British Plant Communities, Vol. 3: grassland and montane communities

British Plant Communities, Vol. 3: grassland and montane communities

TREE vol. 8, no. 12, December 1993 - - Plant Ecology British Plant Communities, Vol. 3: Grassland and Montane Communities edited by J.S. Rodwell, C...

320KB Sizes 4 Downloads 89 Views

TREE vol. 8, no. 12, December



- Plant Ecology British Plant Communities, Vol. 3: Grassland and Montane Communities edited by J.S. Rodwell, Cambridge University Press, 1992. f95.00 hbk (x + 540 pages) ISBN 0 52139766 0 The Park Grass Experiment at Rothamsted, UK, was set up in 1856, three years before the publication of the Origin of Species. Lawes and Gilbert set up the experiment with the idea of determining the combination of fertilizers that would produce the maximum yield of hay. They soon realized, however, that the repeated application of different mineral nutrients had changed the previously uniform grassland into a mosaic of radically different plant communities. It is to the their lasting credit that Lawes and Gilbert realized the potential importance of the pattern they had inadvertently produced, and that they assured the continuity of the experimental treatments to the present day. What the Park Grass experiment demonstrates is that, at a single location, variation in botanical composition is determined by the interplay of mineral nutrient availability and soil acidity. The 89 plots on Park Grass exhibit real, statistically significant, ecologically meaningful differences that reflect the way in which the struggle for existence is played out under different rates of mineral nutrient and lime application. The recent publication of the third in the series of British Plant Communities gives us an opportunity to see the extent to which a project aimed at providing coverage of the British Isles as a whole can deal with mosaics of variation in botanical composition of grasslands at the scale of a few hundred square metres. The British grasslands are divided into three major groups: mesotrophic grasslands (MG), dominated by Arrhenatherum elatius, Lolium perenne or Holcus lanatus and typically found in England and southern and eastern Scotland; calcicolous grasslands (CG), dominated by Festuca ovina, Bromus erectus or Sesleria albicans and found from the wilds of the Durness limestone in the northwest to the chalk cliffs of Dover in the south east; and finally the upland and acid grasslands (U), which comprise a heterogeneous bunch including swards of Nardus stricta, Agrostis curtisii, Agrostis capillaris and various Juncus spp., along with montane fern communities like Cryptogramma crispa, Thelypteris limbosperma and Blechnum spicant and the mossy summit-

heaths dominated by Racomitrium lanuginosum. Each account provides a key to the various numbered communities, and the description of each community follows the now familiar pattern with sections on synonomy, constant and rare species, physiognomy, subcommunities, habitat, zonation and succession, distribution and affinities with continental plant communities. These are followed by species lists showing frequency and abundance and distribution maps for each of the subcommunities. The introductory essays to each of the three main grassland types are packed with ecological information on the effects of grazing, history and grassland management. As it turns out, all of the variation between the 89 Park Grass plots falls within three of the 48 grasslands defined under the National Vegetation Classification (mostly MG5 with MG3 and more rarely MGI). Thus, the variation within defined communities is of the kind created by long-term manipulative experiments. The corollary of this is that the variation between communities is likely to be caused by fundamental (largely abiotic) factors of geology, aspect, drainage and altitude. This is fine, but it does mean that many of the questions about ecological dynamics studied by plant ecologists (such as impact of grazing, plant life history, the importance of seed in recruitment limitation) are not addressed by the classification scheme. It should be said, however, that there is comprehensive discussion of these issues in the text that accompanies each community description. One interesting set of questions posed by the community lists concerns species-rich grasslands like the control plots on Park Grass: what allows the persistence of the rarer species? Why do the rarer species not increase in abundance? What ecosystem function (if any) do they fulfil? It is notable that all of the manurial treatments at Park Grass (additions of nitrogen, phosphorus, potassium or farmyard manure) cause reductions in species richness compared with the control plots; very few new species are found on the fertilized plots that are not present on the controls. In contrast, the addition of lime is typically associated with the maintenance of higher species richness on the unfertilized plots (presumably because it counters the diversity-reducing effects of increasingly acid rainfall). A second set of interesting questions concern the identity of the species that are lost when a particu-


lar grassland occurs in circumstances (as on remote islands) where species diversity is reduced. The grasslands grazed by the Soay sheep on the island of St Kilda fall naturally into the Holcus lanatus-Trifolium repens sub-community of U4 (the FestucaAgrostis-Galium saxatile grasslands). They represent a species-poor variant because a number of characteristic species have never become established on (or never made it to) the island. Grasses like Cynosurus cristatus, Alopecurus pratensis and Dactylis glomerata and herbs like Veronica chamaedrys, Taraxacum officinale, Campanula rotundifolia and Lotus corniculatus are all missing from the otherwise typical U4b community. An unfortunate problem that arises with any classification of plant communities based on the presence of indicator species, is what to do when one or more of those species is missing. For example, the grass Cynosurus cristatus is missing from many of the Rothamsted Park Grass plots that belong to MG5, the Cynosurus cristatus-Centaurea nigra grassland, and neither Festuca ovina nor Galium saxatile are common in the Festuca ovina-Agrostis capillarisGalium saxatile (U4) grasslands in St Kilda. One way of looking at this is to see Murphy in action. Alternatively, you might see it as drawing attention to an important ecological question: to what extent is botanical composition (i.e. species richness and relative abundance) regulated in plant communities? If it is regulated, then what are the mechanisms responsible for regulation? Gleason’s view of communities as ‘accidents’, random assemblages of species thrown together by chance and assembled from a species pool determined largely by tolerances of abiotic factors, does not make predictions about relative abundance, except in so far as species have characteristic sizes, longevities and growth forms. I suspect that relative abundance is not regulated at all at the level of the plant communities; it is the abundance of each individual species that is regulated, and the factors determining the regulation of a given species can vary from place to place to such an extent that there will always be circumstances when a socalled ‘constant species’ may not be able to invade (or having invaded, to persist in) the grassland community that bears its name. The floristically dull and intensively managed lowland agricultural grasslands that are grazed year-round and only infrequently cut for hay or silage 459

TREE vol. 8, no. 12, December

are bundled together into a community called Lolium perenne-Cynasurus cristatus grassland (MG6) and there is a separate category for Lo/&m perenne leys and related-grasslands from which Cynosurus cristatus is absent (MG7). tnterestinglp, white these swards often have extremely low species richness (little more than clover and ryegrass), they can contain as many as- 26 species in a 2 x 2 m sample. This compares favourably with-other grassland communities that are traditionally regarded as being highly species rich [e.g. what used to be called ‘chalk grasslands’ and which are referred to here as Festuca -ovina-Avenula pratensis grassland (CG21, and have a species-richness range 6-47 species at this quadrat size]. It appears that ‘botanical interest’ is not as closely correlated with species richness as one might have thought. These humdrum lowland swards can be just as rich in species as the more attractive chalk grasslands; it’s just that the species involved are run-of-the-mill plants like daisies, docks and plantains rather than orchids, pasque flowers and gentians. I can remember John Harper teasing the audi-

ence at a symposium on conservation biology by suggesting that, as far as he could judge, most of them would prefer it if the abundance of many more plant species were to be drastically reduced, so that they would then have more rarities to get excited about! An endearing feature of the books in this series is their ability to conjure up a tremendous sense of place through the vegetation they describe. The description of CG2a, for example, is redolent of family picnics on the downs above Sittingbourne, with mother in a sensible skirt and father grumbling about the stemless thistles. On the other hand, U17 evokes a dripping Highland cliff, where you can almost feel the horizontal, driving rain and see a huddled group of hairykneed Scotsmen in kilts taking a dram in the shelter of a rock festooned with Luzula sylvatica and Geum rivale. The series also gives a feel for the range of ecological questions that we need to confront before we can say that we understand the structure and function of British Plant Communities. For example, what determines the presence of the ‘constant’ species? Is it simply that they share a require-


ment for the same set of environmental conditions (soil, weather, altitude and aspect) and have such efficient powers of dispersal that they find their way to these places, wherever they appear? Or is it that there are assembly rules (combinations of life histories, growth forms, etc.) which, given the abiotic conditions, mean that only a small subset of species combinations can persist? To what extent are the continued coexistence of the rarer species dependent on the identity, distribution and abundance of the dominant species? This series represents an outstanding achievement of descriptive plant ecology. It shows how the entire British flora is distributed across a limited set of reasonably characteristic communities and, what is more, it describes the ways in which one community type intergrades with another. What we need now is an equally accomplished theoretical plant ecology that can explain why the communities are the way they are. M.J. Crawley Dept of Pure and Applied Biology, Imperial College at Silwood Park, Ascot, UK SL5 7PY

Plant Behaviour The Action Plant by Paul Simons, Blackwell, f25.00 hbk (336 pages) ISBN 13899 4

1992. 0 631

The Action Plant is a stimulating, broad-ranging account of almost everything that can be described as plant behaviour. Most lay people and a fair number of biologists too, it is sad to relate - seem to be under the impression that only animals ‘behave’. Plants are supposed just to sit and take their punishment. Animals, inter alia, feed and seek for food, mate, fight,-or otherwise posture to achieve dominance, defend themselves, and create their own environment. Television, with its brilliant exposition of the animal image in full colour, total movement and the magic of time-lapse, has solidly confirmed this impression. Plants exist as the still-standing, softfocus background to the dramatic animal behaviour shot, whether on the plains of Africa or on the edge of the coral reef. We now know, from careful work stretching back over some 60 years, that the sex life of plants is infinitely 460

more bizarre, more diverse and what can only be described as more ‘kinky’ than anything Soho can manage to inflict upon Homo sapiens. Plants have been practising bonpseudocopulation, prostidage, tution, sado-masochism and sexual deception, amongst other diversions, long before mankind or even the mammals were even thought of in the evolutionary sense. The Sex Life of Plants by Alec Bristow’ opened many eyes to what plants were up to; but nobody was surprised to learn that much of the early work on pseudocopulation by the brilliant Frenchman, Judge M. Pouyane, was either suppressed or studiously ignored because of the supposed veneration of the virginal flower. What did people think flowers were for? Their sex life apart, what Simons demonstrates all too clearly is that even the brilliant work of BurdonSandersot? from 1873 was almost perversely ignored, although his results were in fact exactly paralleled with those in animals and have all proved to be absolutely accurate. Simons rightly considers that sex has already had a good exposure and

concentrates on almost every other aspect of behaviour. Sensitivity, the generation of explosions, hunting and killing, electrical, chemical and physical plant defence, seeing the light, sunbathing, sleeping and rhythmic movements, touchiness, twitchiness and nervousness are all given a good airing. The language sometimes verges on that more normally found in the tabloid press. In my experience plant scientists are a pretty phlegmatic lot and I do not recall any ‘uproar’ when Margulis published her synthesis3 of serial endosymbiosis in 1970. Perhaps I was exceptional in having been brought up on Schimper4, Mereschkowsky5, on plastid inheritance Darlington” and Ris and Plaut’ (who discovered DNA in chloroplasts and mitochondria). The Margulis text seemed to most of us, at the time, nothing more than an elegant restatement of what was broadly-received wisdom. But setting such occasions of hyperbole aside, and ignoring a few, but mainly unimportant mistakes, Simons has covered every single one of his topics of plant behaviour with analytical skill, a nose for the right reference, a