Spatial patterns and succession in a Minnesota southern-boreal forest

Succession was studied in a cold-temperate forest in the Boundary Waters Canoe Area Wilderness (BWCAW) of northeastern Minnesota. The 13 × 18 km study area comprises a complex forest mixture of jack (Pinus banksiana) and other pines, quaking aspen (Populus tremuloides), paper birch (Betula papyrifera), black spruce (Picea mariana), balsam fir (Abies balsamea), and white cedar (Thuja occidentalis) on thin soils over the Canadian Shield bedrock. The main objectives of this study were to examine the relationships between spatial patchiness, spatial scale, and canopy succession in the southern-boreal forest of the BWCAW, and to evaluate under what conditions successional direction may remain stable, converge, or diverge. Knowledge of the successional direction of old forests in the BWCAW that are undergoing demographic transition from even-aged to uneven-aged is important because the landscape now has many old stands as a result of reduced fire frequency. Rotation periods for fires have changed from ≈50-100 yr in presettlement times to >1000 yr since 1910. Analyses were conducted at spatial scales ranging from the individual tree (0.01 ha) to the large stand (16 ha). Two permanent mapped plots (of area 0.53 und 0.56 ha) were established in stands of different age. Fine-scale age structure, successional change, transition from one species to another, and development of small patches (of area <0.25 ha) were studied by means of stand history reconstruction with increment cores, spatial autocorrelation, and analysis of replacement trees in canopy openings. Spatial processes at nested scales of 1, 4, and 16 ha were examined on 15 square 16-ha tracts of upland forest, which are distributed among forests ranging from 15 to 190 yr old. Canopy species composition and patch development over time on these 15 tracts were interpreted on air photos taken in 1934, 1961, and 1991. Thus, the study includes a chronosequence approach, with verification of chronosequence validity by checking patch development processes at more than one time point. This checking was done at small spatial scales by reconstruction of stand history on permanent mapped plots, and at larger spatial scales with sequential air photos of the same locations spanning a 57-year period. Results show that the reduced fire frequency in recent years has changed the dominant successional pathways. When fire frequency was high, jack pine or aspen stands usually burned while still in the even-aged stage of development, and the new trees after the burn were the same species us before. Currently, many stands are undergoing demographic transition from even-aged stands of catastrophic fire origin to uneven-aged stands. This transition parallels a change in canopy composition from jack pine (occasionally red pine (Pinus resinosa)) or aspen to an old-growth multi-aged mixture of black spruce, balsam fir, paper birch, and white cedar. The mechanism that moves this successional path forward is canopy openings, 10-30 m across on average, caused by wind, insect, and disease, that gradually chip away at the relatively uniform canopy of pines and aspen. Successional direction is individualistic in the sense that time and rate of transition from pine/aspen to other species depends on the action of heavy windstorms, insect infestation, and senescence of old pines that create canopy openings. Canopy openings are often filled with one of several species, but if more than one species invades an opening, monodominant patches of each species generally result. Understory-overstory interactions are very weak: the dominant species within each patch is apparently independent of the overstory species that died when the opening was created or the species dominating surrounding patches. During succession, the spatial structure of the stands at the 1-16 ha scales generally changes from a matrix heavily dominated by pine or aspen to a mosaic with relatively large mono-dominant patches that may be remnants of the extensive original matrix, and finally to a mixture with small patches (mean area 35 m², maximum ≈0.1 ha) of black spruce, balsam fir, white cedar, and paper birch. Thus, at 1-16 ha spatial scales, succession leads to convergence on a mixture of species. At smaller spatial scales (e.g., 0,01-01 ha) successional pathways appear to diverge into four community types. The same successional pathways can be reconstructed from historical analysis of individual stands as from a chronosequence of stands; therefore, chronosequences in this area have been stable at least during the lifetime of the current generation of trees.