Positive Plant-plant Interactions Research Articles

Patterns of frequency and density dependence are highly variable in diverse annual flowering plant communities.

Applications of ecological theory to natural communities often assume that competitive, negative density-dependent processes are the only type of interaction important for diversity maintenance. Recent advances suggest that positive interactions within trophic levels (e.g. plant-plant) may also affect plant coexistence. Though positive plant-plant interactions theoretically might result in positive or nonmonotonic frequency or density dependence (FD/DD), less is known about how commonly these patterns occur, or which ecological processes might result in such patterns in natural plant communities. In this study we test for signals of variable frequency and density dependence in annual flowering plant communities in Western Australia, and search for evidence that interactions among plants during flowering might induce positive or nonmonotonic FD/DD in flowering plants. Using four common annual wildflower species, we ask if plant fecundity exhibits positive or nonmonotonic FD/DD, and if pollinator-mediated plant-plant interactions during flowering change patterns of FD/DD relative to pollinator-independent plant interactions. Three species exhibited nonmonotonic (hump-shaped) density dependence, and only one species experienced strictly negative density dependence. Each species exhibited a different pattern of frequency dependence (positive, negative, weakly nonmonotonic, and no detectable frequency dependence). Pollinator-mediated plant-plant interactions during flowering induced both nonmonotonic density dependence and negative frequency dependence in one species. Importantly, the extent of variation in FD/DD observed in our study brings into question the dominance of negative density and frequency dependence in theory, suggesting instead that demographic responses of plants to their communities fall along a continuum of possible density- and frequency-dependent patterns.

Junipers enable heavily browsed rowan saplings to escape ungulates in boreal forest

Recent increases in ungulate herbivore populations have intensified browsing pressure in northern forest ecosystems. High browsing pressure affects recruitment into tree populations and saplings are among the most impacted and critical tree stages. This calls for research on factors that enhance sapling survival and promote recovery of herbivore preferred species. One understudied factor in ungulate dominated boreal forests is the ability of young trees to escape large herbivores by utilising “nurse plants”. First, we examined the impact of ungulate browsing on the abundance and size of rowan (Sorbus aucuparia), a functionally important tree species in an old-growth boreal pine forest, western Norway. The responses were compared between exclosures and ungulate-access plots over ten years (2001–2011) combined with investigations along a browsing intensity gradient (Exp. A). In a second experiment we investigated whether rowan saplings escape browsing and enhance growth and survival by growing within nurse plants. Consequently, we planted rowan saplings in ungulate browsing areas, inside and outside coniferous juniper (Juniper communis) bushes, and analysed the treatment effect on browsing, growth, and survival of rowan saplings across five years (2013–2018; Exp B). Excluding large herbivores resulted in a large increase in rowan saplings which were six times more abundant in exclosures (0.44 ind./m2) compared to ungulate-access plots (0.07 ind./m2) after ten years (Exp. A). The abundance of rowan saplings also decreased with higher browsing intensity. Moreover, the rowans in exclosures grew to almost twice the height of the ungulate-access rowans. The protected rowan saplings experienced significantly less browsing damage, and only 25% of the protected plants experienced heavy browsing or died after the first winter, compared to 70% of the unprotected plants (Exp. B). After five years, 50% more unprotected than protected plants experienced heavy browsing or mortality, and growth in protected plants was nearly 50% greater over the period. The negative impact of ungulate browsing on abundance and growth of rowans can be counteracted by positive plant-plant interactions. Rowan saplings growing within nurse plants showed significantly less browsing and better performance indicating high functional importance of the forest understorey. Forest managed with multi-layered and multi-species understoreys may therefore also be beneficial for tree recruitment in browsed boreal forest landscapes.

Root positioning and trait shifts in Hibbertia racemosa as dependent on its neighbour's nutrient-acquisition strategy.

Nutrient-poor ecosystems globally exhibit high plant diversity. One mechanism enabling the co-existence of species in such ecosystems is facilitation among plants with contrasting nutrient-acquisition strategies. The ecophysiological processes underlying these interactions remain poorly understood. We hypothesized that root positioning plays a role between sympatric species in nutrient-poor vegetation. We investigated how the growth traits of the focal mycorrhizal non-cluster-rooted Hibbertia racemosa change when grown in proximity of non-mycorrhizal Banksia attenuata, which produces cluster roots that increase nutrient availability, compared with growth with conspecifics. Focal plants were placed in the centre of rhizoboxes, and biomass allocation, root system architecture, specific root length (SRL), and leaf nutrient concentration were assessed. When grown with B. attenuata, focal plants decreased root investment, increased root growth towards B. attenuata, and positioned their roots near B. attenuata cluster roots. SRL was greater, and the degree of localized root investment correlated positively with B. attenuata cluster-root biomass. Total nutrient contents in the focal individuals were greater when grown with B. attenuata. Focal plants directed their root growth towards the putatively facilitating neighbour's cluster roots, modifying root traits and investment. Preferential root positioning and root morphological traits play important roles in positive plant-plant interactions.

When facilitation meets clonal integration in forest canopies.

Few studies have explored how - within the same system - clonality and positive plant-plant interactions might interact to regulate plant community composition. Canopy-dwelling epiphytes in species-rich forests provide an ideal system for studying this because many epiphytic vascular plants undertake clonal growth and because vascular epiphytes colonize canopy habitats after the formation of nonvascular epiphyte (i.e. bryophyte and lichen) mats. We investigated how clonal integration of seven dominant vascular epiphytes influenced inter-specific interactions between vascular epiphytes and nonvascular epiphytes in a subtropical montane moist forest in southwest China. Both clonal integration and environmental buffering from nonvascular epiphytes increased survival and growth of vascular epiphytes. The benefits of clonal integration for vascular epiphytes were higher when nonvascular epiphytes were removed. Similarly, facilitation from nonvascular epiphytes played a more important role when clonal integration of vascular epiphytes was eliminated. Overall, clonal integration had greater benefits than inter-specific facilitation. This study provides novel evidence for interactive effects of clonality and facilitation between vascular and nonvascular species, and has implications for our understanding of a wide range of ecosystems where both high levels of clonality and facilitation are expected to occur.

Phosphorus availability and microbial community in the rhizosphere of intercropped cereal and legume along a P-fertilizer gradient

Positive below-ground interactions (facilitation) should be more pronounced when resources limit crop growth, according to the stress-gradient hypothesis. Our aim was to test this hypothesis for intercropped durum wheat and faba bean along a P-fertilizer gradient. A field experiment was conducted in a long-term P-fertilizer trial with three rates of P-fertilization (No, Low and High P). Microbial biomass was assessed by chloroform fumigation-extraction. Quantitative PCR was applied to evaluate the abundance of relevant microbial groups. Phosphorus availability and microbial biomass systematically increased in the rhizosphere compared to bulk soil. P-fertilization resulted in higher abundance of targeted bacterial phyla, whole bacterial and fungal communities, and depressed mycorrhizal colonization of durum wheat, but not faba bean. Microbial biomass carbon significantly increased in the rhizosphere only in P-fertilized treatments, pointing to P limitation of microbial communities. Intercropping yielded a significant effect on rhizosphere microbial properties only at High P. Microbial biomass P increased in the rhizosphere of intercropped faba bean only at No P level, and was thus the sole finding supporting the stress-gradient hypothesis. P-fertilization was the main driver of microbial communities in this field trial, and P-fertilizer application modulated the species-specific effect in the intercrop. Plant performance did not validate the stress-gradient hypothesis as positive plant-plant interactions occurred regardless of the level of P-fertilization.

Can we infer plant facilitation from remote sensing? a test across global drylands.

Facilitation is a major force shaping the structure and diversity of plant communities in terrestrial ecosystems. Detecting positive plant-plant interactions relies on the combination of field experimentation and the demonstration of spatial association between neighboring plants. This has often restricted the study of facilitation to particular sites, limiting the development of systematic assessments of facilitation over regional and global scales. Here we explore whether the frequency of plant spatial associations detected from high-resolution remotely sensed images can be used to infer plant facilitation at the community level in drylands around the globe. We correlated the information from remotely sensed images freely available through Google Earth with detailed field assessments, and used a simple individual-based model to generate patch-size distributions using different assumptions about the type and strength of plant-plant interactions. Most of the patterns found from the remotely sensed images were more right skewed than the patterns from the null model simulating a random distribution. This suggests that the plants in the studied drylands show stronger spatial clustering than expected by chance. We found that positive plant co-occurrence, as measured in the field, was significantly related to the skewness of vegetation patch-size distribution measured using Google Earth images. Our findings suggest that the relative frequency of facilitation may be inferred from spatial pattern signals measured from remotely sensed images, since facilitation often determines positive co-occurrence among neighboring plants. They pave the road for a systematic global assessment of the role of facilitation in terrestrial ecosystems.

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 modeling 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.

Plant-plant interactions, environmental gradients and plant diversity: a global synthesis of community-level studies.

Previous syntheses on the effects of environmental conditions on the outcome of plant-plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities. Here we: i) test how important are facilitative interactions as a driver of community structure, ii) evaluate whether the frequency of positive plant-plant interactions across differing environmental conditions and habitats is predictable, and iii) assess whether thresholds in the response of plant-plant interactions to environmental gradients exists between "moderate" and "extreme" stress levels. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environment relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant-plant interactions for the maintenance of plant diversity. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant-plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.

Environmental conditions and biotic interactions acting together promote phylogenetic randomness in semi-arid plant communities: new methods help to avoid misleading conclusions.

Molecular phylogenies are being increasingly used to better understand the mechanisms structuring natural communities. The prevalent theory is that environmental factors and biotic interactions promote the phylogenetic clustering and overdispersion of plant communities, respectively. However, both environmental filtering and biotic interactions are very likely to interact in most natural communities, jointly affecting community phylogenetic structure. We asked the following question: How do environmental filters and biotic interactions jointly affect the phylogenetic structure of plant communities across environmental gradients? Eleven Stipa tenacissima L. grasslands located along an environmental gradient from central to south-east Spain, which covers the core of the distribution area of this vegetation type in Europe. We jointly evaluated the effects of environmental conditions and plant-plant interactions on the phylogenetic structure -measured with the mean phylogenetic distance index- of the studied communities. As an indicator of environmental conditions, we used a PCA ordination including eight climatic variables. Different metrics were used to measure the following processes: i) competition/facilitation shifts at the entire community level (Species Combination index), and ii) the effect of the microclimatic amelioration provided by the two most important nurses on neighbours composition (similarity indices and comparison of the phylogenetic pattern between canopy patches and bare ground areas). Biotic interactions and, to a less extent, environmental conditions, affected the phylogenetic pattern of the studied communities. While positive plant-plant interactions (both at the community and the scale of individual nurses) increased phylogenetic overdispersion, higher rainfall increased phylogenetic clustering. The opposed effects of environmental conditions and biotic interactions could be the main cause of the overall random phylogenetic structure found in most of these communities. Our results illustrate, for the first time, how an overall random phylogenetic pattern may not be promoted only by the lack of influence of either environmental filtering or biotic interactions, but rather by their joint and opposing effects. They caution about making inferences about the underlying mechanisms shaping plant communities from the sole use of their phylogenetic pattern. We also provide a comprehensive set of easy-to-measure tools to avoid misleading conclusions when interpreting phylogenetic structure data obtained from observational studies.

The advantage of growing on moss: facilitative effects on photosynthetic performance and growth in the cyanobacterial lichen Peltigera rufescens

Facilitative effects and plant-plant interactions are well known for higher plants, but there is a lack of information about their relevance in cryptogams. Additional information about facilitative effects between bryophytes and lichens would be an important contribution to recent research on positive plant-plant interactions, as these can have striking influences not only on the organisation of early successional terrestrial communities but also on succession dynamics by kick-starting ecosystem development through the import of key nutrients. We investigated and quantified these mechanisms between Peltigera rufescens and its associated mosses. Moss-associated thalli had a different morphology that led to several benefits from the association. They had 66% higher net photosynthetic rate and, because the majority of the gas exchange of lichen thalli took place through the lower surface, there was a further increase as the CO(2) concentration was >25% higher beneath moss-associated thalli. Microclimatic measurements showed that mean light levels were substantially lower and temperature extremes slightly ameliorated for moss-associated thalli. As a consequence, desiccation was slower which is, together with an increase in thallus thickness and water storage, the reason for extended periods of optimal net photosynthesis for the moss-associated thalli. All these benefits combined to produce a growth rate of the moss-associated thalli which was significantly higher, twice that of non-associated thalli [0.75 ± 0.4 vs. 0.30 ± 0.1 mm/month (mean ± SD)]. This appears to be the first demonstration of a strong mechanistic basis for facilitative effects between lichens and bryophytes.

Effect of a native tree on seedling establishment of two exotic invasive species in a semiarid ecosystem

Theory predicts that in more stressful environments, positive plant-plant interactions should be more important than negative ones. For instance, in arid and semiarid regions, amelioration of soil drought produced by the shade of established plants could facilitate establishment of other species, in spite of light reduction. However, this theory has not been tested widely in the context of plant invasion. In this paper we evaluated the hypothesis that in a semiarid ecosystem of central Chile, the native tree, Lithrea caustica, should facilitate through positive shading effects, the seedling establishment of two widely planted and invasive forestry species, Pinus radiata and Eucalyptus globulus. We assessed the seedling establishment examining two processes: seedling recruitment (including germination) and subsequent seedling survival. We sowed seeds (to assess recruitment) and planted 8 months old seedlings (to assess seedling survival) of each exotic species under Lithrea patches, open sites and under an artificial shade mimicking Lithrea shading. The study was repeated in a north-facing and a south-facing slope in the study area located in a xeric zone within the distribution range of plantations of these species in central Chile. Our results show that in a north-facing slope Lithrea had positive effects on recruitment of both species, which was produced by shading. These effects were counteracted by negative effects on seedling survival but through a different mechanism, which suggests that Lithrea would have no significant effect on the whole seedling establishment process of Pinus radiata nor Eucalyptus globulus in this habitat. In turn, in a south-facing slope Lithrea had no significant effect on recruitment but had a negative effect on seedling survival, which was not produced by shading. This suggests that in this habitat Lithrea has a negative effect on the seedling establishment of these exotic species. Our results suggest that the effect of the native Lithrea caustica on the seedling establishment of these exotic species is dependent upon the life-cycle phase (recruitment or seedling survival) and habitat even within the same semiarid ecosystem. In contrast to the expected positive effects Lithrea is unlikely to facilitate seedling establishment of these exotic species in this area, and in fact in some habitats this effect could be negative. However, our results also suggest that a common mechanism proposed to resist invasion in forest ecosystems such as shading, probably is not sufficient to inhibit invasion in a semiarid region.

Seasonal patterns of root and shoot interactions in an alpine meadow on the Tibetan Plateau

Numerous studies have showed that the balance between negative and positive plant-plant interactions shifted along environmental gradients. But little is known how the positive or negative plant-plant interactions varied with temporal fluctuating habitat conditions and plant ontogenetic phases. In a 2-year experiment, the four perennial grasses (Kobresia humilis, Stipa aliena, Elymus nutans and Saussurea superba) were grown under four interaction treatments (no root or shoot interaction, only shoot interaction, only root interaction, root and shoot interaction). Intensity of above- and belowground interactions is proposed to vary with the fluctuation of seasonal climatic conditions and soil available nutrients. Here we report measurements of above- and belowground interactions during entire growing season. Correlation between plant interaction intensity and seasonal soil available N as well as habitat climate conditions was also performed. Our experiment found that root interactions had negative effect on plant growth for the four species during growing season. However, both negative and positive shoot interactions occurred among the four species. Despite there being shoot facilitative effect for E. nutans and S. superba, the full interaction was negative, suggested that root interaction take more important role on plant growth than that of shoot interaction. The interaction between root and shoot effect varied as a function of species identity and growth phases. The weak correlation of plant interaction intensity to habitat environmental factors suggested that plant ontogenetic characteristics may be primary factors causing temporal variation in plant interaction.