Where existing regeneration is insufficient or does not include the preferred species to meet restoration goals, planting may be necessary. The optimum type of planting material can vary by species and site constraints. The USDA Forest Service and other organizations have developed a wide variety of materials related to forest tree planting and silviculture (see below Sources of Information on Tree Planting and Silviculture—). The amount of information available is much greater for commercially utilized forest species than for non-commercial species such as California native oaks and other hardwoods.
For large-seeded species such as oaks, direct planting of seed at the restoration site can be effective and cost-efficient. Direct seeding avoids various problems that may be associated with nursery stock (table 3-10). Timing of seed collection, storage conditions, and planting methods vary by species.
Table 3-10—Comparisons between direct seeding and nursery-grown container transplants for restocking
| Factor | Seed | Transplants |
|---|---|---|
Relative cost |
Low. Local seed reduces transportation and handling costs. |
High. Production requires seed collection and seedling maintenance until outplanting. |
Storage |
Low space requirements. Short-lived seed such as acorns can typically be stored for only a few months. Other seed may remain viable for many years under proper storage conditions. |
High space requirement due to size of containers. Plants may be held for multiple years under good growing conditions, but more space and larger containers will be needed as plants grow. |
Planting |
Relatively easy and fast to plant large number of sites. |
Greater effort required for planting, especially if large number of sites are involved or larger stock is used. |
Post planting success |
Varies by species and site preparation. For species such as oaks, high rates of germination and establishment can be obtained and overplanting to obtain adequate survival is relatively inexpensive and fast. |
Varies with stock quality and site conditions. Transplants are often highly susceptible to dry conditions after planting and irrigation may be needed to ensure establishment at some sites. High rates of success can be obtained with many commercial forest species with proper site preparation. |
Root structure |
Root structure adapts to soil conditions as seed germinates and grows. Taproot development typically good. |
Root system is modified strongly by container and is more restricted, requiring adaptation after transplanting. Taproot structure of species with strong taproots (e.g., many oaks) is distorted or eliminated. Root form problems may develop in the container or afterward as a result of poor planting technique. |
Shoot structure |
Shoots typically develop in proper balance with roots and show adaptations to site conditions. |
Shoot to root ratio may be artificially high, leading to water stress after planting. Shoots may be thin and mechanically weak if plants are crowded in the nursery. |
Local germplasm |
Can be obtained by collecting seed in and near restoration site. |
Seed source is unlikely to be local unless custom propagation is done, requiring adequate lead time for growing stock. |
Introduction of pathogens or insect pests |
Typically not an issue for locally collected seed. |
Can pose high risk for introduction of soil diseases such as Phytophthora spp. and movement of some insect pests. |
For small scale projects, it may be possible to locate and transplant naturally-occurring seedlings during the late fall or winter. This requires a plentiful supply of nearby seedlings growing in soil that is not excessively rocky. Seedlings should be small enough that the root system can be easily dug up and moved. Species vary in their ability to survive transplanting. It is possible to transplant relatively large individuals using a tree spade (a truck-mounted device that can move a sapling tree or shrub along with a relatively large volume of soil), but this method is expensive and typically limited to use in developed areas.
Locally-collected natural seedling transplants have the advantage of being from the local plant population. They are likely to be colonized by local species of mycorrhizal fungi, which are important for plant growth and survival. However, locating and transplanting natural seedlings can be time consuming and will disturb the seedling source site. Deleterious organisms, such as root disease pathogens, may also be moved with transplants. If the source site is very close to the restoration site, the risk of introducing new pathogens may be low. Nonetheless, seedling source sites should be inspected for evidence of disease problems before they are selected. Testing of plants or soil may be appropriate to determine if soilborne Phytophthora species or other pathogens are present.
Nursery-grown planting stock is commonly used in forestry and environmental restoration. However, nursery stock can pose a high risk of introducing exotic pathogens, especially Phytophthora species, into forested areas.
From these examples, it is clear that nursery stock infested with any Phytophthora species should not be planted in or adjacent to natural areas. However, no certification program is currently in place to show whether nursery stock is free of pathogenic Phytophthora species. California nursery cleanliness standards are based on annual, visual inspections, so soilborne pathogen are often not detected. To be certain that Phytophthora species are not present, purchasers of nursery stock would need to conduct their own testing, or follow the steps outlined below.
In lieu of specific testing, buyers can inspect nursery production facilities to see whether the cultural practices are sufficient to produce clean stock. To produce nursery plants free from P. cinnamomi and other Phytophthora species, nurseries need to follow prudent sanitation practices, including the use of pathogen-free soil, disease-free propagation stock, and phytosanitary procedures throughout the entire propagation cycle. These principles apply equally to large scale and small scale nurseries, including backyard propagators.
Nursery-grown plants may become infested with Phytophthora species through the use of reclaimed and recirculated irrigation water and movement of infested soil via hoses, tools, pots, hands, shoes, and vehicles. To avoid contamination, pots and flats should never be placed on the ground. Benches, pots, tools, and other items used for propagation need to be routinely disinfested prior to each use. Unless a nursery observes all these practices consistently and rigorously, planting stock can easily become infested.
Contamination risk generally increases the longer a plant is held in the nursery. Hence, the infestation rate of older, larger container planting stock is typically higher than that of younger, smaller container stock. Liner stock grown in the ground (e.g., bare root stock) may be infested at any age if the production field is contaminated.
The presence of Phytophthora infection can be masked in fungicide-treated plants. Fungicide treatment can prevent disease symptoms but will not free plants and container soil of Phytophthora spores. Hold n ursery plants for at least two months to allow fungicide residues to decline before testing for presence of Phytophthora species or symptom expression.
If on-site natural regeneration is inadequate for accomplishing restoration goals, the next best source of planting material is locally-collected seed or planting stock derived from it (table 3-11). Local germplasm represents the result of natural selection under local soil and climate conditions. Trees growing from locally collected seed should be adapted to site conditions and are likely to perform well over time (Millar and Libby 1991).
However, it is important to realize that selection pressure can be high at the seedling stage under natural conditions. High rates of seedling mortality are common in nature, which can exert selection pressure for seedling characteristics that are important for early survival. Such characteristics are likely to be important for long-term stand sustainability. When intensive inputs are used to ensure high rates of seedling survival, this type of selection may not occur.
Table 3-11. Recommendations for the management of genetic resources in forest restoration projects.
| Recommendation | Notes |
|---|---|
Use existing regeneration |
Conserve existing seedling regeneration that has been exposed to selective pressures. |
Use local seed |
For trees such as oaks and tanoaks, trees within the same watershed and/or located within a few to about 10 km of the restoration site may be considered to be within the local seed zone. Wider local zones may apply for species that disperse seed and pollen more widely. |
Match seed source and planting site characteristics |
If collecting seed from within or outside of the local seed zone, match the microclimate (e.g., north slope) and soil characteristics (type and depth) of the seed source site to that of the target restoration site to the degree possible. It may be desirable to collect seed from sites that are hotter and drier than the target site to provide a potentially wider range of climate adaptation variation. |
Ensure genetic diversity in seed collections |
Seed collected for planting should be from a large number of individuals spread widely over the local seed zone. Because of variation in seed production between individuals from year to year, seed collection and planting should be conducted over a series of years. |
Avoid genetic pollution |
Avoid collecting seed from trees likely to be pollinated by horticultural specimens of unknown origin. |
Let natural selection function |
Allow for attrition due to natural selective pressures in restoration plantings. Plant seeds or the smallest planting stock size available and plan for relatively high rates of seedling mortality so that survival will be largely limited to the best-adapted genotypes. |
McCreary, D. 2009 (Revised) Regenerating Rangeland Oaks in California. Publication 21601e. ANR Publications, University of California. Available: http://anrcatalog.ucdavis.edu/Items/21601e.aspx
Silvics of North America, Vol 1. Conifers Available: http://www.na.fs.fed.us/spfo/pubs/silvics_manual/Volume_1/vol1_Table_of_contents.htm
Silvics of North America, Vol 2. Hardwoods http://www.na.fs.fed.us/spfo/pubs/silvics_manual/volume_2/vol2_Table_of_contents.htm