by BNZ LTER monitoring program:

                  *Jamie Hollingsworth (site manager) is responsible for the BNZ LTER climate monitoring program

                  †Monitoring network includes sites throughout interior Alaska