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Silt salt-desert Sand salt-desert
Post burn Shrubunderstory
High elevation
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Spring 2010Fall 2010Spring 2011Fall 2011Spring 2012Fall 2012Average
Dan Harmon and Charlie D. Clements USDA, Agricultural Research Service,
Great Basin Rangelands Research Unit 920 Valley Rd Reno, Nevada 89512
Cheatgrass (Bromus tectorum) management is a formidable task that requires prioritization of efforts. Invading virtually all arid/semiarid habitat types in northwestern Nevada, cheatgrass populations can radically vary by annual weather conditions. We conducted multiple observational experiments to determine to what degree climate vs. soil-habitat vs. heritable traits affect phenology, biomass and seed banks.
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Figure 1. Total and seed biomass per plant. Seed source on the X-axis and grouped by soil type. - a, ab represent significant differences (p<0.05) from group
Soil type Silt Salt Desert Sand Salt Desert Post Burn High elevation Shrub Understory
grams Biomass
Seed (g)
* *
a
Soil type Seed Source High Elevation 66 a 29 b Post Burn 63 ab 63 a Shrub Understory 59 ab 67 a Sand Salt Desert 50 bc 61 a Silt Salt Desert 38 c 61 a
Table 1. Reproductive percent of total biomass. Seed to total biomass ratios responded to soil type and seed source (Table 1). Plants exhibited lower resource allocation to seed production when grown in silt salt desert soils. Cheatgrass seed from higher elevations displayed the lowest percent of seed to total biomass.
Methods In a greenhouse reciprocal garden we tested two treatment variables 1) seed source population (n=5) and 2) soil type (n=5). We measured four response variables: 1) Biomass 2) Seed to biomass ratio 3) Days to flowering and 4) Total life duration. Seed was collected from parent populations and then grown for 2 generations in a greenhouse under equal conditions before experiment to avoid any maternal environment effects. Pots were watered equally (100ml/ 3days)
Biomass
Seed Production Days to flowering differed by seed source. Soil type had little effect on flower timing (sphere size). High elevation seed source exhibited delayed flowering and a long life span. Total life span differed more by soil type than seed source. Salt desert soils lead to shorter life spans *.
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Seed Source Soil Type
Figure 5. Life span on X-Axis. Data label (Sphere size) is
days to flowering.
Life Span Days
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Parent populations
Hawsley series: very deep, excessively drained soils on sand sheets
Playa: fine silt, low permeability. Increased aeolian course soil deposition after cheatgrass invasion, providing germination safe sites, advancing the invasion
Indian Creek series: clayey, shallow, well drained soils derived from mixed igneous rocks
Old Camp series: loamy-skeletal shallow, well drained soils on hillslope
Kyburz & Aldax series: Fine-loamy, moderately deep, well drained soils on uplands
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Figure 3. Cheatgrass garden field results. Samples collected June 1st.
Field common garden
June 1st (%) green vegetation
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Drill row of seed population. (A) Silt salt desert –senesced (B) Pine forest- still green. Photo June 1st
Figure 2. Seed bank (seeds/~.09m2 [1ft2] ) measured the first week
of May and August. Annual variation can be explained by favorable germination conditions. For example 2011 had an abundant germination year beginning with a warm wet October 2010. The high degree of germination led to a small spring 2011 seed bank *. Concurrently with a large dense population (2011), high seed production led to a large Fallⱡ seed bank.
Seed Banks (measured by bioassay technique)
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Discussion Experimental and field observation research found that big sagebrush and post fire habitat is at the greatest risk for fire. Big sage understory soils were the most productive (Figure 1) creating more fuels and increased fire risk while post burn sites had the largest seed banks (Figure 2). Experimentally, silt salt desert soil decreased seed production (Table 1). However, in the field, salt desert plants produced more seed compared to other populations (Figure 3). Summer germination and secondary flowering may also compensate for initial reduced seed production (Figure 4). Sand salt desert maintained the smallest seed banks and had very low establishment during drought years (2012-2013) indicating a possible lower fire risk. Pine forest populations exhibited heritable traits (delayed flowering Figure 5 and increased biomass Figure 1). However a larger biomass was not observed in arid common gardens (Figure 3) indicating a resource availability limitation. The longer green vegetative period of forest populations and lower seed production (Figure 3) compared to salt desert populations, which flowered and senesced ~ a month earlier in common gardens, indicates a lower fire risk for high elevation cheatgrass populations.
Seeds/ft2
(A) Summer germination (June 17th 2009) Silt Salt Desert site, (B) single plant 2nd crop of seed production.
2nd crop seed
1st crop seed
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Summer Germination and Secondary flowering
Secondary flowering and summer germination most commonly occurred on fine silty soils.
Figure 4.
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Post burn ShrubUnderstory
Pine forest
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57 2013 cheatgrass density (plants/ft2)
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Our results found that biomass differed by soil type and seed source . High elevation populations had the largest biomass irrespective of soil medium. Among the lower elevation populations only the salt desert populations ranked the greatest in its own soil*, possibly indicating adaptation to the
harsh salt desert habitat. Understory soils were the most productive + .
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