Cristina Armas

Positive interactions among alpine plants increase with stress

Plants can have positive effects on each other. For example, the accumulation of nutrients, provision of shade, amelioration of disturbance, or protection from herbivores by some species can enhance the performance of neighbouring species. Thus the notion that the distributions and abundances of plant species are independent of other species may be inadequate as a theoretical underpinning for understanding species coexistence and diversity. But there have been no large-scale experiments designed to examine the generality of positive interactions in plant communities and their importance relative to competition. Here we show that the biomass, growth and reproduction of alpine plant species are higher when other plants are nearby. In an experiment conducted in subalpine and alpine plant communities with 115 species in 11 different mountain ranges, we find that competition generally, but not exclusively, dominates interactions at lower elevations where conditions are less physically stressful. In contrast, at high elevations where abiotic stress is high the interactions among plants are predominantly positive. Furthermore, across all high and low sites positive interactions are more important at sites with low temperatures in the early summer, but competition prevails at warmer sites.

MEASURING PLANT INTERACTIONS: A NEW COMPARATIVE INDEX

We propose an index to measure the relative interaction intensity in plants (RII) with strong mathematical and statistical properties which overcome problems shown by other frequently used indices. RII has defined limits [−1, +1]; is symmetrical around zero, with identical absolute values for competition and facilitation; is linear; and does not have discontinuities in its range. It is therefore safe to use in statistical and mathematical operations. RII distribution is approximately normal, with means equal to the true population index and a sampling variance that can be derived. Its strong statistical properties make RII proper for use in parametric meta-analyses. It can be applied to any kind of interaction (from competitive exclusion to symbiosis) and in commonly published ranges of interaction intensity it offers the most consistent results. Because RII uses basic arithmetical operators, it can be scaled up and used to measure multispecific interactions at the community level.

Soil as a mediator in plant-plant interactions in a semi-arid community

Abstract Competition and facilitation may occur simultaneously in plant communities, and the prevalence of either process depends on abiotic conditions. Here we attempt a community-wide approach in the analysis of plant interactions, exploring whether in a semi-arid environment positive or negative interactions predominate and whether there are differences among co-occurring shrub species. Most shrubs in our plot exerted significant effects on their understorey communities, ranging from negative to positive. We found a clear case of interference and another case where the effect was neutral, but facilitation predominated and the biomass of annuals under most shrubs in our community was larger than in gaps. Effects on soil water and fertility were revealed as the primary source of facilitation; the build-up of soil organic matter changed soil physical properties and improved soil water relations. Facilitation by shrubs involved decoupling of soil temperature and moisture. Sheltering from direct radiation had an effect on productivity, but significant differences in understorey biomass did not parallel understorey light environment. A positive balance of the interaction among plants, essentially mediated by changes in soil properties, is the predominant outcome of plant interactions in this semi-arid community. Nomenclature: Castroviejo et al. (1986–2001). Abbreviations: A = photosynthetic rate; Fv/Fm = photochemical efficiency of photosystem II; gs = leaf conductance to water vapour; OM = organic matter; PAR = photosynthetic active radiation; ΨPD = Predawn water potential.

Water release through plant roots: new insights into its consequences at the plant and ecosystem level

Hydraulic redistribution (HR) is the passive movement of water between different soil parts via plant root systems, driven by water potential gradients in the soil-plant interface. New data suggest that HR is a heterogeneous and patchy process. In this review we examine the main biophysical and environmental factors controlling HR and its main implications at the plant, community and ecosystem levels. Experimental evidence and the use of novel modelling approaches suggest that HR may have important implications at the community scale, affecting net primary productivity as well as water and vegetation dynamics. Globally, HR may influence hydrological and biogeochemical cycles and, ultimately, climate.

Ecophysiological significance of chlorophyll loss and reduced photochemical efficiency under extreme aridity in Stipa tenacissima L.

Stipa tenacissima L., a perennial tussock grass widely found in semi-arid environments of the Iberian Peninsula and North Africa, is subjected to multiple stresses during the extreme summer conditions of south-east Spain. We characterised the photoprotective mechanisms of S. tenacissima during the transition from spring to summer and autumn. S. tenacissima experienced a marked water deficit (Ψ{ pd} < -8.4 MPa) and the complete suppression of CO2 assimilation in August, associated with a 72% reduction of maximal photochemical efficiency of PSII (F{ v}/F{ m}). These reduced F{ v}/F{ m} values were related to the pre-dawn maintenance of high levels of epoxidized forms of xanthophyll-cycle pigments (DPS{ pd}, ca. 42% higher than spring values), and with a 60% reduction in the concentration of total chlorophyll (Chl a+b). These changes were associated with a low capacity of dissipation of the excitation energy non-radiatively (measured as NPQ). Leaves showed a complete recovery of F{ v}/F{ m} and xanthophyll and chlorophyll concentrations after the autumn rainfall, which reached levels similar to that of spring. This poikilohydric-type response of S. tenacissima to stress allows for a greater tolerance of water shortage, high temperature and high light intensity, which are typical in these semi-arid environments and accounts for its distinctive opportunistic growth.

Hydraulic lift and tolerance to salinity of semiarid species: consequences for species interactions

The different abilities of plant species to use ephemeral or permanent water sources strongly affect physiological performance and species coexistence in water-limited ecosystems. In addition to withstanding drought, plants in coastal habitats often have to withstand highly saline soils, an additional ecological stress. Here we tested whether observed competitive abilities and C–water relations of two interacting shrub species from an arid coastal system were more related to differences in root architecture or salinity tolerance. We explored water sources of interacting Juniperus phoenicea Guss. and Pistacia lentiscus L. plants by conducting physiology measurements, including water relations, CO2 exchange, photochemical efficiency, sap osmolality, and water and C isotopes. We also conducted parallel soil analyses that included electrical conductivity, humidity, and water isotopes. During drought, Pistacia shrubs relied primarily on permanent salty groundwater, while isolated Juniperus plants took up the scarce and relatively fresh water stored in upper soil layers. As drought progressed further, the physiological activity of Juniperus plants nearly stopped while Pistacia plants were only slightly affected. Juniperus plants growing with Pistacia had stem-water isotopes that matched Pistacia, unlike values for isolated Juniperus plants. This result suggests that Pistacia shrubs supplied water to nearby Juniperus plants through hydraulic lift. This lifted water, however, did not appear to benefit Juniperus plants, as their physiological performance with co-occurring Pistacia plants was poor, including lower water potentials and rates of photosynthesis than isolated plants. Juniperus was more salt sensitive than Pistacia, which withstood salinity levels similar to that of groundwater. Overall, the different abilities of the two species to use salty water appear to drive the outcome of their interaction, resulting in asymmetric competition where Juniperus is negatively affected by Pistacia. Salt also seems to mediate the interaction between the two species, negating the potential positive effects of an additional water source via hydraulic lift.

A role for below‐ground biota in plant–plant facilitation

Summary 1. Plant–plant facilitation is an important driver of plant diversity, which in turn maintains ecosystem multifunctionality and can buffer some negative effects of climate change. Facilitation is classically attributed to the amelioration of environmental extremes and resource availability. Integrating below-ground biota into the positive plant interactions framework should provide a more realistic understanding of this process, enabling us to gain insights into the dynamics of below–above-ground communities. 2. We tested the effect of below-ground biota on the performance of a plant community and of individual species using soil extracts from the understorey of a benefactor plant species and adjacent open spaces. Soil bacteria from extracts and experimental microcosms were analysed using pyrosequencing. 3. Soil biota had a significant effect on the abundance, growth, functional traits and reproductive output of beneficiary plant species through processes that are independent of the direct influence of the benefactor species. Different soil bacterial communities were associated with the benefactor species, the individual beneficiary plant species and the plant community, revealing complex below– above-ground links between plants and soil microbiota. 4. Synthesis. The below-ground biota cultivated by benefactor plant species play a fundamental role in positive interactions between plant species contributing to the preservation of diversity and the evolution of plant communities.

Hydraulic lift promotes selective root foraging in nutrient-rich soil patches

Hydraulic lift (HL) - the passive movement of water through plant roots from deep wet to shallow drier soil layers - can improve root survival in dry soils by providing a source of moisture to shallow roots. It may also enhance plant nutrient capture, though empirical evidence for this is scarce and whether HL promotes the selective placement of roots in nutrient-rich soil enhancing nutrient capture in dry soils remains unknown. We tested this with a split-pot design in which we separated the root system of Retama sphaerocarpa (L.) Boiss shrubs into two pot compartments: a lower, well-watered one; and an upper, drier one. Half the shrubs grew under natural light conditions hence allowed to perform HL, whereas the other half had impaired HL by maintaining continuous illumination at night. Resource-rich (organic matter enriched in 15N and P) and resource-poor soil patches were inserted in the upper compartment after a drought treatment was imposed. Artificial illumination did impair HL at night. Soil moisture in both the whole upper compartment and in soil patches was lower in plants illuminated at night and reduced the allocation of roots to nutrient-rich soil patches at the expense of root growth in nutrient-poor patches (i.e. root foraging precision). Plant nitrogen capture was also lower in shrubs with impaired HL. Overall, these results demonstrate that HL favoured the selective placement of roots in nutrient-rich patches as well as nutrient capture under drought, a process that may secure nutrient capture and maintain plant performance during drought periods.

Selection of forest species for the rehabilitation of disturbed soils in oil fields in the Ecuadorian Amazon

Abstract Soils in the Amazon Basin disturbed by petroleum extraction activities need to be restored to allow for the rehabilitation of these areas and the restoration of the ecosystem services that they can provide. This study explores the performance of saplings of 20 species transplanted to four sites: a paddock and three sites within oil fields that differ in soil substrate contamination and perturbation. In each site we measured sapling survival, possible causes of death, sapling height and diameter at the time of and two years after planting, and the integrated response index. We also analyzed the effects of plants on soil properties. Sapling mortality was limited, with 17 of the 20 species boasting survival rates of over 80%. Saplings in the control site had a higher mortality rate than those in the oil field sites. This was most likely due to competition with and interference of weeds that were more abundant at the control than other sites. Despite the overall low mortality rate, species performance did vary by site, with plants of Flemingia macrophylla, Myrcia aff. fallax, Piptadenia pteroclada, Platymiscium pinnatum, and Zygia longifolia exhibiting the best performance in terms of survival and growth in oil field sites. At the end of the experiment, soil substrates from the oil platform showed increases in pH levels, organic material, Fe, and Zn; whereas substrates contaminated with petroleum showed decreases in hydrocarbon levels ranging from 11 to 22% compared to initial levels before sapling transplanting. Our results shed light on which forest species are most suitable for the rehabilitation of sites disturbed by activities inherently associated with petroleum extraction in the Ecuadorian Amazon.

Enhanced facilitation at the extreme end of the aridity gradient in the Atacama Desert: a community‐level approach

Plant facilitation is now recognized as an important process in severe environments. However, there is still no agreement on how facilitation changes as conditions become increasingly severe. The classic stress gradient hypothesis (SGH) predicts a monotonic increase in facilitation, which rises in frequency as conditions approach the extreme end of the environmental gradient. However, few studies have evaluated the validity of the SGH at the community level, the level at which it was formulated. Moreover, few studies have tested the SGH at either extreme of the gradient, and very few have excluded the effect of livestock on community response to stress. In line with the SGH, we hypothesized that several spatial pattern summary statistics would change monotonically from the least to the most arid sites, indicating increasingly aggregated patterns. In this study, we performed an evaluation of the SGH both within communities of shrub species and across a large portion of the Atacama Desert, and we isolated the abiotic component of the SGH. Our environmental gradient covered an extreme aridity gradient (< 20-130 mm annual precipitation). To perform point pattern analysis, we established 13 sites with environmental conditions representing four distinct levels of this gradient. Further, we conducted species co-occurrence analyses at 19 sites along the gradient. Both sets of analyses showed stronger positive spatial associations among plants at the most extreme end of the gradient. This was true regardless of whether we included all individuals, only small individuals located around large ones, or individuals in species pairs. Moreover, species tended to show greater co-occurrence as environmental severity increased. This increase in aggregation in the plant community seems to correlate with an increase in the strength of positive interspecific interactions, rather than greater clustering within each species. These monotonic increases in species co-occurrence and spatial association in more severe environments are consistent with some of the predictions of SGH, and collectively these results suggest that as the climate becomes more arid, positive species pairs interactions tend to be prevalent in the community.