Local and Regional Variation in Hemlock Seedling Establishment in Forests of the Upper Great Lakes Region, USA

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Successful tree regeneration requires both successful seedling establishment and subsequent survival and growth sufficient to ensure recruitment. We examined patterns of initial seedling establishment in randomly selected stands of eastern hemlock (Tsuga canadensis) distributed across northern Wisconsin and eastern Upper Michigan. In 1990, we counted all hemlock seedlings (defined as 4–29cm tall) and sampled substrate conditions in two 7×7m quadrats of 142 hemlock and hemlock–hardwood stands. In 1996, we resampled 109 of these stands more intensively using six 7×7m quadrats. In 1997, we also intensively surveyed which substrate-supported hemlock seedlings in three 50m2 replicate areas from each of five stands. In 1990, we found no hemlock seedlings at 64 of the 142 stands sampled, and 46% of the stands had more than 100 hemlock seedlings ha−1 (mean density=480±90ha−1). In contrast, only 14 (13%) of the 109 stands resampled in 1996 had no seedlings and 66% had over 100 seedlings ha−1(mean density=840±200ha−1). These results suggest that seedling establishment is patchy, causing the estimated frequency of successful establishment to depend strongly on the area sampled. Seedling abundance within sites was correlated between years, suggesting that site history may in part account for the abundance of seedlings. Analysis of variance to mean ratios reveals that seedlings are indeed strongly clumped at the 7×7m scale or smaller. Such clumping probably results from the tendency for hemlock seedlings to occur most frequently on particular substrates (i.e. decaying wood). Hemlock seedlings occur less frequently in stands with high basal area of sugar maple but do not decline in stands with greater hemlock basal area, suggesting that maple litter may restrict local opportunities for seedling establishment. Seedling densities do not vary among habitat-types, and no other stand or regional variable significantly affected hemlock seedling density. These results suggest that ground surveys are needed to assess hemlock seedling abundance, and that such surveys should be based on quadrats of at least 100m2. Successful hemlock regeneration should capitalize on local patches of existing hemlock seedlings where interference by hardwoods is minimal. Even high initial seedling establishment, however, may not guarantee regeneration in areas where high deer densities preclude recruitment into larger size classes.