Oscar J. Valverde-Barrantes

The distribution of below‐ground traits is explained by intrinsic species differences and intraspecific plasticity in response to root neighbours

Summary 1. Large variation in tree root architecture and morphology has been reported for temperate forest communities. However, it is not clear whether this variation represents adaptation of species to specific soil properties, alternative resource acquisition strategies among co-occurring species, or canalized traits without a strong impact on the success of individuals in different environments. Here, our goal was to test these alternative hypotheses and quantify how community-aggregated and intraspecific root trait variations are explained by biotic versus abiotic mechanisms in a temperate deciduous forest. 2. We conducted our study in an Acer-Fagus-dominated forest in north-east Ohio, USA. Using molecular barcoding techniques, we identified 738 root systems belonging to 14 tree species. We measured seven functional root traits related to root architecture and morphology at the species and community-aggregated levels. 3. Although we found significant relationships between soil resource gradients and root trait distributions, intrinsic differences between coexisting species were more important than soil factors in explaining the distribution of root traits in the community. Additionally, root trait variation at the species level was also influenced by the presence of other species within cores. 4. Community-aggregated variation was more influenced by the combination of species present than soil properties in each sample, suggesting that biotic interactions play an important role in controlling community root trait distribution. 5. Synthesis. We propose that root trait differentiation between coexisting species is the result of inherent differences between species and plasticity-mediated responses to neighbours. Hence, the large variation in root traits reported in temperate forest seems to reflect alternative evolutionary pathways that allow individuals to exploit distinct niches in relatively close proximity.

Impacts of individual tree species on carbon dynamics in a moist tropical forest environment.

In the moist tropical forest biome, which cycles carbon (C) rapidly and stores huge amounts of C, the impacts of individual species on C balances are not well known. In one of the earliest replicated experimental sites for investigating growth of native tropical trees, we examined traits of tree species in relation to their effects on forest C balances, mechanisms of influence, and consequences for C sequestration. The monodominant stands, established in abandoned pasture in 1988 at La Selva Biological Station, Costa Rica, contained five species in a complete randomized block design. Native species were: Hieronyma alchorneoides, Pentaclethra macroloba, Virola koschnyi, and Vochysia guatemalensis. The exotic species was Pinus patula. By 16 years, the lack of differences among species in some attributes suggested strong abiotic control in this environment, where conditions are very favorable for growth, These attributes included aboveground net primary productivity (ANPP), averaging 11.7 Mg C x ha(-1) x yr(-1) across species, and soil organic C (0-100 cm, 167 Mg C/ha). Other traits differed significantly, however, indicating some degree of biological control. In Vochysia plots, both aboveground biomass of 99 Mg C/ha, and belowground biomass of 20 Mg C/ha were 1.8 times that of Virola (P = 0.02 and 0.03, respectively). Differences among species in overstory biomass were not compensated by understory vegetation. Belowground NPP of 4.6 Mg C x ha(-1) yr(-1) in Hieronyma was 2.4 times that of Pinus (P < 0.01). Partitioning of NPP to belowground components in Hieronyma was more than double that of Pinus (P = 0.03). The canopy turnover rate in Hieronyma was 42% faster than that of Virola (P < 0.01). Carbon sequestration, highest in Vochysia (7.4 Mg C x ha(-1) x yr(-1), P = 0.02), averaged 5.2 Mg C x ha(-1) x yr(-1), close to the annual per capita fossil fuel use in the United States of 5.3 Mg C. Our results indicated that differences in species effects on forest C balances were related primarily to differences in growth rates, partitioning of C among biomass components, tissue turnover rates, and tissue chemistry. Inclusion of those biological attributes may be critical for robust modeling of C cycling across the moist tropical forest biome.

Fine root morphology is phylogenetically structured, but nitrogen is related to the plant economics spectrum in temperate trees

Summary Plant functional traits have revealed trade-offs related to life-history adaptations, geographical distributions, and ecosystem processes. Fine roots are essential in plant resource acquisition and play an important role in soil carbon cycling. Nonetheless, root trait variation is still poorly quantified and rarely related to the rest of the plant. We examined chemical and morphological traits of 34 temperate arbuscular mycorrhizal tree species, representing three main angiosperm clades (super-orders asterid, magnoliid and rosid). We tested to what extent fine root chemical and morphological traits were correlated similarly to the leaf economical spectrum (LES) or were structured by ancestral affiliations among species. Root traits did not display the same trade-offs as leaves (e.g. specific root length was not correlated with root N, whereas specific leaf area was correlated with leaf N). Moreover, 75% of below-ground traits were phylogenetically structured according to Pagels λ and Abouheifs Cmean autocorrelation tests, as opposed to 28% of above-ground traits. Magnoliids showed thicker, less branched roots than asterids or rosids, but rosid roots exhibited lower N and higher non-acid-hydrolysable (e.g. lignin) content than other species. In contrast, leaf traits did not differ significantly among super-orders. At the whole-tree level, chemical traits such as nitrogen tissue content and lignin content were correlated between above and below-ground organs. The distribution of root traits in woody temperate trees was better explained by shared ancestry than by the nutrient content and structural trade-offs expected by the LES hypothesis. Root chemistry and morphology differed substantially among species belonging to different super-orders, suggesting deep divergences in resource acquisition strategies among major angiosperm groups. Although we found partial support for the idea of whole-plant integration based on corresponding nitrogen content across all organs (i.e. a plant economics spectrum), our study stresses phylogenetic affiliation as the primary driver of root trait distributions among angiosperms, a pattern that could be easily overlooked based solely on above-ground observations.

Climate, soil and plant functional types as drivers of global fine‐root trait variation

Summary 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. 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. 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. 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.

Aggregated and complementary: symmetric proliferation, overyielding, and mass effects explain fine-root biomass in soil patches in a diverse temperate deciduous forest landscape.

Few studies describe root distributions at the species level in diverse forests, although belowground species interactions and traits are often assumed to affect fine-root biomass (FRB). We used molecular barcoding to study how FRB of trees relates to soil characteristics, species identity, root diversity, and root traits, and how these relationships are affected by proximity to ecotones in a temperate forest landscape. We found that soil patch root biomass increased in response to soil resources across all species, and there was little belowground vertical or horizontal spatial segregation among species. Root traits and species relative abundance did not explain significant variation in FRB after correcting for soil fertility. A positive relationship between phylogenetic diversity and FRB indicated significant belowground overyielding attributable to local root diversity. Finally, variation in FRB explained by soil fertility and diversity was reduced near ecotones, but only because of a reduction in biomass in periodically anoxic areas. These results suggest that symmetric responses to soil properties are coupled with complementary species traits and interactions to explain variation in FRB among soil patches. In addition, landscape-level dispersal among habitats and across ecotones helps explain variation in the strength of these relationships in complex landscapes.

Phylogenetically structured traits in root systems influence arbuscular mycorrhizal colonization in woody angiosperms

Background and aimThere is little quantitative information about the relationship between root traits and the extent of arbuscular mycorrhizal fungi (AMF) colonization. We expected that ancestral species with thick roots will maximize AMF habitat by maintaining similar root traits across root orders (i.e., high root trait integration), whereas more derived species are expected to display a sharp transition from acquisition to structural roots. Moreover, we hypothesized that interspecific morphological differences rather than soil conditions will be the main driver of AMF colonization.MethodsWe analyzed 14 root morphological and chemical traits and AMF colonization rates for the first three root orders of 34 temperate tree species grown in two common gardens. We also collected associated soil to measure the effect of soil conditions on AMF colonization.ResultsThick-root magnoliids showed less variation in root traits along root orders than more-derived angiosperm groups. Variation in stele:root diameter ratio was the best indicator of AMF colonization within and across root orders. Root functional traits rather than soil conditions largely explained the variation in AMF colonization among species.ConclusionsNot only the traits of first order but the entire structuring of the root system varied among plant lineages, suggesting alternative evolutionary strategies of resource acquisition. Understanding evolutionary pathways in belowground organs could open new avenues to understand tree species influence on soil carbon and nutrient cycling.

Placing the Effects of Leaf Litter Diversity on Saprotrophic Microorganisms in the Context of Leaf Type and Habitat

Because of conflicting results in previous studies, it is unclear whether litter diversity has a predictable impact on microbial communities or ecosystem processes. We examined whether effects of litter diversity depend on factors that could confound comparisons among previous studies, including leaf type, habitat type, identity of other leaves in the mixture, and spatial covariance at two scales within habitats. We also examined how litter diversity affects the saprotrophic microbial community using terminal restriction fragment length polymorphism to profile bacterial and fungal community composition, direct microscopy to quantify bacterial biomass, and ergosterol extraction to quantify fungal biomass. We found that leaf mixture diversity was rarely significant as a main effect (only for fungal biomass), but was often significant as an interaction with leaf type (for ash-free dry mass recovered, carbon-to-nitrogen ratio, fungal biomass, and bacterial community composition). Leaf type and habitat were significant as main effects for all response variables. The majority of variance in leaf ash-free dry mass and C/N ratio was explained after accounting for treatment effects and spatial covariation at the meter (block) and centimeter (litterbag) scales. However, a substantial amount of variability in microbial communities was left unexplained and must be driven by factors at other spatial scales or more complex spatiotemporal dynamics. We conclude that litter diversity effects are primarily dependent on leaf type, rather than habitat type or identity of surrounding leaves, which can guide the search for mechanisms underlying effects of litter diversity on ecosystem processes.

Decay of ecosystem differences and decoupling of tree community–soil environment relationships at ecotones

Ecotones are important landscape features where there is a transition between adjoining ecosystems. However, there are few generalized hypotheses about the response of communities to ecotones, except for a proposed increase in species richness that receives varying empirical support. Based on the assumption that transport of abiotic material and dispersal of organism propagules across ecotones are independent processes, we propose the new hypothesis that ecotones decouple community-environment relationships, increasing the importance of spatial structure that is independent of the environment. We tested this hypothesis by examining the effects of ecotones on relationships between trees and soil properties in a temperate deciduous forest. The study area included different landforms defined by topography, hydrology, and geomorphology, which we designated upland, bottomland, and riparian forests. The site also included a mowed herbaceous corridor. We found that soil properties and tree community composition significantly differed among landforms, and thus they could be treated as differing ecosystem types. However, inclusion of plots near ecotones significantly reduced the variance explained by landform due to introduction of increased noise, increased similarity of ecotone plots in different landforms, or both. To examine tree community-soil environment relationships, factorial kriging analysis was used to decompose variation in soil properties into structures associated with differing spatial scales, which were then used as predictors of tree composition using redundancy analysis. In agreement with the ecotone-decoupling hypothesis, we found that ecotones introduced significant unexplained variation into correlations between tree community composition and soil properties. In addition, spatial variation in tree community composition that was independent of soil properties was only detected when ecotones were included in the analysis, and little variation in tree community composition was attributed to small-scale soil property structures. Together, these results indicate that dispersal limitation and mass effects in the tree community take on increased importance near ecotones. We found no consistent changes in tree species richness associated with ecotones, and we suggest that the ecotone- decoupling hypothesis may correspond with a more general community-level pattern that warrants further testing. Decoupling of community-environment relationships near ecotones also has important implications for accuracy of models predicting community distributions from abiotic information.

Carbon Dynamics in the Tropics

Native tree species differed in their effects on aboveand belowground carbon stocks and fluxes in these 16-yrold experimental plantations at La Selva Biological Station, Costa Rica. Results were explained primarily by differences in growth rates, C allocation, turnover rates, and tissue chemistry. In this experiment established in an abandoned pasture, all five tree species had attained biomass amounts similar to that of nearby mature forest, whereas the abandoned pasture control remained in arrested succession. Carbon sequestration averaged 5.2 Mg∙ha-1∙yr-1 across species, close to the annual per capita fossil-fuel use in the United States of 5.3 Mg C.