Fuel reduction thinnings have taken place on 7,000 + ha of oak and chaparral managed by Medford, OR District of BLM since 1996 (map of fuels reduction projects as of 10/2006). Prescriptions include target reductions of stem density by ~ 90% and of canopy cover by ~70%. Thinning is accomplished by hand cut and pile burn or by mechanical mastication.
What effects do treatments have on site conditions 4 – 7 yr post-thinning?
Conditions do differ immediately after the two types of treatment. However, 4 – 7 yr post-treatment, only the obvious and expected site conditions still differ – hand cut sites had more cover of burn pile scars, and masticated sites more cover by woody debris.
Do understory plant communities differ between treated & untreated areas 4 – 7 yr post-treatment?
In response to concerns about high-severity wildfires, land managers in the western United States are carrying out extensive programs of fuel reduction thinning. In some cases, treatments are also intended to facilitate restoration of ecosystems whose composition and functions are known, or presumed, to have been altered by fire suppression. Various treatment methods are used, and these are likely to be differentially successful in achieving fuel reduction and restoration goals. We studied responses of plant communities to two types of fuel reduction treatments in chaparral communities of southwestern OR, where treatments cause radical reductions in canopy cover (see paired photographs, below). Do treatments cause changes in understory communities? Are native species, particularly perennial grasses and forbs, favored by treatments? Alternatively, do treatments result in expansion of weedy species, either native or exotic?
We sampled site and vegetation using 30 sets of paired 50 X 1 m plots, each pair including one treated and one untreated plot. (See a map of the study area.) All treatments had been applied 4 – 7 yr prior to our sampling. The red lines (right) show an example of a paired plot.
We analyzed data using multivariate approaches – a blocked version of multiresponse permutation procedure (MRBP), blocking on pairs; and non-metric multidimensional scaling (NMS) ordination (PC-ORD software was used in analysis).
p = 0.000
A = 0.56 – 0.159
The circles in the ordination figure (right) represent the study sites; the farther the distance between circles, the more dissimilar the sites in terms of herbaceous species composition. Overlay arrows show associations of variables with ordination axes; arrows point toward sites with relatively high values for each variable, and line lengths are proportional to the strength of association. ( Variable labels are defined below.)
Vegetation variables associated (|tau| > 0.3) with unthinned sites:
Higher canopy cover (%Cover); perennial species cover (Perennial); proportions of perennials (PropPerenn), native perennial grasses (PropNPG), exotic annual forbs (PropEAF), and native perennial forbs (PropNPF); and oak and conifer regeneration (OakRegen; joint line for conifer regeneration not shown).
Vegetation variables associated (|tau| > 0.3) with thinned sites:
Higher total herbaceous cover (Herbsum); cover of native species (Native), exotic species (Exotic), annual species (Annual), exotic annual grasses (EAG), and native annual forbs (NAF); proportions of annuals (PropAnnual), exotic annual grasses (PropEAG), and native annual forbs (PropNAF).
These results show strong significance for test of difference between thinned and unthinned sites, but weak effect size.
Overall result from MRBP
MRBP tests for statistical differences between groups, as well as the magnitude of difference (effect size).
Do plant trait groups respond differentially to treatment?
The pie charts below show the proportions of total herbaceous cover comprised of various trait groups in unthinned and thinned sites.
yellow = Exotic Annual Forbs (EAF); pink = Native Perennial Grasses (NPG); aqua = Exotic Annual Grasses (EAG); purple = Native Perennial Forbs (NPF); dark blue = Native Annual Forbs (NAF)
Differences in % cover by trait group (calculated as treated – control) are shown in the bar charts below.
Do effects on communities differ between treatment types?
Uncertain. MRPP comparing within pair (treated minus control) differences between hand cut pile burn and mechanically masticated sites indicated no significant difference (p = 0.84). There was suggestive evidence that effects of the two treatment types did differ within canopy community types (Arctostaphylos-dominated, Ceanothus-dominated, or mixed) but sample sizes were small. This warrants further investigation.
Four to seven years after treatment, both types of treatments were associated with:
Exotic Annual Grass cover
Native Annual Forb cover
Woody debris cover
Exotic Annual Forb cover
Native Perennial Forb cover
Canopy (tree and shrub) cover
No change in:
Species diversity (including both native and exotic species)
Native Perennial Grass cover
Photos were taken by Keith Perchemlides or Debora Coen, unless otherwise noted.
Page by Olivia Duren. Updated 1/2008.
Oak and chaparral ecology and fuels management
in southwest Oregon