Vascular architecture and patchy nutrient availability generate within-plant heterogeneity in plant traits important to herbivores

Within-plant heterogeneity in growth, morphology, and chemistry is ubiquitous, and is commonly attributed to differences in tissue age, light availability, or previous damage by herbivores. Although these factors are important, we argue that plant vascular architecture is an underappreciated determinant of heterogeneity. Vascular architecture can restrict the transport of resources (nutrients, photosynthate, hormones, etc.) to within specific sectors of the plant: this is referred to as sectoriality. Although studies have documented sectoriality in the transport of isotopes and dyes from roots to shoots, the ecological consequences of this sectoriality remain poorly understood. We tested the hypothesis that spatial variation in belowground nutrient availability combined with sectorial transport results in localized "fertilization" of aboveground plant parts and generates heterogeneity in traits important to herbivores. Our split-root experiments with tomato (Lycopersicon esculentum Mill) clearly demonstrate that fertilization to isolated lateral roots generates heterogeneity in leaf morphology, phenolic chemistry, and side-shoot growth. Specifically, leaflets with direct connections to these lateral roots were larger and had lower levels of rutin and chlorogenic acid than did leaflets in other sectors lacking direct vascular connections. Moreover, side-shoot production was greater in the connected sectors. We discuss the implications of this heterogeneity for plant-herbivore interactions.     

The effects of developmental stage and source leaf position on integration and sectorial patterns of carbohydrate movement in an annual plant, Perilla frutescens (Lamiaceae)

A well-integrated plant shows extensive carbohydrate translocation through the plant body. Even in highly integrated plants, however, translocation patterns will be sectorial if vascular tissue restricts carbon movement to sectors along stems. Both integration and sectorial translocation patterns are sensitive to plant architecture and thus may change as a plant develops. These patterns should vary also with the position of the source leaf because leaves at each node are unique in age and vascular relationship to the rest of the plant. I measured the effects of developmental stage and location of the source leaf on integration and sectoriality in an annual plant, Perilla frutescens, by labeling plants with C at one of three leaves and four developmental stages. Stage and source leaf affected both integration and sectoriality. Most notably, integration declined and sectoriality increased during seed fill, when resource demand at each node was high. Furthermore, translocation was least extensive from the leaf supporting the largest number of seeds on its axillary branch. These results suggest that plants are not homogeneous collections of subunits; rather, the role of each leaf in a plant's carbon budget is a function of its age and location on the plant.     

On plant sectoriality, or how to combine the benefits of autonomy and integration

Plant sectoriality implies physiological subdivision of physically coherent plant structures. It is largely determined by vascular structure. Sectorial transport of carbon assimilates, mineral nutrients, water or hormones may be an essential component of plant phenotype in ecological interactions. Most studies of sectoriality have focussed on its effects on plant growth, resource allocation and herbivory. Since sectoriality allows semiautonomous reactions to environmental stimuli to be displayed by different plant parts, it also needs to be considered in discussions of selfishness vs. altruism of plant parts. Future lines of research should include analysis of the genetic basis of sectoriality, investigations into root sectoriality and its effects, studies of the impacts of sectoriality on plant life histories, and analyses of intra- and interpopulation variation in traits related to sectoriality.     

Modeling the influence of differential sectoriality on the photosynthetic responses of understory saplings to patchy light and water availability

Exploitation of patchy light is a key determinant of plant performance in the forest understory. While many adaptive traits are known, the role of stem vasculature in understory photosynthesis is not established. Sectoriality—the degree of vascular constraint to long distance transport—has been hypothesized to limit growth in heterogeneous light. We simulated the photosynthetic potential of sectored and integrated plants in patchy light, as a function of soil water potential (patchy or uniform). We used hydraulic parameters typical of temperate woody species in an Ohm’s law model including a tangential resistance parameter, and simulated cavitation by varying axial resistance of leaves, leaves and roots, or the whole plant. Our results suggest that differential sectoriality will not affect photosynthesis when water is plentiful, but can constrain stomatal conductance at more negative soil water potentials, especially when only a small portion of the crown receives light. This effect is strongest just below the turgor loss point, and depends on axial resistance and soil water heterogeneity. Increased resistance in high light leaves decreases photosynthesis regardless of sectoriality. However, when resistance is increased for leaves and roots or the whole plant, photosynthesis decreases more for sectored than for integrated plants. Moreover, the simulations suggest that sectoriality can further depress photosynthesis when water availability is asymmetrical. These results might explain why integrated species, such as Betula lenta, B. alleghaniensis, and Acer saccharum thrive in the forest understory and grow rapidly into canopy gaps, while sectored species, such as Quercus rubra, do not.     

Plant architecture, sectoriality and plant tolerance to herbivores

The evolution of tolerance is one potential plant response to selection imposed by herbivores. Plant architecture, and in turn, sectoriality may influence a plant's ability to tolerate tissue loss. However, each may either constrain or facilitate a plant's ability to compensate following herbivore attack depending on the plant part damaged and the identity of the damaging herbivore.Plants are limited in their ability to respond to localized damage by chewing insects because carbon does not flow freely from damaged to undamaged plant parts, particularly between branches. Thus, defoliation of individual branches invariably results in decreased growth and reproduction of those branches. Within branches, carbon flow via vascular connections between orthostichies may ameliorate the effects of damage restricted within an orthostichy. Local induction of secondary chemicals to spread damage by folivores throughout a plant's canopy, redistribution of resources within and between IPU's, and delaying reproductive activity until resources have been pooled may all alleviate the constraints on response of plants to grazing.In contrast to the effects of damage by grazers, the metameric construction of plants typically ensures points of regrowth from dormant buds when apical meristems are destroyed either by vertebrate browsers or galling insects. Sectoriality constrains the ability of sap-sucking insects to tap the entire resource base of a plant, thus having a positive effect on plant fitness. However, both the site and timing of attack mitigate the degree of limitation imposed by sectoriality. During peak periods of assimilation, photosynthate flow is mainly over short distances (between sources and sinks within the canopy), and thus sap-sucking insects have a small resource base to draw upon. In contrast, when sucking insects tap into vascular elements in which the flow is from roots to leaves and vice versa, resource availability to the insect (and in turn, potential resource loss from the plant) are only limited by the resources present in those vascular elements.Studies of specific traits in species which demonstrate differential tolerance would greatly add to our understanding of herbivore impacts on plant growth and reproduction. In particular, intraspecific variation in tolerance has been documented for individuals within and among populations with different grazing histories. A number of traits related to sectoriality and architecture probably contribute to such variation in tolerance, and because they are easily manipulated and easily quantified, represent potentially profitable avenues of research. These traits include distribution of leaves and buds, ability to release secondary meristems from dormancy, and the timing of resource movement both before and subsequent to damage.     

How are leaves plumbed inside a branch? Differences in leaf-to-leaf hydraulic sectoriality among six temperate tree species

The transport of water, sugar, and nutrients in trees is restricted to specific vascular pathways, and thus organs may be relatively isolated from one another (i.e. sectored). Strongly sectored leaf-to-leaf pathways have been shown for the transport of sugar and signal molecules within a shoot, but not previously for water transport. The hydraulic sectoriality of leaf-to-leaf pathways was determined for current year shoots of six temperate deciduous tree species (three ring-porous: Castanea dentata, Fraxinus americana, and Quercus rubra, and three diffuse-porous: Acer saccharum, Betula papyrifera, and Liriodendron tulipifera). Hydraulic sectoriality was determined using dye staining and a hydraulic method. In the dye method, leaf blades were removed and dye was forced into the most proximal petiole. For each petiole the vascular traces that were shared with the proximal petiole were counted. For other shoots, measurements were made of the leaf-area-specific hydraulic conductivity for the leaf-to-leaf pathways (k(LL)). In five out of the six species, patterns of sectoriality reflected phyllotaxy; both the sharing of vascular bundles between leaves and k(LL) were higher for orthostichous than non-orthostichous leaf pairs. For each species, leaf-to-leaf sectoriality was determined as the proportional differences between non-orthostichous versus orthostichous leaf pairs in their staining of shared vascular bundles and in their k(LL); for the six species these two indices of sectoriality were strongly correlated (R2=0.94; P <0.002). Species varied 8-fold in their k(LL)-based sectoriality, and ring-porous species were more sectored than diffuse-porous species. Differential leaf-to-leaf sectoriality has implications for species-specific co-ordination of leaf gas exchange and water relations within a branch, especially during fluctuations in irradiance and water and nutrient availability.     

Plant defences and the role of epibiosis in mediating within-plant feeding choices of seagrass consumers

Within-plant variation in susceptibility to herbivory can significantly influence the ecological and evolutionary consequences of plant–herbivore interactions. Seagrasses are marine angiosperms characterised by substantial intra-individual differences in multiple traits, such as nutrients, chemical and structural defences and epibiotic load, all of which can strongly influence herbivore preferences. We quantified the within-plant feeding choices of the two main consumers of the temperate seagrass Posidonia oceanica––the fish Sarpa salpa and the sea urchin Paracentrotus lividus––and determined the plant traits that explained their foraging strategies. We found strong within-plant heterogeneity in both seagrass susceptibility to herbivory and chemical composition, but different consumers exhibited contrasting feeding choices. S. salpa preferred the most nutritious and chemically defended younger leaves, suggesting a full adaptation to consuming this macrophyte and a greater impact of this herbivore on the plant. In contrast, P. lividus consistently preferred the older leaves covered by epibionts, probably attenuating the relative impact of this consumer and helping to explain the weak effects usually recorded for this echinoid in undisturbed meadows. Artificial diet experiments showed that morphology and fine-scale structural defences were the primary determinant of urchin feeding choices, with nutrient content and chemical defences being of secondary importance. Epibiosis did not strongly influence fish feeding, but it did have a strong ‘shared-doom’ effect on urchin consumption. This effect was driven by a distinct preference towards a mixed diet that included both host tissues and their epibiotic community.     

Small-scale heterogeneity shapes grassland diversity in low-to-intermediate resource environments

Questions

Soil resource heterogeneity influences the outcome of plant–plant interactions and, consequently, species co-existence and diversity patterns. The magnitude and direction of heterogeneity effects vary widely, and the processes underlying such variations are not fully understood. In this study, we explored how and under what resource conditions small-scale heterogeneity modulates grassland plant diversity.

Location

Oderhänge Mallnow, Potsdam, Brandenburg, Germany.

Methods

We expanded the individual-based plant community model (IBC-grass) to incorporate dynamic below-ground resource maps, simulating spatial heterogeneity of resource availability. Empirical centimeter-scale data of soil C/N ratio were integrated into the model, accounting for both configurational and compositional heterogeneity. We then analyzed the interplay between small-scale heterogeneity and resource availability on the interaction and co-existence of plant species and overall diversity.

Results

Our results showed significant differences between the low- and high-resource scenarios, with both configurational and compositional heterogeneity having a positive effect on species richness and Simpson's diversity, but only under low-resource conditions. As compositional heterogeneity in the fine-scale C/N ratio increased, we observed a positive shift in Simpson's diversity and species richness, with the highest effects at the highest level of variability tested. We observed little to no effect in nutrient-rich scenarios, and a shift to negative effects at the intermediate resource level. The study demonstrates that site-specific resource levels underpin how fine-scale heterogeneity influences plant diversity and species co-existence, and partly explains the divergent effects recorded in different empirical studies.

Conclusions

This study provides mechanistic insights into the complex relationship between resource heterogeneity and diversity patterns. It highlights the context-dependent effects of small-scale heterogeneity, which can be positive under low-resource, neutral under high-resource, and negative under intermediate-resource conditions. These findings provide a foundation for future investigations into small-scale heterogeneity–diversity relationships, contributing to a deeper understanding of the processes that promote species co-existence in plant communities.     

Spatial patterns of plant richness across treeline ecotones in the Pyrenees reveal different locations for richness and tree cover boundaries

Aim  To forecast the responses of alpine flora to the expected upward shift of treeline ecotones due to climatic warming, we investigated species richness patterns of vascular plants at small spatial scales across elevational transects.
Location  Richness patterns were assessed at local scales along the elevational gradient in two undisturbed treeline ecotones and one disturbed treeline ecotone in the Spanish Pyrenees.
Methods  We placed a rectangular plot (0.3–0.4 ha) in each treeline ecotone. We estimated and described the spatial patterns of plant richness using the point method and Moran's I correlograms. We delineated boundaries based on plant richness and tree cover using moving split windows and wavelet analysis. Then, to determine if floristic and tree cover boundaries were spatially related, overlap statistics were used.
Results  Plant richness increased above the forest limit and was negatively related to tree cover in the undisturbed sites. The mean size of richness patches in one of these sites was 10–15 m. Moving split windows and wavelets detected the sharpest changes in plant richness above the forest limit at both undisturbed sites. Most tree cover and plant richness boundaries were not spatially related.
Main conclusions  The upslope decrease of tree cover may explain the increase of plant richness across alpine treeline ecotones. However, the detection of abrupt richness boundaries well above the forest limit indicates the importance of local environmental heterogeneity to explain the patterns of plant richness at smaller scales. We found highly diverse microsites dominated by alpine species above the forest limit, which should be monitored to describe their response to the predicted upward shift of forests.     

Causes and consequences of sectoriality in the clonal herb Glechoma hederacea

The causes of sectoriality and consequences for clone behaviour are examined using data from the stoloniferous herb Glechoma hederacea. The proximal causes of physiological integration patterns are investigated using anatomical studies, acid fuchsin dye to reveal patterns of xylem continuity between ramets, and ¹⁴C as a label to reveal quantitative photoassimilate translocation patterns in the phloem. Dye movement in the xylem was acropetal and sectorial, and the sectoriality was determined by phyllotaxy. Patterns of ¹⁴C-labelled photoassimilate allocation were qualitatively similar to those of xylem based resources, although there was some basipetal movement of photoassimilate. The patterns of physiological integration and independence between ramets are shown to be governed by rules which depend on vascular continuity and discontinuity between ramets. Physiological support to stolon apices results in acquisition of relative branch autonomy (branches become semi-autonomous integrated physiological units, IPUs).This paper evaluates whether observed physiological integration patterns may be modified by altering normal source-sink relationships or by modifying environmental conditions. An experiment using different defoliation intensities, and different defoliation patterns at the same overall intensity, demonstrated that the precise positions of leaves removed from a clone had unique consequences for its subsequent development. Individual ramets of a given clone may be located in microhabitats of differing quality. An experiment in which competition was either present or absent throughout the space occupied by the clone, or patchy in distribution, showed that G. hederacea did not respond to competition at the whole clone level. Instead, connected stolons (IPUs) responded independently to local competition. Sectoriality may promote the restriction of lethal, localised environmental factors within the affected IPU. A study investigating the uptake and translocation of zinc by clones revealed that quantified patterns of zinc distribution resembled patterns of ¹⁴C movement in the phloem, and that there was no significant transport of zinc from one stolon to another.Although sectorial patterns of resource movement in G. hederacea can be modified in the short term, in the long-term, physiological integration may not allow this species to integrate the effects of environmental heterogeneity. A mobile clonal species with a high growth rate and relatively short-lived ramets, such as G. hederacea, is likely to benefit from a semi-autonomous response to patch quality at the level of the stolon, since the alternative of widespread intra-clonal support may increase the residence time of the clone in unfavourable pathches.     

A Method for Analysing Plant Architecture as it Relates to Fruit Quality Using Three-dimensional Computer Graphics

A method was developed for the spatial analysis of plant architectureas it relates to the within-plant variation in the physical,chemical, and postharvest characteristics of the fruit Computergraphics were used to reconstruct the architectural frameworkand spatial arrangement of the fruit in the canopy of kiwifruitvines (Actinidia deliciosa) trained on two different supportstructures An infra-red beam theodolite was used to obtain thespatial coordinates of the vines components The data files generatedby the theodolite were in turn used with software specificallywritten for the project (MAPIT—Microcomputer Aided PlantImaging Technology) to provide a 3-dimensional reconstructionof the original vines Each fruit was colour coded so that extremesin their attributes could be easily identified and accuratelylocated in the canopy of the vine Patterns were clearly discerniblefor both the pergola and T-bar trained vines The heavier fruitwere located at the apical ends of the canes, while greatersoluble solids concentrations were associated with the smallerfruit located closer to the cordon These patterns were consistentfor all of the vines examined The use of the theodolite coupledwith the computer graphics described in this paper providesa rapid and objective means of accurately describing plant architecture Computer graphics, plant architecture, spatial analysis, theodolite, three-dimensional analysis, fruit quality, Actinidia deliciosa, kiwifruit     

Stage-structured ontogeny in resource populations generates non-additive stabilizing and de-stabilizing forces in populations and communities

Demographic heterogeneity influences how populations respond to density dependent intraspecific competition and trophic interactions. Distinct stages across an organism's development, or ontogeny, are an important example of demographic heterogeneity. In consumer populations, ontogenetic stage structure has been shown to produce categorical differences in population dynamics, community dynamics and even species coexistence compared to models lacking explicit ontogeny. The study of consumer–resource interactions must also consider the ontogenetic stage structure of the resource itself, particularly plants, given their fundamental role at the basis of terrestrial food webs. We incorporate distinct ontogenetic stages of plants into an adaptable multi-stage consumer–resource modeling framework that facilitates studying how stage specific consumers shape trophic dynamics at low trophic levels. We describe the role of density dependent demographic rates in mediating the dynamics of stage-structured plant populations. We then investigate how these demographic rates interact with consumer pressure to influence stability and coexistence in multiple stage-specific consumer–resource interactions. Results detail how density dependent effects across distinct ontogenetic stages in plant development produce non-additivity in the drivers of dynamic stability both in single populations and in consumer–resource settings, challenging the ubiquity of certain traditional ecological dynamic paradigms. We also find categorical differences in the population variability induced by herbivores consuming separate plant stages. Consumer–resource models, such as plant–herbivore interactions, often average out demographic heterogeneity in populations. Here, we show that explicitly including plant demographic heterogeneity through ontogeny yields distinct dynamic expectations for both plants and herbivores compared to traditional consumer–resource formulations. Our results indicate that efforts to understand the demographic effect of herbivores on plant populations may need to also consider the effects of plant demographics on herbivores and the reciprocal relationship between them.     

Resource quantity, not resource heterogeneity, maintains plant diversity

Resource heterogeneity has often been proposed to explain the maintenance of plant species diversity and patterns of species diversity along productivity gradients. Resource heterogeneity should maintain biodiversity by preventing competitive exclusion because different species are superior competitors in different parts of a heterogeneous environment. In natural systems, however, resource heterogeneity covaries with average resource supply rate, making the effect of heterogeneity difficult to isolate. Using a novel experimental approach, we tested the independent effects of resource heterogeneity and average supply rate on plant species diversity. We show that the average supply rate of the most limiting resource controlled species diversity, whereas heterogeneity of this resource had virtually no effect. These findings also suggest that biodiversity declines with increasing productivity because at high enough levels of productivity one resource may always be driven to sufficiently short supply to exclude many species.     

Within-Plant Bottom-Up Effects Mediate Non-Consumptive Impacts of Top-Down Control of Soybean Aphids

There is increasing evidence that top-down controls have strong non-consumptive effects on herbivore populations. However, little is known about how these non-consumptive effects relate to bottom-up influences. Using a series of field trials, we tested how changes in top-down and bottom-up controls at the within-plant scale interact to increase herbivore suppression. In the first experiment, we manipulated access of natural populations of predators (primarily lady beetles) to controlled numbers of A. glycines on upper (i.e. vigorous-growing) versus lower (i.e. slow-growing) soybean nodes and under contrasting plant ages. In a second experiment, we measured aphid dispersion in response to predation. Bottom-up and top-down controls had additive effects on A. glycines population growth. Plant age and within-plant quality had significant bottom-up effects on aphid size and population growth. However, top-down control was the dominant force suppressing aphid population growth, and completely counteracted bottom-up effects at the plant and within-plant scales. The intensity of predation was higher on upper than lower soybean nodes, and resulted in a non-consumptive reduction in aphid population growth because most of the surviving aphids were located on lower plant nodes, where rates of increase were reduced. No effects of predation on aphid dispersal among plants were detected, suggesting an absence of predator avoidance behavior by A. glycines. Our results revealed significant non-consumptive predator impacts on aphids due to the asymmetric intensity of predation at the within-plant scale, suggesting that low numbers of predators are highly effective at suppressing aphid populations.     

Plant Vascular Architecture Determines the Pattern of Herbivore-Induced Systemic Responses in Arabidopsis thaliana

The induction of systemic responses in plants is associated with the connectivity between damaged and undamaged leaves, as determined by vascular architecture. Despite the widespread appreciation for studying variation in induced plant defense, few studies have characterized spatial variability of induction in the model species, Arabidopsis thaliana. Here we show that plant architecture generates fine scale spatial variation in the systemic induction of invertase and phenolic compounds. We examined whether the arrangement of leaves along the stem (phyllotaxy) produces predictable spatial patterns of cell-wall bound and soluble invertase activities, and downstream phenolic accumulation following feeding by the dietary specialist herbivore, Pieris rapae and the generalist, Spodoptera exigua. Responses were measured in leaves within and outside of the damaged orthostichy (leaves sharing direct vascular connections), and compared to those from plants where source-sink transport was disrupted by source leaf removal and by an insertional mutation in a sucrose transporter gene (suc2-1). Following herbivore damage to a single, middle-aged leaf, induction of cell-wall and soluble invertase was most pronounced in young and old leaves within the damaged orthostichy. The pattern of accumulation of phenolics was also predicted by these vascular connections and was, in part, dependent on the presence of source leaves and intact sucrose transporter function. Induction also occurred in leaves outside of the damaged orthostichy, suggesting that mechanisms may exist to overcome vascular constraints in this system. Our results demonstrate that systemic responses vary widely according to orthostichy, are often herbivore-specific, and partially rely on transport between source and sink leaves. We also provide evidence that patterns of induction are more integrated in A. thaliana than previously described. This work highlights the importance of plant vascular architecture in determining patterns of systemic induction, which is likely to be ecologically important to insect herbivores and plant pathogens.     

Refuge-mediated apparent competition in plant–consumer interactions

At the intersection of consumer behaviour and plant competition is the concept of refuge-mediated apparent competition: an indirect interaction whereby plants provide a refuge for a shared consumer, subsequently increasing consumer pressure on another plant species. Here, we use a simple model and empirical examples to develop and illustrate the concept of refuge-mediated apparent competition. We find that the likelihood that an inferior competitor will succeed via refuge-mediated apparent competition is greater when competitors have similar resource requirements and when consumers exhibit a strong response to the refuge and high attack rates on the superior competitor. Refuge-mediated apparent competition may create an emergent Allee effect, such that a species invades only if it is sufficiently abundant to alter consumer impact on resident species. This indirect interaction may help explain unresolved patterns observed in biological invasion, such as the different physical structure of invasive exotic plants, the lag phase, and the failure of restoration efforts. Given the ubiquity of refuge-seeking behaviour by consumers and the ability of consumers to alter the outcome of direct competition among plants, refuge-mediated apparent competition may be an underappreciated mechanism affecting the composition and diversity of plant communities.
Ecology Letters (2010) 13: 11–20     

Lack of strong refuges allows top-down control of soybean aphid by generalist natural enemies

Refuges have been shown to be important mediators of predator–prey interactions, and in particular, have been proposed as a potential mechanism allowing herbivore populations to reach outbreak levels. However, very little research on the role of refuges has been conducted in systems dominated by generalist predators. We investigated the existence of refuges from predation for the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) at multiple scales. This species invaded North America and in spite of previous studies demonstrating strong suppression by generalist natural enemies, its populations periodically cause significant economic losses. Using naturally occurring populations of soybean aphid and its natural enemies, we tested for the presence of A. glycines spatial and dynamic refuges at the within-field, single plant, and within-plant scale. At the within-field level, we found only weak and transient spatial patterns in aphid populations suggesting the lack of spatial refuges at this scale. Similarly, at the plant level we found no individual colonies that escaped predation and aphid suppression was 9- to 28-fold greater in comparison with caged controls regardless of initial aphid density. When high aphid populations were exposed to predation they were rapidly reduced to levels close to the average field density and showed reduced per capita growth rates, indicating an absence of dilution of predation risk at increased aphid density. Finally, we found a significant shift in the distribution of aphids to the lower portions of the plant in the presence of generalist predators, suggesting a partial refuge from predation at the within-plant scale. Overall, we found the naturally occurring community of generalist predators to exert strong top-down suppression of soybean aphid populations at multiple scales, and no evidence that the presence of refuges at the scales studied can lead to outbreak populations. The partial refuge from predation at the within-plant scale revealed in our study may have important consequences for the within-season population dynamics of A. glycines, since it may be associated with low plant quality tradeoffs, and therefore warrants further research.     

Regional and local patterns in plant species richness with respect to resource availability

The hump-shaped relationship between plant species richness and productivity is a well-established and important paradigm. While plot-based species richness patterns on local scales have received much attention, little is currently known about species-based patterns on a regional scale. Using Ellenberg's indicator values for 1802 plant species in central Europe, we assess the patterns in regional species richness with respect to light, water, and mineral nutrient availability – three variables that strongly influence productivity. The results of this analysis are compared to those of published studies on smaller scales leading to the following conclusions:

• 1.

  On a regional scale in central Europe there is a hump-shaped relationship between soil nutrient supply and plant species richness within a given biome.

       
  
  

Resource availability and the trichome defenses of tomato plants

We conducted two experiments to determine how resource availability influenced allocation by tomato (Lycopersicon esculentum) to trichomes, and how different patterns of trichome allocation by plants grown in different resource environments might then influence the behavior of tobacco hornworm (Manduca sexta) caterpillars. In the first experiment we used high and low levels of light and water, and then, using scanning electron microscopy, determined trichome densities on the leaves and stems. We sampled leaves and stems at several places throughout the plant to determine whether there were within-plant differences in allocation to trichomes. The results of the first experiment showed that resource availability influenced allocation to trichome growth. Patterns in high and low-light supported both the growth-differentiation balance hypothesis (GDBH) and the carbon-nutrient balance hypothesis (CNBH). However, the GDBH was not supported by differences among water treatments. Contrary, to predictions of the GDBH, plants with intermediate growth did not have the highest trichome densities, and plants with similar growth differed in trichome density. Possible biological and artifactual explanations are discussed. The first experiment also showed that there was within-plant variation in allocation to trichomes, and that plant resource availability may influence within-plant variation in allocation to trichomes. In the second experiment, we grew plants in high and low-light, and then monitored the behavior of tobacco hornworms on the stems of these plants in the laboratory. This experiment demonstrated that the light environment that tomato plants were grown in influenced the resting behavior of caterpillars. Furthermore, it demonstrated that both glandular and non-glandular trichomes impeded caterpillars from searching for food. Overall, this study indicated that plant resource availability can influence allocation to trichome defenses, and that these differences may affect insect herbivores.     

Patterns of species richness for vascular plants in China's nature reserves

Explaining the heterogeneous distribution of biodiversity across the Earth has long been a challenge to ecologists and biogeographers. Here, we document the patterns of plant species richness for different taxonomic groups in China's nature reserves, and discuss their possible explanations at national and regional scales, using vascular plant richness data coupled with information on climate and topographical variables. We found that water deficit, energy and elevation range (a surrogate of habitat heterogeneity) represent the primary explanations for variation in plant species richness of the nature reserves across China. There are consistent relationships between species richness and climate and habitat heterogeneity for different taxonomic vascular plant groups at the national scale. Habitat heterogeneity is strongly associated with plant richness in all regions, whereas climatic constraints to plant diversity vary regionally. In the regions where energy is abundant or water is scarce, plant richness patterns were determined by water and habitat heterogeneity, whereas in the region with low energy inputs, water interacting with energy, and habitat heterogeneity determined its species richness pattern. Our results also suggest that energy variables alone do not represent the primary predictor of plant richness.