16 • Sawmill & Woodlot

It’s a Jungle Out There!

by Robbo Holleran

Everyone has heard of forest succession.After a disturbance,there is a somewhatpredictable transi-tion from the initial“pioneer plants,”perhaps through several stages,toward a “climaxforest”—the stable,old-growth conditionthat can last foreverand regenerate itselfin its own shade.

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Forest diorama series depicts a central Massachusettes landscape, viewed over time.

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Each soil/climate combination has its own commu-nity of trees to which it is “predestined” if the forest isallowed to develop for centuries. We learned this inhigh-school biology.More modern views theorize that truly stable climax

communities are quite rare and may be just theoretical.There is always some sort of disturbance, and so theforest continually responds in some dynamic state.Whether it is windstorms, fire, introduced species, oreven slow climate change, there is always somethinggoing on. Here in northern New England, we tend toview hemlock—northern hardwoods, hemlock-spruceand hemlock-pine-oak as climax forests on differentsoils and climates. Western hemlock is a common com-ponent of western climax forests, with long-livedspecies such as redwood, Douglas fir, and cedars.Southern climax forests include a wide range of long-lived species, depending on soils, moisture, and cli-mate. These are always mixes, and are constantlychanging over time. Because most ecosystems experi-ence some kind of disturbance, a true climax becomesmerely a theoretical possibility. Modern ecological the-ory recognizes this dynamic progression that neverachieves any stable “equilibrium.”While evidence of succession is fairly obvious, there

is another process at work. My own competing theoryis that the forest is more determined by its response tothe previous disturbance. The key question, in myopinion, is: Are the changes in the current forest condi-tion determined by its trajectory AWAY from the lastdisturbance, or TOWARD its climax state? Extremedisturbance, such as a hot wildfire or severe wind-throw, can change the soil enough to dramatically alterthis trajectory. There are reasonable examples of boththeories, and they are not mutually exclusive, so thetopic makes for great conversation among those whoobserve these things. The modern view of a “dynamic

equilibrium” incorporates this perspective, to someextent. Of course, I hold to the importance of “distur-bance response,” or the article would merely be aboutforest succession.

SuccessionTo brush up on succession, the basic concept is that aforest changes over time, starting with some disturbancelike fire, glacier retreat, major storm damage, or perhapslogging or land clearing for farming. Then, a transitionoccurs with initial colonization of pioneer species, tran-sitional intermediate species and structures, and finallystabilizing in the climax community for that soil and cli-mate. The implication, for some, is that the “climax for-est” is good and everything else is less so. These “cli-max forests” can be an ecological goal, and there isstrong political force that influences land management(particularly on public land) toward this end. Wildernessareas on national forests, along with other “no cuttingzones” such as the Adirondack Park, make up substan-tial acreage in the United States.Part of the myth that climax forests are “good”

comes from the thought that everything “nature” doesis “good” by some definition. Let me remind you, it isa jungle out there. Each tree is competing for sunlightwith its neighbors. In some places, they compete formoisture or nutrients. Some live, some die. But justbecause trees are surviving does not mean they arehealthy. As trees grow, the natural forest always getscrowded unless there is some disturbance. And so,competition for sunlight becomes the major factorstressing tree health, since sunlight is the source ofenergy and sugars for tree growth and health.

Disturbance is Part of the EcologyIn some forest types, the natural community is definedby the particular disturbance to which it is prone. Fire

HARVARD (UNIVERSITY?) FOREST DIORAMA SERIES CREDIT PHRASE..PERHAPS SHOW WEBSITE?????

1. Original, climax-type forest (Hemlock, beech, oak, pine) in Central Massachusetts. 2. Same view showing initial pasture clearingin mid 1700's. Note the complete removal of some forest portions, partial removal of other forests. Much of this landscape wascleared for farming by the mid 1800's. 3. Beginning of pasture abandonment in mid-late 1800's with pine regrowth. 4. Harvest ofinitial pine forest in early 1900's. What will grow back after this disturbance? Will this eventually look like the original picture?

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3 • Sawmill & Woodlot

and flooding are normal examples. Pitch pine-scruboak, lodgepole pine, and ponderosa pine types are allmaintained by fires of various intensity and frequency.Floodplain forest types with sycamore, silver maple,cypress, or gum are maintained by periodic floods. Sothis theory of ecological succession and natural com-munities recognizes that disturbance is a part of theecology.A related viewpoint on silviculture can be called

“natural disturbance forestry.” This consists of model-ing harvesting practices to closely follow natural distur-bance regimes. If wind and ice events tend to blowdown clumps of a few trees or a few acres, then forestrypractices can follow this method. In areas where forestfires or hurricanes create larger disturbances, then har-vesting would mimic these techniques. The theory isthat the resulting forest will be close to a natural forestin species composition and structure, while still beingproductive for timber and other wood products. And“natural” must be good.My own perspective is that the managed forest pro-

vides a great level of benefits, more so than unman-aged forests. In general, we are managing “naturalforests”—forests of native species, with mostly naturalregeneration from nearby seed sources, and usingsome aspects of natural disturbance silviculture. Butintensive management is essential with limited acres.Forestry goals of productivity, tree heath, and valuegrowth are not the same as a “natural forest” in mostcases. This gives us desired results of species composi-tion, structure, and timber quality; and the managedforest is less prone to the drastic swings of natural dis-turbance. These are swings in the population of trees,understory plants, pests, and wildlife. We like things tobe somewhat stable. On the other hand, I am glad thatthere are large tracts of relatively unmanaged land andlarge tracts of super-productive plantation forests.These are all part of the diversity of our forests, forestpractice, and our culture.

Disturbance Response orSuccessional TendencyAll of our forest management techniques, whetherplantations or variations on natural forests and naturaldisturbances, come back to support my view of “distur-bance response” as more important than successionaltendency. When we look at a forest, we are always see-ing the result of the past disturbance. Was it cleared forfarming? After farming, was it abandoned as a plowedfield, partially grazed for a slow reforestation process, orwas it planted to trees? These all jump out to me. Werethere wind or ice storms, disease, or tree harvesting thatshaped the forest we see today? Today’s forests arealways in some state of response to past disturbance. Ofcourse, I work in a region that was mostly cleared forfarming 150 years ago and has had various stages of tim-ber harvesting over the past century. While there is lit-

tle “old growth,” or original undisturbed forest, weactually see a lot of hemlock-northern hardwoods andother communities that are considered climax types.But they are resulting from various disturbances.What about the expression “If you cut softwood,

hardwoods will grow back; if you cut hardwoods, soft-woods grow back”? There is just enough truth in thisthat the expression persists. Many of our softwoodstands are pasture regrowth where softwoods grew (onrich farm soils) because of grazing pressure on hard-woods, competition with grass, and other factors. Pineand spruce are often found on well drained, aban-doned pasture. When these are cut, either heavily orlightly, the regeneration is often mostly hardwoods dueto these soils. When the softwoods finally mature, ahardwood forest will replace it. Cutting hardwoods toregenerate softwoods is less common in my experi-ence, but let me give one example. In hardwoodforests, spruce or hemlock can accumulate in theunderstory for decades. Harvesting the overstory canrelease these saplings and give the impression thatthey resulted from harvesting the hardwoods. Arethese examples of succession or disturbance response?In each case, the forest moves toward its climax typefor the soil and climate, and the previous overstory wasmerely transitional. But they are also examples of dis-turbance response, where the new growth is directlyinfluenced by the amount of sun, shade, disturbance,and seed source.

Coping with ChangeIn a general sense, this is a discussion about how a sys-tem copes with change. The change can be mild orextreme, gradual or at a precise moment. The systemcan either be resilient or fragile. For forest ecosys-tems, what happens on the forest floor will influencewhat happens in the canopy in future decades or cen-

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Drought kill to understory

turies. New seedlings need certain conditions to getoff to a good start. The proportion of sunlight andshade, moisture, soil pH and nutrients, duff and organ-ic coverings, temperature, and seed source are all fac-tors. In a mild disturbance like light-intensity logging,individual tree death from disease or wind damage,only shade-tolerant seedlings will survive. After a fewmore decades, they will be well established in theunderstory and have a chance at growing into the maincanopy if an opportunity (another mild disturbance)arises. This is what we think of as creating our “climaxforest.” More severe disturbances will establish other

species. A hurricane or forest fire will expose subsoiland allow small seeds to flourish. The sunlight willallow shade-intolerants like birch or pine to succeed.Shade-tolerant species may be part of the mix, andoften grow more slowly. Moderate disturbances allowspecies of “intermediate” shade tolerance to be suc-cessful. This points to the idea that the forest is mov-ing “away from” the past disturbance, though it mayalso be “moving toward” some climax condition.Managed forests, by definition, have a cycle ofplanned disturbances (logging) of metered intensity to

achieve some desired result. Climax species and struc-ture might, or might not, be part of this objective.It is often assumed that the climax forest type, for

any given region, will have the greatest species diversi-ty. This, by definition, must be wrong. What we thinkof as “climax” species are the longest-lived trees thattolerate shade or the other stressors of that type, likefire or flood. Short-lived and shade-intolerant treesnormally drop out of the mature and climax types, oroccur only in small numbers where disturbance occurs.Tree species diversity will always be maximized at amoderate level of disturbance. Likewise, forest struc-ture loses some measures of diversity in unmanagedsituations. Unmanaged forests also have some level of distur-

bance, such as fires, storms, drought, or disease. Eventhe “natural forest” may have enough disturbances sothat true climax conditions are never achieved.Climate change is one example of slow change.Whether or not you are a believer in man-caused cli-mate change, it is fairly obvious to me that the forestsindicate both cooler and warmer periods in the past1,000 years or more. Let me mention two examples.Southern Vermont, New Hampshire, andMassachusetts have a few rare black gum swamps.Black gum is a southern species, doing well inMaryland and further south, but surviving as far northas the Great Lakes. These rare swamps were probablycolonized by black gum during a warmer period andhave somehow persisted into colder conditions. Ithelps that these wet areas were avoided by farmersand loggers, and this species tolerates the wet soil,occasional flooding, and grows to over 600 years old insome cases. Another example is an island on the coastof Maine with jack pine, black spruce, tamarack, andalpine birch. These are far-northern species that haveprobably occupied this space since a much cooler peri-od, perhaps the last glacier retreat. The coastal influ-ence makes this warmer than nearby inland Maine.Somehow, these boreal species have persisted and notbeen replaced by other species like red maple and oak,even though these are present on the island. In eachcase, these are examples of relatively undisturbed “cli-max types,” maintained through some change or dis-turbance. As you look at forests in your backyard, your own

woodlot, or as you travel the world, watch for evidencethat the forest is changing. I think you will see that theforest is moving toward some stable climax condition,and that its condition is largely shaped by the previousdisturbance. n

Robbo Holleran is a private consulting forester helpinglandowners meet their goals in Vermont and adjacent areas.His work has him outdoors about 150 days each year, plusplay time. He is one of the authors of the new SilviculturalGuide to Northern Hardwoods.

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After a severe disturbance like a clearcut, hurricane, or forest fire, can the climax forest type

be changed, or just the pathway to it?