Woody material

7.7.1 Quantities of dead wood

Dead wood in a forest is much more variable in time and space than leaf litter and consequently has been the Cinderella of the forest, largely ignored for a long time but now seen to be crucially important. Dead wood appears in many forms, sizes and positions (and so is difficult to measure) including standing dead trees (snags), dead branches in the canopy, and trunks and branches on the ground. It should also include the dead heartwood of living trees but, as George Peterken (1996) points out, this is usually ignored. A useful term for this motley collection is coarse woody debris (CWD) defined as all dead wood over a minimum diameter, typically 2.5 cm although in some studies 7.5 cm is used; anything smaller than this minimum is included in the litter. Coarse woody debris thus includes everything from the largest fallen tree to small branches held in the canopy.

As well as being variable in size and location, CWD is also produced much more irregularly over time than litter. The timing of litter production tends to be fairly regular and predictable (with some exceptions - see Section 7.4), leaves, twigs and reproductive parts being shed in a process largely controlled by the vegetation, overlain by a less predictable amount caused by disease and the like. The production of large pieces of dead wood, on the other hand, is mostly controlled by factors outside the tree and tends to occur more irregularly over the years. In some cases this is fairly predictable over time. For example, in a young stand severe competition and suppression of growth will lead to partial or complete death of weaker individuals at certain points in the stand's history. Successional changes, such as when oaks take over from pioneer birch, will similarly produce a burst of dead wood. Other factors that create dead wood, such as disease, high winds, ice, snow and fire, are less predictable in occurrence. Some of these events produce very large quantities of dead wood such as the New England hurricane of 1938 (Fig. 7.7), the severe windstorm on 12 October 1962 which blew down an estimated 26 million cubic metres of timber in north-western North America and the high winds of 16 October 1987 in southern England which blew over 15 million trees. In the last case, high wind speeds just short of hurricane force were aided by wet soils and a late autumn (trees still had a large 'sail area' of leaves to catch the wind). Despite the external forces governing dead wood production, in some forests there can be a tendency towards seasonality of tree fall due to predictable high winds or the wet season; for example, in the tropical forests of Panama, tree fall is maximal in August through to September. Forest fires are particularly good at producing standing dead wood (snags). Cline et al. (1980) counted 815 snags per hectare after fire in the moist coniferous forest of western Oregon.

Despite the problems of measuring the amount of CWD, and its great variability in timing, estimates of the yearly production of CWD have been made: 0.12-301 ha-1 y-1 in temperate coniferous forests, up to 16tha-1 y-1 in deciduous forests and 5tha-1 y-1 in tropical rain forests of Costa Rica (see Harmon et al., 1986 for more detail). Generally, more CWD is produced in a year in coniferous forests than deciduous forests.

Typically, dead wood in a forest forms up to a quarter of all the above-ground plant biomass. Total accumulations of CWD are typically in the range of 11-501 ha-1 in deciduous forests. Conifer forests commonly hold more CWD than deciduous forests, from 10 to more than 6001 ha-1 although intensively managed forests may have almost none; something around 1001 ha-1 is typical. Tropical forests, with more rapid decomposition, usually have lower amounts of woody accumulation but levels up to 1001 ha-1 are possible in more waterlogged areas of the Amazonian forest. The highest amount of CWD on record appears to be in a 63-year-old messmate eucalypt Eucalyptus obliqua forest in Tasmania regenerating after a fire containing 10891 ha-1 of CWD > 1 cm diameter (Woldendorp and Keenan, 2005). The largest accumulations of CWD in any forest are usually in streams and rivers where waterlogging slows decomposition. In deciduous forests at least, there is a suggestion that accumulations of CWD are higher in cooler regions (Muller and Liu, 1991) with accumulations in eastern North America of 22-321 ha-1 in warmer old-growth forests of Kentucky and 34-491 ha-1 in cooler forests of New Hampshire. This would seem to be due more to slower decomposition rather than a greater annual input of dead wood. If 1001 ha-1 of wood was

Long Island Ecology CenterWind Speeds Deciduous Forests

Figure 7.7 Pictures of the 1938 hurricane at Harvard Forest. On 21 September the 'Great hurricane of 1938' cut a 150-km wide strip through New England, starting at the Long Island coast and moving up through central Connecticut and Massachusetts to north-west Vermont, maintaining a wind speed of 220 km per hour (140 mph) with gusts up to 300 km per hour (186 mph). Along the way it destroyed 8900 homes and damaged another 15000, making 63 000 people homeless and killing 600. Estimations of between

Figure 7.7 Pictures of the 1938 hurricane at Harvard Forest. On 21 September the 'Great hurricane of 1938' cut a 150-km wide strip through New England, starting at the Long Island coast and moving up through central Connecticut and Massachusetts to north-west Vermont, maintaining a wind speed of 220 km per hour (140 mph) with gusts up to 300 km per hour (186 mph). Along the way it destroyed 8900 homes and damaged another 15000, making 63 000 people homeless and killing 600. Estimations of between spread evenly over the forest floor it would amount to 10 kg in each square metre, but the bulk of the wood is in large pieces so the floor is not so cluttered. Typically, less than 5% of the ground will be covered by woody debris although this can rise to around a third cover in very heavily loaded coniferous forests.

Species composition of the CWD does not necessarily reflect the make-up of the canopy. Muller (2003) found in a mixed hardwood forest in the Appalachians of Kentucky that sugar maple Acer saccharum, yellow poplar/ tulip tree Liriodendron tulipifera and black gum Nyssa sylvatica were 50% less abundant than their proportion of living biomass would suggest, and white oak Quercus alba, red oak Q. rubra and black locust/false acacia Robinia pseudoacacia were 50% greater. This is partly explained by ease of decomposition: the first group are rapidly decomposed. But since large woody pieces take so long to decay, the CWD tends also to reflect the past status of the forest and longer-term dynamics.

Snags (standing dead trunks, otherwise known as hulks - Fig. 7.8) are of particular wildlife interest. Up to 25% of dead wood in most forests is standing, sometimes more (up to 40%) but often much less even in old stands that have had time to accumulate dead trees. In the Bialowieza Forest of Poland, one of the most pristine forests in Europe, Bobiec (2002) observed that standing dead wood varied from 3-21% of total CWD. Cline and colleagues (1980) looked at 30 conifer forest stands from 5 to 440 years old in the mountains of Oregon and found the densities of snags over 9 cm diameter decreased with stand age (from 35 to 18 per hectare in stands 120 and 200 years old, respectively) but the mean diameter of snags increased from 13 to 72 cm over stand ages of 35-200 years; larger snags survive longer. This is underlined in Fig. 7.9 which shows the progressive deterioration of a Douglas fir snag. Not all species collapse in such a graceful manner: western red cedar Thuja plicata in the same forests forms a bark-free grey 'buckskin' snag which remains essentially entire until the base is rotted through and it falls in one go 75-125 years after death. In the mixed spruce and fir forests of New Brunswick, Canada,

Caption for Figure 7.7 (cont.)

275 million and 2 billion trees uprooted or damaged have been made, accounting for more than 4 billion board feet of timber (6.5 million cubic metres). The first picture shows the damage in 1938 at Harvard Forest, central Massachusetts and the second shows some of the glut of timber being stored in water in Tom Swamp, Harvard Forest (March 1939) to reduce rotting until it could be used. (Photographs courtesy of Harvard Forest Archives, Harvard University.)

Caption For Rainfotrest

Figure 7.8 Dead 'snags'. The first is a common beech Fagus sylvatica snag (= hulk) in the virgin forest (urwald) of Kyjov, Slovakia and the second shows that fallen trees may not initially be on the ground (taken 1924, Harvard Forest, central Massachusetts). (First photograph by John R. Packham, the second courtesy of Harvard Forest Archives, Harvard University.)

Figure 7.8 Dead 'snags'. The first is a common beech Fagus sylvatica snag (= hulk) in the virgin forest (urwald) of Kyjov, Slovakia and the second shows that fallen trees may not initially be on the ground (taken 1924, Harvard Forest, central Massachusetts). (First photograph by John R. Packham, the second courtesy of Harvard Forest Archives, Harvard University.)

Ww2 German Rockets
Approximate years dead

Figure 7.9 Stages of deterioration of large Douglas fir Pseudotsuga menziesii snags in the Coastal Range mountains of western Oregon. Stage 1 (0-6 years): much activity from wood-boring beetles and hence woodpeckers, some wood decay starting. Stage 2 (7-18 years): fine branches gone and big limbs and top breaking off; larvae of large beetles (e.g. long-horned beetles) penetrating deeply into the wood and increasing decay encouraging hole-nesting birds. Stage 3 (19-50 years): Sapwood extensively decayed and breaking off, heartwood extensively burrowed and decaying. Stage 4 (51-125 years): Sapwood gone and no sound heartwood remaining; vegetation begins to colonize the snag and the mound of broken wood accumulating at the base. Stage 5 (126 years and older): very rotten short trunk remains but stabilized by roots of invading shrubs and trees. Note this time sequence applies to large snags over 50 cm diameter; smaller snags progress through the stages more rapidly: snags 19-47 cm diameter are likely to reach stage 5 in around 60 years, snags 9-18 cm will reach stage 4 in less than 20 years and will fall before reaching stage 5. (From Cline et al. 1980. Journal of Wildlife Management 44.)

50% of dead trees go through this gradual standing deterioration with progressive loss in height while 23% are windthrown intact (Sarah Taylor, pers. comm.).

7.7.2 The role of coarse woody debris in forests

The vital importance of dead wood in forests has been increasingly appreciated over the last decade (e.g. Kirby and Drake, 1993). It is important in carbon budgets of forests and also as an invaluable wildlife resource, especially in streams and rivers where CWD has a major influence in regulating sediment transport and storage. In large rivers, CWD provides a diverse array of habitats and in heavily wooded areas such as the Pacific Northwest is particularly important. Interest in this material is of long standing: some of the most interesting early work on CWD longevity was done in Sweden in the 1930s by Sernander (Box 7.3).

Coarse woody debris is a crucial habitat for terrestrial invertebrates such as bark beetles, wood-boring beetles, termites and carpenter ants, many of which are specialist saproxylic insects living on dead wood. They use it for food, hibernation, shelter and breeding (see Peterken, 1996). Bouget and Duelli (2004), reviewing the effects of windthrow on insects, highlight that the 'dead-wood islands' created are regional hotspots of insect biodiversity. Moreover, 20% of Swedish red-listed beetles are favoured by these extensive windthrows, and only 4% are judged to be harmed (Berg et al., 1994). Large snags, particularly when over 50-60 cm diameter (Mannan et al., 1980), are readily used by birds for feeding and nesting. One example of the many that could be chosen illustrates this: Cline et al. (1980) found that 20% of all bird species in the conifer forests of Oregon featured in Fig. 7.9 depend on snags. In Great Britain, around the same percentage of birds, including the willow tit and the spotted woodpeckers, depend upon dead wood.

Rotting wood is favoured by a number of plants. Mosses and liverworts (see Box 7.4) do particularly well because it is a firm substratum to grow on, holds water and is above the worst of the field layer and litter accumulation which would otherwise cover them up. Fallen nurse logs are renowned as nurseries for tree seedlings, especially in moist forests (such as the Pacific Northwest and the boreal forest) for similar reasons (see Fig. 1.7). Harmon et al. (1986) gives a good review of this phenomenon and notes that shade-tolerant species such as western hemlock and sitka spruce are particularly common on nurse logs. It is not just trees that benefit; in the moist forests of Oregon, red huckleberry Vaccinium parvifolium and salal Gaultheria shallon are commonest in, and dominate, the vegetation growing on broken stumps.

Box 7.3 Change and decay: necrotization of spruce logs and tree regeneration in Fiby urskog, Sweden

Compared with such trees as Scots pine, birch or aspen, Norway spruce Picea abies shows remarkably little resistance to wind, and in the boreo-nemoral (open boreal) primitive forest of Fiby urskog it is frequently blown down leaving storm gaps which facilitate tree regeneration. Sernander (1936) allocated fallen trunks to six necrotization classes, and used their condition in attempts to estimate the dates at which particular storm gaps were formed. Logs which were not penetrated when an iron spike was thrust at them fell into one of the first three categories. Logs in class 1 still had needles on the branches. Bark had begun to fall off logs in class 2, but no epiphytic bryophytes were present. These bryophytes first developed on class 3 logs, from which all bark has fallen. Logs which were penetrated to a depth of 4-6 cm belonged to one of classes 4-6. In logs belonging to class 4 the side branches had collapsed so the log was in contact with the ground; there was also a considerable cover of epiphytic bryophytes, mainly of species not common on the forest floor. Logs in class 5 had begun to disintegrate while those in class 6 were rotten right through and totally covered in bryophytes, particularly large species characteristic of the forest floor such as Dicranum spp., Hylocomium splendens, Pleurozium schreberi, Polytrichum spp. and Rhytidiadelphus triquetrus.

Figure 7.10a shows a 25 x 30m area of Sernander's plot III of 1935 as it was when remapped in 1985 by Hytteborn and Packham (1987). Trees and fallen stems are shown, with fallen aspen Populus tremula being indicated by dashed lines. The necrotization condition of individual logs is shown by a number. The pecked line shows a storm gap, rather new in 1935, whose position has moved somewhat. Many young spruce exist for years as dwarf trees before the creation of storm gaps provides the light required for them to grow comparatively rapidly into adults (see Section 9.3.3). Growth rings of dwarf trees are very narrow and often incomplete (semi-lunar). The size of the symbols indicates the height classes of the live trees present.

Figure 7.10b shows the distribution of seedlings and saplings near the living spruce Z, whose position in the larger area of plot III is shown in (a). The tops of the young aspen and many of the rowan had been grazed by deer. Many aspen shoots subsequently die, with suckers regrowing from the base. Numerous small birch Betula sp. seedlings and saplings were also present, particularly so on the base of the large spruce B, which crushed the fallen log of spruce A which when living shared the same root system as the large living spruce Z (which showed 185 annual rings at a height of 0.35 m). Root grafting is very common in spruce and often occurs when the trees are very young.

Working on a large scale, Sernander allocated a single grade number to each log and thought that a period of 90 years would have to elapse before a log was so disintegrated that no trace of it was obvious on the forest floor. The later

Decomposition and renewal Box 7.3 (cont.)

living dead

Picea

Betula

Prunus

Populus

Sorbus

living dead

Picea

Betula

Prunus

Populus

Sorbus

Populus Tremula Boom

o ( > 20 cm) Populus tremula ▲ Sorbus aucuparia

Bracket fungus 1 m

(Fomes pinicula)

Figure 7.10 Change and decay: necrotization of spruce logs and tree regeneration in Fiby urskog, Sweden.

o ( > 20 cm) Populus tremula ▲ Sorbus aucuparia

Bracket fungus 1 m

(Fomes pinicula)

Figure 7.10 Change and decay: necrotization of spruce logs and tree regeneration in Fiby urskog, Sweden.

investigations of Hytteborn and Packham (1987) showed this to be an overestimate. Eight of the stems which had fallen since 1935 had reached class 5 in 50 years or less. Decomposition rates will in any case differ in particular cases and even within individual logs. Log A was a living tree in 1935, the basal portion of it was adjacent to its old tree base and its now utterly rotten wood entirely covered by a bryophyte mat in which Ptilium crista-castrensis was conspicuous. Regions of the same log on the other side of log B were much less decayed, the first five metres falling into class 4 and parts near the stem apex into class 3.

(Both figures from Hytteborn and Packham, 1987. Arboricultural Journal 11.)

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