Preparing for Summer - Summer World: A Season of Bounty - Bernd Heinrich

Summer World: A Season of Bounty - Bernd Heinrich (2009)

Chapter 1. Preparing for Summer

9 March 2006. THE GROUND IS STILL SNOW-COVERED, but I’ve smelled the first skunk, and the bog is threaded by mink and otter tracks. I’ve heard the first honking of Canada geese. Two big flocks flew over, very high, heading north. The plant life looks unchanged, except that some pussy-willow buds have recently started to show a little more white peeking out over the edges of their dark brown flower bud scales. The first snowdrops, in the pure, unassuming simplicity that I love, are poking their nodding flower heads through the snow. Yesterday evening I heard the first singing of a mourning dove. The first robin is back, long before a worm is in sight. It’s overcast and the forecast says “rain,” but even if snow were predicted I’d expect the male red-winged blackbirds to return any day now.

Spring is on the way, and I think the birds feel it too. Certainly the blue jays do. I was lucky to see their first convocation again this year. I first noticed a crowd of them making a racket at seven AM on the top of the bare branches of an ash tree—the same one where I saw them about this time last year. I counted at least twenty-four, but these jays were coming and going, so maybe there were many more. Those in the top of the tree were bobbing up and down in what looked like energetic knee-bend exercises, and calling at the same time. It did not look as though they were directing their attention in any particular direction or to specific individuals. There were no apparent pairs. I heard at least six to eight different calls, and each of these was given by the whole crowd during any one period of time; they kept “in tune” as the calls changed. I was mesmerized and watched their display for three hours. The top of the one large ash tree seemed to be their stage, the focal point of a dance that extended over a dozen acres. At times there were groups of birds leaving the tree and screaming. They flew in twos and threes and in groups of a dozen or more. Whenever they went—using slow, deliberate wing beats—to or from their main staging area, they changed to a different vocalization. Although the main aggregation broke up at nearly eight AM, a few pairs and individuals stayed around at least another two hours. They registered something that anticipates summer, and I assume their “dance” has something to do with courting and pairing up. Six weeks later, two pairs were still in the vicinity. I saw them busily coming to the edge of my recently dug frog pond to pull rootlets out of the ground for lining their nests.

SUMMER IN THE NORTHERN HEMISPHERE IS SHORT, AND preparation for it is long. Getting an early start in the race to reproduce is critical. The season is anticipated by most organisms through photoperiod, the relative hours of day versus night. The seasons can also potentially be read from the stars. During summer, fall, winter, and spring in the northern hemisphere the North or “Pole” Star, Polaris, is visible as a steady fixed point above the horizon. Its angle to the Earth used to indicate latitude to the mariner. Daily, the constellations turn once around this star, rising in the east and setting in the west. Nearest to it we see the Big Dipper and Little Dipper and Cassiopeia. All three constellations are visible throughout the year, although in winter, when the tilt of the Earth’s northern hemisphere is away from the sun, a new direction of the sky that was blocked during summer comes into view, with other constellations. Now the constellation Orion rises on the eastern horizon in the evening and dominates the southern sky, along with Sirius, a large star. During the summer in the northern hemisphere these winter stars are below the horizon, and the overhead sky is dominated by the Milky Way and three brilliant stars: Vega, Deneb, and Altair, of the constellations Lyra, Cygnus, and Aquila. Together these three stars, the “summer triangle,” are a clear sign of summer. Do the birds know this?

Whether or not any animals can read and interpret the changing seasons from star patterns, and from them anticipate and prepare for the seasons, is not known. We do know, though, that animals use star patterns for navigation during their migrations. Many birds migrate at night, mainly the small songbirds whose rate of energy expenditure is so great that they need the day to refuel more often than large birds do. They watch the stars and recognize the patterns of the night sky. We know from detailed experiments and observations that they orient to the North Star or, more likely (as we do), by the star patterns around it, such as the Big Dipper. On their northward migrations to their summer breeding grounds they fly toward the Big Dipper, just as slaves escaping north at night followed it, code-naming it the “Drinking Gourd.” When the birds return at the end of the summer, Polaris and the Big Dipper are on their backs or shoulders as they fly through the night sky.

Proximally, summer in the northern hemisphere is best defined, as already mentioned, by the period of sunlight and warmth that sustains active life. In the tropics “summer” is essentially endless; there are about 4,320 hours of daylight per year. Here in New England daylight is restricted to about 2,520 hours. And despite the much longer days in the arctic summer, there are fewer of them—not quite half those in New England. However, my calculation is an approximation only. I have for simplicity assumed (1) thirty-day months, (2) twelve months of summer with twelve hours of daylight per day in the tropics, (3) six months of summer with an average of fourteen hours of daylight per day in the temperate zone, and (4) two months of summer with twenty-four hours of light per day in the high arctic.

In early February the worst of the winter is yet to come, even though the days lengthen. On some days when the sun does come out, I hear chickadees singing, blue jays carousing, the great horned owl hooting, and woodpeckers drumming. But the weather, like these activities that depend on it, is unpredictable. In 2006 the spring was unseasonably cold and the fall was unseasonably warm. More snow fell in Vermont that April than had been recorded for more than 100 years. But in early February a raven pair that I know had already refurbished their nest, and the female was incubating a clutch of eggs. A pair of great horned owls then evicted the ravens and took over the nest, and in early April the owl perched on her eggs (or her small young, or both) in the nest mold, which became surrounded by a wall of snow a foot high. The ravens did not renest that year—there wasn’t enough time. They, like the owl, have a narrow window of time. They need to have their young independent by fall. So they get an early start on summer. They need all summer and then some. Nest building and incubation take at least a month; rearing the young takes another two months; and then the young adults need the summer to practice their hunting skills, while there are still numerous young animals around to catch.

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Fig. 2. Leaf and flower buds (in Vermont) of quaking aspen (left) and red maple (right). In each pair of twigs the thinner one bears leaf buds and the thicker one (from the top of the tree) bears flower buds.

The trees prepare for the coming summer nine months in advance, starting in July of the previous year, when they manufacture embryonic shoots, leaves, and flowers and enclose them in buds. They could potentially wait until spring (some, like black locusts, which flower late, do), but for the northern native trees it is apparently better to have at least the shoot-leaf buds ready-made to burst forth at a signal. It is too cold to make them in the winter, and their final signal for the buds to burst forth is warmth. The hitch, though, is that they risk death if they are fooled by any warm spell, such as the usual January thaw. Insects also prepare to be active at specific times in the coming summer. For example, the giant silk moths (Saturniidae) overwinter in the pupal stage, and like tree buds, they shut down their development from pupa to adult through late summer, fall, winter, and spring.

Insect development from the pupal into the adult stage is normally strictly temperature-dependent: the higher the temperature, the faster they become adult. But the overwintering moth pupae can hold back even if they experience warmth. Their rather amazing block to the developmental process can be removed only by subjecting them to both a sufficient length and a sufficient depth of cold. As shown by brain transplants, the developmental blockage originates in the insects’ brain; implanting a chilled “loose” or unconnected brain that has been subjected to the right day length (depending on the species) into the abdomen of a non-chilled pupa will start the process of development as it releases hormones into its host’s blood. Ultimately the pupae, by not activating their normal brain, are preparing and waiting for the next summer, which is about ten months in the future. And only then, at the right time, does the whole population of millions of them emerge relatively synchronously in a week or two to mate and lay their eggs. They must be timely, and quick; they live for only about a week.

Preparing for summer means being able to anticipate the upcoming season, which presupposes knowing (no consciousness is implied) what season you are in. Perhaps one of the most reliable seasonal cues is photoperiod, the relative hours of daylight versus dark in a twenty-four-hour cycle. Throughout late summer and fall the days get shorter, then lengthen again after the winter solstice. Thus, an organism that sees neither the stars nor the angle of the sun could potentially anticipate the approaching summer by registering day lengths.

To measure day length requires the use of a clock that, on our planet, runs on a twenty-four-hour cycle or period. Biological clock mechanisms with approximately this period have by now been demonstrated in one-celled organisms, plants, insects, birds, and mammals. But a clock, even if it has a correct period, is not sufficient to tell time, any more than a watch that has not been set to the local time. Biological clocks must also be set to the correct local time; to do its job, each clock must be sensitive to and synchronized by signals from the environment, in the same way that we set our watches to the time we may hear announced on the radio, or from some other cue.

Like any good watch, a biological clock doesn’t run faster or slower as temperatures increase or decrease, even though the individual chemical reactions that run it presumably do. However, like the windup wristwatches that we used to wear (before batteries), which would run perhaps a minute or two either fast or slow, so that we had to reset them every few days, biological clocks are never totally accurate and also need frequent resetting to the local solar time. For example, a circadian clock that runs fifteen minutes fast per twenty-four-hour day would be off by an hour within four days. But what are the clocks set to? Most biological clocks are set at the signal of either lights-out or lights-on, which in nature would normally be dusk and dawn, respectively. They thus indicate the actual time relatively accurately, despite the fact that their periods may not be exactly twenty-four hours. Once a clock is set and running, appropriately timed behaviors can be “read” from it and will be close to local time.

One of the first to show that the twenty-four-hour clock could be used by an animal to synchronize to the season was Erwin Bünning, in studies with the common white cabbage butterfly, Pieris brassicae. In the summer the caterpillars of this butterfly proceed without pause from the pupa to the adult stage in a couple of weeks, with the exact duration depending on temperature. In the fall the caterpillars still grow normally; but after they have entered the pupal stage, they stop further development regardless of temperature. If they didn’t stop, they would all hatch out when there is no cabbage for their caterpillars to feed on. So they do not continue developing into adults until the following summer. Bünning asked how the animals “know” what season they are in, and what they do about it. He found out that the caterpillars have a clever mechanism involving the use of their daily or twenty-four-hour clock.

Using their circadian clock, the cabbage butterfly larvae begin measuring time at a specific signal: as in most other species, the time of day when darkness turns to light. They then “sample” for the presence or absence of light after measuring off a specific time period—say, about fourteen hours (the exact time differs in populations adapted to various geographical areas). If, for example, in midsummer the day lasts fourteen hours, then they would “see” light when they sampled at their twelve-hour “window,” and then their central nervous system would interpret that as a long day (summer), and continue the normal cascade of hormones to continue their development. However, as the season progresses and the days get shorter, there eventually comes a day when they would experience darkness at the twelve-hour sampling window, and then with no light at that point they would shut down hormone secretion from the brain—until the signal is reversed the next summer, when development would proceed.

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Fig. 3. How an animal may determine the season by the length of day. Based on experiments with the cabbage butterfly caterpillar, using three different photoperiods.

Some organisms do not have access to photoperiod signals. For example, at the equator the photoperiod is an even twelve hours of light and twelve hours of dark all year long. Are animals there clueless about what season they find themselves in? Apparently not, because migrant birds who spend the winter in the tropics “know” when it is time to return north and breed in the summer. And contrary to folklore, the groundhog does not need to come out on 1 February to measure its shadow to decide whether or not to stop hibernating and begin its summer agenda. Even if it did, it would have to know when the first day of February is! Strangely, the groundhog probably does know the approximate date. In the 1960s and 1970s Eric Pengelley and coworkers showed that the ground squirrels (Citellus lateralis) can, in the absence of both light and temperature cues, go into and come out of hibernation according to an internal calendar-clock. Subsequently Eberhard Gwinner showed that European migrant warblers also timed their annual fattening, migration, and breeding schedules with reference to this “circa-annual” rhythm.

One of the most conspicuous and stunningly beautiful seasonal phenomena in the north temperate zone is the flowering and leafing out of the northern forest. Both the flowering and the leafing out determine the insect populations, which in turn make the summer world possible for the majority of the birds and most mammals.

Flowering and tree leafing are precisely scheduled events. By the end of January we’ve had three months of seeing all our trees starkly bare, and we’re still experiencing snowstorms and bitter cold. “Only four months to go” we think then, before the glorious time arrives when the buds break and the trees flower, and are again resplendent in the long-awaited and much-anticipated color, green!

Our impatient waiting is all the harder when we realize that most of the buds are ready-made all along, just biding their time to burst forth. Indeed, they were already fully formed on the trees the summer before, long before the brilliant leaf shows of early October and the shedding of the leaves a week or two later. Buds are embryonic stems with leaves in one package and embryonic flowers in another (as in alders, hazelnut, and birch), or young stems with leaves and flowers encased all together under the same protective leaflike scales (as in most species). All through winter the various types of buds experience and must survive snow, ice, storms, and thaws, and the tree must bear costs for them to have been produced so early. Grouse live for months almost entirely from eating the buds of trembling aspen and birch. Purple finches, pine grosbeaks, turkeys, and squirrels feed on the buds of maple, aspen, firs, and spruce. Red squirrels eat balsam fir and spruce buds (of both leaf and flowers), and indeed they may produce an extra litter of young in apparent anticipation of an episodic spruce cone crop. Although popularly said to be “psychic” and able to “predict the future,” they are not the first but are capable of the second—they get their cue from eating the flower buds that precede a seed crop.

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Fig. 4. Willow twig on 23 October, before the leaves were dropped, showing the “pussy willow” flower buds (along with two tiny leaf buds on the base of each twig, and a portion of the same twig drawn again the following April).

Prepackaging the leaves and flowers into buds the summer before they open normally has advantages that outweigh these costs. The main advantage is probably that it helps the tree to flush out quickly, and thereby to maximize the short growing season of about three months. In those three months the trees must not only produce their photosynthetic machinery, the leaves, but also use them long enough to repay their production costs to make an energy profit. Many animals take advantage of the early bud production, but trees are seldom fooled by a false start—which could occur because of a midwinter thaw, making them lose all their investment. As long as the buds maintain dormancy, they remain safe from freezing. Dormancy and cold-hardiness go together, through an evolved mechanism: the cold-hardiness is achieved in large part by withdrawing water from the tissues. Since water is required for the active processes of growth, development must wait until summer, when it is again safe to become hydrated. But how can the tree “know” when to start up and break bud?

Leaf buds and flower buds often open on very different schedules, even in the same species; and the schedules also differ between species. Most species of northern trees all leaf out at the same time, within roughly two weeks during mid-May in central Vermont and Maine, whereas forest tree flower buds open over a six-month span. Poplars bloom first, in early April, basswood flowers in July, and witch hazel in October. There are only relatively small differences between species in leaf bud opening (with quaking aspen and white birch being first; oaks and ash being last; and beech, maples, and many others being in between).

The buds of different tree species each open according to their specific local schedules, which are dictated by a complex interplay of cues involving hours of daylight, seasonal duration of cold exposure, and warmth. Warmth, as such, is not enough. For example, if sugar maples from the north are transplanted to Georgia, they won’t break bud there, because they receive insufficient chilling. Their strategy of determining whether or not winter has occurred is like that of the previously mentioned silk moth pupa, which also won’t break dormancy unless it (or at least its brain) is chilled for a sufficiently long time.

Although many trees have their primordia for both leaves and flowers packaged into the same bud (for example, apple and other Rosaceae, and viburnums) so that leafing out and flowering occur at about the same time, most of the northern forest trees allocate separate buds for leaves and flowers. This separation of buds appears to be adaptive, because it allows the plant to strategically separate its time of reproduction from the time of leafing out. It thus allows some wind-pollinated trees (the majority of northern trees) to flower a month or more before leafing out, when they can be more easily pollinated because there is less blockage of wind carrying pollen over the flowers. It allows other northern trees, such as bee-pollinated basswood, to be pollinated a month or more after the leaf buds have opened, when in late summer the bee populations have peaked and the bees will search for the flowers among the leaves. Similarly, witch hazel, blossoming in October, takes advantage of the winter months’ pollination that is available then.

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Fig. 5. Leaf and flower buds of quaking aspen, as they appear from the end of summer to early January, with the flower opening in the first week of February, after being kept warm indoors. Center shows speckled alder twig with leaf, and separate male and female flower buds.

How do the buds “know” when to open? Photoperiod has a strong effect, and to try to separate the effect of photoperiod from temperature I bagged (under triple layers of black plastic) half of each of a bush of beaked hazel and speckled alder—two of the earliest-blooming woody plants. I found that the darkness did not retard the flowering times. It seemed as though the buds’ opening is, instead, strictly controlled by temperature. However, this was a very small and select sample—two species of the very earliest-flowering trees—and the leaf buds did not open.

Leaf buds bide their time through the winter, even during thaws. I am impatient. By the winter solstice (21 December), when the nights are longest, I am already anxious to see any little bit of green leaf or colored flower. So, at that time, and in subsequent weeks over the next three months, I have developed the habit of picking some twigs with leaf and flower buds. I bring them into the house, put the stems into water, and wait (and hope) for some to open and show me whether or not they are ready for summer.

In 2006, on the solstice, I brought twigs of a dozen different species of trees and shrubs into the house and set them in water on the windowsill. Then every two weeks I again brought in the same kinds of twigs, and then I noted whether or not any buds opened, or which buds opened, to try to determine whether and when a sudden warming might release the buds’ dormancy.

I had expected that the schedules of the buds’ release might roughly parallel the trees’ normal flowering-leafing schedule, even though all the buds were already preformed the previous fall. To some degree that is what happened. From my first batch of twigs brought in at the solstice, only two of the nonnative species (forsythia and ornamental cherry) opened a few flower buds. Most of the flower buds died and dried, although the twigs remained alive and some of the leaf buds finally opened in February. But alder, willow, beaked hazel, quaking aspen, red maple, and elm brought in during January opened at least some of their flower buds after only six days. And some of the same species brought into the warmth in mid-March (one to three weeks prior to normal blooming outside) also began to expand or open flower buds in about the same time, three to six days later. As in the field, however, their leaf buds remained hesitant to respond to warmth, opening only about a month later. The leaf buds of some tree species, primarily ash, red oak, and sugar maple, showed no response at all even after two months in the warmth.

The restraint in leafing out, although proximally related to potential frost damage, is ultimately probably due to the danger of snow loading that could topple the trees (as discussed later). The risks of frost injury are different for flowers and for leaves. A tree lives for many decades or centuries. It can risk losing its flowers to frost in any one year because energy saved in not fruiting that year can be invested in growth or in fruiting the next year. Losing leaves, on the other hand, results in cutting off energy inflow and stopping growth and hence a falling behind in the competitive growth race for the light.

Tree buds break dormancy owing to local stimulation; prior chilling of one bud on a lilac stem enables it to flower while a neighboring non-chilled bud remains dormant. Similarly, certain chemical vapors applied to one lilac bud will cause it to open while an adjacent untreated bud remains dormant (Denny and Stanton 1928). Therefore, presumably, if a tree is kept in a warm greenhouse all winter it will not leaf out or flower in the spring, although if one branch of it has been protruding to the outside, then that one branch alone will leaf out and bloom. Such simple experiments show that timing—when to renew life for the summer after the long winter—is not left to chance. There are active mechanisms of repression and activation in the development of the buds, and those mechanisms are contingent on cost-benefit ratios. Low temperature plays a large role both in repression and in release, and the timing mechanisms reside in the tissues themselves—not in a central place that then sends signals to the rest of the plant’s body.

The twigs bearing buds that I stuck into a jar on my desk, while snowstorms raged outside and Fahrenheit temperatures dipped into the range below zero, and that then produced leaves and flowers, reminded me of summer to come. Beyond that, they reminded me of overeager runners who have prepared for a big race for over six months, and who are ready and set to wait for more and more specific cues that signal the start. The last “go” signal is a warm temperature pulse. Such pulses are sometimes reliable cues of the beginning of spring, but for the leaf buds, apparently only if they occur in late April or early May.