Population demographics of an invasive weed: responses to soil fertility and cattle management

Abstract

Sericea (Lespedeza cuneata) is an invasive, perennial legume that suppresses native plant species and threatens native grasslands in the Great Plains. Although much is known about L. cuneata, population demographics (survival, growth, reproduction, and population growth rates) have never been assessed over realistic temporal and spatial scales. Population matrix models and perturbation analyses are useful in investigating underlying demographics and determining targeted management options. For two growing seasons, I quantified L. cuneata vital rates across eight, large-scale ranching operations in the Kansas Flint Hills representing seventeen populations, four cattle management regimes and three soil fertility types. Furthermore, I determined how herbicide application affected the population structure. Though soil fertility had no effect on L. cuneata vital rates, seedling survivorship was significantly higher in pastures with stocker than cow-calf cattle. Survivorship was lower for juveniles (~69%) than mature plants (~93%). When exposed to herbicide, the average survivorship across life stages was reduced by 67%. Reproduction was variable but high and on average comprised of 25 chasmogamous (outcross-pollinated) and 235 cleistogamous (self-pollinated) seeds/stem. The proportion of chasmogamous seeds produced/plant increased with increasing soil fertility under cow-calf grazing only. Population growth rates (?) were variable, ranged from 0.8 to 3.9, and emphasized the need to include multiple populations in demographic studies. Given low juvenile survivorship, preliminary results suggest control efforts focus on mature plants regardless of soil fertility type or cattle management. However, these responses need to be verified over additional years to better predict bioeconomic impacts and guide weed management decisions.