Even the Plants Know It's Spring!

Here's a March sight that I always look forward to:

Willow (Salix sp) catkins, Sedge Meadow FP,
Lake Co, IL  3/20/2014

Willow (Salix sp) catkins, Sedge Meadow FP,
Lake Co, IL  3/20/2014

These are willow catkins, just emerging from their buds. Soon, with a little warmth, they'll open up, and then the leaves will follow. This is usually one of the first plants to show that spring is here.

I'm not really sure which willow this is -- there are 400 species in the genus, although most of them don't occur around here. But ID'ing them to species is often quite difficult, regardless, since for many species you need both sexes of flower to key them out. And they're dioecious. That word is Greek for two houses -- in other words, there are separate male and female plants (so it can be rather difficult to say your two samples are from the same species!). If you're thinking that sounds normal, you're not a botanist. In most plants, both male and female structures appear on the same plant, in many cases in the same flower.

Why some plants have evolved a dioecious habit is still unclear. One explanation, to avoid self-fertilization and thus inbreeding, was discounted as early as the late 19th century -- Charles Darwin argued in favor of the idea in 1876, then reconsidered and argued against it in 1877 (1) , rather convincingly. Another explanation, niche partitioning to avoid intersexual competition, appears to have more going for it. (2) A particularly interesting example involves Arctic Willows (Salix arctica), where willows growing in wet areas are mostly females and those growing in dry areas are mostly male. (3) Dawson & Bliss showed that males transpire less water, and are more adaptable to varying water availability, and argue (but don't show) that this is due to differences in reproductive investments.

The marshes at Sedge Meadow Forest Preserve were noisy with Red-winged Blackbirds, Northern Cardinals, and American Robins today. In some respects, spring in the Midwest shouts its arrival with joy. By mid-April, the plants will be doing so. But for now, they whisper of warmth to come, and you have to listen, and look, close to see it.

(1) Darwin, C. (1877). The different forms of flowers on plants of the same species. John Murray, Albemarle Street.

(2) Cox, P. A. (1981). Niche partitioning between sexes of dioecious plants.American Naturalist, 295-307.

(3) Dawson, T. E., & Bliss, L. C. (1989). Patterns of water use and the tissue water relations in the dioecious shrub, Salix arctica: the physiological basis for habitat partitioning between the sexes. Oecologia, 79(3), 332-343.