Sycamore (Acer pseudoplatanus)

they are killed but as water losses are slow, seed desiccation is unlikely to kill many seeds in an oceanic winter climate. Seed viability in sycamore is as high as 95% and germination is optimal at temperatures between 10 and 15°C (Damian & Negrutiu 1973). In nursery conditions sycamore seeds have high germination down to a burial depth of 12cm (Petrovic 1956) and in grassland conditions seeds germinate well in damp situations but not in drought situations unless buried (Chinner 1948).

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Under natural conditions Southwood et al. (1988) stated that seeds germinated well in bare ground or in moderate vegetation as long as the soil is loose and damp. On compacted soils sycamore seeds readily germinate but the rootlet fails to enter the soil and the embryo dies. Such conditions are commonly found in soil with no litter and little soil organic matter.

 

None of the investigated features of the reproductive biology of sycamore appear to include a stage which seriously limits population size and growth of the species. Because in most years sycamore is such a prolific seed producer, but does not appear to spread in the countryside as dramatically as other species (e.g. Rhododendron ponticum), some stage in its life cycle must be limiting its spread.

 

As noted by Helliwell (1965) large crops of young (often only one month-old) seedlings often fail to survive. Sometimes, on the continent, it has been noted that very dense sapling banks disappear, whilst another one nearby did not (Dethioux 1970).

 

Slugs can kill seedlings by eating their stems (Dethioux 1970) and thus cause a very high mortality of seedlings. For instance, Helliwell (1965) found that in woodlands within a month of seedling establishment the majority had been killed, chiefly by slugs. He also noted that the sites where there was the smallest number of seedlings killed, were those with sandy and sandy-loam soils. Ashby (1959) found that heavy mortality of seedlings was caused by slugs at the end of a wet summer and rodents during the first winter. Out of 922 seedlings, 11% survived the summer and only 4% were alive at the end of the winter. Saplings suffer less slug damage because their stems are lignified and only the buds are attacked (Dethioux 1970). Pigott (1991, p 1186) reported that in 1983 sycamore saplings were severely damaged by grey squirrels (Sciurus carolinensis). The cotyledons and, less frequently, the young leaves and growth points of seedlings in old fields were destroyed by grazing, the damage being characteristic of slugs and snails, although small rodents could be important in this habitat (Southwood et al. 1988).

 

Davies & Pepper (1989), in a transplant experiment, reported that sycamore survival in rough grassland was 81% after two years and down to 42% after three years, but when a one metre circle was treated with herbicide 94% survived after three years. After two seasons 86% of saplings in rough grassland had suffered field vole (Microtus agrestis) damage which caused the high casualties observed among saplings. According to Davies & Pepper (1989) field voles, residents of rough grassland, can occur in very large but fluctuating numbers, but are confined to the height of the surrounding vegetation and do not climb. They strip and eat the bark of young trees. 

 

Many other factors affect the growth of seedlings and saplings and depending on their intensity can affect population size, they include:

 1. Seed size: There is a positive correlation between seed weight and seedling weight after 14 weeks (Helliwell 1965).

 2. Nutrients: Sycamore seedlings are very strongly affected by nutrient levels. Van den Driessche & Wareing (1966) found differences in dry weight of an order of magnitude of ten. Saplings grown in nursery or spoil ground grew much better in the close vicinity of Alnus species (Leibundgut 1976, Bradshaw 1989). This would suggest the importance of nitrogen as a limiting nutrient, but the conclusions of the experimental work carried out by Helliwell (1965) and Harrison & Helliwell (1981) clearly show that nitrogen is not important whereas soil phosphorus is an over-riding factor in controlling the growth of young plants.

 3. pH: In sandy soil two year old saplings grown for 82 days had an average height increment of 15.6cm at pH 7; at pH 5 and pH 6 it was 80% of that at pH 7, 68% at pH 4, and only 40% at pH 8 (Sebald 1956). To obtain a sandy soil of pH 8, CaCO3 was added up to a level of 3% and thoroughly mixed. Because sycamore saplings exhibited poor growth in this high pH soil with 3% CaCO3, which is typical of where sycamore is dominant (see Section 4) Sebald (1956) suggests that CaCO3 is detrimental to sycamore growth when thoroughly mixed which is unlikely to be the case in natural conditions. Results of seedling growth experiments in sandy soil at pH 4 by Helliwell (1965) contradict those found by Sebald (1956) as he observed no increase in weight. Although pH is important in growth, Harrison & Helliwell (1981) point out that soil phosphorus is the overriding factor and this could explain the differences in growth observed at pH 4.

 4. Water: According to Dethioux (1970) soils too wet or too dry are detrimental to seedlings. Helliwell (1965) found that seedlings under drought conditions for 20 days wilted. Early spring drought under grasses, as well as grazing, prevent the sycamore invasion of grassland by sycamore (Chinner 1948).

 5. Temperature: The impact of temperature on seedlings grown for three weeks indicates that sycamore seedlings exposed to intermittent low temperatures grew better and more individuals survived long exposure (6-12hrs) to frosty conditions (Damian & Negrutiu 1973). They also found that even hardened seedlings were killed after 4-5 hours at -5°C and this could explain why some years all young seedlings disappear early in the season (e.g. March).

 6. Light: Sycamore seedlings appear capable of adaptation to light intensity at least between relative levels of 25% to 100% (Wassink et al. 1956). Seedling dry weights increase logarithmically with increasing light intensity (Helliwell 1965), and similar increases in height growth (11 to 27cm), in the number of leaves (four to 16) and leaf area (100 to 440 cm2) have been found on two year old saplings at relative light intensities of 1% and 100% (Röhrig 1967). The development of the root system was similarly affected. No data is available on the impact of light quality and the effect of seasons. At most woodland sites growth ceases at light levels of 3% and there is little increase in growth with increasing light intensity, up to 20% level (Helliwell 1965). He suggested that the growth response to light only occurs in nutrient rich soils while Dethioux (1970) claimed that in good habitats light requirements appear to be low. In a Bavarian forest following shelterwood cutting old saplings exhibited strong reaction (in term of biomass increase) following light increases whereas a weak response was observed in young saplings (El Kateb 1991).

Like many other tree species, bud break of saplings takes place before adults (Grime et al. 1988) and this can favour the survival of saplings particularly under the canopy of late flushing trees (e.g. Fraxinus excelsior).

 7. Mycorrhizal associations: Mycorrhizal associations found in sycamore are Vesicular-arbuscular (VA) endophytes (Frankland & Harrison 1985). In woodlands Helliwell (1965) found that none of the seedlings showed the mycorrhizal root formation found in older trees. Garbaye & le Tacon (1986) innoculated potted seedlings with the fungus Glomus mosseae. They were planted out and after a year a difference in height growth of 10cm between seedlings with and without mycorrhizal associations was recorded. This initial difference remained after four years when the saplings had reached a height of 70cm. Research by Frankland & Harrison (1985) suggests that mycorrhizal associations are likely to be important in nutrient poor or chemically unbalanced soils, but Kabre et al. (1982) have pointed out that mycorrhization is important to seedling growth even in rich soils.

 8. Competition: Perennial ryegrass Lolium perenne inhibited root growth and decreased seasonal duration of growth in young sycamore (Richardson 1953) and it is said that sycamore does not do well against bramble (Rubus spp.) competition (Dethioux 1977). It has even been suggested that intra-specific root competition between young sycamores, particularly in dense stands, can be responsible for their death (Dethioux 1970).

Quantitative data on the impact of competition are available from sycamore sapling transplants into sites dominated by grasses carried out by Davies (1987). He found that in undisturbed control areas 1st year growth reached about 1.5cm, with mowing or one application of herbicide it was about 4cm, and with increasing herbicide applications height increment increased up to 40cm. Southwood et al.'s (1988) investigations of an old field succession showed that sycamore became established under a canopy of herbage (25cm high in summer) though few survived reaching a height of 12cm. These saplings were etiolated and had very small leaves. In a recently ploughed field where the herbage was less dense, saplings reached canopy height after three years.
It is clear that sycamore seedlings and saplings suffer from competition, but Buckley's (1984) work indicates that a critical level of herbaceous canopy and patchiness can improve both the survival and growth of established sycamore saplings. He carried out direct seeding on weathered chalk spoil and found that under a sparse cover, including Lolium multiflorum and several legumes, the performance of established seedlings improved.

 9. Interactions: As one would expect many of these factors can interact and some of these interactions have been examined.

     a) Above it was reported that soil phosphorus is the main growth limiting factor and that mycorrhizal associations increase growth. Kabre et al. (1982) showed that greenhouse mycorrhizal associations increased the coefficient of utilisation of phosphorus.

     b) At low light intensity (3.6% of full day light) seedlings do not appear to be able to benefit from increasing amount of soil nutrient (Helliwell 1965).

     c) Davies (1985) found that competition between sycamore saplings and ground vegetation was primarily for moisture and nutrients reducing survival and growth. These effects were greater on soils with poor moisture retention, or where the climate results in high soil moisture deficit.

     d) Apart from competing for resources with seedling and saplings, dense grass vegetation provides the right conditions for the maintenance of large populations of small mammals and molluscs that increase mortality of seedlings and saplings (Southwood et al. 1988).

     e) In grassland conditions most tree species, inclusive of sycamore but with the exception of Quercus robur, fail to become established. A combination of grazing, grass competition for water and spring drought make conditions unsuitable for seed germination and seedling survival. Oak with its large seeds and deep tap root can survive (Chinner 1948).

 

Clearly the stages in the life history of sycamore which limit its population growth and spread are: seedling establishment, and seedling and sapling survival. However, there is not a single factor which will control sycamore establishment. Instead there appear to be a combination of factors (e.g. slug and rodent population size, yearly variation in climate, etc.) which will vary in intensity both temporally and spatially.

 

 Current distribution.

Rodwell et al. (1991) have recently classified the woodlands of Great Britain and found that sycamore was present in 14 of their 25 recognized woodland types. They assert that sycamore is increasing in importance towards the west and the north with a marked association with Ulmus glabra and areas with rainfall in excess of either 762mm/yr or 1000mm/yr (Rodwell et al. 1991, pp. 138 and 255 respectively). They suggest that sycamore is not so much an indicator of human interference but rather of areas of higher rainfall. It is worth noting that sycamore is not recorded in the Quercus petraea and Betula spp. community type (W11) characteristic of western Great Britain, where rainfall is high and soils are free-draining.

 

Sycamore is widely distributed and occurs in 2267 10km squares of the Atlas of the British Flora, and of all tree and shrub species only ash (2344 squares) and Crataegus monogyna are more widely distributed (Perring & Walters 1962).

 

Apart from Sorbus aucuparia, sycamore ascends higher than any other broadleaved species and has been recorded up to an altitude of 480m in Shropshire (Jones 1944). On exposed and often tree-less islands of both the far north (Orkney and Shetlands) and the south-west (Scilly Isles) sycamore is the commonest tree (Low 1987, Davey 1909).

 

Perring & Walters' (1962) sycamore distribution map show that it now occurs in most 10km squares in Ireland, except in Connemara, Mayo and some parts of Central Ireland. In Dublin, sycamore is probably the commonest tree in the inner city (Wyse Jackson & Sheehy Skeffington 1984). Sycamore's altitudinal distribution ranges from sea level up to 280m in Kerry (Scully 1916).

 

In the Lothian Region of Scotland sycamore constitutes 18.4% of the total number of trees in residential areas, 15.3% in lowland rural and 5.5% in upland rural areas. It is the commonest species except in upland areas where soils are poorly drained (Good et al. 1978). In terms of habitats Good et al. (1978) found a large variation in the occurrence of sycamore, it represented only 1% of all the trees found in hedgerows, 2% pastures, 0% in marsh and fens, 1% of industrial spoils, 2% in coniferous woodland, 21% in mixed woodland, 20% in broadleaved woodland, 13% arable fields, 19% in park (commonest tree), 10% in shelterbelts, 8% in scrubs and 7% in gardens. Large geographical variations do occur; for instance in the Galloway region, some parts of Lancashire and near Aviemore sycamore is the commonest hedgerow timber species (Moore et al. 1967).

 

In North Wales sycamore was the 3rd most common roadside tree (14% of the total) (Good & Steele 1981) while in Derbyshire it occurred in small numbers: 2% in brookside and field hedges and 8% in garden hedges (Willmot 1980). Work by Allison & Peterken (1985) suggests that in Avon and Norfolk sycamore is six times more common in built up areas and along highways than in woodlands. Sycamore has often been reported as an important part of the flora of walls (e.g. Payne 1978, Risbeth 1948, Woodell & Rossiter 1959).

 

Sycamore is a common feature of human habitations. In Wales sycamore was commonly planted about farmhouses (Woods 1990), while in the city of Manchester sycamore and other maples represented 11% of the total number of trees surveyed (Wong et al. 1988).

 

In broadleaved high forest of Great Britain sycamore represents 8.8% of the total (Evans 1987) and a similar figure is given for Cumbria where it is the third commonest broadleaved species after oak and birch (Bunce 1989). According to Rackham (1976) the expansion of sycamore has occurred chiefly into highland woods.

 

In a Cumbrian valley Kirby (1986), in a survey, recognized four types of semi-natural woodlands. Quercus petraea woodland (old coppice) was the commonest type, while stands dominated by Betula pubescens, ash and Corylus avellana or Alnus glutinosa were also found. However, sycamore was only present where ash is dominant, mostly on scree slopes.

 

In eastern England sycamore invasion of ancient woods is recent, covers only 0.5% of the woodland area and is more common in ash and elm woods (Rackham 1980). In his investigation of west Suffolk woodlands Bleay (1987) found that sycamore was very common in secondary woodlands and forestry plantations and occurred in half of primary woodlands and deciduous plantations. In woodlands the frequency of sycamore was very variable but at the majority of the sites no tree regeneration was observed. Bleay (1987) found that in some ancient woodlands sycamore regenerated prolifically and sycamore invasion was more commonly found close to the largest anthropogenic centres. He also suggests that sycamore may be more invasive following the decline in woodland management.

 

 Planting and change in abundance

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In Great Britain in terms of volume there has been an increase in sycamore from 2.11 to 2.47 million m3 from 1951 to 1980 according to Forestry Commission surveys and it is the fourth commonest species by volume. All main species except oak and of course elm showed increases (Allison & Peterken 1985).

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The foliage is very tolerant of salt-laden winds, hence it is the most successful planted tree in Outer Isles like Orkney and Shetland. The Sycamore is noted for its tolerance of urban pollution and salt spray, which makes it a popular tree for planting in cities, along roads treated with salt in winter, and in coastal localities. It is cultivated and widely naturalised north of its native range in northern Europe, notably in the British Isles and Scandinavia north to Tromsø, Norway; Reykjavík, Iceland, and Torshavn on the Faroe Islands. In North America, escapes from cultivation are most common in New England, New York City and the Pacific Northwest. It is planted in many temperate parts of the Southern Hemisphere, most commonly in New Zealand and on the Falkland Islands.

 

Recent changes in amenity tree planting in rural landscapes of England and Wales have been documented by Wright (1983) which show that County Councils appear to have dramatically increased their rate of planting of sycamore from 3.2% to 12% of the total number of trees planted within a few years prior to 1981 (Table 2). In contrast, other agencies stopped planting sycamore altogether. 72% of 25 authorities planted sycamore regularly, and planting was common in most of England except in the East and in Wales. Sycamore has also been widely used for land reclamation, particularly spoil heaps (e.g. Jobling 1987).

 

In recent years, because of its high regeneration potential sycamore has been seen as presenting one of the major problems in conservation management plans for the Telford woods (Tobin et al. 1987) and it's profuse regeneration has often been controlled in urban woodlands (Nicholson & Hare 1986) and National Nature Reserves (Gibbons 1990a,b, 1991).