Three Evergreens

 In various posts over the past few months, we’ve talked a lot about deciduous plants — how their leaves die off in the fall, are shed in most species, and, in a few others, are retained throughout the winter. It only seems appropriate then to complete the picture by talking about those plants whose leaves remain green and active throughout the coldest months of the year — the evergreens!

The primary difference between deciduous and evergreen plants is not in whether the plant sheds its leaves, but over what time frame. Deciduous plants tend to be most common in places where there are strong seasonal differences in the availability of crucial resources, such as water or sunlight, which are necessary for maintaining metabolically expensive structures such as leaves. Here in New England, for example, winter temperatures often get below freezing, turning liquid water into ice and making it unavailable for plants to use. At the same time, plants get less access to sunlight due to shorter days and the sun’s lower position in the sky, making the leaves less useful for photosynthesis anyways. By shedding their leaves all at once in the fall, deciduous trees reduce the amount of resources that they need during the winter and are able to store what they do have in more secure locations within their trunks. In the long run, growing their leaves back in the spring, when water and sunlight is more readily available, is less taxing than holding onto them throughout the winter.

Evergreen plants, on the other hand, tend to be found in places where seasonal access to resources is fairly consistent. This can mean that resources are consistently abundant, as is the case for water and sunlight in the canopy of a tropical rainforest, or consistently scarce, as in a patch of nutrient-poor soil or a dry rocky outcropping. In these sorts of environments, the calculus of leaf-loss changes — where resources are consistently abundant, the photosynthetic capacity of leaves can be maintained year-round, while in places where they are consistently scarce, the cost of dropping and regrowing leaves every year will end up being too much. Crucially, this does not mean that evergreen plants never drop their leaves — visit a grove of pine trees and you will find that the forest floor is covered in mat of dead, yellow needles. Instead, evergreen plants drop their leaves slowly and steadily over time, only loosing individual leaves as they grow old and die, rather than all at once during the fall.

One interesting problem that evergreen plants growing in temperate climates have to deal with is water loss. As we’ve just discussed, water can be hard to come by during the winter when it freezes into ice, meaning that plants need to do all they can to conserve it. This is a problem for evergreens because leaves are one of the biggest sources of water loss in most plants. When a plant engages in photosynthesis, it needs carbon dioxide, which it gets through tiny pours in the surface of its leaves called stomata. In addition to collecting carbon dioxide, however, these pours also release water. Evergreens get around this problem by having smaller stomata, which reduces their intake of carbon dioxide and slows down photosynthesis in comparison to deciduous trees, but also reduces water loss. Evergreens also tend to have thick, waxy leaves that resist drying out. 

Here in the temperate United States, we typically think of coniferous trees — those with needles — whenever we hear the word evergreen and, indeed, these are the most common plants with this particular life strategy. They are not, however, the only ones. Below, are three of my favorite, broad-leaved evergreen plants that can be found here in New England. All are pretty common, and you would be likely to find any one of them brightening the path on your next walk in the winter woods. 

Mountain Laurel:

As a more-or-less proud native of Connecticut, I must of course start with our state flower, the mountain laurel (Kalmia latifolia). In the late spring and early summer, mountain laurels bloom with some of what I think are the most beautiful flowers in eastern North America, resembling the delicately rendered, sugary blossoms on a fancy wedding cake. The trees are especially striking during this time of year, but I would argue that they truly find their spotlight as the days get shorter and the stage clears of competing chlorophyll. During the winter, dense thickets of mountain laurel growing here and there on a forested hillside appear as pockets of living green, climbing vigorously over each other on gnarled, fairy-tale trunks towards the low hanging sun. 

Growing to be about ten feet tall, mountain laurels are understory trees which, like many of their fellow species in the heath family (Ericaceae) prefer to grow in acidic soils. This preference likely has a lot to do with the laurel’s evergreen leaves, since certain nutrients like phosphorus and potassium tend to be less biologically available in more acidic soils. This means that while the nutrients are present, they are “locked away” in a chemical form that most plants can’t use. As we’ve already discussed, evergreen leaves are one way in which a plant may adapt to a resource-scarce environment like that which is found in acidic soils, since regrowing their leaves all at once during the spring would be too costly. Mountain laurels and other members of the heath family also rely on a particularly strong relationship with mycorrhizal fungi, which fuse to the tree’s root system and trade extra nutrients for sugars produced via photosynthesis. 

Christmas Fern:

While evergreens can shed dead leaves during any time of year, many will only grow new leaves during the spring and summer when resources are at their most abundant. This still requires less resources than growing back all their leaves each year, since they are generally either replacing ones that they have lost or growing new leaves for newly extended branches. 

The Christmas fern (Polystichum acrostichoides) is an interesting exception to this general trend. Typically found in thick patches in well-drained, shady woodlands with acidic to neutral soils, an individual Christmas fern consists of a clump of fronds emerging from a rootstock in the center. During, the spring and summer, the fronds reach up vertically towards the sun. Then, when winter comes, they are flattened by the weight of snow (as shown in the picture above) and remain that way until they eventually die and decompose over the next summer. Meanwhile, the fern puts out new fronds in the spring which completely replace the older, dying ones. 

While they are lying flat on the ground during the winter, Christmas fern fronds hold down the leaf layer beneath them and, as a result, speed up the process of leaf decomposition. Ferns growing on steep slopes and riverbanks also play an important role in preventing erosion through the thick roots systems that form beneath large colonies, holding the soil around them in place. During colonial times, Christmas ferns were a popular element used to make Christmas wreaths and this, along with their evergreen status, is the reason for their festive name. 

Partridge Berry:

The partridge berry (Mitchella repens) is probably the least well know of the evergreens on this list, not because it is uncommon, but because at less than two inches tall, it simply isn’t a very noticeable plant. Like the other two species on this list, it is well-adapted to live in acidic soils so long as there is a rich layer of organic matter present to capture moisture — partridge berries are very sensitive to drying out. They typically grow in shady forest interiors and, as you could probably imagine given their small size, don’t require a lot of sunlight. In addition to their evergreen leaves, partridge berries have adapted to these tough conditions by growing very slowly. While working on a project recording the phenology of common plants in New Hampshire, I would typically only observe one or two pairs of additional leaves being added to our sample partridge berry plants each year. 

In the spring, partridge berries produce pairs of small, but vibrantly white, trumpet-like flowers that fuse together at the base. The flowers come in two different forms — one in which the pistol (the female part of the flower) is shorter than the stamen (the male part of the flower) and one in which it is longer. This structure prevents the flowers from fertilizing themselves and ensures the genetic diversity of their offspring. Interestingly, the partridge berry’s namesake scarlet fruits are the result of the ovaries of both flowers in a pair fusing together, such that both flowers must be fertilized in order for a berry to form. This unique reproductive strategy is reflected in the berries through two distinctive spots on their surfaces, where the flowers once met. As with their leaves, partridge berries produce only a few berries each year and, as their name implies, these are often fed upon by ground-dwelling birds which help to disperse the seeds in the feces. 

Sources:

Lady Bird Johnson Wildflower Center. (n.d.). Mitchella repens. Lady Bird Johnson Wildflower Center Plant Database. https://www.wildflower.org/plants/result.php?id_plant=MIRE

Lady Bird Johnson Wildflower Center. (n.d.). Polystichum acrostichoides. Lady Bird Johnson Wildflower Center Plant Database. https://www.wildflower.org/plants/result.php?id_plant=poac4

League, Kevin R. (2005). Kalmia latifolia. United States Forest Service Fire Effects Information System. https://www.fs.usda.gov/database/feis/plants/shrub/kallat/all.html

Missouri Department of Conservation (n.d). Christmas fern. Missouri Department of Conservation Field Guide. https://mdc.mo.gov/discover-nature/field-guide/christmas-fern 

Stritch, Larry (n.d). Plant of the week: partridge berry. United States Forest Service. https://www.fs.usda.gov/wildflowers/plant-of-the-week/mitchella_repens.shtml