REGIONAL VARIATION OF ALNUS GLUTINOSA (L.) GAERTN. IN 



BRITAIN 



By D. N. McVean 



The Nature Conservancy, Edinburgh 



Morphological Variation 



In its external morphology Alnus glutinosa is a rather variable species and it is possible 

 that a similar physiological variability exists. The most obvious differences are in habit, 

 leaf size and shape, and catkin size. 



Generally, the alders in the north and west of Britain are shorter and more gnarled 

 than those to the south and east, but this may well be a habitat modification, brought 

 about by exposure to strong winds and growth in less suitable soils, and not genotypic. 

 The multiple-stemmed bush habit is most frequent on waterlogged soils but may be 

 induced by damage to the sapling. 



Leaf outline varies from orbicular through cuneate to ovate or obovate. The apex 

 may be strongly emarginate, rounded or even acutely pointed as in Alnus incana. The 

 leaf outline is fairly constant for each tree, although the shape of the apex may vary. The 

 margin may be undulate with one or two pronounced sets of teeth, or smoothly curving 

 and with only tiny serrations. 



The hairiness of lamina surface, vein angles and petioles is also variable. 



Asymmetric leaves are frequent, and differences between long and short shoot 

 leaves do not appear to be as pronounced as in Betula. 



Pistillate catkins may be globular or elongated, and the free points of the fused scales 

 may or may not be visible. 



Size of seeds (fruits) also varies considerably, and roughly proportionately with 

 catkin size. The range of variation illustrated by Kujala (1924) for Scandinavia is less 

 than that now recognized in Britain (Plate 2). 



Catkins less than 10 mm. long have very tiny seeds (1-5 X 1-0 mm.) which in turn 

 give rise to small, weak seedlings. 



Leaf and catkin sizes were selected for statistical examination in 1949. Leaves 

 were collected at random from spur shoots and from near the apex of long shoots, avoiding 

 immature or obviously deformed specimens. Length was measured from the insertion 

 of the petiole to the distal end of the mid-vein, and breadth at the broadest point at right 

 angles to this line. 



Pistillate catkins could not always be measured at the green-ripe stage, but it was 

 found possible to utilise old catkins provided length was measured from the insertion 

 of the stalk to the tip of the centre scale, ignoring the extra length given by any reflexed 

 basal scales. 



Twenty leaves and twenty catkins from each of five or ten trees per population 

 were measured, and two or three adjacent populations in each district were sampled 

 where possible. This method was used in preference to the collection of one leaf per 

 tree since populations were often small. 



Mean dimensions of leaves and catkins for any one tree in the Arran and Chippenham 

 populations were found to be constant frorn year to year. 



Data from the eighteen populations sampled are summarised in Tables 1 and 2. 

 The location of the populations is shown in Figure 1. The same data have also been 

 presented graphically elsewhere (McVean 1953). 



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