Evergreen plants have been revered in temperate zones throughout history as symbols of hope, strength and everlasting life. So, it’s no surprise that these persistent plants are present in various cultures. Every year during the Holidays in Mad. Sq. Park, we decorate with a large evergreen tree and garland. This year we even added some evergreen boughs to add a bit more color to a washed out winter environment. These cultural decorations may hold different meanings for everyone, but no one can deny the agelessness of an evergreen plant, nor can they deny them as a symbol that spring will bring green yet again.
In temperate climates, broad leaves are both a blessing and a curse for plants. The high surface area found on broad leaf plants allow greater room for photosynthesis, and detrimentally, a greater area for desiccation during periods of freezing and drought. Those of you who were able to visit the New York Botanic Garden, after 2011′s October snow storm, will agree that trees can have structural difficulties in supporting the weight of snow along with a limb full of leaves. These reasons, among others, have caused many broad leaf plants in our climate to lose their leaves in the fall and re-sprout them in the spring when growing conditions are more favorable.
Despite the cold weather limitations to having leaves, not all plants have been forced to lose their leaves for the winter. Evergreen plants have evolved and even produced several advantages over their deciduous relatives. For example, producing an entirely new set of leaves consumes a lot of energy. The energy stored from the previous growing season needs to be accessed while water and nutrients need to be readily available. Also, growing new leaves can be a lengthy process for plants, especially when growing seasons are short. Evergreen plants have the advantage of photosynthesizing as soon as the ground thaws.
Evergreen plants are also well adapted to low nutrient levels because of their leaves’ retention of nutrients. Mountain laurels, Rhododendrons and Pines can be found thriving in highly acidic soils in the northeast. Soil acidity can negatively affect plant nutrient availability, making these soils ideal for evergreen vegetation. Evergreen leaf litter has a higher carbon-nitrogen ratio than deciduous leaf litter and it tends to acidify the soil over time, making survival more difficult for deciduous plants in the same area. Places like the New Jersey Pine Barrens are prime examples of this phenomenon.
Some evergreen plants have evolved to produce needle like leaves. These leaves have strong advantages over broad leaf plants in cold environments. The narrow shape of the leaves limits transpiration. Additionally, these leaves have low sap levels which help prevent damage during freezing temperatures. Finally, the waxy leaves on evergreen plants protect the plant from desiccating when temperatures are freezing or water is scarce.
The conical shape of pines and hollies helps the plants against snow weight. Snow is more likely to slide off branches rather than build up and break under the weight of snow and ice. The shape is also beneficial during photosynthesis. Leaves and branches at the top of the plant are the newest and are the most productive at producing energy. As the tree grows, less sunlight reaches the needles on the lower parts of the tree. The tree sheds these branches as they become unproductive.
Evergreen plants do replace their leaves gradually throughout the year. Some evergreens retain their leaves for only a few months while others like the Bristlecone Pine, wait up to thirty years before replacing their needles. Needless to say, evergreen tendencies are not only limited to woody plans. Herbaceous plants may retain their leaves to protect their crowns from extreme winter conditions.
So next time you take a look at a holly or pine, remember that the ageless beauty of these plants did not come easily. Many well designed adaptations were made in order for evergreens to become the symbol of hope and strength that they are today.
Enjoying the balmy weather,