Plant volumetric allometry and shape above- and belowground

Abstract:

Very little is known about the space occupied belowground by plants in natural systems; however, the size and shape of plant root systems has major implications for our understanding of the earth system and the global cycles of carbon, water, and nutrients. Our aim is to find scaling relationships that can be used to predict the size of plant root systems according to the aboveground size of plants and the environment in which the plants were observed. Generally, much more is known about the size and traits of aboveground plant organs. Furthermore, biomass allocation above- and belowground, commonly referred to as the root:shoot ratio, has been comprehensively studied, whereas the same relationships are not well understood for plant volume or shape. To address these gaps, we created an extensive global dataset including over 2,000 observations of plant volumetric allometry describing the size and shape of plant growth both above- and belowground. The plant profiles ranged from small forbs and grasses to towering redwoods. The volumetric data was produced using a biometric machine learning algorithm, originally created to trace retinal scans, and a novel plant image analysis software to trace and measure plant profile images gathered from across the globe. Our results show that the volume of plant tissues scale nearly isometrically, with an alpha of 0.94, similarly to biomass allometry. We also found significant shifts in plant shape above- and belowground across global water availability gradients, where root systems were deeper and narrower in arid regions. Furthermore, we found that woody plants display much more plasticity in their volumetric allometry and shape compared to herbaceous plants. On average we found that plants were three times wider belowground compared to aboveground which results in a nine times larger surface area explored by roots. This study helps us understand the space taken up by roots in the soil matrix, and how this may change across plant species and climates. The tools developed by this study may also aid other scientists wishing to compute the volumetric allometry of plants via image analysis.