Functional traits determine plant co-occurrence more than environment or evolutionary relatedness in global drylands

Plant–plant interactions are driven by environmental conditions, evolutionary relationships (ER) and the functional traits of the plants involved. However, studies addressing the relative importance of these drivers are rare, but crucial to improve our predictions of the effects of plant–plant interactions on plant communities and of how they respond to differing environmental conditions. To analyze the relative importance of – and interrelationships among – these factors as drivers of plant–plant interactions, we analyzed perennial plant co-occurrence at 106 dryland plant communities established across rainfall gradients in nine countries. We used structural equation modelling to disentangle the relationships between environmental conditions (aridity and soil fertility), functional traits extracted from the literature, and ER, and to assess their relative importance as drivers of the 929 pairwise plant–plant co-occurrence levels measured. Functional traits, specifically facilitated plants’ height and nurse growth form, were of primary importance, and modulated the effect of the environment and ER on plant–plant interactions. Environmental conditions and ER were important mainly for those interactions involving woody and graminoid nurses, respectively. The relative importance of different plant–plant interaction drivers (ER, functional traits, and the environment) varied depending on the region considered, illustrating the difficulty of predicting the outcome of plant–plant interactions at broader spatial scales. In our global-scale study on drylands, plant–plant interactions were more strongly related to functional traits of the species involved than to the environmental variables considered. Thus, moving to a trait-based facilitation/competition approach help to predict that: (1) positive plant–plant interactions are more likely to occur for taller facilitated species in drylands, and (2) plant–plant interactions within woody-dominated ecosystems might be more sensitive to changing environmental conditions than those within grasslands. By providing insights on which species are likely to better perform beneath a given neighbour, our results will also help to succeed in restoration practices involving the use of nurse plants

Aridity modulates N availability in arid and semiarid Mediterranean grasslands

While much is known about the factors that control each component of the terrestrial nitrogen (N) cycle, it is less clear how these factors affect total N availability, the sum of organic and inorganic forms potentially available to microorganisms and plants. This is particularly true for N-poor ecosystems such as drylands, which are highly sensitive to climate change and desertification processes that can lead to the loss of soil nutrients such as N. We evaluated how different climatic, abiotic, plant and nutrient related factors correlate with N availability in semiarid Stipa tenacissima grasslands along a broad aridity gradient from Spain to Tunisia. Aridity had the strongest relationship with N availability, suggesting the importance of abiotic controls on the N cycle in drylands. Aridity appeared to modulate the effects of pH, plant cover and organic C (OC) on N availability. Our results suggest that N transformation rates, which are largely driven by variations in soil moisture, are not the direct drivers of N availability in the studied grasslands. Rather, the strong relationship between aridity and N availability could be driven by indirect effects that operate over long time scales (decades to millennia), including both biotic (e.g. plant cover) and abiotic (e.g. soil OC and pH). If these factors are in fact more important than short-term effects of precipitation on N transformation rates, then we might expect to observe a lagged decrease in N availability in response to increasing aridity. Nevertheless, our results suggest that the increase in aridity predicted with ongoing climate change will reduce N availability in the Mediterranean basin, impacting plant nutrient uptake and net primary production in semiarid grasslands throughout this region

Decoupling of soil nutrient cycles as a function of aridity in global drylands

18 páginas.- 10 figuras.- 72 referencias.- Online Content Any additional Methods, Extended Data display items and Source Data are available in the online version of the paper; references unique to these sections appear only in the online paper..- Puede conseguir el texto completo en el Portal de la producción científica de la Universidad Complutense de Madrid https://produccioncientifica.ucm.es/documentos/5ec78dc52999520a1d557660 .- o en lel respositorio institucional CONICET digital https://ri.conicet.gov.ar/bitstream/handle/11336/29204/CONICET_Digital_Nro.ead4e2ed-0da6-4041-814b-259e8f27bbf6_D.pdf?sequence=5&isAllowed=yThe biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P) are interlinked by primary production, respiration and decomposition in terrestrial ecosystems1. It has been suggested that the C, N and P cycles could become uncoupled under rapid climate change because of the different degrees of control exerted on the supply of these elements by biological and geochemical processes1,2,3,4,5. Climatic controls on biogeochemical cycles are particularly relevant in arid, semi-arid and dry sub-humid ecosystems (drylands) because their biological activity is mainly driven by water availability6,7,8. The increase in aridity predicted for the twenty-first century in many drylands worldwide9,10,11 may therefore threaten the balance between these cycles, differentially affecting the availability of essential nutrients12,13,14. Here we evaluate how aridity affects the balance between C, N and P in soils collected from 224 dryland sites from all continents except Antarctica. We find a negative effect of aridity on the concentration of soil organic C and total N, but a positive effect on the concentration of inorganic P. Aridity is negatively related to plant cover, which may favour the dominance of physical processes such as rock weathering, a major source of P to ecosystems, over biological processes that provide more C and N, such as litter decomposition12,13,14. Our findings suggest that any predicted increase in aridity with climate change will probably reduce the concentrations of N and C in global drylands, but increase that of P. These changes would uncouple the C, N and P cycles in drylands and could negatively affect the provision of key services provided by these ecosystems.This research is supported by the European Research Council (ERC) under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. 242658 (BIOCOM), and by the Ministry of Science and Innovation of the Spanish Government, grant no. CGL2010-21381. CYTED funded networking activities (EPES, Acción 407AC0323). M.D.-B. was supported by a PhD fellowship from the Pablo de Olavide University.Peer reviewe

Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan en Tunisie pré-saharienne : structure du peuplement, réponses et effets biologiques et environnementaux

Ce travail de thèse a pour objectif principal de comprendre la dynamique de l’espèce fondatrice de l’écosystème de la région de Bled Talah, ainsi que les mécanismes susceptibles de mieux comprendre ses interactions avec la végétation compagne. Des mesures dendrométriques accomplies par des essais de détermination d’âge, ont fait ressortir que le peuplement de la forêt du Bou Hedma semble naturellement suivre plutôt une certaine dynamique régressive. C’est ainsi que nos résultats démontrent plutôt la dominance des individus de grandes circonférences (des individus âgés). Les obstacles à la faible régénération naturelle de ce taxon sont, essentiellement, l’infestation des graines par les bruches, la possibilité d’un éventuel effet autoallélopathique et la dominance des interactions négatives (compétition) entre la végétation du Parc National du Bou Hedma et les jeunes plants d’Acacia. Concernant, les interactions biotiques, nos résultats ont permis d’enrichir les débats actuels sur l’importance et la direction des interactions le long des gradients de stress et de perturbation. Une dominance des interactions négatives (compétition), le long des deux gradients a été démontrée, avec une chute de l’importance de la compétition en présence d’herbivorie. Les mesures environnementales et biologiques ont permis de conclure, qu’Acacia tortilis subsp. raddiana, a un effet globalement bénéfique pour le sol, ainsi que pour la végétation naturelle environnante. Compte tenu du rôle clef de cette espèce sous les conditions contraignantes du bioclimat aride, la nécessité de minimiser le risque de la faible régénération naturelle de ce taxon est prioritaire dans les grands programmes de reboisement et de restauration des écosystèmes arides.The main aim of this work were to understand the dynamics of the founding species of the ecosystem of the region of Bled Talah, as well as the mechanisms to understand better its interactions with the vegetation partner. Dendrometric parameters were measured for each tree then determination of age were estimated. This work improves general understanding of the factors explaining the structure of the Acacia tortilis subsp. raddiana. These species naturally seems to follow rather a certain regressive dynamics. The possible reasons for regeneration failure natural regeneration of Acacia tortilis subsp. raddiana were especially the high infestation of seeds by Bruchidius raddianae, the autoallelopathic effetcs and the dominance of the negative interactions (competition) between the vegetation of the National park of Bou Hedma and the young plantations of Acacia tree. The importance and direction of biotic interactions along environmental gradients is still a major debate in plant ecology. Through the study of biotic interactions, our results suggests the dominance of competition between nurses and target species used in this work.The results of this study confirmed the positive effect of Acacia tortilis subsp. raddiana on the understorey vegetation composition and diversity in arid ecosystems. Once, established in the dry area, this species ameliorates microclimate including soil conditions that may be suitable to the regeneration and the growth of other species. In this way, this species will provide beneficial effects to the generating seedlings in the vicinity and therfore will help rehabilitating degraded areas

Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan en Tunisie pré-saharienne : structure du peuplement, réponses et effets biologiques et environnementaux

Ce travail de thèse a pour objectif principal de comprendre la dynamique de l’espèce fondatrice de l’écosystème de la région de Bled Talah, ainsi que les mécanismes susceptibles de mieux comprendre ses interactions avec la végétation compagne. Des mesures dendrométriques accomplies par des essais de détermination d’âge, ont fait ressortir que le peuplement de la forêt du Bou Hedma semble naturellement suivre plutôt une certaine dynamique régressive. C’est ainsi que nos résultats démontrent plutôt la dominance des individus de grandes circonférences (des individus âgés). Les obstacles à la faible régénération naturelle de ce taxon sont, essentiellement, l’infestation des graines par les bruches, la possibilité d’un éventuel effet autoallélopathique et la dominance des interactions négatives (compétition) entre la végétation du Parc National du Bou Hedma et les jeunes plants d’Acacia. Concernant, les interactions biotiques, nos résultats ont permis d’enrichir les débats actuels sur l’importance et la direction des interactions le long des gradients de stress et de perturbation. Une dominance des interactions négatives (compétition), le long des deux gradients a été démontrée, avec une chute de l’importance de la compétition en présence d’herbivorie. Les mesures environnementales et biologiques ont permis de conclure, qu’Acacia tortilis subsp. raddiana, a un effet globalement bénéfique pour le sol, ainsi que pour la végétation naturelle environnante. Compte tenu du rôle clef de cette espèce sous les conditions contraignantes du bioclimat aride, la nécessité de minimiser le risque de la faible régénération naturelle de ce taxon est prioritaire dans les grands programmes de reboisement et de restauration des écosystèmes arides.The main aim of this work were to understand the dynamics of the founding species of the ecosystem of the region of Bled Talah, as well as the mechanisms to understand better its interactions with the vegetation partner. Dendrometric parameters were measured for each tree then determination of age were estimated. This work improves general understanding of the factors explaining the structure of the Acacia tortilis subsp. raddiana. These species naturally seems to follow rather a certain regressive dynamics. The possible reasons for regeneration failure natural regeneration of Acacia tortilis subsp. raddiana were especially the high infestation of seeds by Bruchidius raddianae, the autoallelopathic effetcs and the dominance of the negative interactions (competition) between the vegetation of the National park of Bou Hedma and the young plantations of Acacia tree. The importance and direction of biotic interactions along environmental gradients is still a major debate in plant ecology. Through the study of biotic interactions, our results suggests the dominance of competition between nurses and target species used in this work.The results of this study confirmed the positive effect of Acacia tortilis subsp. raddiana on the understorey vegetation composition and diversity in arid ecosystems. Once, established in the dry area, this species ameliorates microclimate including soil conditions that may be suitable to the regeneration and the growth of other species. In this way, this species will provide beneficial effects to the generating seedlings in the vicinity and therfore will help rehabilitating degraded areas

Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan en Tunisie pré-saharienne : structure du peuplement, réponses et effets biologiques et environnementaux

Ce travail de thèse a pour objectif principal de comprendre la dynamique de l’espèce fondatrice de l’écosystème de la région de Bled Talah, ainsi que les mécanismes susceptibles de mieux comprendre ses interactions avec la végétation compagne. Des mesures dendrométriques accomplies par des essais de détermination d’âge, ont fait ressortir que le peuplement de la forêt du Bou Hedma semble naturellement suivre plutôt une certaine dynamique régressive. C’est ainsi que nos résultats démontrent plutôt la dominance des individus de grandes circonférences (des individus âgés). Les obstacles à la faible régénération naturelle de ce taxon sont, essentiellement, l’infestation des graines par les bruches, la possibilité d’un éventuel effet autoallélopathique et la dominance des interactions négatives (compétition) entre la végétation du Parc National du Bou Hedma et les jeunes plants d’Acacia. Concernant, les interactions biotiques, nos résultats ont permis d’enrichir les débats actuels sur l’importance et la direction des interactions le long des gradients de stress et de perturbation. Une dominance des interactions négatives (compétition), le long des deux gradients a été démontrée, avec une chute de l’importance de la compétition en présence d’herbivorie. Les mesures environnementales et biologiques ont permis de conclure, qu’Acacia tortilis subsp. raddiana, a un effet globalement bénéfique pour le sol, ainsi que pour la végétation naturelle environnante. Compte tenu du rôle clef de cette espèce sous les conditions contraignantes du bioclimat aride, la nécessité de minimiser le risque de la faible régénération naturelle de ce taxon est prioritaire dans les grands programmes de reboisement et de restauration des écosystèmes arides.The main aim of this work were to understand the dynamics of the founding species of the ecosystem of the region of Bled Talah, as well as the mechanisms to understand better its interactions with the vegetation partner. Dendrometric parameters were measured for each tree then determination of age were estimated. This work improves general understanding of the factors explaining the structure of the Acacia tortilis subsp. raddiana. These species naturally seems to follow rather a certain regressive dynamics. The possible reasons for regeneration failure natural regeneration of Acacia tortilis subsp. raddiana were especially the high infestation of seeds by Bruchidius raddianae, the autoallelopathic effetcs and the dominance of the negative interactions (competition) between the vegetation of the National park of Bou Hedma and the young plantations of Acacia tree. The importance and direction of biotic interactions along environmental gradients is still a major debate in plant ecology. Through the study of biotic interactions, our results suggests the dominance of competition between nurses and target species used in this work.The results of this study confirmed the positive effect of Acacia tortilis subsp. raddiana on the understorey vegetation composition and diversity in arid ecosystems. Once, established in the dry area, this species ameliorates microclimate including soil conditions that may be suitable to the regeneration and the growth of other species. In this way, this species will provide beneficial effects to the generating seedlings in the vicinity and therfore will help rehabilitating degraded areas

Acacia tortilis (Forssk.) subsp. raddiana (Savi) Brenan as a foundation species: a test from the arid zone of Tunisia

The geographic distribution of Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan in Tunisia, is limited to the Bou-Hedma region (Bled Talah) where the National Park of Bou-Hedma, a UNESCO (1986) biosphere reserve, is located. We have tested the hypothesis that A. tortilis acts as a foundation species as a result of the improvement of soil conditions under its canopy (soil water availability and nutrient enrichment). The herbaceous community (floristic composition, species density, species richness) and soil characteristics (soil nutrients and soil water) were sampled using the quadrat method beneath and between 16 adult trees of A. tortilis with both northwards and southwards (south and north sides) and eight open areas among the trees. Results showed that A. tortilis trees improve soil water availability, the nutrient status of soil (organic matter, total carbon, total nitrogen, extractable phosphorus), and has a positive effect on the understorey vegetation under arid conditions. No significant differences in species richness were observed between the north subhabitats and the south subhabitats

Acacia tortilis subsp raddiana in the North African arid zone : the obstacles to natural regeneration

L’objectif du présent travail consiste à étudier les causes de la faible régénération naturelle d’Acacia tortilis subsp. raddiana (Fabaceae). Des recherches ont été effectuées in situ, avec observations et examen sous la canopée de 300 individus adultes et de taille moyenne en 2006-2007. En outre, nous avons étudié le taux d’infestation des semences par Bruchidius raddianae et B. aurivillii. Des observations ont aussi été réalisées en laboratoire afin de tester un éventuel effet autotoxique de l’extrait de feuillage sur des semences en incubation. À partir d’une solution mère initiale (50% de feuilles + 50% d’eau distillée), différentes concentrations ont été obtenues. Trente graines intactes d’A. tortilis ont été placées en boîtes de Petri, à raison de 8 répétitions, et arrosées avec les solutions puis placées à l’étuve à l’obscurité et à une température de 30 °C. En conditions naturelles, la germination est quasi nulle. Le taux d’infestation des graines par les bruches est anormalement élevé, atteignant 83%, ce qui peut entre autres expliquer la faible régénération naturelle. L’inhibition de la germination augmente avec l’accroissement de la concentration de l’extrait foliaire, avec des résultats hautement significatifs à 100% de concentration.The main objective of this study is to determine the possible reasons for regeneration failure of Acacia tortilis subsp. raddiana (Fabaceae). We conducted some laboratory experiments, simulating natural conditions. We estimated rate of infestation of seeds by Bruchidius raddianae and B. aurivillii and tested the autotoxicity effect of leaf extracts (different concentrations) of A. tortilis in the laboratory at 30 °C (optimum germination temperature). Results show that the inhibitory effect on germination increase with the increased leaf extract concentration. At 100% concentration, a significant reduction of germination rate was found compared to other concentrations. On the other hand, the study also revealed a high infestation rate of seeds by B. raddianae and B. aurivillii which reduces the possibility in inhibits natural regeneration in A. tortilis

The influence of Acacia tortilis (Forssk.) Subsp raddiana (Savi) and livestock grazing on grass species composition, yield and soil nutrients in and environments of South Tunisia

The objectives of the study, conducted during the 2003/2004 growing season in the National Park of Bou Hedma (South Tunisia), were to quantify the effects of the single-woody species Acacia tortilis subsp. raddiana on grass species composition, on total plant cover, on density of perennial species, on dry matter (DM) yield and on soil nutrients at lightly and heavily grazed sites. In each study site, two subhabitats were distinguished, i.e. under tree canopies and open grasslands. In the lightly grazed site, the nutrient status of soil (organic matter, total N, extractable P, K+, Ca2+, Na+, Mg2+) under Acacia raddiana canopy, was found to be significantly higher (p<0.05) than under the open grassland. In the same way, total plant cover (p<0.05), density of perennial species (p<0.01) and DM yield (p<0.01) were significantly higher under tree canopies in the lightly grazed site. Heavy grazing proved to exert a strong overriding effect over the positive influences of the woody plants. For most studied parameters, a non-significant difference was recorded between canopied and uncanopied subhabitats. Some palatable species were frequently found under trees. In the heavily grazed site, these species are being replaced by less desirable species. This emphasizes the importance of conservation stocking rates and proper pasture management