# Hydrologic regulation of plant rooting depth: breakthrough or observational conundrum?

Alain Pierret, Guillaume Lacombe

<figure><img src="image_1.png" /></figure>

## To cite this version:

Alain Pierret, Guillaume Lacombe. Hydrologic regulation of plant rooting depth: breakthrough or observational conundrum?. Proceedings of the National Academy of Sciences of the United States of America, 2018, 115 (12), pp.E2669-E2670. 10.1073/pnas.1801721115. hal-03990284

HAL Id: hal-03990284

https://hal.sorbonne-universite.fr/hal-03990284v1

Submitted on 5 Dec 2024

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

# Hydrologic regulation of plant rooting depth: Breakthrough or observational conundrum?

Alain Pierret<sup>a,1</sup> and Guillaume Lacombe<sup>b</sup>

In PNAS, Fan et al. (1) propose that landscape-scale hydrologic convergence along topography is the main driver of rooting depth. Fan et al. (1) base their assessment on a compilation of published reports and the development of an inverse modeling applied at the global scale.

One of the central hypotheses of Fan et al. (1) is that the water table depth (WTD) both determines—as a readily available water supply—and restricts—due to lack of oxygen below the water table—rooting depth, as indicated by the alignment of points with 1:1 line in their figure 3F. However, some points in their figure 3F correspond to roots growing more than 10 m deeper than the WTD. Their figure S6 also shows that *Picea* and *Eucalyptus* roots grow below the WTD. Supporting information provided by Fan et al. (1) reveals that observer-expectancy bias (2) hampers the informative value of their dataset: "most excavations [were] terminated at arbitrary depths"; "the deepest roots were discovered accidentally"; and rooting profiles were "likely under-sampled." While stated, it is unclear how these strong limitations have been accounted for by Fan et al. (1).

Root growth within the water table exists in both temperate and tropical plant species and plant adaptations to waterlogged conditions are well documented (3, 4): hypertrophy of lenticels; development of aerenchyma and intercellular spaces; deposition of suberin in cell walls. Furthermore, the saturated zone is not necessarily oxygen depleted: high concentrations of dissolved oxygen in shallow water tables

flowing through sandy material have been reported (5). Fan et al. (1) never consider how dissolved oxygen and nutritive solutes transported by lateral ground-water flows can shape an environment conducive to root growth within the water table, as previously evidenced (6, 7).

Fan et al. (1) also hypothesize that plants preferentially take up water from the most readily available sources. Not only does this hypothesis contradict a putative absence of rooting at depths that exceed the WTD but it also overlooks the fact that trees do not always use the most readily available water (6). Root growth in saturated layers is often offset by enhanced access to nutrients (7, 8), and nutrient uptake rates at depth can be higher than that observed near the surface (9). Fine roots can also grow deep in fractured bedrock, where, under predominantly saturated conditions, they contribute to mineral alteration through respiration-induced acidification of the soil solution (10).

In conclusion, the analysis and modeling effort of Fan et al. (1) represent an original contribution that sheds new light on the determinants of plant rooting depths. However, we advocate that there is a pressing need to resolve the observational conundrum that we identify in this Letter and that new data on plant rooting depth, free of observer-expectancy effects, are still required to significantly improve our understanding and predictive capacity of the role of plants with regard to global hydrogeochemical cycles.

1. Fan Y, Miguez-Macho G, Jobbágy EG, Jackson RB, Otero-Casal C (2017) Hydrologic regulation of plant rooting depth. *Proc Natl Acad Sci USA* 114:10572–10577.

2. Ralph DF, Hatch Ralph M (1983) On the psychology of watching birds: The problem of observer-expectancy bias. *Auk* 100:755–757.

3. Parolin P, et al. (2004) Central Amazonian floodplain forests: Tree adaptations in a pulsing system. *Bot Rev* 70:357–380.

4. Dittert K, Wötzel J, Sattelmacher B (2006) Responses of *Alnus glutinosa* to anaerobic conditions—mechanisms and rate of oxygen flux into the roots. *Plant Biol* (Stuttg) 8:212–223.

5. Prince KR, Franke OL, Reilly TH (1988) Quantitative assessment of the shallow ground-water flow system associated with Connetquot Brook, Long Island, New York. United States Geological Survey Water-Supply Paper (US Government Printing Office, Washington, DC), 2309.

<footnote><sup>a</sup>Institut d'Écologie et des Sciences de l'Environnement de Paris (Institut de Recherche pour le Développement-Sorbonne Universités-Université Pierre et Marie Curie-CNRS-Institut National de la Recherche Agronomique-Université Paris Diderot-Université de Paris-Est Créteil), Department of Agricultural Land Management, Vientiane, Lao Democratic People's Republic; and <sup>b</sup>International Water Management Institute, Southeast Asia Regional Office, Vientiane, Lao People's Democratic Republic</footnote>

<footnote>Author contributions: A.P. and G.L. wrote the paper.</footnote>

<footnote>The authors declare no conflict of interest.</footnote>

<footnote>Published under the PNAS license.</footnote>

<footnote n="1"><sup>1</sup>To whom correspondence should be addressed. Email: alain.pierret@ird.fr.</footnote>

<footnote>Published online February 27, 2018.</footnote>
6 Bowling DR, Schulze ES, Hall SJ (2016) Revisiting streamside trees that do not use stream water: Can the two water worlds hypothesis and snowpack isotopic effects explain a missing water source? *Ecohydrol* 10:e1771.

7 Lintern M, Anand R, Ryan C, Paterson D (2013) Natural gold particles in eucalyptus leaves and their relevance to exploration for buried gold deposits. *Nat Commun* 4:2614.

8 Armstrong W, Justin SHFW, Beckett PM, Lythe S (1991) Root adaptation to soil waterlogging. *Aquat Bot* 39:57-73.

9 da Silva EV, et al. (2011) Functional specialization of eucalyptus fine roots: Contrasting potential uptake rates for nitrogen, potassium and calcium tracers at varying soil depths. *Funct Ecol* 25:996-1006.

10 Hasenmueller EA, et al. (2017) Weathering of rock to regolith: The activity of deep roots in bedrock fractures. *Geoderma* 300:11-31.