Saturday, May 23, 2009

5-23-09 "Timberline" by Robert Monahan, Dartmouth College Forester and a cofounder of the Mt. Washington Observatory

This is the title page of Bob "Gramps" Monahan's master's thesis that he wrote in 1930-1931 when he was attending Yale Forestry School. I like the wood cut of the krummholz that he did himself. Reprinted in Volume 19 (old system of indexing), Appalachia, 1933; pg 402.

In defining timberline Monahan quotes Chittenden and other researchers who first observed that balsam fir (Abies balsamea) grows to altitudes of 5500 feet in the Presidential Range and that from 4900 feet to 5500 feet in elevation the fir produced few seed cones. Reproduction then, occurs mostly by the mechanical means referred to as “layering” in which the lowest branches of the fir that are closest to the ground send down roots into the soil and anchor the limb and the limb eventually becomes an autonomous tree.

Balsam fir, along with dwarf paper birch (Betula papyrifera) and black spruce, (Pices marinara) are the three most important tree species at timberline according to Monahan but he refines that by stating that “balsam fir and paper birch are the principal species at timberline”. To that he adds “balsam fir is the outstanding species.”

In terms of adaptations he notes that the roots of the fir grow to a greater length without branching giving it an advantage over black spruce on dry, exposed locations. Due to the more shallow roots of the fir, he states, it can absorb its moisture in the wetter upper layer of the soil. Lastly, germination requirements for the fir are simple as it only needs a layer of moss in which to germinate. Fir can establish itself in many habitats where other species cannot.

Some terminology needed for understanding some of the prominent features of the timberline. The first is the word “Krummholz” that’s German for “twisted or gnarled wood”. Trees growing in the severe conditions at timberline are often twisted and form thick mats close to the ground and are called krummholz. Fell-field defines a “rocky flat or plateau in arctic, or subarctic regions or on alpine summits of mountains” according to one of Monahan’s sources. Fell-fields shouldn’t be confused with “feldsmeer”, the large areas of weathered and broken rocks that are common on or near the summits of the Presidential Range.

The Climatic Conditions and Their Effect on Timberline

The following are direct quotes from Monahan’s paper:

“Climatic conditions must be recognized as the principal cause of limitations in the altitudinal distribution of trees on high mountains and for the peculiarities of tree growth at and near timberline; in particular, conditions of air temperature, solar radiation, atmospheric humidity, precipitation, and air movement.

“There are local physiographic conditions which explain a deviation in the altitude reached by tree growth but in general this vegetation continues upwards until the climatic complex becomes too acute and equilibrium between growth requirements and climatic factors is destroyed.

“It should be understood that no one of these factors, per se, can limit tree growth; for example the adverse effect of lower temperatures may be offset by the increasing precipitation. The series operates in a complex to such an extent that the individual effect of each factor is very difficult, if not impossible, to measure.

“As might be expected, the actual altitude of timberline varies with latitude. In general, in traveling northward, it occurs at increasingly lower elevations, and toward the northern limit it is but little above sea level. It is further influenced by other general climatic relations: whether for instance, the range is located in a region of continental climate marked by lack of uniformity, high and low temperature extremes, irregularly distributed and moderate precipitation, strong insolation, great loss of heat by radiation at night and in the winter, and relatively slight humidity; or whether, on the other hand it is located in a region near the sea-coast where the maritime climate is more moderate and the annual precipitation higher.

“ The climate of the Presidential Range is neither continental nor maritime, but a variation between the two. The Presidential Range has been described by several writers as “an arctic island in the temperate zone,” having the same climate as Labrador at 60 degrees north latitude. It is hoped that the current records of the Mt. Washington Observatory may provide a more complete definition. (Ed. note. Here’s a plug for the MWO. Apparently Monahan either wrote this paper while working at the Obs or at one of the AMC huts during a summer off from grad school.)

“It should be readily appreciated that the climatic conditions found on high mountains are far different from those that prevail in the lowlands. This change is accounted for directly by the diminution in the atmospheric pressure as the altitude increases, which indirectly influences the other climatic factors of heat, light, and precipitation.

“The climate plays a leading part in affecting the variations in the forest cover ahs been established by numerous ecologists. This paper will indicate the result of the operation of these various climatic influences at timberline and will attempt to emphasize those which may be considered the determining factors.” (end of quotes.)

Monahan, from this point on, analyzes the primary influences he feels play a role in creating the timberline beginning with Air Temperature. This he explains was long-thought to be the direct cause of timberline and there was a lot of direct evidence that timberline more or less paralleled certain isotherms.

Air temperature certainly has a major role in the life process of trees directly and indirectly, he points out, as temperature effects evaporation, transpiration, and might counteract the effects of lower barometric pressure and accelerated air movement. A lengthy discussion of the effect of altitude on temperature and the effect of temperature on tree growth with the following observation of note:

“As tree growth occurs chiefly during the night, it is logical to suppose that the low nocturnal temperature greatly inhibits growth and this relation may be an explanation of the prostrate form characteristic of trees growing the zone of cold nights.”


“In the true alpine zone the growing season, in which the plant must complete all its vegetative and reproductive processes, is reduced from five months to three weeks. This short season is responsible for the rarity of annual plants. Even when annual plants are transplanted from the lowlands they will frequently develop a perennial habit. The brief period of growth is also one cause of the prevalence of evergreen trees among the timberline species, because the evergreens are ready to assimilate at the first opportunity where as the assimilation of the deciduous trees cannot begin until the leaves have developed.”

Monahan then concludes:

“I believe that, rigorous as the temperature conditions on Mt, Washington may be shown to be, cold alone will not directly limit tree growth. The greatest importance of this factor rests in its keeping the soil frozen during the long periods throughout the year. During this time the moisture in the soil is literally locked up and non-available, with the result that the trees are growing on a physiologically dry site. To make the conditions more adverse for the trees, strong desiccating winds are blowing much of the time, which tend to ‘lick up’ whatever moisture may be available in the tree. Timber growth at treeline must therefore be very conservative in its water relations. At such times, when the transpiration loss, augmented by the constant air movement, cannot be offset by the supply of water in the soil, the tree is persisting under conditions that my ultimately result in death”

In a short sequence of other factors solar radiation, atmospheric humidity, precipitation, and air movement, Monahan discusses the contributions of each and its effect on the timberline. Solar radiation and humidity in the White Mountains do not limit tree growth at higher altitude in the Presidential Range as they might in mountains in more arid regions, he observes. Precipitation is important and he remarks that the highest level of precipitation is achieved during the summer months in the White Mountains including the Presidential Range which is advantageous for the trees as this is their growing season. Otherwise, he says, the total annual rainfall on Mt. Washington averages a little more than 80 inches so one could not conclude that precipitation is a limiting factor on tree growth.

Air Movement, “the daily alternation of winds descending to the valleys and ascending to the mountains, coupled with the normal increasing air passage with greater altitude, produces a constant movement of the air at high altitudes. This behavior of air currents represents a factor of considerable importance in limiting tree growth. The crux of the wind relations depends upon the ability of the individual trees to develop enough foliage, despite the adverse wind conditions, to carry on photosynthesis,” he states.

In addition wind increases rates of transpiration loss and the evaporation of soil moisture which are important factors in the success of failure of the timberline trees. Wind plays an important role in the morphology, the shape and structure of the trees, and wind also plays a role in the dispersal of seeds. Wind, also, may be responsible for “dry killing” trees at timberline because the winds are so severe and there is no moisture available, Monahan observes.

He goes on to say that trees adapt to the wind, along the “tension belt” by growing in areas protected from the wind making use of topography and physiographic features like rock outcroppings, boulders, and the sides of ravines. He notes that, “Not only the height but also the spread of these more fortunate pioneers is measured in terms of the dimensions of the sheltering boulder or ledge.”

He observes “the side of the rock around which the winds sweep with the greater velocity is readily indicated by cross-section of the trunk, for the growth rings against the direction the greater velocity are compressed many times closer than those in more favorable quadrants.

It should be borne in mind that it is the dryness rather the high velocity of the wind, which causes such a marked effect at, treelike. The combination of dry winds blowing across the mountain slopes at great speed forms and exceedingly critical condition.”

The list of factors in Monahan’s discussion that impact timberline includes Soil composition, soil moisture, surface and depth of soil, and soil temperature. Of these, pertaining to the Presidential Range, he remarks that “inadequate soil moisture, especially in the winter, may well become the limiting factor in the upward extension of tree vegetation.” He also puts importance on the affect of soil temperature on the distribution of trees at timberline altitude because of the effect on the length of the growing season as it “does not allow the seeds adequate time to ripen.”

“The frozen condition of the soil for long periods may explain the success of the balsam fir on Mt. Washington, “ he writes” for the zone with the most constant temperature is at the surface of the soil, this favoring the shallow-rooted balsam, the roots of which feed in the upper horizon.”

Monahan goes on to discuss slope and exposure, air drainage, snow deposits, altitude, and forest fires and their impact on timberline saying that in the order of importance individual or as a group they do not have more than a negligible impact on the formation of timberline in the White Mountains. Instead he concludes:

“The several factors influencing timberline have been discussed in detail and the most important conditions emphasized. I have attempted to stress the necessity of considering each factor not only from the view of its individual effect on tree growth but also in its relation to the complex of factors whose combine influence governs the altitudinal extension of timberline.
I do not claim to have solved this problem, which numerous investigators have failed to explain fully. I do, however, take this opportunity to emphasize one set of condition which must be considered in the case of the Presidential Range. It has been pointed out that snow cover on the upper slopes is surprisingly thin and therefore provides no protection to the soil beneath. Whatever moisture may be in the soil is frozen and consequently not available to the trees during long periods. Simultaneously strong dry winds are sweeping across and promoting a high transpiration loss that the tree is unable to sustain through further absorption of soil moisture. The result is that the tree literally dies of thirst. The leader may be killed, or the entire tree may die, if the period during which these conditions prevail is prolonged.
There are other factors which produce a marked effect, but I believe that the chain of circumstances offered in the preceding paragraph is the most logical explanation for the formation of timberline on the Presidential Range.”

The End

(Ed. Note: I’m enclosing several photos of my own to help illustrate Monahan’s thesis points. The black and white photos contained in his Appalachia article did not photocopy well.)

A photo taken from Jefferson's Knee of Mt. Adams showing the irregularities in the timberline that is often interrupted by the feldsmeer (areas of broken, weathered rock), slide tracks, and also where the balsam has taken advantage of a protected slope to advance higher in a few areas.

This photo is towards the summit of Mt. Jefferson, also from Jefferson's Knee, and in the foreground shows a "fell field", or rocky plateau, characteristic of the arctic. The large "fan" of balsam extending to the left and below the area of bare rock on the slope above the fell-field can partially be explained by the snow patch shown in the next photo which each year fills the "bowl" created by the area of bare rock .

This is a photo of the famous Mt. Jefferson snow patch that sometimes lingers until late June and early July. The snow patch is sitting in the middle of the rocky opening shown in the previous photo. This photo was taken from the summit of Mt. Adams on July 17, 1969, the year of the heaviest snowfall ever recorded in the White Mountains. But the idea is that the balsam growing on the eastern side of Mt. Jefferson at 5100 feet are benefited by the extra moisture provided by the lingering snow even if it doesn't always linger until July.

This is krummholz growing on Franconia Ridge at 5063 feet and establishing the higher advance of tree growth in that location. The photo was meant to show how snow packs in around the trees and protects them from the wind and provides added moisture to the soil around the trees' roots in the spring.

This is another example of a fell-field. It's called Bigelow's Lawn and is located on the southeast side of Mt. Washington. The kummholz in the foreground in balsam fir. The large, distinct green patch of krummhoz on the lower part of the summit cone is growing in an area where large amounts of snow, the "upper snow field", accumulates and remain well into May and sometimes June providing moisture for the trees and alpine plants.

This photo shows the approximate timberline across the western flank of Mt. Washington. It is roughly at the 5,000 foot elevation. This is an area of high winds throughout the year.

This photo is looking acrosss the timberline on the Franconia Ridge with Mt. Lincoln (5,106 ')
in the background. Here you can see trees, black spruce and balsams thinning out but advancing up the slope and creating an irregular line. This is an area of high winds through out the year as well.

This is a photo of the timberline on Mt. Adam's north slope. It also thins out gradually and balsams seem to advance up slope in an irregular line at just below 5000 feet. Again, this is an area of very high winds.

A hiker below the summit of Mt. Adams navigating the feldsmeer with crampons on. Generally speaking, the feldsmeer is only a feature of the northern summits of the Presidential Range; Mts Madison, Adams, Jefferson, Clay and Washington. It is most visible above timberline and therefore a feature of the alpine zone of the summits just named. It most likely exists below timberline in some areas and is obscured by the forest growing there.

If you look back up to the photo of Bigelow Lawn and the summit Mt. Washington you can see that most of the cone of is covered by feldsmeer and there is a funny story about this prominent and defining feature of the mountain. The story involves a man by the name of Russell Hodgson who was actually called "Casey" because his father had been an engineer on the Boston and Maine Rail Road and somehow Casey got that nickname from his father's profession. Casey had many jobs around the mountains. He worked for the US Forest Service, for the AMC, and for a number of years was an observer at the Mount Washington Weather Observatory (the Obs). In fact, he and Bob Monahan were good chums. Anyway, one afternoon in July 1961, Casey was out in front of the observatory taking the 2 pm weather observation when a man came running up to him. "Mister!, Mister!," he yelled at Casey and then with some urgency in his voice asked, "can you tell me where all these rocks come from?" Casey looked around calmly as if surveying all the rocks (the feldsmeer) individually and he looked back at the guy and replied, "The glacier brought 'em." Then the guy looked around a little more at all the rocks while Casey finished the 2 pm observation. "Well," the guy then asked Casey, "where's the glacier now?" Without missing a beat Casey, with a slight air of impatience, replied, "Gone back for another load." That's a true story.

A lovely photo of Mt. Adams from the cone of Mt. Madison taken in early April. In the lower center of the photo you can see Star Lake and just to the right of that you can see the advanced line of balsam that has managed to invade this col as the trees receive direct protection from the wind from Mt. Adams. The area in the col is moist, if not wet, most of the summer. The elevation of timberline here is also 5,000 feet, roughly.

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