Grégoire T. Freschet, Oscar J. Valverde‐Barrantes, Caroline M. Tucker, Joseph M. Craine, M. Luke McCormack, Cyrille Violle, Florian Fort, Christopher B. Blackwood, Katherine R. Urban‐Mead, Colleen M. Iversen, Anne Bonis, Louise H. Comas, Johannes H. C. Cornelissen, Ming Dong, Dali Guo, Sarah E. Hobbie, Robert J. Holdaway, Steven W. Kembel, Naoki Makita, Vladimir G. Onipchenko, Catherine Picon‐Cochard, Peter B. Reich, Enrique G. de la Riva, Stuart W. Smith, Nadejda A. Soudzilovskaia, Mark G. Tjoelker, David A. Wardle, Catherine Roumet

1. Ecosystem functioning relies heavily on below‐ground processes, which are largely regulated by plant fine‐roots and their functional traits. However, our knowledge of fine‐root trait distribution relies to date on local‐ and regional‐scale studies with limited numbers of species, growth forms and environmental variation.
2. We compiled a world‐wide fine‐root trait dataset, featuring 1115 species from contrasting climatic areas, phylogeny and growth forms to test a series of hypotheses pertaining to the influence of plant functional types, soil and climate variables, and the degree of manipulation of plant growing conditions on species fine‐root trait variation. Most particularly, we tested the competing hypotheses that fine‐root traits typical of faster return on investment would be most strongly associated with conditions of limiting versus favourable soil resource availability. We accounted for both data source and species phylogenetic relatedness.
3. We demonstrate that: (i) Climate conditions promoting soil fertility relate negatively to fine‐root traits favouring fast soil resource acquisition, with a particularly strong positive effect of temperature on fine‐root diameter and negative effect on specific root length (SRL), and a negative effect of rainfall on root nitrogen concentration; (ii) Soil bulk density strongly influences species fine‐root morphology, by favouring thicker, denser fine‐roots; (iii) Fine‐roots from herbaceous species are on average finer and have higher SRL than those of woody species, and N₂‐fixing capacity positively relates to root nitrogen; and (iv) Plants growing in pots have higher SRL than those grown in the field.
4. Synthesis. This study reveals both the large variation in fine‐root traits encountered globally and the relevance of several key plant functional types and soil and climate variables for explaining a substantial part of this variation. Climate, particularly temperature, and plant functional types were the two strongest predictors of fine‐root trait variation. High trait variation occurred at local scales, suggesting that wide‐ranging below‐ground resource economics strategies are viable within most climatic areas and soil conditions.
   

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Root traits are related to plant water-use among rangeland Mediterranean species

F. Fort ^ab, F. Volaire ^c, L. Guilioni ^d, K. Barkaoui ^e, M-L. Navas ^a, C. Roumet ^b

^a Montpellier SupAgro, CEFE UMR 5175, Université de Montpellier – Université Paul Valéry – EPHE, 1919 route de Mende, 34293 Montpellier Cedex 5, France; ^b CNRS, CEFE UMR 5175, Université de Montpellier – Université Paul Valéry – EPHE, 1919 route de Mende, 34293 Montpellier Cedex 5, France; ^c INRA, CEFE UMR 5175, Université de Montpellier – Université Paul Valéry – EPHE, 1919 route de Mende, 34293 Montpellier Cedex 5, France; ^d Montpellier SupAgro, Département biologie et écologie, 2 place Pierre Viala, 34060 Montpellier Cedex 2, France

In the context of a global increase in the frequency and intensity of droughts under climate change, root traits need to be explored to better understand their influence on water-use strategies of plants and communities. Perennial Mediterranean herbaceous species are an interesting model since they exhibit various spatial and temporal water-use strategies with contrasting adaptive responses to drought.

Combining a functional trait-based approach with a water balance model, we tested whether root functional traits are related to spatial and temporal water-use among 12 Mediterranean rangeland species. Each species was grown in monoculture in a common garden. Soil water content was monitored for ten months along the entire soil profile in each monoculture. These measurements were combined with climatic variables in a water balance model that assessed the amount and dynamics of water uptake of each species on a daily basis. Root morphological traits were measured at two soil depths (shallow and deep soil) and root biomass was measured along the soil profile to estimate species rooting depth.

We found that species with thin roots in shallow soil layers maximised water uptake in a short period following the peak of spring biomass production, while they used large amounts of water during periods of low water availability in summer. Conversely, species with coarse roots took up less water during the peak-growing season but maintained water uptake over a longer period of time and consumed less water during periods of low water availability. Deep roots with large diameters improved species’ ability to reach water from deep soil. Root biomass allocation in the deep soil layer was positively correlated to the amount of water consumed during periods of low water availability. Our results highlight that root traits influence spatial and temporal water-use among Mediterranean rangeland species. However, root traits account for the amount of water consumption during dry periods but not during the entire growing season.

In press, Functional Ecology

A poster accepted : Grassland root functional parameters vary according to a community-level resource acquisition-conservation trade-off

Florian Fort¹, Pablo Cruz², Eric Lecloux², Claire Jouany²

¹ Montpellier SupAgro, UMR 5175 CEFE, Université de Montpellier – Université Paul Valéry – EPHE, 1919 route de Mende, F-34293 Montpellier Cedex 5, France; ² INRA, UMR1248 AGIR, INRA – Université de Toulouse – INPT, F-31326 Castanet-Tolosan, France

The fundamental trade-off between fine root trait attributes related to resource acquisition and conservation is well documented at species and community levels. However, relations remain unclear between this trade-off and communities’ adaptation to environmental factors. As a result we ask: i) How do fertility and soil depth influence the communities’ position along the root acquisition-conservation trade-off? ii) How does root position along the soil profile influence its functional parameter?

We have assessed grassland botanical composition and measured communities’ root functional parameters (at plot and soil-layer levels) in 16 plots differentiated by the amounts of N and P fertilisers applied over 16 years and a soil depth gradient. Structural equation models were used to analyse relations among environmental factors, botanical composition and root functional parameters.

Botanical composition and plot-level root functional parameters vary according to fertility and soil depth. Communities from low fertility plots display high root tissue density, low specific root length (SRL) and low root length density (RLD), while communities from high fertility plots display opposite parameter values. Roots display different parameter values with soil depth. Roots in the surface horizon have small diameters and high SRL and RLD, while roots in deep horizons have large diameters and low SRL.

Our results demonstrate that the root resource management strategy varies according to fertility, communities from fertile plots are displaying more acquisitive strategies than the others. Root parameters variation with soil depth suggests intensive soil exploration, a high nutrient acquisition capacity in the surface horizon and a high water transport capacity per root length unit in deep horizon. We hypothesise that root specialisation enables species to be competitive for both nutrient and water uptakes. Our study provides evidence that fertility and soil depth modified root functional parameters in agreement with predictions from the economics spectrum theory.

Photography H. Bessière

Grassland root functional parameters vary according to a community-level resource acquisition-conservation trade-off

Florian Fort, Pablo Cruz, Eric Lecloux, Leandro Bittencourt de Oliveira, Ciprian Stroia, Jean-Pierre Theau, Claire Jouany

Summary

This study has for objective to test how do soil depth gradient and fertility changes influence communities’ below-ground parameters on a long-term experiment set on a French temperate grassland.

Our results support the hypothesis that fertile communities are dominated by acquisitive species and highlight the fact that soil depth has a strong effect on functional parameters.

Accepted for publication in Journal of Vegetation Science

A functional characterisation of a wide range of cover crop species: growth and nitrogen acquisition rates, leaf traits and ecological strategies.

Hélène Tribouillois¹, Florian Fort¹, Pablo Cruz¹, Raphaël Charles², Olivier Flores³, Eric Garnier⁴ and Eric Justes*¹

¹ INRA, UMR AGIR, 24 Chemin de Borde Rouge – Auzeville, CS 52627, 31326 Castanet-Tolosan Cedex, France; ² Agroscope, Institute of Plant Production Sciences, 50 Route de Duillier, CP 1012, CH-1260 Nyon 1, Suisse; ³ Université de la Réunion/CIRAD, UMR—Peuplements Végétaux et Bioagresseurs en Milieu Tropical, 97410 Saint Pierre, France; ⁴Centre d’Ecologie Fonctionnelle et Evolutive (UMR 5175), CNRS – Université de Montpellier – Université Paul-Valéry Montpellier – EPHE, 1919, route de Mende, 34293 Montpellier Cedex 5, France

Summary

Cover crops can produce ecosystem services during the fallow period, as reducing nitrate leaching and producing green manure. Crop growth rate (CGR) and crop nitrogen acquisition rate (CNR) can be used as two indicators of the ability of cover crops to produce these services in agrosystems.

We used leaf functional traits to characterise the growth strategies of 36 cover crops as an approach to assess their ability to grow and acquire N rapidly. We measured specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen content (LNC) and leaf area (LA) and we evaluated their relevance to characterise CGR and CNR. Cover crop species were positioned along the Leaf Economics Spectrum (LES), the SLA-LDMC plane, and the CSR triangle of plant strategies.

LA was positively correlated with CGR and CNR, while LDMC was negatively correlated with CNR. All cover crops could be classified as resource-acquisitive species from their relative position on the LES and the SLA-LDMC plane. Most cover crops were located along the Competition/Ruderality axis in the CSR triangle. In particular, Brassicaceae species were classified as very competitive, which was consistent with their high CGR and CNR. Leaf functional traits, especially LA and LDMC, allowed to differentiate some cover crops strategies related to their ability to grow and acquire N. LDMC was lower and LNC was higher in cover crop than in wild species, pointing to an efficient acquisitive syndrome in the former, corresponding to the high resource availability found in agrosystems. Combining several leaf traits explained approximately half of the CGR and CNR variances, which might be considered insufficient to precisely characterise and rank cover crop species for agronomic purposes. We hypothesised that may be the consequence of domestication process, which has reduced the range of plant strategies and modified the leaf trait syndrome in cultivated species.

Keywords: catch crop; domesticated species; green manure; functional traits; nitrogen; resource-use strategy

Abstract

Fabaceae species play a key role in ecosystem functioning through their capacity to fix atmospheric nitrogen via their symbiosis with Rhizobium bacteria. To increase benefits of using Fabaceae in agricultural systems, it is necessary to find ways to evaluate species or genotypes having potential adaptations to sub-optimal growth conditions.
We evaluated the relevance of phylogenetic distance, absolute trait distance and hierarchical trait distance for comparing the adaptation of 13 grassland Fabaceae species to different habitats, i.e. ecological niches. We measured a wide range of functional traits (root traits, leaf traits and whole plant traits) in these species. Species phylogenetic and ecological distances were assessed from a species-level phylogenetic tree and species’ ecological indicator values, respectively.
We demonstrated that differences in ecological niches between grassland Fabaceae species were related more to their hierarchical trait distances than to their phylogenetic distances. We showed that grassland Fabaceae functional traits tend to converge among species with the same ecological requirements. Species with acquisitive root strategies (thin roots, shallow root systems) are competitive species adapted to non-stressful meadows, while conservative ones (coarse roots, deep root systems) are able to tolerate stressful continental climates. In contrast, acquisitive species appeared to be able to tolerate low soil-P availability, while conservative ones need high P availability.
Finally we highlight that traits converge along the ecological gradient, providing the assumption that species with similar root-trait values are better able to coexist, regardless of their phylogenetic distance.

Fort F., Cruz P., Catrice O., Stroia C., Delbrut A., Luzarreta M., Jouany C. Root functional trait syndromes and plasticity drive the ability of grassland Fabaceae to tolerate water and phosphorus shortage.  Environmental and Experimental Botany

Abstract

Water, N and P shortages are three major factors that limit plant development worldwide. Understanding which plant strategies allow them to cope with these stresses may encourage the selection of species or genotypes better able to make the most of non-optimal growth conditions in grassland agrosystems.

To test which strategies allowed species to be more efficient under P and water shortages, we grew 13 Fabaceae species under four growth conditions in a greenhouse experiment that resulted from crossing two levels of both P and water availability.

Ten root functional traits were measured under the four growth conditions, including cross-sectional area occupied by aerenchyma, mycorrhizal rate, root hair length, and specific root length (SRL). We determined relations between species’ root trait values and their plasticity and response to P and/or water stress in terms of total biomass production and stress intensity.

Results showed that mycorrhizal rates were negatively related to biomass production in the four growth conditions. Long root hairs and high aerenchyma production were respectively associated with high biomass production in growth conditions without and with P stress, respectively. Species with acquisitive strategies (high SRL) were less impacted by P and water stress than those with conservative ones (low SRL). Additionally, increased SRL and root hair length decreased the impact of P stress on species’ performances.

Results showed that grassland Fabaceae display a wide range of root functional strategies (i.e. root functional trait attributes and plasticity), which explain their performances under P and/or water shortage.

Keywords: Drought, Fabaceae, GLM, PCA, Phosphorus, Root functional trait

PSP5 will be a multidisciplinary event, gathering plant nutritionists (plant physiology, genetics and systems biology), agronomists, ecologists, biogeochemists and soil scientists from worldwide, fostering scientific exchanges across discipline boundaries, in order to face the challenge of phosphorus limitations in many agroecosystems and terrestrial ecosystems.
PSP5 will be immediately followed by the 4th Sustainable Phosphorus Summit to be held in Montpellier from 1st to 3rd September 2014, where global concerns about Phosphorus sustainability will be discussed. You will thus have the opportunity to attend both events and be part of this unique P week.

Philippe Hinsinger, UMR Eco&Sols, Montpellier SupAgro-CIRAD-INRA-IRD