Data from: Where do we expect to find deep plant roots?

Plant roots have been observed up to 70 meters in depth – What would
compel a plant to root so deep? Earlier work shows that the climate, soil
and drainage all affect rooting depth, but with conflicting results. For
example, both the deepest and shallowest roots are found in arid regions.
We compiled >2,400 globally distributed rooting-depth observations
of individual plants and applied simple correlation analysis to assess the
impact of global climate, local topography and substrate, and individual
plant size, and their combinations controlling where and why plants root
deep. The provided dataset includes 2,458 observations of maximum rooting
depths of individual plants, and their abiotic and biotic variables across
>130 plant families, including 2,021 observations from Fan et al.
(2017)  and additional 437 observations synthesized as part of
this study. Using this data, we found that at the global scale, deep roots
are driven by climate. Both concentrated wet periods and prolonged drought
are required to drive deep roots, and we find the deepest roots in
semi-arid climates with strong precipitation seasonality or interannual
variability. At the landscape scale, drainage modulates rooting depth. An
accessible water table facilitates deep roots at midslopes but it is too
deep to impact roots further upslope. Instead, the deep vadose zone
moisture reserve is the primary driver for deep rooting. Thus the deepest
roots are observed on well-drained uplands with deep vadose zones under
climates with distinct wet and dry periods. At the plot scale, substrate
structure and hydraulic properties modulate deep rooting – B-horizons
limit deep roots, while woody plants often root below the bedrock surface,
provided it is fractured. At the individual plant scale, deep roots are
limited to high-biomass woody plants. Together, these findings sharpen our
understanding of where and why plants root deep, highlighting
intersections of climate, drainage, terrain and biomass and identifying
conditions where deep roots may serve as a lifeline during prolonged
drought, meanwhile weathering rock, sequestering carbon, and bringing the
living world far deeper than the conventional “root zone”.

人们已观测到植物根系最深可达70米——究竟是什么驱使植物将根系扎根至如此深度？过往研究表明，气候、土壤与排水条件均会影响根系深度，但相关结论存在矛盾。例如，最深与最浅的根系均发现于干旱区域。本研究整合了全球范围内2400余条单株植物的根系深度观测数据，并通过简单相关分析，评估了全球气候、局地地形与基质、单株植物体型及其组合对植物扎根深度的空间分布与驱动机制的影响。本数据集包含2458条单株植物最大根系深度的观测记录，覆盖130余个植物科，同时附带对应的非生物与生物变量；其中2021条数据源自Fan等（2017）的研究，剩余437条数据为本研究新合成的观测结果。基于该数据集，本研究发现：在全球尺度上，深根分布由气候主导。植物形成深根需要同时满足集中湿季与长期干旱两个条件；最深根系分布于降水具有强季节波动或年际变率的半干旱气候区。在景观尺度上，排水条件调控根系深度：可及的地下水位会促进坡中部区域的植物形成深根，但当地下水位过深时，则无法对上坡区域的根系产生影响。此时，深层包气带（vadose zone）水分储备是驱动深根的主要因素。因此，最深根系分布于兼具干湿交替气候、深层包气带与良好排水条件的高地。在样地尺度上，基质结构与水力特性调控深根发育：B层土壤（B-horizons）会限制深根延伸，而木本植物通常可在裂隙基岩下方扎根。在单株尺度上，深根仅见于高生物量的木本植物。综上，本研究细化了植物深根的分布区域与驱动机制，明确了气候、排水、地形与生物量之间的交互作用，并识别出在长期干旱期间深根可作为植物生命线的环境条件；同时，深根可参与岩石风化、碳固存，并将生命活动拓展至传统“根系层”之外的更深地层。