Ecology of coarse woody debris in temperate ecosystems

Woody debris is an important, but often neglected component of manyterrestrial and aquatic ecosystems. Coarse woody debris (CWD), primar-ily in the form of standing dead trees and downed holes and large branches, is abundant in many natural forest and stream ecosystems,forming major structural features with many crucial ecological functions—as habitat for organisms, in energy flow and nutrient cycling, andby influencing soil and sediment transport and storage.
CWD includes a wide variety of types and sizes of materials. Types ofCWD include snags, logs, chunks of wood (which result from disintegra-tion of larger snags and logs), large branches, and coarse roots. The sizeused to define CWD has varied widely among studies, making exact com-parisons difficult. Typical minimum diameters are 7.5-15 cm in western North American studies and 2.5-7.5 cm elsewhere. Some ecologists (e.g.,Christensen, 1977) make no distinction between coarse and fine woodydebris. For the purposes of this article, we consider CWD as being anywoody material >2.5 cm in diameter, although many of the studies thatwe review consider only larger material.
A brief review of the roles of CWD highlights its importance. CWD ishabitat for many species, including autotrophs and heterotrophs. "Nurselogs" are a widely recognized example of logs acting as habitat for auto-trophs (Kirk, 1966). Indeed, in some environments, such as the rain for-ests of the Pacific Northwest, logs are the major site of tree seedlingestablishment (McKee et al., 1982). Snags (also called standing deadtrees) are used by many animal taxa, particularly birds; snags are ofemerging concern in wildlife research and management (e.g., Davis et al.,1983). In contrast, the use of logs by forest vertebrates has not been as widely appreciated (e.g., Maser et al., 1979), nor has the dependence ofsome vertebrates, such as salamanders, on CWD (Maser and Trappe,1984). Invertebrate use of and dependence on both snags and logs is well known to entomologists. Finally, many decomposer bacteria and fungiutilize CWD as an energy and nutrient source as well as a habitat(Frankland et al., 1982; Swift, 1977a).
The importance of CWD in energy flow and nutrient cycles of ecosys-tems has not always been appreciated by ecologists. Low in nutrientconcentration and slow to decompose, these materials are ignored in many ecological studies despite the large amounts of organic matter rep-resented. Although CWD may be a nutrient sink in the short run, thesematerials can be a major long-term source of both energy and nutrients inmany ecosystems (Larsen et al., 1978; McFee and Stone, 1966; Triskaand Cromack, 1980; Triska et al., 1984). Furthermore, although intrinsi-cally poor in nitrogen (Merrill and Cowling, 1966), nitrogen fixation inCWD is an important source of this limiting clement in both terrestrial andaquatic ecosystems (Cornaby and Waide, 1973; Sharp and Milbank,1973).
In stream and river systems, CWD has a major influence on geomorphic processes (Swanson et al., 1982a.b). Woody structures arc critical inregulating sediment transport and storage. Debris accumulations in smalland moderate-sized streams dissipate energy and store organic and inor-ganic sediment. In large streams and rivers, CWD provides a diverse array of habitats that significantly influence biological productivity (e.g.,Franklin et a!., 1982; Sedell and Frogatt, 1984; Sedell et al., 1982).
CWD is biologically important to the freshwater and estuarine ecosys-tems in which it occurs. In forested streams, CWD serves as an energyand nutrient source, a site for nitrogen fixation, and habitat for organisms(Triska et al., 1982). CWD influences the physical structure of foreststreams, creating a variety of habitats—debris dams, plunge pools, andgravel and sandbars; as much as 50% of the habitat in small forestedstreams of the Pacific Northwest may be provided or controlled by CWD(Swanson and Lienkaemper, 1978). CWD is instrumental in retaining or-ganic material, particularly forest litter, in small streams until it can be utilized by aquatic organisms (Bilby and Likens, 1980; Cummins, 1979).
Despite the role of CWD as habitat, an influence on geomorphic pro-cesses, and an integral component of energy flow and nutrient cycles, thismaterial has often been ignored. Some of this "neglect" may have arisen because of the many problems associated with the study of CWD; it varies widely in space and time, which creates sampling difficulties, and ismassive, making manipulative experiments difficult. The rate of CWD production is difficult to measure, requiring long periods of observation over large areas. Decomposition is slow and also requires long periods of time for investigation.
This article partially parallels the structure of a forest-ecosystem compartment model (Fig. 1). We emphasize CWD in temperate forest and stream ecosystems and include only selected references on CWD in tropi-cal and boreal ecosystems for comparative purposes. In Section II, therate at which CWD is added to ecosystems via tree mortality and break-age of stems and branches, as well as by physical transport from adjacentecosystems, is considered. As soon as CWD is created, biological andphysical processes begin to degrade it. Section III examines the processesinvolved in decay and decomposition rates of CWD. Of these, respirationand fragmentation have received most study, whereas leaching, burial,and physical transport have received least. In Section IV, we examinehow rates of tree death and decomposition as well as disturbances toforests control the amounts of CWD in temperate ecosystems. The func-tional importance of CWD depends not only on the amount of CWD, butalso on its distribution in terms of size, spatial arrangement, degree of decay, species, and position (i.e., snags versus logs). In Section V, wefocus on the influence that CWD exerts on organisms and processeswithin ecosystems, as a plant, vertebrate, and invertebrate habitat, aswell as its role in nutrient cycles and in controlling geomorphic processes.The ecological behavior and significance of CWD varies markedly be-tween aquatic and terrestrial environments and between managed and"natural" lands. The article concludes with a comparison of these environments.