Is it best to add native shrubs to a coastal sage scrub restoration project as seeds or as seedlings?

Abstract

Ecological restoration frequently involves the addition of native plants, but the effectiveness (in terms of plant growth, plant survival, and cost) of using seeds versus container plants has not been studied in many plant communities. It is also not known if plant success would vary by species or based on functional traits. To answer these questions, we added several shrub species to a coastal sage scrub restoration site as seeds or as seedlings in a randomized block design. We measured percent cover, density, species richness, size, survival, and costs. Over the two years of the study, shrubs added to the site as seeds grew more and continued to have greater density than plants added from containers. Seeded plots also had greater native species richness than planted plots. However, shrubs from containers had higher survival rates, and percent cover was comparable between the planted and seeded treatments. Responses varied by species depending on functional traits, with deep-rooted evergreen species establishing better from container plants. Our cost analysis showed that it is more expensive to use container plants than seed, with most of the costs attributed to labor and supplies needed to grow plants. Our measurements of shrub density, survival, species richness, and growth in two years in our experimental plots lead us to conclude that coastal sage scrub restoration with seeds is optimal for increasing density and species richness with limited funds, yet the addition of some species from container plants may be necessary if key species are desired as part of the project objectives.

Fig 1. Monthly rainfall totals for each year of the study.

In addition to monthly rainfall totals for each year of the study, seeding and planting events are also noted on the month that the event occurred. Data are from the Corona del Mar Weather Station (NWS station E3141).

Fig 2. Average values by treatment for all native shrubs added to the site.

Average plot values for all native shrubs added to the site including A) total native percent cover, B) total native density, C) native species richness, and D) relative change in height. Bars represent mean +/− 1 SE.

Fig 3. Average values for each species in the two different treatments.

Dots indicating average values for each shrub mix species in each treatment with bars representing mean +/− 1 SE. Data shown includes A) native percent cover, B) native density, C) final height, D) final width, E) relative height change, F) relative width change, G) percent survivorship, H) estimated time to death. Species with very low sample sizes were excluded from these analyses.

Fig 4. Density and species richness over time.

A) Density and B) species richness values from four different time points during which these values were measured. Bars represent means +/− 1 SE.