Environment Matters: An update on our first growing season with Ponderosa Pine planted at SEGA sites.
This June we planted well over one hundred Ponderosa pine seedlings at two SEGA sites on the Kaibab Plateau, just north of the Grand Canyon. The trees we planted started from seeds collected near Flagstaff, Arizona. To fully understand the story of these trees, I first want to step back and think about the larger story of a trees life.
Trees, like almost all plants, acquire energy from the sun and assimilate carbon from atmospheric carbon dioxide to make sugars, or food. This processes is known as photosynthesis. To truly make the system work, plants need a variety of essential nutrients, including water, nitrogen, potassium and many others. Much like how we acquire these nutrients from our diets, trees forage the soil to find these critical elements.
In many cases, trees find it difficult to acquire these nutrients in the soil on their own, so they seek help from a variety of soil organisms, especially mycorrhizal fungi. These fungi are a specific type of fungus that live inside of plant roots. They obtain their food directly from the plant’s carbon, and exchange give the plant soil nutrients and water that was otherwise difficult for plant roots to obtain. To be clear, these organisms incur a carbon cost to the plant, they are not free, but when a plant is in need of something, they are almost always worth it.
Climate change brings an interesting tidbit to this dynamic. Models continue to predict warmer conditions for much of the southwestern United States, but there is less clarity on how precipitation will change into the future. Regardless of this uncertainty, warmer conditions mean high evaporation rates, and this means less water available to plants during dry periods. Plants faced with shifts in their climate have three simple possible responses, they can die, adapt and persist, or move to a more favorable environment. How then, can we expect plant-soil microbe interactions to change as we observe shifts in our weather and climate patterns? If a plant were to migrate somewhere new, is it dependent on its microbes moving as well?
To address these questions, we planted trees in large tree pots, forcing them to grow with either the microbes they have always lived with (home team), or forced them to grow with an unique assemblage of soil microbes (away team). We then planted these trees at sites that are 1) the same site the plant seed came from 2) A site that is warmer and drier, thus simulate climate warming and 3) a site that is cooler and wetter. Our primary hypothesis was that plants would invest more carbon into their home team soil microbes, regardless of environment, and in turn would receive greater benefits.
Interestingly, the pattern we are observing is quite different and much more context dependent. Our findings so far (see figure 1) indicate that plants growing in warm dry environments grow much more when they are paired with their home team soil organisms. In fact, their growth is nearly identical to the same plants growing at their site of origin, suggesting that the soil microbes buffer against these stressful growing conditions. Oddly, nearly the opposite pattern is observed at the cool, wet site. At this site plants paired with their home team soil microbes are actually slightly smaller than those without their microbes. At this site, it could be that water is abundant enough in the soil that investing carbon into their microbial partners has little pay-off because the plant could easily obtain water from the soil without help from the microbes.
Figure 1: Aboveground biomass of ponderosa pine growing with different soil organisms communities at three sites along a temperature and moisture gradient. The bars indicate standard error of the mean. Notice how much smaller plants are when grown in sterile potting soil. Also, the benefits of having the home-team soil organisms in warm-dry sites is exceptionally large.
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