March is often the month when the snow cover retreats. After several months of white ground cover we are suddenly confronted with browns, tans and greens. Lots of old leaves and dead plants are brown and tan, but what about those green plants? Who are they and why are they green?
Some plants, it turns out, are evergreen. Nearly all of the conifers maintain their needles year-round, the tamarack being the exception in this area. But down at ground level there are some smaller plants that remain green just like the conifers. Who are they?
It depends on where you are looking. In the swamps it is the Labrador tea, leatherleaf, bog rosemary and bog laurel. All are members of the Ericaceae family. On higher ground evergreen plants include trailing arbutus, goldthread, bearberry, pipsissewa wintergreen, round-leaved hepatica, and the pyrolas, to name a few of the more common ones.
So why be evergreen? What advantages does it confer and why don’t all plants evolve to be evergreen? Most plants that become evergreen are growing in less than perfect conditions. Perhaps the soils are too acid, or too dry, or they may lack certain necessary minerals, most specifically phosphorous. The absence of phosphorous has been linked to the development of evergreen leaves.
One might think that the cost in terms of energy used to produce a leaf would be higher in evergreens than in deciduous plants. Oddly this is not necessarily the case. Evergreens produce leaves built for the long haul. They use lots of lignin and fibers along with chemicals designed to defend the leaf from attack. Deciduous plants create leaves with high quantities of proteins and photosynthetic enzymes. Depending on the location and the type of leaves these energy costs turn out to be relatively equal.
On richer soils deciduous plants dominate. It is much simpler to develop leaves that are designed to function for the short term. They can be larger, thinner, and less well defended since their loss is not fatal to the plant in most cases. The ready availability of nutrients in the soil allows the plant to grow new leaves easily. Though it would seem that having evergreen leaves would allow the plant to photosynthesize all the time, evergreen leaves tend to be tough, waxy, thicker, and smaller. These characteristics tend to preserve the plant’s water and nutrients, while at the same time reducing the amount of photosynthate produced. So they may husband the plants existing nutrients and water but at the cost of producing less food.
The advantages of being evergreen include the slow, but continuous loss of leaves. This ensures a constant supply of nutrients as the leaves lost decompose. Because the leaves are usually thicker, stronger, and chemically better defended than deciduous leaves they decompose slowly, further delaying the loss of the nutrients. The presence of leaves year-round also aids the plant community by allowing any precipitation to leach small quantities of nutrients directly from the leaves to the soil. In soils that are nutritionally challenged this slow release often coincides with the nutritional uptake of the plants. A balance develops between the food lost and the food required. In deciduous systems the nutrients are dumped into the soils in one big leaf fall at the end of the growing season. Often these nutrients are leached away before the plants can take advantage of them. When this happens on poor soils deciduous plants are not able to thrive.
Because evergreen plants are so effective when it comes to retaining the nutrients they pick up from the soil they tend to do well on soils where nutrient loads are light. Conversely in fertile soils evergreen species cannot accumulate as much food as deciduous species in the same amount of time. Litter decomposition in nutrient poor and acid soils is slow. And evergreen leaves tend to add acids to the soils as they decompose further favoring those plants that are parsimonious with their resources. So when you walk about this spring and notice those bright, lovely green leaves think about cost benefits and soil chemistry, or just enjoy the moment and realize that there is more than one path to success.
John Latimer is well known throughout northern Minnesota for his phenology work. John will deliver the keynote presentation at the March 2012 Minnesota Family Woodlands Conference and appears weekly on KAXE radio in Grand Rapids, and audio and twitter archives are available here. We hope his work will be a frequent feature on MyMinnesotaWoods. This article also appeared in the Duluth Senior Journal. It is printed with the author’s permission.