Matt Erickson
11/6 Env. Stu.

Final Internship Report: Forest Blowdowns

Forest blowdowns occur naturally in forests all over the world. In the Tollgate area of the Umatilla National Forest, severe high winds during winter storms in 1989 and 2000 blew over several stands of trees seemingly located in a random, sporadic assortment around the area. My primary goal for this internship was to identify the factors, which contributed and caused these windfalls. Additionally, I wanted to discover if these windfalls were detrimental to the National Forest to begin with, since most natural blowdowns are environmentally profitable to the local habitat. I feel I accomplished these goals in that I identified several contributing factors as well as several evidence-supported theories about the environmental profitability of blowdowns.

In most cases, windfalls are helpful in reviving environmental habitat. For example, in Alaska, “small scale blowdowns perpetuate the uneven age structure of old growth forests.” (Carstensen, p.151) By stratifying the age and development of these tree stands, these small-scale windfalls perpetuate old growth. The tree stand as a whole will not live and die in the same time frame, rather, individual trees will grow at different ages or in successive generations. Windfalls also create large patches of sunlight, good for understory plants needing sunlight to thrive. This promotes good groundcover and vertical diversity. It also increases seedling density in the ecosystem (NISE project, p.2). Thus, windfall is a natural way of promoting good groundcover, vertical diversity and stratification of tree age and development. Yet in contrast to these “small scale blowdowns,” larger scale ones can cause loss of animal habitat, erosion and soil depletion due to topsoil being ripped out of the ground by fallen trees. Increased wind speed as a result of less standing trees available for wind retardation is another consequence.

I feel that in the specific case of the Umatilla National Forest, windfall is overall environmentally degrading solely because of man’s intervention in the area, which exacerbates the natural windfall damage. For example, it is possible that severe windfall in the UMF can be attributed to logging (previous clear cutting and present tree thinning) in and around the forest. As mentioned above, tree loss causes increased wind speed, leading to further windfall, causing the trees to progressively get thinned and the wind progressively stronger. In addition, this somewhat protected windfall area in the National Forest is home to various animals (ex-white tailed deer). Regionally, the National Forest is roughly the deer’s only habitat to begin with and is increasingly becoming an isolated “island” of habitat due to constant human expansion.

Even large preserves are likely to suffer disturbances large enough to lead to drastic reductions in species numbers… when the landscape was less altered by human impact, that was not likely…but now, with nature preserves more and more isolated from each other, [they become] islands of natural areas surrounded by seas of developed land (NISE project p.2).

Thus, depletion of species numbers as a result of windfall is exaggerated, since animals have fewer options for “backup” habitats to reside in when their original one has suffered from windfall. In addition, unlike the Alaska windfalls mentioned above, these blowdowns are not just several small, isolated spots or stands. They encompass tens if not hundreds of acres throughout the forest, creating large patches of animal habitat destruction. Thus, I feel that although windfall is a natural occurrence, usually beneficial to the environment, there is evidence that UNF blowdowns may be a result of man’s logging and increased development, making it an unnatural occurrence. Thus I feel that since these blowdowns can possibly be attributed to man’s interference with nature, it is a wise idea to be concerned with these windfalls’ possibly problematic effects

My primary goal of this internship was to determine the cause and workings of these windfalls. In my initial objectives, I chiefly wanted to learn the cause of these blowdowns, using the GIS system to overlay maps and find correlations between soil types, topography, and weather systems. I feel I accomplished this goal in that I developed several possible explanations for windfall and was able to support these ideas with substantial evidence using the GIS system.

After visiting several windfalls and conversing with Betsy and Earl Weaver, I feel that windfall is a result of several combinations of factors. Specific factors include soil type, soil moisture and depth, topography and elevation, and different tree species. First of all, every overturned tree (regardless of species) we observed had a similar and peculiar root structure. From examination, we found that each trees’ roots at both sites visited grew down about two feet into the soil, and then abruptly, almost perpendicular to the tree itself, grew outwards laterally, creating a wide but shallow root structure. According to Earl Weaver, this was due to the fact that the rich topsoil utilized by the trees was from one of the many historical Mt. Mazama volcano eruptions roughly 6,800 years ago, which left a thin layer of rich ash for trees to grow in. On average, this ash settled in the Blue Mountains at about a two-foot thick layer on top of a clay called Lacustrine sediment. Lacustrine sediment was created when earlier volcanic ash fell, came into contact with water, and hardened into clay called Tuff or Tuffacious sandstone. This clay is now what forms the hard barrier layer just underneath the rich volcanic topsoil, in many cases preventing tree roots from entering. Yet according to Earl, all over the Blue Mountains, Mazama’s ash was blown around by wind, which created varying depths in different areas. Even today, in layers of ash substantially less than two feet deep, growing trees are forced to produce roots, penetrating the hard Lacustrine clay, since the thin topsoil layer is not adequate to sustain trees itself. None of the downed trees we examined had any significant root structure penetrating into the clay. Thus we concluded that one factor contributing to windfall is the fact that downed coniferous trees are unable to root in the solid clay (probably due to growing in predominantly thicker ash layers) and are thus easily unearthed in the ash, which is 50% air space, providing even less compaction and stability. Our theory was further supported by the fact that this Lacustrine sediment, covered by Mazama ash, was the primary stratified soil type found in the Tollgate area blowdowns by the GIS system (Map #1).

I also reasoned that moist or muddy soil was a factor in windfalls. For example, in the second blowdown site we visited near Jubilee Lake, much of the wind-fallen stand visited was located on a relatively flat, plateau-like area with a shallow, muddy gully that resembled a swampy creek bed. Here many fallen trees, mostly spruces and firs, were located in relatively swampy soil, the water making the dirt much more soft and pliable. In addition, hardly any of the trees a few feet higher in elevation that surrounded the shallow creek bed were blown over.

In another example, the GIS system recorded annual precipitation (in inches) of the Tollgate area (Map #2). The map shows that the primary areas of large windfall occur in a region of relatively high rainfall for the area (50 to 55 inches/year). This further convinced us that increasingly moist soil resulted in more soil destabilization, leading to blowdowns.

A third factor seemingly leading to blowdowns is that of topography. It is a general fact that higher winds usually occur on the tops of ridge lines, which is why, for example, the electrical generating windmills near Wallula are all located on the tops of ridges in order to utilize the maximum wind speed and power in the area. In looking at the topographic GIS map #3, we see that the substantial primary windfall sites are all generally located on ridgelines or high elevation plateaus. Thus, areas of the most extreme wind speed on average in the Blues correlate well with the areas of the most windfall.

A fourth aspect of determining windfall can be seen in certain tree species. For example, Firs, Spruce, and Ponderosa pine were all susceptible to blowdowns, Spruce being the most fallen tree species. Yet, in the two sites visited, no Western Larch blowdowns were seen. In the case of the swamp-like site near Jubilee Lake, the entire stand of Firs and various other trees blew over, except for the few larch interspersed in the area. We concluded that this was a result of the larch’s relatively small branch and needle crown. In addition, all of the severe wind storms in the Blue Mountains in recent history have occurred in late fall and throughout the winter. The Larch is deciduous and has thus lost its needles by late October/early November. Needles and leaves greatly add to the sail effect in trees, in which branches and foliage act as sails or wind catchers, which cause the tree to be bent and even uprooted given a strong enough wind gust. Thus Firs, Spruces and Pines, with their fuller, year-round crowns are more susceptible to these winter storms. As a result, we concluded that different tree species are more susceptible to windfall than others.

In short, I think it is a good idea for future interns to see the blowdown sites a little earlier in the fall (I got lucky and had a beautiful day in the end of October), when the weather is sure to be cooperative. I would also recommend doing research before physically going to Tollgate as I did. This way, future interns will have a much more educated idea of what they are looking at, and will be able to observe and draw better conclusions from the physical evidence they encounter. In conclusion, I would recommend future interns examine how natural factors in geology, climatology, and topography can all combine to affect the stability of trees in the Umatilla National Forest. Interns should also examine how man’s increased deforestation and development has possibly amplified tree instability, due to growing wind speeds, which cause more severe windfall and habitat loss in the region. Thus, I would recommend this internship for those who are interested in both man’s and nature’s influence on forest and animal habitat around Walla Walla.


1. 1996. Carstensen, R. Southeast Alaska. In R. Kirk (Ed.), The Enduring Forests

2. A NISE project funded by NsF Clearcutting, Natures Way,

Key Contacts

1. Betsy –(509) 522-6056 (National Forest Service, Walla Walla)

2. National Weather Service, Pendleton – (541) 276-0103, 276-7832

3. Earle Rother – (541) 278-3734