Positive, Negative and Net Effects of Shrub-Tree Seedling Interactions in Red Pine Ecosystems

In forested ecosystems, young tree seedlings interact with already established vegetation such as adult trees, shrubs, herbs, understory trees, and other juvenile trees. The extent and nature of these interactions can affect growth rates of tree seedlings and their probability of survival. Thick shrub understories are thought to deter regeneration through increased mortality and slow growth of tree seedlings and saplings. After canopy loss or removal, development of dense shrub thickets may decouple tree regeneration responses from those expected based on overstory conditions alone. To explore mechanisms of interaction between tree seedlings and shrubs, we manipulated the magnitude of above- and belowground interactions and monitored survival and growth of 6 tree species (Pinus resinosa, P. banksiana, P. strobus, Acer rubrum, Quercus rubra & Betula papyrifera) common in the Great Lakes region. We found evidence of both competition and facilitation that depended on tree species identity and ecological context (e.g. growing in open or forest interior). Shrubs reduced light availability by 27-28% in closed canopy and gap sites and reduced N availability by \u3e50% in gaps In closed canopy sites, the presence of shade reduced survival of Pinus banksiana, P. resinosa and Betula papyrifera. In gap sites, the presence of roots reduced survival of all species and reduced growth of A. rubrum, P. banksiana and P. strobus. The presence of shade increased survival and growth of Acer rubrum, Betula papyrifera, and Pinus strobus in gap sites but only when roots were not present. For some species (e.g. Acer rubrum, Betula papyrifera), competitive effects of roots appear balanced by facilitative effects of shade leading to no net effect of shrubs on seedling growth or survival