Benjamin Franklin’s original daylight saving concept centered around the idea that humans could be more productive if only we had more daylight in which to get tasks done. Remaining productive in the winter, as any Alaskan knows, can be challenging. During the dark of winter, our botanical friends also rest from their primary task of converting sunlight into productive growth. However, plants will soon green up as the first days of spring lengthen into summer.
Chlorophyll-bearing plants absorb atmospheric carbon dioxide and convert it to sugars essential to growth through the process of photosynthesis. Inorganic carbon dioxide and water are transformed (or fixed) into oxygen and organic compounds like protein, fat, and carbohydrates. Plants that fix carbon are autotrophs or, literally, “self-nourishing.” The rest of us heterotrophs, or “other-nourishing” creatures, mustconsume autotrophs for the food we need to survive, either directly from a plant or somewhere up the food chain.
As long as we have sunlight, carbon dioxide and plants, we’ll have photosynthesis and food production. We know that days will always get longer in the spring, and that plants will lushly unfurl soon after. What we don’t know in a changing climate is how higher temperatures or more carbon dioxide might affect our northern ecosystems, especially when we think about how plants produce their food.
The two most common metabolic processes for fixing carbon are known as C3 and C4 pathways. In Alaska, most of our familiar local plants use the C3 pathway, including white spruce, mountain hemlock, willows, fireweed and alders. C3 plants require ample carbon dioxide in the atmosphere, abundant ground water, and moderate temperature or light levels. These plants originated over 500 million years ago, and today represent 95 percent of earth’s biomass.
Carbon dioxide enters the plant through stomata, or leaf pores. A catalyzing enzyme bonds the carbon dioxide using water into a six-carbon chain, which is then broken down into two three-carbon molecules (hence the name C3), releasing water and oxygen in the process through the open stomata. The smaller carbon chains then synthesize into glucose and more complex sugar molecules like starch. Plants produce all their parts this way, including fruits and crops that humans and other animals eat.
At high temperatures, the enzyme loses efficiency and burns more oxygen in C3 systems, requiring more energy. The plant suffers net losses of carbon and nitrogen, limiting their ability to grow and thrive. In dry conditions, C3 plants must shut their stomata to reduce water diffusion (transpiration) or they face dehydration and death. Closed stomata prevent carbon dioxide from entering the leaves and stops photosynthesis. C3 plants can lose up to 97 percent of their absorbed water to transpiration.
C4 plants, in contrast, have a more efficient photosynthetic process which conserves energy and water in high temperatures or drought conditions. C4 plants fix a three-carbon compound into a four-carbon compound (hence the name C4) using an enzyme in a deep cell layer away from stomata. This means that C4 plants can photosynthesize with stomata either opened or closed, unlike C3 plants which must keep stomata open for photosynthesis and risk dehydration.
The four-carbon compound is next rapidly converted into the same sugar products. C4 plants can therefore fix carbon at a much higher rate than C3 plants. Many cultivated food species are C4 plants including sugarcane, corn, sorghum, millet, and amaranth. In Alaska, native C4 plants are rare but include some wetland sedge family members such as rushes (Juncus), spikerushes (Eleocharis), beaked sedge (Rhyncospora) and coastal saltmarsh grasses like saltgrass (Distichlis). Many C4 plants are able to switch to a C3 pathway in cooler, drier conditions.
Until recently, C4 plants were thought to be newcomers to the autotroph scene, getting their evolutionary start during a steep drop in carbon dioxide levels 23 million years ago. However, recent research with radio-isotope dating of fossil pollen suggests that C4 grasses were present before the carbon level drop. By the late Miocene, 5 to 11 million years ago, C4 plants dominated several ecosystems.
So what caused the rise in C4 plants, and why aren’t they more prominent on earth today? One theory is that the C4 rise in the Miocene was a result of shifts in climate and fire regime, leading to deforestation in warm areas and a surge in C4 grasslands globally. As the climate cooled, animals that grazed on grass were replaced with those that browsed woody vegetation, and herbaceous C4 species weren’t able to compete. C3 plants, with fewer enzymes and specialized parts, and greater size had a head start in cooler, moister environments with average light levels. Fire-dominated and water-limited shrub-grassland systems still contain the highest proportion of C4 grasses today.
Today, although less than 5 percent of the world’s terrestrial plants use the C4 pathway, they fix about 30 percent of terrestrial carbon. So we know that C4 plants play a role in regulating carbon dioxide levels on earth. Climate change theorists wonder if increasing the proportion of C4 plants might absorb some of the excess carbon dioxide driving our global temperatures up.
Right here on the Kenai Peninsula we have seen major changes in the wake of spruce bark beetle and a warming climate. Anticipating what happens next is a big part of understanding which species are able to grow here now and in the future. Do we see a C4 field of corn where spruce forest used to be, happily fixing high levels of carbon? Probably not. But we may see more grasslands naturally appearing, or other shifts in shrub-forest-grassland patterns. Regardless of what’s growing out there, we’ll be happy to see our green friends each spring as daylight returns to the Kenai.
Dr. Elizabeth (“Libby”) Bella is an ecologist at Kenai National Wildlife Refuge. You can find more information about the Refuge at http://kenai.fws.gov or http://www.facebook.com/kenainationalwildliferefuge.