Fan et al. 2017. Hydrologic regulation of plant rooting depth. Proceedings of the National Academy of Sciences 114(40):10572-10577. Doi: 10.1073/pnas.1712381114
You may not have ever thought about it, but the depth to which plants reach into the ground to find water (termed “rooting depth”) is extremely important for understanding the world’s carbon and water cycles. Rooting depth constrains the depth at which plants can reach sources of water during dry periods or in dry climates. If trees cannot find sources of water to drink or “transpire” they will reduce their rate of photosynthesis, leading to decreased carbon uptake and water release, which can affect future climate conditions. Thus to have better estimates of future global climate, we need to know about roots and their distribution in the soil.
Unfortunately, roots are underground and extremely difficult to observe compared to other metrics like leaf area, tree height, or tree age. Investigation of rooting depth necessitates disruptive excavation of the plant, which is feasible for smaller annual grasses and shrubs but not for trees. Trees are so large that achieving a robust sample size of rooting depth observations through excavation is extremely costly and time consuming. Some researchers have taken advantage of trees being blown over from storms to investigate rooting depth while others have used data from trees that were scheduled to be removed for other reasons. Even with these opportunistic approaches, we still have collected very few observations of rooting depth from trees, especially within different climate zones so our understanding of what factors control rooting depth is basic at best.
How do we go about investigating rooting depth?
Us scientists want to identify environmental factors that can explain the global variability of rooting depth. If we can determine what factors influence rooting behavior, we can improve global estimates of rooting depth variability, and use this information to improve earth system models of carbon and water flux.
Previous studies have attempted to synthesize rooting depth observations to identify what environmental factors control rooting depth. These studies have concluded that climate and soil type are generally important. Climate is important because it is a large scale predictor of how much rain will fall in any one area. Soil type determines how easily water can travel (infiltrate) into the soil after a rain event. Soil type also affects how far water from lower in the soil profile (the groundwater table) travels upwards due to suction pressure from small soil pores – a process called “capillary rise” (Fig. 1; see an explanation of this process here). Capillary rise occurs to a greater extent in small textured soils compared to coarse textured soils. On the other hand, water can infiltrate to a greater depth in coarse textured soils relative to fine textured soils. These two characteristics, climate and soil type, both play a role in controlling how much water is available to plant roots.
However, a new paper, Fan et al. (2017), shows us that this is not the whole story. For instance one might assume that the deepest roots should be found in deserts and the shallowest roots in humid tropical areas. In reality, observations have shown that both the shallowest and deepest roots are found in deserts. How could this be? Fan et al. (2017) showed that although both climate and soil type affect plants’ rooting depth, the strongest predictor of rooting depth is actually water table depth (Fig. 2).
So what is water table depth? Well, water table depth is the distance from the ground surface to the soil depth where all soil pores are full of water or fully “saturated”. The study demonstrates that water table depth is an important proxy of topography or elevation, with well-drained uplands having large water table depths (groundwater is deep below ground) and lowlands having small water table depths (groundwater is shallow) (Fig. 3). Climate also plays a large role in controlling rooting depth because it largely controls how far rain is able to infiltrate into the soil through controlling total amount of rain annually. The authors argue that although soil type is still important climate and water table depth are the primary controllers of rooting behavior. To show this, researchers used a modeling technique in which they calculated the soil water supply profile driven by precipitation and evapotranspiration (the combined amount of water evaporated from the soil and the water transpired by plants), topography, and soil texture. They then compared the global frequency of occurrence of modeled rooting depths to the frequency of occurrence of observed rooting depth frequency to ensure that their model was similar to the observed rooting depth data.
Climate and water table depth interaction
This study explained rooting behavior as a response to both infiltration depth and capillary rise. In desert climates, plants can have both shallow and deep roots depending on topography (Fig. 4). Roots of plants located in the lowlands of desert climates are shallow because the water table is high even though rain amount is low while roots in uplands are also shallow because the groundwater depth is too deep to ever be reached. In between these extremes there are dimorphic roots (both shallow and deep parts for an individual plant) that are able to take advantage of rain derived moisture when it is available but also tap groundwater resources during prolonged drought periods. Roots of plants living in areas with seasonally arid climates can be both shallow (in lowlands) and dimorphic as well with upland plants showing rooting behavior that depends on the depth to which rain can infiltrate during the wet season. In humid climates, roots are generally shallow because rainfall is abundant regardless of topographic position.
Roots are an important connection between the atmosphere and the land. Unfortunately our ability to observe rooting behavior of plants is limited because they are buried beneath the soil. However this study and others have utilized our physiological knowledge of plant behavior and observable environmental conditions to improve our ability to understand and predict dark, mysterious world of roots. This increased knowledge of roots will improve our understanding not only of how our climate works now, but also about how landscapes may respond to future climate changes.