Nuttle et al. 2013

Post date: Apr 15, 2013 6:20:11 PM

This paper (subscription required) describes the results from a five year experiment, tracking over 28,000 saplings (!) in a deciduous forest in the eastern US, with manipulations of surface fire activity, deer browsing, and forest gap creation. The short version of this paper is that both disturbance by surface fire and disturbance by canopy gap creation can increase the diversity of forest stands… but only when deer are excluded from the plots.

Here’s the problem in a nutshell: eastern deciduous forests used to be diverse, and dominated by a mix of oak, chestnut, and hemlock. Now, they’re almost exclusively beech. Many scientists have suggested that this shift in composition and decline in diversity is due to fire exclusion, or a lack of canopy gaps, or deer overabundance and increased herbivory. There have been many studies with conflicting results. So, the question remains: which type or types of ecological disturbance are responsible for these changes? The authors assert that because deer are so over-abundant in these forests, browsing by deer is the overwhelming control on tree seedling and sapling survival. Because Nuttle et al.’s experiment is fully-crossed, they’re also able to show that, if deer are overabundant, neither fire nor gap creation will affect species richness or composition in these stands. On its own, excluding browsers has a big effect on species composition, richness and diversity. Fire or gap creation alone don’t have this effect. But using fire or gap creation after deer exclusion makes even bigger changes in the plant communities. The suggestion, then, is that to increase species diversity in eastern forests, where deer populations are far denser than they have been historically, the first order of business is to reduce the deer population. Only then can other treatments – like prescribed fire and canopy gap creation – be effective. But those other treatments can be quite effective, provided, again, that herbivory has been reduced.

There is a lot to like about this paper. Beyond a truly beautiful presentation of their data (I love the colorful stacked bars in figure 2), the authors directly tackle opposing results from previous scientists’ work. They’re able to say definitively why other studies reached erroneous conclusions because they actually manipulated the three relevant factors, and they tracked individuals seedlings and saplings for long enough to see some of the effects of these factors. I can’t guess at how much work this experiment was – how many personnel, and what the cost was. But I’m sure it was expensive. And it’s just the kind of ecological research that needs to happen to draw conclusions that are more solid than the typical vague, qualified and hand-waving answers that we usually get in ecological research.