Vineyard weather is famously fickle, and it seems like every season brings surprises. This May, naturalists, gardeners, and others with a stake in the Wild Side all agree, the surprise is the earliest spring in recent memory.
How early is hard to state precisely. But forsythia bloomed at the very beginning of April, perhaps two weeks ahead of its usual schedule. Oaks were leafing out by the end of April, again weeks before one would expect. On the bird front, I heard my first chipping sparrow singing on April 12, a week early for this species, a short-distance migrant that usually appears quite reliably within a day or two of April 20. And field sparrows and towhees arrived on their territories in the state forest in early April, more like two weeks ahead of schedule. Of the 20 butterfly species I’ve observed so far, nearly all first appeared close to their record early dates, and new early records were established for about a quarter of these species. Most notable was a brown elfin Judy Holland found on April 5, beating the old record by ten days. Spring always catches me by surprise, but this time early is for real.
A look at National Weather Service climate data show what all these plants and animals are reacting to. I looked up March and April records for Providence, Rhode Island — not exactly the Vineyard, but close enough to approximate our conditions. Both months averaged about six degrees warmer than their historical norm; in March, every day except three was warmer than average, and in April, every day but five. Every single day in March made it above freezing; in the third week of the month, a run of days in the 60s and 70s (more like the 50s and 60s on the Island) set records. And the month of April stayed entirely frost-free. This last fact may be the most telling, since it is the presence of liquid water in the ground and in their tissues that allows plants to begin growing. In essence, our growing season began weeks ahead of schedule, and the rest of the natural world responded.
Is this a result of global warming? Well, you can’t really say that. Weather has always varied from year to year, and extreme events have always occurred from time to time. Safest in a case like this season, is to say that some essentially random convergence of forces pushed warm southern weather to our region much earlier than usual. But it’s also fair to say that climate change is gradually improving the odds of exceptional weather occurring in any given year: as the atmosphere warms and contains more energy, weather will average both warmer and more volatile in the future. Powerful storms, drought, extreme rainfall, and even, paradoxically, extreme cold snaps are likely to be as much a part of the picture as warmer weather.
For many species, the early spring is likely to prove beneficial. Many birds, for example, started nesting early and may have time to crank out a second or third batch of young, or at least have better chances of success if they lose nestlings to crows or cats and have to start again. The result could be an increase in the nesting population next year. Oak trees still suffering from the stress caused by caterpillar defoliation can surely use an extended growing season to rebuild their resources.
But the full picture isn’t quite that simple. The effect of an unusual season on any particular species depends in part on how that species interacts with its resources, and in part on how the future plays out.
Here’s a hypothetical example to show what I mean. Juvenal’s duskywing, a small, dark butterfly, emerges as an adult in early spring, laying eggs that hatch into caterpillars that feed frenetically for a few weeks and then go dormant until the following spring. To survive the winter and then carry out the energetic process of changing into an adult, these butterflies rely on the decidedly finite amount of energy they’re able to store as caterpillars.
If they start their life cycle very early one year, and then the following spring runs very late, their stored resources will have to last several weeks longer than usual. It’s possible that fewer caterpillars than usual would make it through this extended dormant period without running out of gas; a seemingly bountiful early season might actually contribute to higher than usual mortality for such a species before the next spring.
Or consider the likely event that spring will eventually do what is usually does on the Vineyard: you know, get crummy. Remember last June? Remember all the other years when you’ve cursed your way through a miserable May? If this happens, it spells trouble for many animals, which will have set in motion processes that they can’t easily slow or stop. The simpler biology of plants makes it easier for them to halt their growth temporarily in cold weather, and because of this, plants and the animals that depend on them could easily end up disastrously out of synch.
Consider the sleepy duskywing, a close relative of Juvenal’s, which was one of several species appearing at a record early date (they were common by April 21) this spring. This insect, as a caterpillar, eats only the young leaves of scrub oak, which is notoriously late to leaf out; some years, Vineyard scrub oak thickets are bare sticks into early June. In mild parts of the Island, this hardy oak is already producing leaves, but in the state forest, which is the population center for both scrub oak and sleepy duskywings, the buds are just starting to swell.
Faced with a spell of cold weather, the scrub oak buds will simply slow their development until conditions improve. But the duskywing can’t put the brakes on so easily. The adults, which may all have emerged by now, have a finite time window in which to lay their eggs, and the eggs will then hatch on an irrevocable schedule. If there are no suitable leaves for the caterpillars to eat when they hatch, they’ll starve — a process made all the faster if the weather is chilly. What seems like a promising start for this butterfly could still turn into a year of very low productivity.
Thinking about this exceptional season has driven home two points in my mind. First, “global warming” is something of a misnomer: our future holds weather that will be not just increasingly warm on average, but also increasingly chaotic. And this may complicate the lives of many forms of wildlife. And second, every year is different, and weather is constantly changing. So get out there and enjoy what we’ve got, while we’ve got it!