Trade-off between plant growth and defense? A comparison of sagebrush populations

Volatile-Mediated Signalling Between Potato Plants in Response to Insect Herbivory is not Contingent on Soil Nutrients

Plant-plant signalling via volatile organic compounds (VOCs) has been studied intensively, but its contingency on abiotic conditions (e.g., soil nutrients, drought, warming) is poorly understood. To address this gap, we carried out a greenhouse experiment testing whether soil nutrients influenced signalling between potato (Solanum tuberosum) plants in response to insect leaf herbivory by the generalist caterpillar Spodoptera exigua. We placed pairs of plants in plastic cages, where one plant acted as a VOC emitter and the other as a receiver. We factorially manipulated soil nutrients for both emitter and receiver plants, namely: unfertilized (baseline soil nutrients) vs. fertilized (augmented nutrients). Then, to test for signalling effects, half of the emitters within each fertilization level were damaged by S. exigua larvae and the other half remained undamaged. Three days after placing larvae, we collected VOCs from emitter plants to test for herbivory and fertilization effects on VOC emissions and placed S. exigua larvae on receivers to test for signalling effects on leaf consumption and larval mass gain as proxies of induced resistance. We found that herbivory increased total VOC emissions and altered VOC composition by emitter plants, but these effects were not contingent on fertilization. In addition, bioassay results showed that receivers exposed to VOCs from herbivore-damaged emitters had lower levels of herbivory compared to receivers exposed to undamaged emitters. However, and consistent with VOC results, fertilization did not influence herbivore-induced signalling effects on receiver resistance to herbivory. In sum, we found evidence of S. exigua-induced signalling effects on resistance to herbivory in potato plants but such effects were not affected by increased soil nutrients. These results call for further work testing signalling effects under broader range of nutrient concentration levels (including nutrient limitation), teasing apart the effects of specific nutrients, and incorporating other abiotic factors likely to interact or covary with soil nutrients.

Historic exposure to herbivores, not constitutive traits, explains plant tolerance to herbivory in the case of two Medicago species (Fabaceae)

Mechanisms that allow plants to survive and reproduce after herbivory are considered to play a key role in plant evolution. In this study, we evaluated how tolerance varies in species with different historic exposure to herbivores considering ontogeny. We exposed the range-restricted species Medicago citrina and its closely related and widespread species M. arborea to one and two herbivory simulations (80 % aerial biomass loss). Physiological and growth parameters related to tolerance capacity were assessed to evaluate constitutive values (without herbivory) and induced tolerance after damage. Constitutive traits were not always related to greater tolerance, and each species compensated for herbivory through different traits. Herbivory damage only led to mortality in M. citrina; adults exhibited root biomass loss and increased oxidative stress after damage, but also compensated aerial biomass. Despite seedlings showed a lower death percentage than adults after herbivory in M. citrina, they showed less capacity to recover control values than adults. Moderate tolerance to M. arborea herbivory and low tolerance to M. citrina is found. Thus, although the constitutive characteristics are maintained in the lineage, the tolerance of plants decreases in M. citrina. That represents how plants respond to the lack of pressure from herbivores in their habitat.

Growth-defense trade-offs masked in unadmixed populations are revealed by hybridization

Organisms are constantly challenged by pathogens and pests, which can drive the evolution of growth-defense strategies. Plant stomata are essential for gas exchange during photosynthesis and conceptually lie at the intersection of the physiological demands of growth and exposure to foliar fungal pathogens. Generations of natural selection for locally adapted growth-defense strategies can eliminate variation between traits, potentially masking trade-offs and selection conflicts that may have existed in the past. Hybrid populations offer a unique opportunity to reset the clock on selection and to study potentially maladaptive trait variation before selection removes it. We study the interactions of growth, stomatal, ecopysiological, and disease resistance traits in poplars (Populus) after infection by the leaf rust Melampsora medusae. Phenotypes were measured in a common garden and genotyped at 227K SNPs. We isolate the effects of hybridization on trait variance, discover correlations between stomatal, ecophysiology, and disease resistance, examine trade-offs and selection conflicts, and explore the evolution of growth-defense strategies potentially mediated by selection for stomatal traits on the upper leaf surface. These results suggest an important role for stomata in determining growth-defense strategies in organisms susceptible to foliar pathogens, and reinforces the contribution of hybridization studies toward our understanding of trait evolution.

Signal transduction in artichoke [Cynara cardunculus L. subsp. scolymus (L.) Hayek] callus and cell suspension cultures under nutritional stress

Stimulated production of secondary phenolic metabolites and proline was studied by using cell cultures of artichoke [Cynara cardunculus L. subsp. scolymus (L.) Hayek] submitted to nutritional stress. Artichoke cell cultures accumulated phenolic secondary metabolites in a pattern similar to that seen in artichoke leaves and heads (capitula). This paper shows that both callus and cell suspension cultures under nutritional stress accumulated phenolic compounds and proline, at the same time their biomass production was negatively affected by nutrient deficiency. The results obtained strongly suggest that plant tissues respond to nutrient deprivation by a defensive costly mechanism, which determines the establishment of a mechanism of trade-off between growth and adaptive response. Furthermore, the results of this research suggest that perception of abiotic stress and increased phenolic metabolites are linked by a sequence of biochemical processes that also involves the intracellular free proline and the oxidative pentose phosphate pathway. The main conclusion of this paper is that, once calli and cell suspension cultures respond to nutrient deficiency, in acclimated cells the establishment of a negative correlation between primary metabolism (growth) and secondary metabolism (defence compounds) is observed.

Pre-infestation of Tomato Plants by Aphids Modulates Transmission-Acquisition Relationship among Whiteflies, Tomato Yellow Leaf Curl Virus (TYLCV) and Plants

Herbivory defense systems in plants are largely regulated by jasmonate-(JA) and salicylate-(SA) signaling pathways. Such defense mechanisms may impact insect feeding dynamic, may also affect the transmission-acquisition relationship among virus, plants and vectoring insects. In the context of the tomato - whitefly - Tomato Yellow Leaf Curl Virus (TYLCV) biological model, we tested the impact of pre-infesting plants with a non-vector insect (aphid Myzus persicae) on feeding dynamics of a vector insect (whitefly Bemisia tabaci) as well as virus transmission-acquisition. We showed that an aphid herbivory period of 0-48 h led to a transient systemic increase of virus concentration in the host plant (root, stem, and leaf), with the same pattern observed in whiteflies feeding on aphid-infested plants. We used real-time quantitative PCR to study the expression of key genes of the SA- and JA-signaling pathways, as well as electrical penetration graph (EPG) to characterize the impact of aphid pre-infestation on whitefly feeding during TYLCV transmission (whitefly to tomato) and acquisition (tomato to whitefly). The impact of the duration of aphid pre-infestation (0, 24, or 48 h) on phloem feeding by whitefly (E2) during the transmission phase was similar to that of global whitefly feeding behavior (E1, E2 and probing duration) during the acquisition phase. In addition, we observed that a longer phase of aphid pre-infestation prior to virus transmission by whitefly led to the up-regulation and down-regulation of SA- and JA-signaling pathway genes, respectively. These results demonstrated a significant impact of aphid pre-infestation on the tomato - whitefly - TYLCV system. Transmission and acquisition of TYLCV was positively correlated with feeding activity of B. tabaci, and both were mediated by the SA- and JA-pathways. TYLCV concentration during the transmission phases was modulated by up- and down-regulation of SA- and JA-pathways, respectively. The two pathways were inconsistent during the acquisition phase; SA- related genes were up-regulated, whereas those up- and down-stream of the JA pathway showed a more complex relationship. These findings enhance our understanding of plant - herbivore - virus interactions, which are potentially important for development of ecologically sound pest and pathogen management programs.

Transcriptomic and metabolomics responses to elevated cell wall invertase activity during tomato fruit set

Fruit set is a developmental transition from ovaries to fruitlets that determines yield potential. Cell wall invertase (CWIN) is essential for fruit and seed set, but the underlying molecular basis remains elusive. We addressed this issue by using CWIN-elevated transgenic tomato, focusing on ovaries and fruitlets at 2 d before and after anthesis, respectively. RNAseq analyses revealed that ovaries and fruitlets exhibited remarkable differences in their transcriptomic responses to elevated CWIN activity. Ovaries 2 d before anthesis were far more responsive to elevated CWIN activity compared with the fruitlets. We identified several previously unknown pathways that were up-regulated by elevated CWIN activity during fruit set. The most notable of these were expression of genes for defence, ethylene synthesis and the cell cycle along with a large number of cell wall-related genes. By contrast, expression of photosynthetic, protein degradation and some receptor-like kinase genes were generally decreased as compared with the wild type ovaries. GC-MS analyses revealed that 22 out of 24 amino acids exhibited reduced levels in the RNAi ovaries as compared with that in the wild type, probably owing to a down-regulated expression of protein degradation genes. Overall, the data indicate that (i) ovaries are much more sensitive to metabolic intervention than fruitlets; (ii) high CWIN activity could promote fruit set by improving resistance against pathogens and altering cell cycle and cell wall synthesis.

Grapevine Grafting: Scion Transcript Profiling and Defense-Related Metabolites Induced by Rootstocks

Rootstocks are among the main factors that influence grape development as well as fruit and wine composition. In this work, rootstock/scion interactions were studied using transcriptomic and metabolic approaches on leaves of the "Gaglioppo" variety, grafted onto 13 different rootstocks growing in the same vineyard. The whole leaf transcriptome of "Gaglioppo" grafted onto five selected rootstocks showed high variability in gene expression. In particular, significant modulation of transcripts linked to primary and secondary metabolism was observed. Interestingly, genes and metabolites involved in defense responses (e.g., stilbenes and defense genes) were strongly activated particularly in the GAG-41B combination, characterized in addition by the down-regulation of abscisic acid (ABA) metabolism. On the contrary, the leaves of "Gaglioppo" grafted onto 1103 Paulsen showed an opposite regulations of those transcripts and metabolites, together with the greater sensitivity to downy mildew in a preliminary in vitro assay. This study carried out an extensive transcriptomic analysis of rootstock effects on scion leaves, helping to unravel this complex interaction, and suggesting an interesting correlation among constitutive stilbenes, ABA compound, and disease susceptibility to a fungal pathogen.

Carbon Fluxes between Primary Metabolism and Phenolic Pathway in Plant Tissues under Stress

Higher plants synthesize an amazing diversity of phenolic secondary metabolites. Phenolics are defined secondary metabolites or natural products because, originally, they were considered not essential for plant growth and development. Plant phenolics, like other natural compounds, provide the plant with specific adaptations to changing environmental conditions and, therefore, they are essential for plant defense mechanisms. Plant defensive traits are costly for plants due to the energy drain from growth toward defensive metabolite production. Being limited with environmental resources, plants have to decide how allocate these resources to various competing functions. This decision brings about trade-offs, i.e., promoting some functions by neglecting others as an inverse relationship. Many studies have been carried out in order to link an evaluation of plant performance (in terms of growth rate) with levels of defense-related metabolites. Available results suggest that environmental stresses and stress-induced phenolics could be linked by a transduction pathway that involves: (i) the proline redox cycle; (ii) the stimulated oxidative pentose phosphate pathway; and, in turn, (iii) the reduced growth of plant tissues.

The importance of ecological costs for the evolution of plant defense against herbivory

Plant defense against herbivory comes at a cost, which can be either direct (reducing resources available for growth and reproduction) or indirect (through reducing ecological performance, for example intraspecific competitiveness). While direct costs have been well studied in theoretical models, ecological costs have received almost no attention. In this study we compare models with a direct trade-off (reduced growth rate) to models with an ecological trade-off (reduced competitive ability), using a combination of adaptive dynamics and simulations. In addition, we study the dependence of the level of defense that can evolve on the type of defense (directly by reducing consumption, or indirectly by inducing herbivore mortality (toxicity)), and on the type of herbivore against which the plant is defending itself (generalists or specialists). We find three major results: First, for both direct and ecological costs, defense only evolves if the benefit to the plant is direct (through reducing consumption). Second, the type of cost has a major effect on the evolutionary dynamics: direct costs always lead to a single optimal strategy against herbivores, but ecological costs can lead to branching and the coexistence of non-defending and defending plants; however, coexistence is only possible when defending against generalist herbivores. Finally, we find that fast-growing plants invest less than slow-growing plants when defending against generalist herbivores, as predicted by the Resource Availability Hypothesis, but invest more than slow-growing plants when defending against specialists. Our results clearly show that assumptions about ecological interactions are crucial for understanding the evolution of defense against herbivores.

Direct and indirect impacts of infestation of tomato plant by Myzus persicae (Hemiptera: Aphididae) on Bemisia tabaci (Hemiptera: Aleyrodidae)

The impacts of infestation by the green peach aphid (Myzus persicae) on sweetpotato whitefly (Bemisia tabaci) settling on tomato were determined in seven separate experiments with whole plants and with detached leaves through manipulation of four factors: durations of aphid infestation, density of aphids, intervals between aphid removal after different durations of infestation and the time of whitefly release, and leaf positions on the plants. The results demonstrated that B. tabaci preferred to settle on the plant leaves that had not been infested by aphids when they had a choice. The plant leaves on which aphids were still present (direct effect) had fewer whiteflies than those previously infested by aphids (indirect effect). The whiteflies were able to settle on the plant which aphids had previously infested, and also could settle on leaves with aphids if no uninfested plants were available. Tests of direct factors revealed that duration of aphid infestation had a stronger effect on whitefly landing preference than aphid density; whitefly preference was the least when 20 aphids fed on the leaves for 72 h. Tests of indirect effects revealed that the major factor that affected whitefly preference for a host plant was the interval between the time of aphid removal after infestation and the time of whitefly release. The importance of the four factors that affected the induced plant defense against whiteflies can be arranged in the following order: time intervals between aphid removal and whitefly release > durations of aphid infestation > density of aphids > leaf positions on the plants. In conclusion, the density of aphid infestation and time for which they were feeding influenced the production of induced compounds by tomatoes, the whitefly responses to the plants, and reduced interspecific competition.

Metabolic efficiency underpins performance trade-offs in growth of Arabidopsis thaliana

Growth often involves a trade-off between the performance of contending tasks; metabolic plasticity can play an important role. Here we grow 97 Arabidopsis thaliana accessions in three conditions with a differing supply of carbon and nitrogen and identify a trade-off between two tasks required for rosette growth: increasing the physical size and increasing the protein concentration. We employ the Pareto performance frontier concept to rank accessions based on their multitask performance; only a few accessions achieve a good trade-off under all three growth conditions. We determine metabolic efficiency in each accession and condition by using metabolite levels and activities of enzymes involved in growth and protein synthesis. We demonstrate that accessions with high metabolic efficiency lie closer to the performance frontier and show increased metabolic plasticity. We illustrate how public domain data can be used to search for additional contending tasks, which may underlie the sub-optimality in some accessions.

Additive genetic variation in resistance traits of an exotic pine species: little evidence for constraints on evolution of resistance against native herbivores

The apparent failure of invasions by alien pines in Europe has been explained by the co-occurrence of native pine congeners supporting herbivores that might easily recognize the new plants as hosts. Previous studies have reported that exotic pines show reduced tolerance and capacity to induce resistance to those native herbivores. We hypothesize that limited genetic variation in resistance to native herbivores and the existence of evolutionary trade-offs between growth and resistance could represent additional potential constraints on the evolution of invasiveness of exotic pines outside their natural range. In this paper, we examined genetic variation for constitutive and induced chemical defences (measured as non-volatile resin in the stem and total phenolics in the needles) and resistance to two major native generalist herbivores of pines in cafeteria bioassays (the phloem-feeder Hylobius abietis and the defoliator Thaumetopoea pityocampa) using half-sib families drawn from a sample of the population of Pinus radiata introduced to Spain in the mid-19th century. We found (i) significant genetic variation, with moderate-to-high narrow-sense heritabilities for both the production of constitutive non-volatile resin and induced total phenolics, and for constitutive resistance against T. pityocampa in bioassays, (ii) no evolutionary trade-offs between plant resistance and growth traits or between the production of different quantitative chemical defences and (iii) a positive genetic correlation between constitutive resistance to the two studied herbivores. Overall, results of our study indicate that the exotic pine P. radiata has limited genetic constraints on the evolution of resistance against herbivores in its introduced range, suggesting that, at least in terms of interactions with these enemies, this pine species has potential to become invasive in the future.

Community specificity: life and afterlife effects of genes

Community-level genetic specificity results when individual genotypes or populations of the same species support different communities. Our review of the literature shows that genetic specificity exhibits both life and afterlife effects; it is a widespread phenomenon occurring in diverse taxonomic groups, aquatic to terrestrial ecosystems, and species-poor to species-rich systems. Such specificity affects species interactions, evolution, ecosystem processes and leads to community feedbacks on the performance of the individuals expressing the traits. Thus, genetic specificity by communities appears to be fundamentally important, suggesting that specificity is a major driver of the biodiversity and stability of the world's ecosystems.

Coping with herbivory at the juvenile stage: responses to defoliation and stem browsing in the African savanna tree Colophospermum mopane

Responses of plants to herbivory are dependent on the type of damage and the ontogenetic stage of the plant. We compared the effects of stem pruning and defoliation on seedlings of Colophospermum mopane, an ecologically important tree species widely distributed in southern Africa. The growth of 160 greenhouse-grown juveniles were measured for 6-mo after germination and then 6-mo after treatments including 50% defoliation, 100% defoliation, 50% stem pruning and controls. Pruning resulted in 30% reductions in total leaf area, height and biomass. Partial defoliation resulted in 30% reductions in total leaf area and plant biomass. However, complete defoliation resulted in a 30% increase in biomass production, a doubling in leaf and lateral branch number, a 45% reduction in leaf size, and no change in total leaf area. Thus, completely defoliated seedlings showed greater performance than those that were only partially defoliated, indicating that C. mopane has become adapted to the chronic and severe defoliation inflicted by Imbrasia belina caterpillars. Comparison of our results with other studies indicates that C. mopane seedlings are less herbivory-tolerant than adults and that pruning has more negative effects than defoliation. Thus, seedling browsers may constrain recruitment in C. mopane, influencing its population dynamics and abundance.

Evolution of prey in ecological time reduces the effect size of predators in experimental microcosms

Ecologists have long studied the effect of predators on prey population abundance while evolutionary biologists have measured prey trait evolution in response to predation. Ecological and evolutionary processes were generally thought to occur on different time scales, but recent evidence suggests that evolution may alter the ecological effects of predation over the course of ecological experiments. We used a protozoan and its mosquito-larvae predator, naturally found in the water-filled leaves of pitcher plants, to examine the effect of prey evolution on predator-prey interactions. In experiments conducted over 12 days (approximately 50 prey generations, but less than one predator generation), we measured a decrease in the effect of mosquito larvae predators on protozoa prey populations. In a separate set of experiments, we found that the presence of predators corresponded with evolution of smaller cell size and increased population growth rate. In ecological experiments, two situations commonly occur: strong selection pressure applied by the treatment itself and discrepancies in generation times of associate species. Our results suggest that in either situation, the resulting evolutionary patterns may lead to dramatic and important changes in ecological effect size.

Habitat partitioning among neotropical pioneers: a consequence of differential susceptibility to browsing herbivores?

Four species of fast-growing pioneer tree species in the genus Cecropia exist in the forests in central Panama. Cecropia insignis is dominant in old-growth forests but absent from nearby secondary forests; C. obtusifolia, and C. peltata are abundant in secondary forests but rare in old-growth forest, and C. longipes is uncommon in both. To determine whether Cecropia habitat associations are a consequence of local dispersal or differences in recruitment success, we grew seedlings of these species in common gardens in large treefall gaps in secondary and old-growth forest. In contrast to the observed adult distribution, only C. insignis grew significantly over 16 months in secondary forests; remaining species were heavily browsed by herbivores. C. insignis also grew and survived best in old-growth forest. Differences in susceptibility to herbivory did not result from an ant defence mutualism; none of the plants were colonised by ants during the experiment. To test whether C. insignis, the species least susceptible to herbivory, trades off investment in growth in favour of defence, we also grew the four Cecropia species in a screened growing house under light conditions comparable to large forest gaps. Contrary to expectation, species growth rates were similar; only C. peltata grew significantly faster than C. insignis. These results suggest that (1) conditions in ~40-year-old secondary forests no longer support the recruitment of Cecropia species, which are canopy dominants there; and (2) among congeners, differences in plant traits with little apparent cost to growth can have large impacts on recruitment by affecting palatability to herbivores.

Differential effects of defoliation by mopane caterpillars and pruning by African elephants on the regrowth of Colophospermum mopane foliage

Plant responses to herbivory vary depending on herbivory type, yet the comparative effects of defoliation (e.g. by insects) and pruning (e.g. by large mammals) on a single tree species are poorly documented. We investigated this in the Northern Province of South Africa by comparing the regrowth of Colophospermum mopane trees previously defoliated by caterpillars or pruned by elephants, the two main browsers of C. mopane foliage. Shoots were up to 160% and 125% longer after natural (elephant) and simulated pruning and leaves ~25% longer in regrowth after natural pruning (n = 13–15 trees per treatment). Shoot density and chemical defences in leaves (tannin:protein ratio and total polyphenolic concentration) were, however, no different from control trees. Simulated defoliation resulted in statistically insignificant changes to regrowth in terms of leaf and shoot size (both slightly decreased) and shoot density (slightly increased). Natural (caterpillar) defoliation, however, resulted in regrowth with significantly decreased shoot and leaf size (about 50% and 20% of control lengths, respectively), as well as decreased leaf chemical defence. Shoot and leaf length were longer on trees flushing for the first time after pruning and late-season defoliation had a greater negative impact than mid-season defoliation. Despite the differences in regrowth characteristics after pruning and defoliation, mopane plants showed no apparent trade-off in investment between tolerance and resistance after either herbivory type, as neither regrowth nor chemical defence occurred at the expense of the other.

Plant defense, growth, and habitat: a comparative assessment of constitutive and induced resistance

The growth rate (GR) hypothesis relates the evolution of plant defense to resource availability and predicts that plants that have evolved in abiotically stressful environments grow inherently more slowly and are more constitutively resistant to herbivory than plants from more productive habitats. Stress-adapted plants are also predicted to have reduced inducibility, but this prediction has not been previously tested. To evaluate this hypothesis, I compared the growth of nine species of herbaceous plants from Missouri glade habitats to congeners from more productive non-glade habitats. I also conducted bioassays using larvae of the generalist herbivore Spodoptera exigua to estimate constitutive and inducible resistance in these congeners. Glade congeners tended to grow more slowly and have higher constitutive resistance and lower inducibility than non-glade species. However, none of these comparisons was statistically significant due to the conflicting response of one congeneric pair (Salvia azurea and S. lyrata). Analyses without this genus were consistent with the GR hypothesis, as were analyses that categorized congeners by relative growth rate. These results highlight the complexity in searching for factors that determine plant growth rates and resistance traits across multiple genera and support the hypothesis that both constitutive and induced resistance may be influenced by selection on traits that alter plant growth rates. Future studies should attempt to determine whether variation in inducibility is better explained by habitat or relative plant growth rates.

Consequences of insect herbivory on grape fine root systems with different growth rates

Herbivory tolerance has been linked to plant growth rate where plants with fast growth rates are hypothesized to be more tolerant of herbivory than slower-growing plants. Evidence supporting this theory has been taken primarily from observations of aboveground organs but rarely from roots. Grapevines differing in overall rates of new root production, were studied in Napa Valley, California over two growing seasons in an established vineyard infested with the sucking insect, grape phylloxera (Daktulosphaira vitifoliae Fitch). The experimental vineyard allowed for the comparison of two root systems that differed in rates of new root tip production (a 'fast grower', Vitis berlandieri x Vitis rupestris cv. 1103P, and a slower-growing stock, Vitis riparia x Vitis rupestris cv. 101-14 Mgt). Each root system was grafted with a genetically identical shoot system (Vitis vinifera cv. Merlot). Using minirhizotrons, we did not observe any evidence of spatial or temporal avoidance of insect populations by root growth. Insect infestations were abundant throughout the soil profile, and seasonal peaks in phylloxera populations generally closely followed peaks in new root production. Our data supported the hypothesis that insect infestation was proportional to the number of growing tips, as indicated by similar per cent infestation in spite of a threefold difference in root tip production. In addition, infested roots of the fast-growing rootstock exhibited somewhat shorter median lifespans (60 d) than the slower-growing rootstock (85 d). Lifespans of uninfested roots were similar for the two rootstocks (200 d). As a consequence of greater root mortality of younger roots, infested root populations in the fast-growing rootstock had an older age structure. While there does not seem to be a trade-off between potential growth rate and relative rate of root infestation in these cultivars, our study indicates that a fast-growing root system may more readily shed infested roots that are presumably less effective in water and nutrient uptake. Thus, differences in root tip production may be linked to differences in the way plants cope with roots that are infested by sucking insects.

Evidence that fungal pathogens inhibit recruitment of a shade-intolerant tree, white birch ( Betula papyrifera), in understory habitats

Evidence from tropical forests suggests understory habitats are associated with a high risk of disease, which may prevent the establishment of vulnerable tree species; in contrast, canopy gaps can act as refuges from these pathogens. However, few studies have investigated the impacts of pathogens on regeneration in temperate forests. To determine whether losses to fungi of seeds of Betula papyrifera, a light-loving species, varied between habitats that differed in their degree of openness, we applied fungicide to seeds buried in old fields, treefall gaps, and forest understory sites. We found that the application of fungicide significantly reduced losses in all habitats, relative to control values. This effect was habitat-dependent: the benefit of fungicide was greater in forest understory than in openings. This suggests that B. papyrifera is prevented from establishing in understory environments in part by its susceptibility to pathogen attack, and not solely because of a high light requirement.

Extensive summer water pulses do not necessarily lead to canopy growth of Great Basin and northern Mojave Desert shrubs

Plant species and functionally related species groups from arid and semi-arid habitats vary in their capacity to take up summer precipitation, acquire nitrogen quickly after summer precipitation, and subsequently respond with ecophysiological changes (e.g. water and nitrogen relations, gas exchange). For species that respond ecophysiologically, the use of summer precipitation is generally assumed to affect long-term plant growth and thus alter competitive interactions that structure plant communities and determine potential responses to climate change. We assessed ecophysiological and growth responses to large short-term irrigation pulses over one to three growing seasons for several widespread Great Basin and northern Mojave Desert shrub species: Chrysothamnus nauseosus, Sarcobatus vermiculatus, Atriplex confertifolia, and A. parryi. We compared control and watered plants in nine case studies that encompassed adults of all four species, juveniles for three of the species, and two sites for two of the species. In every comparison, plants used summer water pulses to improve plant water status or increase rates of functioning as indicated by other ecophysiological characters. Species and life history stage responses of ecophysiological parameters (leaf N, delta15N, delta13C, gas exchange, sap flow) were consistent with several previous short-term studies. However, use of summer water pulses did not affect canopy growth in eight out of nine comparisons, despite the range of species, growth stages, and site conditions. Summer water pulses affected canopy growth only for C. nauseosus adults. The general lack of growth effects for these species might be due to close proximity of groundwater at these sites, co-limitation by nutrients, or inability to respond due to phenological canalization. An understanding of the connections between short-term ecophysiological responses and growth, for different habitats and species, is critical for determining the significance of summer precipitation for desert community dynamics.

Performance of the cyclic autumnal moth, Epirrita autumnata, in relation to birch mast seeding

The mast depression hypothesis has been put forward to explain the 9- to 10-year population cycle of the autumnal moth (Epirrita autumnata; Lepidoptera: Geometridae) in northern Fennoscandia. We analysed long-term data from Finnish Lapland in order to evaluate the critical assumption of the mast depression hypothesis: that better individual performance of herbivores, followed by high annual growth rate of populations, occurs in the year following mast seeding of the host, the mountain birch ( Betula pubescens ssp. czerepanovii). Since mast seeding has been suggested to occur at the expense of chemical defence against herbivores, we bioassayed the quality of birch leaves from the same trees by means of yearly growth trials with autumnal moth larvae. We also measured the size of wild adults as a determinant of potential fecundity of the species in different years. The relative growth rate of larvae was poorer in post-mast years compared to other years, rather than better as assumed by the hypothesis. Conversely, a slight indication of the increase in potential fecundity was observed due to the somewhat larger adult size in post-mast years. Population growth rate estimates, however, showed that the increase in fecundity would have to be much higher to facilitate population increase towards a cyclic peak with outbreak density. Accordingly, our two data sets do not support the assumption of a higher annual growth rate in autumnal moth populations subsequent to mast seeding of the host, thereby contradicting the predictions of the mast depression hypothesis. Temperatures, when indexed by the North Atlantic Oscillation and accumulated thermal sums, were observed to correlate with the abundance or rate of population change of the autumnal moth. The factors underlying the regular population cycles of the autumnal moth, however, remain unidentified. Overall, we suggest that the causal agents in cyclic insect population dynamics should be clarified by field experimentation, since trophic interactions are complex and are further modified by abiotic factors such as climate.