Many species interactions are visible – a bee visits a flower, a lion chases a gazelle, a mistletoe perches atop a tree. However, a whole host of other interactions remain invisible. These interactions may include microbes or other organisms that are too small to see, or the interactions may occur belowground, hidden from sight. Plants engage in extensive belowground interactions with other plants, soil arthropods, microbes, and fungi. Plants also serve as a link between aboveground and belowground subsystems, an area that is currently receiving much attention in ecological research.

If you want to know why and how plants interact with each other belowground then read this paragraph. If you’ve got that under control, of if you don’t like broad statements about ecosystem ecology, skip it. No matter how many nutrients a plant takes in, one of those nutrients will be the limiting factor. So what nutrient do plants need? There’s carbon, phosphorous, light, nitrogen, and a range of “micronutrients” that can be ignored for now. As for carbon, there’s plenty of CO₂ in the atmosphere – no problem there. Phosphorous comes from rocks, so unless the soil is very old* phosphorous is not likely to be the primary limiter of plant growth. Light can limit plant growth in some systems (imagine a small sapling in the rainforest), but still isn’t the most common limiter, which leaves nitrogen. Although plants add nitrogen to the soil over time, there’s never really enough to go around, and nitrogen is the nutrient that limits plant growth in most ecosystems. So as a very general rule, interacting plants are engaging in a contest for the same nutrient – and that contest comes down to who can deploy more roots, explore more volume of soil, find nitrogen hotspots, and efficiently use the nitrogen that is taken up.

Because nitrogen is so important, most competition between plants is likely to take place belowground; this may mean that the biggest impact neighboring plants have on each other’s traits is belowground as well. In a paper just published in Ecology and Evolution, we manipulated the possibility of belowground interactions (see image at top) and compared the importance of direct genetic effects (how genes determine the phenotype of the organism in which they are expressed) and indirect genetic effects (how a focal individual’s genes determine the phenotype of interacting individuals) for plant biomass production. We found that direct genetic effects were more important for aboveground biomass regardless of whether belowground interactions were allowed or not. When belowground interactions were experimentally excluded (by inserting a watertight, airtight divider in the soil between neighboring plants), direct genetic effects were still more important for aboveground biomass, but indirect genetic effects were more important for belowground biomass. The most interesting result here is that, under certain conditions, the expression of genes in a neighbor can have more effect on a focal individual’s phenotype than the expression of genes in the focal individual. That’s a big issue for traditional evolutionary models, which assume a direct relationship between an individual’s genotype and its phenotype.

Recently, a big issue in community genetics has been trying to evaluate how important genetic variation is relative to other ecological factors. Our results inform this debate for at least 2 reasons. First, most studies examine only aboveground interactions and could miss the importance effects that interacting plants have on each other’s belowground structures. Second, the effects of genetic variation are not only, so both direct and indirect pathways should be considered to accurately determine the importance of genetic variation.