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.

In Iceland, mountain birch dominates indigenous woodlands and scrub communities. For use in inventories of the natural birch population, we derived single parameter aboveground biomass functions from a stratified random sample encompassing the entire native birch population. We evaluated the accuracy of these models on independent data from the same population and used regressions of log-transformed predicted versus observed values and compared slope and intercept parameters against the 1:1 line. We propose that the level of accuracy of allometric models might be quantified by the size of Theil's random error component (U e) and the normality of residual variances might be a decisive test of acceptable functions. The commonly used allometric power function without intercept proved highly accurate for diameters at ground level but was biased for diameters measured at 0.5 m up the stem. We compared both non-linear regressions and log-transformed linear regression techniques. The latter produced more accurate models especially for applications to small diameter trees. Power functions with intercept and diameters measured 0.5 m above ground produced accurate estimates, except for trees with diameters less than 50 mm. We suggest allometric models for general use in Iceland for inventories of native birch woodlands and scrub.

The only native tree species in Iceland forming woodland is the mountain birch. Since human settlement in the 9 th century AD, natural birch woodlands have decreased from around 28% to 1.5% of the terrestrial area. In this paper we estimated and compared aboveground woody biomass stocks of two sample inventories

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At coastal sites, trees are exposed to marine aerosols that may cause foliar necrosis and shoot dieback, which can result in deformed crowns and contorted stems. A six-year study of leaf primordia in terminal buds of black cottonwood trees (Populus trichocarpa Torr. & Gray) on Heimaey Island off the south coast of Iceland was undertaken to elucidate the physiological events associated with salt-deposition-related bud failure. Leaf and bud lengths, dry mass, water content and chloride concentrations were monitored and related to four phenological stages: (1) bud set; (2) dormancy induction; (3) dormancy release; and (4) bud break. The trees set buds in July and shed their leaves by late September. Leaf primordia generally stopped growing by September 10 +/- 22 days and attained midwinter water content in late September. Leaf growth commenced in the terminal buds by March 2 +/- 16 days, but mean dates of bud swelling and bud break were April 29 +/- 19 and May 10 +/- 12 days. In summer and until November, chloride concentrations in leaf primordia were low, but increasing. Chloride concentrations remained stable from December to February, even though the dormant trees were exposed to large amounts of marine aerosols. In February and March, three events occurred more or less simultaneously: (1) leaf extension growth commenced; (2) chloride concentration surged in the leaf primordia; and (3) the leaf primordia began to hydrate. Following dormancy release, growth and hydration of leaf primordia were negatively related to chloride concentration in the leaf primordia, with inhibition of leaf growth, tissue hydration and chloride acquisition occurring at a chloride concentration threshold estimated at 7.3 mg Cl- g(-1) tissue water. Necrosis of leaf primordia was observed above 14 mg Cl- g(-1) tissue water. Growth and hydration of leaves at bud break in mid-May was explained by a three-parameter logistic model of chloride concentration in leaf primordia at the end of March. By mid-May, 90% of all buds remained non-necrotic, but only 56% the terminal buds had broken. Salt alone explained the observed growth suppression of leaf primordia in the buds and the resultant failure of terminal buds to break by mid-May.