Managing semi-arid oak forests (Quercus brantii Lindl.): Mature oak trees of different dimensions create contrasted microhabitats influencing seedling quality.

We investigated the influence of mature of oak trees of various dimensions on soil properties, acorn and oak seedling characteristics in semi-arid forests in western Iran. A total of 24 oak trees were selected in comparable site conditions according to three size categories: small trees (DBH< 20 cm), medium trees (DBH: 20-50 cm) and large trees (DBH> 50 cm). Soil properties, light availability below canopy, acorn dimensions and weight, various below- and above-ground seedling morphological traits were measured. Besides, a seedling quality index (SQI) was also produced as an integrative measure of the seedling response. We found an increasing light availability from small trees to large trees (1512-103 µmol m-2 s-1) and soil fertility was largely improved from small trees to large trees: soil organic carbon (1.33-2.2%), available phosphorus (12.9-18.1 ppm) and potassium (301.2-470.4 ppm). However, soil properties did not significantly differ between medium and large trees. In contrast, acorn weight and dimensions as well as many seedling traits, including the aerial and belowground biomass and the SQI, were the highest in the medium tree category. To fully explore the relationships among our large set of variables, we produced a partial least square path model which explained 72% of the variation of SQI across the three tree classes. To conclude, we identified a clear effect by mature trees which provided favourable conditions for seedling establishment, but that effect was mediated by tree size and optimal conditions were found below the canopy of medium trees.

Linkage between plant species diversity and soil-based functions along a post-agricultural succession is influenced by the vegetative forms.

There is a growing body of knowledge that ecosystem functions, in particular, soil-based ecosystem functions, are related to biodiversity. However, how plant species diversity influences soil-based functions along post-agricultural secondary succession is still a largely ignored question in Mediterranean semi-arid conditions. Therefore, we used the plant functional group approach to investigate the relationships between plant species diversity indices and soil-based functions including microbial biomass carbon (MBC), basal respiration (BR), and carbon sequestration (CS) across three different stages of the vegetation succession corresponding to ~ 5 years after agricultural abandonment, ~ 15 years after abandonment, and oak forests which represent the terminal stage. We also tested if these relationships are supported by the niche complementarity and selection effect hypotheses. The results showed that soil-based functions significantly increased with time since abandonment as BR, MBC, and CS increased respectively by 1.7, 1.5, and 2.7 times across the three successional stages. We also found strong correlations between the diversity indices and the soil-based functions BR, MBC, and CS which were positive for richness (R2 values 0.75, 0.74, and 0.75) and Shannon diversity (R2 values 0.61, 0.58, and 0.61) but negative for evenness (R2 values 0.38, 0.38, and 0.36 for, respectively). Similarly, richness and Shannon diversity of the different plant functional groups positively correlated with soil-based functions. However, contrasting results were found for evenness which positively correlated with soil-based functions for perennial grass only. We suggested that increasing the diversity of plant species and facilitating dominant species would be needed to improve the soil-based ecosystem functions after abandonment of degraded soils. This study also revealed that the mechanisms behind the relationships between biodiversity and ecosystem functions were influenced by the vegetative forms.

The Impact of Competition and Allelopathy on the Trade-Off between Plant Defense and Growth in Two Contrasting Tree Species.

In contrast to plant-animal interactions, the conceptual framework regarding the impact of secondary metabolites in mediating plant-plant interference is currently less well defined. Here, we address hypotheses about the role of chemically-mediated plant-plant interference (i.e., allelopathy) as a driver of Mediterranean forest dynamics. Growth and defense abilities of a pioneer (Pinus halepensis) and a late-successional (Quercus pubescens) Mediterranean forest species were evaluated under three different plant interference conditions: (i) allelopathy simulated by application of aqueous needle extracts of Pinus, (ii) resource competition created by the physical presence of a neighboring species (Pinus or Quercus), and (iii) a combination of both allelopathy and competition. After 24 months of experimentation in simulated field conditions, Quercus was more affected by plant interference treatments than was Pinus, and a hierarchical response to biotic interference (allelopathy < competition < allelopathy + competition) was observed in terms of relative impact on growth and plant defense. Both species modulated their respective metabolic profiles according to plant interference treatment and thus their inherent chemical defense status, resulting in a physiological trade-off between plant growth and production of defense metabolites. For Quercus, an increase in secondary metabolite production and a decrease in plant growth were observed in all treatments. In contrast, this trade-off in Pinus was only observed in competition and allelopathy + competition treatments. Although Pinus and Quercus expressed differential responses when subjected to a single interference condition, either allelopathy or competition, species responses were similar or positively correlated when strong interference conditions (allelopathy + competition) were imposed.

Six-year time course of light-use efficiency, carbon gain and growth of beech saplings (Fagus sylvatica) planted under a Scots pine (Pinus sylvestris) shelterwood.

Two-year-old Fagus sylvatica L. saplings were planted under the cover of a Pinus sylvestris L. stand in the French Massif Central. The stand was differentially thinned to obtain a gradient of transmitted photosynthetically active radiation (PAR(t); 0-0.35). Eighteen Fagus saplings were sampled in this gradient, and their growth (basal stem diameter increment) was recorded over six years. Over the same period, morphological parameters (leaf area, number and arrangement in space) were monitored by 3D-digitization. Photosynthetic parameters were estimated with a portable gas-exchange analyzer. Photosynthesis was mainly related to light availability, whereas sapling morphology was mainly driven by sapling size. Annual stem diameter increment was related to the amount of light-intercepting foliage (silhouette to total leaf area ratio (STAR) x total sapling leaf area (LA)) and light availability above the saplings (PAR(t)). However, light-use efficiency, i.e., the slope of the relationship between STAR x LA x PAR(t) and stem diameter increment, decreased over time as a result of a relative decrease in the proportion of photosynthetic tissues to total sapling biomass.