PS 62-67 - Fire and invasive species: The burning quest for truth

Thursday, August 11, 2011
Exhibit Hall 3, Austin Convention Center
L. Poulos, Department of Biology, University of Oregon, Eugene, OR, Bitty A. Roy, Institute of Ecology and Evolution, University of Oregon, Eugene, OR and B. Thomas, McKenzie River Ranger District

Invasive species that cause ecosystem-level change are particularly dangerous.  One way to cause ecosystem collapse is by altering fire regimes.   Here we present methodology and an experimental design for disentangling four questions regarding the ways that fire and invasive species interact: (1) The risk of fire and fire severity may increase in well-established areas from an accumulation of finer fuels due to non-native species biomass.  (2) Conversely, because some invasive species have different phenology than other vegetation in the invaded environment, they might stay green late into the fire season, having a dampening effect on fire risk and decreasing fire severity. (3) Fire itself may facilitate the spread of invasive species by exposing the soil, increasing light levels due to a reduction in canopy and by increasing seed dispersal on crews and equipment in the case of prescribed fire. (4) Fire may have a negative effect on the spread of invasives by killing established plants and seeds.


To address these questions we are working with Brachypodium sylvaticum, a shade-tolerant grass, which is an aggressive invasive species native to temperate Eurasia. Classified as a noxious weed in CA, OR and WA, and also recently found in NY, MO and VA, it has the potential to alter forest fire regimes by adding a thick thatch layer to the forest understory.  We have divided two sites (N8 and C16) in the Willamette National Forest into eight ±2 hectare plots with a 2m wide belt transect running down the middle. Half the plots are fire treatment and half are controls. On each transect there are two 1 m2 plots containing B. sylvaticum (“Brachypodium plots”) and four without B. sylvaticum (“dispersal plots”). Each site thus has 16 plots with B. sylvaticum (8 burned, 8 unburned) and 32 dispersal plots without B. sylvaticum (16 burned, 16 unburned).  We chose sites that are very different in terms of cover, density and fuels to increase the generality of the methodology. For example, N8 started with a 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).  The fire will be set this spring.  Our experimental design with pre and post burn measurements is a good way to address fire interactions with invasive species.

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