The Role of Nitrogen Fixation and Climbing in Competitive Interactions Between Bird Vetch and Native Plants Diane Wagner, University of Alaska Fairbanks, Institute of Arctic Biology and Department of Biology and Wildlife The invasive species bird vetch (Vicia cracca) is common on Alaska roadsides, grasslands, and low-density forest, where it often outcompetes native vegetation. Using experiments and surveys, I investigated the contribution of two life history traits, nitrogen fixation and climbing, on vetch's ability to grow rapidly and compete successfully. When cultivated from seed in a greenhouse alongside five native legume species and in the presence of both native and non-native soil microbes, vetch accumulated twice the biomass of native plants over the same period of time, despite allocating proportionally less biomass to roots. One explanation for this pattern is that vetch plants were far more likely to form root nodules indicative of symbiosis with nitrogen fixing bacteria. When grown in sterile medium inoculated only with native microbes, vetch plants grew larger and produced a much greater mass of root nodules than individuals of the native Astragalus alpinum. These results suggest that, though non-native, vetch forms highly effective symbioses with local nitrogen-fixing bacteria. Aboveground, vetch plants grow vertically by clinging to woody shrubs and small trees. By climbing vertically, vetch plants increase access to light and enhance growth rates relative to vetch plants prevented from climbing. In a survey of shrubs and small trees across six invaded sites in Fairbanks, climbing by vetch vines reduced light to the structural host by 26 percent. Small Populus trees in an invaded field site grew less and were more likely to die within a year than trees from which vetch vines were experimentally deflected. The results suggest that successful competition by vetch is supported by effective symbioses with local microbes and a strategy of robbing light from structural hosts. Primary Learning Outcomes 1. Though lacking a long history in Interior Alaska, bird vetch was more likely to form symbioses with local nitrogen-fixing microbes than the native legume species tested. 2. Climbing by bird vetch increases access to light and can reduce growth rate and survivorship of the woody structural host. 3. Effective symbiosis with local soil microbes and a climbing growth habit support high growth rates and competitiveness by bird vetch.
The role of nitrogen fixation and climbing in competitive interactions between bird vetch and native plants Diane Wagner Biology & Wildlife, Institute of Arctic Biology University of Alaska Fairbanks Student contributors: Amanda Spenser, Alexandria Wenninger, Jenifer Wheeler, Savannah Yunkers Annual Invasive Species Workshop 24 October 2017 K. Gobroski
Bird vetch (Vicia cracca) Native to Europe and Asia in open habitats First report in North America 1860 Rampart Experiment Station on Yukon River in 1909 Later to Fairbanks and Palmer Experiment Stations Noxious weed in Alaska Cadbury 1971; Aarssen et al. 1986; Klebesadel 1980
3 m
V. cracca can establish under conditions of low disturbance and fertility (Thompson et al. 2001)
Traits That Facilitate Establishment Nitrogen fixation Climbing Allelopathy
n = 65, vetch > natives at both nutrient levels P<0.05 Growth Potential Relative to Native Total biomass (g) 7 6 5 4 3 2 Fabaceae Pale low nutrients Dark moderate nutrients 1 0
n = 65, species effect P=0.002, nutrient effect P=0.04 Investment in Roots Root biomass (%) 35 30 Pale low nutrients Dark moderate nutrients 25 20 15 10 5 0
Nodulation % Plants with root nodules 100 80 60 40 20 0 Total biomass (g) 5 4 3 2 1 0 0 25 50 75 100 % Plants with nodules 0 0 0 n = 65, species effect
Use of Local Microbial Symbionts Total biomass (g) 10 8 6 a b b Nodule mass (mg) 250 200 150 a 4 100 b 2 0 50 0 c n = 19, species effect
Nitrogen Fixation Summary Nitrogen fixation supports high growth rates by V. cracca relative to native legumes V. cracca appears to use local microbial symbionts more effectively than native legume species Light is key to symbiotic N fixation as well as growth
Bird vetch (Vicia cracca)
Advantages of Climbing PAR (μmol m -2 sec -1 ) 1400 1200 1000 V. cracca biomass (g) 250 200 800 150 600 100 86 cm 400 36 cm 200 50 0 15 30 50 70 Height above ground (cm) 0 Vetch on aspen Vetch alone
Vetch Shades the Structural Host Average 27% (± 4 SE) decrease in PAR to host due to V. cracca n = 6 sites, 32 trees and shrubs
What is the effect of climbing vetch on host performance?
Aboveground Competition Experiment Natural Setting Study plot grassland slope colonized by small aspen trees (0.5 1 m) 2 treatments: Vetch contacting aspen deflected to side (not pulled) Vetch unmanipulated Measured host mortality and change in height & stem diameter (growth) 13 months later n = 40 aspen
Mortality
n = 40, P = 0.02 Mortality 30 25 Percent mortality 20 15 10 5 0 Vetch Present Vetch Deflected
Growth of Survivors 60 1.6 Height change (cm) 50 40 30 20 10 Diameter change (mm) 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Vetch Present Vetch Deflected 0 Vetch Present Vetch Deflected
Conclusions Effective light capture through climbing combined with efficient nodulation support high intrinsic growth rates and competitiveness by V. cracca On forest edge or interior, V. cracca has the potential to alter recruitment dynamics of native trees