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Team IDs invasive tree dispersal patterns on Great Plains prairies
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As woody vegetation marches across grasslands — encroaching on prairies that wildlife and ranchers alike have come to depend on — ecologists are studying exactly how that invasive vegetation is populating and transforming formerly intact landscapes.
Mitigating the spread of invasive vegetation, including the eastern redcedar tree now threatening Nebraska’s prairies, means understanding the dispersal of seeds that eventually mature into new trees and bear seeds of their own. To date, though, no studies have analyzed how that dispersal may be shaping the patterns of encroachment seen on the Great Plains.
Husker researchers Dillon Fogarty, Dirac Twidwell and Robert Peterson recently investigated the issue at two sites in the Nebraska Sandhills: the Nebraska National Forest at Halsey and the Samuel R. McKelvie National Forest near Valentine. Both forests were hand-planted in the early 1900s and remain surrounded by the mixed-grass prairie native to the area.
The team began by randomly picking five treeless points around each of the two forests, then drawing lines from each of those 10 points to the nearest eastern redcedar planting — the oldest feasible source of seeds in the respective vicinity. Fogarty, Twidwell and Peterson proceeded to walk those 10 transects while marking the location, sex and height of any eastern redcedar tree they encountered. By using tree height as a proxy for age, the researchers were able to chronicle the encroachment of eastern redcedar over time.
Of the 961 eastern redcedar trees sampled by the team, more than half were growing within just 40 feet of the nearest seed-bearing tree, with their density declining rapidly beyond that distance. And more than 95% were located within 200 yards of the nearest seed source, delineating a zone in which grasslands are most vulnerable to woodland conversion.
Though most eastern redcedar occurred near seed sources, the farthest-flung outliers were found more than half a mile from those sources. The team suspects that the long-distance outliers stem from grassland birds foraging on eastern redcedar’s berry-like cones before flying to areas of treeless prairie and expelling the seeds.
Mature eastern redcedar trees produce up to 1.5 million seeds a year. Fogarty said the species’ one-two punch — with local seed dispersal driving swift conversion to woodland as long-distance outliers speed expansion across large tracts of land — makes eastern redcedar a serious threat to Nebraska’s prairies.
Efforts to defend Nebraska’s already under-siege prairie against the further encroachment of eastern redcedar should especially focus on areas within 200 yards of mature trees, the researchers said. At the same time, managers should keep an eye on treeless expanses residing far from invasive vegetation. Early detection and rapid responses that eradicate the long-distance outliers from remote tracts will prove critical to safeguarding what remains of Nebraska’s pristine prairie, the team said.