Are ash and sycamore ecologically similar? |
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| Invasive Woody Plants | It has often been suggested that ash (Fraxinus excelsior) and sycamore are ecologically similar (e.g. Oakali 1966a,b). As pointed out by Watt (1925), it is true that many general features such as the frequency of seed production, the age at which this occurs and the adaptations for dispersal are much the same. It is worth investigating this question further as we have seen in Section 5 that sycamore appears to compete and possibly occasionally displace ash but only in the Yorkshire ash woods and in the gaps of beech woods. Ash is a species which has drawn as much interest as sycamore and a large amount of information is available for the purpose of comparison. The ecology of ash has been reviewed by Wardle (1961) and Grime et al. (1988), and some aspects are very well documented including its light requirements (Gardner 1975), seedling and sapling ecology (van Miegroet & Lust 1972, van Miegroet et al. 1981), and its silviculture (Thill 1970). The respective ranges of ash and sycamore are quite different - the centre of their distributions is in central Europe but ash occurs at higher latitudes and further to the north west and north east whereas sycamore occurs at higher altitudes in the Alps. In the British Isles according to Perring & Walters (1962) both species now have a similar distribution. The work by Blackstock (unpublished data) on regeneration in Irish forestry plantation shows a very different pattern of distribution of Sycamore in Ireland. Whilst the probability of occurrence of sycamore increases from the south west to the north east, that of ash shows a trend related to altitude. Below an altitude of 60m the probability of occurrence of ash increases towards the east, but to the south-east it is at an altitude between 60 and 180m and to the south west it is above 180m. The reasons for these differences are unclear. In Ireland both species are spreading and Tomlinson et al. (1987) suggested that they are both replacing other tree species. This appears to be true for Great Britain as well (e.g. Merton 1970). Some of the species attributes of sycamore and ash are given in Table 1 and it can be seen that some of them appear to be similar whilst others show clear differences. The main characteristics differentiating the two species, inclusive of data not given in Table 1, are summarized below. 1. Water: Ash tolerates inundation for five times as many days a year as sycamore (Dister 1983) and is often described as more tolerant of poor drainage (e.g. Kassas 1952). In central Europe ash is associated with forests with high water tables often close to ground level (Clot 1990). This habitat requirement is reflected in the water consumption per unit of leaf area, ash consuming about 10% more than sycamore (Braun 1976). 2. Light: Sycamore cast a much denser shade than ash. Their respective requirements at various stages in the life cycles are poorly documented, but often commented upon, and most authors would agree that sycamore tolerates denser shade than ash (e.g. Watt 1925, Thill 1975). 3. Growth: Mycorrhizal associations enhance growth in sycamore at the seedling stage whereas in ash they have a strong impact on growth rates in saplings (Garbaye & le Tacon 1986). Stem diameter increase in sycamore starts and ends much later than in ash, which even starts growing prior to bud opening (Geissler 1971). Growth rates are depressed earlier in ash than sycamore enabling the latter species to gain dominance (Merton 1970). 4. Architecture: During stormy weather large branches or stems of ash can easily break, whereas this is rarely the case with sycamore. 5. Regeneration and grazing: Seed fall and seed dormancy are the main differences between the respective reproductive biologies of ash and sycamore. In sycamore all seeds will fall in the autumn in Ireland (but in the Alps part of the seed crop falls the following spring, Montemartini 1906), whilst in ash the majority of the seeds remain on the tree throughout winter. Seed germination in ash is delayed by one year compared to sycamore. Assuming that ash and sycamore show similar cycles of seed production, there could be a tendency for these two species to regenerate in alternate years. This could be important in habitats where ground vegetation recovers rapidly following disturbance or simply give a competitive advantage. In places this process could explain the maintenance of both species in the same habitats. Seed predation in sycamore is important, chiefly by woodmice (Apodemus sylvaticus), which do not readily eat ash keys (Watts 1968). Sycamore seedlings have been described as immune to rabbit attack whereas ash is often grazed, but following grazing ash possesses a greater recuperative capacity (Watt 1925). Ash seedling establishment appears to be possible in not too dense grassy vegetation (see Section 5.4) whilst this is not the case in sycamore (but see Brotherton 1973 for opposite view). The latter species often requires some sort of facilitation to become established (see Section 5.7). 6. Conservation value: Elton (1966, p 201) has described ash as having a rather small associated fauna, but carries more epiphytes than sycamore (Harding & Rose 1986). This comparison between ash and sycamore shows that there are some clear ecological differences between the two species. Some environmental factors (e.g. tolerance of flooding) will limit the distribution of sycamore more than that of ash, whilst the timing of seed production, the length of seed dormancy, seed predators and grazers will differentially affect the establishment of the two species. Table 1. Some of the known species attributes of the invasive sycamore, of the ecologically similar ash
(Fraxinus excelsior) and of Maesopsis eminii, an invader of moist tropical forest in Tanzania.
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| Copyright © 1999 Pierre Binggeli. All rights reserved. |