Managing contingency in semiarid grassland restoration through repeated planting

摘要

The success of grassland restoration in semiarid regions is contingent on good establishment years. Here it is asked whether success rate can be increased by planting repeatedly among years, or by consistently using high planting densities. I planted seeds of a dominant native perennial grass, Elymus lanceolatus, at five densities (range: 30–3000 seeds/m2) in each of 3 years. Elymus seedling emergence and cover increased significantly with planting density. Cover after three growing seasons was maximized (i.e. not significantly different from the greatest cover) at planting densities of 300–600 seeds/m2. On the other hand, low germination in a dry warm year resulted in density having no effect on cover 3 years later. Two favorable planting years showed trajectories to native dominance after three growing seasons. In contrast, one unfavorable planting year showed a trajectory of non-native dominance. Emergence was increased modestly by both herbivore and interspecific neighbor removal. Overall, the most economical process for ensuring success (assuming that seed costs are high and planting and site-preparation costs are low) may be to plant at moderate density (300–600 seeds/m2) in repeated years until a favorable year is encountered.