Species interactions are the foundations of ecological science. As early as pre-school, we begin building food webs and discussing the basic principles of species survival and interactions between living and non-living parts of an ecosystem. We know herbivores eat plants and prefer ‘tasty’ ones; plants compete for light, nutrients, water and space; and historical land management impacts on future actions. What we still don’t know is the importance of the interactions between these core elements. Which of these elements is the most important for land managers to take into consideration when they are trying to re-plant a forest, given that all three impact on seedling survival and systemic budget shortages?
There are many choices when attempting to re-grow forests but common principles are applied: controlling competing vegetation with brush cutting, herbicides and burning; and fencing off areas from vertebrate herbivores. The spread and dominance of invasive plants has made vegetation management practices more challenging because of their remarkably high survival, growth and reproduction rates. There is the compounding issue that many invasive plant species are not preferred forage for herbivores with native species often being more palatable, the basis of the commonly heralded ‘Enemy Release’ hypothesis.
A new study led by Daniel Johnson at Indiana University, has tested the interacting roles of competition from an invasive annual grass (Microstegium vimineum), pressure from vertebrate herbivores and the legacy of site management history on the growth and survival of four native tree species (Acer saccharum, Quercus alba, Quercus rubra and Quercus Shumardii). Daniel set up a split-plot experiment where he killed the invasive grass with herbicides, he fenced off the plots from vertebrate herbivores such as deer, and applied these two treatments in all factorial combinations. This split-plot design was set up at three sites that had recently been harvested for timber and three sites where harvesting occurred at least 20 years previously.
Some results found by Daniel and collaborators were predictable¾removing the invasive grass and excluding vertebrate herbivores increased seedling survival. We know that herbivores forage on plants and a fast growing grass species will compete with a small tree seedling for water, nutrients and light.
What was surprising, was the finding that competition from the invasive grass trumped herbivory. Plots where the invasive grass was removed and herbivory was not excluded showed a lower survival and growth rate than the interaction of these treatments. This suggests that the invasive grass may provide some shelter from herbivory for the seedlings and that some cover or alternative fodder for the herbivores could be beneficial for seedling growth. But the treatment where the invasive grass was not removed but herbivory excluded was not significantly different to the control plots (no treatment) suggesting that competition from the invasive grass provides greater pressure than herbivory. This study also revealed that site management history trumped competition and herbivory depending on the native tree species. The growth and survival of the seedlings of the oaks were greater at the recently harvested sites.
The study byJohnson et al. is the perfect example of how an apparently simple but well-thought-out and painstaking experimental design can help solve complex questions of ecological interactions for both theoretical and practical benefits. Understanding these interactions can guide land managers on the most important to concentrate given limited budgets and time for key priorities like reforestation.
Guest post by Jennifer Firn, Associate Editor, Journal of Applied Ecology.