The role of trees in nourishing the soil has long been understood by ecologists, but the extent to which trees control belowground processes remains unclear. This is due in large part to the difficulties associated with measuring root activity and the dynamic interactions between roots, mycorrhizal fungi, and the soil environment.
To develop a better understanding of how photosynthates, the sugars produced by photosynthesis, influence patterns of growth and metabolic activity in the soil, a group of ecologists in Sweden decided to conduct a large scale girdling experiment. Tree girlding involves stripping tree bark all the way to the depth of the tree’s xylem, the long tubes that transport water from the soil to the leaves using negative pressure and the difference in water potential between the soil and the atmosphere. Xylem is located well inside the living tissue of a tree, but most importantly for a girdling experiment, it is located directly inside the phloem. Phloem is the piping that runs down the tree and transports photosynthates produced in the leaves via gravity into the rest of the plant. Thus by stripping a tree down to its xylem, the researchers are removing phloem and cutting off the supply of photosynthates to the roots and the soil. This allows them to study the effect of photosynthate removal on the soil without influencing roots, water transport or any other soil parameters.
In this large-scale experiment conducted in a boreal Scots pine forest in northern Sweden, six 900m2 plots were chosen and rouglhy 120 trees per plot were girdled. In each plot, half of the trees were girdled early in June and half in late August. Early and late girdling were used to detect whether phenological (seasonal) differences affected root photosynthate production.
The results of this experiment were striking. In the early-girdled plots, soil respiration declined by 27% compared to control plots within the first five days of girdling. (Remember, “soil respiration” refers to the amount of CO2 released by the soil. It is a proxy for total soil metabolic activity, including the respiration of microbes, fungi, nematodes and other small soil invertebrates, and even roots themselves! Thus more respiration = healthier, more metabolically active soil.) By the end of the growing season, roughly 50% less respiration had occurred in the early-girdled plots. The occurrence of ectomychorizal fungi, an essential nutrient-acquiring symbiote for most plants, was also dramatically reduced. The late-girdled plots responded even faster, with respiration declining almost 40% within the first 5 days of girdling. Interestingly, the researchers found less response to girdling toward the edge of the girdled plots, indicating that soil organisms here were acquiring some photosynthates from neighboring, ungirdled trees.
The more rapid declines of soil respiration in the late girdling plots fit with our current understanding of how trees use their photosynthates. Early in the growing season, and especially in conifer forests, trees allocate most new photosynthates towards shoot, bud and needle production. Simultaneously, trees begin tapping their root carbon stores to enhance above ground growth. Later in the growing season more photosynthates are sent belowground to nourish the soil community. By this point root carbon stores are also diminished. Girdling trees later in the growing season cuts off carbon supply to the soil at precisely the moment when it is being ramped up.
The flux of photosynthates to the roots has a big impact on belowground processes, with a clear seasonal component. In moving forward in our understanding of whole ecosystem carbon balances, understanding that trees are conduit, connecting the earth to the atmosphere and transforming both in the process, will, I believe, be an essential paradigm to adopt.
Ho¨gberg P, Nordgren A, Buchmann N et al. (2001) Large-scale forest girdling shows that photosynthesis drives soil respiration. Nature, 411, 789–792.