Howie, Having seen the replies, I could not resist=20 making one more comment. Call me the=20 "non-adaptationist scrooge" if you will. One=20 thing no one has mentioned is that a likely=20 reason why most of the composites in the Astereae=20 we see flowering in the East right now are yellow=20 (and I would also argue blue and white, e.g.,=20 Aster and Boltonia), but not red, is that this=20 group lacks the floral structure/arrangement to=20 support bird pollination. So, red ray flowers=20 most likely were not common EVEN IN THE ANCESTORS=20 of this group. In other words, the lower=20 proportion of red flowers in the fall (in the=20 East, at least) may largely be the result of a=20 phylogenetic constraint within the Astereae=20 subfamily of composites, which represents a large=20 proportion of the plants flowering right now in=20 the East.
Sounds like a good question for a class. Steve At 9:49 AM -0400 9/19/06, Howie Neufeld wrote: >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 > >=ECInteresting 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.=EE > >=ECComposites (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.=EE > >=ECIronic 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).=EE > >=ECI 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.=EE > >=ECI 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?=EE > >=ECAn 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...=EE > >=ECA 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?=EE > >=ECMost 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?=EE > >=ECAnd 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.= =EE > >=ECI asked my botany professor this exact same question 10 years ago. He di= d >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.=EE > >=ECI 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!=EE > >=ECIn our area, depending on where you are at (I am including most of Orego= n), >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.=EE > >=ECI 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).=EE > > > > > >-- >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 -- Department of Biology PO Box 1848 University of Mississippi University, Mississippi 38677-1848 Brewer web page - http://home.olemiss.edu/~jbrewer/ =46AX - 662-915-5144 Phone - 662-915-1077
