Bonanza Creek LTER

Atmospheric Warming Reduces Forest Production in High Latitudes

The boreal forest is one of the world's largest biomes, covering nearly 17% of the earth's land surface area. This cold northern region has warmed significantly in recent decades. Computer models of global climatic warming from greenhouse gasses project that some of the largest amounts of warming would occur in the boreal region. Nearly all models of climatic warming assume that tree growth, and thus uptake of the greenhouse gas carbon dioxide, will increase in the boreal forest under the warmer conditions.

White spruce is the dominant forest cover on about 25% of the Alaska boreal forest. Although white spruce occupies floodplains and cold treeline sites, much of this forest type in Alaska occurs on dry upland sites across low-elevation basins in the interior of the state. On upland sites in Interior Alaska, radial growth of white spruce is highly negatively related to summer temperature. In other words, it grows least in years with warm summers and most in years with cool summers. This is an unexpected finding, because treeline white spruce at the edge of the tundra have produced many important climate reconstructions based on the positive relationship between summer temperature and radial growth on those sites. The temperature control of white spruce growth in Interior Alaska is consistent throughout the entire period of Fairbanks climate record, which dates to 1906. The negative relationship of radial growth to summer temperature is associated with temperature-induced drought stress as confirmed at Bonanza Creek LTER by three properties of tree-rings (1) ring-width, (2) 13C discrimination, and (3) maximum latewood density. The period since the mid-1970s has been the longest sustained interval of warm summers in Interior Alaska, and lowest radial growth of white spruce. The mid 20th century was an usually favorable period of cool summers and high radial growth. Radial growth in the warm late 20th century is about half the level of the cool mid 20th century on a sustained basis. These results suggest that high-latitude warming and associated drying will not enhance carbon uptake and sequestration in upland white spruce forests, as currently projected by global and regional models.

Fire v. Logging in the Arctic: Identifying the lesser of the evils

Fire and logging are the two major disturbances in upland boreal forests of North America, which is a largely disturbance-driven ecosystem. Fire has long played a crucial role in the ecology of the boreal forest of interior Alaska represented by BNZ- LTER, and many plants species depend upon it. On the other hand logging only started in interior Alaska about 1900. Forest managers need to know to what degree disturbance by logging does or does not create effects similar to fire.

Recently burned plots supported abundant cover of a few apparent fire specialist species that were present in only minor amounts on logged sites. Burned plots exhibited higher species turnover from stage to stage and among all stages than logged plots. Species dominant in young burned plots were nearly absent at later stages, while early dominants in logged sites were common mature forest species that increased in each subsequent stage Greater soil organic layer depth in the boreal forest is associated with cooler soil temperatures, reduced nutrient availability, and lowered site productivity. Logged sites appear to begin and continue succession with a greater share of the original mature forest understory plants, while burned sites initiate succession with more distinctive and specialized plant species. Most of the species at risk of decline in abundance from logging compared to burning by wildland fire in central Alaska are associated with early successional conditions. These results suggest that forest managers should consider broadcast burning of at least some logged sites.