Sow it Grows

In my last post, we examined the reasoning behind the seasonal color change of leaves. But why do some leaves fall and others don’t? The answer (as usual) is evolution. Everything in nature revolves around resource scarcity—that is, there are a limited number of resources in the environment for which organisms must compete. Deciduous and coniferous trees have different strategies for living and propagating to maximize their survival in different conditions.

Deciduous, or broadleaf, trees use a sprint strategy. During the warmer, wetter seasons they produce and use leaves with a large, flat surface area arranged in large canopies. The leaves absorb all available light during favorable conditions with no worry of low temperature or precipitation. When seasons turn cold and dry, the trees rescue available nutrients from the delicate leaf organ and shed the remnants. The tree then remains dormant until conditions are favorable for new growth, and the cycle starts again. This approach to survival works well in temperate and tropical climates by utilizing the abundance of both water and warmth. But higher climbs and latitudes change the availability of these resources, so deciduous trees are typically absent.

If we think about the problems with harsher conditions—dryness, cold, shorter growing seasons—it’s easy to understand why. Deciduous leaves are efficient at photosynthesis but also wasteful; they require a greater amount of ambient water to sustain themselves due to high water loss via evaporation. The greater loss of water means that arid environments suck out moisture from the plants too quickly for them to flourish. Also, the increased surface area of deciduous leaves not only allows the most amount of sunlight to be absorbed but also the most wind and frost to freeze, making them far too delicate for frigid climates.

This is where conifers come in. As opposed to the fast, expending behavior of their deciduous cousins, conifers use an unhurried tactic of long-lived durability. The leaves of conifers are typically referred to as “needles” for their slim, cylindrical appearance that punctuates in a point. The needles are covered by a waxy cuticle which prevents the evaporative problem found in deciduous leaves. Cacti use a similar method for retaining water in arid climates. The limited surface area of needles reduces the overall photosynthetic volume of the conifer, but the fact that it’s able to photosynthesize all year balances out the light requirements. Coniferous trees benefit from the reduced surface area because it makes them less susceptible to freezing. Conifers also produce antifreeze—proteins alter ice crystal formation to nullify its damaging effects. Some frog species exhibit a similar sugar-related mechanism that allows them to be frozen solid during winter! Needles will fall off eventually, but this is due to age and not as a result of strategy as with the broadleaf trees. New growth occurs directly after. Moving to the macro perspective, the conifer’s cone shape aids in displacing frosty films too. Snow is shed due to the tapered positioning of conifer branches, allowing them to mitigate problems associated with heavy accumulation of ice and snow.

Each strategy works best in certain conditions, although there is some overlap in environments and in strategy, like deciduous conifers which share attributes of both types. Unlike the story of “The Tortoise and the Hare,” both these strategies serve their respective species equally in the race for resources. It comes down to adapting to conditions, which is what evolution is all about.