Selecting SeedManagers must evaluate the available seed supply to determine which is most appropriate for their project site. Many agencies have policies emphasizing the use of genetically local plant materials. In evaluating the suitability of native plant materials, there are two main components to consider: (1) the geographic origin of the material, and (2) what has been done to the material since it was collected.
Seed Origin The prevailing theory is that the best choice for plant material is from the closest donor site possible. Material that originated far away may not be adapted to the site and may not survive, or if it does survive, it may diminish the genetic integrity of the local population. The questions arise, however – how close is close enough? – and how far is too far? Unfortunately, we lack the information required to make scientifically-supported decisions for most species.
Geographic distance is not the same as genetic distance, however, and may not be the best indicator. Seed from a similar set of environmental conditions 100 miles away may be more suitable than seed from only 10 miles away but from a totally different environment. Similarly, elevation exerts an important influence on plant genetics, as it is so strongly correlated with variables such as minimum temperatures, precipitation, and days without snow pack.
Plant Material Development Not all plant materials are created equal, and it is important to be aware of the developmental history of specific plant materials in order to make informed decisions about their use. Humans have a long history of improving plants, and many plants have been developed for use in wildland rehabilitation. The impact of plant material development on genetic diversity is a relatively new area of research, and very little is known at present. Two main areas of research are the genetic consequences of cultivation and the genetic composition of the progeny when wild and cultivated materials are grown together.
The longer a plant is removed from its native environment, the more it is subjected to artificial selection, whether intentional or not. Often plant breeders will single out and/or hybridize select plants for multiple generations, resulting in a plant that bears the desired traits, but may not much resemble its wild progenitors. The hallmarks of a cultivar (cultivated variety) are that the material is distinct, uniform, and stable, which comes at the expense of genetic diversity. Even without overt selection, plants that are cultivated in an agricultural operation will undergo “domestication selection,” as some genotypes will be favored over others. For instance, genotypes that reproduce later in the season may not have produced mature seed when the field is harvested. In order to keep cultivated plant materials as close to the wild type as possible, the material should be increased in as few generations as possible. When evaluating a commercial source of a plant material, ask about its development.
Releases Several categories of ‘releases’ are now available. A ‘release’ is a term used to describe a plant material that has been selected by an agency (often a federal agency and/or a university) and ‘released’ to the public for use. The release process is essentially a way of marketing a plant material with specific known traits, giving it a trade name, and making it an identifiable product. Historically materials were released as cultivars after several rounds of development and testing. When an ‘improved’ product was attained that the developing agencies could stand behind (i.e. distinct, uniform, and stable), the product would formally be released. Cultivars are still being developed and released, and still bear the expectation that the product will perform as advertised.
In order to meet the changing demands of the marketplace, where ecological concerns are becoming more prominent, releases now include ‘pre-variety germplasm.’ Pre-variety germplasm (PVG) refers to a plant material that has not undergone the full schedule of testing and data collecting required to qualify as a full-fledged variety. The most rudimentary PVG is ‘Source-Identified,’ a material of known origin that has not undergone selection or testing. ‘Selected class PVG’ is the next level, where a particular material has been identified from several collections as having promise for conservation use. ‘Selected’ can mean that specific traits have been identified or individuals in the population that possessed that trait were collected and cultivated. ‘Tested class PVG’ is the next level, where the material has been grown for another generation and the desired traits have proven heritable. The final step in developing a cultivar is testing that material in several locations over multiple years to monitor its performance and determine its area of adaptation.
Wildland collection Although wild-collected seed has the highest genetic identity with the local populations, it may not be the most economical or sustainable choice of native plant materials. Heavy, repeated collection could deplete resident seed sources and negatively impact the regeneration of natural populations. While cultivation of wild-collected seed does inevitably change the genetics of the population, it is not known how great this shift is – further research may find it to be inconsequential. Cultivated seed can be produced much more economically than wild collected seed, and therefore greater quantities are made available for revegetation efforts. Some species, however, especially shrubs and trees, do not lend themselves to cultivation and are best collected from the wild.