Now that we've looked at the traditional methods of logging and site preparation in forests of the Pacific Northwest (PNW), let's look at some of the consequences of these methods:
Consequences of slash disposal:
The removal of organic material when slash (often with duff and top soil) is piled and burned has important consequences for post-harvest site fertility. As we saw when we were talking about agricultural issues, soil organic material is important for nutrient retention on the site (through its charged sites that "hang onto" nutrient cations such as calcium and magnesium and because it improves the water holding capacity of the soil, lessening runoff of water [and soil!]). It is also important in that it tends to keep the soil well-aerated; this along with the improved water holding capacity makes the soil more suitable for soil-inhabiting creatures that are decomposers (freeing nutrients), for fungi (such as the mycorrhizae discussed previously) and for the regenerating trees. This organic matter would otherwise, of course, furnish a source of slow release fertilizers as it decomposed.
Many of the nutrients that were in the slash, duff, and top soil do get returned to the site after burning, of course. However, they will not be scattered all over the site but, rather, concentrated in piles. Further, temperatures can get very hot in the burn piles, so much of the nitrogen that was in the material is volatilized (given off as a gas).
The equipment used to pile the slash also, of course, increases soil disturbance and compaction. Thus, there can be increased erosion problems after slash piling, with consequent loss of site productivity via erosion and compaction of the soil.
Broadcast burning, while avoiding some of these problems, can be dangerous (if the fire isn't contained), and can also sterilize the top layers of the soil if temperatures are high enough. Soil is a good insulator though, so seeds, roots, spores, and organisms that are under the top couple of inches of soil often survive.
Both methods reduce ground cover by litter, herbs, grasses, and shrubs, so increase vulnerability of the site to erosion by water and wind (as we saw in agroecosystems). Losses of water from a logged site often increase greatly in the first few years after harvest, in part because vegetation that would otherwise take up much of the water is so reduced. Vegetation often transpires at least a third of the yearly precipitation input (less in the winter; more during the active growing season). Increased runoff causes:
1. increased erosion
2. increased flooding downstream (this is partly why Thailand
has declared a moratorium on forest cutting!)
3. increased nutrient losses (with soil)
4. effects on the recipient aquatic systems
5. decreased water availability for regenerating trees and other
vegetation.
Note also that site preparation also often includes treatment of the site with herbicides, to decrease competition with the desired conifer species. This, of course, results in less ground cover on the site, so can aggravate soil and water losses; without herbicide treatment, the vegetation on the site acts like a sponge, taking up water and nutrients as they become available, and then slowly releasing the nutrients as they die and decompose. Herbicide treatment basically increases the time required for the biota to reestablish control over water and nutrient dynamics. (You may have observed muddy creeks below recently cut over areas? How many muddy creeks have you seen below undisturbed natural forests? This contrast offers an obvious, visible expression of the disruption of soil, water, and nutrient dynamics that is associated with conventional logging practices - a disruption that often lasts for several (e.g., at least 4 - 10) years.
Both methods opf slash disposal (piling and burning and broadcast burning) release nutrients much faster than they would be released if slash were left to decompose gradually. This rapid release, of course, furnishes the regenerating trees with a quick pulse of nutrients, but basically, more nutrients are often available than can be taken up by the little trees. The consequence is nutrient loss with runoff and soil erosion.
These soil and nutrient losses are particularly problematic on steep slopes - and are associated with poor forestry practices, such as excessive disturbance of the forest floor with skidding and yarding of the logs. (I'm sure that many of you have seen sites that meet this description, if you've kept your eyes open while passing through the Coast or Cascades Ranges of Oregon - or elsewhere in the PNW.) Skidding and yarding disturb the forest floor physically, making it more vulnerable to losses, particularly when the skid lines run up and down the slope.
Decomposition rates often increase following logging, because the soil tends to be warmer (since the shading vegetation is removed) and because there is an influx of organic material and increased availability of water and inorganic nutrients (since there aren't many plants there to use these soon after logging). Soil disturbance associated with logging can also increase rates of microbial activity. Since, however, there is less vegetation there to take up the nutrients released by this decomposition, and since erosion and runoff are often increased after logging, many of the nutrients released by this decomposition are "lost" from the system.
Roads, built to access and remove timber, are another cause of increased rates of erosion associated with logging. In fact, excessive and poorly designed roads are often the leading cause of accelerated erosion following logging. The road systems built in support of timber harvesting are much more extensive than most people realize. In Washington, each square mile of commercial forest is supported by about 5 miles of roads. In the US national forests overall, there are over 570,000 km of logging roads -- a network that is eight times the size of the US interstate highway system. These roads in national forests cover about 1.4 mill hectares (over 3 million acres); this is acres of roads themselves! In the PNW, 8% of the area cleared as part of logging operations is cleared for the construction of roads. Particularly when roads are constructed on steep grades, or are not supplied with culverts so that water flows along the road instead of under it, erosion can be extreme. The compacted surface of a road is ideal for water (and associated soil) movement.
The consequences of increased rates of soil erosion are probably quite obvious, but just in case they aren't, there are consequences for loss of nutrients from the site and for loss of physical structure and amount of soil, which must be maintained if we expect reasonable tree regeneration to occur. Accelerated rates of soil erosion also have effects on aquatic systems. They are associated with increased siltation, which covers gravels that salmon need to spawn in and decreases the diffusion of oxygen into the gravels.
Both methods of slash disposal (piling and burning and broadcase burning) also release CO 2 more rapidly than it would be released by gradual decomposition. This release of CO 2 has implications for global climate change, as we've seen previously.
Nutrient consequences of removing timber itself:
There are also, of course, direct consequences of timber harvest for the nutrient status of the site; nutrients stored in the woody biomass are removed. The fraction of site nutrients that is removed this way depends on how much of the above ground material is removed and also depends on soil fertility -- how rich are soils in nutrients compared to the biomass removed? In turn, soil fertility depends on the nature of the parent material and on how long it was since the site was last logged -- the "rotation period." The rotation period influences how long a time there has been for decomposition and abiotic inputs to restore nutrients to soils. The rotation period also influences how much time has there been for nutrients to be sequestered in the wood. It is best to plan for rotation lengths that are at least as long as the "ecological rotation length;" the period over which harvested nutrients are replenished. This would, of course, increase the sustainability of harvesting.
METHODS OF STUDYING NUTRIENT, SOIL, AND WATER CHANGES THAT RESULT FROM LOGGING AND SITE PREPARATION:
If we want to figure out how important the nutrient, water, and soil losses are to the the site -- and thus how sustainable logging is likely to be - we can use simulation models in which nutrient (or water or soil) compartment sizes and the fluxes between these compartments are modeled (based on sampling of the compartments and measures or estimates of the fluxes), as we talked about at the very beginning of the term, when we talked about ecosystem models.
EXPERIMENTAL FORESTS:
An alternative approach that has been used at a few sites, is to actually measure and monitor the compartments and fluxes over time in the real world, under various management scenarios. One of the most famous of these large scale experimental studies has taken place at the Hubbard Brook Ecosystem in New Hampshire - see the Likens et al. reference on your supplementary reading list for more information on the site and how the experiments were carried out.
The Hubbard Brook Ecosystem is in the lower elevations of the mountains in New Hampshire. Watersheds within the ecosystem constitute units of study . (A watershed is all of the land that drains into a particular creek or river; as in a mountain drainage basin.) The Hubbard Brook system contains several small watersheds with clearly defined boundaries (ridges) between them. Each is underlain by an impermeable rock substrate so nothing can leave the system by leaching to groundwater; all exports (except minor losses by wind, animals, seeds, etc) leave via the stream that drains it. Inputs into the ecosystem come through atmospheric deposition (bulk; that is, wet and dry) and a minor weathering input from bedrock. These features make it possible for scientists to measure essentially all of the important inputs and outputs from each watershed. This makes it an ideal situation for contrasting nutrient and water losses between undisturbed watersheds and logged ones; researchers can log a watershed and study the subsequent nutrient dynamics. What they've found at Hubbard Brook, in very general terms, is that losses of nutrients to streams increase relative to losses from the undisturbed reference areas in the first few years following the disturbance. These losses stay at several times the "normal" for 4 - 10 years after logging, depending on the particular watershed and the logging and site preparation practices used. In general, the losses are fairly small in comparison to the total nutrient budget of the site, so are unlikely to significantly influence site productivity unless (1) the harvest is very intensive with a high proportion of the watershed disturbed and lack of regeneration or (2) rotations are very short.
You may have heard of the H. J. Andrews Experimental Forest near us? Indeed, there is an experimental forest in Oregon not far from us where the functioning of forest ecosystems has been studied since 1948. It is located about 50 km east of Eugene in the Blue River district of the Willamette National Forest. It represents the region's coniferous forests and associated wildlife and stream systems, and does contain some old-growth forest. It is funded partly by the National Science Foundation's "LTER" (Long Term Ecological Research Program). The program recognizes the need for long-term data (decades to centuries) on the structure and functioning of ecosystems, which is very important. People have trouble doing long term studies because the funding for research is often relatively short term. The LTER sites are given a relatively secure funding base (subject to congressional whim and evaluation, based on the research output of the site) to carry out long term investigations on ecosystems and effects of various manipulations on the ecosystem compartments and processes.
At the H.J. Andrews forest, the focus of research has changed over the decades. In the 1950's, research centered on systems for constructing roads and harvesting old-growth forests. In the 1960's, research began to focus on effects of logging on sediment, water, and nutrient losses from small watersheds. As at Hubbard Brook, described above, one watershed was harvested using conventional methods (conventional for the times), one was harvested using smaller patch cuts, and another was left undisturbed, as a control. (Watershed 1 was clearcut in 1962-66 and broadcast burned in 1967; watershed 3 had roads constructed in 1959 (6% of its area) and was clearcut in three patches (25%) in 1963; while watershed 2 was left undisturbed as a control.) Scientists have been studying vegetation changes and water, sediment and nutrient yield since then periodically on permanently marked plots. During the 1970's, the focus shifted somewhat to basic (versus applied) studies on how the intact systems worked, particularly in old-growth forests. During the 1980's and 1990's, basic studies on intact forests and differences from managed forests continued. One motivation for the work has been that, if we can learn what sustains the productivity of intact systems, we might be able to apply that understanding to our managed stands to increase their sustainability for timber production and for other ecosystem attributes.
LONGER TERM CONSEQUENCES OF FOREST HARVESTING:
Many of the disruptions associated with logging and site preparation are fairly temporary. For example, as we just saw, the most dramatic changes in nutrient retentiveness and soil erosion usually last for a few years only. Not so temporary, however, are effects of these practices on forest structure and habitat fragmentation. These changes tend to be much longer term, yet, as we'll see later, can be decreased through the changes in forest management that are now taking place.
Typical forestry practices also have effects on the species diversity of forest ecosystems; including the trees. In the early to mid-1900's, much logged land was left to regenerate naturally -- seed blew in from nearby, or loggers left seed trees to supply seed for regeneration. However, there were some real failures -- especially on sites that had suffered extensive soil disturbance and erosion. Partly as a consequence of this past reliance on natural regeneration (and its sometimes failures), managers began increasingly to replant logged sites. There was a tendency to plant "monocultures" - for example, in our region, all Douglas- fir. Further, the tendency was to plant seedlings that originated from a restricted number of seed trees; those with good form or that were fast-growing. Thus, the seedlings were probably more uniform genetically than those that would have resulted from natural regeneration.
What might be some dangers in this approach - planting monocultures of relatively homogeneous plants? As we saw in agriculture, genetic homogeneity increases dangers from pests and diseases. Further, forests dominated by one species of tree lack the genetic diversity to cope with the variable weather that we may increasingly experience with climate change. "Monocultures" also have implications for other aspects of the ecosystem. Lowered diversity of the overstory will result in lower diversity of other life forms. Some birds, mammals, decomposers, etc. are fairly specialized for certain tree species - and may not be able to survive where these species are lacking. The general lack of stability associated with monocultures also could affect the associated ecosystem. For example, if all overstory trees are killed by a particular disease, repercussions will run throughout the ecosystem.
When sites are replanted instead of relying on natural regeneration, more even-aged stands generally result. With natural regeneration, bare sites are slowly colonized over time, whereas with planting, available sites are mostly occupied at once. This difference between planted stands and those that result from natural regeneration has implications for structural and age class diversity in the forest. We saw previously how important structural (and compositional) diversity can be for many aspects of forest ecosystems; this diversity is nearly lost in many planted stands.
DO CLEARCUTTING AND SLASH BURNING DIFFER FROM NATURAL DISTURBANCES, SUCH AS FIRE?
You often hear the argument that clear cutting is an ecologically-sound practice, because it is "like a wild fire;" that is, that it is a "natural way" to regenerate forests. However, this claim overlooks some very important distinctions between clear cutting and associated site preparation versus forest fires!