Nonperiodic grassland restoration management can promote native woody shrub encroachment

Abstract

Woody species encroachment is increasingly displacing grasslands, negatively impacting regional plant richness and reducing economic productivity from grazing. Although intermediate disturbance has been found to reduce woody species encroachment and maximize species diversity, ecological restoration can often lead to many small, infrequent disturbances. These small disturbances may not be strong enough to limit woody encroachment, and instead may promote invasion. Drought may slow encroachment, but adjustments in key functional traits may allow for persistent woody invasion. Baccharis pilularis is a woody shrub native to western North America, but has been shown to have higher recruitment following nonperiodic disturbances and be invasive in native grasslands. To address the extent of woody invasion following limited restoration actions, I quantified natural B. pilularis recruitment and cover at an invaded coastal California grassland in plots after experimental restoration (singular planting and nonnative species control efforts) and extreme drought conditions (60% rain exclusion) 6 years posttreatment. For traits, I measured B. pilularis specific leaf area, major vein length per unit area, leaf thickness, and lobedness 4 years posttreatment and stem diameter 5 years posttreatment. Native shrub encroachment by B. pilularis was higher in restored plots compared to nonrestored plots, which had zero recruitment. Drought reduced B. pilularis recruitment but not cover and resulted in adjustments in leaf thickness and major vein length per area. Results suggest that planting and other singular restoration activities (i.e. invasive species control) in coastal grasslands can cause small, infrequent disturbances that promote native woody shrub encroachment.