Spatiotemporal origin of soil water taken up by vegetation

Vegetation modulates Earth's water, energy and carbon cycles. How its functions might change in the future largely depends on how it copes with droughts(1-4). There is evidence that, in places and times of drought, vegetation shifts water uptake to deeper soil(5-7) and rock(8,9) moisture as well as groundwater(10-12). Here we differentiate and assess plant use of four types of water sources: precipitation in the current month (source 1), past precipitation stored in deeper unsaturated soils and/or rocks (source 2), past precipitation stored in groundwater (source 3, locally recharged) and groundwater from precipitation fallen on uplands via river-groundwater convergence toward lowlands (source 4, remotely recharged). We examine global and seasonal patterns and drivers in plant uptake of the four sources using inverse modelling and isotope-based estimates. We find that (1), globally and annually, 70% of plant transpiration relies on source 1, 18% relies on source 2, only 1% relies on source 3 and 10% relies on source 4; (2) regionally and seasonally, source 1 is only 19% in semi-arid, 32% in Mediterranean and 17% in winter-dry tropics in the driest months; and (3) at landscape scales, source 2, taken up by deep roots in the deep vadose zone, is critical in uplands in dry months, but source 4 is up to 47% in valleys where riparian forests and desert oases are found. Because the four sources originate from different places and times, move at different spatiotemporal scales and respond with different sensitivity to climate and anthropogenic forces, understanding the space and time origins of plant water sources can inform ecosystem management and Earth system models on the critical hydrological pathways linking precipitation to vegetation. Global inverse modelling of plant water acquisition depth and isotope-based plant water use estimates demonstrate globally prevalent use of precipitation from distant sources, and that water-stressed ecosystems are well suited to using past and remote precipitation.

Automated summary

Global and seasonal patterns show that the majority of plant transpiration relies on current month precipitation, with only a small percentage relying on past precipitation stored in deeper soils, groundwater, or precipitation fallen on uplands. Different regions and landscapes exhibit varying reliance on different water sources, with deep vadose zone and valleys playing critical roles in plant water uptake. Water-stressed ecosystems are shown to effectively utilize past and remote precipitation.