Monday, October 20, 2014

Fruit "Strategies"

One of the amazing things I learned from my encounter with the Peterson Field Guide to Eastern Forests concerns fruiting strategies.  From the plant's point of view, fruit is the way to give the kids (inside the seeds) the best chance possible for survival.  Among those plants which produce edible fruit in order to recruit animals to help with this, there are at least three different strategies in operation. 

The first is to get out front with a sweet fruit to attract resident birds and mammals, who will seek out this fruit for its sweetness, poop out the seeds probably some distance from the parent plant, and thereby "plant" the babies with a helpful accompaniment of fertilizer.  These fruits attract us, as well, so you can probably name a few: blackberries and raspberries (Rubus), and cherries and their relatives (Prunus).

Black raspberry (Rubus occidentalis): a sweet, early season fruit.

A second strategy is to produce a fruit high in fat that will be valued by migrating birds because of its high energy density.  Instead of sweet, these fruits are typically sour or pungent.  Such "high quality fruit" fruit is more expensive for the plant to produce, but recruiting migratory birds insures that, whatever the future holds, your children will at least be well-traveled!  Because migrating birds will often be in unfamiliar territory, many of these "high quality fruit" bearers have leaves that turn earlier in the fall, providing visual "foliar flags" that announce, "here's the good stuff!" to all comers.  Most of the "comers" are thrushes of one sort or another, catbirds and waxwings.  Around here, the high quality fruit-producers are spicebush, flowering and gray dogwoods, Virginia creeper, sassafras, and magnolia. 

I didn't think to take a closer look at the dogwood above at the time, so can't say what it is.
The one below is flowering dogwood (Cornus florida), which bears a high-fat fruit.

 Virginia creeper (Parthenocissus quinquefolia
is displaying its foliar flag, but has no fruit to advertise.

A third strategy is a miserly one that relies on animals' need for food in the lean months of winter and early spring, when better food is scarce.  This fruit is neither sweet nor high in fat, so is cheaper for the plants to produce.  Low quality fruit is the last chosen, often remaining on the plant into the spring.  Besides starving residents, these fruits will help migrating birds on their way back north.  Around here, the low-quality fruits are hawthorns, sumacs, chokecherry, greenbriars, roses, maple-leaved viburnum, foxgrape, poison ivy, hollies, redcedar, and bayberry.  

 The rose hips of this multiflora rose are low in boh sugar and fat,
and so may be hanging around for a loonng time.

Since reading the Field Guide to Eastern Forests, I have waited for fall to see if I could discern the high-fat fruit strategy, in particular.*  But I'm afraid migratory birds aren't keen on landing here in the urb: the flowering dogwood I've been able to reach has seen the fruit falling uneaten, or fattening squirrels.  I will have to visit Ponkapoag and its nice spice bush and sassafras populations and see if they're getting much traffic.

*Yes, strategy is a confusing word here, since it makes it seem that plants can make plans!  In reality, what looks like conscious planning is really the cumulative effect of variation and differential reproductive success: these modes of life diverged from common ancestors by mutations, and developed and still exist because the plants that have them succeeded and are still in the game. 

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