INTRODUCTION
One of the most important forestry regulations in British Columbia concerns reforestation. A new forest must be quickly established on every harvested hectare. Everyone has heard of reforestation, but have you ever considered the science and technology behind planting seedlings? Reforestation is just one branch of silviculture, the forest science dedicated to growing forests.
If harvested sites are left alone they will eventually regenerate, but it may take many years before new seedlings take hold. Through silvicultural practices such as site preparation and immediate planting after harvesting, that length of time can be cut to two or three years. Immediate planting with high quality seedlings enables the new forest to become established quickly, and gives the trees a jump on fast-growing weeds. Other silvicultural practices such as weeding and fertilizing can speed up the time it takes for trees to grow to maturity.
To grow the forest back requires an understanding of genetics, soil science, ecology, tree physiology and more! In this Forestry Note we are going to look at the background of those seedlings being planted - a peek at the science and technology employed to get that healthy seedling to the tree planter. Then we will look ahead to what the future has in store for those seedlings.
PART 1: BEHIND THE SEEDLING SCENE
Tree habitat
If you were to walk around forests in B.C., what might be the first thing you would notice? Other than the natural beauty, you might be impressed by the variety of forest types. There are different species of trees and plants in different places. If we are going to manage forests, we need to look at how nature grows forests. What makes a spot good for one species of tree but not for another. What influences the forest to be different in one place than another?
The science that studies these questions is called ecology. We can look at the ecology of a whole forest or just of one tree. The ecology of an individual tree is the interactions between the tree and its environment: soil, sun, climate, slope, bacteria, weeds, and animals. Each species of tree has evolved to fit a particular niche in the environment. Some species of trees are always found growing in rich moist soils, while others are always found in drier soils with lower nutrient levels. Some trees are good competitors fighting it out with other plants for rich spots to grow, while others tolerate more difficult habitats to avoid competition.
Years of careful study have yielded large amounts of data about each species of tree. Analyzing this information defines the ranges of habitat a tree will be found in: a combination of geography, elevation, soil type, light intensity and climate. This collection of ecological information is called the silvics of a tree. Silvics characterizes the kind of habitat where this species tree will grow well naturally. Such information is the basis for the practice of silviculture.
Cones and seeds
"Follow nature" is often the rule in forestry. The mix of trees that nature selects to grow in an area are usually the best fit for that particular habitat. The silvics of a tree species is one part of information bank needed to make good reforestation decisions. The other type of data needed is the specific ecological information about the forest where the the harvest is planned: forest ecosystem type, soil types, slope position, brush potential and any special regulatory requirements. A silviculture forester will choose which species to use in reforestation based on the fit between the specific environment and the silvics of various trees.
Once a forester makes the decision about what types of trees (what mix of species etc.) are the best choice, then the type of harvest is designed. The harvest must create the condition required for the chosen species to grow (e.g. shade or sun) as well as meet requirements for wildlife habitat, recreation and other values.
Before any trees are harvested, silviculture foresters must put in their seedling requests to the Ministry of Forests Seed Centre. Seedlings are ordered up to three years prior to planting. When placing orders, foresters must specify species, elevation, seed zone and type of seedling required. Seedlings can be grown to different ages and sizes.
During the late summer and fall, forest workers collect mature cones from selected wild stands or from private seed orchards throughout the province. Conifers have both male and female cones on them. The male cones contain pollen, which is blown by the wind to other trees and fertilizes the female cones. The female cones are the larger cones you might find and collect from the forest floor. Once the seed is ripe the female cones will open and the seeds will fall out and flutter to the ground. Cone pickers search the forests for trees that have a good cone crop and whose seeds are ripe. Cones can be collected in a number of ways:
- a helicopter can drop a basket over the tree and pull off the cones
- machines can shake the cones from standing trees
- workers can climb trees to pick the cones
- workers can rake the cones from harvested trees.
Some species produce good cone crops only rarely (i.e. once every 7-10 years). When there is a good cone crop, every effort is made to collect as much seed as possible. Information about the location (geography, seed zone, elevation, etc) of every cone batch is carefully noted.
All seeds are taken to the Ministry of Forests Seed Centre in the Fraser Valley for processing and storage. The cones are dried in a kiln and then put in a tumbler to shake the seeds out. The seed passes through special equipment to remove seed wings, debris and any empty or damaged seeds. Samples of each seed lot are x-rayed and tested for quality. Then the seeds are stored at 18 ° C in vaults until a request for them is made.
When seedlings are requested, the seeds are removed from cold storage, soaked and then chilled. This simulates the cold of winter and helps them to sprout. The seeds are then sent to a nursery to be grown to the standards specified by the forester, who has made the request.
The nursery provides special growing conditions that enable the young seedlings to grow faster and stronger than they would on their own. Most nurseries have automated systems that control heat, light, moisture and fertilizer for each batch of seedlings. The seedlings must be raised to survive under the conditions of their eventual growing sites, so the nursery raises them according to very specific standards such as height, stem caliper (thickness) and vigour. Once they meet these standards the trees are ready to be transported to the planting site.
Genetics
Foresters want seed from certain species of trees, but also want seeds that come from an area very similar in elevation and geography to the proposed harvest site to ensure the trees will grow well. As well, the forester wants the seeds to be from trees that are healthy and strong. What determines if a seed will be healthy and strong, and grow well at specific elevation and geographic ranges? These characteristics are inherited from the seed's parents. The science that studies inheritance is called genetics.
Forest genetics is an important component of silviculture. The demand for wood products is increasing yet the land available to produce timber is decreasing. We want to use the land as efficiently as possible by growing better trees, more quickly. Genetics is key! The idea, however, is not a new one. Plant and animal breeders have been doing this for centuries. For example, by selectively breeding top racehorses, the next generation of horses will be faster. Similarly Canada's wheat industry has for decades been breeding plants that will grow faster and be more disease resistant.
The area of silviculture that focuses on tree breeding is called tree improvement.
"Tree Improvement is the process of selecting trees with desirable genetic traits such as faster growth, straighter stems, better wood quality and insect and disease resistance." (Ministry of Forests)
Natural selection shapes the characteristics passed on by the parents so that the seeds are suited to the environmental conditions of that area. When collecting wild seed the geographic position of the seed tree must be noted. As well, cone collectors will try to select trees that exhibit good characteristics such as health, strength, straightness, speed of growth and limited branching. These are called plus trees.
Searching for wild cone crops from plus trees of certain species is sometimes difficult. In some cases, species don't produce cones very often. For example spruce trees can take twelve years to produce seed. In other cases, insects may ruin the seeds before the cones can be picked. This means that wild seed is not always from the best trees and so may not always be of good quality. However, Forest Managers want to have seed that is top quality and will produce fast growing disease resistant trees. Because they cannot always depend on the quality of wild seed, seed orchards have been developed to produce genetically superior seed. Good seed equals good trees!
Tree breeders search the forests for those outstanding plus trees, ones that are healthy and well formed. They take cuttings from the branch tips, graft them to host trees in the seed orchard, and then selectively mate these trees to produce superior seed. The seedlings that result from this careful breeding program are planted at test sites to be measured and evaluated. The ones that prove to be outstanding will be used in reforestation programs. Any parent trees that prove to be unsatisfactory will be pulled out of the orchard. The orchard trees are pruned to be short, so cone picking is much easier than in the wild, where trees can be over 100 hundred meters tall!
What does "superior seed" do for reforestation? The trees from this seed will establish themselves and grow much faster than wild seed. A clearcut planted with super seedlings may be ready to harvest 20 years sooner than one planted with wild seedlings. Seed is also selected for resistance to disease. White Pine from superior seed will resist a disease called blister rust, while almost all wild White Pine trees in B.C. will die from this serious tree infestation.
Some critics of seed orchards claim that seed orchards have less genetic diversity than natural stands. In fact quite the opposite can be true. Most people don't realize that trees in any given natural stand are closely related, coming from a small number of parent trees. In seed orchards the plus trees are selected from a broad range of forests. Ultimately when these genetically distinct parents are crossed there is a much broader genetic base.
About 15% of seed today comes from seed orchards with 85% coming from wild seed. Within the next decade, the complete reverse is expected to be true, with over 80 % of the seed coming from seed orchards.
Biotechnology
Some trees are difficult to grow as seedlings. Some have seed that is hard to find or will not grow well in nurseries. Spruce and Yellow Cedar are two examples of trees where different methods have had to be developed.
Rooted cuttings are young trees that are grown from tiny pieces of branches from another tree. By using rooting hormone, roots can sprout from one end of the branch, and a new tree is started. You can do this at home with many houseplants. The branch bits come from:
- Donor trees specially grown as a "hedging orchard". A hedging orchard can be used for 5 to 7 years to produce cuttings.
- Donor seedlings. This preferred method is commonly used with interior spruce. The donor seedling is grown for one year with maximum branching encouraged. Up to 100 cuttings can be taken from the donor with proper cultural practices.
With yellow cedar we have even gone a step further using biotechnology. A process of tissue culture has been developed where branch material is cloned in a test tube, producing thousands of new trees from that one original branch.
Another use of biotechnology is the use of "somatic embryogenesis (SE)". This procedure is a type of tissue culture where thousands of individuals are cloned from a seed. SE may well become the main biotechnique of the future. Since SE is an expensive procedure, in British Columbia it is likely to be used primarily for pest resistance. For example, SE would be used for developing weevil resistant lines of Sitka and interior spruces. While British Columbia has some of the most advanced embryogenesis companies in the world, these techniques are used more commonly in the United States and New Zealand.
Getting Ready to Plant!
As you can see, wild seed, improved seed, wild seedlings, improved seedlings, rooted cuttings and clones all play an important role in reforestation programs in B.C. Many factors are considered before any seed is collected or seedlings are grown, and all this planning occurs before any trees are harvested. Ecology, genetics and nursery technology all play important roles in getting the right seedling ready for planting.
While the seedlings are being prepared at the nursery, the forest is harvested and the ground is prepared to receive the new seedlings. In a process called site preparation, good seedbeds for direct seeding and/or plantable spots for tree planters are created. Tools such as fire, machines or herbicides can be used to clear the area and encourage tree growth. Site preparation speeds up regeneration. Some of the methods of site preparation include tilling, raking and the creation of mounds to provide warmer places for roots to grow. Slash burning and herbicides can also be effective site preparation tools, but they are more limited in their use due to environmental concerns.
PART 2: FROM SEEDLING TO FOREST
It has taken a lot of people and science to get that special mix of seedlings to the tree planter. However this is just part one of the reforestation story. There are years of special treatment ahead!
Tree planters are highly trained and supervised, so each seedling is given a good chance to flourish. The mix of tree seedlings in a tree planter's bag must each be planted in the perfect spot: a drier hump for one species, a wetter depression for another and a resistant species for the root rot area. Sometimes tree planters add fertilizer when they plant to give those new trees a little boost.
Foresters conduct surveys or "payment plots" to ensure the tree planters have followed the silviculture plan and that they have planted the seedlings properly. If not, the planters may be paid less or even lose their pay altogether. It is very important to start these seedlings out right!
The seedlings will be regularly monitored throughout their lifetime. Monitoring involves checking for disease, insect problems and animal damage. Growth rates and brush competition are also measured until the seedlings reach harvest age, to see if stand tending (below) may be required.
In nature, the majority of seeds produced do not survive to be mature trees. Competition with other plants and damage from insects and animals take a huge toll. However, silvicultural practices such as site preparation, planting and stand tending dramatically improve the survival rate in managed forests.
Stand tending is a group of nurturing activities carried out at different stages in the life of a stand of trees. Treatments such as juvenile spacing, weeding/brushing, conifer release, commercial thinning, fertilizing, site rehabilitation and pruning are often carried out to improve the growth, quality and value of the remaining trees. Stand tending improves the vigour and health of the stand, and therefore shortens the time it takes until the trees reach harvestable age.
A forest will grow to the limits of the available light, soil nutrients and water. However the growth can be directed into herbs, shrubs or trees. Clearly, the forest manager prefers that the majority of growth to be used for trees. Stand tending controls where the growth will go and so helps to:
- maximize the value of wood grown on a site by controlling the species of plants
- maximize tree size by controlling the density of the trees
- reduce the losses to competition, insects, disease and fire
- reduce the time to harvest
A forest responds to stand tending activities according to the site, the tree species, the tree's age and its condition. It is possible in some cases to reach harvestable age 25% faster with intensive stand tending. In other situations there has been little or no improvement. Recent data is raising questions over the value of juvenile spacing in B.C. forests. The cost of treatments and the responses in stand growth and value must be considered carefully. Tending activities are expensive and the time span is very long from the initial investment until profits are realized at harvest. To be economically viable these activities must produce a measurable profit.
Regardless of economics, however, there are other situations where our knowledge and understanding of stand tending techniques will be valuable. For example these same techniques can be used in wildlife habitat management, forage production, protection and in creating viewscapes.
SUMMARY
Silviculture, the art and science of growing a forest, spans many sciences and utilizes leading edge technology on many fronts. The parallels to farming are strong in the sense of managing a crop. However a forest is far more complex than a farm, both ecologically and in terms of the many uses and values we all expect from a forest. Forests also take much longer to grow, so the time between the investment in silviculture and the potential profit from selling the crop can be fifty years or longer. That's a long time to wait for an economic return and so the decisions made in this important industry must be carefully considered.
As the human population increases so does the demand for forest products. However the land available for production of these products is decreasing. This puts more pressure on forest managers and scientists to continually improve the silviculture knowledge base. Finding ways to grow high quality trees quickly, utilize the productivity of the land efficiently and do it all economically is of critical importance to the future of the forest products industry, and to all Canadians who enjoy the wild and natural beauty of British Columbia.
For more information see the Ministry of Forest web site:
References:
All Things Considered MOF Forestry Handbook: UBC press Tree Improvement website MOF Forem-Introduction to Silviculture module COFI Balancing Act Hamish Kimmins Managing Your Woodland FRDA II