Competition For Soil Resources Research Articles

Positive and negative plant−plant interactions influence seedling establishment at both high and low elevations

Deciphering how plants interact with each other across environmental gradients is important to understand plant community assembly, as well as potential future plant responses to environmental change. Plant−plant interactions are expected to shift from predominantly negative (i.e. competition) to predominantly positive (i.e. facilitation) along gradients of environmental severity. However, most experiments examine the net effects of interactions by growing plants in either the presence or absence of neighbours, thereby neglecting the interplay of both negative and positive effects acting simultaneously within communities. To partially unravel these effects, we tested how the seedling establishment of 10 mountain grassland plants varied in the presence versus absence of plant communities at two sites along an elevation gradient. We created a third experimental treatment (using plastic plant mats to mimic surrounding vegetation) that retained the main hypothesised benefits of plant neighbours (microsite amelioration), while reducing a key negative effect (competition for soil resources). In contrast to our expectations, we found evidence for net positive effects of vegetation at the low elevation site, and net negative effects at the high elevation site. Interestingly, the negative effects of plant neighbours at high elevation were driven by high establishment rates of low elevation grasses in bare soil plots. At both sites, establishment rates were highest in artificial vegetation (after excluding two low elevation grasses at the high elevation site), indicating that positive effects of above-ground vegetation are partially offset by their negative effects. Our results demonstrate that both competition and facilitation act jointly to affect community structure across environmental gradients, while emphasising that competition can be strong also at higher elevations in temperate mountain regions. Consequently, plant−plant interactions are likely to influence the establishment of new, and persistence of resident, species in mountain plant communities as environments change.

Under-Vine Vegetation Mitigates the Impacts of Excessive Precipitation in Vineyards.

Excessive precipitation events have greatly increased in several grape growing regions due to human-caused climate change. These heavy downpours result in a myriad of problems in the vineyard including soil aggregate breakdown, soil runoff, nutrient leaching, excessive vine vegetative growth, and diseased fruit. The negative impacts of excessive precipitation events on vineyards are exacerbated by the maintenance of bare soil under the vines. Exposure of bare soil results in soil erosion and runoff which pollutes nearby watersheds; raindrops weaken and break apart soil aggregates, leading to increased soil erosivity and contributing to the formation of surface crusts. In addition to excessive precipitation events, some grape growing regions can be characterized by fertile soils. The availability of ample water and nutrients can lead to highly vigorous vines with shoot growth continuing through harvest. Long shoots and large leaves result in shaded fruit, a humid vine microclimate, and excessive cluster rot. In this review, we examined how either natural (i.e., resident) or seeded under-vine vegetation (UVV) can help mitigate many of the problems associated with excessive precipitation. Through providing vegetative coverage to reduce the force of raindrops, increasing soil organic matter and enhancing soil microbial diversity, UVV can reduce the soil degradation and off-site impacts caused by excessive precipitation events. Through competition for soil resources, UVV can reduce excessive vegetative growth of vines and decrease cluster rot incidence and severity, although grapevine response to UVV can be highly variable. We discussed recent advances in understanding below and aboveground vine response and acclimation to UVV and presented current evidence of factors influencing the impact of UVV on vine growth and productivity to assist practitioners in making informed decisions and maximize the ecosystem services provided by UVV.

Temperate Forests Dominated by Arbuscular or Ectomycorrhizal Fungi Are Characterized by Strong Shifts from Saprotrophic to Mycorrhizal Fungi with Increasing Soil Depth.

In temperate and boreal forests, competition for soil resources between free-living saprotrophs and ectomycorrhizal (EcM) fungi has been suggested to restrict saprotrophic fungal dominance to the most superficial organic soil horizons in forests dominated by EcM trees. By contrast, lower niche overlap with arbuscular mycorrhizal (AM) fungi could allow fungal saprotrophs to maintain this dominance into deeper soil horizons in AM-dominated forests. Here we used a natural gradient of adjacent forest patches that were dominated by either AM or EcM trees, or a mixture of both to determine how fungal communities characterized with high-throughput amplicon sequencing change across organic and mineral soil horizons. We found a general shift from saprotrophic to mycorrhizal fungal dominance with increasing soil depth in all forest mycorrhizal types, especially in organic horizons. Vertical changes in soil chemistry, including pH, organic matter, exchangeable cations, and extractable phosphorus, coincided with shifts in fungal community composition. Although fungal communities and soil chemistry differed among adjacent forest mycorrhizal types, variations were stronger within a given soil profile, pointing to the importance of considering horizons when characterizing soil fungal communities. Our results also suggest that in temperate forests, vertical shifts from saprotrophic to mycorrhizal fungi within organic and mineral horizons occur similarly in both ectomycorrhizal and arbuscular mycorrhizal forests.

Manual Cultivation Operations in Poplar Stands: A Characterization of Job Difficulty and Risks of Health Impairment.

Short rotation poplar forests are a viable alternative in producing high quality wood for industrial applications. Their success depends on timely and high-quality implementation of a series of operations. Weed control operations are implemented to favor the trees in their competition for soil resources, and cultivation is an option typically used in many European countries. For the moment, a complete mechanization of such operations is virtually impossible, and they still require an intensive use of manual labor. Since information on work difficulty and risks in manual cultivation operations is limited, this study aimed to characterize this job. Evaluation was made in terms of work efficiency, cardiovascular workload, work intensity and postural risks by implementing a time and motion study combined with heart rate measurements, accelerometry and whole-body postural analysis. Work efficiency was particularly low even if the share of effective work time was high (70% of the observation time). Job was characterized as moderate to high intensity, which resulted into a moderate to high cardiovascular strain. While the postural analysis indicated rather small risks, the main problem was found for the back postures assumed during the work. Improvements should aim to extend mechanization, train the workers and appropriately design rest breaks.

Species‐specific size vulnerabilities in a competitive arena: Nutrient heterogeneity and soil fertility alter plant competitive size asymmetries

Abstract The size dependence of competitive interactions is starting to be highlighted as an important driver of species diversity within communities; however, it is still unknown whether all species are equally impacted by size‐asymmetric competition and what resources drive it. Here, we test species‐specific responses to size‐asymmetric competition under various soil environments by manipulating plant size as well as soil fertility, nutrient heterogeneity and the initial suppression of microbial communities within a three‐species community. Competition was primarily size‐asymmetric; however, the degree to which competition was size‐asymmetric was species‐specific, where three out of five soil treatments resulted in size‐asymmetric competition in some species but size‐symmetric competition in others. Overall, soil fertility and nutrient heterogeneity altered the degree of size‐asymmetric competition, while the initial suppression of microbial communities had no effect. Contrary to past predictions, when they had an effect, increased soil fertility and the presence of a high‐quality patch reduced the degree of size‐asymmetric competition, while the presence of a low‐quality patch increased it. This suggests that the role nutrient heterogeneity plays on the degree of size‐asymmetric competition is dependent on the quality and location of the patch relative to an individual's neighbours. These findings challenge the current understanding that competition for soil resources is always size‐symmetric and demonstrate that species within the same community may not experience the same degree of size‐dependent competition. These differential responses suggest we must consider variations in tolerance and suppression between species during size‐dependent plant–plant interactions, which may promote species coexistence as all small individuals may not be negatively impacted during size‐dependent competition. A plain language summary is available for this article.

Influence of low light intensity on growth and biomass allocation, leaf photosynthesis and canopy radiation interception and use in two forage species of Centrosema (DC.) Benth.

AbstractSustainable silvopastoral systems rely on the capacity of forage species to withstand shade, competition for soil resources and grazing. On a field‐plot experiment, we studied the effects of artificial shade on the performance of two tropical forage legumes: Centrosema molle (CIAT 15160) and C. macrocarpum (CIAT 5713). Acclimation to shade in these two species was investigated at individual leaf and whole canopies, to explain differences in the efficiency of radiation interception and use, and thus forage yield. Under 30% sunlight, midday leaf net photosynthesis (A) was reduced by 32%–44%, although instantaneous light use efficiency increased up to 44%–140%. Almost 50% increase in leaf specific area was accompanied by 61% in total chlorophyll content. From analysis of the light‐response curves, net photosynthesis at light saturation was only reduced (by 36%) in C. macrocarpum. Efficiency of chloroplast photosystem II (PSII) was unaffected by shade. Canopy structure for light interception was not modified by shade, although conversion into biomass and final yield were significantly reduced (40%–43%) in C. macrocarpum. Results are discussed in terms of factors affecting potential acclimation to shade and the agronomic implications of using both forage legumes in mixed pastures under tree canopies.

Is Competition for Soil Resources by Carex pensylvanica Restricting Tree Seedling Growth in a Temperate Northern Hardwood Forest?

In response to ecological disturbances, sedge species like Carex pensylvanica form dense monocultures on the forest floor. These “sedge mats” have been shown to severely inhibit plant growth, and limit understory species diversity. In recent years, concern has grown that they may also be restricting the regeneration of economically valuable tree species like sugar maple through belowground competition. To determine if and how Carex pensylvanica may be impacting tree seedling growth through belowground competition I located and exclosed 44 tree seedlings in areas of representatively dense sedge at two forest sites at the Queen’s University Biological Station. I removed sedge from half the plots, and measured soil resource availability and seedling growth response at all plots throughout the growing season. I predicted that sedge would negatively impact the growth of tree seedlings by decreasing the availability of soil resources. I found that the presence of sedge did not affect seedling growth over one growing season, but that it did impact soil resource availability by increasing the availability of surface soil moisture and decreasing the availability of soil nitrate, changes which may have implications for seedling growth beyond the single growing season studied. My results were also site specific, indicating that location is important when managing sedge impact on tree regeneration. Understanding the impact of sedge on tree seedling regeneration is important for predicting changes in the trajectory of forest communities and for informing the management of economically valuable species like sugar maple.

Trait-based characterisation of soil exploitation strategies of banana, weeds and cover plant species.

Cover plants can be introduced in cropping systems to provide agroecosystem services, including weed control via competition for resources. There is currently no consensus on how to identify the best cover plant species, while trait-based approaches are promising for screening plant species due to their agroecosystem service provision potential. This study was carried out to characterize soil exploitation strategies of cover plant species in banana agroecosystems using a trait-based approach, and in turn identify cover plant species with a high weed control potential via competition for soil resources in banana cropping systems. A field experiment was conducted on 17 cover plant species, two weed species and two banana cultivars grown individually. Four functional traits were measured. Two of them (i.e., the size of the zone explored by roots and the root impact density) were used to characterize root system soil exploration patterns. Two other traits (i.e., specific root length and root diameter) were used to characterize resource acquisition within the soil zone explored by the roots. All studied traits exhibited marked variations among species. The findings suggested a trade-off between the abilities of species to develop a limited number of large diameter roots exploring a large soil zone versus many thin roots exploring a smaller soil zone. Three soil-resource exploitation strategies were identified among species: (i) with large diameter roots that explore a large soil zone; (ii) with small diameter roots and a high specific length that explore a smaller soil zone; and (iii) with a high total root-impact density and an intermediate specific root length that explore the uppermost soil layers. Interestingly, in our panel of species, no correlations with regard to belowground and aboveground strategies were noted: species with an acquisitive belowground strategy could display an acquisitive or a conservative aboveground strategy. The findings of this study illustrated that a trait-based approach could be used to identify plant species with potential for competing with weeds, while minimising competition with banana. Six of the 17 studied cover crop species were identified as having this potential. The next step will be to assess them for their weed control performances in banana cropping systems with low reliance on herbicides.

Partitioning of Competition for Resources Between Soybean and Corn as Competitor Plant

ABSTRACT Plants compete for resources present below and above the soil surface. The objective of this work was to separate the individual effects of the competition for resources between soybean and corn as competitor plants. The experiment was conducted in a greenhouse, in 2014/2015. The treatments consisted of soybean cultivars (TEC 5718 and TEC 6029) in competition conditions with corn (no competition, competition for soil resources, competition for solar radiation and total competition). The variables evaluated were plant height at 7, 14, 21, 28, 35 and 42 days after emergence (DAE), stem/culm diameter, leaf area, shoot dry mass, root dry mass and the chlorophyll index, at 42 (DAE). The competition for soil resources between soybean and corn is pronounced, being that short cultivars with determinate growth habit, such as TEC 5718, invest more in root biomass, specific leaf area and leaf area ratio when in competition. The soybean cultivars do not suppress corn, but allow it to benefit when associated to its root system, increasing the shoot and root dry mass, leaf area and chlorophyll index.

New procedure for the simulation of belowground competition can improve the performance of forest simulation models

The major part of existing models of belowground competition in mixed forest stands is limited in explaining the spatial distribution of roots as a response to competitive pressure from neighbours and heterogeneity of soil properties. We are presenting a new spatially explicit and multi-layered discrete model of belowground competition, RootInt (ROOTs INTake). It describes spatial distribution of belowground biomass and allows simulation of competition between trees for soil nutrients. The tree-specific area of root zone is calculated on the basis of stem diameter, with site-specific modifiers to account for the effect of soil fertility and moisture. The shape of root zone is dependent on the amount of available nitrogen in the current cell, distance between this cell and the stem base, and the mass of roots of other plants. RootInt was incorporated into ecosystem model EFIMOD to refine the existing description of belowground competition in forest stands with multiple cohorts and tree species. The results of simulation showed that bringing more complexity into structure of stand (including initial spatial locations of trees, species composition and age structure, vertical structure of canopy) resulted in higher spatial variation in competition intensity, as well as in higher rates of resource uptake. This indicates that stands with complex canopy structure had high plasticity in their root systems and were adapted to intensive competition for soil resources.

Dominance by an obligate annual affects the morphological characteristics and biomass production of a planted wetland macrophyte community

Aims biodiversity–ecosystem function experiments can test for causal relationships between planting diversity and community productivity. Planting diversity is routinely introduced as a design element in created wetlands, yet substantive support for the finding that early diversity positively affects ecosystem functioning is lacking for wetlands. We conducted a 2-year diversity–productivity experiment using freshwater wetland mesocosms to investigate community biomass production as affected by planted macrophyte functional richness. Methods a richness gradient of macrophytes in four emergent wetland plant functional groups was established in freshwater mesocosms for two consecutive years. species-specific aboveground morphological traits of plant size were measured at peak growth in both years; rooting depth was measured for each species in the second year. aboveground biomass (agb) and belowground biomass (bgb) were harvested after peak growth in the second year; first year agb was estimated from morphological traits in constructed regression equations. Net richness effects (i.e. both complementarity effects and selection effects) were calculated using an additive partitioning method. Important Findings species richness had a positive effect on community agb relative to monocultures in the first year. in the second year, mean agb was significantly reduced by competition in the most species-rich mixtures and all mixtures underyielded relative to the average monoculture. Competition for soil resources was weaker belowground, whereby root distribution at depths >20 cm was reduced at the highest richness levels but overall bgb production was not affected. Changes in species biomass were strongly reflected by variation in species morphological traits, and species above and belowground performances were highly correlated. The obligate annual (Eleocharis obtusa), a dominant competitor, significantly contributed to the depression of perennial species’ growth in the second growing season. To foster primary productivity with macrophyte richness in early successional communities of created wetlands where ruderal strategies are favored and competition may be stronger than species complementarity, unsystematic planting designs such as clustering the same or similar species could provide protection for some individuals. additionally, engineering design elements fostering spatial or temporal environmental variability (e.g. microtopography) in newly created wetlands helps diversify the responses of wetland macrophyte species to their environment and could allow for greater complementarity in biomass production.

Temporal and spatial distribution of roots as affected by interspecific interactions in a young walnut/wheat alley cropping system in northwest China

Intercropping Persian walnut (Juglans regia L.) and wheat (Triticum aestivum Linn.) in northwest China is receiving increasing attention because it offers potential advantages for improving land-use efficiency, increasing economic returns to farmers and increased sustainability in crop production. We carried out a two-year field intercropping experiment, using a system composed of young Persian walnut trees with wheat to determine the root competitive interactions between the two species, aboveground biomass and the economic yield of each component of the intercropping system at a spacing of 4 × 6 m in Hetian old oasis, south Xinjiang province, northwest China. Roots were sampled by auger in the Persian walnut tree/wheat intercropping system and in the sole wheat and sole walnut systems, down to a depth of 1 m in the soil profile. The results show that the roots of the trees spread under the crop and that the roots of intercropped wheat tended to have a shallower distribution in the soil profile. The roots of both intercropped walnut and wheat had a lower root length density and root diameter and a higher specific root length at all soil depths than those of sole walnut trees and wheat in 2012. Although root competition belowground led to decreases in yield and biomass of both crops, intercropping of Persian walnut and wheat improved the utilization of plant growth resources (LER > 1) as compared with sole crops. Tree height, diameter at breast height and mean radius of crown of intercropped trees in both intercropping years had lower values when compared with the monoculture. We conclude that interspecific interactions occurred largely as below-ground competition between the species investigated. Competition for soil resources was the primary limiting factor for wheat productivity. Further research is needed to examine the dynamic processes of species interactions in intercropping systems with trees of increasing age.

Non‐specific phospholipase C5 and diacylglycerol promote lateral root development under mild salt stress in Arabidopsis

Developing a robust root system is crucial to plant survival and competition for soil resources. Here we report that the non-specific phospholipase C5 (NPC5) and its derived lipid mediator diacylglycerol (DAG) mediate lateral root (LR) development during salt stress in Arabidopsis thaliana. T-DNA knockout mutant npc5-1 produced few to no LR under mild NaCl stress, whereas overexpression of NPC5 increased LR number. Roots of npc5-1 contained a lower level of DAG than wild type, whereas NPC5 overexpressor exhibited an increase in DAG level. Application of DAG, but not phosphatidic acid, fully restored LR growth of npc5-1 to that of wild type under NaCl stress. NPC5 expression was significantly induced in Arabidopsis seedlings treated with NaCl. Npc5-1 was less responsive to auxin-mediated root growth than the wild type. These results indicate that NPC5 mediates LR development in response to salt stress and suggest that DAG functions as a lipid mediator in the stress signalling.

Architectural characteristics of roots in typical coastal psammophytes of South China

Aims Roots are an important part of plants; they not only facilitate the utilization of soil water and nutrients, but also successively anchor the plants against wind and water wave erosion. This study was conducted to determine the adaptive strategy of root architectural characteristics in different plant life forms, in order to assist the selec- tion of most suitable plant species for the coastal dune restoration. Methods After a detailed field investigation of sand dunes in South China, four species were sampled on a damaged coastal sand dune, including Casuarina equisetifolia, Ipomoea pes-caprae, Spinifex littoreus and Cyno- don dactylon, and roots were excavated for analyzing their architectural characteristics such as root bifurcation ratio, root link length, root topological index and root-shoot ratio. Important findings There were great differences as well as similarities in the root architectural characteristics among the four plant species. The total root bifurcation ratios in Spinifex littoreus and Cynodon dactylon were apparently higher than in other two species, and was lowest in Ipomoea pes-caprae, indicating that herbaceous plants are more competitive for soil resources by having a higher root bifurcation ratio. With except of Casuarina equisetifolia, all other three species were easily sand buried due to their flatten and extended shoots, which might further improve the transportation efficiency of soil resources. The average root link lengths of the four species were smaller in coastal sandy dunes than which in inland areas; Casuarina equisetifolia had the greatest average root link length of 19.25 cm among the four species studied, and greater proportion of transportation roots than other three species. This demonstrated that an increase in average root link length and proportion of transportation roots would favor the reduction of root competition for soil resources and high efficiency of resources transporta- tion. Ipomoea pes-caprae had a smaller proportion of the assimilation roots, but the adventitious roots on the shoot compensated for the low assimilation efficiency of resources. All the four species showed a dichotomous structure, of which the herbaceous species had more similar root architectures, indicating that herbaceous species are less affected by limitation of environmental resources and better to colonize the coastal dunes if being used in restoration. Compared with species in inland areas, there are greater differences for the species in coastal sandy areas in the acquisition of soil resources and spatial expansion, indicating different adaptive strategies among the

Indirect competition for pollinators is weak compared to direct resource competition: pollination and performance in the face of an invader

Invasive plants have the potential to reduce native plant abundance through both direct and indirect interactions. Direct interactions, such as competition for soil resources, and indirect interactions, such as competition for shared pollinators, have been shown to influence native plant performance; however, we know much less about how these interactions influence native plant abundance in the field. While direct competitive interactions are often assumed to drive declines in native abundance, an evaluation of their influence relative to indirect mechanisms is needed to more fully understand invasive plant impacts. We quantified the direct effects of resource competition by the invasive perennial forb, Euphorbia esula (Euphorbiaceae), on the recruitment, subsequent performance, and ultimate adult abundance of the native annual, Clarkia pulchella (Onagraceae). We contrast these direct effects with those that indirectly resulted from competition for shared pollinators. Although E. esula dramatically reduced pollinator visitation to C. pulchella, plants were only weakly pollen-limited. Pollen supplementation increased the number of seeds per fruit from 41.28 to 46.38. Seed addition experiments revealed that the impacts of ameliorating pollen limitation only increased potential recruitment by 12.3 %. In contrast, seed addition experiments that ameliorated direct competition with E. esula resulted in an increase in potential future recruitment of 574 %. Our results show that, while the indirect effects of competition for pollinators can influence plant abundance, its effects are dwarfed by the magnitude of direct effects of competition for resources.

Small‐scale grassland assembly patterns differ above and below the soil surface

The existence of deterministic assembly rules for plant communities remains an important and unresolved topic in ecology. Most studies examining community assembly have sampled aboveground species diversity and composition. However, plants also coexist belowground, and many coexistence theories invoke belowground competition as an explanation for aboveground patterns. We used next-generation sequencing that enables the identification of roots and rhizomes from mixed-species samples to measure coexisting species at small scales in temperate grasslands. We used comparable data from above (conventional methods) and below (molecular techniques) the soil surface (0.1 x 0.1 x 0.1 m volume). To detect evidence for nonrandom patterns in the direction of biotic or abiotic assembly processes, we used three assembly rules tests (richness variance, guild proportionality, and species co-occurrence indices) as well as pairwise association tests. We found support for biotic assembly rules aboveground, with lower variance in species richness than expected and more negative species associations. Belowground plant communities were structured more by abiotic processes, with greater variability in richness and guild proportionality than expected. Belowground assembly is largely driven by abiotic processes, with little evidence for competition-driven assembly, and this has implications for plant coexistence theories that are based on competition for soil resources.

Carbon Addition as a Technique for Controlling Exotic Species in Pacific Northwest Prairies

Competition for soil resources is a driving force in plant community ecology. Therefore, amendments that alter soil resources should affect vegetation dynamics and are potential tools for controlling exotic species. We tested the effects of two types of carbon addition, sugar and activated carbon (AC), on an exotic-dominated grassland plant community. Sugar stimulates the microbial community and temporarily reduces plant available nitrogen (N), while AC adsorbs plant available N. Six experimental plots and three carbon treatments were used: sugar (1000 g m-2; 42% C), AC (420 g m-2; 100% C), and control. Treatments were applied in a split-plot design in spring 2008. Aboveground biomass, plant cover, species richness, and plant basal area were tracked for two growing seasons. Although total biomass was not affected by C addition, the distribution of biomass among life forms was affected: carbon addition reduced forb biomass but had no effect on grasses or legumes. Total cover was lower in sugar-treated plots, and sugar also altered the proportional composition of that cover by reducing the abundance of forbs, especially in the first year. Five species were particularly sensitive to sugar addition: Myosotis discolor, Sonchus asper, Taraxacum officinale, Valerianella locusta, and Vulpia bromoides. Sugar also reduced plant basal area in the first year. These results suggest that sugar and, to a lesser extent, AC could be useful management tools. Sugar could be applied to areas where herbicide use is undesirable, reducing the abundance of exotic forbs and providing a window of opportunity for native species establishment.

Litter inputs and plant interactions affect nectar sugar content

Summary 1. Declines in availability of plant resources to pollinators are a major cause of pollinator loss. The management of plant communities to enhance floral resources is often proposed as a way to sustain pollinator populations. Nectar, the main energetic resource for pollinators, plays a central role in behaviour and composition of pollinator communities. Abiotic and biotic factors are known to influence nectar traits at both the species and community levels, but the impact of plant community composition itself has never been investigated. 2. Below‐ground interactions in plant communities can induce changes in plant development through (i) plant‐derived litter amendment of the soil and (ii) competition for soil resources between plants. We tested how plant below‐ground interactions affect above‐ground nectar traits involved in plant attractiveness to pollinators. 3. A short‐term pot experiment was carried out with three temperate grassland species Mimulus guttatus, Lamium amplexicaule, and Medicago sativa, showing distinct litter stoichiometry and competitive abilities for soil resources. Litter amendment (none, mono and tri‐specific litter) and plant interaction treatments (monocultures, two‐ and three‐species mixtures) were crossed in a factorial design. 4. Litter amendment to the soil led to an increase in total nectar sugar content in L. amplexicaule plants but not in the two other species. We also found that the presence of M. guttatus, a competitive species, reduced the total nectar sugar content in L. amplexicaule through a concomitant decrease in nectar volume per flower and in floral display size, but not in other species. Species‐specific responses of nectar traits to variation in soil nitrogen availability were thus observed, suggesting consequences for plant species and community attractiveness to pollinators. However, we did not find evidence that the legume M. sativa affected nectar traits of any neighbouring plants. 5. Synthesis. Our results demonstrate that litter inputs and competition between plants for soil resources can alter nectar traits linked to plant attractiveness to pollinators. This supports the idea that below‐ground plant–plant interactions for soil resources can influence above‐ground plant–plant interactions for pollination services. This offers promising perspectives in studying the role of below‐ground–above‐ground interactions on higher trophic levels.

The role of photosynthesis in the recruitment of juvenile Quercus gambelii into mature Q. gambelii communities

Abstract Previous work with Quercus gambelii (Gambel's oak), a widespread oak species from the mountains of the western United States and northern Mexico, showed high juvenile density (< 3 cm in DBH [diameter at breast height] or less than 150 cm in height), but few or no small (3–5 cm in DBH) tree sized plants, suggesting a recruitment bottleneck. We postulate that there could be several factors preventing the recruitment of juveniles into the adult population, including reduced light levels, herbivory, competition for soil resources, or a combination of these factors. In this study, we evaluated the response of leaves of Q. gambelii to various light levels. Surface light levels and leaf gas exchange rates were measured for sun (open) and shade (understory) Q. gambelii seedlings in Q. gambelii communities in the Lincoln National Forest, New Mexico, USA. Mean daily photosynthetic flux densities (PFD) in the understory during the monsoon period (wet season) were 40 ± 46 µmol m−2 sec−1, or 6% of the open co...

Silvopastoralism in New Zealand: review of effects of evergreen and deciduous trees on pasture dynamics

Complex interactions between livestock, trees and pasture occur in silvopastoral systems. Between trees and pasture, competition for soil resources (nutrients and water) occurs, becoming especially relevant when one of them is in scarce supply. Trees reduce light and water reaching the understorey layers according to tree density and canopy size. However, they may ameliorate extreme climatological features (reducing wind speed and evapotranspiration, and alleviating extreme temperatures), and improve soil properties, for example, deciduous tree litter may contribute to increased pH and soil nutrient concentrations. During tree establishment, there are generally negligible effects on pasture, irrespective of tree type. However, there is a decline in pasture production and nutritive value under shade with increasing tree age and higher stand density. Under the same conditions, deciduous trees affect pasture later (extinction point of pasture occurs at 85% of canopy closure) than evergreen trees (about 67% for Pinus radiata D. Don). This is mainly because deciduous trees have a leafless period that enables pasture recovery, and their litter smothers pasture less intensely because of its relatively fast decomposition. Silvopastoral studies conducted in New Zealand are reviewed to discuss these effects, and differences in the effects of evergreen and deciduous trees are shown using the examples of P. radiata, and Populus and Salix spp. respectively, which exist in many temperate countries. Future research needs are outlined.