Survival of tree seedlings on different species of decaying wood maintains tree distribution in Michigan hemlock-hardwood forests

摘要

1. Seedlings of some tree species are strongly associated with decaying wood in the germination or establishment period. Despite recognition of this pattern in forests around the world, few studies have compared the ability of different species of decaying wood to support seedlings. 2. We measured the abundance, survival, and age distribution of seedlings of Tsuga canadensis (eastern hemlock), Betula alleghaniensis (yellow birch), and Acer saccharum (sugar maple) among forest floor substrates at field sites in Upper Michigan, USA. Seedlings were censused on decaying wood of the same three species and on soil to determine whether species of decaying wood and soil differ in their ability to support seedlings. We hypothesized that Tsuga and Betula seedling abundance and survival are higher on Tsuga wood than on Acer wood or soil. 3. Independent of seed fall, light and size of wood piece, Tsuga wood generally supported the highest densities of first-year and established (> 1 year-old) seedlings of Betula and Tsuga. Averaged over three sites, established seedlings per metre square on Tsuga wood, Betula wood, Acer wood and soil were 0.42, 0.21, 0.08 and 0.01, respectively, for Tsuga, 0.60, 0.15, 0.10 and 0.01, respectively, for Betula and 0.09, 0.03, 0.04 and 0.98, respectively for Acer. Long-term seedling survival was also greatest on Tsuga wood which supported seedlings as old as 13 years, while on Acer wood seedlings > 3 years-old were rare. Despite a strong general pattern in seedling densities, there were large site differences perhaps related to water availability. 4. Synthesis. We conclude that the strong association of Tsuga and Betula seedlings with Tsuga decaying wood and not with Acer wood or soil, combined with the distribution of Tsuga wood, helps explain (i) the close Tsuga-Betula association, (ii) the maintenance of distinct, temporally stable Tsuga- and Acer-dominated patches; and (iii) the decline of Tsuga in selectively logged forests as mature trees are harvested and decaying Tsuga wood is scarce. Differences between Tsuga and Acer wood may reflect general differences in the suitability of conifer vs. angiosperm wood for seedling establishment.