Plant species diversity on logged versus burned sites in central Alaska
Introduction
Fire and logging are two major disturbances in upland boreal forests of North America (Johnson, 1992), including Alaska (Viereck and Schandelmeier, 1980). Fire has long played a crucial role in the ecology of the North American boreal ecosystem, and many species of plants have evolved to depend upon this disturbance for their survival (Shafi and Yarranton, 1973). Localized logging started in interior Alaska about 1900, occurred at low levels during the middle of the 20th century, and has increased since the 1970s (Wurtz and Gasbarro, 1996).
The goal of forest policies in many boreal regions is to simultaneously develop sustainable productivity and maintain biodiversity within the framework of tested and reliable resource management techniques (Lamas and Fries, 1995, Riley, 1995). This new emphasis reflects concerns for biodiversity as logging has become widespread and natural fire has been greatly diminished or eliminated in parts of the boreal world.
In Sweden, 286 species, including vertebrates, invertebrates, vascular plants, bryophytes, lichens, and fungi, are threatened due to land use changes in forestry, especially increased logging and fire suppression (Nilsson and Ericson, 1992). Logging on short rotations has reduced many of the structural components, spatial patterns, and processes affecting plant species diversity in the post-fire disturbed Fennoscandian boreal forest (Esseen et al., 1992). The near elimination of fire in the Scandinavian landscape has made post-fire succession stands, burnt substrates, and coarse woody debris rare.
The objectives of this study were to determine whether logged versus burned sites across a range of times since disturbance in central Alaska differ in (1) overall plant species richness, (2) successional trajectories, and (3) species diversity at particular stand development stages.
Section snippets
Study area and sample site selection
All plots were located between 65°50′ and 63°20′N latitude and 142°30′ and 150°00′W longitude in the Tanana and Yukon River drainages of central Alaska. Sampling was restricted to typical commercial forest sites, which are generally south-facing slopes with inceptisols formed in deep permafrost-free loess deposits, generally overlying Precambrian schist. In this region moderately sloping southerly aspects primarily support mixed stands of aspen (Populus tremuloides Michx.), paper birch (Betula
Organic layer and canopy cover
Organic layer depth was consistently greater at each stage on logged plots compared to burned plots and overall (Fig. 2). Organic depth of was significantly different between disturbances (P>0.000, 1 d.f., F=47.6) and within each disturbance (P>0.001, 3 d.f., F=11.0). The interaction of within-disturbance versus between-disturbance effects on organic layer depth was not significant (P>0.48, 3 d.f., F=0.86). Overall canopy cover across all stages was not significantly different between disturbance
Discussion
One of the most significant findings of this study is the clear and consistent difference in organic layer depths at all stages between the two disturbance types. In central Alaska deep organic layers are poor seedbeds (large number of seeds required to produce each germinant) for a variety of species including the dominant forest trees (Zasada et al., 1992). Complete removal of the organic layer through fire eliminates vegetative reproductive structures and creates a site requiring plant
Implications for management
We have established that organic layer depth is consistently greater on logged sites in central Alaska than on burned sites. Because greater organic depth is associated with cooler soil temperatures, reduced nutrient availability, and lowered site productivity (Van Cleve and Viereck, 1981, Van Cleve and Yarie, 1986, Curran and Ballard, 1990), we recommend that managers consider broadcast burning of at least some logged sites. The benefits we expect from this treatment would include greater
Acknowledgements
Funding for this project was provided by the Natural Resource Fund of the University of Alaska, National Science Foundation Grant DEB #9211769 in support of the Bonanza Creek Long-Term Ecological Research site, and the McIntire-Stennis Cooperative Forestry Research Program. Access and location assistance were provided by the Alaska Department of Natural Resources, Division of Forestry. The USDI BLM Alaska Fire Service provided fire records. Joy Jenkins and Robert Solomon assisted in the
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