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Large contribution of woody plant expansion to recent greening of the Northern Great Plains

Linking two consequential vegetative changes across one of the last intact grasslands on Earth.

According to the World Wild Fund for Nature (formerly World Wildlife Fund, or WWF), the Northern Great Plains are one of the 142 most biologically important terrestrial ecoregions on Earth. The Northern Great Plains, spanning portions of five US states and two Canadian providences, make this list because the ecoregion is home to one of the largest and last remaining relatively intact temperate grasslands globally. For scale, this relatively intact area is just larger than the state of California. The Northern Great Plains are a conservation target largely due to the grasslands' extirpation through conversion to agriculture.

However, other large-scale social and ecological transitions occur throughout the region. (For more on the social-ecological dynamics and conservation strategies of the NGP, see this post!)

The Northern Great Plains. The top right inset (c) is the grassland portion of the ecoregion.

One of those ecological transitions is a march towards a more woody-dominated vegetation regime across the grasslands known as woody plant expansion (also woody plant encroachment, elsewhere), or WPE. WPE has large consequences for the ecology and function of the ecosystem, thus it is important to understand where it is occurring, why it is occurring, and examine the potential for continued change. Documentation of WPE in the Northern Great Plains has largely been circumscribed to anecdote, despite being well studied across many other grasslands across North America. As such, we sought to conduct the first large-scale, high spatial resolution examination of WPE across the Northern Great Plains.

However, a second large-scale ecological phenomenon is occurring across grasslands across the globe simultaneously, including the Northern Great Plains. Large areas of grassland have increased in vegetative greenness. Increased greenness, or greening, is a proxy for increased vegetative productivity. This could be due to individual plant growth, or due to large-scale vegetative changes. This greening phenomenon is largely caused by increasing precipitation, temperature, and/or CO2 concentrations. Increased CO2 concentrations in the atmosphere lead to higher water use efficiency (More CO2 into the plant, less H2O lost from the plant), which can cause a plant to be more productive per amount of water. Pair this with increased water availability (i.e., increased precipitation) and increased growing season length (i.e., increased temperatures). The result is a far more productive grassland than in past decades (although nitrogen may potentially constrain continued growth in the future. For more on that, see this post!).

Thus, we know then that the Northern Great Plains has undergone some degree of WPE (at least anecdotally) and has simultaneously experienced large amounts of greening. Naturally, this raised a fundamental question: To what degree are greening and WPE linked?

We examined both vegetative changes and their potential causes across the California-sized grasslands of the Northern Great Plains of North America. We found that, across this imperiled grassland, nearly half of the grasslands in the Northern Great Plains experienced increased tree cover (read WPE), and over half underwent greening over the past 20 years. We also found that these changes are largely driven by the absence of wildfire and changing climate, as previously speculated. Moreover, regarding our fundamental question, we reveal a strong coupling between the two vegetative changes; WPE was positively associated and spatially coherent with a large proportion of recent greening.

Greening is measured here as the proportional change in NDVI; Woody plant expansion as the proportional change in percent tree cover. The dark grey shaded area is grassland portion of the ecoregion.

Our results highlight that WPE is a large component of vegetative greening in the NGP and imply that it may be an underestimated component of greening trends in other semi-arid biomes. Furthermore, agriculture may not be the only threat to the persistence of grasslands in the future. Whether or not this is a desirable outcome, however, we will let the managers and conservation organizations of the ecoregion decide.

In (a), the concurrent greening and WPE extent can be observed. In (b), we see the relationship between greening and WPE. In (c), we see the average amount of greening as defined by the novel greening index used in our study.

Authors: Bryce Currey, Dave B. McWethy, Nickolas R. Fox, E.N. Jack Brookshire

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