Summer World_ A Season of Bounty - Bernd Heinrich [68]
The reason for the bird’s change of behavior from spring to summer depended on tree physiology and, unlike the sapsuckers, I had not considered the fluid mechanics of the trees’ “plumbing.” The early-arriving sapsuckers were accessing the sugar maple sap that was going up in the wood (xylem) of the tree, when it is apparently under pressure and comes out through any tiny puncture. Later in the season—and only then—when the xylem flow is low, and when our own traditional maple sugaring stops, they start taking sap from phloem in birches. The phloem, a living layer of the inner bark, transports sucrose and other products of the leaves down to the trunk and the roots. So the birches did not yield the rich phloem sap until they had put on their leaves, when they were transporting nutrients down.
On a birch, the sapsuckers make a large aggregation of wounds in a very distinctive pattern. Each individual wound is a somewhat square quarter-inch to half-inch bare patch where the bark is removed down to but never into the wood. Series of these bare patches are placed neatly one above the other in vertical rows, and many such vertical rows are created simultaneously side by side, all around the trunk of the tree, girdling it. Any one such lick site lasts for only three or four years before all of its phloem flow has been interrupted and the tree dies.
Fluid mechanics also explains why the sapsuckers end up making huge destructive lick sites on birches. When a sapsucker takes out a patch of bark on a birch tree in the summer after the leaves are on, the sap oozes down from above and eventually gets routed around the wound after the tree responds by plugging the open phloem channels. There is then still just as much phloem sap that descends down the trunk of the tree, and the bird can tap it as before by simply opening another patch of bark above where the tree had attempted to stanch the hemorrhage. Alternatively, more fluid is rerouted laterally, immediately adjacent to the original hole. The woodpecker may also take the next patch of bark out there, and so the area of little bare patches expands both vertically and laterally, gradually encircling the whole tree trunk as the woodpecker keeps a step ahead of the tree’s defense. This results in a band of dead tissue on the tree—a situation analogous to putting more and more stones into the middle of a stream to try to stop the flow, which isn’t stopped but instead, for a while, actually increases laterally. It pays the woodpecker to keep working on the same tree as the phloem flow becomes more narrowly channeled. Eventually, however, the next few taps will cut the flow off entirely, and then the tree dies. Luckily for the woodpeckers, they can simply drill into the next tree.
The isolated sap licks on birch trees are magnets for life during the summer. In 2004, when the woodpeckers established a lick station in a large birch near my cabin, I built a platform of boards in a maple tree adjacent to it, at about the same level twenty feet up. I sat there often to watch. In one perhaps typical watch (six to seven AM on 7 July 2005), when I again visited my sapsucker lick to “take its pulse” I saw five bald-faced hornets at one time. All these wasps were usually grouped at the same little square sap hole lick on one branch. Four red squirrels came for their sweets. I saw nineteen hummingbird visits (apparently, all by females or fledged young). There were eleven sapsucker visits, with up to four adults present, once, at the same time. I heard the contrasts of the resonant drone of the hornet queens versus the high whine of their smaller workers, as well as the deep hum of the ruby-throats; and also the sapsucker’s drumming, which seemed incongruous in the summer.
Fig. 28a. Juvenile sapsucker at a typical late-summer phloem sap lick, on birch.
Fig. 28b. Typical sapsucker xylem lick in spring, on a sugar maple. Note the tap hole at top center