Francisco M. Padilla

The role of nurse plants in the restoration of degraded environments

Traditional ecological models have focused mainly on competition between plants, but recent research has shown that some plants benefit from closely associated neighbors, a phenomenon known as facilitation. There is increasing experimental evidence suggesting that facilitation has a place in mainstream ecological theory, but it also has a practical side when applied to the restoration of degraded environments, particularly dry-lands, alpine, or other limiting habitats. Where restoration fails because of harsh environmental conditions or intense herbivory, species that minimize these effects could be used to improve performance in nearby target species. Although there are few examples of the application of this “nursing” procedure worldwide, experimental data are promising, and show enhanced plant survival and growth in areas close to nurse plants. We discuss the potential for including nurse plants in restoration management procedures to improve the success rate of such projects.

Variability in amount and frequency of water supply affects roots but not growth of arid shrubs

Rainfall and soil moisture variability have a strong effect on plant survival and seed germination in arid environments, yet very little is known about the effects on roots and growth of woody seedlings. Here we focused on the effects of variability in both amount and frequency of water supply on juvenile root and leaf functional traits and growth of seven Mediterranean shrub species occurring in arid SE Spain, Anthyllis cytisoides, Atriplex halimus, Ephedra fragilis, Genista umbellata, Lycium intricatum, Retama sphaerocarpa, and Salsola oppositifolia. In a 14-month greenhouse experiment we manipulated water supply expecting that reduced water amount and pulses of watering of different magnitude affected functional traits and seedling growth, even if the amount of water provided was the same. Different watering patterns altered soil drying dynamics, with reduced supply of water amount and frequent watering becoming the driest treatment. We found that roots of all species responded to alterations in water supply by changing biomass allocation patterns (i.e., higher root-to-shoot mass [R:S] ratio in droughted plants), and by altering fine roots diameter, measured in terms of specific root length. Indeed, differences in growth rate among species were significantly linked to fine roots diameter and biomass allocation, which relates to uptake capacity of roots. However, relative growth rate and leaf traits such as specific leaf area were insensitive, likely because prolonged droughts over longer periods of time seem necessary to constraint growth in all these arid shrubs.

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.

Early Root Overproduction Not Triggered by Nutrients Decisive for Competitive Success Belowground

Background Theory predicts that plant species win competition for a shared resource by more quickly preempting the resource in hotspots and by depleting resource levels to lower concentrations than its competitors. Competition in natural grasslands largely occurs belowground, but information regarding root interactions is limited, as molecular methods quantifying species abundance belowground have only recently become available. Principal Findings In monoculture, the grass Festuca rubra had higher root densities and a faster rate of soil nitrate depletion than Plantago lanceolata, projecting the first as a better competitor for nutrients. However, Festuca lost in competition with Plantago. Plantago not only replaced the lower root mass of its competitor, but strongly overproduced roots: with only half of the plants in mixture than in monoculture, Plantago root densities in mixture were similar or higher than those in its monocultures. These responses occurred equally in a nutrient-rich and nutrient-poor soil layer, and commenced immediately at the start of the experiment when root densities were still low and soil nutrient concentrations high. Conclusions/Significance Our results suggest that species may achieve competitive superiority for nutrients by root growth stimulation prior to nutrient depletion, induced by the presence of a competitor species, rather than by a better ability to compete for nutrients per se. The root overproduction by which interspecific neighbors are suppressed independent of nutrient acquisition is consistent with predictions from game theory. Our results emphasize that root competition may be driven by other mechanisms than is currently assumed. The long-term consequences of these mechanisms for community dynamics are discussed.

Land-use changes and carbon sequestration through the twentieth century in a Mediterranean mountain ecosystem: implications for land management.

Ecosystems in the western Mediterranean basin have undergone intense changes in land use throughout the centuries, resulting in areas with severe alterations. Today, most these areas have become sensitive to human activity, prone to profound changes in land-use configuration and ecosystem services. A consensus exists amongst stakeholders that ecosystem services must be preserved but managerial strategies that help to preserve them while ensuring sustainability are often inadequate. To provide a basis for measuring implications of land-use change on carbon sequestration services, changes in land use and associated carbon sequestration potential throughout the 20th century in a rural area at the foothills of the Sierra Nevada range (SE Spain) were explored. We found that forest systems replaced dryland farming and pastures from the middle of the century onwards as a result of agricultural abandonment and afforestation programs. The area has always acted as a carbon sink with sequestration rates ranging from 28,961 t CO(2) year(-1) in 1921 to 60,635 t CO(2) year(-1) in 1995, mirroring changes in land use. Conversion from pastures to woodland, for example, accounted for an increase in carbon sequestration above 30,000 t CO(2) year(-1) by the end of the century. However, intensive deforestation would imply a decrease of approximately 66% of the bulk CO(2) fixed. In our study area, woodland conservation is essential to maintain the ecosystem services that underlie carbon sequestration. Our essay could inspire policymakers to better achieve goals of increasing carbon sequestration rates and sustainability within protected areas.

Root plasticity maintains growth of temperate grassland species under pulsed water supply

Background and aimsThe frequency of rain is predicted to change in high latitude areas with more precipitation in heavy, intense events interspersed by longer dry periods. These changes will modify soil drying cycles with unknown consequences for plant performance of temperate species.MethodsWe studied plant growth and root traits of juveniles of four grasses and four dicots growing in a greenhouse, when supplying the same total amount of water given either regular every other day or pulsed once a week.ResultsPulsed water supply replenished soil moisture immediately after watering, but caused substantial drought stress at the end of the watering cycle, whereas regular watering caused more moderate but consistent drought. Grasses had lower water use efficiency in the pulsed watering compared to regular watering, whereas dicots showed no difference. Both grasses and dicots developed thinner roots, thus higher specific root length, and greater root length in the pulsed watering. Growth of dicots was slightly increased under pulsed watering.ConclusionsTemperate species coped with pulsed water supply by eliciting two responses: i) persistent shoot growth, most likely by maximizing growth at peaks of soil moisture, thus compensating for slower growth during drought periods; ii) plasticity of root traits related to increased resource uptake. Both responses likely account for subtle improvement of growth under changed water supply conditions.

Woody species of a semi-arid community are only moderately resistant to cavitation

Vulnerability to drought-induced cavitation and seasonal water relations of six shrub species with different functional traits (deep v. superficial roots; evergreen v. summer deciduous; leaves v. cladodes) were measured in a semi-arid plant community strongly limited by water availability. The underlying hypotheses were that species would differ in their hydraulic properties and resistance to drought, reflecting different adaptations to a common environment and that individual adaptations may involve tradeoffs that would cause hydraulic properties to co-vary. Species experiencing the lowest minimum leaf water potentials generally had lower stomatal conductance, but they were not more resistant to xylem embolism than species with higher leaf water potentials and stomatal conductance. Overall, the studied species were more vulnerable to xylem embolism than expected and experienced high rates of native embolism and percent of leafless branches during summer drought. However, recovery rates from leafless branches were also high. Xylem resistance to embolism varied between species but had no relationship with minimum leaf water potential, suggesting that (i) adaptation to arid environments does not necessarily imply high resistance to embolism; and (ii) the costs associated with embolism resistance can be compensated by other components of the hydraulic strategy of a given species.

Diversity effects on root length production and loss in an experimental grassland community

1.Advances in root ecology have revealed that root standing biomass is higher in species-rich plant communities than in species-poor communities. Currently, we do not know whether this below-ground diversity effect is the result of enhanced root production or reduced root mortality or both, which is essential information to understand ecosystem functioning, as it determines C sequestration and N dynamics in soil. 2.Minirhizotron observations were combined with root coring in five different plant communities (four monocultures and the respective mixture). Molecular markers were used to quantitatively determine species abundance in mixed root biomass samples in order to track shifts in below-ground species composition. In addition, a litterbag experiment was performed to study root decomposition independent of root mortality. 3.Root length production was greater and root length loss was lower in the mixture than expected from monocultures in all years. Simulations suggest that at least two species must have had reduced losses in mixture compared to monoculture. However, the diversity effects on root mortality may partially be explained by selection effects as the species with the longest root life span became dominant in the mixtures. Root length loss from minirhizotrons was very low; the combination of minirhizotron length measurements with root biomass estimates from coring suggested underestimation of root loss in minirhizotrons over time. Root decomposition was not affected by diversity. 4.Diversity enhanced root length production and decreased root loss, resulting in below-ground overyielding. With decomposition unaffected, our results suggest that root mortality is reduced with increasing diversity. Future studies have to reveal the generality of our observations in larger scale biodiversity experiments by using species having a wider variety of root traits.

Assessing crop N status of fertigated vegetable crops using plant and soil monitoring techniques

Abstract Evaluation of crop N status will assist optimal N management of intensive vegetable production. Simple procedures for monitoring crop N status such as petiole sap [NO 3 −–N], leaf N content and soil solution [NO 3 −] were evaluated with indeterminate tomato and muskmelon. Their sensitivity to assess crop N status throughout each crop was evaluated using linear regression analysis against nitrogen nutrition index (NNI) and crop N content. NNI is the ratio between the actual and the critical crop N contents (critical N content is the minimum N content necessary to achieve maximum growth), and is an established indicator of crop N status. Nutrient solutions with four different N concentrations (treatments N1–N4) were applied throughout each crop. Average applied N concentrations were 1, 5, 13 and 22 mmol L−1 in tomato, and 2, 7, 13 and 21 mmol L−1 in muskmelon. Respective rates of N were 23, 147, 421 and 672 kg N ha−1 in tomato, and 28, 124, 245 and 380 kg N ha−1 in muskmelon. For each N treatment in each crop, petiole sap [NO 3 −–N] was relatively constant throughout the crop. During both crops, there were very significant (P < 0.001) linear relationships between both petiole sap [NO 3 −–N] and leaf N content with NNI and with crop N content. In indeterminate tomato, petiole sap [NO 3 −–N] was very strongly linearly related to NNI (R2 = 0.88–0.95, P < 0.001) with very similar slope and intercept values on all dates. Very similar relationships were obtained from published data of processing tomato. A single linear regression (R2 = 0.77, P < 0.001) described the relationship between sap [NO 3 −–N] and NNI for both indeterminate and processing tomato, each grown under very different conditions. A single sap [NO 3 −–N] sufficiency value of 1050 mg N L−1 was subsequently derived for optimal crop N nutrition (at NNI = 1) of tomato grown under different conditions. In muskmelon, petiole sap [NO 3 −–N] was strongly linearly related to NNI (R2 = 0.75 – 0.88, P < 0.001) with very similar slope and intercept values for much of the crop (44–72 DAT, days after transplanting). A single linear relationship between sap [NO 3 −–N] and NNI (R2 = 0.77, P < 0.001) was derived for this period, but sap sufficiency values could not be derived for muskmelon as NNI values were >1. Relationships between petiole sap [NO 3 −–N] with crop N content, and leaf N content with both NNI and crop N content had variable slopes and intercept values during the indeterminate tomato and the muskmelon crops. Soil solution [NO 3 −] in the root zone was not a sensitive indicator of crop N status. Of the three systems examined for monitoring crop/soil N status, petiole sap [NO 3 −–N] is suggested to be the most useful because of its sensitivity to crop N status and because it can be rapidly analysed on the farm.

Tools and Strategies for Sustainable Nitrogen Fertilisation of Vegetable Crops

In intensive vegetable production, N fertiliser applications often contribute to a supply of N that appreciably exceeds crop N requirements resulting in the loss of N to the environment which can result in NO3− contamination of water bodies. There is a range of tools and strategies that can assist vegetable growers to improve N management. These include various methods based on soil analysis or estimation of the soil N supply, N balance calculations, methods based on plant analysis, methods based on monitoring crops with optical sensors, and the use of computerised decision support systems based on simulation models or data bases. Use of these tools has been demonstrated to appreciably reduce fertiliser N application and N losses while maintaining production. The selection of tools to be used by a grower will be influenced by factors such as availability, the grower’s technical level, and economic considerations. For fertigation systems with high frequency N application, a combination of a planning method such as a decision support system with a monitoring method is recommended. Additional tools that can assist in demonstrating to stakeholders the benefit of improved N management are simulation models that provide scenario analysis. Fundamental strategies for improving N fertiliser management are to consider all N sources such as root zone soil mineral N and N mineralised from organic materials, and to partition N application so that it coincides with crop N demand.