Theory of seed mix design with applications to ecological restoration

Authors: Rinella, Matthew - Matt; JAMES, JEREMY - California Polytechnic State University

Submitted to: Restoration Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2024
Publication Date: 7/31/2024
Citation: Rinella, M.J., James, J.J. 2024. Theory of seed mix design with applications to ecological restoration. Restoration Ecology. Article e14234. https://doi.org/10.1111/rec.14234.
DOI: https://doi.org/10.1111/rec.14234

Interpretive Summary: Revegetating degraded ecosystems is important and difficult. One factor hindering revegetation is uncertainty about which plant species will establish. This paper attempts to account for this uncertainty in the design of seed mixes. We started by developing the theory governing relationships between seed mix richness and plant densities. This allowed us to prove a principle: With the total seed rate fixed and species and varieties chosen randomly, evenly allocating seed to all available species and varieties minimizes chances of low densities. In addition to entire seed mixes, this principle applies to partitions within seed mixes. For example, evenly allocating grass and shrub seed rates to all available grasses and shrubs minimizes chances both groups develop low densities. According to our theoretical development, the more survival probabilities vary among species and varieties, the more increasing richness reduces chances of low densities. To quantify effects of richness, we measured survival probabilities of many species and varieties in two degraded ecosystems. With the total seed rate fixed, species-poor mixes often suffered high chances of generating exceedingly low densities when species-rich mixes did not. We recommend seeding as many species and varieties as possible that are compatible with management objectives, because this reduces chances of low densities and establishment failures. This recommendation marks a strong departure from current practice.

Technical Abstract: Revegetating degraded ecosystems is important and difficult. One factor hindering revegetation is uncertainty about which plant species will establish at densities sufficient to colonize degraded sites. This paper attempts to account for this uncertainty in the design of seed mixes and other planting mixes (e.g. root stock). We started by developing the theory governing relationships between seed mix richness and plant densities. This allowed us to prove a principle: With the total seed rate fixed and species and varieties chosen randomly, evenly allocating seed to all available species and varieties minimizes chances of low densities. In addition to entire seed mixes, this principle applies to partitions within seed mixes. For example, evenly allocating grass and shrub seed rates to all available grasses and shrubs minimizes chances both groups develop low densities. Partitioning allows generalization of the principle to nonrandom species selection, which occurs when there is prior knowledge of species performance. According to our theoretical development, the more survival probabilities vary among species and varieties, the more increasing richness reduces chances of low densities. To quantify effects of richness, we measured survival probabilities of many species and varieties in two degraded ecosystems. With the total seed rate fixed, species-poor mixes often suffered high chances of generating exceedingly low densities when species-rich mixes did not. We recommend seeding as many species and varieties as possible that are compatible with management objectives, because this reduces chances of low densities and establishment failures. This recommendation marks a strong departure from current practice.