SMALL-SCALE PLANTING OF ENGELMANN OAK TREES
T. V. St. John, Ph.D.
This pamphlet was developed through a research project funded by the California Department of Forestry and Fire Prevention (CDF); carried out by T. V. St. John, J. M. Evans, J. W. Bohn (Tree of Life Nursery), and T. A. Scott (U.C. Cooperative Extension, University of California, Riverside). It includes information from a variety of additional sources, including other CDF research projects.
Engelmann oak (Quercus Engelmannii) woodlands
only occur in the foothills of San Diego, Riverside, Orange, and Los
Angeles counties, and cover the smallest range of any oak species in
California. About two-thirds of Engelmann oak woodlands are on private
lands. To maintain the current distribution of the species, resource
managers have been trying to develop restoration techniques to enhance
existing woodlands. In this paper we present recommendations for
small-scale replanting of Engelmann oak woodlands, with emphasis on the
oaks rather than on understory or other woodland components. The methods
suggested here represent current knowledge about Engelmann oak
restoration, focusing on the
Engelmann oak woodlands occur primarily on private lands, and as such, are subject to conversion into rural and urban housing. Although oaks are highly desirable landscape features, construction in oak woodlands causes loss of adult trees and deleterious alteration of the soil. One solution to this loss has been the restoration of small areas to compensate for the loss of trees during construction. Although this cannot be considered mitigation for the loss of a woodland, replanting helps to ensure that oaks will remain part of the landscape.
Natural regeneration of Engelmann oak is poor
over most of its range, because of grazing and competition from exotic
annual grasses. Native animals also take young oaks, especially rabbits,
deer, ground squirrels, gophers, and cattle. By protecting seedlings from
grazing and weed competition, seedling mortality can be reduced
Most Engelmann oak woodlands occur between 2300 ft and 4200 ft, often on mesas or southwest-facing slopes, in areas dominated by grasslands or sage scrub vegetation. They also can be found in live oak (Quercus agrifolia) woodlands along river valleys, and rarely with black oaks (Quercus kelloggii) or pines. Engelmann oak occupies the margins between mesic canyon bottoms and drier scrub/grassland areas. It is better than live oak at germinating and growing in low moisture conditions. It never seems to dominate riverbeds or streams, but requires some moisture at depth in the soil.
The upper elevation range of the Engelmann oak is restricted by winter temperatures. Although the trees seem to be cold tolerant, their acorns and seedlings are damaged by frost. About 60 to 80 % of the Engelmann oak woodlands occur on south-facing slopes at higher elevations. Temperatures in the range of Engelmann oak drop below freezing in the winter but not far below. The natural climate for Engelmann oak is found in the Sunset New Western Garden book as zones 18 through 23.
Engelmann oaks rarely grow on the coastal plains, where rainfall, soils, and substrate moisture do not compensate for evapotranspiration. In general, Engelmann oak woodlands are found where winter rains exceed 18 inches. Isolated stands of Engelmann oaks can be found along the Banner Grade (San Diego County) and in the Pauba Valley near surface moisture or springs. Remnant populations on the coastal terraces (and Santa Catalina Island) appear to be protected by fogs and overcast during summer drought.
Fire can alter the distribution of Engelmann oaks. Unlike live oaks which re-sprout from the canopy, Engelmann oaks either die or re-sprout from the root crown after a fire. They are unlikely to maintain stand dominance in an area with frequent fires, because of their slow growth patterns.
Droughts and over-use of range lands around the turn of the century may have altered the distribution of Engelmann oaks. As a result, there are suitable sites for this species that are currently unoccupied. In particular, annual grasslands near existing Engelmann oaks may provide the most promising locations for replanting. Sites with other deep-rooted species, such as laurel sumac, redberry, sugar bush, and toyon may also indicate that soils could support Engelmann oaks.
Chaparral sites may not provide the proper soil conditions and there is a potential for mortality through intense fire.
The best indicator of a suitable site is the presence of naturally-occurring Engelmann oaks. This does not guarantee the suitability of the site, however, since Engelmann oak commonly gives way to grassland, chaparral, or coastal sage scrub over a distance of only a few feet. The reason may be topography that rises above deep moisture, a change in soil type, or the presence of a hardpan that prevents root penetration.
The best results will be realized by planting in relatively undisturbed native vegetation. The problems of planting on a graded site are much more difficult, and in that case we recommend using a professional habitat restoration consultant. The methods presented here relate to enhancement of existing habitat rather than replanting of graded sites.
A less obvious problem is the natural partnership between the roots of oak trees and certain kinds of woodland fungi. The fungi may not be in the soil unless you are planting close to established trees. Without their root-inhabiting fungi, it is unlikely that Engelmann oak seedlings will survive past their first few months.
The deep roots penetrate several feet into the ground, where they find moisture in cracks in the bedrock, in deep soil layers, or in some cases, in a water table. This requirement for deep soil moisture seems to be usual among southern California oaks. If the soil is too shallow, or if there is no moisture at depth, oaks do not grow there. Clay soils hold more water than other soils, and Engelmann oak is often found on clays.
In nature, very few acorns become mature trees. The acorns that have the best statistical chance are those that happen to land in "safe sites:" locally protected spots that offer light shade, protection from herbivores, and favorable hydrological conditions. Success of artificial planting can be dramatically improved by arranging for all the acorns or seedlings to start off in "safe sites."
In our recommended procedures, screen cylinders provide light shade and protection from herbivory. We further recommend that most plantings be in spots that provide locally favorable hydrology. To find these spots, consider the ways in which surface runoff pools along the edges of rocks and logs. Water is channeled to a relatively small soil area on the rock's perimeter. Make some observations in a rain storm, or pour water over the rocks to find the most favored spots.
Most natural regeneration takes place at the margins of the canopies of large trees. The canopies probably act to concentrate rainfall much as rocks do, and also offer shade and shelter from herbivores.
All plant material should originate on site, or as close to the site as possible. Plants vary genetically over their natural ranges; the importance of local material rests largely on its suitability for local conditions. Local oak seedlings come directly from acorns collected nearby, from container plants grown under special contract with a native plant nursery, or by moving natural seedlings.
We recommend planting with pre-germinated, freshly collected acorns. While there is always mortality with direct seeding, the method is cheap and fast enough that over-planting can usually compensate for the expected losses.
Engelmann acorns begin to mature in September,
and most have dropped by December. Only green acorns, still on the tree,
should be picked. Those that turn brown on the tree have been killed by
insect larvae. Those collected from the ground include many that are dead
or insect-infested. Acorns may be picked as soon as they have reached
their full size. This is indicated by their shape relative to the cap;
immature acorns are short, while mature ones have grown to about one and
one-half times their diameter. Acorns that readily separate from the cap
are fully mature, but they may be picked somewhat
Flotation is a good way to do a preliminary
sorting of acorns. Most of the floating acorns fail to germinate, while
most of those that sink are good. If the supply is plentiful, choose
relatively large acorns rather than small ones. The acorns will begin to
germinate soon after collecting, but can be slowed somewhat by
refrigeration. By storing them until they begin to germinate, one can
assure that all planted acorns are alive. The acorns can be stored in peat
moss or potting medium, contained in loose-weave sacks in a cool place.
Acorns cannot be stored from one year to the next. When it is time to
plant, choose only pre-germinated acorns with 1/8 to 1 inch of root.
Planting must be carried out after the first fall rains and before about
March 1. It is important that the surrounding soil be moist from rainfall
or pre-irrigation. Place the acorns with the root tip pointing downward,
at a depth of about two inches. It is a good idea to plant
Engelmann oaks are available in a range of container sizes from several specialty nurseries in southern California. Generally, the smaller container sizes are better suited for wild land planting, and the larger sizes better suited for garden or horticultural planting. Engelmann oaks develop a deep tap root very early in life, before the seedling even appears above ground. In containers, the tap root is not allowed to develop. This is a less serious problem with young seedlings in small containers, which can quickly resume tap root growth once moved to native soil. Larger plants may have lost their ability to form the tap root, although they will form "sinkers" if other factors allow them to survive long enough. Sinkers are functionally similar to tap roots; they form on the larger horizontal support roots, and eventually replace the tap root even in wild trees.
The advantages of containerized plants include reduced susceptibility to shading from weeds, improved resistance to some kinds of herbivore damage, and escape from some of the factors that cause mortality at the acorn stage. Disadvantages include higher cost and the difficulty of obtaining plants of local genetic origin. Depending upon quality of the container plants and the methods of planting, it may be more difficult for container plants to establish a root system with a normal configuration.
Efforts to transplant natural seedlings are sometimes successful, in spite of the fact that the tap root of Engelmann oak seedlings quickly grows to a yard or more in length. The prospects for success diminish rapidly with increasing size. We do not recommend moving seedlings, because any gain in oak woodland at the new location must be paid for by losses at the original location. Nevertheless, some transplanting of seedlings may be take place when the donor site is to be destroyed at a construction site. The success rate may be low, but any that survive will be of local genetic stock and will carry with them mycorrhizal fungi and other beneficial soil organisms.
Any transplanting should take place only when the soil is moist and evapotranspiration is low, that is, during the winter months. Maintain an intact soil volume around the seedling, even though the entire root system may not be included. Move the seedling into moist soil, and provide shade and moisture until the seedling shows signs of new growth.
The soil should be moist from rain at the time of planting, but in a poor rain year can be moistened artificially to a depth of two or three feet. Each acorn is placed in a shallow depression and covered with about two inches of soil.
We do not recommend amending the soil when planting oaks among existing native vegetation. This standard landscape practice has limited use in habitat restoration, since it encourages the roots to develop only within the volume of the original planting hole. Engelmann oaks have a shallow root system in addition to the deep tap root, and it is important that those shallow roots spread to some distance from the trunk. Digging stimulates germination of dormant weed seeds, providing another reason to use the smallest possible planting holes.
The time of planting is very important. Acorns may be planted in fall or winter, when there has been enough rain to moisten the soil to a depth of about two feet. Any container plant installation should take place in mid-winter.
With either acorns or container plants, it is important that the roots be able to find their symbiotic mycorrhizal fungi. If the soil is undisturbed and is near mature oaks (either Engelmann or coast live oak), assume that the fungi are present. If the soil has been disturbed, or if planting is more than a few feet from the canopy of an established oak tree, it is best to do a low-technology inoculation. Dig up a small amount of soil (a tablespoon or so for each planting hole) from under the canopy of a large oak tree. Put it below the acorn in the planting hole.
Each planting site should be enclosed by a cylinder of aluminum window screen, 1/4" hardware cloth, or 1/2" poultry netting. The cylinder is buried six inches below ground for support and extends one foot or more above ground for protection from deer and rabbits. Exclusion of gophers requires that the screen extend a foot below ground. It is sometimes more practical to trap gophers than to exclude them.
Grasshoppers can pose serious problems in some cases, as when the oaks have been planted among exotic annual grasses. Where grasshopper damage is a danger, a window screen cylinder can be stapled shut at the top. Once the seedlings emerge from the screen or rocks, they will be exposed to rabbits, deer, and other herbivores. Engelmann oak foliage provides a meal that is less tasty than many of the alternatives, and will often be rejected if fresh grass or palatable shrubs are available. However, they can be severely damaged if other plants are not present. They will be particularly attractive to herbivores if they have been watered while surrounding plants are dry. Larger plants can survive considerable damage, and even seedlings can re-grow after one or two defoliations.
Weeds cause a number of problems for native plants, including competition for water and light. The tap root of Engelmann oak grows so quickly that most weeds compete for water only during the weeks after germination. After that, the oak is drawing water from a different soil volume. Dense shade is detrimental, but light shade, even from weeds, may be beneficial. Weed seeds germinate readily on disturbed soils, and the disturbance needed to install the screen and bury the acorn will be enough to cause significant weed growth. It is common to see the screen cylinder bursting with weeds, while the soil outside the screen remains relatively free of them.
We have not yet found an entirely satisfactory
solution to this problem. An herbicide may be used in moderation before
the oak seedling appears above ground. If the oak seedlings appear before
the weeds can be treated, we recommend clipping the weeds above the level
of the oak seedling, by working some clippers through the side seam of the
screen. During the first spring, it is probably a poor idea to pull weeds
out of the ground, since there is a good chance of breaking off the oak
seedling. By the second spring, the oak seedling has a deep, woody root
and a tough stem, and most kinds of weeds can be pulled with little danger
to the oak. Weeds outside the screen can be cut or pulled if they appear
to be casting deep shade,
Irrigation systems are expensive and unreliable, and are impractical on remote sites. Watering favors weeds more than oaks, which are adapted by nature to survive summer drought. Thus we discourage the installation of irrigation systems, even drip systems unless the property owner intends to dedicate considerable energy to weed removal. Instead, counteract drought by proper selection of planting sites, care in planting technique, proper timing, and over-planting to compensate for mortality.
The acorns may be watered at the time of planting, but is not required thereafter unless rainfall is extremely low. It will usually be more practical to water by hand rather than from an irrigation system. Summer watering is probably harmless if done no more than once a month, but should not be necessary if other things have been done correctly. No fertilizer should be used in undisturbed soil. We have found that fertilization, like frequent watering, favors weeds at the expense of the oak seedlings. Fertilization also suppresses mycorrhizae, without which the oak trees can never become self-sufficient.
Tree mortality decreases steadily with age, so the older a seedling becomes, the greater its chances of reaching maturity. Trees that have survived the first summer without irrigation have passed the most difficult test. Engelmann oaks often appear to have died during the first summer, but later resprout from the ground or from the "dead" stem. It is common for them not appear at all the first spring, then come up as healthy seedlings the second or even third year.
In our experiments, by far the most important factor determining seedling survival was plot location. By the second summer, oak seedlings were present in one fourth of the control (no treatment) screens at our best sites and in none of the screens at our worst sites. The differences may have been related to moisture holding capacity of the soil. The soil moisture at the best sites was not evident at the surface, but rather at a yard or more in depth, which is well within reach of the long Engelmann tap root. The much lower success elsewhere may have been related to seasonal drying throughout the upper soil profile.
In most cases, the point of replanting is to replace trees that must be destroyed during construction. One would not want to plant so many that the number will one day greatly exceed the natural tree density of the area. However, planting too many trees is probably a better mistake than planting too few. The number to be planted is essentially an estimate of the number of acorns or container plants required to produce each mature tree. That number varies widely with planting location and technique. We can make some preliminary estimates of the number of acorns needed, assuming that planting will be in relatively undisturbed soil currently supporting native vegetation, within the natural range of Engelmann oak. At our best site it would have taken a four to one planting ratio to get the desired number at the beginning of the second summer. There will be some mortality in subsequent years, so we suggest a six to one planting ratio for a margin of safety.
The relatively low six to one ratio applies at the best sites. At our worst sites, all trees died and there were no planting ratios that would have produced trees. In order to keep planting within the realm of practical possibility, we recommend a forty to one planting ratio from acorns on most soils. The ratio from containers could be lower, say ten to one, from small containers, but replanting will be required in many instances. Because these ratios are technically below the best estimates given by our research, it is important that a large share of the planting done under the edges of existing tree canopies (either Engelmann or coast live oak) and at the edges of rocks that can provide water collection.
Planting from acorns is relatively fast and inexpensive, so high planting ratios are not terribly burdensome. However, when the highest ratios are to be used, the property owner may want to experiment with larger screened areas that allow multiple plantings. A rabbit-proof fence can be made of one inch mesh wire, turned out six inches at the bottom and held flat with rocks, and extending three feet or more above ground. A lid across the top will also exclude deer, and may be cost-effective if a few such exclosures can replace numerous individual screen cylinders. Remember that large exclosures will complicate weeding and other maintenance.
Some later replanting may be necessary if mortality becomes unexpectedly high. The first year's experience will probably indicate the methods and planting locations that are most appropriate for each site, and those methods can be emphasized in subsequent years.
Tree of Life Nursery and the investigators thank the project collaborators for making available our experimental sites. In particular we thank Dr. Gary Bell of The Nature Conservancy's Santa Rosa Plateau preserve; the Rancho Mission Viejo company, Dawn Lawson of the Natural Resources Office, USMC Camp Pendleton; Ron Woyczak and Roger Wong, USDA Forest Service, Palomar District; Jane Fyer, President, Tenaja Property Owners' Association; and David and Maureen Selzler, Tenaja Property Owners' Association.
We thank Mr. W. B. Tyson of Land Restoration Associates, San Diego, for sharing some of his extensive experience with us, especially during development of the proposal. This project was funded by the California Department of Forestry and Fire Prevention.