Microstegium vimineum’s germination, survival, and growth in closed-canopy deciduous forests

Background/Question/Methods Microstegium vimineum (Japanese stiltgrass) is a shade-tolerant exotic grass capable of invading closed-canopy eastern deciduous forests of the U.S.  However, its growth under shade is reduced compared to its growth under high-light conditions, such as roadsides.  Our research evaluated the germination, survival, and growth response of M. vimineum planted in fenced forests under four relatively low light levels: 1) closed-canopy, 1% of full sunlight), 2) low light, 2%, 3) medium light, 4%, and 4) high light, 5%.  These approximate light levels were achieved by removing subcanopy trees 10 years ago, at which time the control, low, medium, and high light levels were approximately 2%, 4%, 8%, and 12% of full sunlight, respectively.  Current light levels are typical of closed canopy deciduous forests, however, they differ by increased understory and subcanopy re-growth in the higher light treatments. We measured shoot heights of transplanted two-week old seedlings (5.9 cm average height) and tallied germinations in buried pots with and without litter cover every two weeks from early June until early August of 2009 (six measurement periods, before flowering). We then removed, dried, and weighed both shoots and roots.  Our goal was to evaluate M. vimineum growth under a variety of closed-canopy conditions.
Results/Conclusions Germination rates without litter were higher in the high-light treatment than the control treatment, but below 8%.  Germination rates with litter showed no significant difference among light levels (all below 6%). Mortality was due to herbivory (snails and small rodents).  Shoots of the low-light treatment were significantly taller than those in the medium and control treatments. The high-light treatment did not differ significantly from the other treatments.  The plants grown under the low-light treatment showed less herbivory than all other treatments.  By the last week, low-light treatments averaged 30 shoots 15 cm or taller, while control, medium, and high-light treatments averaged 15, 21, and 16 shoots, respectively.  The tallest shoot occurred in the high-light treatment at 30.0 cm, with the second tallest in the low-light treatment at 29.5 cm.  Shoot and root dry weights (analyzed both with and without shoots with herbivory) did not differ significantly among the light treatments though the low-light treatment had the highest shoot and root dry weights.  Microstegium vimineum survival and growth are impacted by herbivory in forested areas with greater understory cover, which suggests a healthy understory layer may not only keep light levels down but also facilitate herbivory of M. vimineum stems.