Understanding Forest Succession To Improve Your Woodlot

Let's start by looking at what kind of forest develops when trees first invade an area of cleared land like an abandoned pasture. The first stage in our area (here in the North East) is when the field grows up to brush. Brush may include an assortment of small bushes, goldenrod, aster, thorn apple, wild apple and often raspberries (brambles). This first stage provides good habitat for small animals and the raspberries attract birds, raccoons, and bears.

Partially open fields are a harsh environment for tree seedlings. The intense summer sun can create periods of very dry conditions especially in the top layers of soil where seedlings first take root. Many tree species that normally live in the partial shade of a forest can not survive in full sunlight. Only trees that are specially adapted to survive under these conditions can grow. Grey birch, cherry, poplar (aspen), pine, and wild apple trees are trees that can get a foothold here. These "pioneer species" of trees have the ability to invade and colonize open areas.

As we walk through this pioneer forest we are first struck with the fact that the initial stage of a pioneer forest looks rather thin. There are often large spaces between the trees that are starting to grow in the clearing. The trees here are competing more with the harsh environment than they are with each other. Small seedlings with their shallow root system are under the most strain. Once a tree gets established with deeper roots and more leaves then growth accelerates rapidly.

Birch trees take advantage of their fast growth by sending up multiple main stems. A typical birch may have 3, 4 or even 5 main stems. The trees bush out and shade out a large area under their leaves. Poplars have a different reaction to their advantaged position. They put all their growth into a single stem, made with a very light wood that grows exceptionally fast and tall compared to other trees.

As time goes on, the clearing begins to fill in with other trees. We can see the birches continue to spread out taking a wide circle of sun with their multiple stems. The poplars grow fast, rapidly becoming the tallest trees. Pine trees are not far behind the poplars.

Now, as we walk through the young forest of 20 years old, we are struck by the fact that it is starting to look like a forest. Gone are the open areas where the sun beat down all day. Instead of patches of grasses, we find areas of shade and the beginnings of a forest floor cover here and there with last years’ leaves. In this new environment of partial shade, a fundamental change is taking place. Here and there we can see new trees moving into this partial shade environment. The pioneer trees that endured the harsh conditions of full sun and dry conditions have created a new protected zone in their partial shade. Tree seeds from the surrounding forest have fallen in this area and found the conditions just like they are used to in a forest. Hard Maples, Red Maples, Yellow Birches, Ashes, Beeches and other shade tolerant trees are starting to take a foothold. In the next few years an under story of small trees begins to form under the larger pioneer trees.

Thirty to forty years after the initial invasion of the pioneers, the Birches still shade a wide circle around their multiple stems but outside that circle the new comers are now beginning to grow taller than the birch. The Poplars are still the tallest trees but their fast growth and light wood is starting to take a toll with broken branches and an occasional black canker growth near the top of some of the trees. The smaller apples and Juneberry trees are struggling to find light in the increasing shadow. Pines are still doing well, though not quite as tall as the poplars. The more shade tolerant trees underneath them continue their relentless growth upward.

This stage marks the beginning of the transition from a pioneer forest to a more mature forest. Many woodlots will contain this mixture of trees. Older poplars, still the tallest trees, are starting to show decline. Smaller pioneer trees like birch, apple and Juneberry are dead or dying, shaded out by the taller trees. It also is the stage where you can begin to notice that pioneer species are no longer found in the under story. The sun loving trees that 1st moved into the clearing have now created conditions of shade that make it very difficult for their own offspring to grow in the same area as their parents thrived in. They can not compete with the shade tolerant newcomers from the surrounding forest.

The trees that take root in the under story of a pioneer forest gradually overtake the trees that originally colonized the clearing. Poplars rarely live beyond 40 to 60 years. Birch are covered in shade and gradually die out. Some trees like black cherry can continue into the next forest but find it difficult to reproduce in the new shaded forest.

Pioneer species need open spaces and clearings to grow. Clearings occur naturally in all forest systems. Openings can occur from forest fires (started by lightning), heavy winds, insect damage, or ice storms. Pioneer species have been invading newly disturbed areas for eons of time, so they have adjusted to deal with the fact that they eventually shade out their own seedlings. The seeds of pioneer species have special characteristics that allow them to travel far from the parent trees into new clearings in distant places. As they fade in the under story of the forest they created, their seeds will find their way to new clearings in other distant areas.

Pioneer species have different strategies for moving their seeds to new areas. Poplar (close relative to cottonwood tree) produces white cottony fluff each year in great abundance. On careful examination the fluff will reveal very tiny (pin head size) seeds enclosed in the cotton. An abundance of light windborne seed allows poplar to colonize new clearings or disturbed areas even at far distances from the original site. Birch has a similar strategy with small winged seeds borne by the wind. Cherry has a different delivery method. Birds love the cherries and they carry the hard indigestible seeds in their droppings to other areas. Although fewer cherries are produced than the small seeds of birch or poplar, the large seeds delivered in a packet of fertilizer from the bird droppings give the seeds a good start in life.

Rain can pose a problem for wind borne seeds. Wet seeds can be too heavy to travel far from the parent tree. Pine trees have evolved a special adaptation to protect their seed from release into the rain. The cones are sensitive to water. Once the seeds have matured and are ready for release the pine cones begin to open as they dry out. If the cone gets wet it will close and hold its seeds. Later, when conditions dry out again the cones will reopen to allow seed release. (Find a pine cone under a tree and put it in a bucket of water. It will close. Take the pine cone out, let it dry and watch it open over a period of a few days.) Pine cones are often at the top of the tree to take advantage of the best wind conditions. Having water sensitive cones limits release of seeds to periods of the best condition: dry air with wind.

Once the pioneer species move into a clearing they use it as a staging area to colonize new areas by taking full advantage of their dominate position to produce abundant seed crops. The far reaching seeds spread out to new areas for many years. By the time the parent trees are overshadowed by the more shade tolerant trees many of their descendants are already taking root in new distant clearings.

For example, in our woodlot, about 1/4 of the forest is in some stage of pioneer succession. With our new view of forest succession let's see how this influences what trees we decide to harvest. When we see grey birch trees surrounded by taller young hardwood (like maple, or ash) we see a birch tree in decline. It is just a question of time before that birch will die from being overshadowed by the trees around it. Any one who has ever grabbed a standing dead birch and had the tree crumble in your hands knows it doesn't take long for a birch to rot after it dies. If you ever plan to burn this wood you better harvest it now before it is gone. Birch stored in the sun makes a nice firewood and the wax in the bark makes a sweet smelling smoke and a fast starting fire. Burn it within a year or two or it may begin to rot if not split and stacked in full sun.

Our decision to cut the birch is based on a new understanding of the natural succession of a forest as it matures. By harvesting the birch the only impact we had on the forest was to speed up by a few years the natural succession process and in the process we harvested firewood that otherwise would have rotted.

In our woodlot there are times where we have decided to slow down this natural progress. Apple trees that once grew on this land (both wild and descendants of domesticated apples) are often found in a pioneer forest. Like the paper birches they are doomed to be overshadowed by the more mature forest trees. This may not always be in the best interest of the forest ecosystem. Apples trees produce abundant crops in the Fall when many animals are trying to put on as much fat as they can before the winter. Many animals love to fill up on apples. Deer, raccoons, turkeys, squirrels, ruffed grouse, birds, and bears can not pass up an apple tree in the fall. Deer, for example, need as much fat as they can get because they often are forced to feed on browse that is low in nutrition during the worst of the winter. Without adequate fat stored in the fall many deer won't make it through to the Spring.

We found a few acres of our forest with many apple trees just starting to be overshadowed by faster growing hardwoods. Given the value of apples to wildlife (including people) we decided to set back this forest succession and remove all the competing hardwoods and make this area into a natural apple orchard. In the process we harvested a whole year’s supply of firewood. The thing to remember about apples is that every apple tree is different. You may be surprised to know that Macintosh apples are all clones. Cuttings were taken off the original Mac tree (from some farmer in the North East). Apple trees breed true only by grafting, so all your Mac apple trees are cuttings from trees grafted from the original Mac tree. Branch cuttings are grafted to other apple root stocks to keep the Mac line of apples going.

So apples trees that grow from the seed for fallen apples are always different from the parent tree. Sometimes they can be very different (sour, sweet, firm, mushy, etc). So we pick the ones we like and leave the rest for the animals. With about 50 apple trees in our natural orchard there are bumper crops for animals almost every year. (Individual apple trees naturally produce a crop of apples on average every other year, but with trees on different schedules there are usually some apples with good crops each year. A failure of the apple crop is usually caused by a cold spring that limits bee activity or a frost that damages the apple blooms before they set fruit.) The thinning of hardwoods from the apple area has resulted in a large increase in apple yield and the newly revived orchard has become a major feeding area for the deer in our area. Cutting down the hardwoods in the apple area and setting back the natural progress of the forest (to a more mature mixed hardwood forest) has dramatically increased the carrying capacity of this land for many animals. Deer, raccoon, birds, grouse, turkey, and even the occasional bear have all benefited. We have an electric fence around our garden (see: [ Controlling Animals In The Garden ]) to make sure the deer attracted to the apples don't decide to extend their feeding into the gardens.

Let look on how to use our knowledge of forest succession for making harvesting decisions. Poplar trees remain the tallest trees in the early stages of the new forest but continue to decline. In our area the decline is mainly due to a disease that strikes the older trees by forming black canker areas near the top of the tree that eventually kills the top and the tree. Poplar wood, as we said earlier, is a very light wood (when dry) and burns too fast without any coals to be a good firewood when hardwoods are available. It makes really good kindling, but there is only so much kindling you can use. It is used for pulp in making magazines but the price paid is usually not worth the effort. It does have a use for making log cabins as long as the logs are not used as the bottom logs on the foundation (it rots if wet). There is a poplar log cabin near us that is still going strong after 25 years. Poplar wood is a very light wood (full of air spaces) and probably provides good insulation. It's best to use good diameter logs (a foot or more) and cut 2 sides flat. They may twist when drying so some people build with them while they are still green (before they dry out). Get help from someone in your area who knows your local conditions before undertaking a project with poplar. We leave the best poplar (straight, good diameter), and girdle the crooked or diseased ones by cutting the bark with a chain saw just slightly into the wood. Do 2 rings to make sure. This will kill the tree, allows the surrounding hardwood to grow faster (speed up the succession) and the standing dead tree provides really good habitat for tree dwelling animals like woodpeckers, owls, tree nesting birds and occasionally raccoons.

When the shade tolerant trees finally take over, they do not mark the end to the succession. Often the trees that initially come into an area came from trees that were nearby to supply the seeds so they may not represent the best adapted trees to this phase of forest succession. Over time the mixture of trees in a maturing forest will change. The climax forest is a mixture of trees that can reproduce in the mixed shade conditions of the forest. In our area the climax forest is the mixed northern hardwood forest. In colder areas the hardwoods tend to disappear in favor of evergreens.

Once the forest begins to move towards a mature mix you need to know the preferences of tree types in your area. You also need to decide what type of trees you want to favor in order to decide on a harvest plan. For example, black cherry is a really nice hardwood with many uses. It also makes a good firewood. Normally cherry likes sun and can enter as a pioneer species. If you selectively harvest individual hardwood trees widely spread out in your forest you will be selecting against cherry trees because they need sun to get started in a forest. If you selectively cut trees in small groups then you can create small clearings with sun that will allow cherries a chance to reproduce even in a maturing forest. Your method of cutting will determine if you will have cherry in your forest in the future. Most other hardwoods can reproduce in the shade of a maturing forest to maintain the climax mixture.

Some trees like hemlock (evergreen) can tolerate shade for extended periods of time. There have been cases of hemlock trees a few inches in diameter being over 100 years old. They get by with as little light as possible by barely growing and just waiting for other trees to die and give them a place in the sun. Once a hemlock recovers from over shading it can then grow to an age of 300 or more years if left uncut under the right conditions.

We found it useful to set out some general goals to guide our forest improvement program:

-SUSTAINABLE YIELD: Harvest your forest so you never run out of wood (See: [ Managing Your Woodlot For Sustainable Yield ]) and [ Snowmobile Sled Firewood System Home Page ])

-ENCOURAGE DIVERSITY: Don't put all your eggs in one basket. If a new plant disease hits trees (like Dutch elm disease) you will have other trees that are not impacted. In a natural system more diversity means more system stability. Most woodlots have been cut over many years. Often the best species were removed leaving the less desirable species to reproduce. Encouraging diversity could involve an effort to select for trees that more accurately reflect the mix of trees that naturally occurred in your area. We have very few oak trees in our woodlot. In the past oak was highly prized for lumber. We try to leave any oaks we find to encourage seed production. Our most plentiful species is red maple. When we have a choice we always cut red maple over other less common hardwoods. Ash trees are currently in decline in our area, so we try to favor those that still look healthy.

-IMPROVE FOREST FOR FIREWOOD PRODUCTION AND LIMITED USE FOR BUILDING: Selective cutting of diseased trees lessens the impact of insect damage. Removing crooked trees leave the straight trees to mature. Straight trees make better lumber and are easier to process into firewood. Leaving white pine (for lumber) and the straightest poplars leaves a reserve of potential building material for the future (both for us and those who come after us).

-INCREASE CARRYING CAPACITY OF LAND TO SUPPORT WILDLIFE POPULATIONS: Animals are an important part of a forest ecosystem. Trees like Beech, cherry, apple provide good feed for a wide variety of animals and birds. Small clearings provide browse for deer, cover for small animals and raspberries for feed. Consider the option of including small clearings in your harvest plans. Besides the enjoyment of having interesting animal populations, there is a very practical consideration. Even if you don’t normally hunt, if economic times get more difficult then you may need to consider the option of harvesting, deer, rabbits, and turkey from your woodlot just to feed your family. Increasing the carrying capacity of your land actually allows more animals to live there than would have been possible without your improvements. Woodlot improvement provides a reserve for both the animals and your family.

-RECREATIONAL USE: Protecting some particularly interesting or scenic areas of your woodlot from harvest may be something you choose to do. The network of roads or trails can provide opportunity for walks, cross-country ski trails, snowshoe trails, ATV or snowmobile access. Seeing a few deer in your walk can add to any experience.

-COPPICING: By cutting certain trees that can successfully sprout from the stump and grow to a harvestable size rapidly you can increase firewood production and make harvesting more intensive and easier to carry out. Look up coppicing in forest management books or online for more detailed information on tree species that can be coppiced and how to manage harvests. This practice has long been established in Europe.

When harvesting your woodlot set up your own set of goals and get to know your trees. With a basic knowledge of plant succession you can work to improve your forest while supplying your family with firewood into the future and for generations to come.

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