Coarse root architecture: Neighbourhood and abiotic environmental effects on five tropical tree species growing in mixtures and monocultures

Most biodiversity-ecosystem function research focusses above-ground; if it considers roots, it considers fine roots, while coarse roots remain a "black box". We utilized the Sardinilla experiment in Panama, the oldest tropical tree diversity experiment, to test how species mixtures at the plot and neighbourhood scales and abiotic environmental characteristics affected 7 coarse root traits of 5 tropical tree species. Sardinilla was planted in 2001 with a gradient of native tree species richness of two-, three- and five-species mixtures and their respective monocultures. Root systems of 128 mature trees were excavated 0.5 m radially and one coarse root was fully excavated; unexcavated root lengths were predicted using species-specific allometric relationships. Root traits included the observed traits primary root length, diameter, and number, the number of secondary roots per meter of primary root, predicted root system extent and total predicted length of roots, and coarse root biomass. Tree size scaled positively with most root traits. Species identity (conspecific or heterospecific) and size of near neighbours were found to explain more variation (63.0%) in root traits than abiotic environmental characteristics (30.4%). Trees in waterlogged plots grew fewer, longer roots. Trees reduced root system extent but grew more roots when growing with conspecifics or growing in plots of high soil nitrogen and greater depth to water table. These findings suggest strategies employed by tropical tree species to both reduce competition with neighbours and make the most of local abiotic conditions, bringing us closer to understanding the role of coarse roots in the context of BEF research in forests.

大多数生物多样性 - 生态系统功能研究聚焦于地上部分；如果考虑根系，也只是考虑细根，而粗根仍然是一个“黑箱”。我们利用巴拿马的萨尔迪尼利亚实验（这是最古老的热带树木多样性实验）来测试样地和邻域尺度的物种混合以及非生物环境特征如何影响5种热带树木的7种粗根性状。萨尔迪尼利亚实验于2001年开展，设置了包含2种、3种和5种本地树种的混合以及相应单一种植的物种丰富度梯度。对128棵成熟树木的根系在半径0.5米范围内进行挖掘，并完整挖掘出一条粗根；利用物种特异性的异速生长关系预测未挖掘的根长。根系性状包括观测到的主根长度、直径和数量等性状，每米主根上的次生根数量，预测的根系范围和根系总预测长度，以及粗根生物量。树木大小与大多数根系性状呈正相关。研究发现，近邻的物种特性（同种或异种）和大小对根系性状的解释变异（63.0%）比非生物环境特征（30.4%）更多。在积水样地中的树木长出的根更少、更长。当树木与同种植物一起生长，或者在土壤氮含量高且地下水位更深的样地中生长时，树木会减小根系范围但长出更多的根。这些发现表明热带树木物种所采用的策略既能减少与邻株的竞争，又能充分利用当地的非生物条件，使我们更接近理解粗根在森林生物多样性 - 生态系统功能研究背景下的作用。