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Which stage in the sycamore life cycle limits its spread?

Tree Autecology and Biology

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East Usambaras

Pitcairn Islands

SEED

Sycamore seeds are readily eaten by wood mice (Apodemus sylvaticus) as soon as they fall to the ground but only when acorns or beech nuts are rare (Ashby 1959, Watts 1968, Montgomery et al. 1991). Helliwell (1965) carried out sowing experiments of sycamore seeds in spring in 11 English woodlands. Within a month nearly all seeds had been eaten in six of the woods while in the other no predation took place. Thus the impact of small rodents on sycamore seeds can be dramatic but varies strongly both in space and time.

Seeds are also affected by environmental conditions. If seed moisture content drops below 45% 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).

Under natural conditions Southwood et al. (1988) stated that seeds germinated well in bare ground or in moderate vegetation. My observations indicate that this is true 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.

Buckley (1984) sowed sycamore seeds on weathered chalk spoil under total vegetation cover and recorded a very low (2-3%) germination rate. He suggested that the dense vegetation was "apparently pre-empting available sites for germination".

SEEDLING AND SAPLING

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).

CONCLUSION

From the above review it can be concluded that many factors affect the survival of seeds, seedlings and saplings and the key factors causing heavy mortality appear to be:

1. seeds: Predation by small rodents and soil compaction and dryness preventing successful germination;
2. seedlings: slug grazing and sharp spring frosts, spring drought;
3. saplings: field vole stem barking in grassland, competition for light and water by mainly grassy vegetation.

Other factors such as phosphorus requirements, mycorrhizal associations, pH and grazing by herbivores (e.g. rabbits and other larger mammals) will reduce juvenile sycamore competitive ability.

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.