As the Christmas season approaches here in the U.S., evergreen plants like spruce (for Christmas trees) and holly become culturally important. What does it mean to be evergreen?
The tea plant is an evergreen plant, meaning that it is green year-round. However, the term "evergreen" usually carries some additional meaning or connotation: the term "evergreen" is typically used to refer to plants that are green year-round in areas where a number of the other plants are deciduous (losing their leaves). The following photo shows an example familiar to people living in wet temperate climates like the eastern U.S., an evergreen pine tree in the midst of a bunch of deciduous trees in winter:
Evergreen plants are not the same as plants with needles (coniferous plants). There are broadleaf evergreens, like the tea plant, as well as deciduous needle plants, such as cypress, larch, and dawn redwood trees.
The fact that the tea plant is evergreen tells us several things about the nature of this plant, and in this post, I will explain a few things about what being evergreen means for a plant, from an ecological perspective.
Why are plants deciduous?
The short answer for why some plants lose their leaves and then later regrow them, rather than keeping them year-round, is that in certain climates, the environment or conditions are not beneficial for keeping the leaves year round. These unfavorable conditions usually take the form of certain periods of time that are either too cold or too dry. In the eastern U.S., the main seasonal stressor on plants is the cold winter. In other regions, such as parts of Sri Lanka or Africa, or even parts of the southwestern U.S., the main seasonal stressor is drought. Asia, including China, India, and southeast Asia, critical areas in tea production, are interesting in that they have both cold and dry conditions occurring together: this whole region has a strongly seasonal precipitation pattern, with dry winters.
When it is cold or dry, plants risk being damaged by the cold or drought. Plants can maintain their leaves under these conditions, but it takes resources, such as reserves of water, or expenditure of energy. Plants, like businesses, are faced with a cost and a benefit to keeping their leaves. Since plants produce energy from sunlight, and light levels are lower in the winter, the benefit of keeping leaves in the cold season is lower. The same is true of dry periods; most plants will not benefit much from keeping their leaves through dry periods -- even though there is ample sunlight, the plants usually just go dormant because they do not have enough water to continually photosynthesize.
The leaves of deciduous plants, like those of this pawpaw (Asimina triloba), tend to have relatively delicate or thin leaves that are light green in color, contrasting with the tougher and darker green leaves of evergreen plants. Why?
Think of a person trying to keep their home warm. People can choose to keep their home warm by burning more fuel, or they can invest more energy into insulating their home, allowing them to maintain the same amount of warmth while burning less fuel. Investing in insulating one's home is costly up-front, but pays off in the long-run. For this reason, homeowners and property owners are likely to invest in insulation, but short-term renters are unlikely to make similar investments.
Similarly, plants can keep their leaves safe through cold or dry periods by expending energy or water (a short-term expenditure), or by investing in a more robust leaf structure. The more robust leaf structure may include waxy coatings that help hold water in the leaf, conserving water during dry periods, and it may include resins and other chemicals that allow the leaf to operate at lower temperatures without expending energy. Evergreen plants tend to have tough leaves: they tend to be thick, leathery or waxy in texture, and tend to be very dark green in color.
The following leaves from Camellia sasanqua, a close relative of the tea plant, are a typical example. This plant, like all Camellias, has leaves adapted to both cold and dry conditions in winter:
Building a more robust leaf is costly to plants: firm structures and waxy coatings in particular are very energy-intensive to manufacture. If a plant is to get a good return-on-investment on a robust leaf structure, it needs to keep the leaf for a longer period of time, typically 2 or more years, so that it can gain back enough energy from photosynthesis to make the investment worthwhile. Like a business that needs to earn revenue to survive, a plant will die if it does not earn back its investment of energy through photosynthesis.
Why are some plants evergreen?
The explanation above establishes that plants are deciduous in climates with cold or dry seasons because the plants are not able to gain more than they expend by keeping their leaves year-round. If this is true, then why does one encounter some evergreen plants even in climates with periodic cold or dry seasons? And why do the coldest climates, such as the Boreal forests of northern Canada and high altitudes in warmer climates consist mostly of evergreen needle trees like spruce and fir?
The main factor is actually availability of nutrients, such as nitrogen. Deciduous trees drop their entire set of leaves every year, and need to resprout them. Building leaves not only takes energy, but also takes nutrients. Although the energy balance is in their favor, the balance of nutrients may not be. If the plants are growing in a nutrient rich environment, such as most of the forests of the Eastern or Midwestern U.S., the loss of nutrients is not much of an issue: the plant can just pick up new nutrients through its roots. But in areas with fewer nutrients in the soil, the yearly loss of nutrients is too crippling to the plant.
This phenomenon explains the dominance of evergreens in rocky outcroppings with poor soil, and on coastal areas and in pine barrens with sandy soil. It also explains the dominance of evergreens in cold climates, because as the climate gets colder, leaf litter breaks down more slowly, and eventually a point is reached at which few of the nutrients in the leaf litter are actually available to the plants growing in the soil.
Back to the tea plant:
The tea plant is evergreen, and it is a broad-leaf evergreen which grows tough, waxy leaves, quite unlike the tender, newly-sprouted leaves used to produce most of the tea we drink. Its leaves are adapted to get through both cold and dry winters characteristic of the Asian climates where it is native, and, in a broader sense, its evergreen nature is an adaptation to growing in nutrient-poor conditions, such as the rocky outcroppings of the Wuyi mountains.
What do you think?
Do you find the topic or phenomenon of plants being evergreen as interesting as I do? Do you have any thoughts to add?