EXPERIMENTS ON THE CAUSES OF ALTITUDINAL DIFFERENCES IN THE LEAF NUTRIENT CONTENTS, SIZE AND DELTA-C-13 OF ALCHEMILLA-ALPINA

This paper describes experiments carried out to investigate the causes of high leaf nitrogen concentrations and high delta(13)C values in Al chemilla alpina L. growing at high altitudes. We investigated whether genetic adaptation, high levels of nitrogen input or low temperatures could account for these trends. In a field experiment, plants from two altitudes in the Scottish Highlands were transplanted to Great Dun Fe ll, a site in the Pennines of northern England. The experimental desig n was fully factorial: two altitudinal origins x two altitudes of grow th x two nitrogen levels. A second experiment used a controlled enviro nment to test the effects of temperature alone. The effects of altitud e in the field transplant experiment were very similar to those in nat urally growing plants. Leaf nitrogen concentration and delta(13)C were both higher at the high altitude, whilst growth declined and nitrogen per leaf was unaffected. An increase in potassium concentration with altitude was also found. Nitrogen addition caused increased leaf nitro gen concentrations but also increased nitrogen per leaf; delta(13)C wa s not affected and potassium and phosphorus concentrations decreased. The addition of nitrogen also increased mortality. Altitude of origin had relatively few effects but the population from the higher altitude did have a higher specific leaf area. Low temperature in the controll ed environment caused increased delta(13)C, decreased leaf size and in creased nitrogen and carbon contents, although the effect was less cle ar than the effects of altitude in the field. Gas exchange measurement s suggested that the delta(13)C effect was caused by a reduction in st omatal conductance. We conclude that the effects of altitude on this s pecies are principally the result of direct environmental modification s to growth rather than genetic adaptation. Of the various factors tha t change with altitude, temperature and a short growing season are par ticularly important; enhanced nitrogen supply through atmospheric depo sition promotes increasing leaf nitrogen concentrations but must be co nsidered in conjunction with other variables.