In the last two-plus decades, landowners, land managers, and scientists have made significant progress addressing woody plant encroachment in America’s grasslands. Through dedicated research, new mapping tools, and  refined management guidelines honed through on-the-ground experience, the collective understanding of the ecological processes, impacts, and removal benefits regarding woody plants and grasslands has greatly improved since the late 1990s. However, gaps in the understanding of woody plant encroachment remain.

New research led by WLFW-affiliated researcher, Dillon Fogarty at North Dakota State University, is helping fill in these gaps around how trees reinvade grasslands after tree-removal treatments. The team’s findings help managers develop restoration plans that provide durable and long-term conservation results.

Most research on the ecological processes of encroachment has focused on how woody plants, like eastern redcedar, initially invade grasslands. Fogarty and his team examined how woody plants reinvade grasslands after mature trees have been killed using prescribed fire.

Fogarty and his team found that woody plants reinvade grasslands at a significantly faster rate than during the initial invasion process. This finding means that grasslands where trees have been removed are highly vulnerable to reinvasion and require follow-up management to prevent sites from quickly transitioning back to eastern redcedar dominance.

The team conducted their study in the Loess Canyons Experimental Landscape, where landowners have been partnering to use prescribed fire to halt, and even reverse, woody encroachment across ownership boundaries since 2002.

Specifically, the team found that with reinvasion, eastern redcedar density recovered in 4-11 years and then continued to increase beyond the density range found in nearby eastern redcedar woodlands. In nearby grasslands experiencing invasion for the first time, tree density remained relatively low and stable until 11 years post-fire.

Tree cover likewise increased faster with reinvasion compared to adjacent grasslands experiencing invasion. With reinvasion, cover increased rapidly after seven years post-fire. At 17 years post-fire, tree cover was predicted to be about 55% compared to 84-87% in nearby eastern redcedar woodlands.

In adjacent grasslands experiencing invasion for the first time, cover remained low and stable until 14 years post-fire, when it began to increase. At 17 years post-fire, tree cover in previously uninvaded grasslands was only 2.2%, well below grasslands undergoing reinvasion.

The team also found that, initially, invasion and reinvasion were readily distinguishable based on the former grassland-woodland ecotonal boundary; however, over time, this became less distinct as woodland patches quickly recovered and then started to expand into surrounding grasslands.

The researchers note that woody plant invasion causes major changes in grassland ecosystems, some of which persist after tree removal and are suspected to play an important role in the speeding up of reinvasion. These include seeds left in the soil that readily sprout following restoration, skeleton trees left after prescribed fire that provide avian perches and boost avian seed deposition, and soil-microbial changes that may facilitate seedling recruitment.

For managers and landowners, the evidence of rapid reinvasion following fire-based restoration treatments highlights the need for improved, long-term management plans that account for reinvasion’s faster timeline. This is especially true for western grasslands where reinvasion will happen faster than initial invasion, though not as fast as reinvasion in the Great Plains. The researchers note that the relatively slow rates of initial invasion can be managed with less intensive treatments compared to areas undergoing restoration, which underscores the need to defend core, intact grasslands from being invaded in the first place.

Through its framework for conservation action in the Great Plains grasslands biome, Working Lands for Wildlife is working with partners and landowners to address woody plant encroachment in the Great Plains. This adaptive and flexible framework makes it possible to incorporate research like this into conservation planning, leading to improved and longer-lasting conservation outcomes.

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