Form and function of grass ring patterns in arid grasslands: the role of abiotic controls

Ring-shaped growth patterns commonly occur in resource-limited arid and semi-arid environments. The spatial distribution, geometry, and scale of vegetation growth patterns result from interactions between biotic and abiotic processes, and, in turn, affect the spatial patterns of soil moisture, sediment transport, and nutrient dynamics in aridland ecosystems. Even though grass ring patterns are observed worldwide, a comprehensive understanding of the biotic and abiotic processes that lead to the formation, growth and breakup of these rings is still lacking. Our studies on patterns of infiltration and soil properties of blue grama (Bouteloua gracilis) grass rings in the northern Chihuahuan desert indicate that ring patterns result from the interaction between clonal growth mechanisms and abiotic factors such as hydrological and aeolian processes. These processes result in a negative feedback between sediment deposition and vegetation growth inside the bunch grass, which leads to grass die back at the center of the grass clump. We summarize these interactions in a simple theoretical and conceptual model that integrates key biotic and abiotic processes in ring formation, growth and decline.     

Effects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review

Productivity of seagrasses can be controlled by physiological processes, as well as various biotic and abiotic factors that influence plant metabolism. Light, temperature, and inorganic nutrients affect biochemical processes of organisms, and are considered as major factors controlling seagrass growth. Minimum light requirements for seagrass growth vary among species due to unique physiological and morphological adaptations of each species, and within species due to photo-acclimation to local light regimes. Seagrasses can enhance light harvesting efficiencies through photo-acclimation during low light conditions, and thus plants growing near their depth limit may have higher photosynthetic efficiencies. Annual temperatures, which are highly predictable in aquatic systems, play an important role in controlling site specific seasonal seagrass growth. Furthermore, both thermal adaptation and thermal tolerance contribute greatly to seagrass global distributions. The optimal growth temperature for temperate species range between 11.5 °C and 26 °C, whereas the optimal growth temperature for tropical/subtropical species is between 23 °C and 32 °C. However, productivity in persistent seagrasses is likely controlled by nutrient availability, including both water column and sediment nutrients. It has been demonstrated that seagrasses can assimilate nutrients through both leaf and root tissues, often with equal uptake contributions from water column and sediment nutrients. Seagrasses use HCO₃⁻ inefficiently as a carbon source, thus photosynthesis is not always saturated with respect to DIC at natural seawater concentrations leading to carbon limitation for seagrass growth. Our understanding of growth dynamics in seagrasses, as it relates to main environmental factors such as light, temperature, and nutrient availability, is critical for effective conservation and management of seagrass habitats.     

The role of reproductive plant traits and biotic interactions in the dynamics of semi-arid plant communities

The dynamics of semi-arid plant communities are determined by the interplay between competition and facilitation among plants. The sign and strength of these biotic interactions depend on plant traits. However, the relationships between plant traits and biotic interactions, and the consequences for plant communities are still poorly understood. Our objective here was to investigate, with a modelling approach, the role of plant reproductive traits on biotic interactions, and the consequences for processes such as plant succession and invasion. The dynamics of two plant types were modelled with a spatially-explicit integrodifferential model: (1) a plant with seed dispersal (colonizer of bare soil) and (2) a plant with local vegetative propagation (local competitor). Both plant types were involved in facilitation due to a local positive feedback between vegetation biomass and soil water availability, which promoted establishment and growth. Plants in the system also competed for limited water. The efficiency in water acquisition (dependent on reproductive and growth plant traits) determined which plant type dominated the community at the steady state. Facilitative interactions between plant types also played an important role in the community dynamics, promoting establishment in the driest conditions and recovery from low biomass. Plants with vegetative propagation took advantage of the ability of seed dispersers to establish on bare soil from a low initial biomass. Seed dispersers were good invaders, maintained high biomass at intermediate and high rainfall and showed a high ability in taking profit from the positive feedback originated by plants with vegetative propagation under the driest conditions. However, seed dispersers lost competitiveness with an increasing investment in fecundity. All together, our results showed that reproductive plant traits can affect the balance between facilitative and competitive interactions. Understanding this effect of plant traits on biotic interactions provides insights in processes such as plant succession and shrub encroachment.     

Microbial-mediated feedbacks of leaf litter on invasive plant growth and interspecific competition

Background and Aims

In line with the Stress Gradient Hypothesis, studies of facilitation have tended to focus on plant–plant interactions (biotic nurses), while the relative role of abiotic nurses has been little studied. We assessed the role of biotic and abiotic nurses, and their interaction, on soil enhancement and the consequential performance of a native annual grass, Dactyloctenium radulans.

Methods

We used a growth chamber study with two levels of water application to compare the performance of D. radulans growing in soil from foraging pits of the Short-beaked echidna (Tachyglossus aculeatus; abiotic nurse) and non-pit soil from either under tree canopies (biotic nurse) or surrounding open areas.

Results

All measures of plant performance were more pronounced under the high than the low water treatment. The greatest differences between pit and surface Microsites occurred under the low water application, reinforcing our view that facilitatory effects are greater in resource-limited environments. Despite tree canopy soil having greater N, there was no significant effect on plant performance, nor any significant interaction with Microsite.

Conclusions

Our study provides strong evidence that foraging pits enhance soil properties and this soil, in turn, facilitates plant growth; and supports previous work documenting the positive effect of nurse-protégé interactions under greater levels of abiotic stress.     

Assemblage of a semi-arid annual plant community: abiotic and biotic filters act hierarchically

The study of species coexistence and community assembly has been a hot topic in ecology for decades. Disentangling the hierarchical role of abiotic and biotic filters is crucial to understand community assembly processes. The most critical environmental factor in semi-arid environments is known to be water availability, and perennials are usually described as nurses that create milder local conditions and expand the niche range of several species. We aimed to broaden this view by jointly evaluating how biological soil crusts (BSCs), water availability, perennial species (presence/absence of Stipa tenacissima) and plant-plant interactions shape a semi-arid annual plant community. The presence and cover of annual species was monitored during three years of contrasting climate. Water stress acted as the primary filter determining the species pool available for plant community assembly. Stipa and BSCs acted as secondary filters by modulating the effects of water availability. At extremely harsh environmental conditions, Stipa exerted a negative effect on the annual plant community, while at more benign conditions it increased annual community richness. Biological soil crusts exerted a contradictory effect depending on climate and on the presence of Stipa, favoring annuals in the most adverse conditions but showing repulsion at higher water availability conditions. Finally, interactions among co-occurring annuals shaped species richness and diversity of the final annual plant assembly. This study sheds light on the processes determining the assembly of annual communities and highlights the importance of Biological Soil Crusts and of interactions among annual plants on the final outcome of the species assembly.     

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.     

Influence of nutrients,disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.     

Microhabitat selection in the common lizard: implications of biotic interactions,age, sex,local processes,and model transferability among populations

Modeling species' habitat requirements are crucial to assess impacts of global change, for conservation efforts and to test mechanisms driving species presence. While the influence of abiotic factors has been widely examined, the importance of biotic factors and biotic interactions, and the potential implications of local processes are not well understood. Testing their importance requires additional knowledge and analyses at local habitat scale. Here, we recorded the locations of species presence at the microhabitat scale and measured abiotic and biotic parameters in three different common lizard (Zootoca vivipara) populations using a standardized sampling protocol. Thereafter, space use models and cross‐evaluations among populations were run to infer local processes and estimate the importance of biotic parameters, biotic interactions, sex, and age. Biotic parameters explained more variation than abiotic parameters, and intraspecific interactions significantly predicted the spatial distribution. Significant differences among populations in the relationship between abiotic parameters and lizard distribution, and the greater model transferability within populations than between populations are in line with effects predicted by local adaptation and/or phenotypic plasticity. These results underline the importance of including biotic parameters and biotic interactions in space use models at the population level. There were significant differences in space use between sexes, and between adults and yearlings, the latter showing no association with the measured parameters. Consequently, predictive habitat models at the population level taking into account different sexes and age classes are required to understand a specie's ecological requirements and to allow for precise conservation strategies. Our study therefore stresses that future predictive habitat models at the population level and their transferability should take these parameters into account.     

Abiotic effects predominate under prolonged livestock‐induced disturbance

Despite the widespread recognition that disturbance by livestock affects multiple indices of landscape health, few studies have examined their effects on both biotic and abiotic processes. We examined the effects of livestock disturbance on soil, vascular plants and reptiles across a disturbance gradient in a semi‐arid Australian woodland. Our gradient ranged from long‐ungrazed water remote sites, through intermediately grazed recovering sites, to currently grazed sites close to water. Our aim was to examine the nature of the effects of grazing‐induced disturbance on biotic and abiotic processes along the gradient. We detected small biotic effects, but no abiotic effects, at low levels of disturbance (intermediate sites). We could not detect a consistent biotic effect on plants or reptiles along the gradient, except between the extreme disturbances. In contrast, we recorded substantial reductions in abiotic structure and function at the most disturbed sites. Structural changes included reductions in the cover of shrub hummocks and increases in bare soil, and reductions in cryptogamic soil crusts. Structural changes were associated with declines in function (soil stability and nutrient indices). Our data are consistent with the notion that abiotic effects predominate at high levels of disturbance in rangelands. Given the extent of abiotic modification at currently grazed sites, the cover of abiotic elements such as hummocks and soil surfaces would seem a better indicator of the long‐term effect of grazing‐induced disturbance than biotic components. The extent of disturbance at currently grazed sites across large areas of rangeland suggests that autogenic recovery will be protracted.     

Regulation of Microbial Community Composition and Activity by Soil Nutrient Availability, Soil pH, and Herbivory in the Tundra

Soil nitrogen (N) availability and pH constitute major abiotic controls over microbial community composition and activity in tundra ecosystems. On the other hand, mammalian grazers form an important biotic factor influencing resource coupling between plants and soil microorganisms. To investigate individual effects and interactions among soil nutrients, pH, and grazing on tundra soils, we performed factorial treatments of fertilization, liming, and grazer exclusion in the field for 3 years at 2 contrasting tundra habitats, acidic (N-poor) and non-acidic (N-rich) tundra heaths. The effects of all treatments were small in the non-acidic tundra heaths. In the acidic tundra heaths, fertilization decreased the fungal:bacterial ratio as analyzed by soil PLFAs, but there were no effects of liming. Fertilization increased soil N concentrations more drastically in ungrazed than grazed plots, and in parallel, fertilization decreased the fungal:bacterial ratio to a greater extent in the ungrazed plots. Liming, on the other hand, partly negated the effects of fertilization on both soil N concentrations and PLFAs. Fertilization drastically increased the activity of phenol oxidase, a microbial enzyme synthesized for degradation of soil phenols, in grazed plots, but had no effect in ungrazed plots. Taken together, our results demonstrate that grazers have the potential to regulate the fungal:bacterial ratio in soils through influencing N availability for the soil microorganisms.     

Bacterial viability assessment by flow cytometry analysis in soil

Soil microhabitats and their heterogeneity are often considered to be among the most important factors affecting soil biotic communities. The microbial commu-nity has become one of the most important links in soil nutrient cycles and trophic components due to its role in biological processes, spatial and temporal dynamics, and physiological adaptation. Sandy-soil desert systems are characterized by fast water infiltration during the rainy season, high salinity, and low moisture availability in the upper soil layers. Plants have developed different ecophy-siological adaptations in order to cope with this harsh environment. The Tamarix aphylla is known to be one of the most commonly adapted plants, exhibiting a mechan-ism for secretion of excess salts as aggregates through its leaves. These leaves aggregate beneath the plant, creating 'islands of salinity'. Soil biotic components are, therefore, exposed to extreme abiotic stress conditions in this niche. The goal of this study was to examine the effect of T. aphylla on the live/dead bacterial population ratio on a spatial and temporal scale. The results emphasize the effect of abiotic factors, which changed on temporal as well as spatial scales, and also on the size of the active soil bacterial community, which fluctuated between 1.44% and 25.4% in summer and winter, respectively. The results of this study elucidate the importance of moisture availability and the 'island-of-salinity' effect on the active microbial community in a sandy desert system.     

The importance of disturbance by fire and other abiotic and biotic factors in driving cheatgrass invasion varies based on invasion stage

Disturbances create fluctuations in resource availability that alter abiotic and biotic constraints. Exotic invader response may be due to multiple factors related to disturbance regimes and complex interactions between other small- and large-scale abiotic and biotic processes that may vary across invasion stages. We explore how cheatgrass responds to both frequency and season of prescribed burning for a 10-year period in ponderosa pine forested stands. To understand interactions of fire disturbance, other abiotic factors, biotic resistance, and propagule pressure, we use long-term data from different spatial scales representing different invasion stages (local establishment or spread and broader scale extent/impact) to model cheatgrass dynamics. We found that after 10 years, cheatgrass cover increased with fall burning regardless of burn frequency (1 burn vs. 3 burns). There was no evidence that cheatgrass invasion is decreasing through time even in areas burned only once. Factors important for explaining local fine-scale cheatgrass establishment and spread, and broader scale extent/impact varied. The spatial extent of the first burns facilitated fine-scale cheatgrass establishment while bare soil cover constrained establishment. Biotic resistance, in the form of native annual forb cover, constrained fine-scale cheatgrass spread. Initial cheatgrass abundance in 2002, a factor related to propagule pressure, was key for explaining the broader scale extent/impact of cheatgrass by 2012. Biotic resistance, in the form of native perennial grass cover, constrained extent/impact but only when initial cheatgrass abundance was low. Our findings regarding factors affecting invasion dynamics may be useful to consider for future restoration and conservation efforts in burned ponderosa pine forests.     

Factors shaping community assemblages and species co‐occurrence of different trophic levels

Species assemblages are the results of various processes, including dispersion and habitat filtering. Disentangling the effects of these different processes is challenging for statistical analysis, especially when biotic interactions should be considered. In this study, we used plants (producers) and leafhoppers (phytophagous) as model organisms, and we investigated the relative importance of abiotic versus biotic factors that shape community assemblages, and we infer on their biotic interactions by applying three‐step statistical analysis. We applied a novel statistical analysis, that is, multiblock Redundancy Analysis (mbRA, step 1) and showed that 51.8% and 54.1% of the overall variation in plant and leafhopper assemblages are, respectively, explained by the two multiblock models. The most important blocks of variables to explain the variations in plant and leafhopper assemblages were local topography and biotic factors. Variation partitioning analysis (step 2) showed that pure abiotic filtering and pure biotic processes were relatively less important than their combinations, suggesting that biotic relationships are strongly structured by abiotic conditions. Pairwise co‐occurrence analysis (step 3) on generalist leafhoppers and the most common plants identified 40 segregated species pairs (mainly between plant species) and 16 aggregated pairs (mainly between leafhopper species). Pairwise analysis on specialist leafhoppers and potential host plants clearly revealed aggregated patterns. Plant segregation suggests heterogeneous resource availability and competitive interactions, while leafhopper aggregation suggests host feeding differentiation at the local level, different feeding microhabitats on host plants, and similar environmental requirements of the species. Using the novel mbRA, we disentangle for the first time the relative importance of more than five distinct groups of variables shaping local species communities. We highlighted the important role of abiotic processes mediated by bottom‐up effects of plants on leafhopper communities. Our results revealed that in‐field structure diversification and trophic interactions are the main factors causing the co‐occurrence patterns observed.     

Intraspecific phenotypic differences in fish affect ecosystem processes as much as bottom–up factors

Evolution of life history traits can occur rapidly and has the potential to influence ecological processes, which can also be shaped by abiotic and biotic factors. Few studies have shown that life history phenotype can affect ecological processes as much as commonly studied biotic ecological variables, but currently we do not know how the ecological effects of life history phenotype compare in size to the effects of abiotic factors, or whether the ecological effects of phenotypes are sensitive to variability in abiotic conditions. Using a factorial mesocosm experiment we compared the ecosystem effects of guppy Poecilia reticulata life history phenotypes in two light treatments representing a four‐fold difference in light levels, which was comparable to upstream downstream differences in light availability in Trinidadian streams. Light and phenotype had significant effects on similar aspects of ecosystem function. Whereas light had a stronger effect on ecosystem structure (algal and invertebrate stocks) than phenotype, phenotype and light had nearly equal effects on many ecosystem processes (nutrient recycling, nutrient fluxes, ecosystem metabolism and leaf litter decomposition). Light had a stronger effect on most guppy life history traits and guppy fitness than differences between phenotypes. The effect of light on these traits was consistent with higher availability of food resources in the high light treatments. Interactions between light and phenotype were weak for the majority of response variables suggesting that abiotic variability did not alter the mechanisms by which phenotypes affect ecosystem function. We conclude that subtle phenotypic differences in consumers can affect ecosystem processes as much as meaningful variability in abiotic factors which until recently were thought to be the primary drivers of ecosystem function in nature. However, despite its effects on traits and the ecosystem, light did not alter the effect of guppy phenotype on ecosystem function.     

Incorporating biotic factors in species distribution modeling: are interactions with soil microbes important?

It is increasingly recognized that species distributions are driven by both abiotic factors and biotic interactions. Despite much recent work incorporating competition, predation, and mutualism into species distribution models (SDMs), the focus has been confined to aboveground macroscopic interactions. Biotic interactions between plants and soil microbial communities are understudied as potentially important drivers of plant distributions. Some soil bacteria promote plant growth by cycling nutrients, while others are pathogenic; thus they have a high potential for influencing plant occurrence. We investigated the influence of soil bacterial clades on the distributions of bryophytes and 12 vascular plant species in a high elevation talus‐field ecosystem in the Rocky Mountain Front Range, Colorado, USA. We used an information‐theoretic criterion (AICc) modeling approach to compare SDMs with the following different sets of predictors: abiotic variables, abiotic variables and other plant abundances, abiotic variables and soil bacteria clade relative abundances, and a full model with abiotic factors, plant abundances, and bacteria relative abundances. We predicted that bacteria would influence plant distributions both positively and negatively, and that these interactions would improve prediction of plant species distributions. We found that inclusion of either plant or bacteria biotic predictors generally improved the fit, deviance explained, and predictive power of the SDMs, and for the majority of the species, adding information on both other plants and bacteria yielded the best model. Interactions between the modeled species and biotic predictors were both positive and negative, suggesting the presence of competition, parasitism, and facilitation. While our results indicate that plant–plant co‐occurrences are a stronger driver of plant distributions than plant–bacteria co‐occurrences, they also show that bacteria can explain parts of plant distributions that remain unexplained by abiotic and plant predictors. Our results provide further support for including biotic factors in SDMs, and suggest that belowground factors be considered as well.     

Strain and vegetation effects on local limiting resources explain the outcomes of biotic interactions

Positive interactions are hypothesized to increase with stress (stress-gradient hypothesis, “SGH”), which is defined in terms of standing biomass at the community level. However, recent evidence suggests that facilitation may decrease or remain constant as stress increases. Several reasons for this discrepancy are possible: (i) the outcomes of biotic interactions depend on the component of the fitness considered; (ii) they are influenced by how vegetation affects local limiting resources; (iii) within a particular community, only species that are deviated from their physiological optima are likely to be facilitated. In a removal experiment, we quantified the deviations of species in subalpine grassland from their physiological optima, defined here as species-level “strain”, and examined whether strain and vegetation effects on local resources can explain the outcome of biotic interactions. The experiment was performed along a gradient of standing biomass driven by contrasting land use and resource availability, and used five grass species with contrasting traits and ecological optima. Strain for each species was estimated by comparing growth without vegetation (target species only submitted to local abiotic factors) to growth in optimal conditions (under controlled conditions in an experimental garden). The outcomes of biotic interactions, recorded in terms of survival and growth, could be predicted from the data about strain and vegetation effects on local limiting resources (light and water). Only highly strained species were affected by facilitation, which occurred when the surrounding vegetation alleviated the constraining factors. On the other hand, standing biomass was poorly related with the outcomes of biotic interactions. The “SGH” was only partially validated with growth data when strain and vegetation effects co-varied with standing biomass. As a consequence, strain (at species level) represents a mechanistic basis which could improve the prediction of the outcomes of biotic interactions along ecological gradients.     

Differential effects of interspecific interactions and water availability on survival, growth and fecundity of three congeneric grassland herbs

Fitness of individual plants and of populations depends on the rates of survival, growth and fecundity. This study tested whether vital rates were differentially affected by biotic interactions and water availability. The effects of manipulations of above-ground competition (through clipping) and water availability (through water addition) on the vital rates of seedlings of three species (Viola elatior Fries, Viola pumila Chaix and Viola stagnina Kit.) were analysed in dry, mesic and wet grasslands. Water addition and grassland type had the largest effects on survival (accounting for 41 and 24% of total variation, respectively) across species. Height growth rate was positively affected by grassland type (19%) and water addition (12%) and varied among species (8%), while leaf accumulation rates and reproduction were affected by grassland type and clipping. The data suggested facilitative effects of the canopy on seedling survival in the dry grassland. This study presents evidence that environmental conditions and biotic interactions may have differential effects on seedling survival, growth and reproduction. The findings highlight the complex interplay between spatial and temporal environmental variation and biotic interactions in structuring plant communities.     

Photosynthesis, photorespiration, and light signalling in defence responses

Visible light is the basic energetic driver of plant biomass production through photosynthesis. The constantly fluctuating availability of light and other environmental factors means that the photosynthetic apparatus must be able to operate in a dynamic fashion appropriate to the prevailing conditions. Dynamic regulation is achieved through an array of homeostatic control mechanisms that both respond to and influence cellular energy and reductant status. In addition, light availability and quality are continuously monitored by plants through photoreceptors. Outside the laboratory growth room, it is within the context of complex changes in energy and signalling status that plants must regulate pathways to deal with biotic challenges, and this can be influenced by changes in the highly energetic photosynthetic pathways and in the turnover of the photosynthetic machinery. Because of this, defence responses are neither simple nor easily predictable, but rather conditioned by the nutritional and signalling status of the plant cell. This review discusses recent data and emerging concepts of how recognized defence pathways interact with and are influenced by light-dependent processes. Particular emphasis is placed on the potential roles of the chloroplast, photorespiration, and photoreceptor-associated pathways in regulating the outcome of interactions between plants and pathogenic organisms.     

Nutrient and water availability alter belowground patterns of biomass allocation, carbon partitioning, and ectomycorrhizal abundance in Betula nigra

In managed settings, seedlings are often fertilized with the objective of enhancing establishment, growth, and survival. However, responses of seedlings to fertilization can increase their susceptibility to abiotic stresses such as drought. Seedlings acclimate to variation in soil resources by reallocating carbon among different physiological processes and compartments, such as above versus belowground growth, secondary metabolism, and support of ectomycorrhizal fungi (EMF). We examined the effects of nutrient and water availability on carbon allocation to above and belowground growth of river birch (Betula nigra), as well as partitioning among root sugars, starch, phenolics, lignin, and EMF abundance. As nutrient availability increased, total plant biomass and total leaf area increased, while percent root biomass decreased. Root sugars, total root phenolics and EMF abundance responded quadratically to nutrient availability, being lowest at intermediate fertility levels. Decreased water availability reduced total leaf area and root phenolics relative to well-watered controls. No interactions between nutrient and water availability treatments were detected, which may have been due to the moderate degree of drought stress imposed in the low water treatment. Our results indicate that nutrient and water availability significantly alter patterns of carbon allocation and partitioning in roots of Betula nigra seedlings. The potential effects of these responses on stress tolerance are discussed.     

Effects of pollen load, parasitoids and the environment on pre-dispersal seed predation in the cleistogamous Ruellia nudiflora

Few studies have simultaneously addressed the effects of biotic and abiotic factors on pre-dispersal seed predation (PSP). Plant–seed predator interactions may be influenced by natural enemies and pollinators (the latter through changes in fruit or seed traits), and the activity of pre-dispersal seed predators and their natural enemies may both be affected by the abiotic environment. Additionally, in the case of cleistogamous plants with fruit dimorphism, PSP may be biased towards larger and more seeded chasmogamous (CH) fruits [relative to the smaller cleistogamous (CL) fruits], and the effects of biotic and abiotic factors may be contingent upon this fruit dimorphism. We studied PSP in the cleistogamous Ruellia nudiflora using a split-plot experimental design and asked the following: (1) is PSP biased towards CH fruits and is there an effect of pollen load on PSP? (2) Do parasitoids influence PSP and is their effect influenced by pollen load or fruit type? And (3) do light and water availability modify PSP and parasitoid effects? PSP was higher for CH relative to CL fruits, and under low water availability it was lower for pollen-supplemented CH fruits relative to open-pollinated CH fruits. Parasitoids were not influenced by abiotic conditions, but their negative effect on PSP was stronger for pollen-supplemented CH fruits. Overall, we show that fruit dimorphism, abiotic factors and natural enemies affect PSP, and that these effects can be non-additive.