The in situ root chamber: A novel tool for the experimental analysis of root competition in forest soils

Competition between the roots of mature trees in mixed forests is not well understood because adequate methods for studying this interaction under field conditions are not yet available. We present a novel root chamber (size: 90×70×30 mm) that allows growth monitoring of individual tree fine roots in the soil while altering root competition situations experimentally. Fine roots of mature trees that were carefully uncovered from the soil were allowed to grow for several months in the chamber which contained soil material from the root's close proximity. Fine root increment was quantified by optical root length determination at the beginning and the end of the experiment. By placing individual fine roots of a tree species together with a second conspecific or allospecific root, the chamber allows one to simulate conditions of intra- and interspecific competition, and to test hypotheses on intensity and direction of root competition in the soil of mixed forests.We investigated the applicability of the root chamber in a mature beech–oak mixed forest in which beech is known to be a superior competitor above-ground. One-hundred and six chambers with different combinations of beech and oak fine roots were exposed in the soil for 180 or 438 d. In two-species chambers, which contained one beech and one oak root allowing for interspecific competition, beech fine roots grew significantly faster than oak roots. Furthermore, beech roots tended to show a higher growth rate in two-species chambers than in single-species chambers (two beech roots: intraspecific competition). In contrast, oak roots tended to grow slower when placed together with beech than when growing together with oak. By expressing the competitive strength of beech and oak roots with the relative competition intensity (RCI) index, evidence of asymmetric interspecific root competition in favour of beech was obtained.The potentials of this technique are related to the fact that replicated experiments with fine roots of adult trees can be conducted in the field; a certain artificiality, which is inherent to all rhizosphere experiments, represents the main limitation. From this study we conclude that while there are some limitations, in situ root chambers represent an important step towards the experimental analysis of root competition in forests.