Belowground competition, aboveground competition, and aboveground facilitation: Complex interactions among canopy trees, tree seedlings, and shrubs in red pine ecosystems

Background/Question/Methods

In forested ecosystems, young tree seedlings interact with already established vegetation such as adult trees, shrubs, and other juvenile trees. In many forest ecosystems, thick shrub understories deter regeneration through increased mortality and slowed growth of tree seedlings and saplings. 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.  Manipulations included: removal of shrubs, tieback of shrubs, removal of shrubs and addition of shade, unmanipulated shrubs.

Results/Conclusions

We found that shrubs had significant effects on survival and growth and found evidence of both competition and facilitation that depended on tree species identity and ecological context (e.g. growing in open or forest interior). White pine (Pinus strobus) had significantly higher survival in forest compared to the open; removing shrubs increased survival in both treatments. In contrast, jack pine (Pinus banksiana) and red oak (Quercus rubra) had higher survival in large gaps but this effect was significant only in the absence of shrubs. Red maple (Acer rubrum) showed evidence of facilitation as it had higher survival in gaps when grown under shrub cover. White pine also showed higher survival in gaps under shrub cover but only when roots were not present (e.g. under simulated shade only). Addition of shade in the forest understory (due to shrubs or shadecloth) decreased survival in shade intolerant jack and red pine (Pinus resinosa). Finally, it appears that roots always have a negative or competitive effect. Jack pine showed decreased survival in open plots where shrub roots were present and white pine had lower survival in all plots with roots regardless whether they were in open of forest conditions. Our results highlight the complexity of species interactions in forested ecosystems.