Root to Shoot

As technology advances, our knowledge of global ecology improves. In 2015 we used databases and satellite imagery to calculate how many trees live on our planet. For the first time, we showed that three trillion trees are distributed on Earth’s surface. And although satellite imagery is getting more and more precise, grey areas persist. Specifically, satellites can’t reveal what’s happening below-ground.

While a plant’s trunk and branches—the shoot—grow up and to capture CO2 and light, the roots grow down into the soil to collect nutrients and water essential for the development of all types of vegetation. We can easily see how big a tree is above ground, but it is more difficult to appreciate the size of the root system.

So, we wondered: How much does a tree need to develop below ground to meet its needs? Is the proportion of aerial biomass (the shoot) to root the same for any kind of vegetation? Does root development depend more on plant species or environmental conditions?

In the publication presented here we studied the root-to-shoot ratio in trees, shrubs, and grasses, in locations all over the globe to highlight the drivers of root development.

In this study, we used a large dataset including below- vs. above-ground biomass measurements done in forests, grasslands, and shrublands from all over the globe. In total, 17,817 biomass measurements were compiled and cross-referenced with 63 different ecological parameters (climate, topography, soil, etc.) to study the root-to-shoot ratio in each biome and the environmental factors affecting it. Thanks to this data, we developed a vegetation model able to predict the geographic variations in the root-to-shoot ratio for each vegetation type around the world.

All over the globe, and within all vegetation types, roots were more developed in dry and/or cold environments. Forests have the longest roots in boreal regions and seasonally dry tropical and subtropical regions (e.g. California or the Mediterranean region); grasslands have roots twice as long in temperate environments like montane and temperate steppe regions (e.g. Qinghai-Tibetana and Mongolian Plateaus); and shrublands have deeper roots in the boreal and arid biomes. Altogether, our new vegetation model predicts that on average, the biomass of forests, shrublands, and grasslands is 22%, 47% and 67% below-ground, respectively.

In our study, it appears that independent of the species or vegetation type, roots develop mainly according to temperature and water availability. This result supports the idea that a plant invests more into building roots in cold regions where nutrients are scarce or in dry regions where water availability is limited. But in dense forests where competition is mainly for light, plants will limit their investment in roots and favour the shoot.

Other important factors affecting root development depend on soil characteristics such as sand content and depth before bedrock. Roots were observed to be longer in sandy and deep soil. The necessity for deeper roots in sandy soil can be explained by the limited water availability typically associated with this kind of soil and also by an increased need for stability. A lack of soil depth before bedrock usually correlates with short roots in the associated vegetation.

For the first time, our study provides a model able to predict vegetation’s proportion of root biomass, depending on its environment, at any point on the globe. On average, 24% of all vegetation on Earth is located below ground. Previous global geochemical cycling studies overlooked this significant portion of the world’s biomass.

Our study provides a new tool for taking steps toward better modeling global carbon cycling and better assessing the impact of climate change on terrestrial carbon distributions. As we continue to expand our understanding of global above-ground biomass, our predictive model will account for the quarter of the total plant carbon stored below-ground and improve predictions about the role of total plant biomass on Earth under current and future climate scenarios.