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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: http://www.jstor.org/stable/2256824 .

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VOLUME 44, 2 JULY 1956

ECOLOGY OF ALNUS GLUTINOSA (L.) GAERTN.

V. NOTES ON SOME BRITISH ALDER POPULATIONS

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,

THE NEW FOREST VALLEYS

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A later (1938) unpublished description by Godwin and Clapham of the Beaulieu and Burley bogs in general, and of Matley bog in particular, gives rather a different picture.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



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 numerous 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 conditions. 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.



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, expressed 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 AND HEDNEY'S BOTTOM, NORFOLK

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.



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.

NORTH WALES

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



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 woodland have formed a close canopy.

SUTHERLAND

The alder occurs throughout the north of the county as the usual stream-side lines of small, gnarled trees. At the head of Strath Vagastie where the gradient is steep and the river cuts a gorge through the gneisses, alder occur freely on the rock ledges, but only within the probable range of flood waters, though wind-borne seeds must be deposited on the higher ledges. Betula pubescens, Sorbus aucuparia and Populus tremula grow on the higher ledges and their seedlings may be seen on the peat slopes above.

On flatter ground downstream above Altnaharra, where the river floods out over areas of sand and shingle, large populations of alder saplings are found in pure stand, associated species on the shingle being Calluna vulgaris, Thymus drucei, Lotus corniculatus, Erica spp., Festuca ovina, etc. In 1950 the youngest sapling groups were 7 years old and the oldest 15-20 years, so that a marked increase in regeneration has recently taken place, the alders upstream acting as the seed parents.

A remarkable fragment of open alderwood covers the south-facing slope of Meall a'Bhata east of the narrows between Loch a'Choire and Loch a'Bheallaich, from 275-366 m. O.D. The trees are all veterans and about twelve of the total ninety have been enclosed in a plantation of Larix, Alnus glutinosa and A. incana.

Seedlings and saplings are absent even within the sheep- and deer-proof fence of the plantation. Seeds collected in 1950 were without embryos.

The vegetation of the slopes is bent-fescue grassland with some Trifolium repens in places. Large parts of the lower slopes are occupied by dense Pteridium and there are areas dominated by Deschampsia caespitosa, Juncus effusus, Molinia and Nardus. Soil appeared to be a deep fertile brown earth, though a profile could not be examined, extending from the Juncus flushes of the valley floor to where continuous peat cover begins at the remarkably high altitude of 460 m. O.D. The alders show no association with the flushes and streams on the slope.

It is rather difficult to account for the presence of such vegetation at this-altitude in the north of Sutherland. The rocks of Meall a'Bhata consist partly of an ultra- basic intrusion which might help to explain the soil type, and the ruins of a crofting township in the glen below show that the effect of human influence may have been considerable (Mackenzie, 1914).

The spreading pioneer crowns of the surviving trees show that they completed most of their growth at a time when the woodland was already opening out and degenerating. It is much less likely that the trees were established in open formation as they are now, and never formed part of a denser wood.

LOCH TREIG, INVERNESS-SHIRE

An open alder-birch wood, very similar to that of Meall a'Bhata, lies on the west slope of Garbh Bheinn above the loch. The alders have the same oak-like pioneer form and are dying off in the same way as the Sutherland trees. The average age of the birches seems to be less than that of the alders and few dead or dying trees were seen. The altitudinal limit of the birch lies at about 610 m. O.D. and that of the



326 Ecology of Alnus glutinosa alders at somewhere under 460 m. The alders bear catkins up to the altitudinal limit but many of these have withered off each season.

Birch regeneration is active, though sheep keep the saplings trimmed. There are no alder seedlings above the Glasgow-Mallaig railway line which crosses the area at 305 m. O.D. Below the line alder saplings were seen in Molinia-Myrica flushes at 270 m., sheep-grazed and probably 4-5 years old.

Soil and herbaceous vegetation resemble those of Meall a'Bhata, though sections made visible by wave action along the shores of the loch reveal an underlying indurated till which probably impedes drainage in wet seasons.

GLEN NEVIS

Alder is an abundant tree from sea level almost to the water-shed at Tom an Eite at 331 m. O.D. It also extends up the tributary streams and is found more rarely in soft spots on the hill slopes. In the bottom of the glen below Cam Dearg alders form extensive woods on alluvial fans from the steep slopes, and on the river alluvial flats. Between the alluvial areas, on waterlogged Molinia-Myrica ground, the few trees are only 3-4 m. high. Regeneration here is abundant and continuous but the saplings fail to develop properly and give rise to thickets of scrub.

A soil profile revealed on the undercut river bank shows alternate layers of mineral downwash and pure acid peat, thus indicating that the above facies on alluvium and bog may also alternate in time.

Typical areas of the alder scrub are:

(1) Molinia-Myrica-Sphagnum bog of about 2 acres covered with stunted alders, about 1 m. high, and bearing catkins. Local raised patches of silt within this area carry young trees 2-3 m. tall and probably of the same age as both the stunted scrub and the tall straight trees of the larger alluvial populations.

The Sphagnum round scattered pools of iron-stained water was found to be covered with healthy alder seedlings in their first year, but older seedlings were dying off in large numbers. The pH of the standing water was found to be 5.0 and that of the water expressed from Sphagnum 4.0 by colorimetric estimation. Samples of soil and moss removed in waxed cartons gave glass-electrode readings of 5.2 for the peat and 4.69 for the Sphagnum 48 hours later.

Saplings were found to be sparsely nodulated while the nodulation of adjacent AMyrica bushes appeared to be normal.

(2) A partially closed thicket of Alnus and Myrica, 1-2 acres in extent, with some Salix atrocinerea and Betula pubescens, the alders 1-2 m. tall and bearing catkins. The age of the bushes is uncertain, and the thickened stem bases with their many living and dead shoots indicate that such a community may persist for many years, The field layer consists of tufted Molinia and Deschampsia caespitosa with subordinate Sphagnum, Juncus, Succisa, Filipendula, Narthecium, etc.

Sheep are excluded from these areas and the vegetation has not been burned for some time. Similar communities, modified by grazing and burning, have since been recognized throughout Glen Nevis and elsewhere.

Alderwood also occurs at an altitude of 152 m. O.D. at the head of lower Glen Nevis where the Nevis Water emerges from the gorge. It covers 4-5 acres and is of considerable interest since it appears to represent the rejuvenation of a wood of the



D. N. MCVEAN 327

Meall a'Bhata and Loch Treig type. Old pioneer trees are scattered throughout and concentrated along the tributary streams of the Nevis Water.

Extension of the wood has been in stages so that it consists of several adjacent patches of uniformly aged trees, the youngest being in the late thicket stage, 6-9 m. tall, 7-15 cm. in diameter and possibly 20-25 years old. First-year seedlings were abundant in 1951.

The wood is still extending sporadically uphill above the footpath into a boulder- strewn Molinietum with some birch scrub. The soil is a well drained and stony brown earth overlying fluvio-glacial gravel and the floor of the wood is predominantly grassy with much fern and bracken.

FLORISTICS AND SUCCESSION

About 243 species of vascular plants are recorded for British alderwoods (McVean, 1953). Continental authors (Kujala, Steffen, Schoenichen, Hock and Raunkiaer) list about 306 species of which 21 are absent from the British flora and 10 occur as aliens. Hock (1897) points out that many species accompany Alnus glutinosa throughout its range so that the floristic composition of alderwoods is everywhere very uniform as compared, for example, with stands of Quercus. In the New World the companion species of A. maritima, A. serrulata, etc., are closely related taxono- mically to those of A. glutinosa in the Old World.

In Britain itself there is a tendency for several species of a genus to replace one another in different parts of the country, e.g. species of Carex, Circaea, Galium and Lysimachia, though many species such as Ranunculus repens, Galium palustre and Fili- pendula ulmaria are wide ranging.

The flora of alderwoods is everywhere strongly influenced by the composition of the vegetation it has succeeded. About 50 per cent of the alderwood flora can also be recorded from the fen and marsh associations colonized by the alder. On the other hand, only about 5 per cent are woodland species, even in the broad sense, and there are no distinctive societies comparable with the Mercurialis society of oakwood or the Allium-Ficaria society of ashwood.

Soil water and base content are the factors obviously determining the aspect of the field layer, and Table 1 has been compiled to illustrate this with some character- istic species. Sharp divisions do not, in fact, exist, and the full range of many of the species has not been expressed.

Alder communities are at their most distinctive on the wetter, more base-rich habitats, while the drier and less basic types merge progressively with those of oak, ash and birch. The former bear a closer resemblance to Continental alderwoods in floristics and physiography, while the alder-birch woods of our hill slopes appear to be unique.

The alder is primarily a pioneer and opportunist species, and is capable of direct colonization of even the rawest of soil material. At Dolgarrog, North Wales, alder has successfully colonized bare stretches of the finer material laid down by the flood waters from a burst reservoir in 1925. The colonization of road embankments in the west of Scotland has already been referred to.

In the same way the species acts as a pioneer on hydroseres, being capable of colonizing at very early stages in the primary succession if good seed is available, and liable to invade at any stage up to the point at which a closed carr of other species



328 Ecology of Alnus glutinosa

Table 1. Effect of soil water and base content on the subordinate species of alder communities

Increasing water

1 2 3 +

Ilex aquifolium Betula pubescens Salix capraea Betula pubescens Molinia caerulea Myrica gale Sorbus aucuparia Sphagnum spp. Molinia caerulea Agrostis spp. Carex binervis Narthecium ossifragum

a Anthoxanthum odoratum Juncus spp. Eriophorum angustifolium Galium saxatile Luzula multiflora Sphagnum spp. Dryopteris spp. Dryopteris spp. Pinguicula vulgaris Pteridium aquilinum Cirsium palustre Potamogeton polygonifolius

Fraxinus excelsior Fraxinus excelsior Salix spp. Corylus avellana Quercus spp. Ligustrum vulgare

, Prunus spinosa Crepis paludosa Hydrocotyle vulgaris r Oxalis acetosella Chrysosplenium oppositifolium Caltha palustris b Prunella vulgaris Myosotis scorpioides Filipendula ulmaria

Agrostis stolonifera Lysimachia nemorum Mentha aquatica d Poa trivialis Athyrium filix-femina Epilobium hirsutum

Deschampsia caespitosa Dryopteris spp. Galium palustre i- Luzula maxima Carex remota

Sambucus nigra Betula pubescens Salix atrocinerea Hedera helix Salix atrocinerea Carex acutiformis Urtica dioica Viburnum opulus Eupatorium cannabinum

c Galium aparine Calamagrostis lanceolata Filipendula ulmaria Ajuga reptans Eupatorium cannabinum Phragmites communis Bryonia dioica Phragmites communis Solanum dulcamara Glechoma hederacea Thelypteris palustris Thelypteris palustris (Bare ground) Solanum dulcamara Peucedanum palustre

Carex acutiformis + Lysimachia vulgaris

Community la corresponds to alder-birch wood on siliceous grassland slopes, seasonally wet due to an impervious subsoil, but with no tendency to peat formation. The woods are usually grazed. The Meall a'Bhata and Loch Treig woods are remnants of this type.

Community lb corresponds to alder-hazel wood on soils richer in bases, but again with impeded drainage. The woods are less often grazed and have a higher proportion of forbs, perhaps for this reason. The Afon Ddu wood approximates to it.

Community c corresponds to the drier types of fen woodland produced by drainage of fen carr or by planting alders on drained fen peat or other base-rich soil. Many of the valley fen woods of the East Anglian Breckland are of this type.

Community 2a corresponds to alder-birch wood on slopes more continuously flushed by water poorer in bases than la, and showing a tendency to acid peat formation in places. This is exempli- fied by many hill alderwoods at intermediate altitudes, e.g. along the west bank of Loch Lomond.

Community 2b corresponds to base-rich and moist ash-alder wood. Fragments of this type are to be seen in Nant Gwynant, North Wales, and along the Great Glen of Scotland.

Community 2c corresponds to typical fen carr (Lambert, 1951).

Community 3a corresponds most closely to the Finnish lettokorpe and lettorame (Jallas and Okko, 1951) which are often hummocky, scrub-covered acid fens. In Britain only traces of the community can be found, since the present regime of land management, with its repeated burning and continuous grazing, prevents its formation on the Molinia-Myrica bogs that approximate to it.

Community 3b corresponds to alder-willow carr or marsh alderwood developed on waterlogged but less base-rich sites than fen communities, and with no tendency to fen or acid peat formation. The Matley and Lyndhurst carrs approximate to it in places.

Community 3c corresponds to typical swamp carr (Lambert, 1951).



D. N. MCVEAN 329

has been established. Alder carr does not succeed an earlier Salix or Rhamnus carr, though these species may all colonize simultaneously, and pure alder carr eventually results from the greater vigour and longevity of the alders.

PRIMARY SUCCESSION SECONDARY SUCCESSION ATER

MOWING MARSH, ETC- r FLOATING MATN TMARSH -- -

(C. ACUTIFORMIS SILTING MOWIN SOIDE C. PANICUMNLATA MOLINIA-MYRICA

OR GRAZING -- SOLID FEN C PANICULTA ? I FEN ALDER CARR- ALDER CARR j ALDER SCRUB

^OALDER >MIXED WOOD (CLIMAX) 0O ALDER-WILLOW ? WOOD (CLIMAX) ,, CARR%? RAISED

4

4 II

CLIMAX MIXED WOOD BO

SOILS OF IMPEDED DRAINAGE

FEN SUCCESSION L FLUSH GRASSLAND ALDER WOOD I (CLIMAX) (ABOVE) ILS ORA t

Fig. 1. The place of alder in marsh and fen successions. (Based on the work of Lambert (1951) and Pearsall (1918).)

The possibility of climax alder carr must remain undecided. It is probable that, with constant variation in land and sea levels the theoretical autogenous succession seldom exists. The observation made at Heron's carr suggests ways in which climax alder carr might come about, the essential feature being a balance in the rate of accumulation and oxidation of plant remains to leave a sufficient area of colonizable surface above water, and edaphic conditions unfavourable enough to dwarf the trees and allow sufficient light penetration for regeneration from seed. It is thought that

vegetative regeneration alone would not suffice to perpetuate the community. Godwin and Bharucha (1932) point out that peat destruction may predominate under Salix and Rhamnus carr.

The breakdown of Lambert's swamp carr to open water is regarded as a distinct

possibility, even in the absence of land subsidence or marine transgression. Parts of

Hedney's Bottom form the nearest approach to this at the present day. Invasion by the alder of pure Phragmitetum, Cladietum, etc., is slow, but more

opportunities for colonization are given by mixed communities, largely for mechanical reasons, e.g. the tops of Carex paniculata tussocks on floating Phragmites mat, and the

funnel-shaped depressions made by Phragmites stems in Molinia litter. For the same reasons fen successions, deflected by grazing, mowing, etc., favour more rapid alder colonization.

The alder is not adapted for the invasion of grassland, either directly or after a shrub phase. Neglected grass in which drainage has become interrupted and given rise to seasonal surface wetness is, however, ideal.

White (1932) ascribes the lack of bush colonization on the fens of north Armagh to the extremely wide fluctuations of the water-table between winter and summer



330 Ecology of Alnus glutinosa levels. Where the water-table is slow to fall in the spring this could prevent the colonization even of alder and willow, and similar phenomena in other areas may help to account for the absence of alder in apparently suitable habitats.

SUMMARY

A description of certain alder populations is given which illustrate points in the ecology of the species.

Some notes of floristics and succession are presented: Table 1 and Fig. 1 summarize the relationships of the ground flora of alderwoods to soil water content and base status, and the part played by alder in hydroseres and on soils of impeded drainage.

REFERENCES BROOKS, C. and GLASSPOOLE, J. (1928). British Floods and Droughts, p. 177. London. GODWIN, H. and BHARUCHA, F. R. (1932). Studies in the ecology of Wicken Fen. II. J. Ecol., 20, 157. HOCK, F. (1897). Pflanzen der Schwarzerlebestande Norddeutchlands. Bot. Jahr. Engler, Bd. 22. JALLAS, J. and OKKO, V. (1951). Botanical and geological analysis of the Cladium mariscus at Joroinen.

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(Received October 1st, 1954)'

