Ecology of Alnus Glutinosa (L.) Gaertn: V. Notes on Some British Alder Populations

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  • Ecology of Alnus Glutinosa (L.) Gaertn: V. Notes on Some British Alder PopulationsAuthor(s): D. N. McVeanSource: Journal of Ecology, Vol. 44, No. 2 (Jul., 1956), pp. 321-330Published by: British Ecological SocietyStable URL: .Accessed: 22/11/2014 03:03

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  • VOLUME 44, 2 JULY 1956



    BY D. N. McVEAN (The Nature Conservancy, Edinburgh)

    (With one Figure in the Text)

    Floristic lists from the areas described here will be found in McVean (1953), Tables 3 and 4. The following descriptions enlarge upon the short summary given there for the more informative populations,


    A brief topographical and ecological account of the valley bogs of the New Forest is given by Rankin (1911) and by Moss, Rankin and Tansley (1910).

    In these accounts alder is described as forming strips of carr and woodland along the course of the streams, flanked by successive zones of Phragmites, Molinia and the Sphagnum bog that extends to the Calluna heath of the valley walls. The impres- sion is conveyed that the alderwoods were then degenerating and retrogressing to acid bog as Sphagnum and Eriophorum extended in from either side to occupy the wood- land floor. The alder thickets are said to contain numerous rotting trunks of fallen trees, and 'in the course of time, probably as a result of decreased aeration, marshy vegetation gains the upper hand, the former woodland becomes submerged and leaves only a sub-fossil tree layer in the peat of the newly formed moor'. Regeneration of the alders is not specifically mentioned.

    A later (1938) unpublished description by Godwin and Clapham of the Beaulieu and Burley bogs in general, and of Matley bog in particular, gives rather a different picture.

    The conspicuous features are now said to be the narrow marginal woodland fringe of Betula, Ilex, Salix, Frangula and Alnus marking the transition from the broad boggy floors of the valleys to the Calluna heath on the gently sloping valley sides, the alder carr along the central streams, and the belts of fen and bog between these woody zones. The marginal woodland belt was sometimes entirely replaced by Myrica gale, and the carr was frequently bordered by a strip of the same species. Signs of artificial drainage of the alder carr were noted, and this was taken as a possible cause of the tendency shown by the alders to extend over their bordering 'splash zone' with its plants of acid fen or marsh.

    Higher acidity and the more sluggish flow of water in Denny bog were presumed to be at least partly responsible for the less definite zonation shown here, and for the total absence of alder. The Burley bogs are, however, described as having a higher base status and richer flora, yet being without alders.

    A rapid inspection of these same bogs was made in the summer of 1950, with particular attention to the status of the alder. While the general impression was similar to that gained by the 1938 investigators, certain changes in detail had occurred in the intervening twelve years.

    B J.E. 321

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  • 322 Ecology of Alnus glutinosa Along the course of the Beaulieu river, at Matley Passage, the edge of the alder

    carr was not sharply defined, and scattered, stunted alders, 3-4 m. high, with numer- ous saplings and seedlings, extended over the flanking Molinia-Myrica bog almost to the damp heath zone. Clumps of thicket with Betula, Ilex, Pinus, Alnus, Rosa and Rubus certainly occurred in the bog, but the sharpness of the zonation had been lost, and it seemed that the streamside woodland belt had linked up irregularly with the marginal one.

    Further downstream the transition from alder swamp to Calluna heath was more abrupt, and a 2-3 m. strip of Molinia and Myrica might alone separate the two. This strengthened the impression that the alders had spread laterally. Heath fires must constitute an important factor here as young alder shoots from burnt stumps were found among Calluna in the damp heath zone.

    In the alder carr and woodland itself only seedlings of the current year, with a few in their second season, were found. This is normal in such situations and it is difficult to see how alder carr could have replaced an earlier willow carr as Godwin and Clapham suggested. Salix, Betula and Frangula bushes are more frequent marginally in those strips of carr, and only a few survive in the close alder canopy, but these are certainly the contemporaries of the surrounding alders.

    Still further downstream, where the railway crosses the river, alder carr extends for some distance into an extensive Molinia-Myrica bog. Alder regeneration is here very active, seedlings and saplings of all ages being found on the outskirts of the carr, though only first- and second-year seedlings in the denser shade. The young seedlings in the bog appeared healthy and apparently tolerant of the edaphic condi- tions. Old, many-branched alder snags, 1-2 m. high, and with abundant basal shoots, occurred over a range of 30 m. from the wood edge, but in view of the abundant evidence of periodic fires nearby it is hard to say if these snags are comparable with the stunted alders observed on similar ground in Scotland or are merely the result of fire damage.

    In places fires have swept into the carr and destroyed many of the older trees. Others, though scorched along the greater parts of their stems, have sent up abundant new shoots. Seedlings and saplings are plentiful in the burnt carr, and it seems that the temporary advance of the bog will be checked very quickly.

    North of the Lyndhurst-Beaulieu road three strips of alder carr extend downhill to a tributary of the Beaulieu river and at right-angles to it.

    They mark the course of streams, now lacking a definite channel, which arise as quaking Molinia bogs at the heads of the carrs. As before, the alders are often separated from the heath by only a narrow fringe of Molinia and Myrica. Where this fringing bog widens uphill, Phragmites communis may form marginal relict reed-beds.

    Typically, a core of old trees mark the original stream line, and successively younger zones of alder are found to the outside. The remains of Phragmites, Myrica and Molinia under the fringing thicket of saplings provides evidence of rapid alder colonization. In places young trees form the entire width of the strip.

    A possible explanation of the different situations observed in 1911, 1938 and 1950 is that the proportions of bog and carr at any time depend upon the amount of flushing with circim-neutral or calcareous stream water that has been taking place. Periods of heavy rainfall and flooding would assist the extension of the carr over the bog, and periods of low rainfall would lead to drying out and acidification of the bog, with perhaps its advance streamwards.

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  • D. N. MCVEAN 323

    The position found by Rankin could be regarded as a bog maximum with great extension of Eriophorum and possible absence of alder regeneration. The level of the ground-water may then have risen, and the alder succeeded in colonizing the bog while Eriophorum decreased. This is not in accord with the speculation of Godwin and Clapham that the spread of the alder may have been induced by artificial drainage. It has been shown in Part III of this account (McVean, 1955) that drainage can only result in increased regeneration of the alder where the original water level is above the soil surface in spring and summer. Influence of drainage on the sub- sequent growth of the trees is another matter.

    The tendency of the carrs to regenerate peripherally only would make the effect of acidification more immediate, the trees being checked in their spread and dying off in the rear.

    Godwin and Clapham give the following pH values for water samples along one of their transect lines at Matley: Sphagnum bog 4.7, marginal wood 5.1, alder carr 5.9, stream 7.4.

    Several pH determinations were made colorimetrically in 1950, and lower figures than these obtained. The standing water of the bog was everywhere about 4.0 and the water of the carr 5.0. The river water was not tested.

    Rainfall records for the area were examined in the hope of discovering some evidence for the climatic control of fen and bog cycles. Total annual rainfall, ex- pressed as percentages of the normal (1880-1918) are given below. It will be seen that the period 1916-38 was slightly wetter than 1938-48.

    Year Rainfall Year Rainfall 1880-1915 100 1938 88

    1916 115 1939 120 1917 90 1941 96 1918 100 1942 93 1921 55 1943 84 1922 110 1944 85 1924 127 1945 81 1925 130 1946 119 1926 100 1947 79 1928 120 1948 103 1929 105 1930 109 1931 95 1932 100 1933 75 1934 105

    It is of course realized that the liability of the bogs to flooding may have little connec- tion with total rainfall but be more closely correlated with summer rain, or even a tendency for concentrated rainfall and drainage activity in the area.

    Brooks and Glaspoole (1928) give rainfall periodicities in Britain as showing 52, 37, 11, 9, 5 and 2 year cycles. The present phenomenon fits in best with the first two of these.


    Heron's carr is developed partly as a fen carr on tongues, of solid peat stretching out from the upland, and partly as swamp and semi-swamp carr on a floating mat of vegetation (Lambert, 1951). The fen carr trees are tall with no signs of die-back, but those of the floating mat are poorly developed (2-3 m. tall) and with little foliage.

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  • 324 Ecology of Alnus glutinosa This open, well-lighted carr allows seedlings and basal shoots to develop into thin poles which contribute to the canopy before dying back in their turn. This is the only example of the internal regeneration of an alder stand known to the author.

    Between Ranworth marshes and the confluence of the rivers Ant and Bure there is a large area, considered to be an extinct broad by Lambert (1951), carrying mori- bund open alderwood. A number of the trees are completely dead, the remainder showing extensive die-back, and in June 1952 many living shoots were seen to be attacked by a fungus (Taphrina sp.) that could not be isolated in culture.

    The trees may originally have been harmed by the subsidence of the floating mat, but the ground is now reasonably firm in places and the water-table no higher than in many areas outside the broads where trees are healthy.

    Sapling root systems were examined here and found to be entirely superficial in contrast to those from the waterlogged areas of Chippenham fen (McVean, 1953).

    An immediately sub-surface sample of peat from the area gave a positive sulphide reaction (cf. Table 2, McVean, 1953), and it may be that the alder is particularly sensitive to high concentrations of this substance, though tolerating a reducing medium.


    South of the Conway valley alderwood occupies the north-facing slopes of Moel Eilio at 800-1000 ft. (244-305 m.) O.D. The soil is a gley, peaty in places, and developed on glacial drift.

    Tree cover is patchy with many clearings containing Juncus-Sphagnum flushes in which there is little sign of alder regeneration. Regeneration was noted along ditches and where trees had been felled, but the saplings were sheep grazed. Much of the wood consists of multiple-stemmed scrub 3-6 m. high, with occasional trees of over 10 m., so that it may have been cut over at some time. It gives way to closed birch scrub on the upper slopes.

    On the south slope of the hill towards the Afon Ddu the alders reach a greater altitude and only a little open birch scrub lies above them. This valley is exposed to particularly severe winds from the south-west so that the alders and hazels are wind-cut and show die-back of the exposed branches. This is not unknown in coastal populations but unique inland. Drainage is again impeded by an indurated gley subsoil.

    Alder regeneration is practically absent. A few saplings that appeared to have been killed by the wind were noted in 1951, while others had been badly damaged on the windward side. On the other slopes the alderwood is interspersed with areas of Corylus, Prunus spinosa and Juncus flushes. Trees on the outskirts of the woodland areas are shorter than those in shelter so that the patches of woodland are lenticular in cross-section.

    Further down the Afon Ddu, in shelter at about 153 m. O.D., the alders attain 15 m. in height, and oak and elm come in with many additions to the ground flora.

    The general impression is that the wood is dying off at the higher levels. A particular extensive population of alder is developed, at Pont Croesor, Tre-

    madoc, on land reclaimed from the sea in the eighteenth century, formerly the Traeth Mawr. The open alder scrub extends to over 200 acres, mainly within the fork of the Afon Glaslyn, and the entire area is still liable to winter flooding. The high water-table results in continuous alder regeneration though grazing by horses

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  • D. N. MCVEAN 325

    and cattle has kept the growth open and consequent exposure to sea winds has limited the size of the trees and the viability of the seed set. Local patches of wood- land have formed a close canopy.


    The alder occurs throughout the north of the county as the usual stream-side lines of small, gn...


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