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The taxonomic position of this monogeneric Family, based on DNA analysis and assessment, is still actively discussed. The debate centres on whether it should be included in the Sapindaceae or whether it merits a separate status. Currently it is separated – containing only the genus Acer and with the often associated genus Dipteronia, still, being included with the Sapindaceae.

 

Acer

 

The Maples (Acer) are a numerous and diverse genus of woody plants – currently about 150 species are recognised but some of these demonstrate wide variability. For taxonomic purposes the genus is divided into about sixteen Sections - which has the advantage of grouping similar species - and hence their possible seed dormancy breaking treatments - together.

 

The genus occurs throughout the Northern Hemisphere from sub-arctic conditions to sub-tropical conditions, occurring south of the equator only in South Asia. The genus contains species which have evolved to occupy specialist environmental niches to species with conventional distributions and to a few species (eg A. saccharum) which have an extensive geographic range and which vary clinally, quite dramatically, throughout the range.

 

They are all deciduous but a few species (all from warm temperate climates) do not absciss their leaves until the new crop is formed and these are described as semi-evergreen.

 

The species, covered within these notes, are chiefly limited to those which occur in conventional temperate climates.

 

The fruit of this genus consists of a one seeded, winged samara (often in conjoined pairs and occasionally in threes); flowering is normally in the spring with the fruits maturing in the autumn and generally dispersing quickly. The seed is usually short lived which is a typical characteristic of seeds which store their food reserves predominantly as non-carbohydrates. Long term storage is therefore problematical – it is suggested that a relatively slow drying procedure is the best option - as this, in effect, probably causes less damage to the microscopic structures of the seed, nevertheless rehydration is, even then, usually less than satisfactory at best. Short to medium term storage is achieved by sealing fresh, ripened fruits (at the dispersal stage) in a polythene bag, to conserve moisture, and then maintaining the sample at a low temperature (3̊C). The food reserve is stored in the cotyledons and there is no endosperm.

 

The production of viable seed crops in many species, when grown in cultivation in the UK, in relation to individual plants, is a matter of conjecture as many of the species are functionally dioecious and therefore need more than one plant, of a distinct genetic constitution, for cross pollination to occur. Other species are monoecious with the sexes being located in different parts of the tree and then sometimes with the different sex flowers maturing at different times. It is for these reasons that it is often prudent and necessary to obtain seeds from native stands or single species groups of seedlings in cultivation.

 

There are a number of groups of species, within the genus Acer, which are closely related and if the fruits are collected from specimens, within collections, where related species are growing within close proximity, the genetic status of the seeds should be treated with suspicion - as there is a high risk of hybridity occurring in the offspring.

 

The majority of species do not present any difficulties in the production of seedlings although virtually all of those species which disperse their seeds in the autumn exhibit an endogenous embryo dormancy which requires a period of chilling for its elimination.

 

As would be expected for such a numerous genus, there has been little critical observation conducted on the chilling requirement of most of the species. The results from those which have been assessed (and these are chiefly species from temperate to cold temperate distributions) seem to suggest that a period of 84 days at 3ºC provides the optimal treatment for a rapid and synchronised emergence when an acceptable temperature for germination is reached - although shorter periods may achieve the effect, the emergence at germination is often erratic and certainly slower.

 

The embryo, of many of these cold temperate species (eg A. saccharum), will develop to the stage of radicle emergence at the same low temperatures once the cold stratification treatment has been completed ie after the dormancy control has been eliminated. It is therefore important to monitor the progress of any sample which is being stratified so that the sample can be transferred, if the radicle does emerge, to suitable temperatures for efficient germination, this is especially relevant if no previous experience is available as to time scales. It is probable that the radicle will only develop very slowly if maintained at low temperature but it would not be prudent to leave seeds in such a condition as they will inevitably deteriorate in some degree. In general Maples germinate most effectively and quickly when provided with an environment of 20˚C.

 

The threshold temperature for chilling has not been critically assessed for most species and it is quite possible that species from warmer environments (eg A. paxii, A. pectinatum and A. sikkimense) will respond at higher temperatures - the threshold for these species could be as high as 7ºC or possibly even 10˚C.

 

The germination of those maples (eg A.campestre, A. griseum, A. pensylvanicum and A. circinatum) which develop a significantly impermeable seed coat to water uptake, as part of their reproductive strategy, is normally delayed until the second, or later, spring after an autumn dispersal. It requires the warmth of at least one summer, in the soil, to enhance the degradation of the seed coat to a level at which the seed is capable of imbibing and when it will then respond to the chilling period provided by the following winter – which, in turn, will then eliminate the endogenous embryo dormancy.

 

Experience, however, has suggested that this attenuated time scale could be mitigated, by the propagator, by collecting the fruits (samaras) at an immature stage ie before the case has hardened to a stage at which imbibition is prevented. The seed will have developed to a complete, quiescent and mature condition relatively early in the season and it is when this stage has been reached that the hard seed coat begins to develop – if the fruits are collected at this crucial, early stage the hard seed coat can be avoided. This is usually when the samaras are just changing from green to yellowish green. It would appear that the samaras are still transferring photosynthate to the seed while they remain green. At this stage, when collected, the seeds should be stored under water conserving conditions (so that any further drying is prevented) – subsequently the seeds can be successfully imbibed. They can then be subjected to a chilling treatment with the expectation that germination will occur in the following spring.

 

Section Rubra

Those species in the Section Rubra (A. rubrum, A. saccharinum and A. pycnanthum) which flower very early in the year and set their fruit in the summer (under normal conditions) exhibit a transient viability which is an indication that once the fruits are shed, and find suitable conditions, germination is almost immediate. Thus there are no dormancy controls. Under normal nursery conditions the seeds are sown fresh and without any drying and will germinate rapidly and uniformly. The storage of these seeds is therefore not worthwhile and fresh seed needs to be sourced each year - however A. rubrum has been successfully stored when fresh seed is sealed in a polythene bag (in order to maintain the water status) and stored at low temperature (c2ºC), under this regime satisfactory viability has been retained for as much as twelve months. Acer rubrum has a considerable geographic range and the dormancy controls from north to south of the distribution varies dramatically - the northern distributions requiring a chilling period before germination will occur.

 

Section Macrantha

The Snake Bark Maples Section consists of up to 18 species. It occurs across Eastern Asia from the Eastern Himalayas, through China and Taiwan, to Japan, with one, eccentrically situated, species in Eastern North America (A. pensylvanicum). This grouping contains some highly ornamental small trees and large shrubs – all of which, in some degree, exhibit the characteristic striated bark patterns and many also develop a red colouration to the shoots and foliage during the spring. The greatest concentration of species is in Japan – A. capillipes, A. caudatifolium, A. crataegifolium, A. micranthum, A. morifolium, A. rufinerve A. tegmentosum, and A. tschonoskii. They are of varying hardiness – depending on their provenance – A. morifolium (which is rare in cultivation) and the very ornamental A. tegmentosum are from southerly latitudes and require a mild niche to succeed in the UK. Acer laxiflorum occurs in Western China, A. davidii (which now includes A. grosseri and A. hersii – although these latter two variants ‘come true’ if collected from single ‘species’ stands) and A. maximowiczii are both found in Central China and A.forrestii in Southern China. Acer pectinatum and A. sikkimense (syn A. hookeri) are both from the Eastern Himalayas and are usually found in warm temperate situations and would be regarded as verging on the half hardy - although there has been an introduction in recent years (from a high elevation) of an apparently more cold hardy provenance of A. pectinatum. This group of Maples shed their fruits during the autumn and do not develop a hard seed coat as a normal process - however if, when, the seeds are collected and allowed even to marginally dry then a hard seed coat develops which, although not intractable, is sufficient to delay imbibition; if the seeds are not dispersed quickly they tend to dry on the tree and develop similarly. Thus the usual advice to achieve their successful propagation is relevant - ie collect the fruits well before they mature, while the samaras are still yellow-brown and process them without drying. If a dry seed coat has developed it can usually be mitigated by a warm water soak. A. pensylvanicum is exceptional in that, in its natural environment, it develops a hard seed coat, which will delay germination for at least a year, as a matter of course. The solution is necessarily to collect and process the seeds as the samaras reach the stage at which they begin to turn yellow. All the species from cold temperate provenance will respond to a chilling procedure of 84 days at 3˚C. The species from warmer provenances will respond to 56 days at below 7˚C.

 

Section Trifoliata

The trifoliate maples which occur in two series (Acer griseum, A. maximowiczianum (A. nikoense) and A. triflorum and secondly A. mandschuricum (with A. sutchuenense) present a particular issue insofar as the extremely hard, woody seedcases of the samaras are often void and the number of viable seeds produced is often extremely low (less than 10%). This may be a natural quirk or may be due to the self incompatibility of single specimens and even where seed is collected from a group of trees, in cultivation, they may be grafted and therefore likely to be clonal - so the same result applies. Even if samples with acceptable levels of viability (< 30%) are found the seed case is very hard and intractable and will require substantial warm stratification to overcome the problem. The embryo when exposed to water and imbibed still exhibits an endogenous dormancy needing at least 84 days (at below 3˚C) of chilling to overcome. This group represents another classic instance of the need to attempt to avoid seed coat dormancy by collecting the samaras at a very early stage; maintaining in a moist condition and warm stratifying immediately if the case is already beginning to harden.

 

Section Platanoidea

This Section occurs through Europe and Asia in cold temperate climates and is recognised a) by the production of a milky sap and b) the seedcases which are round and flat - A. ambiguum (often given as part of A. pictum), A. amplum (Central China), A. campestre (Western Europe), A.cappadocicum  (Cauacsus to the Himalayas), A. lobelii, A. miyabei (Japan), A. pictum (syn A. mono) (widespread in China, Manchuria, Japan and Korea), A. platanoides (widespread through Europe and the Caucasus) and Acer truncatum (Northern China), inter alia, all belong in this grouping.

 

Acer campestre and A. truncatum both exhibit a hard seed coat condition as a normal course of maturation and although warm stratification will overcome the problem it is easier to harvest the seeds as the samara colour turns from green to yellow and avoid the issue. All the species have a chilling requirement and this is provided by 84 days at 3˚C. The remaining species should not develop any impermeability of the seed coat if they are harvested as soon as they are mature and either stored or sown without further drying. If the seed coat has dried it can usually be made permeable by soaking in warm water.

 

Section Acer

This Section is the second largest and is divided into three series. The first series contains the various Sycamore look-alikes - Acer caesium, A. giraldii, A. heldreichii, A. pseudoplatanus, A. trautvetteri and A. velutinum (inc A. vanvolxemii). The second grouping consists of a series of species occurring around the Mediterranean basin – Acer granatense, A. hyrcanum. A. monspessulanum (inc A. trilobatum), A. obtusifolium (inc A. syriacum), A. opalus and A. sempervirens – and all are adapted to that particular climatic biome.

 

The species from these two groups do not develop any significant hard seed coat problem even with drying but it would nevertheless be prudent to soak the seeds in warm water to ensure imbibition. They will all need some degree of chilling (at 3˚C) but for the warmer climate species this will be shorter (c56 days) than the cold temperate species (84 days).

 

The third series is limited to the various Sugar Maples. The Sugar Maple, Acer saccharum, exhibits a wide clinal morphological and physiological variation across a substantial geographical and climatic spread in North America. It is regarded as ‘typical’ in the North East where it is a major contributor to the ‘fall colour’ in that region – it becomes (either as a separate species or as a sub-species) A. leucoderme in Georgia and the Piedmont region across into Eastern Texas and A. floridanum (=A. barbatum) in the far South East; a variant A. nigrum occurs in the Mid-West and a disjunct population of a drought tolerant variant (an ecotype), the ‘Caddo Maple’, occurs in Oklahoma; A. skutchii is found in southern Texas and adjacent Mexico and despite its provenance appears to be reasonably hardy. In the Rockies and South to Texas from the Big Bend to the Edwards Plateau this population is, nowadays, designated as a distinct species - A. grandidentatum.

 

These ‘species’ do not in general develop a hard seed coat which prevents imbibition but if the seed has been dried it would be prudent to give the seed a warm water soak to ensure imbibiton – especially A. grandidentatum which night just develop the condition to a significant level if the seeds are dried too extensively. The chilling requirement will vary according to provenance. At the most northerly A. saccharum requires 84 days at 3˚C and will germinate to radicle extension at that temperature as well - so that chilling samples should be regularly monitored. For the most southerly species (A. floridanum and A. skutchii) chilling for 30 days at 7˚C will suffice.

 

Section Ginnala

This small section comprises - A. aidzuense (part of A. tataricum?), A. ginnala (China and Japan), A. tataricum (South Eastern Europe and Asia Minor) and A. semenovii (part of A.ginnala?).

 

These shrubby species develop a hard seed coat and so should be harvested as the samaras turn colour from green. They are then processed without further drying and chilled at 3˚C for 84 days. Germination will be most effective at 20˚C.

 

Section Pentaphylla

This section consists of one series containing, inter alia, A. buergerianum, A. coriaciefolium. A. oblongum, and A. paxii (of which the latter three are semi-evergreen) and a series containing only A. pentaphylla. Only A. buergerianum would be described as reasonably hardy (although it does have a distribution which covers milder areas and where it will become semi-evergreen) – the remainder require a mild niche for successful cultivation in the UK.

 

The Himalayan and Southern Chinese, semi-evergreen species, all tend to be frost susceptible in some degree and therefore need a favoured site to succeed and fruit. Their warmer provenance dictates that the chilling requirement is less severe (56 days at 7˚C), germination will require at least 20˚C for a rapid emergence.

 

Section Palmata

This Section constitutes the largest segment of the genus and contains about fifty species which are divided into three Series.

 

The first series contains, inter alia, the well known Vine Maple, A. circinatum from Western North America and the Japanese A. japonicum, A. palmatum and A. sieboldianum – these species are generally cold hardy and are characterised by developing a hard seed coat condition.

 

These species benefit from collection at an early stage (ie before the seed coat hardens) and treating them, without delay, to a chilling regime of 84 days at below 3˚C. A temperature then of 20˚C for germination will see emergence in 7 to 10 days. 

 

The second series contains several less well known species which, in general, are not fully hardy - but the semi-evergreen, simple leaved A. fabri and A. laevigatum are seen in cultivation. If the seeds are maintained without drying and are then imbibed, they will respond to chilling for 56 days at below 7˚C and a germination temperature of 20˚C.

 

The third series is the most numerous and the species are found throughout Eastern Asia, they are also generally less hardy but A. campbellii, A. erianthum, A. flabellatum, A. oliverianum, A.sinense and A. wilsonii are seen regularly in cultivation in mild situations. The same remarks, relating to germination, as for the previous series apply.

 

Section Glabra

This section includes A. acuminatum, A. argutum, A. barbinerve, A. stachyophyllum and A. glabrum.

 

The available evidence suggests that these species are functionally dioecious and therefore more than one tree is required to ensure good seed crops.

 

These species will, in general, develop a sufficiently hard seed coat to delay germination for one year and it would therefore be prudent to collect the samaras at the immature stage – before imbibing and providing a chill for 84 days at 3˚C.

 

Section Indivisa

The Japanese A. carpinifolium, with its Hornbeam-like leaves, hardly looks like a Maple and is the only species in this Section. It has the potential to develop a hard seed coat and should be collected at an early stage before the pericarp hardens, it will then require chilling for 84 days at 3˚C.

 

Section Negundo

This section comprises three similar species which are however separated into two series – the first contains only A.negundo which is widespread in North America and the second which includes A. henryi from China and A. cissifolium from Japan. They are all large shrubs to small trees and all are dioecious. They flower and fruit profusely when both sexes of parent are present for suitable pollination.

 

The samaras are relatively small, narrow and linear. The samaras will dry sufficiently while on the tree, before dispersal, to develop a hard seed coat condition and this will be enhanced by dry storage.  Practically this can be avoided by collection of ‘green’ seed however if only dispersed or stored seed is available, it would be prudent to scarify and then warm water soak. In many samples the proportion of filled seeds is often low.

 

The imbibed seed will require a period of chilling (84 days at 3˚C) to permit germination.

 

Section Lithocarpa

This section includes A. diabolicum (Japan), A. franchetti (Central China) and A. sterculiaceum (syn A. villosum) (the Himalayas) in one series and A. macrophyllum (Pacific coast of North America) in a second. These species do not usually develop a hard seed coat and only require chilling for 84 days at 3˚C after a warm water soak.

 

Section Parviflora

This Section contains A. caudatum, A. spicatum, A, ukurudense, A. distylum and A. nipponicum. All the species have the potential to develop a hard seed coat condition and hence should be collected when the samaras are turning colour to yellow and then processed and chilled (84 days at 3˚C) without delay.

 

Summary

 

In summary the Maples show three possible dormancy scenarios:-

a) no constraints to germination when conditions are suitable;

b) the presence of a hard seed coat condition which can be intractable or as a result of marginally  excessive drying. This condition is best avoided by the collection of the samaras as the colour changes from green to yellow during the late summer to early autumn – this is the most suitable policy to adopt if the condition for a particular species is not known;

c) the existence of an endogenous embryo dormancy requiring a period of chilling to mitigate. All the species, despite their varying threshold temperatures and periods of exposure can be universally treated by chilling at 3˚C for 84 days and then monitoring the sample for radicle emergence.

 

The temperatures required for optimum germination will, in general, be in the range 16 to 22˚C with an economic, standard protocol being fixed at 20˚C.