DIFFERENTIAL GROWTH-RESPONSES OF CASSIOPE-TETRAGONA, AN ARCTIC DWARF-SHRUB, TO ENVIRONMENTAL PERTURBATIONS AMONG 3 CONTRASTING HIGH SITES AND SUB-ARCTIC SITES

Three populations of Cassiope tetragona (Ericaceae) were subjected to in situ environmental perturbations simulating predictions of global w arming. The populations were selected to represent different parts of the range of the species, one growing in a high arctic coastal heath a t Ny-Alesund (Svalbard, northern part of the species' range), one at a subarctic fellfield at 1150 m a.s.l. at Abisko, Swedish Lapland, and one in a subarctic tree-line heath at 450 m a.s.l. at Abisko, southern part of the species' range. The manipulations included nutrient addit ion, shading and two levels of temperature enhancement using passive g reenhouses. The micrometeorological effects of the shading treatment w as similar to that of a mountain birch canopy and the temperature enha ncement treatments had the desired effect to increase the average air temperature by 2-4-degrees-C. Greenhouses which had a gap between the soil and the greenhouse plastic were particularly successful in creati ng the desired climatic perturbation without causing extreme maximum t emperatures or other unwanted side-effects. The environmental manipula tions caused strikingly different responses in the vegetative growth p attern of main shoots of C. tetragona among the three populations: at the subarctic tree-line heath, nutrient addition caused a substantial increase in growth, whereas it was the temperature enhancement treatme nts that caused increases, although smaller, at the subarctic fellfiel d and the high arctic heath sites. At the high arctic site, we also fo und growth reduced in response to shading, but at the subarctic sites, and particularly at the tree-line heath site, shading caused a marked etiolation of the shoots. Hence, different factors seem to produce ve ry different responses in the vegetative growth of C. tetragona in dif ferent parts of its geographical range. We conclude that competition f or nutrients and light are the main limiting factors for the growth of Cassiope tetragona near the lower distributional limit (LODIL) of the species, but that temperature is the main limiting factor in the nort hern parts of its range, and at high altitudes in the southern parts o f its range. We also suggest that the direct effect of predicted futur e climatic warming on the growth of Cassiope tetragona will increase t owards the north, whereas a possible indirect effect of increasing nut rient availability following a temperature increase will be the main e ffect in the southern and lower parts of its range. These responses co uld, however, be modified by shading from other species responding to environmental change by increased growth.