Deciduous Decisions

In the plant world there are two types of trees: deciduous and evergreen. Deciduous trees give us spectacular displays of fall color before shedding their leaves. Evergreens provide a shadow of summer with a splash of green during the bleak winter days. Both types of trees use the leaves (broad leaf or needle leaf) for photosynthesis, but obviously the deciduous tree evolved to opt out of photosynthesis during winter while the evergreens evolved to work a longer season. Which tree has the competitive advantage until the spring greenup?

Life in the wild is all about two things: making babies and being energy efficient. Depending on the environment and situation, being deciduous or evergreen offers a competitive advantage by saving energy, but of course it also comes at a price.

Deciduous trees typically live in areas with temperature fluctuations. Here in the northeast, we have a nice mix of deciduous trees: maples, oaks, hickories, poplars, and more. We also have four distinct seasons. Winters can be snowy, and for a tree that spreads it branches quite a distance from its trunk, not having leaves to catch a heavy snowfall could be the difference between losing limbs or not. Evergreens compensate for snowfall by having a conical shape and shorter branches, with the longest at the bottom. The branch structure allows snow to fall to the next level of branches relatively quickly.

Summer deciduous

While the loss of leaves saves branches, it costs the tree energy. A tree must have enough energy stored up for next spring’s new leaves by fall. Because photosynthesis uses energy (much like chewing and digesting your food burns calories), deciduous trees evolved to shed leaves when there is less sunlight, not enough to be worth the energy expense to generate some more energy. While the deciduous trees are standing idly through the winter, evergreens take advantage of their ever green leaves (either needles or broad leaves in the tropics) and photosynthesize for a longer season or year-round in the tropics. They gain a little more energy that way, and we’ll see why next.

Late fall deciduous

Those expendable deciduous leaves are fairly flimsy compared to a pine needle or palm frond, which are built to last. Less energy is required to build a full set of maple leaves than pine needles. More of the maple leaf is dedicated to photosynthesis than a pine needle, which needs more compounds for structure and defense. Unit for unit, a maple leaf generates more energy than a pine needle. Evergreens, conifers especially, use that conical structure to capture more sunlight to maximize energy production.

Summer evergreen conifers

Deciduous trees thrive in nutrient-rich soil. If there are plenty of nutrients to absorb, they can afford to shed leaves. They often draw remaining nutrients from the leaves back into the trunk before shedding leaves. They even contribute to the nutrient cycle with their fallen decaying leaves. Evergreens have adapted to nutrient-poor soil. They keep their leaves because of a lack of available nutrients. If you look at how green and full of life the tropical rainforests and dense forests of the Pacific Northwest are, you may be shocked at how poor the soil actually is. The nutrients are usually absorbed right out of the dead material into the upper layer of soil and into the living trees.

Winter evergreen conifers

The different leaf styles also lead to different strategies for water conservation. When stressed due to dry conditions, deciduous trees will shed their leaves early to stop water loss. During photosynthesis, water is drawn from the roots to the leaves to make it happen, and the trees exhale water vapor. Evergreens keep their leaves, but they have a waxy cuticle on them that helps prevent water loss.

Hopefully you have a better understanding of why trees do what they do. With fall here and winter on the way I think we can appreciate both leaf styles for the splashes of color they give us. How is the leaf situation in your neighborhood?
This week's information comes from an article by Frances C Smith in the journal Maine Naturalist. 
Frances C Smith: Smith, F. (1993). Evergreen vs. Deciduous Woody Plants: Which Wins Where. Maine Naturalist, 1(4), 205-212. doi:10.2307/3858181