Shift from short-term competition to facilitation with drought stress is due to a decrease in long-term facilitation

Disentangling short- and long-term neighbour effects, using both removal and observational methods within a single experiment, has strongly improved our understanding of the driving mechanisms of plant–plant interactions. However, there has been no attempt to assess two important underlying processes of their changes along gradients, either environmental-severity (changes in target performance without neighbours) or neighbour-traits (changes in performance with neighbours) effects, the former previously shown in alpine communities to be involved in competition and the latter in facilitation. We addressed this goal in an experiment conducted in continental saline depressions (sebkhas) from the Mediterranean arid climate of central Tunisia. We quantified short- and long-term effects of dominant shrubs, transplanting three target grass species in open, nurse and removed-nurse microhabitats of two habitats of different salinity levels in height sebkhas. The design extended greographically from central Tunisia to the Libyan border, 500 km southeastward. We used the relative interaction index to calculate short- and long-term effects before and after the dry summer seasons and environmental-severity and neighbour-trait effects. Short-term effects were slightly negative and long-term effects strongly positive before the dry summer season in the two habitats. Short-term effects switched to positive with increasing drought stress, due to an environmental-severity effect, whereas long-term effects decreased due to a neighbour-trait effect. Salinity did not affect neither short- nor long-term shrub effects. Soil moisture measurements showed that both changes were due to vanishing shrub soil engineering-effects during the summer drought. We conclude that an increase in short-term facilitation with increasing drought stress through time, apparently supporting the stress gradient hypothesis, might be due to a decrease in long-term facilitation. Thus, we recommend using, as much as possible, both the removal and observational methods in experiments assessing changes in plant–plant interactions along stress gradients to avoid wrong conclusions.     

Polymorphism of Cenchrus ciliaris L. a perennial grass of arid zones

This study deals with the phenotypic polymorphism of twenty accessions of Cenchrus ciliaris L. (buffelgrass) collected in various climatic areas of Tunisia. Six seedlings per accession were cultivated according to a common garden experiment design, and 33 phenological or morphological variables were measured during one cycle of growth. The observed differences among accessions were not significant for the seedling stage, but they were significant for all other studied variables of the vegetative and reproductive growth, dry matter and habit type. The principal component analysis provided further support for the important fixed genetic polymorphism of this species. On the other hand, a hierarchical classification was used to classify accessions in four groups, but we did not observe a clear geographical pattern associated with these groups. In conclusion, our study revealed considerable genotypic variation for C. ciliaris across Tunisia, which could be preserved by the agamospermous system of reproduction. Finally, among the groups from hierarchical classification, Mezzouna‐08 accession may be a good choice for the restoration of degraded Tunisian ecosystems, thanks to its high aboveground biomass productivity.     

Growth phenology and water potential patterns of Periploca angustifolia in relation to water availability

Periploca angustifoliaLabill., is a multipurpose shrub also used to rehabilitate drylands. It is essential to understand its phenological patterns under arid conditions. In order to understand how plants manage extreme drought, this study investigates the relationship between phenological traits of P. angustifolia Labill., water potential and climatic conditions. During two successive growing seasons (Sep 2009–Augst 2011) phenological patterns, soil water content and water potentials were measured monthly for plants of Periploca angustifoliaLabill growing under rain-fed conditions (Annual rainfall = 168 mm) at the pastoretum of the Arid Regions Institute in southern Tunisia. The Ψmd decreased progressively and concomitantly with increasing seasonal drought, reaching the lowest values in late summer (down to –3 MPa). Water potentials of P. angustifoliaLabill were affected by spatial and temporal variations in soil water content. The increased diurnal amplitude values (∆ψ > −1.3 MPa) explained the especially high biological activity of this species during the dry season. Our results clearly demonstrate that P. angustifolia Labill is a drought-tolerant species reaching low water potentials during the driest months of summer. These characteristics make this shrub as promising for rehabilitating degraded arid ecosystems.     

Aboveground biomass estimation of the shrubs, Echiochilon fruticosum (Desf.) and Helianthemum kahiricum (Del.) in the arid zone rangelands of Tunisia

The knowledge of the plant biomass is very important for the assessment of the rangeland productivity. It could help to select the appropriate species for the improvement of natural ecosystems (rehabilitation, restoration and seedling). By examining different correlations between the biomass production and the volume parameters of two North African shrub species of high range value ( Echiochilon fruticosum Desf. and Helianthemum kahiricum Del.), we aimed to establish the appropriate regression models, which could be useful for the prediction of the productivity of these species. The data showed a significant relationship between the total biomass (TB) production and the mean diameter (MD) of the studied species ( R ² = 0.65 for Echiochilon and R ² = 0.75 for Helianthemum ). Likewise, annual fresh production (leaves and current-year shoots) was well correlated with MD of Helianthemum ( R ² = 0.82). However, the correlation between these two parameters was relatively low for Echiochilon ( R ² = 0.42).