Empirical and provisional seed transfer zones for the Mojave Desert and surrounding regions

Empirical evidence that local adaptation and home-site advantages are frequent across plant taxa has increased recognition that local adaptation plays a fundamental role in shaping plant responses to seed transfer (i.e., when plant materials are introduced to a site during restoration). Use of maladapted plant materials can lead to poor project outcomes if propagules exhibit poor germination or seedling survival, low population fitness, inbreeding or outbreeding depression. Accounting for local adaptation in restoration designs is a promising approach for increasing revegetation effectiveness, particularly in drylands where extreme environmental variation potentially drives high levels of selection. Consequently, demand has increased for genetically appropriate plant materials for use in arid land restoration. Over the past several decades, both the scope of restoration treatments and the use of native species have increased on public lands in the western United States. Coordinated efforts among public agencies, universities, and other partners are focused on developing a sustainable supply of native seeds to meet future demands.

Empirical Seed Transfer Zones
Seed provenance strategies that incorporate spatially explicit genetic information promote effective revegetation by reducing genetic risk factors, such as maladaptation, outbreeding, and inbreeding depression. This project uses genetic analyses to develop empirical seed transfer zones for species that have been genotyped in the Mojave Desert. Empirical seed transfer zones delineate areas within which seeds may be transferred with limited risk of maladaptation. Seed transfer zones for each species are based on a multivariate spatial model linking variation in potentially adaptive genetic loci with gradients in climate, including variation in precipitation and temperature. The full details of these analyses are described in the accompanying publication (Shryock et al. 2017; https://doi.org/10.1002/eap.1447).

Provisional Seed Transfer Zones
Despite considerable effort, genetic guidelines are still lacking for numerous plant species that are desirable targets for restoration. In place of species-specific genetic studies, provisional seed transfer zones have been the preferred alternative for seed-sourcing on public lands. Provisional seed transfer zones use climate variation as a proxy for local adaptation and are intended to aid seed transfer decisions when genetic information is not available for a species.

A secondary goal of this project was to develop improved provisional seed transfer zones for arid regions of the desert southwest (including the Southern Great Basin, Colorado Plateau, Mojave Desert, and Sonoran Desert) through a robust multivariate analysis. Although species and populations differ with respect to the mechanisms shaping local adaptation, multivariate climate models capture a holistic representation of the variation in climate and are most likely to represent relevant environmental variation across different species and functional types. The provisional seed transfer zones provided here are based on climate variables of known importance to desert plants, including averages, extremes, and ranges of variability in both precipitation and temperature. The full details of this analysis are described in an accompanying publication (Shryock et al. 2018; https://doi.org/10.1002/ecs2.2453).

References:
Shryock, D.F., Havrilla, C.A., DeFalco, L.A., Esque, T.C., Custer, N.A. and Wood, T.E., 2017. Landscape genetic approaches to guide native plant restoration in the Mojave Desert. Ecological Applications, 27(2), pp.429-445.

Shryock, D.F., DeFalco, L.A. and Esque, T.C., 2018. Spatial decision-support tools to guide restoration and seed-sourcing in the Desert Southwest. Ecosphere, 9(10), p.e02453.