Cycles and fluctuations

There have been surprisingly few long-term studies that have monitored changes in the composition of plant communities over more than three to five years. As can be seen from Figure 4.7, the performance of a species over such a period really gives very little information about what it is actually doing over periods of decades. Those longer studies that have been carried out tend to confirm what most gardeners know by experience: plants tend to have good years and bad years, determined primarily by weather conditions, and perennial plants (both woody and herbaceous) tend to become over­mature and require rejuvenation in due course.

Figure 4.7 is taken from the Bibury dataset, one of the longest continuous studies of herbaceous vegetation in the wild, which is taken from productive grassland vegetation on a roadside verge in the south of England. The figure shows the yearly performance of a large stand of rosebay willowherb (Chamerion angustifolium), a vigorous tall perennial ‘competitor’ that forms large spreading clumps that tend to exclude other species, and which makes a

Cycles and fluctuations

Comparison of maximum shoot height (♦) and shoot biomass of rosebay willowherb over the period 1959-1996 in the Bibury road verges. No measurements were made in 1961 (from Dunnett and Willis (2000))

Cycles and fluctuations

4.8

Comparison of the performance of C. angus tifolium, Arrhenatherum elatius and Stachys sylvatica over the period 1959-1996 in the Bibury road verges. No measurements were made in 1961 (from Dunnett and Willis (2000))

dramatic display of tall pink flower spikes in mid to late summer.

Figure 4.7 indicates that there is considerable variation in both the height and bulk (biomass) of this species from year to year. The main factor determining these changes (in the absence of changes in management) is yearly differences in weather patterns. But there is also dramatic longer-term change, with a period of peak performance over the 12 years from 1964 to 1976. This rise and fall (which may be cyclical) is typical of the behaviour of many herbaceous perennials. Such processes in vegetation are again generally the result of the influence of dominant species going through four distinct phases (Watt 1947): pioneer (establishment of a species), building (growth to peak biomass),

Cycles and fluctuations

4.8 Succession on railway sidings abandoned in Sheffield—grassland is being invaded by scrub and birch woodland

mature and degeneracy (breakdown of dominance and invasion by other competing species).

Whilst the increase in biomass and the lateral extent of the stand can be readily explained in terms of rapid clonal extension and elimination of subordinate species—C. angustifolium has a high potential for dominance (Grime 1973)—the sudden decline and break-up of the stand are less easy to interpret. One explanation for the change in performance of perennials in this way may be a progressive decline in vigour of the stand as resources are accumulated in living and dead components of the biomass, resulting in reduced nutrient supply (Watt 1947). Other explanations may include responses to extreme weather (such as drought), herbivory, allelopathy or disease.

The influence of such fluctuations is not limited to the individual species, but is played out through interactions with other species in the same community. Figure 4.8 illustrates the interplay of C. angustifolium with other components of the system at Bibury. Arrhenatherum elatius (False Oat Grass) is another ‘competitor’ and clearly benefits from the collapse in vigour of C. angustifolium, but the performance of Stachys sylvatica (Hedge Woundwort), which grows on the shady edges of the stand of C. angustifolium mirrors the performance of the willowherb.

There are very few such studies that demonstrate interactions between species over extended periods, but they generally indicate the major influence of dominant species on the behaviour of subordinates, and the overriding effect of climatic factors in causing yearly fluctuations in the abundance of different species (Watt 1971).