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1 The Bibury long-term data set contains information on annual fluctuations in the abundance of over 100 grasses and forbs in roadside verge vegetation over the period from 1958 to the present. Monitoring has been carried out every July by the same individual. The data set represents a unique long-term record of the dynamics of a complete plant community.
2 Records for the most abundant taxa (including bare ground and litter) were used to determine the effect of climate variability on the year-to-year performance of the selected species. Residuals about the long-term mean log biomass of each species (de-trended where the species showed a significant increase or decrease in abundance over time) were correlated against indices of interannual climate variability. Plant and weather records were compared over 3-month seasonal periods (March–May, June–August, September–November, December–February) or 6-month seasonal periods (March–August, September–February), with time lags of 0, 1 and 2 years.
3 Principal components analysis (PCA) was used to formulate annual weather indices, using either conventional weather variables (temperature, rainfall and sunshine) or the Lamb catalogue of daily weather types.
4 Between 5% and 70% more correlations were observed than might be expected to occur by chance, depending on the season and the PCA index, indicating markedly non-random plant–weather relationships. Total vegetation production was positively correlated with minimum spring temperature. The distribution of correlations was generally evenly distributed across the three lag periods.
5 In general, those species favoured by environmental stress or disturbance were promoted following warm dry springs and summers, whereas those favoured by more productive conditions were promoted following a wet growing season.
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- Site description
- Materials and methods
Annual monitoring of mesotrophic grassland vegetation in wide road verges near Bibury, Gloucestershire, started by A. J. Willis and E. W. Yemm in 1958, has resulted in a unique record of the performance of individual species within the verges, and of the vegetation as a whole, over a very long period. Results from the first 30 years of the study, concerning the management of roadside vegetation through the application of herbicides and a growth regulator, have been widely quoted and applied (Willis 1972; Willis 1988). However, in the late 1980s, as interest increased in the possible effects of climate change on the functioning of ecosystems, the Bibury records had a new lease of life with the full realization that data from the untreated control plots could yield valuable insights into the relations between plant performance and climate (Grime et al. 1994).
Despite the increasing number of long-term studies now being published (e.g. Dodd et al. 1995; Fitter et al. 1995; Sparks & Carey 1995) the Bibury data set has several features that make it unique. The information has been collected by a single recorder over the entire period, using a consistent sampling method. The records are continuous and contain information on the long-term dynamics of a complete plant community.
The aim of this study was to investigate the influence of weather on annual fluctuations in plant performance using the Bibury data set. To achieve this, annual vegetation records were compared with time series both of basic meteorological data (individual weather variables) and of higher-level meteorological data (synoptic patterns). We determined whether plant performance was related to weather in the various seasons and over a range of time lags between occurrence and observation. Observed relations are discussed not only in terms of individual species’ responses but also with regard to functional types and plant life strategies.
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- Site description
- Materials and methods
The vegetation records were taken from permanent plots set up along a 700-m stretch of Akeman Street, originally a Roman road, near Bibury, Gloucestershire (National Grid reference SP 119048). The site was initially chosen for the exceptional width of its road verges, the apparent homogeneity of its vegetation, and because of the fairly light traffic along this length of minor road. The original experiments were established to monitor the effects of spraying the herbicide 2,4-D and the plant growth regulator maleic hydrazide onto the vegetation of road verges. The controls for these experiments consisted of eight plots of unsprayed vegetation, each 20 m long and 3.3–5.5 m wide. Data from these untreated control plots were used in the analysis described below to investigate plant–weather relationships. The plots can be divided into two groups, plots 1–6, which form controls to the maleic hydrazide treatments, with and without 2,4-D, and plots 7 and 8, which are at the western end of the experimental site in what appears to be a more ancient part of the verge and which form controls to the 2,4-D-only treatments. Each plot consists of a rectangular area within which is set a permanent quadrat of 1.0 × 0.5 m, in most cases about 2.0 m from the roadside edge of the plot.
GEOLOGY AND SOILS
The parent geological stratum of the Bibury area is primarily oolitic limestone. The depth of the soil generally varies between 300 mm and 800 mm, although in places it can be as little as 100 mm, and pH values lie within the range 7.5–7.9. Pollution and contamination from vehicle emissions fall off rapidly with distance from the road edge. In a study of the verges of roads with very heavy traffic (chiefly A roads), Akbar (1997) found that contamination and pollution were sharply reduced with distance from the road, with concentrations of most heavy metal elements only significantly above normal within the border zone of a road (up to 1 m from the edge.) The permanent quadrats at Bibury, which are all situated well-back from the road, which is subject to only light traffic, are therefore not likely to be affected.
The vegetation of the Bibury road verges can be characterized under the National Vegetation Classification (Rodwell 1992) as an Arrhenatheretum elatioris grassland (MG1). Two grass species, Arrhenatherum elatius (nomenclature follows Stace 1997) and Dactylis glomerata, are the major components of the vegetation and comprise 40% of the total above-ground biomass. The large Umbellifers Anthriscus sylvestris and Heracleum sphondylium are the most conspicuous herbs. Well over a hundred taxa have been recorded from the Bibury road verges but only the most abundant taxa (listed in Table 1), including bare ground and litter, were included in this analysis.
Table 1. Long-term trends, the presence of autocorrelation and a functional classification of major taxa in the Bibury data set. Taxa in plots 1–6 are represented by A and taxa in plots 7 and 8 are represented by B
| ||Taxa exhibiting long-term increase in abundance||Taxa exhibiting long-term decrease in abundance||Taxa that do not exhibit autocorrelation||Functional type sensuGrime 1979 (from Hunt et al. 1993)|
|Litter|| || || B|| |
|Bare ground|| || || A B|| |
|Achillea millefolium|| || || || CR|
|Agrostis stolonifera|| || || || CR|
|Alopecurus pratensis (1–6 only)|| || || A|| CSR|
|Anthriscus sylvestris|| || || B|| CR|
|Arrhenatherum elatius||A|| || || C|
|Brachypodium pinnatum (7 and 8 only)|| || B|| B|| SC|
|Bromopsis erecta|| || || B|| SC|
|Centaurea nigra (7 and 8 only)||B|| || || S|
|Cirsium arvense|| || A|| B|| CR|
|Convolvulus arvensis|| || A|| || CR|
|Cruciata laevipes|| || A|| A B|| CSR|
|Dactylis glomerata||B|| || || C|
|Elytrigia repens|| || A B || || CR|
|Festuca arundinacea (1–6 only)||A|| || B|| CSR|
|Festuca rubra|| || A|| B|| CSR|
|Galium aparine|| || || B|| CR|
|Galium verum||A|| || || SC|
|Glechoma hederacea|| || A B|| || CSR|
|Heracleum sphondylium|| || || B|| CR|
|Hypericum maculatum (7 and 8 only)|| || B|| || CSR|
|Knautia arvensis||A B|| || A|| CSR|
|Lolium perenne||B|| || A|| CR|
|Odontites vernus||A B|| || A B|| R|
|Phleum bertolonii (1–6 only)||A|| || A B|| CSR|
|Plantago lanceolata||B|| || || CSR|
|Poa pratensis|| || A|| A B|| CSR|
|Potentilla reptans (7 and 8 only)||B|| || B|| CR|
|Ranunculus repens||A B|| || || CR|
|Rumex crispus||B|| || A B|| CSR|
|Taraxacum officinale agg.||B|| || B|| CSR|
|Tragopogon pratensis (7 and 8 only)||B|| || B|| CR|
|Trifolium pratense|| || || A B|| CSR|
|Trifolium repens|| || || || SC|
|Trisetum flavescens|| || || A|| CSR|
|Urtica dioica|| || A B|| || C|
|Veronica chamaedrys|| || || B|| S|
|Vicia sativa||A|| ||B||R|
Although the roadside verges were chosen as an experimental site partly because of the uniformity of their vegetation, some variation is present. A distinction can be made between plots 1–6 and plots 7 and 8. The vegetation in plots 1–6 tends towards the more vigorous, less diverse Festuca rubra and Urtica dioica subcommunities (MG1a and MG1b, respectively), while the vegetation in plots 7 and 8 is more typical of the Centaurea nigra subcommunity (MG1f). A fuller account of the floristic composition of the verges is given by Yemm & Willis (1962).
A metre-wide strip of the verges adjacent to the road is mown in late spring to maintain visibility, and the rest of the verge is usually ‘topped’ with a flail mower in November each year at a height of approximately 0.5 m.
Over the long-term monitoring period there has been no decrease in species-richness in plots 1–6, while in plots 7 and 8 there has been a significant increase in the number of species recorded (r = 0.432). The abundance of different species has fluctuated considerably. Regressions of log biomass against time indicate that 16 species have shown a significant increase in abundance, 10 a significant decrease, while 12 have shown no overall change (Table 1). Of those species which have significantly increased in abundance, such as the annuals Odontites vernus and Vicia sativa and the perennials Lolium perenne, Ranunculus repens, Rumex crispus and Taraxacum officinale agg., a number are advantaged by some disturbance, while others, such as Arrhenatherum elatius, Centaurea nigra, Festuca arundinacea, Galium verum and Knautia arvensis, exhibit a degree of drought tolerance. Species that have shown a significant decrease in abundance include a number that can tolerate some shade, such as Cruciata laevipes and Glechoma hederacea, or which are not typical of dry grassland, such as Poa pratensis and Urtica dioica. These trends may reflect the long-term trends in summer rainfall discussed below and shown in Fig. 1.
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- Site description
- Materials and methods
More correlations were observed between the interannual abundance of the different taxa with interannual weather variation than are expected to occur by chance alone, strongly suggesting that non-random relations exist between plant and weather variables. The use of PCA-derived indices allows objective classification of plant response to seasonal weather conditions in a way that is not possible when using individual weather variables with a large vegetation data set.
The use of the two indices (CWV and LDWT) enables a more comprehensive picture to be built up of plant–weather relationships in the Bibury road verges than the use of each index alone would. This study extends the previously very limited application of LDWT to biological data. Masterman et al. (1996) used LDWT to model the autumn migration of the bird cherry aphid, Rhopalosiphum padi.Aebisher et al. (1988) compared the relationship between abundance of organisms at different trophic levels in the Atlantic Ocean over the period 1958–88 with the annual frequency of westerly weather: striking similarities were apparent between the frequency of westerly weather and biological abundance. We have demonstrated that LDWT can also be applied to terrestrial vegetation dynamics. Overall, nine taxa exhibited comparable responses to both indices in the same season: bare ground, Achillea millefolium, Brachypodium pinnatum, Bromopsis erecta, Dactylis glomerata, Elytrigia repens, Taraxacum officinale agg., Veronica chamaedrys and Vicia sativa. Of these correlations (11 in all), for all but two, the significance of the correlations with the LDWT index was greater than with the CWV index. The CWV index characterizes seasons according to the relative frequencies of individual weather variables, while the LDWT index relates these to prevailing synoptic conditions. As such, LDWT have potential use in predictive studies of the future effects of climate changes on vegetation performance. More research is needed to investigate the relationships between LDWT and other terrestrial data sets. Meanwhile, this study suggests that they give added value when used in conjunction with CWV.
TOTAL VEGETATION BIOMASS
A number of other workers have demonstrated that the total biomass production of grassland systems is linked to rainfall, either in the current season or the previous season, for example in English hay meadows (Smith 1960), and the Park Grass Experiments (Silvertown 1980; Silvertown et al. 1994). The latter authors found a positive relation with rainfall in the growing period prior to hay cut in the Park Grass plots. However, when the annual total above-ground production at Bibury was correlated with annual fluctuations in individual weather variables, rainfall was not found to be a significant controlling factor. Instead, mean spring minimum temperature was found to be the most important variable. The Bibury vegetation differs from the other examples given above in that it is not cut for hay but is instead lightly ‘topped’ at the end of the growing season. It may therefore be less dependent on summer rainfall to boost regrowth after cutting. It is likely that minimum temperatures in spring dictate the relative growth rates of the vigorous competitive species that dominate the Bibury verges, given that in most years moisture reserves at depth are likely to remain sufficient to support plant growth in the early part of the growing season, and that many of the most abundant species have an early phenology. The negative correlations with the PCA indices for autumn and winter combined and for spring and summer combined, both with a lag of t-2, are likely to reflect the performance of dominant species, such as Dactylis glomerata, in the sward.
RESPONSES OF INDIVIDUAL TAXA
A number of similarities are apparent in the responses of individual taxa to fluctuations in the PCA index for both weather types and conventional weather variables for the 3-month spring and summer periods, and for the 6-month spring and summer combined period. This is as expected, given the similar climatic characteristics of positive and negative amplitudes of both indices over these periods. The amount of bare ground in the system is promoted following positive years for both indices in spring. Similarly the amount of litter in the system is increased following warm dry springs and summers. The amount of litter would be expected to increase following a dry season, and this is not incompatible with an increased frequency of gaps at ground level in tall productive vegetation such as that in the Bibury verges. Species promoted by dry springs and summers include fairly deep-rooted species such as Centaurea nigra, Convolvulus arvensis, Taraxacum officinale agg. (illustrated in Fig. 2), Trifolium pratense and Rumex crispus. Other plants promoted include the vigorous drought-tolerant grasses, Arrhenatherum elatius, Brachypodium pinnatum, Elytrigia repens and Festuca arundinacea, and the annuals Odontites vernus and Vicia sativa. Species that are retarded by dry springs and summers include grasses typical of productive sites, such as Dactylis glomerata, Lolium perenne and Poa pratensis. Other species that are retarded are associated with shady or damp grassland and include Cruciata laevipes, Galium aparine, Glechoma hederacea and Ranunculus repens. Bromopsis erecta is also retarded by dry springs and summers at Bibury. This observation supports results from European populations of B. erecta, which are favoured by moist springs and retarded by a dry season (Bornkamm 1961). There are some exceptions to this general rule. For example, Galium verum, a species of well-drained calcareous soils, would be expected to be favoured by dry spring and summer conditions but it is in fact retarded.
Species that are promoted following mild or wet autumns and winters include those of damp or shady grassland, such as Alopecurus pratensis, Cruciata laevipes, Glechoma hederacea and Hypericum maculatum. Another example is Veronica chamaedrys, a perennial herb of moist grassland, which can form an understorey in tall grassland. It is promoted by wet autumns and winters at Bibury. This promotion may be enhanced by the species’ phenology: rapid shoot growth is restricted to spring and autumn (Grime et al. 1988). Bromopsis erecta is also promoted by mild winters and this may also be linked to phenology, it being a grass of early phenology, capable of growth during winter (Speddings & Dickmans 1972). Species promoted by cool dry autumns and winters include a number that are also favoured by dry springs and summers at Bibury, such as Rumex crispus, Taraxacum officinale agg. and Trisetum flavescens.
RESPONSES OF FUNCTIONAL GROUPS
It appears that species’ life histories and habitat distributions can give some indication of their response to fluctuations in weather in mixed vegetation such as that in the Bibury verges. This is supported by the responses of the aggregated functional groups shown in Table 7. Warm, dry springs and summers promote those species that are adapted to environmental stress or disturbance [competitive ruderals (CR), ruderals (R) and stress-tolerators (S)], while species adapted to more productive conditions are retarded [competitors (C) and stress-tolerant competitors (CS)]. If, as predicted under climate change scenarios (e.g. CCIRG 1991), such summers become more frequent in southern Britain, the relative abundance of the more ruderal components of the Bibury vegetation might increase (Fig. 4). Those groups partially linked to the competitive strategy respond positively to mild wet winters, which, in effect, extend the productive growing season. The future balance between wet winters and dry summers will therefore be decisive in determining the relative abundance of components of the Bibury vegetation (Hunt et al. 1993).
Figure 4. Functional groups retarded and promoted following warm dry springs and summers. The two environmental dimensions are: increasing stress, from top left to bottom right; increasing disturbance, from top right to bottom left. Those groups more tolerant of environmental stress or disturbance are promoted following such seasons, while competitors are promoted following cooler, wetter summers. After Hunt & Cornelissen (1997).
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Interactions between plant performance and weather are complex: different weather variables may differentially affect flowering, seed production and vegetative growth. Plant responses may be direct or mediated through competition, and responses may lag behind weather conditions by one or more growing seasons. A striking feature of the results from Bibury is that the total number of correlations between plant performance and weather with a lag of 1 or 2 years is greater than that which occurs with weather in the current season, although overall there is a relatively even distribution of correlations over the three lag periods (0, −1 and −2 years). Time lags of more than 1 year between cause and effect are common features of natural systems (Magnuson 1990). Herben et al. (1995), using data from permanent grassland plots over a period of 10 years, also found large year-to-year variation in species’ performance that could be correlated with weather variables, and which also involved lagged responses. They noted that different measures of plant performance (biomass of ‘modules’ and number of modules) resulted in correlations with different variables over different periods for the same species, and suggested that this may be the mechanism by which time lags arise.
An example of the complexities involved in interpreting lagged responses is the relation between Dactylis glomerata and warm, dry springs and summers (Fig. 3). The same response (a negative relationship with a 2-year lag) is found with both PCA indices and in both vegetation series. Experiments with D. glomerata in mixed communities have shown that following productive spring conditions (warm and moist) the grass produces lush vegetative growth at the expense of inflorescences, and that the resulting large tussocks cause weakening of the less dominant members of the community; this effect is carried over into subsequent years (Dunnett 1996). The sensitivity to spring and summer drought may be linked to carbohydrate metabolism: the carbohydrate reserves of northern populations of D. glomerata (Volaire 1995; Volaire & Gandoin 1996) under long and intense drought are continuously utilized and not replenished, causing mortality and lack of persistence. Such effects may not be apparent in the field until seasons following the drought event. It may, of course, be that the observed effects are not direct responses but are mediated through interspecific competition. It is probable that many of the lagged responses of minor components of the Bibury vegetation represent reactions to the performance of dominants such as D. glomerata.
Field results obtained from long-term monitoring studies, such as those obtained from the Bibury verges, do not allow distinctions to be made between direct plant responses to weather variables and those mediated through interspecific competition. It has been suggested that positive feedback, favouring better competitors, may result in a magnification of the effects of climatic perturbations in fertile, productive habitats (Silvertown et al. 1994). In a series of microcosm experiments with synthesized grassland communities using transplants from the Bibury verges, we have shown that this is indeed the case. Asymmetric interspecific competition resulted in amplification of climatic signals, with more productive species being disproportionately promoted by favourable growing conditions (N. P. Dunnett & J. P. Grime, unpublished data).
The long-term record from the Bibury verges shows that even within what may be thought of as relatively stable vegetation there can be marked interannual fluctuations in species’ abundance. At least part of this variation can be explained as a response to climatic variability. Many of the observed responses match what is known of the species concerned through their natural history and published accounts. Several of the most frequent Bibury species have been subjected to climate manipulations under controlled conditions and their responses are the same as those observed in the field (Dunnett 1996). The Bibury data set therefore has significant potential in modelling the response of individual taxa, functional groups and the vegetation as a whole to any future climate scenario.