Box 74 Mosses and liverworts on rotting wood

Managed and unmanaged forests differ in many ways, particularly in terms of their biodiversity. Eighty plant species, of which 50 are mosses and liverworts (bryophytes), are acutely threatened by intensive forest management in Sweden, with another 72 that are vulnerable (40 being bryophytes). Andersson and Hytteborn (1991) investigated the bryophyte populations of an area of the natural forest of Fiby urskog dominated by Norway spruce, and of a managed forest about one kilometre away. The managed forest was clearcut for charcoal production in the 1930s, thinned in 1973/4, and fertilized with ammonium nitrate (NH4NO3) in 1979. Some 55% of the trees were Scots pine, the remaining being Norway spruce with a few birch; aspen occurred only in Fiby urskog. Eight 10 x 20 m sample areas were employed in each forest. Records were made from the decaying wood of the stumps and logs, but not the standing dead snags, in each plot. In the managed forest, decaying wood on the ground consisted mainly of stumps and tree tops from the thinnings, whereas Fiby urskog contained many fallen logs whose decay is described in Box 7.3.

Altogether a total of 54 bryophyte species, belonging to four different groups, were found on decaying wood. Of these 48 came from Fiby, compared with 33 from the managed forest (six of which were not found in the Fiby plots). The primeval forest had a larger amount of dead wood, much of it consisting of large-diameter logs that were absent from the managed forest. The presence of particular species was related to the type of tree involved as well as its degree of decay. Eight decay classes were used, starting with wood that was hard and parts of logs with intact bark and finishing with completely soft material. Facultative epiphytes, e.g. Ptilidium pulcherrimum, were the first to colonize decaying rotting wood, being followed by epixylic specialists (living on the outside of wood) and then by epigaeics (living at the soil surface), such as Hylocomium splendens and Rhytidiadelphus triquetrus. Opportunistic generalists including Hypnum cupressiforme and Brachythecium rutabulum showed a more irregular pattern of occurrence. There were 16 epixylic specialists, plants such as Aulacomnium androgynum dependent on decaying wood, in the Fiby plots, but only five of these were present in the managed forest.

Coarse woody debris has an important part to play in carbon budgets of forests (see Chapter 11) but much less of a role in nutrient budgets simply because wood is so poor in nutrients. Laiho and Prescott (2004), in a review of the subject in northern coniferous forests, point out that CWD contributes 3-73% of above-ground litter input, but < 20% of N, P, K (potassium), and Ca (calcium). Although up to 54% of accumulated organic matter (including that in the soil) is CWD, it contributes < 5% of the N, < 10% of the P, and

< 25% of the K, Ca, and Mg (magnesium). As discussed above with litter, CWD is initially a sink for N and P, becoming a source as decay progresses but it still plays a minor role in overall availability. Moreover, due to the low initial N levels, wood can be a net sink for N for many decades.

The importance of maintaining adequate amounts of dead wood in fully functioning and highly diverse ecosystems is emphasized in Section 3.8, which deals with veteran trees. The unquestionable value of CWD for wildlife has led to many recommendations for forest management to maximize the amount of dead wood (see Chapter 10), despite the human desire for order and tidiness in forests. These recommendations include such notions as leaving mature trees to die, rather than being harvested, because they provide the most wildlife-friendly CWD, and keeping forked trees as they are likely to have a higher biodiversity of fungi (Heilmann-Clausen and Christensen, 2003). This train of thought has led to suggestions that dead wood should not just be kept, it should be created by deliberate mutilation. Ways of mutilating trees are many and, to quote George Peterken (1996, p. 420)'... can be achieved by sawing rot holes and ring-barking, or by more exciting techniques such as fire, exploding the crown from the trunk or holding a vandals' convention'. These techniques do work. A major fire at Ashtead Common in Surrey, England seriously scorched and damaged a significant proportion of the 2000 oak pollards and many maiden trees. The subsequent aim was to retain as much standing dead oak as possible while rendering the standing trees safe, so chainsaws were used to remove dangerous branches, and replicate the shattered ends of storm-damaged limbs in a technique known as coronet cutting.

7.7.3 Ecology of wood decomposition

Wood is difficult to digest not just because it is chemically tough but because it is so poor in nutrients. It is composed of 40-55% cellulose, 25-40% hemi-celluloses and 18-35% lignin (conifers having a greater proportion of lignin than hardwoods). Wood is thus high in structural carbohydrates (which require specialized enzymes to break them down) but poor in such elements as nitrogen; wood has 0.03-0.1% N (by mass) compared to 1-5% in foliage.

Compared with litter, freshly dead wood loses little of its weight by initial leaching, partly because wood is low in soluble substances but also because of the low surface area to volume ratio. Physical fragmentation is, however, an important part of wood decomposition; breaking into smaller pieces increases the surface area exposed to the biological agents of decay. Snags are most prone to breaking up since they are exposed more to changes in temperature and high winds as they stand vertically. As shown in Fig. 7.9 the thinnest parts of snags are broken off first. Thus, twigs and small branches are rapidly lost (within a handful of years) followed by progressively fatter parts of the trunk. Most snags will eventually fall once the roots are weakened within a decade or so, usually precipitated by strong winds, although as outlined above, there are exceptions with western red cedar standing for over a century and bristlecone pines for millennia. When the snag hits the ground, if it is sufficiently rotted it will shatter upon impact. At the other end of the size spectrum, individual fibres are physically separated and washed off the outside of sun-exposed logs. Fire is capable of consuming large amounts of CWD but since larger logs can be almost completely fire-proof(due to their low surface area to volume ratio) its influence is not always as much as might be expected. Tinker and Knight (2001) looked at CWD consumption during a fire in Yellowstone National Park and estimated that only 8% of CWD (above 7.5 cm diameter) was consumed by the fire and another 8% turned into charcoal. Moreover, fires also produce more CWD by killing branches and whole trees, and Tinker and Knight concluded from a computer simulation that over 1000 years a fire-return interval of 100 years resulted in more CWD than a 200- or 300-year interval.

Wood decomposition proceeds at an impossibly slow rate without the intervention of biological agents. Of these the two most important groups are burrowing invertebrates and particularly fungal rots. Fallen trees go through a process of collapsing and settling. At first the trunk may be supported above the ground by the branches, but as these rot it settles into contact with the ground, keeping it moister. The bark is gradually lost and as the log begins to rot it settles further, increasing its contact with the ground, changing its suitability for microbes, invertebrates and vertebrates. Thus, as decomposition progresses, there is a succession of different organisms involved.

Heliovarra and Vaisanen (1984) recognized four phases of insect succession on fallen wood in temperate forests. Phase A starts with short-term feeding on bark by bark beetles and longhorned beetles (Scolytidae, Cerambycidae). Phase B is composed of species living under the bark and in the surface layer of the wood. By this time the bark has fallen and Phase C follows, a long stage of several decades of wood-inhabiting species. In the final and longest Phase D, the wood-inhabiting species are replaced by animals living under the shelter of decaying logs, such as soil insects, snails and centipedes. During this stage other animals like frogs, salamanders (see Box 7.5) and snakes burrow under the log and moles and shrews tunnel in and around the log foraging for their prey. Finally the stem breaks up and merges into the soil organic matter. Burrowing insects, especially termites and the larvae oflarge beetles, can make sizeable channels into the wood and contribute greatly to decomposition (see Box 7.6). However, the main

Box 7.5 Newts, salamanders and dead wood

Amphibians such as newts and salamanders (strictly speaking, newts are a type of salamander) are often ignored in forest ecology as interesting but minor players. Yet, in the hardwood forests of Hubbard Brook Experimental Forest in New Hampshire, Burton and Likens (1975) found around 2950 salamanders ha-1, which may not sound many but this works out at more salamanders in the forest than either birds or small mammals. More importantly, the biomass of the salamanders was more than two and a half times that of all the birds during the peak of the breeding season, and equal to the biomass of all the mice and shrews.

The red-backed salamander made up almost 95% of the salamander biomass in these forests. Experimental removal of this species from similar forests by Wyman (1998) showed that the salamanders significantly reduced the decomposition of litter by 11-17%. The most likely cause is that they eat a significant number of the leaf fragmenters such as beetles, millipedes, snails and insect larvae.

Many forest salamanders are associated with the dead wood of streams and dry land. The red-backed salamander in particular is associated with a thick leaf litter and decaying logs or stumps. Thus, anything that affects CWD will affect the salamanders. Bury (2004) looked at whether salamanders and other amphibians and reptiles were endangered by prescribed burning and thinning for fire control since it reduces the amount of CWD and so reduces cover. Bury's evidence, however, suggests that gentle fires themselves have little effect because big pieces of wood are not consumed in normal fires, and during hot dry conditions suitable to fire, the animals are deep underground.

Huge Red Backed Salamander
Figure 7.11 Newts, salamanders and dead wood. The red-backed salamander Plethodon cinereus of New England, USA. (Photograph by Brooks Mathewson).

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