Fig. 9.23 Cassiope hypnoides and possible responses to seasonal differences in relation to climatic warming. For explanation see Fig. 9.21.
(4) The polar willow (Salix polaris), shown here for comparison, also exhibits differential effects between summer and winter temperature changes, with winter warming being the most disadvantageous and winter cooling the most advantageous for its presence in north-western Europe (Fig. 9.24).
9.8.2 Possible migration behaviour
The maps in Figs. 9.21-9.24 offer insights into the possible effects of temperature on species distribution.
(1) Species migration is sensitive to existing temperature seasonality and the superimposed seasonality of temperature change.
(2) Migration cannot be predicted from annual mean temperature alone.
(3) For the same patterns of change, in different parts of their ranges, species could migrate in opposite directions.
(4) Seasonality gradients may present barriers to migration notwithstanding overall warming.
(5) The effects of changes in temperature and seasonality may affect species either directly or indirectly by changing the nature of competition with other cohabiting species.
Some physiological explanation is required to explain why milder oceanic conditions should be disadvantageous for woody species. The Norwegian plant ecologist Eilif Dahl was one of the first to distinguish between the positive and negative effects of oceanic conditions on alpine species. The relatively species-poor mountain flora of the Scottish Highlands and south-west Norway were considered by Dahl to be due to mild periods of winter weather that encouraged premature spring growth, causing severe dieback of non-hardy shoots. Dahl first drew attention to Norwegian montane species such as Rhododendron lapponicum and Aconitum septentrionale that are absent from more oceanic mountains and described them as south-west coast avoiders (Dahl, 1951, 1990).
A study of Vaccinium myrtillus in north-eastern Sweden (Ogren, 1996) noted that after a warmer
Salix polaris p=
>0.7-0.55 I >0.55-0.35 ■ >0.35-0.15 '>0.15-0.05 <0.05
Probability of occurrence at 1961-1990 temperatures
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