Wind storms

Constant wind causes damage, deformation and, as described in Section 3.5, even death when accompanied by temperature extremes. Here we consider the major natural disturbances by wind which often result in very extensive felling. This can take the form of 'a mosaic or patchwork which is a state of constant change' of the kind described by Cooper (1913) on the Isle Royale, in the northwest of Lake Superior, where old wind throws regenerated as new ones developed. Fiby urskog, near Uppsala, still shows the storm-gap structure developed on unstable boulder and marly moraines described by Sernander (1936), who in 1935 mapped five plots of which Plot 1 is shown in Fig. 9.10a with a superimposed re-map of the same 50m x 50m area in 1988/90. The second mapping shows the same basic structure, but the pattern is different because regeneration has occurred in some areas, while in others mature trees have fallen. Sernander allocated a degree of decay to fallen trunks, on which spruce seeds often germinate. His necrotization scale runs from 1-6 (see Box 7.3); in stage 6 the area is usually entirely covered by moss, but a row of young trees may mark the line of the decayed trunk. Plots 1, 2 and 3 were remapped in 1988/90 and the decay states of the logs on the forest floor then

Gap throughout

Canopy throughout

Gap in 1935; canopy in 1988/90

Canopy in 1935; gap in 1988/90

Gap throughout

Canopy throughout

Gap in 1935; canopy in 1988/90

1

1

-n

i

1

i

1

2

50 m Decomposition class

Figure 9.10 Long-term results from the Fiby Forest, near Uppsala, Sweden. (a) shows an area originally mapped by Sernander in 1935 showing storm gaps resulting from windthrow of Norway spruce. Other important trees include Scots pine, birch and aspen. This same Plot 1 was remapped in 1988/90 by Hytteborn et al. (1993) and the results obtained in both time periods are shown in the figure. Open areas are gaps on both occasions; dark shaded areas are gaps in 1988/90 but canopy matrix in 1935; the stippled area is a gap in 1935 but canopy in 1988/90; light shading indicates canopy matrix mapped on both occasions. (b) Distribution of fallen logs in the six decomposition classes in Plots 1, 2 and 3 at Fiby urskog in 1935 (n = 169) and 1988/90 (n = 303). Decomposition increases from class 1 to 6; see Box 7.3 for a detailed description of the classes. (Redrawn from Hytteborn et al., 1993. In Small-scale Natural Disturbance and Tree Regeneration in Boreal Forests (Liu Qinghong). Ph.D. thesis, University of Uppsala, Sweden.)

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and in 1935 are also compared in Fig. 9.10b. There is a much higher proportion of logs in classes 1 and 2 in 1988/90 and the total number of fallen trunks is also larger. Sernander's thesis of 1936 was of great conservation importance in that besides describing the greater susceptibility (in contrast to pine, birch and aspen) of spruce to windthrow and the role of dwarf trees in its regeneration, it was also in direct confrontation with the views of Hesselman, who considered the forest of small scientific value. Had Sernander lost the argument the forest, its unique communities and the important epiphytic lichens Lobaria pulmonaria and Nephroma lusitacum would have been destroyed.

Figures 9.11 and 9.12 also pertain to the small but valuable old-growth forest of Fiby urskog; two of the small trees concerned grew on the granite ridge rather than the moraine, while the large Norway spruce was released by one of the numerous canopy gaps formed by the storm of 1795. This storm was particularly notable but records indicate that all spruce forests in Uppland are hit by major tree-felling storms at least five times every century, indeed Fiby urskog and Granskar lost many spruce in the successive storms of December 1931 and February 1932. There is also considerable interest in the small-scale natural disturbance and tree regeneration in the Norway spruce forests studied by Hytteborn et al. (1993) at Fiby urskog and Svartnasudden, northern Sweden (see Section 9.2.2 also). Both these old-growth forests have been free of fire for at least 200 years. Tree death was caused by storm felling, fungal infection and insect attacks, sometimes in combination. Gap sizes in Fiby

1980

Mature spruce (this tree was over 210 years old, was 28 m tall and had a dbh of 38 cm)

1980

Mature spruce (this tree was over 210 years old, was 28 m tall and had a dbh of 38 cm)

(8 1900

1950

1980

Fallen pine from Goat Ridge

Figure 9.11 Environmentally controlled variation in radial increment in two trees from Fiby urskog, near Uppsala, Sweden. Annual rings of the mature spruce Picea abies show a marked increase in width immediately after the great storm of 1795. It appears that the tree was favoured by storm gaps torn in the forest canopy at this time. The fallen Scots pine Pinus sylvestris was only 3.9 m high and its roots were superficial, being developed in a mat of humus and vegetation (including lichens) overlying solid granite. Stem diameter where the trunk was bored (10 cm above ground) was only 6.5 cm, yet the increment core had over 80 annual rings, some of them paper-thin. (From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 9.12 Dwarf spruce Picea abies which was considerably less than the 1.3 m height from which age increment cores are usually taken. Tree rings taken at ground level showed that this dwarf tree from Fiby urskog, Sweden had an age of at least 43 years. Note the prominent lateral branches and weak leading shoot. Dwarf trees beneath heavy shade retain a wonderful latent vitality. When growing on a suitable substrate they change their form and grow rapidly upwards if a canopy gap appears. (Photograph by Roland Moberg. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, fig. 3.3. With kind permission of Springer Science and Business Media.)

Figure 9.12 Dwarf spruce Picea abies which was considerably less than the 1.3 m height from which age increment cores are usually taken. Tree rings taken at ground level showed that this dwarf tree from Fiby urskog, Sweden had an age of at least 43 years. Note the prominent lateral branches and weak leading shoot. Dwarf trees beneath heavy shade retain a wonderful latent vitality. When growing on a suitable substrate they change their form and grow rapidly upwards if a canopy gap appears. (Photograph by Roland Moberg. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, fig. 3.3. With kind permission of Springer Science and Business Media.)

urskog ranged from 9-3000 m2 and 98% of the total gap area consisted of gaps less than 250 m2 in area. Most gaps resulted from the fall of a tree or a small group of trees, but the area of such gaps tended to increase with time. Unlike Scots pine and deciduous trees, Norway spruce regenerated in small gaps here. Tree diversity was maintained by gap enlargement and the occasional formation of large gaps.

At the opposite extreme is mass tree mortality. The wind storm that hit northern France on 26-27 December 1999 has the dubious distinction of reaching the Guinness Book of Records as the largest number of trees 'destroyed in a storm' when 270 million trees were felled or broken. Other dramatic storm effects can lead to mangrove peat collapse as at Bay Islands, Honduras, after Hurricane Mitch in October 1998 (Cahoon et al., 2003). In some Caribbean mangrove forests on oceanic islands, soil development primarily occurs by the formation of peat derived from mangrove roots, since a continental source of sediment is absent. The continued stability, and even existence, of such forests depends upon the production of thick mangrove peat at a rate considerably greater than loss of elevation due to decomposition of organic matter, for the forest has to maintain itself in the face of local sea-level rise. Powerful storms are known to have caused mass mortality of mangrove forests in the Caribbean and south-west Florida over a prolonged period, but recent study involving detailed measurements of elevation, accretion and root production has actually demonstrated the validity of the concept of mangrove peat collapse put forward in the past.

Low-, medium- and high-wind impact sites were investigated on the two most oceanic of the Bay islands. The impact of Hurricane Mitch was particularly great in Mangrove Bight, Guanaja, causing virtually complete mortality across all tree diameter classes in most places. Trunks of adult red mangroves Rhizophora mangle were broken and adult black mangrove Avicennia germinans trunks were uprooted. Of the 311 ha of mangrove forests possessed by the island only 3% survived. The mangroves of Roatan, where wind impact was low to medium, were much less severely affected. Here the dominant species were red and black mangrove, with some white mangroves Laguncularia racemosa and scattered individuals of buttonwood Conocarpus erectus. Most of the mangroves survived the strong winds and tidal inundation caused by Hurricane Mitch, but considerable areas of black mangrove forest on the north shore were killed by defoliation and toppling of the trees. When mangroves are killed peat collapse is likely to continue for some years. Recovery is made more precarious by the fact that successful seedlings cannot develop if the substrate level is at too low an elevation.

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