1. COMPOSITION -- the array of plant and animal species present in an ecosystem (diversity in one sense of the word). Few species are tied exclusively to old growth, however for many old growth provides optimal habitat (say for nesting or foraging). For some groups, old-growth forests support higher diversity (more species) than younger forests (for example, tree species or epiphytes - plants that live on trees or other plants, but use them only for substrate - that is, aren't parasitic on them), such as lichens and mosses. The scientific advisory team under Jack Ward Thomas (part of President Clinton's "forest summit" process) estimated that 482 species were at risk from rates and patterns of forest harvesting that prevailed before the listing of the Northern potted Owl. That is, these species were tied to some degree to old growth forests (35 mammals, 38 birds, 21 reptiles/amphibians/ 149 invertebratess/150 plant)s
2. STRUCTURE -- refers to the spatial arrangement of various components of the ecosystem, including the spacing of trees, heights of various canopy levels, sizes (average and variance) of live and dead trees, abundance and size of coarse woody debris. These structural aspects of old growth are vitally important. In fact, the unique compositional and functional characteristics normally associated with old-growth forests are mainly related to the distinctive structural features associated with old forests. The structural features of old growth may be more influential on composition and function of these forests than is the actual age of stands.
3. FUNCTION -- refers to ecosystem properties, such as nutrient cycling, control over erosion and water movement through the system, productivity, and so forth.
Four structural components are considered to have overwhelming importance:
Each of these is discussed below.
Compositional roles: relate to what species live in them or depend on them in some way. These big old trees support distinctive communities of plants and animals living on their branches. The upper surfaces of large branches are often covered with an organic "soil" that is several cm thick (derived from fallen needles, etc) and that supports entire communities of epiphytes (plants that live on other plants - use them as substrate - without being parasitic or pathogenic), such as lichens, mosses, ferns, and, in the case of these old trees, even shrubs. Branches and cavities in the trunk provide nesting sites for mammals and birds.
Functional roles: relate largely to their influence on ecosystem functions (nutrient cycling, energy capture and flow, and so forth). These large old trees influence the flow of nutrients, carbon and water through the system. For example, some of the epiphytic lichens (such as Lobaria oregana) are nitrogen-fixers. They contain a cyanobacterium as one of their symbionts, with the cyanobacterium providing the nitrogen fixation. The amount of fixed nitrogen that these provide to the forest ecosystem is actually quite substantial. How does it get from the lichens to the forest? Through leaching, litter fall and decomposition. In old forests of the PNW, these lichens fix about 2.5 - 4.5 pounds of nitrogen per acre per year (some estimates are higher). By comparison, US farmers generally apply about 130 pounds of nitrogen per acre for corn production.
Epiphytes in these trees also remove soluble mineral elements from water flowing over them and trap dust and litter fragments. These (or nutrients derived from them) ultimately reach the forest floor, but often after first being processed in the canopy. The trees themselves are important, directly, in the nutrient dynamics of these forests; each tree represents a sink for organic material and nutrients in the short run, but a storehouse in the long run.
These trees are one of the primary sites for photosynthesis (production of the food base on which the rest of the system depends). Sometimes people think that these old trees are no longer growing (or accomplishing net photosynthesis), but this isn't true. They are still growing; just not very fast.
Finally, these big old trees also serve as the source for snags and down logs, discussed below.
These were the first dead component of natural forests that foresters paid attention to, mostly because they were perceived as constituting fire and safety hazards (they were called "widow makers" because of the dangers that they posed to loggers and others working in the woods). However, their roles in influencing the species composition of forests have recently been recognized. They have tremendous value to a variety of wildlife species, through their provision of habitat. Large standing dead trees (greater than 20 inches in diameter at breast height ["dbh"] and more than 65 feet tall) are particularly useful to wildlife. These are often referred to as "wildlife trees." Many current cutting prescriptions require leaving more than two wildlife trees per acre after harvest, and, if standing dead trees don't occur naturally on a site, they are sometimes created by blowing the tops out of living trees. The density (abundance) and diversity of hole-nesting birds is positively related to snag diameter in a forest; smaller snags do not provide suitable habitat for some animal species. Snags are the primary location for cavities that are used by at least 63 species of vertebrates (39 birds and 24 mammals). Snags are used for nesting and overwintering, food storage, and furnish sites for courtship rituals. Insects, some of which are valuable in controlling forest pests, also find homes in snags. Standing dead trees are needed in a variety of decay classes to meet all needs.
Under natural conditions, large snags are not strictly an attribute of old growth. In many cases, snags would be carried over after disturbances (or produced by them). For example, young growth forests that regenerate after natural disturbances such as wind storms or fires often contain snags, left over from the former stand. Conventional forestry did not leave snags, but that is changing, as foresters learn more about their important compositional and functional roles in forests (and as laws mandating that some be left come into force!).
The functional roles played by standing dead trees include the storage and gradual release of nutrients and carbon that occurs from them. This release is similar to that occuring from logs on the forest floor, but standing dead trees actually tend to decay more rapidly than down logs, so the releases from snags tend to occur more rapidly Snags also, in many cases, furnish coarse woody debris to the forest floor (or to streams) when they fall over or blow down.
These are a characteristic feature of old-growth forests in the PNW, as you know if you've ever tried to walk through such a forest without a trail to follow. They may cover as much as 30% of the forest floor (and sometimes it seems like more!)
Their compositional role related largely to the habitat they provide. They are essential for many invertebrates, vertebrates and fungi, furnishing lookout sites, sites for feeding and reproduction, protection and cover, sources and storage sites for food, and bedding. They maintain a high moisture content even by the end of a hot dry summer, so they provide useful habitat for amphibians (e.g., salamanders). They are also important after disturbances such as fire, and in the early stages of succession. The persistence of large logs through and after forest disturbance is important in providing continuity in the ecosystem, both in terms of habitat continuity and in terms of their functional roles (see below). Large logs on land may contribute to the reestablishment of animal populations after disturbance by providing pathways along which small mammals venture into clearcuts or other disturbed areas.
Logs also an important site for supporting the reproduction of some tree species, such as western hemlock or Sitka spruce. Perhaps you've seen these "nurse logs," with seedlings and saplings of these tree species growing on them? In some cases, you can tell mature trees originated on logs that have since decomposed - the trees will be growing in a row, often with roots that arch down and out from the tree trunk, reflecting that they grew over and in a nurse log in their early days. These tree species often regenerate best in rotten wood - their roots often can't readily reach through the duff (organic matter on the surface of the soil in forests) to the mineral soil, so they dry out, but since the logs stay moist all summer, seedlings and young trees can survive on them. Species such as western hemlock and spruce are important in these forests, as they are relatively shade tolerant and form the replacement trees as the older canopy trees (often Douglas-fir, which is not very shade tolerant) die and leave gaps.
Logs on the forest floor also are important for the reestablishment of tree seedlings on bared areas as well. The survival and growth of new trees depends on their being able to develop appropriate mycorrhizal associations. (Mycorrhiza = fungus root; these are fungi that live in a symbiotic association with plant roots, "helping" the plant take up nutrients and water "in exchange for" getting carbohydrates from the plant. Hyphae of the fungi spread through the soil in a mat, hence they can "reach" nutrients and water that bare roots do not necessarily reach. These fungi are saprophytes - that is, they "feed on" [decay] dead material.) Many of the species of fungi that form this relationship with plant roots have obligate relationships with trees (that is, cannot survive without them), and so disappear from cut over areas shortly after the trees are removed (especially if the site is scarified, compacted and burned post-harvest). (Hopefully I'll get the notes about that kind of "site preparation up by the end of this term, and then I'll have a link to those notes). In such cases, it can take time for the site to be reinoculated with the fungi either naturally or via humans. However - here is the role for the coarse woody debris -- the down logs can provide mycorrhizal habitat and reservoirs, so that the fungi can survive until tree seedlings are reestablished.
An Example of Intricate Interconnections Among Species:
In fact, this is just one reflection of an intricate web of interconnections among species that is typical of those found in old-growth systems. If I was fancy with this web site (and had more time) I'd draw a picture to illustrate the following, but that will have to wait….I would draw a cycle, showing interconnections among the following players, discussed below: fungi, trees, squirrels, nitrogen-fixing bacteria, and the Northern Spotted Owl.
A. The fungal connection. The trees need the fungi and the fungi need the trees (as just described). Further, nitrogen-fixing bacteria (Azospirillum species) occur inside the mycorrhizae and get food from the fungi and provide it and the tree with fixed nitrogen.
B. The fruiting body -squirrel connection. Many of the fungi that form mycorrhizae with trees produce hypogeous fruiting bodies - such as truffles. These, as the name "hypogeous" implies, are subterranean, so that their spores are not readily dispersed by the wind. Rather, they depend on small mammals that eat them to disperse them. They are an important part of the diet of several mammals, notably for this case, the northern flying squirrel which nests in the trees but comes down at night and forages for fungi (it finds them because they are - to a squirrel - good and smelly). In mild climates, these fruiting bodies may be available year round, but in forests of the Cascades, squirrels may switch to eating lichens in winter. When the squirrels eat the fungi, of course, they also eat the nitrogen-fixing bacteria that live with them.
C. The squirrel pellet connection. Th squirrels' fecal pellets then contain spores of the fungi (which are still viable) and nitrogen- fixing bacteria. As the squirrels scamper about the forest, they scatter their fecal pellets, which serve as a "pill" of symbionts, dispersed in the forest as the squirrel moves around. Thus, they reinoculate down logs, disturbed patches of soil, and so forth, with these symbionts.
D. The spotted owl connection. Squirrels are important components of the diet of the spotted owl, which are characteristic members of the old-growth forest community.
Systems like this interconnect the forest -- live trees provide nesting sites for the squirrels and owls, are interactive with the fungi, are habitat for lichens, and are source material for the down logs on land. In turn, the trees depend on these components
This example points out that it is impossible, in many cases, to make a complete separation between compositional and functional roles played by these structural features. For example, the abundance of logs influences the abundance of mycorrhizal fungi (a compositional role), and these mycorrhizal fungi are important decomposers and important in nutrient cycles, thus the logs contribute to that function.
Coarse woody debris has other functional roles as well. It serves to bridge disturbances, by providing a continuing source of nutrients and energy to the system. These logs, for example, hold major pools of nutrients, such as nitrogen and phosphorus. In some systems, there is approximately 192 pounds of nitrogen per acre stored in these logs - competitive with the amount applied yearly in US corn production. Of course, it isn't released all at once; the logs disappear slowly, thus the nutrients that they contain are released gradually. Models of decomposition suggest that it takes 480 - 580 years for a 30" (diameter) log to become 90% decayed! Thus they have an important nutrient cycling function: they are sinks or storage compartments for energy and nutrients as well as sources of these over the long term. Over the short term, one could consider nutrients and carbon as being "tied up" in these logs, but over the long term, they are gradually released. Thus, they serve as a long term bridge between the old forest and the regenerating forest. They are also sites in which substantial bacterial nitrogen fixation is believed to take place. Coarse woody debris also provides surface stability, protecting the soils from erosive forces.
Thus, in an old-growth forest, carbon and nutrient cycling is a tight, detritus-based system. Detritus decomposes slowly by a long succession of decomposers, there is slow release of energy and nutrients, and plants rapidly take up mineralized nutrients. The system is thus highly conservative of nutrients; very retentive. This is evidences by the very low levels of nutrients in waters that drain these forests.
Logs enter streams by blowdown, landslides, avalanches, and so forth (and , of course, trees growing along streams eventually die and may topple over into the stream). Logs in streams have important influences on the species composition of streams, as they increase the variety of habitats that are available in the stream, compared to that found in even-gradient channels. Pools form behind the logs, and the logs create riffles; different species find optimum habitat in each of these sets of conditions. Further, the wood itself serves as habitat for a variety of organisms.
These logs in streams also play important functional roles. They enhance the stability of stream banks and of stream beds. In small streams, debris dams dissipate much of the energy of the water in little waterfalls. Thus, considerable energy is dissipated in a small proportion of the stream's length, so less energy is available for erosion of the stream bed and banks.
The major input of nutrients and energy into a stream ecosystem is organic litter from the adjacent forest -- needles, buds, twigs, etc. A stream system retains some of these materials, which then furnish energy and nutrients for organisms that live in the streams. The fraction of this material that remains in the stream (and furnished energy and nutrients) depends, in part, on the rate of water movement through the system. Dams created by woody debris act as sieves and deposit zones are created in the pools behind the dams. Thus, logs, by creating pools of relatively slow-flowing water, slow the routing of organic material through system, allowing more time for it to be processed (decomposed) in the stream (and thus retaining food for the stream inhabitants for longer than it would otherwise be retained).
ALL OF THESE FEATURES:
(1) Provide a legacy from the previous stand; that is carryover in terms of species composition for the creatures that depend on these features, for nutrients and for carbon (energy).
and
(2) Provide soil stability and tight nutrient cycling and retention
IN NATURAL (UNMANAGED) STANDS, THESE STRUCTURAL FEATURES AREN'T NECESSARILY ASSOCIATED UNIQUELY WITH OLD GROWTH
For example, remnant trees are often left after fire or other kinds of natural disturbances. In the next section of notes (or in lecture, if I don't have time to bring the notes up on this site by the end of the term), we'll talk about ways that forest management can change (is changing), fostering these features even in young managed stands.
To move to a discussion of current changes in forest management in the Pacific Northwest, click PNW. OR, click the Contents box at the bottom to return to the master table of contents for this BI 301 web site, "deforestation" to return to the list of topics in this particular section, or "navigate" for reminders on how to move about within and among these pages.
Page maintained by Patricia S. Muir. Last updated November 19, 2002.