Study Sites & Design: Bonanza Creek Experimental Forest

Credit: Roger Ruess

The Bonanza Creek Experimental Forest (BCEF) was established in 1963 is a 50 km2 experimental forest located approximately 20 km southwest of Fairbanks, Alaska and located on land owned by the State of Alaska. The Forest is within the Tanana Valley State Forest, a unit managed by the Division of Forestry, State of Alaska. The area encompasses typical vegetation and landforms of interior Alaska and includes a section of the Tanana River floodplain and adjacent uplands. These uplands form the southern limit of the Tanana-Yukon uplands, rising to a ridge crest of 470 m. The vegetation, a mosaic of forest and non-forest types, corresponds to four broad topographic zones: upland hills and ridges, lowland toeslopes and valley bottoms, old Tanana River terraces, and the active floodplain.

Research has been conducted in the BCEF since the mid 1950s and has included studies focused on white spruce seed production, development of growth and yield tables for white spruce, aspen, and birch, silvicultural of white spruce, sprout and sap production in birch stands, effects of red squirrel foraging on white spruce cone and seed production, nutrient relationships in birch and black spruce stands, and monitoring of insect and pathogen outbreaks. More recently BCEF has been the site of studies of floodplain soil moisture dynamics and formation of salt crust on freshly-deposited alluvium (Salt Affected Soils), forest reestablishment following wildfire (Rosie Creek Burn), insect and disease dynamics following wildfire, and tree species provenence tests. In 1987, BCEF joined the network of Long-term Ecological Research (LTER) program, studying successional processes in the boreal forest of interior Alaska. The LTER research program at Bonanza Creek Experimental Forest is designed to study ecosystem structure and function through examination of controls over successional processes in taiga forests of interior Alaska. This study tests hypotheses in two successional sequences: primary succession on the floodplain of the Tanana River (6 successional stage with 3 replicates per stage), and secondary succession following wildfire in upland forest (4 successional stage).

Credit: Jeremy Jones

Successional stages and turning points

We selected "turning points" in the succession sequences where critical changes in ecosystem structure are accompanied by functional changes, which have far-reaching effects on ecosystem development. At six turning points in the floodplain succession and three turning points in the upland succession three replicates of experimental plots were established for experimental projects and to follow the natural changes occurring in ecosystem structure and function.

The trajectory of succession is sensitive to environment, propagule availability, legacies associated with prefire vegetation, and disturbance severity. Primary succession predominates in the active floodplain. After initial establishment, competition, facilitation, and herbivory interact to drive successional. Ecosystem controls change at key turning points (thresholds), where a shift in dominance of plant functional types radically alters the physical and chemical environment that govern ecosystem processes and disturbance probability. In the floodplain, intense herbivory by moose initially constrains canopy development, creating an ecosystem dominated by physical controls over soil water movement, surface evaporation and gypsum accumulation at the soil surface. Colonization by alder shifts the system from physical to biological control, adds 60-70% of the nitrogen that accumulates during succession, and causes herbivory to change from a deterrent to an accelerator of succession by eliminating palatable early successional species. Other key turning points include (1) a shift to balsam poplar dominance, where changes in productive potential and litter chemistry enhance NPP and nitrogen cycling rates and (2) the shift to white spruce dominance, where mosses grow rapidly in the absence of smothering broadleaved litter, reduce nutrient cycling rates by sequestering nutrients in low-quality litter, and increase fire probability by producing resinous fuels that dry quickly. In late-successional black spruce stands, root turnover governs nutrient supply.

Secondary succession is the rule in the uplands. Self-replacement, in which the prefire tree species returns to dominance shortly after fire, generally occurs in extreme environments, whereas succession with multiple stages is more common in intermediate sites. Late-successional conifers establish during the initial 1-2 decades after fire, but their establishment success is sensitive to the depth of the organic mat remaining after fire, understory species composition, and seed availability from on-site serotinous cones (black spruce) or off-site seed sources (white spruce). Variations in fire frequency or severity can alter plant regeneration feedbacks that stabilize community composition and cause rapid shifts in forest cover types. Changes in any of these processes could alter vegetation composition and successional trajectory.

At each of the sites in Bonanza Creek Experimental Forest, a 50x60 meter permanent "control plot" was established to provide a control for experiments and to monitor vegetation change. Within each control plot 20 vegetation plots are sampled. Each vegetation plot consists of a 1 m2 plot for measurement of herb, lichen, moss, and low shrub cover estimates, and a 4 m2 plot for measurement of shrubs and tree seedlings. In addition all trees and shrubs having a breast height diameter of 2.5 cm or larger are tagged and mapped. Ten trees of each species within the reference stand are also equipped with band dendrometers for measuring annual diameter growth at breast height. In young successional stands the vegetation plots are monitored every two years; in mature types they are monitored every five years. In addition, litter trays have been placed in each reference stand and seed traps in one of each of the eight successional stages. At four points around the perimeter of each reference stand the forest floor and mineral soil profile was described and sampled using standard procedures. Bulk samples of both materials were obtained for physical and chemical analysis. These assessments are repeated at 10-year intervals.

Study sites at Bonanza Creek Experimental Forest are named according to the habitat, successional stage the site was at when the site was established, and the replicate. There are two main naming schemes. In the first scheme the site is described numerically. In the second naming scheme the sites are coded according to the following scheme: Habitat (FP=Floodplain, UP=Upland), successional stage (numeric), replicate (A-C).

The Tanana River

The Tanana River is the largest tributary of the Yukon River and flows northwest from headwaters near the Canadian Border for nearly 1000 km.Bordered by the Alaska Range to the south and the Yukon-Tanana Upland physiographic province to the north (Wahrhaftig 1965), the Tanana River valley lowland covers 17,400 km2. The Tanana River occupies a structurally-controlled basin extending below sea level and is filled by Tertiary and Quaternary fluvial/glaciofluvial deposits as much as 100 to 250 m thick.

During the Delta Glaciation of the late Pleistocene, the increased discharge and sediment load from the glaciers of the Alaska Range caused the Tanana River and its tributaries to aggrade rapidly, damming the lower reaches of several valleys of the Yukon-Tanana Uplands. A period of downcutting by the Tanana followed the end of the Delta Glaciation, forming an upper terrace. During the Donnelly Glaciation, the Tanana River once again aggraded; the resulting floodplain was not built up as high as during the Delta Glaciation. Following the end of the Donnelly Glaciation, the Tanana cut down again, forming a second, lower terrace, whose age has been estimated at 10,000 years before present.

Active and recently abandoned floodplain terraces adjacent to the Bonanza Creek Experimental Forest are only around 3000 years old. Preliminary mapping in the BCEF area of the floodplain confirms this basic chronology and shows that the islands contain floodplain deposits 1000 yrs old. However, most of the islands are considerably younger, less than 400 yrs old.

About 85% of the discharge of the Tanana River is derived from north-flowing, glacier-fed rivers originating in the Alaska Range. Four large rivers provide about half the total Tanana discharge, and two of these, the Nabesna and the Delta, lie upstream from the study locale. The south-flowing tributaries from the Yukon-Tanana upland provide only about 15% of the total discharge and all lie upstream from the study locale. Flooding is common during winter breakup, when ice dams form, and during the summer in response to large rainstorms or during periods of warm temperature and increased glacial melt. Summer floods commonly lead to high bank erosion and deposition of new point bars.

Activities within the area interfering with long-term research plots is restricted through the lease agreement. Research development within the area is managed by BNZ staff. Permission to conduct research can be obtained by contacting the lead principal investigators or the site manager.

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