Emily Wedel, from Kansas State University’s Konza Prairie Biological Station, describes her article: Divergent resource-use strategies of encroaching shrubs: Can traits predict encroachment success in tallgrass prairie?
Background: Attack of the shrubs
The expansive grasslands that once dominated North America’s Central Great Plains have all but disappeared. The remaining grass-dominated regions that avoided conversion to row-crop agriculture are now being consumed by a thick layer of trees and shrubs, displacing herbaceous species. The expansion of woody plants into grassy ecosystems, known as woody encroachment, is a global phenomenon resulting from the complex interactions between increased atmospheric CO2 concentrations and the suppression of fire and browsing that historically limited tree cover. While woody encroachment is typically due to the expansion of native species, not all native shrubs are expanding at the same rate. Our study quantified whether the most abundant encroaching species in tallgrass prairie have similar traits that promote their success or unique traits that differentiate their carbon- and water-use strategies.
The Study: Characterizing the growth strategies of woody encroaching species
We quantified carbon gain and drought tolerance traits for the seven most abundant encroaching shrubs at Konza Prairie Biological Station (Manhattan, Kansas, USA). Konza Prairie is an experimental landscape with over 30 years of plant species composition data in response to long-term fire and grazing manipulations. The focal woody species spanned an order of magnitude in abundance and have encroached at vastly different rates. We measured aboveground allocation to leaf and stem mass and quantified detailed leaf-level physiological traits, such as photosynthetic capacity and turgor loss point. Together, these traits give deeper insight into interspecific differences in shrub physiology to assess whether the most abundant encroachers have similar or different growth strategies.
Key Findings: There is more than one way to encroach successfully
We found that there are two major types of encroaching shrubs in this ecosystem: tall woody species that form dense thickets versus shorter species that allocate more biomass to leaves and have higher photosynthetic rates. Two of the most abundant encroaching species had the most extreme values for several traits, including height:diameter ratios, maximum photosynthetic rates, and wood density. However, these two species varied in opposite directions. The relative advantage of these opposing growth strategies varies across disturbance gradients. Tall shrub species with dense canopies can shade out grasses and further promote their growth by suppressing fire. In contrast, the shorter species with higher photosynthetic rates co-exist with grasses and recover quickly after fire. These results highlight the diversity of growth strategies among encroaching species within a single grassland site. We also highlight that single traits rarely explain species abundances, and measuring a suite of traits directly related to plant physiology, such as carbon assimilation, can more clearly capture functional differences among co-existing species.
Implications: A diverse shrub community is likely difficult to manage
Our study suggests that niche differences among a community of woody species promote the rapid encroachment of several shrub species at a single site. We emphasize that woody encroachment is not a ‘one-size-fits-all’ strategy, and managing these shrubs likely requires an equally diverse approach. The high diversity of functional types among encroaching shrubs may drive a high ‘response diversity’ to management practices and future climate scenarios. If either of these drivers changes in a way that harms the currently dominant woody species, there is likely another species in the corner waiting to take its place. By understanding the physiological strategies of encroaching shrubs, we can better predict which plant species will increase under future climate and land-use scenarios and inform potential management efforts to reduce woody cover.
