Smithsonian Tropical Research Institute, United States
Tropical tree roots make up a significant portion of tree total biomass, but root systems are often overlooked in biomass estimates. Moreover, the vertical and horizontal extent of roots in the soil is not well understood. Belowground interactions among trees are known to occur before above-ground interactions, yet the scale and influence of these interactions are not fully understood, especially in relation to abiotic factors.
We excavated 40 trees representing 6 species down to 2mm root diameters to quantify the contribution of root biomass to total biomass. Our results showed that root biomass comprised more than 26% of the total biomass. Following the initial root excavation, an additional 150 trees were scanned by a terrestrial laser and 128 of those trees’ roots were excavated to investigate tree root extent. We found that tree roots were 2.6+/-1.1 meters wider than the crowns and that roots could travel horizontally more than 3 times the distance of the crown extent. Further, we found that root architecture, including. compactness, volume, depth, and distance, varied among species, even those within the same guild.
We collected data on various abiotic factors, including depth of water, slope, elemental nitrogen, organic carbon content (%), soil acidity, and soil bulk dry, and found that more variation in crown and root system traits was explained by biotic factors (65.8%) than abiotic factors (26.0%) at the plot level. Further, tree diameter and species identity (conspecific or heterospecific) of neighbouring trees were found to explain more variation (63.0%) in root traits than abiotic environmental characteristics (30.4%), highlighting the importance of competition in shaping tree morphology, both above and below ground.
Our findings underscore the need to consider root systems in carbon estimates and reveal how competition with neighbouring trees influences crown and root compactness in young tropical trees. By investigating below-ground interactions, we gained a better understanding of how trees interact and compete in their environment.
