The Birch Family - Betulaceae

 

The Birch Family consists of six genera of deciduous trees and shrubs mainly occurring in the temperate climates of the Northern Hemisphere but with a couple of species (of Alders) being found down the Andes into the Southern Hemisphere. In general they are found as pioneer or early succession species in their various habitats and thus often depend for their success on developing symbiotic relationships. Some Birch species are adept at surviving in cold, hostile environments.

 

From the seed propagator’s point of view many have evolved their reproductive strategies to be opportunistic and to take account of the various possibilities that the dispersed seed may encounter – so that more than one option may be available.

 

The Family is, currently, divided into two sub-families the Betuloideae (Alnus and Betula) and the Coryloideae (Carpinus, Ostrya, Ostryopsis and Corylus).

 

Betuloideae

 

This sub-family (Alnus and Betula) has developed similar seed germination strategies in both genera – this allows the seed to respond to the particular micro-environment into which it is dispersed at any particular time. Both genera also, being chiefly pioneer species, have developed symbiotic relationships to suit their particular environmental niches – many Betula species rely on mycorrhizal associations and Alnus, because of its adaptation to wet situations, has developed an association with the nitrogen fixing actinobacterium Frankia in root nodules.  

 

Betula

 

The Birches (Betula) are an extensive genus of some fifty to one hundred species – depending on the taxonomic treatment of the genus. The genus has a circumpolar distribution in the Northern Hemisphere – ranging from the sub-arctic to virtually the subtropical (but there, at reasonably high elevations in more or less warm temperate situations). The species from temperate climates are mainly pioneer species and are relatively short lived.

 

Three species are native to the British Isles and the two tree types are commonly produced from seed – for specimen planting, as rootstocks for the ornamental variants and for sylvicultural use. Many species with valuable ornamental characteristics, have been introduced to cultivation and are found extensively in Parks, Gardens and Arboreta in the UK.  The ornamental value is in their usually medium size and lightweight structure of stems and leaves, the highly coloured bark and autumn foliage colours. There are specialist monographs on the genus which will validate the niceties of nomenclature.

 

Birch species tend to flower before foliage bud break and the flowers (the inflorescences are elongated, pendant and cone-like) are wind pollinated. The seeds of Birch are produced in cone-like structures described as strobiles. These mature in the late autumn when they dry out and break up while still on the tree. Thus the seeds are dispersed in the wind.  Each tree flowers profusely at an early age and can produce very large quantities of seed. The seeds are small, lightweight and usually brown when mature and are winged. Seed collection is best achieved, therefore, just prior to the stage at which the strobiles begin to disintegrate. In practice it is possible to collect the cones when they are still greenish. These are spread out on a dry sheet in a shallow layer (to prevent overheating) and gently dried until they break up when rubbed. To some extent the seeds can be separated from much of the cone scale by fanning but the resultant sample will still contain a fair amount of debris; thus it is inevitable that making seed counts will be difficult and tedious.

 

The seeds do not retain good levels of viability for long and the seed is best processed and sown soon after collection. The seeds can be stored (in the short term) in an airtight container, at cool temperature, for several months without detriment.

 

The individual seeds are small and nut-like and the viability can be variable and difficult to assess. Hence in practice sowing the seed, plus the accompanying debris, is usually achieved by broadcasting it fairly thinly onto a seed tray so that the seedlings can be pricked off into plugs or suitable containers soon after germination. Thus growth is not constrained by a high seedling density – should it occur.

 

Although the seeds of Birch are normally dispersed in the autumn the seeds are often liberated sparingly over time during the early part of the winter and for various reasons may face a variety of environmental scenarios in terms of priming (the elimination of endogenous dormancy) to allow germination in the spring.

 

The successful germination of Birch seeds in nature is therefore governed by a number of alternative strategies – which the plant has derived to take account of the particular conditions which are experienced in the natural environment in which a particular individual seed is sited in that particular season. This reflects where the seed lands (ie the influence of shade), whether it imbibes relatively quickly and whether a sufficient chill occurs (the potential insulation effected by snow cover). Thus the imbibed seeds respond with the particular strategy which reflects the environmental conditions that the seed experiences – all, however, are organised to allow germination to occur in the Spring. The three factors which influence success therefore involve chilling, the presence/absence of an adequate light intensity and the provision of a high threshold germination temperature.

 

1. The autumn dispersal of seeds and the consequent exposure of the imbibed seed to chilling allows germination in the Spring without further intervention – hence if the imbibed seed is chilled artificially and then sown in controlled conditions and covered (ie with the exclusion of light) it is possible to achieve germination at conventional temperatures (<15ºC).

 

2. If the seeds are dispersed in the autumn, become covered and do not receive a sufficient chill then germination fails - however the seeds may well germinate when the soil temperature eventually exceeds 20ºC.

 

3. If the seeds are dispersed to an area without shade and do not become covered then they will germinate at conventional temperatures without prior chilling (ie after an exposure to a sufficient period of an adequate light intensity).

 

It is probable that the temperature threshold for chilling will vary according to the parameters of the climate experienced by the species. Those species from northern provenances will require temperatures of around 3ºC for a period, this period is usually recommended as 84 days for the achievement of a satisfactory germination - although as little as 56 days has proved effective with some species. Those species from warm provenances appear to respond at temperatures as high as 7ºC for as little as 28 days.

 

For a universal treatment it is probable that chilling the imbibed seeds for a period of 56 days at a temperature of 3ºC, sowing the seeds onto damp compost in a seed tray, covering with grit and then providing a minimum temperature of 15ºC will achieve germination in less than 30 days. As the viability is variable and the ability to make a reliable seed count is difficult, it is prudent to be prepared to prick off the seedlings very soon after germination especially if the density of seedlings is high. It is thus prudent to ensure that the germination medium is of a suitable consistency for the effective transfer without damage to the root system.

 

Birches are generally pioneer species and are often fairly heavily dependent on the development of mycorrhizal associations for successful growth rates. Thus it is sensible to inoculate the compost with suitable mycorrhizae or site the seedlings in a situation where spore rain could provide inoculation.

 

As befits pioneer species the rate of growth of seedlings in their first season of growth is relatively substantial – it is necessary to get some leaves into a good position for photosynthetic activity.

 

Ashburner, K and McAllister, H A (2013) The Genus Betula; Kew Publishing.

 

Alnus

 

The Alders (Alnus) are a, relatively, small genus of about thirty species of, chiefly deciduous, trees and shrubs which are distributed throughout the temperate climatic regions of the Northern Hemisphere but also occur down the Andes into Argentina. In general the species are adapted to growing in wet situations where they are often one of the pioneer woody plants. The Alders are not, in general, as decorative as the Birches but they produce good, strong green foliage which makes them eminently suitable for windbreaks and has the advantage that they are remarkably free of pests and diseases – especially those which affect fruit trees - and hence they have value for protecting orchards.

 

The native Alder (Alnus glutinosa) is found prolifically along water courses and in wet places where it can develop into a substantially sized tree.

 

The male and female flowers are pendulous catkin-like structures which are produced separately on the same tree during the autumn but which do not open until just before, or at, bud break in the Spring and are then mainly pollinated by wind. The female flowers develop into the cone-like fruits known as strobiles which contain the seeds - unlike the strobiles of Birch, those of Alder are woody and the cones are retained on the tree where they open as they dry during the autumn. The seeds are shaken out by the wind and this can occur continuously throughout the winter. Alders tend to produce prolific seed crops from an early age. The strobiles remain on the tree for some time after seed dispersal.

 

The most effective way to collect the seed is to cut those branchlets carrying a heavy population of intact strobiles just before they open and then hang them up to dry - in a paper sack - in a warm, dry environment. With such treatment they will open readily and the seeds can then be shaken out into the bottom of the sack. As the cones do not break up the sample produced in this way is reasonably clean. Seed is produced prolifically and - as might be anticipated - usually has a fair amount of void seeds in the sample.

 

The seeds are small nut like structures with a small wing and when ready to be dispersed are red-brown in colour.

 

Alders have developed, effectively, the same strategies as the Birches for ensuring germination of at least some of the seeds in the Spring which, after dispersal, finds its way to the soil and becomes imbibed:-

 

1. Seeds, which have not been subject to exposure to cold but have been exposed to light, will germinate when temperatures reach 10˚C however at this low level germination is slow, erratic and with only a small proportion of the sample responding; at higher temperatures the speed of germination increases, the emergence becomes more uniform and a greater proportion of the sample responds. At temperatures above 20˚C germination becomes commercially acceptable insofar as emergence is both rapid (c14 days) and reasonably synchronised, with a fair proportion of the seeds responding. Thus seed germination is delayed until the advent of suitable conditions for survival.

 

2. Seeds which have been covered or sufficiently shaded to prevent sufficient light accessing the seed will germinate when the soil temperature reaches 20˚C or more.

 

3. If the seed is chilled by exposure to cold then subsequent germination at lower temperatures is improved - but most satisfactory results still come from temperatures at around 20˚C when emergence levels show the best and most economic response. The period of chill and the threshold temperature are not fixed parameters as even relatively short periods of chill (eg 21 days at 5˚C) will achieve some improvement in germination levels; however as the period of chill is increased and the threshold lowered to 2 to 3˚C then responses improve dramatically – the most economically effective response appears to be 56 to 63 days at 3˚C although longer periods will still exhibit some improvement. At this level of chill the subsequent temperature range at which improved emergence will occur becomes lower although the best results will still occur in the range 15 to 20˚C.

 

Thus imbibition of the seed followed by 56 days stratification at 3˚C and then germination at 18˚C will usually cause emergence in 14+ days. This protocol seems to work well for the cold temperate species.

 

Those species which occur in warm temperate climate areas (eg A. rhombifolia and A. rubra) will respond to shorter periods of chill with higher threshold temperatures – however in the particular parts of their native habitats without cold winters, they are more likely to respond to the quick rise in soil temperatures in the spring - which can quickly reach 20˚C in the early part of the year.

 

All Alnus species depend for their normal pattern of growth on developing a symbiotic relationship with the nitrogen fixing actinobacterium Frankia. This bacterium causes the development of large, easily visible, nodules on the roots of the tree. Care should be taken to ensure that the compost is suitably inoculated.

 

Seedling growth in the first season is usually substantial if space or nutrition is not limiting.

 

Coryloideae – the Hornbeams, Hop-Hornbeams and Hazel

 

The Hornbeams (Carpinus) and the Hop-Hornbeams (Ostrya) together with Ostryopsis and the Hazels (Corylus) make up the Coryloideae – one of the two sub-families of the family Betulaceae.

 

The first two genera are dealt with together here as the seed construction and treatment is similar in both genera. Both genera are deciduous and produce good autumn colour – usually in shades of yellow. The wood of both genera is heavy and hard and is sought after for a number of uses where durability and ‘hardness’ are needed.

 

Carpinus and Ostrya

 

The genus Carpinus contains some thirty species, of chiefly small deciduous trees, which occur across the Northern Hemisphere with the majority being native to China and Eastern Asia. Carpinus betulus (the European or Common Hornbeam) occurs widely in Europe including the British Isles; it is used as a hedging plant, for coppicing and for the production of rootstocks for grafting the many ornamental variants. Other species which are cultivated in the UK as ornamentals or as arboretum subjects include C. caroliniana from the Eastern United States, C. japonica from Japan and the very decorative C. fangiana from China which has only been introduced in the last twenty years or so.

 

The pendulous male and female catkins are produced separately on the tree during the early Spring and pollination is by wind. The more or less erect fruiting cluster is made up of several (10-30) nutlets each in a samara-like fruit with an asymmetrical wing. Each seed is 3 to 6mm long and has a hard seed coat when fully mature.

 

The genus Ostrya consists of eight or so species of small, deciduous trees which are native to Southern Europe, parts of Southern Asia and North America down into Central America. They generally produce a conical, sometimes spreading, crown and can become decorative trees for use in areas with limited space. The leaves are similar to Birch.

 

The male and female flowers are produced separately and are pendulous, catkin-like structures which are produced in the Spring and are wind pollinated. The fruits are usually towards the end of the branchlets and consist of an elongated, pendulous cluster of six to twenty seeds, each of which is a small nut (2 to 4mm long) enclosed in a samara type fruit – when green the whole cluster resembles the Hop fruit and thus gives it its common name. Ostrya carpinifolia from Southern Europe, the American O. virginiana and O. japonica from Japan will all succeed well in the UK. Other species may succeed in mild niches but generally are not successful otherwise.

 

Fruiting of all the species is generally prolific with each tree producing vast number of seeds every year - however as is common in such cases the number of void seeds is often considerable.

 

The fruiting clusters, of both of these genera, if collected whole when mature, will break up into individual seeds quite easily after marginal drying. The fruit is a nut encased in a samara - the wings of which are slightly twisted to increase the range of dispersal on the wind. The samaras are borne in clusters towards the tips of the branchlets, When they are ripe the clusters disintegrate and the individual fruits are dispersed in the late autumn (+/-November) by wind. The samaras are initially green then change to yellow and as they dry out become brown. At this stage the nut will have developed a hard case - which under natural conditions, in the soil, takes a summer season to degrade to a condition which will allow water to be imbibed. This strategy delays germination until the second spring after dispersal as the embryo also exhibits an endogenous dormancy condition which requires a period of chilling for mitigation.

 

Degrading the hard seed coat can be achieved by a warm stratification but the period required will depend on the degree to which the seed coat had dried. Separating the seeds from the fruits sufficiently to use a digestion process is too tedious. Thus the prudent option is to collect the seeds when they are still green to just changing to yellow and prevent any further drying - at this stage the seed coat will permit the passage of water and the seeds will imbibe and the chilling process can be undertaken straight away. If the seed has already advanced into some degree of hardness then this can usually be dealt with by giving the seed a warm moist stratification (c20˚C) for a month.

 

The chilling requirement, for those species which have a temperate distribution, is reasonably constant although, as might be anticipated for subjects which produce a hard seed coat the recommendations found in the literature vary considerably – largely, probably, because the seed coat has not been completely degraded at the beginning of the treatment. However experience and reliable sources would suggest that a period of 63 days at 3˚C will usually be sufficient.

 

The seeds are big enough to handle and sow individually - however as the separation of viable seeds from the detritus and void seeds is not easy it is sensible to sow the seed broadcast onto a seed tray of suitable compost and cover with grit so that the seedlings can be lifted and potted on into a suitable container as soon after germination as is feasible.

 

Seeds of these species will germinate most satisfactorily at temperatures in the region of 20˚C.

 

Corylus

 

This genus Corylus is a member of the Family Betulaceae and the sub-family Coryloideae and is thus closely allied to the hornbeams and hop-hornbeams but differs significantly in the size of the seed.

 

Corylus consists (depending on its taxonomic treatment) of about fifteen species of deciduous, small trees and shrubs. They occur in the temperate climates of the Northern Hemisphere through Europe, Asia and North America – some of the species are morphologically very similar and differ only in their distinct geographical distribution – thus they form discernible groups:-

a) multi-stemmed shrubs with a leafy sheath to the nut - C. avellana (Southern Europe), C. heterophylla (Asia), C. americana (East and Central North America), C. californica (South West United States), C. yunnanensis (China);

b) multi-stemmed shrubs with the leafy sheath much longer than the nut - the ‘Beaked Hazels - C. cornuta (North America), C. sieboldiana (Eastern Asia), C. maxima (Southeastern Europe), C. colchica (Caucasus);

c) single stemmed, small trees with a stiff and bristly sheath to the nut - C. colurna (Western Asia), C. jacquemontii (The Himalayas), C. chinensis (Central Asia), C. fargesii (Western China), C. wangii (Southwestern China) and

d) multi-stemmed shrub with the nut enclosed in a fruit similar to a chestnut - C. ferox (syn tibetica) (Himalayas to Southwestern China).

 

The species are readily grown from seed. Seedlings of the relevant species may be required as rootstocks for grafting the decorative variants although these can be produced by layering which obviates any rootstock suckering problem. The Hazel Nuts, Cob Nuts and Filberts have a small but important niche in fruit production in the UK and for this purpose it is necessary to produce rootstocks for grafting the relevant varieties.

 

The Hazel Nut (Corylus avellana) is native to the UK and occurs widely in hedgerows and woodlands – it is often grown as a coppice to provide the flexible shoots which are used for basket making but also for use as pea sticks and stakes.

 

The various alien species can produce seed crops in the British Isles but the summer is often not warm enough to produce good enough crops every year to ensure the required supply. However supplies of reliably viable seed can be obtained from commercial sources although it will inevitably have been dried - which may have implications for the dormancy controls developed.

 

When suitable and adequate seed crops become available they can be collected in the autumn by picking up from the ground around the parents – however the critters also find them attractive and they generally disappear very quickly. Hence it is normal to pick the seeds before they become attractive to predators. Soundness of the seed is usually good although in some years weevil infestation can be significant – damaged seed can be separated by flotation. The seeds can usually be stored, after marginal drying, for two or three years at c3˚C. However drying of the seed can cause the erratic development of dormancy conditions which will be reflected in the eventual germination success.

 

The fruits consist of a nut with a hard pericarp enclosed to some degree in an involucre which may be soft and leafy or rigid and spiny. The removal of this involucre will be determined by the maturity of the seed at collection or dispersal - ie as it dries and separates from the nut.

 

To avoid difficult dormancy problems it is best to harvest the seeds in the ‘half-brown’ stage – which implies about August; at this time the embryo and food reserve has been completed and the pericarp is hardening and the involucre is just beginning to turn brown. At this stage neither the embryo nor the endosperm has developed any endogenous dormancy control. The seeds can thus be stored as collected without any further drying (ie under water conserving conditions) at 2-3˚C for several weeks and then subjected to suitable temperatures for germination.

 

Seeds collected at maturity may have developed a sufficiently hard seed coat to prevent imbibition and will have begun to develop a chilling requirement – the degree of this can be variable and will probably need 84 days at 3˚C before the condition is mitigated.

 

Germination of the seed is most prolific at around 20˚C.