Hello all: As a long time retired pollination ecologist I want to offer a cryptic summary of my research observations (unpublished) that seem relevant to the question asked by Howie Neufeld. Back in 1968, I spent a glorious summer with my wife and two small children (and an assistant) studying the pollination ecology of the flora in the prairie, boreal, and alpine ecosystems of Banff National Park, Alberta. This focussed mainly on bagging experiments to determine breeding systems (insect exclusion work and hand pollination) and on visitation by various pollinators (flower flies, butterflies, various bees, wasps). One aspect of the field research involved taking photographs of the flowers of every species (regardless of the colour of petals, sepals or floral bracts) both in b&w and through a filter that excluded virtually all light except in (invisible) ultraviolet (using a Kodak Wratten 18A filter which is a deep purple/black colour to the human eye). This did not include wind pollinated species. Film was developed in the usual way. In September 1968 I went to teach systematics at the botany department at Berkeley and continued photographing flowers of many families (from 'primitive' to 'advanced') opportunistically, including some tropical ones at Botanical Gardens in southern California. So, I ended up with a collection of photographs of UV reflectance and absorption patterns in some hundreds of flowers of many families. Unfortunately, I soon left my research into pollination ecology and went to work for an environmental organization in Canada. Hence, almost none of the findings were ever published, but I still have many of the photos up in the attic.
My field observations in many ecosystems across North America is that while fall flowering yellows do seem to predominate in some ecosystems, there are many yellow flowering species that come out from the early spring and through the summer. There are many yellow flowering species in the prairies in mid summer. Here in the northeast in spring or summer we have the ubiquitous dandelions, Trout Lily (Erythronium), the shrub Dirca palustris (snow still on the ground), Basswood (Tilia), Lithospermum, and many others. While the dozen or so goldenrod species tend to dominate the fall flora, there are also plenty of asters with their purple, blue and white flowers as someone pointed out. My observations suggest that the evolved perception capabilities of the higher pollinating insects (mainly Hymenoptera) is a key to the abundance of yellows versus the purple/blues...throughout the flowering season. These perception capabilities have co-evolved with specific light reflecting characteristics of the majority of flowers. The colour spectrum visible to these pollinating insects is a critical factor. Thus, whereas humans see three primary colours (with opposites being violet/indigo vs yellow, blue vs orange, and red vs green), these insects see the green world around them as variable shades of grey (or perhaps variable shades of green?). But they also see a brilliant ultraviolet (reflection of sunlight) and deep 'cyan or black' (strong absroption of sunlight). These two 'colours' (ultraviolet and 'cyan or black') are opposites in the colour spectrum of these insects). It so happens that yellow flowers either reflect ultraviolet (in which case these insects see them as brilliant points of light against a sea of variable grey (greenery to us) but many yellow flowers strongly absorb ultraviolet light totally or nearly so (in which case these insects see such flowers as "black or cyan holes" against the variable shades of grey foliage). It also happens that the blue/purple flowers and whites are absorbers of UV so presumably these can also be perceived and identified from the background. I plotted the intensity of reflectance/absorption characteristics against frequency of occurence (using the UV photographs from the wide range of Angiosperm familes, including some primitive Ranales). I don't recall the sample size just now but what emerged was a histogram having a frequency distribution showing a distinct U shape. On the left of the X axis were all yellow flowers (including many heterstylous species of different families) and on the right of the X axis were the whites, pinks, reds and purples (including some purple hetersytyled species). In the trough at the bottom of the x axis fell the few Ranales I was able to find in greenhouses or botanical gardens in southern California plus a variety of other flowers. I took this data to indicate that UV refectors and UV absorbers were derived specializations responding to foraging pollinators. What does this have to do with the greater abundance of yellow flowers in the fall? ...I have no idea, except to say that some enterprising pollination ecologist might take an interest in this kind of work. With digital photography, who knows what surprising insights about co-evolution would emerge. I have been away from pollination ecology for a long time and much more must be known about the above describe phenomenon must be known by now.. Cheers, Ted Howie Neufeld wrote: >Dear All - I have been asked by a colleague if I know why yellow flowers >seem to predominate in the fall, especially in open habitats, and I said >I don't have any good theories at this time. Does anybody out there >know about or have any theories? > It could also be that yellow doesn't really dominate, but that >certain species (goldenrods for example) are simply super abundant and >they flower in the fall and "appear" to dominate. >Thanks! >Howie Neufeld > > > -- Ted Mosquin Box 279, Lanark, Ontario K0G 1K0 Canada Anthology of Ecospheric Ethics <www.ecospherics.net <http://www.ecospherics.net>> A Manifesto for Earth (3 files at:<www.ecospherics.net <http://www.ecospherics.net>>)
