I like Howie's nudging the discussion into the area of physiological
ecology. When I addressed issues of pollination biology is was partly to
suggest that they couldn't tell the whole story.
Mark Winterstein's conribution about the habitat diversity is
valuable because it lets us southerners know that specialization on various
habitats may be important driver of speciation. However, then we must ask
why alder and birch species don't map onto the same range of habitats. And
just to bring pollination dynamics back in, efficient long-range dispersal
of pollen and seed would tend to work against speciation by promoting high
levels of gene flow.
I also like Howie's description of how habitat-dependent conditions
for photosynthetic efficiency could lead to weak wood. I don't know if it
is true for arctic willow, but I have heard that for some more southerly
willows having weak twigs is in fact a means of asexual reproduction; twigs
break off readily and are carried by water till they lodge on a bank or
sandbar and start growing.
In an earlier post in the thread, Scott Chamberlain suggested that
willow is diverse in the Arctic because it underwent an adaptive radiation
there. I suppose he meant as opposed to having achieved species richness in
more southerly conditions and THEN invading the Arctic; otherwise his
comment still raises the question, "Yeah, but WHY?" Did giant tree ferns
form the swamp-forests of the Carboniferous because they were uniquely
equipped physiologically to do so, or did they do it because they happened
to be there and no arborescent gymnosperms or dicots were waiting around to
show them how it should really be done? How do historical considerations
like that tie in with other explanations of adaptive radiations. Will we
ever be able to even name all the relevant factors, let alone tell which
were actually most important in the process?
Martin Meiss
2008/8/27 Howie Neufeld <[EMAIL PROTECTED]>
> Dear All - I find it quite interesting that the majority (if not all) of
> the responses to David's question about why Salix predominate at high
> latitudes and altitudes are dominated by population ecologists, discussing
> mostly pollination syndromes, whereas contributions by ecophysiologists are
> entirely absent. One can not fully understand the distribution of a species
> without also considering it's physiological tolerances to biotic and abiotic
> stresses.
> This could especially involve aspects of hydraulic architecture (avoiding
> freezing-induced embolisms), phenology (leafing out only when the
> probability of xylem freezing is mostly past), achieving high rates of
> photosynthesis due to a short growing season, producing a high ratio of
> leaves to biomass to maximize whole plant carbon uptake,
> biochemical/anatomical/physiological tolerances or avoidance of flooding
> stress, and so on. These suites of ecophysiological/biomechanical
> adaptations might also have a major influence on reproductive strategies,
> yet there is little in the literature relating ecophysiological adaptations
> to reproductive adaptations; rather, the two are most often viewed as
> separate magisteria (to badly paraphrase Stephen J. Gould).
> To give but one example - if Salix do indeed have high rates of
> photosynthesis either on a leaf or whole plant basis, it would suggest they
> also have high hydraulic conductivities, which in turn would support high
> stomatal conductivities. This would mean they might have relatively weak
> wood (large lumens, thin walls). That in turn means a lowered mechanical
> advantage, or in other words, the tree can not support a lot of weight
> (think hanging, pendulous willow branches on some species). That in turn
> might mean producing small seeds which are highly dispersable, rather than 5
> lb coconuts. Just a thought exercise - I'm sure someone smarter than me
> (I?) can find logical flaws in this argument. The point is that we
> should be considering all aspects of a species ecology, not just how it has
> sex.
> Howie Neufeld
>
>
> --
> 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<http://www.appstate.edu/%7Eneufeldhs/index.html>
>
> Tel: 828-262-2683
> Fax: 828-262-2127
>
