Deciduous vs. Coniferous

Have you ever wondered why some trees have leaves while others have “needles”?

There are two basic systems for trees, and each has its own advantages and drawbacks. And of course, as can only be expected – there are exceptions to all the “rules!”

Two types of coniferous tree needles. (MPF via Wiki)

The purpose of the leaves on a tree is twofold: the leaves help to facilitate the exchange of gases (they “inhale” carbon dioxide” and “exhale” oxygen; now go thank a tree), and to collect sunlight for photosynthesis. Small openings on the leaves, called stomata (singular: stoma), can open and close, allowing the passage of CO2 into the tree’s circulatory system. CO2 and sunlight combine to make essential sugars for the plant, but the open stomata also lose water during respiration.

In a tree’s ideal world, it would be bright and sunny all year long with high humidity. That way, the leaves could collect lots of sunlight without losing a lot of water, and not have to survive a winter. Since that only happens in the tropics, where competition for those amazingly available resources is high, trees have evolved some pretty neat methods of surviving winters in the rest of the world.

Deciduous, broadleaf trees in winter in the Mid-Atlantic region. Author photo.

Let’s do some basic vocab first.

Deciduous = the dropping of a part that is no longer needed, in this case leaves

Coniferous = bearing pinecones, most coniferous are evergreen

Evergreen = retaining leaves year round, therefore remaining “forever green”

Broadleaf = a thin, broad leaf structure with a good deal of surface area

Needle = a thin, long modified leaf typical of conifers

Hardwood = another commonly used word for deciduous, broadleaf trees

Softwood = usually refers to coniferous trees

So the difference in survival strategies between coniferous and deciduous trees boils down to two basic things: energy and soil.

Most broadleaf trees live in areas where the soil is pretty nutritious. This is critical for deciduous trees because it takes a big nutrient boost to produce hundreds or thousands of new leafy suncatchers. And broadleaves do, in fact, collect an enormous amount of sunlight compared to their coniferous cousins, meaning that they can photosynthesize at high rates through the warm season. Then, when autumn rolls around, the leaves can be dropped essentially on top of the tree’s roots, where they’ll recharge the soil with nutrients after composting. During the winter, most of the energy in the tree moves into its roots and it has a vastly reduced need for food, water, and growth during that time.

Beech tree leaves. (the Cat via Wiki)

Coniferous trees (cone-bearing, as opposed to singular seed-bearing), on the other hand, are typically evergreen. They retain their needles year-round, replacing them slowly throughout the year rather than all at once. Needles have a variety of benefits: they are smaller, more watertight and more windproof, and can photosynthesize all year long. Needles don’t collect a lot of sunlight themselves, but overall the tree can continue photosynthesizing at a reduced rate whenever sunlight is available during winter months. Needles, with their reduced surface area, are harder to destroy and less tasty to insects. Since conifers don’t drop all their needles at once, they don’t need a big nutrient boost in the spring – which is good, because conifers typically inhabit areas with poor soils and less water than their deciduous cousins.

Monterey Pine cone. (Fir0002/Flagstaffotos)

This is why you see so many deciduous forests in the Eastern half of North America – where the soil is rich and there’s more readily available water – and more coniferous forests in the Western half, where soils tend to be poorer and the climate is more arid. Deciduous trees seem to have a “live fast and hard” system while coniferous trees seem to have a “slow and steady” lifestyle. Both work, and well!

There are some exceptions to these very general guidelines, however, and they’re definitely worth mentioning.

Tamaracks and Larches (Larix species) are deciduous conifers. They’re found all over northern North America and contribute to the beautiful autumn displays as their needles turn a golden yellow before falling off. No one’s exactly sure what the benefit of losing needles might be to this conifer, but perhaps Larix species meet the “hard and fast” and “slow and steady” lifestyle somewhere in the middle.

Lovely yellow Larch in Autumn. (Credit: NW Ridgeback Blog: Nala, Justin, and Amber.)

Most deciduous trees don’t need much photosynthesizing during the winter because they go into a type of dormancy, but the Quaking Aspen (Populus tremuloides) is one deciduous tree that has adapted and learned to thrive in the poorer soils and colder climate of the Western US and Canada. The bark of the tree’s trunks contain chlorophyll and can photosynthesize in the winter months (when temps are above 27degreesF anyway). Now, it’s not terribly efficient (similar to shade-adapted leaves), but it helps.

Aspen bark. (Credit: What Tree Is it? by the Ohio Public Library Information Network.)

This, sadly, is the best photo I could find of green aspen bark. If you live near aspens, you can do a quick trick to see the green for yourself: lick your thumb and rub it on the bark. The faint yellow-green of the chlorophyll will magically appear before your eyes! I do not think the tree in the photo above had been licked, but it should have been.

There’s also something called a “Live Oak,” which is an evergreen oak tree (Quercus virginiana). Most oaks are deciduous, but the Live Oak retains most of its leaves year-round. As you could imagine, these trees are typically found growing in warm, humid climes of the Southeastern US and the Gulf of Mexico.

Live Oak, Quercus virginiana (Wiki)

Pretty cool, huh?