Mechanisms of patch maintenance in old-growth hemlock-hardwood forests

Old-growth forest remnants in Upper Michigan, USA, exhibit a conspicuous patchy mosaic of eastern hemlock- (Tsuga canadensis (L.) Carr.) or hardwood-dominated stands. These patches have been hypothesized to modify plant-resource availability in a manner that favors the recruitment of conspecific seedlings or excludes seedlings of other species, thereby reinforcing the patchy forest mosaic. We studied the effect of hemlock and hardwood patches on light and soil nitrogen availability in two mapped 1.96-ha plots. The greater canopy depth and more recalcitrant litter of hemlock patches were expected to reduce light and nitrogen availability compared to the hardwood patches. We also investigated the growth rate and three-year survival of naturally-established and planted seedlings of hemlock and the primary hardwood species, sugar maple (Acer saccharum Marsh.), and related them to patch type or light and nitrogen availability. We hypothesized that the low light and nutrient availability predicted in the hemlock patches would decrease hemlock and sugar maple seedling growth rate and survival when compared to hardwood patches, but that decreases would be greater for sugar maple seedlings.; The summer light and nitrogen availability were similar between patch types. Light availability when deciduous species were leafless was much higher in hardwood patches than hemlock patches. Related soil measurements done at four old-growth sites found that pH decreased with increasing hemlock dominance as did nitrification at two sites.; The survival of planted seedlings was higher in the hardwood patches than in the hemlock patches for both species. Also, both planted and naturally-occurring seedlings of hemlock had higher survival than those of sugar maple. Hemlock had higher survival than sugar maple at similar low light availability for both planted and naturally-established seedlings, but they had similar growth rates. Naturally-established hemlock seedlings were found primarily on rotting wood of hemlock and yellow birch (Betula alleghaniensis Britt.). The spatial distribution of hemlock seedlings appears to be related primarily to favorable germination sites, such as rotting wood or soil hummocks. Sugar maple germinants establish ubiquitously, therefore their spatial distribution as seedlings seems related primarily to differential survival in the hemlock and hardwood patches.