Dr. Steve Archer, a biologist and ecologist currently teaching at the University of Arizona, studies the interactions between grasses and woody plants in relationship to soils, climate, fire and grazing.
Science Moab host Kristina Young spoke to Dr. Archer about how some ecosystems are shifting from being primarily made up of grasses to primarily woody plants and shrubs. This shift has wide-reaching effects on the desert systems we know and love.
This interview has been edited for length and clarity.
Science Moab: When an area transitions from being grassland to woodland, what else changes in the system?
Archer: As you start to add woody plants to these ecosystems, you’re starting to change the habitat of the plants and animals that evolved with those systems. So things like pronghorn or rattlesnakes or many species of grassland birds start to disappear when woody plants start to come in. So that’s a concern from a biodiversity conservation standpoint.
Also when the vegetation changes, the fundamental function of ecological systems changes as well. Changes that occur include how much water the vegetation is using, how much carbon is sequestered, how much biomass is produced over time and how much biomass is allocated to the below-ground structures like roots versus above-ground structures like leaves and stems. So you’re fundamentally changing the land cover. And when that land cover changes, so does how that land surface interacts with the near-surface meteorology.
Science Moab: Is it a sign that a system is degraded if we see these woody plants?
Archer: It depends on your definition and what your values are. There are some definite negative consequences, but there are also tradeoffs involved. For example, when woody plants encroach some systems, the evidence suggests that we’ll get substantially more carbon sequestration. So depending on what value society places on these different products from our ecosystems, that’s something we have to factor into the decisions we make.
Science Moab: As the climate continues to change, what could the future look like for grasslands?
Archer: One thing that might happen under future climates if we get longer windows of droughts or more frequent droughts, is that the system may favor woody plants that are more adapted to drier conditions relative to herbaceous plants like grasses. Woody plants have some adaptations going for them that may help them survive as we get to warmer, drier conditions.
Woody plant encroachment has already occurred in a lot of our ecosystems. As the climate continues to change, we may see a change in the kinds of woody plants that are dominating the landscape and we might also see some of the current woody vegetation start to suffer. Maybe we start to see a shift to plants like succulents like cacti which could become more advantaged under future climatic conditions.
The other thing that’s happening across many parts of the western United States is the invasion of exotic non-native grasses: annual grasses like cheatgrass in the cold deserts and things like buffalo grass and love grass in the hot deserts. Our historic concerns over woody plant encroachment may give way to concerns over the invasions of these exotic grasses which change the fire cycle. So we may have many ecosystems that went from a native perennial grassland to a shrub-invaded or tree-invaded grassland. And then the next step might be domination by cheatgrass and other annual grasses.
To learn more and listen to the rest of Dr. Steve Archer’s interview, visit www.sciencemoab.org/dynamic-desert.
Ecologist Dr. Steve Archer talks about climate and changing ecosystems
