One of the complaints people make during winter and early spring in New England is how much they miss the color of foliage and flowers from the warmer months. In past NCF Science blog articles we’ve explored some of Nantucket’s evergreens (such as pine, cedar, and American holly), looked at the underlying architecture and details of deciduous twigs and branches, and learned about some little green guys in the form of club mosses.
There’s a whole subtle world of photosynthesis taking place in winter, ranging from cushions of moss to coatings or beards of lichen festooning the treetops. And even the trees, shrubs, and vines we think of as dormant in winter have some interesting things going on long before t-shirt weather arrives!
True, photosynthesis works a lot better in the growing season, which is mainly from May to October on Nantucket. At lower temperatures, but still above freezing–from 32˚F-50 ˚F–photosynthesis can still happen, but it’s slower. Also, winter air is colder and holds less moisture, so plants have to be protected from the “winter drought” brought on by cold drying winds and frost crystals forming inside their tissues, which can keep water from flowing freely. They need to regulate moisture loss and do that by closing their stomata on evergreen needles, which halts photosynthesis as then they are no longer taking in CO2 and releasing able to release oxygen. But during a time period when the weather warms up a bit (let’s face it, much of January this year on Nantucket) and there is plenty of moisture, plants can make food.
However, most of our trees, shrubs, and vines have shed their leaves by mid-late October, so what’s a deciduous plant to do? It turns out that some species have chlorophyll in a layer just under the protective surface coating of their bark, especially on young saplings and on the most recent growth of twigs from the previous summer. This part of the tree “greens or reddens up” when the temperatures and humidity are favorable, especially later in the season and on rainy days when the sun breaks through.
Sassafras trees (Sassafras albidum) are a prime example. Sassafras saplings have a thin smooth green bark when they are sprouting from the forest floor, and on a late winter or early spring walk, you can easily see a shift in color to green in the treetops of sassafras stands, making them stand out from nearby stands of tupelo (Nyssa sylvatica) which remain a dark battleship gray. Sassafras trees are poised to “make hay while the sun shines” even in winter; when we have a stretch of a day or two with the warmer temperatures of mud season — which happens a lot here in the fickle weather of our offshore island — they can harness some sunshine to help carry themselves through the season. And you will notice that the buds, which contain leaves and flowers, start to expand, adding even more color to the treetops.
Greenbriars (Smilax rotundifolia and Smilax glauca) are native lianas (plants which grow trailing and climbing stems like a vine, but with woody stems) with a similar talent for winter photosynthesis. While the canopy trees and shrubs are leafless, greenbriar lives up to its name with a dense green tangle of smooth stems that bear stout, sharp prickles. Often found in the understory of forests or in dense shrub thickets, greenbriar’s evergreen stems give it an edge over species that lack the ability to photosynthesize in cold weather—such as those with thick opaque, or flaky bark–like fox grape (Vitis labrusca). Deer actually enjoy devouring greenbriar stems, presumably by nibbling between the spines, and birds feed on the dark purple-black fruit into late winter when many more favored foods are long gone. Walking through the woods in winter or early spring, the green tangle, often up to six feet high, is a warning to keep out; the thickets are great shelter for smaller wildlife but a hazard to tender skinned humans!
Another color that may catch your eye in the winter landscape is the red stems and twigs of black huckleberry (Gaylussacia baccata) stems in our shrublands or the tips of red maple (Acer rubrum) and highbush blueberry (Vaccinium corymbosum) branches in our forests.
Huckleberry and lowbush blueberry grow in large patches, expanding outward by sending out roots that send up new stems; winter is a great time to look at these patterns of different clonal plants on the landscape. Sometimes the dramatic red color of the stems really stands out, perhaps due to the lighting or a recent rain. The shift toward more vivid colors may also be a clue that the plants are not strictly dormant. Red pigments are more of a mystery in the plant world than chlorophyll, but are thought to be protective, a type of sunscreen that may come from their antioxidant status. The rich red or red-purple colors often contain anthocyanin. People have long hypothesized that the red pigments are protective as they are common in new, tender leaves that are just unfurling (such as on red maple or shadbush) but then, why would some plants just have new green leaves? And in the spring, black huckleberry bushes sport green leaves and rich ruby red blossoms!
Checking back into some research on this topic, I found a recent experiment where researchers looked at several species, including a red-stemmed shrub, red osier dogwood (Cornus stolonifera, =Swida sericea) which is a common shrub in New England, albeit not native to Nantucket. The researchers (Gould et al 2010) did find evidence supporting the hypothesis that the red pigmentation was photoprotective (meaning that it acted as a type of sunscreen) for red osier dogwood and a few other plant species.
But why would it be a favorable trait to have red stem coloration in the winter months, when the sun is less intense, and the days are shorter? It could be that without leaves, more of the stems are exposed to direct sunlight, while during the growing season they would be shaded by the leaves, so they need some sun protection. Another of the findings of the research was that the photosynthesis of the red stems was less inhibited during cold temperatures (4˚C, or 49˚F) than in the green stems. So red stems may make it easier for a plant to photosynthesize while leafless in winter.
Keep an eye out for the color shifts in our trees and shrubs as the days get longer and the buds begin to swell. Areas in the Middle Moors are great to view expanses of black huckleberry, while the wooded swamps at Squam Swamp and Squam Farm are a great place to get a close look at red maple twigs and the red maple buds as they expand and bloom.
Squam and Norwood are also a great place to check out sassafras stands from a distance and along trail edges, and to steer clear of the walls of prickly greenbriar!