What makes a community
susceptible to weed invasion?
Characteristics of invasive species
Characteristics of ecosystem
Relationship between invasive species and ecosystem
e.g., lock and key models (Heger and Trepl 2003)
Idaho
Logan
Nevada
Utah
Soil Texture
Mean particle size distribution
(soil texture) per site
Jackson
Cove
Bowhuis
Holbrook
Stocker
Grandine P.
Grandine B.
Roe
Wight
Stone
E. B. Pine
Russell
Grouse Cr.
Yost II
Yost 1
Lynn
EY
Narrows
Darby
Bridge
Jim Sage
HPP
Cassia
Idahome
Warm
Sandrock
Onaqui
S.B. Pine
Lofgren
Lookout
Boulter
Buckskin
Red Butte
Wilkins
Toana
Ward
Brush
Bell Canyon
Soil texture varies widely among sites,
though 90% are loam or silt loam.
Site name
Tex ture Class Distribution
1%
1%
2%
4%
silt loam
1%
loam
3%
loam/ silt loam
silt y clay loam
25%
loam/ clay loam
63%
clay loam
sandy loam
0%
20%
40%
60%
80%
Percent Sand, Silt and Clay
100%
sandy clay loam
Vegetation Diversity
25
Average grass, forb and shrub species
Shrub spp
per site
20
Forb spp
15
Grass spp
10
5
0
29
22
20
27
8
37
6
30
24
33
15
Site Number
10
35
1
18
9
12
38
4
What combinations of plant materials
are best to resist weed invasion?
Tom Monaco and Eamonn Leonard
Objective
• Evaluate the effect of species, growth form, and
disturbance on weed resistance.
Lambers et al. 1998
• Research Questions
1. Does a mix of three morphologically distinct growth
forms provide greater weed resistance than a single species ?
2. Is resistance to invasion greater in plots that contain
species of same growth form as the invader?
3. Does disturbance similarly facilitate invasion in
grass, forb, and shrub plots?
Study site:
Millville, in northern UT, Cache County - Soil at this site is a
Ricks gravelly loam series.
Established 600 plots in 2003
Mixes and monocultures of 12 species in 20 combinations
7 plot types used in this experiment
Low and high diversity plots containing:
Grasses
Forbs
Shrubs
Mix of all 3
30 replicates of each plot type arranged in a completely
randomized design (240 total plots).
Disturbance and introducing weed seed
Removed 4 plants in Fall 2004 (low-diversity only)
Added seed of Bromus tectorum and
Isatis tinctoria (~400 seeds/species)
in November 2004 and 2005.
Density of weeds evaluated
in Summer 2005 and 2006).
Disturbance similarly increased invasion in grass, forb,
and shrub plots.
Seedling density
Treatment
2005
2006
Downy brome
Intact
Disturbed
28 b
47 a
139 b
230 a
Dyer’s woad
Intact
Disturbed
17 b
51 a
11 b
23 a
• Conclusions
1. Does a mix of three morphologically distinct growth
forms provide greater weed resistance than a single species ?
No - grass plots best resisted weed emergence
2. Is resistance to invasion greater in plots that contain
species of same growth form as the invader?
Yes, but only for downy brome
3. Does disturbance similarly facilitate invasion in
grass, forb, and shrub plots?
Yes
Acknowledgments
We thank Justin Williams, Kevin Connors, Lowell
Gardner, Travis Taylor, Tonya Shoemaker, Kevin
Gunnell, and Jamin Johanson for assisting with field
research.