A Parent beech
A Seech sapling • Ash sapling
□ Oak sapling Quercus robur V Spindle Euonymus europaeus
Figure 9.7 Top: Distribution of phases in space when an old beechwood has been left to itself and has trees of all ages. Beech Fagus sylvatica is forming a reproduction circle in the gap. Vegetation that subsequently develops in a gap goes through the original succession in minature, with the understorey dominated by bare ground, then wood-sorrel (Oxalis) and finally bramble (Rubus ). Since gaps appear at different times, all the successional phases can be seen in the woodland. (Redrawn from Watt, 1947. Journal of Ecology 35, Blackwell Publishing.) Bottom: Detail of a reproduction circle in a gap which typically has saplings of ash Fraxinus excelsior in the centre and those of beech and other trees round the margins. (Redrawn from Watt, 1925. Journal of Ecology 13, Blackwell Publishing.)
virtually no field layer beneath it. This bare stage is followed by the Oxalis stage when the light reaching the ground beneath more mature beech is sufficient to allow shade-tolerant wood-sorrel Oxalis acetosella to establish. Only when very much more light reaches the ground can the bramble of the Rubus stage develop. As trees eventually die, the vegetation in the resulting gaps frequently goes through stages similar to those that occurred in the original succession when the forest developed from open ground. This process, in which woodland goes through repeated cycles of similar sequences of development on a small-scale in gaps, is known as cyclic change (or turnover of vegetation).
In most woodlands and forests successional development does not follow a tight and exact prescribed pattern; modern work emphasizes that there is room for variation in what grows in each gap. In terms of general forest theory Watt himself was well aware of the various influences that could cause the outcome of gap regeneration to vary (see Packham et al, 1992, Section 5.1). With regard to the reproduction circle shown in Fig. 9.7, for example, the size of the gap is important because beech mast formed in high forest drops almost vertically and is largely confined to the margins of wide gaps, the centres of which are often occupied by a dense growth of common ash whose seed is wind-dispersed. The occasional beech seedling has little chance of establishing in the central core of ash until the taller growth and deeper shade of the young peripheral beech, together with the gradual extension of the parent beech canopy, tend to suppress the more light-demanding ash. The outcome varies; in at least some cases the ash in the centre of a gap grows up fast enough to avoid suppression, while in narrow gaps the whole floor is sometimes seeded by beech whose seedlings and saplings may not meet with competition from any other tree.
Numerous studies have now been completed that show that many factors can change the gap-phase succession in a forest gap. So much so, that while climate, soil and topography determine the large-scale regional vegetation (e.g. oak forest) it is the small-scale gaps that produce most of the local variation that controls the proportions in which the various species grow.
The starting point is to consider how new plants arise in gaps. Many seeds will arrive as seed rain from surrounding trees. In some pines such seed is released from canopy seed banks as a result of fire (see Sections 3.7.1, 4.6.2). Pioneer species tend to have light, wind-blown seeds that go further than the heavy seeds of primary species. This is not always clear-cut since a number of trees with heavy seeds use animals to transport the seeds; perhaps the best example is the movement of some pine and oak seeds by corvid birds.
New plants can also arise from a soil seed bank (see Section 4.6.2), where seeds become buried under litter and humus, remaining viable for sometimes centuries until stimulated to germinate. It may seem paradoxical at first but it is normal that the most frequent species in the mature vegetation are largely absent from the seed bank and vice versa (Olmsted and Curtis, 1947; Bossuyt et al., 2002). Rather, the soil seed bank is composed largely of early succes-sional species and those of disturbed areas, waiting for another gap to appear. In north-west Europe, common species in woodland soil seed banks are the wild raspberry (Rubus idaeus), gorse (Ulex spp.) and heather (Calluna vulgaris), all representative of early successional stages, with seed densities of up to 1000, 30 000 and almost 70 000 seeds m-2, respectively (Thompson et al., 1997). There are some apparent exceptions such as in acid Scots pine woodland in Scotland with an understorey of heather, and a soil seed bank of a mean density of 83 000 seeds m~2, 96% of which is heather (Miller and Cummins, 2003). However, this large heather seed bank is probably still a relict of an early woodland stage where heather was much more abundant, aided by the longevity of buried heather seeds (up to 150 years in moist peat).
Most soil seed banks are made up of herbaceous and shrubby species while trees are largely absent. This is primarily because most trees have comparatively large seeds that are readily found and predated on the ground. Supporting this is that trees with small seeds, such as birch, are found in seed banks. In Europe, birch has been seen regularly to have hundreds of seeds m~2, and exceptionally recorded as up to 12 800 m-2 for silver birch (Betula pendula). Most viable seeds are near the surface, declining in number with depth and changing in species composition (a facet of the relative longevity of different species). Thus the removal of increasing depth of organic matter by fire or mechanical means will result in a different density and species composition of seedlings.
Existing but damaged trees can also be part of the successional dynamics by producing new growth from buds surviving on branches, trunks and roots -the bud bank. Peterken (1996) provides an illustration of vigorous sprouts arising from the horizontal trunk of a wind-thrown small-leaved lime Tilia cordata. This species also sprouts vigorously from the bases of old trunks; in some cases thickets of saplings have developed vegetatively from the crowns of fallen trees. Few conifers can do this but it is a common means of regrowth in hardwoods. A final variable is the use of a seedling bank by some shade-tolerant species. Here, small, suppressed saplings persist under heavy shade growing very slowly. When a gap appears they have a head start in the race for dominance.
Taking all these variables into account, it is perhaps not surprising that successional pathways may not always be the same. For example, gap size is important. In small gaps created by one tree falling, shade-tolerant trees (the later-successional or primary species) such as beech or firs are more likely to do best and dominate. In larger gaps, early-successional, pioneer trees, such as birch and willow, which invade quickly from light wind-borne seeds and grow quickly, are likely to dominate, giving way later to the shade-tolerant trees. Thus, in small gaps the successional pathway can be truncated, leaving out the earlier stages. In subalpine forests of central Japan, Narukawa and Yamamoto (2001) found that Veitch fir (Abies veitchii) and especially Maries' fir (A. mariesii) formed seedling banks under the canopy while the Hondo spruce (Picea jezoensis var. hondoensis) and Japanese hemlock (Tsuga diversifolia) were restricted to gaps. In small openings the relatively few fir seedlings would dominate. In larger gaps the spruce and hemlock outnumber the fir seedlings and are more likely to reach the canopy and persist. In very large gaps with high mineral soil exposure, even birch (Betula ermanii and B. corylifolia) would be able to hold its own (Kohyama, 1984; Yamamoto, 1993).
Larger gaps also vary across their width, with gradients of increasing light and decreasing moisture towards the middle having an effect. The bulk of new seedlings at the edge of big gaps may reflect the high seed rain from surrounding trees, and the seedling bank doing well in partial shade. By contrast, the centre of large gaps will be less dependent upon seed input (except perhaps those from pioneer trees with lighter seed that travel further) and the seedling bank (which may not be able to compete with dense vegetation in light, dry conditions), and more dependent upon soil seed banks and vegetative sprouts from the bud bank. There will also be less below-ground competition away from the root systems of the large trees at the edge. This can be quite crucial; Barberis and Tanner (2005) found that in the seasonal rain forest of Panama, trenching to remove competition from roots of established plants increased tree seedling growth, attributed to increased nutrients in the wet season and increased water and nutrients in the dry season. Other factors may also help establishment; for example, in large gaps the sudden increase in light and temperature may lead to photoinhibition of the understorey vegetation (Houter and Pons, 2005), making it less competitive.
Further variability in seedling establishment is produced by small-scale heterogeneity of the forest floor since seedlings will be successful only if they are in favourable microsites. Kuuluvainen and Juntunen (1998) found in a Scots pine forest in Finland that the pits and mounds of bare mineral soil created by falling trees were important for establishment. Although these bare sites covered just 8.4% of the forest, they held 60% of pine and 91% of birch seedlings and saplings. Bare mineral soil offers less competition and a more constant water supply than the surrounding humus-rich forest floor. Gaps with more bare soil allow birch a better chance to establish and grow quickly enough to persist in the canopy, thus altering the successional pathway. Rotting 'nurse' logs also provide excellent conditions for seedling establishment in moist forests; in this case their main value is in raising the seedlings above the shade of dense understorey vegetation (see Fig. 1.7). Suitable microsites become increasingly important the denser the understorey vegetation and the deeper the litter and humus layer (see Kuuluvainen, 1994 for a review of Finnish boreal forests).
Hytteborn et al. (1993) studied small-scale natural disturbance and tree regeneration in two Swedish boreal forests. The canopy of the central boreal forest of Svartnasudden, south of Umea, was relatively sparse and sapling distribution was related to microsite pattern. That of the boreo-nemoral forest of Fiby urskog, near Uppsala, was more pronounced; here sapling distribution was influenced by both microsite pattern and canopy gaps. Both forests are of the Picea abies-Vaccinium myrtillus type and have a forest floor covered by a thick layer of moss in which Pleurozium schreberi, Hylocomium splendens, Ptilium crista-castrensis, Dicranum spp. and Polytrichum spp. are prominent. The nature of the microsites where tree regeneration occurs varies with the species concerned. More Norway spruce saplings grow on boulders than on soil, but they are concentrated on highly decomposed logs where they are more successful than on other microsites, whereas goat willow Salix caprea and the birches aggregate in the tip-up pits and on the root plates of fallen trees. Aspen Populus tremula produces root suckers so its vegetative reproduction is independent of microsites.
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