Does Gulf Stream position affect vegetation dynamics in Western Europe

It is widely accepted that the climate enjoyed by maritime western Europe is strongly affected by the warming influence of the North Atlantic Gulf Stream. Until recently, however, little account has been taken of the fact that the latitudinal position of the Gulf Stream varies continuously. In theory, such displacements could generate year-toyear variation in crop yields and in the productivity and relative abundance of component species in natural vegetation. Here we test this hypothesis by examining the productivity of above-ground vegetation, monitored annually in permanent plots at Bibury (Gloucestershire, southern England) over the period 1966-1993, in relation to changes in latitudinal position of the Gulf Stream. The long-term field experiment on the road verges at Bibury (Yemm and Willis 1962, Willis 1972, 1988) has recently provided an unforeseen opportunity to examine year-to-year variations in species composition in permanent plots experiencing a standardized management regime (late autumn mowing). We examined inter-annual variation in mean total shoot biomass in each of the main Bibury species in relation to a Gulf Stream northerliness index (see legend to Table 1). Total productivity of above-ground vegetation was positively correlated with Gulf Stream northerliness (Fig. la). Annual species were negatively correlated with northerliness during certain periods, biennials showed no clear relations, and perennial species were positively correlated. Shoot biomass was positively correlated with northward displacement of the Gulf Stream in 10 species, showed a negative relation in three other species, and had no consistent effect on the remaining 10. Different species showed significant correlations with Gulf Stream movements at different times of the year (e.g. Fig. lb), and some species showed correlations with movements in the preceding year or in the year before that. Table 1 classifies the Bibury species according to response to Gulf Stream northerliness, life history and ecology (ecological data from Grime et al. 1988). Species favoured by northerly tracks include robust, perennial grasses (Bromopsis erect, Dactylis glomerata, Elytrigia repens and Festuca rubra). At Bibury, and across Western Europe, these grow rapidly in early spring and are major contributors to community biomass. The similarly responsive dicotyledons (Achillea millefolium, Gahum verum, Knautia arvensis, Plantago lanceolata and Vicia sativa) have relatively later phenologies and fairly deep root systems. Species declining in biomass with Gulf Stream northerliness (Anisantha sterilis, Galium aparine and Urtica dioica) are more heterogeneous ecologically, but are either annuals and/or have field distributions associated with damp or shaded locations where water relations may be important (Grime et al. 1988). The circumstantial evidence is that variation in Gulf Stream position is associated with changes in the relative abundance of components of the vegetation. Because these differ in phenology and rooting depth, moisture supply appears to be implicated, though the precise mechanism is unclear. Large-scale changes in atmospheric circulation are known to affect both the position of the Gulf Stream and the weather in the United Kingdom. However, it has been suggested by Taylor and Stephens (1980) that displacement of the Gulf Stream may directly influence the movement of cyclones across the North Atlantic and hence affect the frequency of anticyclones to the west of the UK (Taylor 1995a). Clearly, both of these actors may affect rainfall patterns. To test this assertion, we examined relations between Bibury weather and Gulf Stream position (see legend to Fig. 1). The relation for local atmospheric pressure appears in Fig. lc, showing that high pressure is clearly related to northward displacement. Analyses also confirm, as expected, that high pressure is associated with higher mean temperatures, more sunshine hours, and lower rainfall. We conclude (a) that the overall productivity of Bibury vegetation and the performance of certain individual spe-