Iván Prieto

Complementary effects of species and genetic diversity on productivity and stability of sown grasslands.

Plant species diversity regulates the productivity1–3 and stability2,4 of natural ecosystems, along with their resilience to disturbance5,6. The influence of species diversity on the productivity of agronomic systems is less clear7–10. Plant genetic diversity is also suspected to influence ecosystem function3,11–14, although empirical evidence is scarce. Given the large range of genotypes that can be generated per species through artificial selection, genetic diversity is a potentially important leverage of productivity in cultivated systems. Here we assess the effect of species and genetic diversity on the production and sustainable supply of livestock fodder in sown grasslands, comprising single and multispecies assemblages characterized by different levels of genetic diversity, exposed to drought and non-drought conditions. Multispecies assemblages proved more productive than monocultures when subject to drought, regardless of the number of genotypes per species present. Conversely, the temporal stability of production increased only with the number of genotypes present under both drought and non-drought conditions, and was unaffected by the number of species. We conclude that taxonomic and genetic diversity can play complementary roles when it comes to optimizing livestock fodder production in managed grasslands, and suggest that both levels of diversity should be considered in plant breeding programmes designed to boost the productivity and resilience of managed grasslands in the face of increasing environmental hazards.

Competition with winter crops induces deeper rooting of walnut trees in a Mediterranean alley cropping agroforestry system

Background and aimsCharacterising the spatial distribution of tree fine roots (diameteru2009≤u20092xa0mm) is fundamental for a better understanding of belowground functioning when tree are grown with associated crops in agroforestry systems. Our aim was to compare fine root distributions and orientations in trees grown in an alley cropping agroforestry stand with those in a tree monoculture.MethodsFieldwork was conducted in two adjacent 17xa0year old hybrid walnut (Juglans regiau2009×u2009nigra L.) stands in southern France: the agroforestry stand was intercropped with durum wheat (Triticum turgidum L. subsp. durum) whereas the tree monoculture had a natural understorey. Root intercepts were mapped to a depth of 150xa0cm on trench walls in both stands, and to a depth of 400xa0cm in the agroforestry stand in order to characterise tree root distribution below the crop’s maximum rooting depth. Soil cubes were then extracted to assess three dimensional root orientation and to establish a predictive model of root length densities (RLD) derived from root intersection densities (RID).ResultsIn the tree monoculture, root mapping demonstrated a very high tree RID in the top 50xa0cm and a slight decrease in RID with increasing soil depth. However, in the agroforestry stand, RID was significantly lower at 50xa0cm, tree roots colonized deeper soil layers and were more vertically oriented. In the agroforestry stand, RID and RLD were greater within the tree row than in the inter-row.ConclusionsFine roots of intercropped walnut trees grew significantly deeper, indicating a strong plasticity in root distribution. This plasticity reduced direct root competition from the crop, enabling trees to access deeper water tables not available to crop roots.

Root functional parameters predict fine root decomposability at the community level

Root quality is one of the main drivers of fine root decomposition, an important process controlling soil carbon (C) and nutrient cycling in most terrestrial ecosystems. Root quality is defined by chemical and morphological traits, which differ across species and thus communities. This trait variation is assumed to follow a trade-off between resource acquisition and conservation (i.e. the root economics spectrum). To what extent root quality or the economics spectrum influence fine root decomposition rates at the community level remains poorly understood, particularly within the context of land use change.nChanges in land use induce shifts in plant community composition, which also affect root distribution within the soil profile, resulting in changes in root quality. We hypothesize that at the community level, i) root decomposability is driven by community root functional parameters (i.e. root traits measured at the community level), ii) changes in root functional parameters among land use types and with soil depth translate into changes in root decomposability.nWe collected shallow and deep fine roots (< 2 mm) from 20 plant communities across contrasting land use types in 7 sites worldwide, ranging from agricultural crops to natural forests and determined their decomposition rate in standard conditions. Fine root quality was related to known values of functional parameters for these communities, including carbon (C), nitrogen (N) and lignin concentrations.nA combination of chemical functional parameters (lignin, C and N concentrations) best explained root decomposition rates at the community level, whereas root economics remained a poorer predictor of decomposability rates. Among land use gradients, roots from agricultural and agroforestry communities decomposed faster than roots from forest sites. Across and within plant communities, a consistently greater decomposability in shallow roots was observed. Both land use and depth effects were explained by changes in root chemical traits at the community level.nSynthesis. Our results suggest that the conversion of plant communities from forests to agricultural lands leads to changes in root functional parameters, that drastically increase root decomposition rates and may lead to major soil C losses, especially in shallow soil layers.

Disentangling the Litter Quality and Soil Microbial Contribution to Leaf and Fine Root Litter Decomposition Responses to Reduced Rainfall

Climate change-induced rainfall reductions in Mediterranean forests negatively affect the decomposition of plant litter through decreased soil moisture. However, the indirect effects of reduced precipitation on litter decomposition through changes in litter quality and soil microbial communities are poorly studied. This is especially the case for fine root litter, which contributes importantly to forests plant biomass. Here we analyzed the effects of long-term (11xa0years) rainfall exclusion (29% reduction) on leaf and fine root litter quality, soil microbial biomass, and microbial community-level physiological profiles in a Mediterranean holm oak forest. Additionally, we reciprocally transplanted soils and litter among the control and reduced rainfall treatments in the laboratory, and analyzed litter decomposition and its responses to a simulated extreme drought event. The decreased soil microbial biomass and altered physiological profiles with reduced rainfall promoted lower fine root—but not leaf—litter decomposition. Both leaf and root litter, from the reduced rainfall treatment, decomposed faster than those from the control treatment. The impact of the extreme drought event on fine root litter decomposition was higher in soils from the control treatment compared to soils subjected to long-term rainfall exclusion. Our results suggest contrasting mechanisms driving drought indirect effects on above-(for example, changes in litter quality) and belowground (for example, shifts in soil microbial community) litter decomposition, even within a single tree species. Quantifying the contribution of these mechanisms relative to the direct soil moisture-effect is critical for an accurate integration of litter decomposition into ecosystem carbon dynamics in Mediterranean forests under climate change.

Unexpected phenology and lifespan of shallow and deep fine roots of walnut trees grown in a silvoarable Mediterranean agroforestry system

Background and AimsFine roots play a major role in the global carbon cycle through respiration, exudation and decomposition processes, but their dynamics are poorly understood. Current estimates of root dynamics have principally been observed in shallow soil horizons (<1xa0m), and mainly in forest systems. We studied walnut (Juglans regiau2009×u2009nigra L.) fine root dynamics in an agroforestry system in a Mediterranean climate, with a focus on deep soils (down to 5xa0m), and root dynamics throughout the year.MethodsSixteen minirhizotron tubes were installed in a soil pit, at depths of 0.0–0.7, 1.0–1.7, 2.5–3.2 and 4.0–4.7xa0m and at two distances from the nearest trees (2 and 5xa0m). Fine root (diameteru2009≤u20092xa0mm) dynamics were recorded across three diameter classes every 3xa0weeks for 1xa0year to determine their phenology and turnover in relation to soil depth, root diameter and distance from the tree row.ResultsDeep (>2.5xa0m) root growth occurred at two distinct periods, at bud break in spring and throughout the winter i.e., after leaf fall. In contrast, shallow roots grew mainly during the spring-summer period. Maximum root elongation rates ranged from 1 to 2xa0cmxa0day−1 depending on soil depth. Most root mortality occurred in upper soil layers whereas only 10xa0% of fine roots below 4xa0m died over the study period. Fine root lifespan was longer in thicker and in deeper roots with the lifespan of the thinnest roots (0.0–0.5xa0mm) increasing from 129xa0days in the topsoil to 190 at depthsu2009>u20092.5xa0m.ConclusionsThe unexpected growth of very deep fine roots during the winter months, which is unusual for a deciduous tree species, suggests that deep and shallow roots share different physiological strategies and that current estimates based on the shortest root growth periods (i.e., during spring and summer) may be underestimating root production. Although high fine root turnover rates might partially result from the minirhizotron approach used, our results help gain insight into some of the factors driving soil organic carbon content.

Soil aggregate stability in Mediterranean and tropical agro-ecosystems: effect of plant roots and soil characteristics

AimsOur aim was to determine the effect of soil characteristics and root traits on soil aggregate stability at an inter- and intra-site scale in a range of agro-ecosystems. We also evaluated the effect of soil depth and the type of land use on aggregate stability.MethodsSoil aggregate stability, soil physicochemical properties and fine root traits were measured along land use gradients (from monocultures to agroforestry systems and forests), at two soil depths at four sites (Mediterranean and tropical climates) with contrasting soils (Andosol, Ferralsol, Leptosol and Fluvisol).ResultsAggregate stability was much lower in deep than in surface soil layers, likely linked to lower soil organic carbon (SOC) and lower root mass density (RMD). Locally, and consistently in all sites, land use intensification degrades soil aggregate stability, mainly in surface soil layers. Soil organic carbon, cation exchange capacity and root traits: water-soluble compounds, lignin and medium root length proportion were the most important drivers of aggregate stability at the inter-site level, whereas SOC, root mass and root length densities (RMD, RLD) were the main drivers at the intra-site level.ConclusionsOverall, the data suggest different controls on soil aggregate stability globally (soil) and locally (roots). Conversion from forests to agricultural land will likely lead to greater C losses through a loss of aggregate stability and increased soil erosion.

Leaf carbon and oxygen isotopes are coordinated with the leaf economics spectrum in Mediterranean rangeland species

The leaf economics spectrum (LES) describes covariation in traits relevant to carbon and nutrient economics across plant species, but much less is known about the relationship between the LES and leaf water economy. We propose an approach combining the measurement of two leaf traits related to water-use economy, leaf carbon (δ13C) and oxygen (δ18O) isotopic composition, and the measurement of leaf morphological and nutrient traits to investigate the link between leaf carbon and nutrient economics and water use. nWe tested the relationships between leaf traits linked to carbon and nutrient use within the LES and water-use traits using leaf δ18O as a proxy of stomatal conductance (gs) and δ13C as a proxy of intrinsic water-use efficiency (WUEi) across 15 Mediterranean rangeland species grown in an irrigated common garden and in a natural rangeland in Southern France. nThe target species spanned a wide range of variation in leaf morphological and nutrient trait values and a wide range of leaf δ18O and δ13C values. Principal component analysis revealed multiple associations among leaf morphology, nutrients and isotopic composition, with the first axis alone explaining 56.0% of the total variation across species. Leaf δ18O and δ13C covaried with leaf morphology and leaf nutrient concentrations along a single resource-use axis. Species with high leaf δ18O and δ13C (low gs and high WUEi) exhibited a resource-conservative strategy (high leaf dry matter content, low leaf N, P and K), whereas species with low leaf δ18O and δ13C (high gs and low WUEi) showed a more resource-acquisitive strategy (high specific leaf area and leaf N, P and K). These leaf trait syndromes and resource-use strategies were strongly conserved across sites with contrasting environmental conditions, indicating that foliar δ18O and δ13C can be included as an integral part of the LES for this set of rangeland species. nOverall, the data suggest a tight coupling and coordination between water, carbon and nutrient-use strategies across herbaceous plant species. A dual δ18O and δ13C isotope approach combined with LES trait measurements is a promising tool to more comprehensively assess the diversity of resource-use strategies among coexisting plant species. n n nA plain language summary is available for this article.

Five species, many genotypes, broad phenotypic diversity: When agronomy meets functional ecology

PREMISE OF THE STUDYnCurrent ecological theory can provide insight into the causes and impacts of plant domestication. However, just how domestication has impacted intraspecific genetic variability (ITV) is unknown. We used 50 ecotypes and 35 cultivars from five grassland species to explore how selection drives functional trait coordination and genetic differentiation.nnnMETHODSnWe quantified the extent of genetic diversity among different sets of functional traits and determined how much genetic diversity has been generated within populations of natural ecotypes and selected cultivars.nnnKEY RESULTSnIn general, the cultivars were larger (e.g., greater height, faster growth rates) and had larger and thinner leaves (greater SLA). We found large (average 63%) and trait-dependent (ranging from 14% for LNC to 95.8% for growth rate) genetic variability. The relative extent of genetic variability was greater for whole-plant than for organ-level traits. This pattern was consistent within ecotypes and within cultivars. However, ecotypes presented greater ITV variability.nnnCONCLUSIONSnThe results indicated that genetic diversity is large in domesticated species with contrasting levels of heritability among functional traits and that selection for high yield has led to indirect selection of some associated leaf traits. These findings open the way to define which target traits should be the focus in selection programs, especially in the context of community-level selection.

Decomposition rates of fine roots from three herbaceous perennial species: combined effect of root mixture composition and living plant community

AimsIn most ecosystems, plant roots from different species decompose in mixtures and in the presence of living roots; however much root decomposition research has focused on how roots of individual species or artificial mixtures decompose in the absence of living plants. We thus examined two poorly studied components of root litter decomposition: 1) whether decomposition of root mixtures can be predicted from the sum of the decomposition rates of each component species and 2) how living plants influence rates of root decomposition.MethodsDecomposition rates of roots from three perennial herbaceous Mediterranean species grown in monocultures and in two- and three-species mixtures were determined after a one-year incubation period under their living community and in non-vegetated soil (bare soil). Soil respiration in the presence of glucose (substrate induced respiration, SIR) was measured in each plant community and in bare soil.ResultsDecomposition rates of root mixtures cannot be predicted from decomposition rates of the component species, both additive and non-additive effects were observed; the presence of low quality roots of Carex humilis in mixtures strongly negatively influenced root decomposition. The presence of living plants stimulated root decomposition in monocultures and two-species communities, likely through an enhanced microbial activity (SIR) under plant communities.ConclusionThis study highlights that root decomposition cannot be predicted from decomposition rates of the component species and is more influenced by endogenous factors or root litter functional composition than by plant community composition.