Cause of patch formation was investigated on a 7.2 ha study area in Sylvania Wilderness Area, a primary forest remnant in Upper Michigan comprising a mosaic of hemlock, sugar maple, and mixed—forest patches. Spatial autocorrelation analysis of the stem map indicated that, although most species pairs have a neutral association between canopy trees and understory trees of other species, hemlock and sugar maple canopy trees both have strong positive self association and negative reciprocal association with each other. No species pairs have a positive reciprocal association on regeneration with each other. MOSAIC, a Markov simulation model in which transition probabilities depend on neighborhood species composition, shows that the negative reciprocal association between hemlock and sugar maple of the intensity observed in this study, could lead to spatial separation into monodominant patches over long time periods (3000 yr). The mixed—forest patches are along spatial continua of varying steepness between sugar maple and hemlock patches. Interactions sugar maple and hemlock overstory and understory trees, along with the pattern of invasion of hemlock, provide a reasonable explanation for the patch structure. Pedological, topographical, and disturbance history differences do not coincide with the location of patches within upland forests on the study area.
The Landscape Reconstruction Algorithm (LRA) overcomes some of the fundamental problems in pollen analysis for quantitative reconstruction of vegetation. LRA first uses the REVEALS model to estimate regional vegetation using pollen data from large sites and then the LOVE model to estimate vegetation composition within the relevant source area of pollen (RSAP) at small sites by subtracting the background pollen estimated from the regional vegetation composition. This study tests LRA using training data from forest hollows in northern Michigan (35 sites) and northwestern Wisconsin (43 sites). In northern Michigan, surface pollen from 152-ha and 332-ha lakes is used for REVEALS. Because of the lack of pollen data from large lakes in northwestern Wisconsin, we use pollen from 21 hollows randomly selected from the 43 sites for REVEALS. RSAP indirectly estimated by LRA is comparable to the expected value in each region. A regression analysis and permutation test validate that the LRA-based vegetation reconstruction is significantly more accurate than pollen percentages alone in both regions. Even though the site selection in northwestern Wisconsin is not ideal, the results are robust. The LRA is a significant step forward in quantitative reconstruction of vegetation.
The record of forest invasion by eastern hemlock (Tsuga canadensis) during the course of Holocene migration provides useful information about invasion processes in temperate forest, a system that has been invaded by few exotic species. We used fossil pollen preserved in small forest hollows, which record forest composition on the scale of 1–3 ha, to study hemlock invasion of forests in the Sylvania Wilderness in the upper peninsula of Michigan, where there is now a mosaic of 3–30 ha stands dominated either by hemlock or by sugar maple (Acer saccharum) and basswood (Tilia americana). Fossil pollen was interpreted by comparison with 66 surface samples from small hollows in Michigan and Wisconsin, using three different statistical methods: pollen ratios, a dissimilarity index, and canonical variates analysis (CVA) ordination. We found that four hemlock stands along a 10‐km transect across Sylvania originated as patches of white pine (Pinus strobus) forest that were invaded by hemlock ∼3000 yr ago (calibrated 14C dating) when hemlock expanded its range in northern Michigan. Invasion occurred at about the same time (within 800 yr) at all sites but was not associated with disturbance at any site. Over the next several thousand years hemlock coexisted with white pine, but eventually, at a time that differs from site to site, hemlock became dominant, and white pine disappeared from all but one of the four stands. These changes were apparently driven by climate changes over the last 4000 yr that caused the water table to rise (Brugam and Johnson 1997). The history of four nearby maple stands is more variable and less well understood. Unlike the hemlock stands, three of the four maple patches were not dominated by pine at the time of hemlock invasion, but instead had abundant oak (Quercus) and/or maple. Two of these stands were not invaded by hemlock, and the third, if invaded at all, was invaded for a few centuries by low densities of hemlock trees. Thus invasion by hemlock was sensitive to the species composition of the resident forest. Sugar maple and basswood increased in these stands, and by 2000 to 800 yr ago, depending on site, they resembled modern maple stands. The fourth patch was invaded by hemlock, but it was converted to a maple stand 1000–500 yr ago. A wood layer in the sediment is evidence that a catastrophic windstorm may have been responsible.
Methods of interpreting pollen assemblages in sediment were examined using surface samples from 66 small forest hollows in Michigan and Wisconsin. All canopy trees in the surrounding 50 m were measured to provide detailed information about the source vegetation of each surface pollen assemblage. Basal area of trees in each forest sample was used to classify them into six stand types: hemlock-dominated, sugar maple/hemlock mixed, sugar maple-dominated, and ash-, oak-, and pine-dominated stands. Various statistical procedures were tested to learn which was most successful in sorting the pollen assemblages into appropriate vegetation groups. Two ordination techniques – detrended correspondence analysis (DCA) and canonical variate analysis (CVA) – give similar overall results, although CVA more successfully separated assemblages from hemlock stands from those of sugar maple-dominated stands. Squared chord distance,0.05 also successfully identified samples from the same forest type. After stand dominants have been identified from pollen assem blages using multivariate methods, a further determination of stand composition is sometimes possible using ratios of pollen counts of individual taxa. Ratios can be calibrated by comparison with species abundances around surface samples. For instance, ratios of pine to hemlock pollen can indicate the abundance of pine within a stand dominated by hemlock, whereas pine pollen percentages alone are affected by variable abundance of other species.
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