Geographic variation in seed-setting by heather (Calluna vulgaris (L.) Hull) in the Scottish Highlands

Aim To assess the influence of oceanicity, altitude and latitude on spatial and temporal variations in the fecundity of heather (Calluna vulgaris (L.) Hull) in Scotland. Location Mountain study areas located throughout the Highlands of Scotland. Methods Seed-set was assessed as the number of seeds per heather capsule. A t each mountain, the mean number of seeds per capsule was recorded at five sites spaced at 150 m height intervals up to heather's upper altitudinal li mit. Germinability was measured by incubating seeds at 20 degreesC after ch illing at 2-3 degreesC for 6 weeks. Collateral cultivation of heather cutti ngs taken from sites below 450 m and above 900 m determined whether seed-se tting and germinability were under genetic or environmental control. Results Neither seed-setting nor germinability was under genetic control. S eed-setting generally declined with increasing site oceanicity and altitude but the effect of latitude was ill-defined. Mean maximum temperature and t otal rainfall during August-September were the principal environmental cont rols. Germinability varied less widely but tended to be least towards heath er's upper altitudinal limit. Main conclusions Seed-setting by heather varied in both space and time, dep ending mainly on late summer warmth. It was always copious below about 600 m but was sparse or even failed altogether on higher ground, particularly i n the hyperoceanic west Highlands.