Dear All - Here are the summaries of the replies to my question about yellow flowers in the fall. A brief "summary" of the summaries shows the following major thoughts with my comments in quotes: 1. Perhaps the fall migration of hummingbirds, which prefer red flowers, selects for other colors at this time, and perhaps yellow. "I like this one, but why should this select for yellow preferentially?" 2. Yellows predominate in the Composites, which one person said originated largely in South America, and perhaps drives their fall flowering. Increasing xeric conditions may also select for this group of plants. "Perhaps, but then why did the Composites settle on yellow flowers?" 3. Shorter day lengths and solar angles may select for yellow flowers at this time of the year. "I can't see that this should be the case, but I can't rule it out either." 4. It could be that human disturbance has created habitats that favor fall flowering plants, which just happen to have mostly yellow flowers. "This is quite possible, and suggests that there may not be a dominant color by season, but that yellow predominates now due to human influences." 5. In the fall, pollinators may be relatively scarce, and perhaps yellow flowers do a better job at attracting them. "One person wrote that the predominance of white flowers in the spring does the same at that time of year. This is a very testable hypothesis." 6. Color predominance may be regional - several people pointed out that different colors dominate in their particular area.
Howie Neufeld Here is are the replies: Reasons for Dominance of Yellow Flowers in the Fall Interesting question. I look forward to hearing what responses you get. One possibility might be that, since hummingbirds seem to be largely attracted to red flowers (I think), and since hummingbirds are probably gone south by early fall, their absence might have some effect. Composites (which are usually yellow flowering) tend to bloom in late summer and fall. This may be an ancient vestige of their origination in South America (see Bremer 1994. Asteraceae). Or it may be that composites are often adapted to more xeric environments than the average flowering plant. In western North America, the summer and fall are increasingly dry for the most part. Ironic that you should find yellows more common. I've wondered why purple flowers seem to dominate in late fall in Alaska, predominantly in alpine areas: hare bell, fireweed, several asters, lupine, monkshood, and gentian to name a few. Needless to say, I'd consider the pollinators and/or light spectrum at that time of year (and at a given latitude). I don't have an adaptationist explanation for you, but a phylogenetic answer is that there are a lot of fall-flowering yellow composites (e.g., Solidago, Pityopsis, Chrysopsis, Heterotheca, Bidens, Helianthus, Silphium) and many of these and other Asteraceae are adapted for disturbed or open habitats. Close relatives of these, such as Aster, which do not produce yellow flowers, also flower in the fall. I think you may have answered your own question, though I'm no expert on this subject. Many of the fall flowers here in northern Utah are yellow and all of the common yellow ones--sunflowers, goldenrod, and rabbitbrush--are composites. So there is a strong phylogenetic component combined with the tendency of many of these composites to be very common species. Now that you've raised the question, I wonder what the ultimate explanation (i.e., Why yellow and not some other color?) for this pattern is? An interesting question. One hypothesis suggested for abundance of white flowers in the early spring flora is selection for a syndrome to maximize visitation by pollinators. During the limited time frame between thaw and canopy closure spring ephemeral flora need quick and "loyal" pollinators. Diversity in floral displays could select specialized plant-pollinator relationships. Could we apply this to yellow flowers in Fall? Maybe/maybe not. The overabundance of goldenrods, Bidens sp. etc in many habitats does present a different scenario than the spring ephemeral flora. Though the domination in many fields is related to human activities, land management, prior farming etc... A wild conjecture: Might it have anything to do with the effect of daylight length on pigments, as with the changes of leaf colour in other plants at this time? Most flower forms and colours are related to specific type of pollinators (think about the red-poppy guild in the Middle east that caters to large beetles as pollinators). So, my first question would be, what are the pollinators of these plants, and are they particularly active in fall? And don't forget, what appears plain yellow to us may not appear the same to an insect or bird with color vision that extends outside the human range. I asked my botany professor this exact same question 10 years ago. He did not know the answer and I was too occupied with the rest of my undergraduate course work to delve into it myself... My thoughts are perhaps it also has to do with the wavelengths of the spectrum and season. It may be a ltitle farfetched, but who knows? Perhaps the yellow flowers are the ones that benefit most from more intense or concentrated or [insert your own word here] wavelengths of the light spectrum during the fall season. Yellow, of course, being useless since it is the color reflected back to us. I would like to add something to the "pollinator idea". For some agricultural practices, I saw people using yellow plastic flags to attract bugs, and they seem to work pretty well. Farmers add something sticky to catch the bug on the flag, but I don't think this glue has a fragrance that attracts them. Anyway, maybe yellow is a good color to attract bugs, and this color increases the possibility of pollination for plants with no specific pollinators. Maybe this is related to the time of the year and bug's population dynamics too. I assume in summer the amount of bugs in this area should be high, but decrease while summer turns to fall. Then, if the availability of possible pollinators is low in fall, it should be a good idea to increase the probability to get pollinated using any bug available in the area, attracting them with the color of petals. But I think other questions can show up with this hypothesis. Thanks for the questions. It was fun to think about it! In our area, depending on where you are at (I am including most of Oregon), yellow can dominate in spring (leguminous shrubs such as Scotch brooms and gorse) on disturbed hillsides in western Oregon, or riparian/mesic meadows when the blues and whites of the Camass lillies give way to buttercups (Ranunculus). Later spring into summer brings various Rosaceae species such as Potentillas and Geums, as well as the composites in Arnica, Balsamorhiza, Solidago, Tragopogon, and a plant I can't remember the geneology or latin name, Nevada shooting star. Depending on where you are, there are also a lot of legumes as Melilotus and various lupines. Dandelions make the underside of the chin yellow from spring through summer. In central and eastern Oregon (and I will presume through much of the Great Basin) we are now experiencing a shrub layer making many square miles yellow, and creating havoc for the sinuses of many of our field workers. Rabbit Brush (Ericameria and Chrysothamnus spp.) are doing their thing now. I don't see any other yellow flowers. However, an often affiliated shrub (and common in the Ponderosa pine uplands), Purshia tridentata (bitterbrush), lightens the landscape with lemon in the spring. I just thought I'd share some examples. I think the yellow flower 'dominance' may be regional. Here in central Texas, we do have yellow flowers in the fall (usually Guttierezia spp.), but the dominant colors are actually white (from Euphorbia marginata or E. bicolor) or green (Ambrosia spp.). On the other hand, in the spring, yellows and reds dominate (from Coreopsis spp., Gaillardia pulchella, and Ratibida columnifera). -- Dr. Howard S. Neufeld, Professor Department of Biology 572 Rivers Street Appalachian State University Boone, NC 28608 email: [EMAIL PROTECTED] departmental webpage: http://www.biology.appstate.edu/faculty/neufeldhs.htm personal webpage: http://www.appstate.edu/~neufeldhs/index.html Tel: 828-262-2683 Fax: 828-262-2127
