Angela Illuminati

The least‐cost economic theory of photosynthesis (LCE) postulates that leaf nitrogen (N) modulates the balance between carbon gain and water loss in plants, with N‐rich leaves exhibiting higher photosynthetic efficiency and lower transpiration rates than N‐poor leaves. However, the optimal balance of water and N acquisition and use may differ substantially among coexisting species, influenced by factors such as evolutionary history. Understanding how co‐occurring species balance water and nutrient use during photosynthesis, and how this shapes eco‐physiological performance and growth is ecologically relevant, particularly in water‐limited environments. Here, we investigate whether the LCE theory can explain differences in leaf water‐use traits among 23 naturally co‐occurring Mediterranean shrubland species growing under optimal, non‐limiting water and nutrient conditions. Specifically, we evaluated the effects of leaf N content on both inter‐ and intraspecific variation of three leaf traits related to water use: δ ¹³ C (proxy for intrinsic water‐use efficiency, WUEi), δ ¹⁸ O (proxy for stomatal conductance) and IMS, that is integrated metabolic strategy, (index integrating both isotopes to estimate leaf metabolic efficiency) accounting for potential phylogenetic constraints. We also examined how these traits influenced plant relative growth rate across species. Our 23 perennial species exhibited large interspecific variation in leaf traits related to water and nutrient use. Mean leaf N content ranged from 2.4 to 6% across species and was positively associated with both δ ¹⁸ O and IMS, revealing tighter stomatal regulation and enhanced metabolic efficiency in N‐rich leaves. Phylogenetic mixed models confirmed that leaf N was the main driver of variation in leaf water flux (δ ¹⁸ O) and metabolic efficiency (IMS) across individuals. Interspecific variation in plant growth rate was largely driven by leaf N and IMS. Synthesis. The large functional diversity in our dryland plant community results in significant variations in water and nutrient input mixture optimization across co‐occurring species. Our findings provide experimental evidence that increased nitrogen investment in leaves is associated, both across and within species, with enhanced photosynthetic efficiency and reduced water loss under controlled, non‐limiting conditions. Furthermore, they support IMS as an integrative index of the trade‐off between carbon assimilation and water loss.

Background and Aims: Plant–plant interactions are crucial in Mediterranean ecosystems for coping with drought conditions, but it remains unclear how these interactions are modulated during plant establishment. This study examined how drought influences plant performance during early life stages and whether interactions depend on functional similarity and phylogenetic distance to the neighbourhood. Methods: We conducted a common garden experiment using seeds of 20 perennial species from a Mediterranean shrubland (central Spain). Four of these perennials (focal species) grew during their early life stage either alone, with conspecifics or with individuals of the remaining perennials, in both a control and drought treatments. Four functional traits were measured in all focal species individuals to estimate interaction patterns through the relative intensity index (RII). We then tested whether RII values were related to the phylogenetic or functional similarity between species. Key Results: Drought and neighbour presence induced negative effects on plant performance during early life stages, suggesting competitive interactions under these situations. Both RIIbiomass and RIIFv/Fm were significantly more negative under drought conditions, indicating that water scarcity results in individuals with lower biomass and reduced Fv/Fm, an indicator of photosynthetic efficiency, when grown with neighbours. The relationship between interaction patterns and phylogenetic or functional relatedness was species-specific, with some focal species competing more intensely when they grew with similar species (according to the limiting similarity hypothesis), and other species showing the opposite pattern (consistent with the competition–trait hierarchy hypothesis). Conclusions: These results challenge the idea that lower phylogenetic or functional similarity always reduces competition, highlighting the need for future research to consider the complexity of plant interactions between neighbours and across ontogeny.

Despite decades of research, a comprehensive understanding of trait coordination at the whole‐plant level remains elusive. Furthermore, while the link between above‐ground growth rates and leaf traits related to nutrient use is well established, much less is known about the below‐ground compartment. Herbs and woody species exhibit distinct above‐ground growth and nutrient‐use strategies, but whether these differences extend to root traits below‐ground remains unclear. We carried out a common garden experiment with 23 perennials (7 herbs and 16 woody species) coexisting in a Mediterranean shrubland and measured 17 above‐ground and below‐ground traits related to growth, nutrient use and size. We analysed the links between growth rate and nutrient use focusing especially on roots and considering potential differences between herbs and woody species. We also combined plant size data from experimental juveniles and field‐sampled adults to determine the effects of life stage on whole‐plant phenotypic integration. We found a significant relationship between growth rates (both above‐ground and below‐ground) and root nutrient‐use strategies. Root diameter was negatively associated to growth rate only in herbs. Specific root area and root tissue density were positively and negatively correlated with growth rate, respectively, in both herbs and woody species. Moreover, we found significant differences in roots traits between herbs and woody species. Plant growth rate and root nutrient‐use strategies were both positively associated to plant size (height and above‐ground diameter) in juveniles, while a negative relationship was observed in adults. Our work provides insights on the links between whole‐plant growth rate and nutrient‐use strategies in species from a dry Mediterranan shrubland. Specifically, we show that whole‐plant growth rate is strongly associated to root traits, with more acquisitive root nutrient‐use strategies related to faster growth rates, which in turn translated to greater plant size in juveniles but smaller in adults. Our results also highlight root functional differences and contrasting patterns of whole‐plant phenotypic integration between herbs and woody species, offering deeper insights into species coexistence in species‐rich dry Mediterranean environments. Read the free Plain Language Summary for this article on the Journal blog.

Water is the most limiting resource for plant survival and growth in arid environments, but the diversity of water‐use strategies among coexisting species in dryland communities is not well understood. There is also growing interest in assessing whether a whole‐plant coordination exists between traits related to water‐use and the leaf economic spectrum (LES). We used water stable isotopes (δ²H, δ¹⁸O) to quantify water uptake proportions from different soil depths by 24 species in a Mediterranean shrubland. Leaf traits associated with water‐use efficiency, stomatal regulation (δ¹³C, δ¹⁸O) and the LES (SLA, N, P, K concentrations) were also measured. We assessed potential trade‐offs between the above‐mentioned leaf traits, water uptake depth and their relationship with species abundance. We found distinct ecohydrological niche segregation among coexisting species. Bayesian models showed that our shrubland species used a median of 37% of shallow soil water (0–30 cm) and 63% of deep water (30–100 cm). Still, water source proportions varied considerably among species, as shallow soil water‐use ranged from a minimum of 6.4% to a maximum of 68%. Interspecific variability in foliar carbon investment (SLA) and nutrient concentrations was remarkably high, indicating diverse nutrient‐use strategies along the LES. Leaf δ¹⁸O, δ¹³C and δ¹⁵N values also differed widely among species, revealing differences in stomatal regulation, water‐use efficiency and nitrogen acquisition mechanisms. After accounting for evolutionary history effects, water uptake depth was coordinated with the LES: species using shallower soil water from fertile topsoil layers exhibited a more acquisitive carbon‐ and nutrient‐use strategy, whereas water uptake from deeper but less fertile soil layers was linked to a more conservative nutrient‐use strategy. Leaf‐level water‐use traits significantly influenced species abundance, as water‐savers with tight stomatal regulation and high water‐use efficiency were dominant. Synthesis. Greater utilisation of water stored in nutrient‐rich topsoil layers favoured a more acquisitive nutrient‐use strategy, whereas a deeper water uptake pattern appeared to constrain access to nutrients. Our findings thus suggest a largely inescapable trade‐off and coordination between soil water uptake depth and carbon‐ and nutrient‐use strategies in low‐fertility drylands.

Summary: Roots are assumed to play a major role in structuring soil microbial communities, but most studies exploring the relationships between microbes and plants at the community level have only used aboveground plant distribution as a proxy. However, a decoupling between belowground and aboveground plant components may occur due to differential spreading of plant canopies and root systems. Thus, soil microbe–plant links are not completely understood. Using a combination of DNA metabarcoding and spatially explicit sampling at the plant neighbourhood scale, we assessed the influence of the plant root community on soil bacterial and fungal diversity (species richness, composition and β‐diversity) in a dry Mediterranean scrubland. We found that root composition and biomass, but not richness, predict unique fractions of variation in microbial richness and composition. Moreover, bacterial β‐diversity was related to root β‐diversity, while fungal β‐diversity was related to aboveground plant β‐diversity, suggesting that plants differently influence both microbial groups. Our study highlights the role of plant distribution both belowground and aboveground, soil properties and other spatially structured factors in explaining the heterogeneity in soil microbial diversity. These results also show that incorporating data on both plant community compartments will further our understanding of the relationships between soil microbial and plant communities.