A rapid and accurate visual assessment of nectar production can reveal patterns of temporal variation in Banksia ericifolia (Proteaceae)

Abstract

Banksia species are conspicuous in Australian heathlands and woodlands and provide a major source of nectar for a range of vertebrate and invertebrate pollinators. However, nectar production by Banksia is difficult to measure because inflorescences comprise large numbers (hundreds to thousands) of tiny flowers. Our aim was to quantify variation in nectar volume among B. ericifolia L.f. plants, but existing techniques were too time consuming or required destructive sampling of inflorescences. We estimated nectar volume by counting the number and estimating the size of nectar drops on a sample of flowers on each inflorescence. Our technique was fast (10 min per inflorescence) and a highly significant positive relationship was found between visual estimations and direct measures (capillary tubes and micropipettes) of nectar volume (r = 0.92; P ≤ 0.0001). This technique thus provides an alternative to current techniques for measuring nectar volume in plants that have large numbers of small flowers arranged in dense inflorescences. By using this technique, we found significant variation among days in nectar volume produced by each of five B. ericifolia plants (P ≤ 0.02). Furthermore, days of high and low nectar production were synchronised among plants. We concluded that the temporal variation found may be the result of fluctuations in environmental variables, rather than the age of an inflorescence. Indeed, nectar volume was found to be significantly correlated with a range of weather variables including daily maximum temperature (°C), precipitation (mm), wind speed (km h^–1) and relative humidity (%). However, larger-scale and longer-term studies are needed to adequately examine any relationship between environmental variables and nectar production.