Invasion and fire: A look at the interactions between prescribed fire and the invasive grass, Brachypodium sylvaticum, in the Willamette National Forest, OR

Background/Question/Methods

Brachypodium sylvaticum, a shade-tolerant, aggressive invasive grass native to temperate Eurasia, is classified as a noxious weed in CA, OR and WA, and could possibly cause ecosystem collapse by altering fire regimes.  To examine interactions with fire we divided two sites in the Willamette National Forest into eight units, and randomly selected half for treatment with prescribed fire. Each site has 16 plots with B. sylvaticum (8 burned, 8 unburned) and 32 dispersal plots without B. sylvaticum (16 burned, 16 unburned).  There are four possible outcomes:  (1) Fire severity and intensity may increase in well-established areas with the accumulation of finer fuels from B. sylvaticum.  (2) Conversely, because B. sylvaticum stays green late into the fire season, it could have a dampening effect and decrease fire severity and intensity. (3) Fire may facilitate the spread of B. sylvaticum by exposing the soil, increasing light levels due to a reduction in canopy and by increasing seed dispersal on crews and equipment. (4) Fire may control the invasive by killing established plants and seeds.

In a complementary greenhouse study, we sampled soil cores from every plot to assess whether fire affects the seed bank of B. sylvaticum. Soil cores were mixed with a combination sand and sphagnum substrate and randomly arranged in the greenhouse. Brachypodium sylvaticum germination was compared between fire treatment plots and controls.

Results/Conclusions

The fires were set in the spring of 2011. Pre-burn, site 2 had higher density of B. sylvaticum (33±10 vs. 1±0.61/linear m, P<0.0001), more canopy cover (28±0.8% vs. 22±1.4%, P=0.0003), and more cover by other plants (43%± 4.41 vs. 27.89%±2.46, P=0.0007). We found that with a spring burn B. sylvaticum density decreases fire severity, χ2 = 11.92, 63 and P = 0.0077, with differences between the sites. The fire treatment significantly decreased B. sylvaticum germination in comparison with the control (0.14±0.07/plot burned versus 0.30±0.16/plot controls, P=0.0153) without a significant difference between the sites.

Future analyses: Does fire influence the abundance and dispersal of B. sylvaticum?