AOBPreview originally published online on September 16, 2004
Annals of Botany 2004 94(5):753-762; doi:10.1093/aob/mch200
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Annals of Botany 94/5, © Annals of Botany Company 2004; all rights reserved
Stature of Sub-arctic Birch in Relation to Growth Rate, Lifespan and Tree Form
Icelandic Institute of Natural History, POB 5320, 125 Reykjavik, Iceland, Iceland Forest Research Station, Mógilsá Kjalarnes, 116 Reykjavik, Iceland and Icelandic Horticultural College, Reykir, 810 Hveragerði, Iceland
* For correspondence: Icelandic Institute of Natural History. E-mail hjalti{at}ni.is
Received: 10 June 2004 Returned for revision: 5 July 2004 Accepted: 4 August 2004 Published electronically: 16 September 2004
• Background and Aims Sub-arctic mountain birch Betula pubescens var. pumila communities in the North Atlantic region are of variable stature, ranging from prostrate scrubs to forests with trees up to 12 m high. Four hypotheses were tested, relating growth and population characteristics of sub-arctic birch woodland and scrub to tree stature; i.e. the variable stature of birch woods is due to differences in (1) the mean growth rate; (2) the age-related patterns of growth rate; (3) the life expectancy of stems; or (4) the tree form.
• Methods A stratified random sample of 300 birch trees was drawn from the total population of indigenous birch woodlands and scrub in Iceland, yielding 286 valid sample genets. The population was divided into three sub-populations with dominant trees 0–2, 2–4 and 4–12 m tall, referred to as birch scrub, birch scrub-woodland and birch forest, respectively.
• Key Results Trees in the scrub population were of more contorted growth form than birch in the scrub-woodland and forest populations. Mean growth rates, mean age and median life expectancies increased significantly with sub-population of greater tree stature. At the population level, annual increment and longevity of birch stems was apparently interrelated as the stems in vigorously growing birch sub-populations had a longer life expectancy than those of slower growth. However, no difference was observed between sub-populations in age-related patterns of extension growth rate.
• Conclusions The results were consistent with hypotheses (1), (3) and (4), but hypothesis (2) was rejected. Hence, mountain birch of more vigorous growth attains a greater stature than birch of lesser increment due to faster extension growth rate and a longer lifespan. In addition, the more contorted stem form of scrub populations contributes to their low stature.
Key words: Betula pubescens var. pumila, stature, height, growth, increment, population dynamics, life span, tree line, sub-arctic
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