The idea that ecosystems can be restored on the basis of plant functional traits is captivating. Restoration is time consuming, expensive, and outcomes can be uncertain. Furthermore, restoration of plant communities in particular can require endless maintenance, as exotic species continue to invade despite our best efforts.
For restoration through functional traits to work, however, we need to understand some key ecosystem and species features. For example, what are the abiotic conditions that allow certain plant species to invade (e.g. increased light or nutrient availability)? And, what are the plant traits that allow exotic species to invade an ecosystem (e.g. faster growth under increased light/nutrients)? If we can identify these drivers and response traits, we may be able to alter the drivers in a way that reduces or eliminates invaders.
Sometimes, ecologists are able to identify these traits and drivers bang-on. For example, based on previous research demonstrating that invasive grasses were reduced in the presence of shade in wet Hawaiian forests, Funk and McDaniel (2010) used plant functional traits related to light tolerance to successfully predict the physiology, growth and survival of native and exotic species exposed to shade. Native species had traits that indicated they could better cope with shady conditions (lower photosynthetic rate, higher quantum yield) while exotic species had traits related to rapid growth in high-light conditions. Indeed, as predicted, low light reduced invasive biomass by 80%, and while native species were less impacted (0-56%). By understanding the relevant abiotic filters and related traits, invasive species could be reduced or elevated and native species could flourish through the application of something as simple as a shade structure.
Sometimes, though, ecologists are mistaken about the importance of a given filter. One particular example is the role of nitrogen in plant invasion. Nitrogen additions to the soil have been shown to facilitate plant invasion in a number of ecosystems, thus, invaders must be better able to use nitrogen and so outcompete the natives, who can’t use nitrogen as efficiently (or so the thinking goes). Treatments which reduce soil carbon have been implemented to reduce or eliminate exotic species with very mixed success (see Perry et al. 2010 for an excellent review of this technique).
So. If nitrogen addition facilitates invasion, and invasive species are better able to use excess nitrogen, then ecologists should be able to identify traits associated with nitrogen acquisition and use these to predict which species suffer when nitrogen is experimentally reduced in the system…
Steers et al. (2011) decided to test both the utility of a trait-based approach, and the prevailing theory that nitrogen controls invasion, in a desert shrub ecosystem that has experienced elevated nitrogen due to fire and anthropogenic sources. This system has also been invaded by exotic annual plants that are thought to be nitrogen-loving…a common combination familiar to any practitioner of restoration.
Steers et al. measured traits relevant to nitrogen-use on 11 native and 3 exotic species. Over three years, they applied two levels of sugar (yep, regular old table sugar) to the soil surface of experimental plots. Sugar quickly stimulates the microbes in the soil (delicious sugar) causing them to take up nitrogen in the soil and convert it to a form unusable to plants. Sugar is ideal in this study, because it quickly disappears from the system as it is taken up by microbes, so researchers can be sure it observed results aren’t some persistent effect of sugar just lying around. Each year, the researchers sampled the soil to look for changes in soil nitrogen and they measure the cover, density, and biomass of plants in plots that had been treated with sugar and in control plots, which had received no treatments.
As expected, sugar significantly lowered the amount of nitrogen present in the soil, and thus the amount of nitrogen available to plants. And, in the first year of measurement, invasive species performed more poorly than native species in the sugar treated plots. In the second year, no plants germinated, and so, no measurements were taken. But in the third year, a different trend emerged. This time, both exotic species and some native species had reduced performance in the sugar treated plots. What gives?
When plant traits were examined, it turned out that native species didn’t differ much from exotic species in terms of traits related to nitrogen acquisition. In fact, some of the native species were found to have higher requirements for nitrogen that the exotics. So, nitrogen-use traits are not necessarily the traits we should be looking at. Instead, it seems that timing is everything in this system.
You see, in year 1, when exotic species were most impacted by nitrogen reduction, exotic species were the first to emerge at the beginning of the wet season. In year three, native species were the first to emerge. Sugar is a rapidly digested carbon source, and thus leads to very rapid reductions in soil nitrogen. So, whichever group of plants emerged first was hardest hit by reduced nitrogen, regardless of their traits.
Steers et al. conclude that traits related to rapid emergence and growth seemed to be better predictors of success when nitrogen is reduced, and that, on the whole, exotic species in this system tend to germinate faster and mature more quickly than natives, making them more susceptible to appropriately timed reductions in nitrogen.
Trait-based approaches to restoration do seem very promising, but they require a deep and nuanced understanding of the systems under study. Sometimes, the prevailing thoughts are only partially correct, and careful testing of underlying assumptions reveals the complex set of mechanism at play in plant invasion.
Funk, J. L., & McDaniel, S. (2010). Altering Light Availability to Restore Invaded Forest: The Predictive Role of Plant Traits. Restoration Ecology, 18(6), 865–872. doi:10.1111/j.1526-100X.2008.00515.x
Perry, L., Blumenthal, D., Monaco, T., Paschke, M., & Redente, E. (n.d.). Immobilizing nitrogen to control plant invasion. Oecologia, 163(1), 13–24. doi:10.1007/s00442-010-1580-x
Steers R.J., Funk J.L. & Allen E.B. (2011). Can resource-use traits predict native vs. exotic plant success in carbon amended soils?, Ecological Applications, 21 (4) 1211-1224. DOI: 10.1890/09-2345.1
17 December, 2013