How plant functional traits (e.g., seed mass) drive species abundance within communities remains an unsolved question. Borrowing concepts from natural selection theory, we propose that trait-abundance relationships can generally correspond to one of three modes of trait selection: directional (a rectilinear relationship, where species at one end of a trait axis are most abundant), stabilizing (an n-shaped relationship), and disruptive (a u-shaped relationship). Stabilizing selection (i.e., the functional convergence of abundant species) would result from positive density-dependent interactions (e.g., facilitation) or due to generalized trade-offs in resource acquisition/use, while disruptive selection (i.e., the divergence of abundant species) would result from negative density-dependent interactions (e.g., competition) or due to environmental heterogeneity. These selection modes can be interpreted as proxies for community-level trait-fitness functions, which establish the degree to which traits are truly "functional". We searched for selection modes in a desert annual-plant community in Argentina (which was divided into winter and summer guilds) to test the hypothesis that the relative importance of disruptive mechanisms (competition, disturbances) decreases with the increase of abiotic stress, a stabilizing agent. Average density was analyzed as a function of eight traits generally linked to resource acquisition and competitive ability (maximum plant height, leaf size, specific leaf area, specific root length), resource retention and stress tolerance (leaf dissection, leaf dry matter content, specific root volume), and regeneration (seed mass) using multiple quadratic-regression models. Trait selection was stabilizing and/or directional when the environment was harshest (winter) and disruptive and/or directional when conditions were milder (summer). Selection patterns differed between guilds for two important traits: plant height and seed mass. These results suggest that abiotic stress may drive within-community functional convergence independently of the trait considered, opposing the view that some traits may be inherently convergent while others divergent. Our quadratic model-based approach provides standardized metrics of both linear and nonlinear selection that may allow simple comparisons among communities subjected to contrasting environmental conditions. These concepts, rooted in natural selection theory, may clarify the functional link between traits and species abundance, and thus help untangle the contributions of deterministic and stochastic processes on community assembly.

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Question: For a desert where winter is the driest, harshest season we asked: does the effect of dominant shrubs (Bulnesia retama) on annual species depend on (1) the functional traits of the latter, (2) the season of the year, or (3) the activity of livestock?Location: A low-density goat farm in central-northern Monte Desert, Argentina. Methods:We estimated the effect of shrubs using a log response ratio based on annual species population sizes underneath shrub canopies and in open spaces. We collected density data of annual species in 18 visits between Aug 2010 and Apr 2013 in permanent 50-cm square areas laid out according to a split-plot design, in which the activity of livestock (fenced and unfenced; plots were 10-m squares) was the main factor, and microsite type (shrub and open) was the subordinate factor, with 20 replicates for each combination. We also gathered data on eight functional traits (characterizing whole plants, leaves, roots and seeds) from annual plants collected in the study site following standardized protocols.Results: Annual species with acquisitive attributes (high specific leaf area, intermediate-to-low leaf dry matter content, large leaves and high specific root volume) were more benefited by shrubs compared to species with the opposite, relatively conservative attributes. Facilitative influences of shrubs were pervasive during winter, while competitive influences increased in frequency during summer, when total plant density was higher. This pattern was not affected by livestock.Conclusions: The outcome of species interactions depended on the interplay between plant strategies and abiotic stress: facilitated species were mostly acquisitive, and shrub facilitation was more important during the harshest season (winter). Specific root volume, along with widely used functional traits (specific leaf area, leaf dry matter content, leaf size) delineated such strategies. This underlines the importance of considering below-ground traits when studying plant-plant interactions. Single traits achieved superior explanatory power of shrub effects than composite ones (i.e. principal component axes based on single traits), reinforcing the idea that single functional traits are themselves meaningful indicators of complex physiological trade-offs that ultimately affect community structure and dynamics.

Woody plant encroachment of savanna ecosystems has been related to altered disturbance regimes, mainly fire suppression and herbivore exclusion. In contrast, neighbourhood interactions among resident and colonising woody species have received little attention, despite their likely influence on the pattern and rate of tree establishment. We examined how resident palm trees (Butia yatay) and established adults of two riparian forest tree species (Allophylus edulis and Sebastiania commersoniana) influenced seed arrival and seedling performance of the latter two species in a humid savanna of east-central Argentina. Seed traps and seedlings of both riparian species were placed in herbaceous openings, and beneath palm, conspecific and heterospecific adult trees in two unburned savanna patches, and were monitored for 2 years. Only seeds of the bird-dispersed Allophylus arrived in palm microsites, yet survival of Allophylus seedlings near adult palms was limited by animal damage through trampling and burrowing, a non-trophic mechanism of apparent competition. Seeds of both riparian species dispersed into conspecific microsites, although adult trees selectively reduced growth of conspecific seedlings, a pattern consistent with the "escape hypothesis". Further, survival of Sebastiania increased in the moister Allophylus microsites, suggesting a one-way facilitative interaction between woody colonisers. Our results indicate that dispersal facilitation by resident savanna trees may be critical to riparian species invasion after fire suppression. Distance-dependent effects of conspecific and heterospecific adult trees could contribute to shape the subsequent dynamics of woody seedling establishment. Overall, we show that indirect interactions can play a prominent role in savanna encroachment by non-resident woody species.