The Deal Behind Evergreens

Alas, a challenger approaches!

My employers wished that I incorporate more sciencey articles into our written material. After staring at our bare garden framework for a bit, this was about the best I could come up with. (Please leave the evolutionary time scale out of this. I'm aware that conifers come first.)

Evergreen plants have been revered in temperate zones throughout history as symbols of hope, strength and everlasting life. It’s no surprise that these persistent plants play such an important part in various cultures. This winter the park was decorated with a large evergreen tree, garland, and boughs and while I'll admit that I humbugged through all of it, the park did look rather nice. I still can't support the concept of a Christmas tree, but that's just me. Needless to say, these cultural decorations 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 checked out New York Botanic Garden after last year’s freak October snow storm will agree that trees can have structural difficulties in supporting snow weight along with a full limb of leaves. These two reasons have caused many broad leaf plants in our climate to lose their leaves in the fall and resprout them in the spring when growing conditions are favorable.

Despite the cold weather limitations to having leaves, not all plants have decided to go bare for the winter. Evergreen plants have evolved and even produced several advantages over their deciduous relatives. Producing an entirely new set of leaves is very energy consuming. Energy stored from the previous growing season needs to be accessed while water and nutrients need to be readily available. Leafing out can be a lengthy process for plants in short growing seasons. Evergreen plants can catch the worm by photosynthesizing as soon as the ground thaws.

Evergreen plants are also well adapted to low nutrient levels because of their leaf retention. 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 because narrow shape of the leaves limits transpiration and they have low sap levels which help prevent damage during freezing temperatures. Leaves on evergreen plants are often waxy. This coating protects 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, be reminded of that crazy old relative who’s not only been through everything, but also knows how to make lemons into lemonade.