Resistance by perennial grass communities to the invasion and establishment of musk thistle

Background/Question/Methods

Musk thistle (Carduus nutans L.), an introduced broadleaf weed species of European origin, is commonly found throughout much of North America. The occurrence of musk thistle populations in perennial grasslands is thought to be related to the opportunistic growth habit of the invasive species and the functional diversity of cool- and warm-season perennial grass communities. Field studies were conducted at the University of Nebraska-Lincoln West Central Research & Extension Center to determine the ability of a diverse perennial grass community to resist invasion and establishment by musk thistle. In plots of bare ground and established cool- and warm-season perennial grass communities, musk thistle seed was planted by hand to establish population densities representative of naturally occurring stands. A mowing treatment to simulate grazing was applied to half the perennial grass plots. Between April and October, measurements were recorded for plant cover, belowground root growth, light penetration and soil moisture in bare ground and perennial grass communities.  

Results/Conclusions

Musk thistle cover peaked at 90% and 38% in October in bare ground and grazed warm-season grass plots, respectively. In the non-grazed plots, musk thistle cover was less than 1% in June and by August 0% cover observed in both cool- and warm-season grass communities. Cool-season grasses were actively growing prior to planting musk thistle seed in late April, yet light penetration (% PAR) was similar for grazed (62%) and non-grazed (58%) plots. By mid-June, the warm-season grasses emerged from dormancy and light penetration began to decline significantly in the non-grazed plots. In the cool- and warm-season grass plots, soil moisture in the upper 10 cm was constant for the entire season except for the grazed warm-season plots later in the season. Prior to June, belowground root activity was greater in the cool-season grass communities (4.42 mm live roots cm^-2) compared to warm-season grass communities (0.30 mm live roots cm^-2) at depths of 0-15cm. These results suggest that the early season belowground root activity by the cool-season perennial grasses prevented musk thistle seedlings from establishing beyond the seedling stage, even though adequate soil moisture and light were available.