Relative Importance of Environmental Stress and Herbivory in Reducing Litter Fall in a Semiarid Woodland

We examined the impact of soil stress (low water and nutrient availabilities) and two keystone insect herbivores on pinyon pine (Pinus edulis) needle litterfall. We compared trees growing on two distinct soil types: volcanic cinders, which exhibit pronounced water and nutrient limitation, and sandy-loam soils, which have higher water-storage capacity and nutrient availability. Using two long-term herbivore removal experiments (15 and 18 years, respectively), we also examined the effects of the pinyon needle scale (Matsucoccus acalyptus, which attacks juvenile trees) and the stem-boring moth (Dioryctria albovittella, which attacks mature trees) on pinyon litterfall. These herbivores reach high densities on cinder soils but are absent or occur at much lower levels on sandy-loam soils. Four years of litterfall measurements showed four major patterns. First, independent of herbivory, needle litterfall was 20% lower under trees on high-stress cinder soils than on sandy-loam soils. Second, in agreement with the negative impact of scales on tree growth (that is, a 30% decline in stem growth), trees with scale infestations had 25% lower litterfall rates than trees resistant to scale; however, 15 years of scale-insect removal did not significantly increase needle litterfall. This implies possible intrinsic differences in litter production between scale-resistant and scale-susceptible trees. Third, in contrast with significant negative effects of moth herbivory on tree growth (that is, a 27% decline in stem growth), moth herbivory had no effect on needle litterfall. This, along with increased stem density in moth-susceptible trees, may be evidence of compensatory production. Fourth, there were strong year by soil type and year by scale herbivory interactions, such that in some years the effect on litterfall can be obscured or reversed by some other factor. In summary, soil stress has a strong and predictable effect on needle litterfall, whereas the relationship between insect herbivory and needle litterfall is weaker and depends on the individual herbivore. These effects, however, are mediated by other environmental factors that have considerable annual variation.     

THE SIGNIFICANCE OF OUTCROSSING IN AN INTIMATE PLANT-HERBIVORE RELATIONSHIP. I. DOES OUTCROSSING PROVIDE AN ESCAPE FROM HERBIVORES ADAPTED TO THE PARENT PLANT?

Sexual reproduction may be advantageous for hosts that are preyed on or parasitized by enemies that are highly adapted to them. Sexual reproduction can create rare genotypes that may escape predation by virtue of rarity and can create variable progeny that may escape predation if enemies are specialized to only one genotype of host. Populations of the herbivorous thrips, Apterothrips apteris, have been shown to be adapted to individual Erigeron glaucus clones. Here, we show that thrips adapted to the parental clone could better use plant progeny of the “home” clone produced through selfing than progeny derived from selfing of other clones. Thus, despite recombination, progeny produced by selfing presented a resource that was similar to the parental phenotype with respect to use by adapted thrips. We also show that E. glaucus susceptibility to thrips has a genetic basis and then ask whether outcrossing provides a means for E. glaucus clones to escape attack by adapted thrips. When we compared the success of thrips on progeny produced by selfing or outcrossing of the home clone, we found that the merits or disadvantages associated with outcrossing were dependent on the susceptibility to infestation of the parental clones. Selfing by clones characterized by low infestations of thrips appeared to preserve resistant genotypes; all outcrossed progeny had, on average, higher infestation levels than selfed progeny. In contrast, outcrossed progeny of clones characterized by high infestations of thrips had either the same thrips density as progeny from selfing, when the pollen donor was a highly infested clone, or lower density, when the pollen donor was a low infestation clone. The advantages of outcrossing were caused by the alleles contributed to progeny rather than to progeny variability or rarity.     

Drought responses of conifers in ecotone forests of northern Arizona: tree ring growth and leaf δ<Superscript>13</Superscript>C

We sought to understand differences in tree response to meteorological drought among species and soil types at two ecotone forests in northern Arizona, the pinyon-juniper woodland/ponderosa pine ecotone, and the higher elevation, wetter, ponderosa pine/mixed conifer ecotone. We used two approaches that provide different information about drought response: the ratio of standardized radial growth in wet years to dry years (W:D) for the period between years 1950 and 2000 as a measure of growth response to drought, and ¹³C in leaves formed in non-drought (2001) and drought (2002) years as a measure of change in water use efficiency (WUE) in response to drought. W:D and leaf ¹³C response to drought for Pinus edulis and P. ponderosa did not differ for trees growing on coarse-texture soils derived from cinders compared with finer textured soils derived from flow basalts or sedimentary rocks. P. ponderosa growing near its low elevation range limit at the pinyon-juniper woodland/ponderosa pine ecotone had a greater growth response to drought (higher W:D) and a larger increase in WUE in response to drought than co-occurring P. edulis growing near its high elevation range limit. P. flexilis and Pseudotsuga menziesii growing near their low elevation range limit at the ponderosa pine/mixed conifer ecotone had a larger growth response to drought than co-occurring P. ponderosa growing near its high elevation range limit. Increases in WUE in response to drought were similar for all species at the ponderosa pine/mixed conifer ecotone. Low elevation populations of P. ponderosa had greater growth response to drought than high-elevation populations, whereas populations had a similar increase in WUE in response to drought. Our findings of different responses to drought among co-occurring tree species and between low- and high-elevation populations are interpreted in the context of drought impacts on montane coniferous forests of the southwestern USA.     

Interactions between hosts and non-hosts of Pinus spp. and Matsucoccus josephi: anatomical responses of stem to infestation

Artificial infestation of seven pine species revealed that Matsucoccus josephi (Homoptera: Margarodidae) completes its development on Pinus brutia , P. eldarica and P. halepersis , whereas only a small number of the scale larvae infesting P. cunariensis and P. pined reached the second instar and none developed to the adult stage. M. josephi crawlers died while trying to feed on P. radiata and P. pinaster. Histological examination of the infested plant sections indicated that necrosis in shoot and stem barks of P. eldarica and P. halepensis occurs just before or after the scale completes its development. Injury reached the cambium, and traumatic resin ducts; parenchyma cells instead of tracheids and curved tracheids were formed. In P. brutia wound periderm was observed in the stem section but not in the shoot cortex following scale development. Wound periderm was also formed in the stem cortex of P. canariensis and P. pinena whereas no pathological changes were detected in the infested cortex of P. pinaster and P. radiata.
Mechanical lesions caused by needle punctures resulted in the production of parenchyma cells instead of tracheids in all four pine species investigated. The response of the pine cortex was non-specific; however several similarities in the response to scale infestation, such as the formation of resin ducts, were observed. We hypothesize that the response of the pine tissue to the scale saliva is a hypersensitive reaction. It is suggested that the development of Matsucoccus species on particular pine species is related to the scale's inability to trigger the defence system of the host while the larvae are feeding.     

Chronic herbivory negatively impacts cone and seed production,seed quality and seedling growth of susceptible pinyon pines

Although herbivory often reduces the reproduction of attacked trees, few studies have examined how naturally occurring insect-resistant and susceptible trees differ in their reproduction, nor have these effects been experimentally examined through long-term herbivore removals. In addition, few studies have examined the effects of herbivory on the quality of seeds produced and the implications of reduced seed quality on seedling establishment. We evaluated the impact of chronic herbivory by the stem-boring moth, Dioryctria albovittella, on cone and seed production of the pinyon pine (Pinus edulis) during two mast years. Three patterns emerged. First, moth herbivory was associated with reductions in cone production, viable seed production and seed mass. Specifically, pinyons susceptible to moth attack had 93–95% lower cone production, and surviving cones produced 31–37% fewer viable seeds, resulting in a 96–97% reduction in whole tree viable seed production. In addition, surviving seeds from susceptible trees had 18% lower mass than resistant trees. Second, long-term experimental removal of the herbivore resulted in increased rates of cone and seed production and quality, indicating that moth herbivory was the driver of these reductions. Third, seed size was positively associated with seed germination and seedling biomass and height, suggesting that trees suffering chronic herbivory produce poorer quality offspring. Thus, the resistance traits of pinyons can affect the quality of offspring, which in turn may affect subsequent seedling establishment and population dynamics.     

Lifespans of fungal rhizomorphs under nitrogen fertilization in a pinyon-juniper woodland

The lifespan of individual microbes in the soil influences nutrient cycling rates as well as population dynamics, but their responses to global change factors such as anthropogenic nitrogen deposition have been challenging to quantify in situ. We used minirhizotron images to track the abundance and turnover rate of individual fungal rhizomorphs under nitrogen fertilization in a pinyon-juniper woodland in New Mexico. We hypothesized that increases in nitrogen availability would alter rhizomorph lifespan and abundance. Sequential images were collected over eight sampling dates from November 1997 to August 1999, and a total of 278 rhizomorphs were examined. We found that neither standing stocks nor lifespans of rhizomorphs differed significantly between treatments. Lifespans of rhizomorphs lasted eleven months on average, indicating that nutrient immobilization in these structures could last for longer than a growing season in these sites.     

Contribution to the taxonomy of Pinus uncinata (Pinaceae) based on cone characters

Eight natural populations of Pinus uncinata Ramond from the Spanish Pyrenees, five of P. mugo Turra from the Carpathians, the Sudethians and Abruzzian Apennines, three of P. sylvestris L. from the Central-European Lowland and the Iberian Peninsula and one of P. uliginosa Neumann from SW Poland have been compared biometrically in 16 cone characters. P. uncinata appears the most distant from all other taxa, differing by having the largest and most asymmetric cones and the thickest apophyses with the most prominent umbo. In cone characters, P. mugo is more similar to P. sylvestris than to P. uncinata. Nevertheless, P. mugo and P. sylvestris remain distant from each other in a number of morphological characters examined. P. uliginosa has cones intermediate between the other three taxa and is moreover very variable. The locality of P. uliginosa in the Silesian Lowland may be of a relict character and conserve genes from P. uncinata.     

Evidence for mutualist limitation: the impacts of conspecific density on the mycorrhizal inoculum potential of woodland soils

The ability of seedlings to establish can depend on the availability of appropriate mycorrhizal fungal inoculum. The possibility that mycorrhizal mutualists limit the distribution of seedlings may depend on the prevalence of the plant hosts that form the same type of mycorrhizal association as the target seedling species and thus provide inoculum. We tested this hypothesis by measuring ectomycorrhizal (EM) fine root distribution and conducting an EM inoculum potential bioassay along a gradient of EM host density in a pinyon–juniper woodland where pinyon is the only EM fungal host while juniper and other plant species are hosts for arbuscular mycorrhizal (AM) fungi. We found that pinyon fine roots were significantly less abundant than juniper roots both in areas dominated aboveground by juniper and in areas where pinyon and juniper were co-dominant. Pinyon seedlings establishing in pinyon–juniper zones are thus more likely to encounter AM than EM fungi. Our bioassay confirmed this result. Pinyon seedlings were six times less likely to be colonized by EM fungi when grown in soil from juniper-dominated zones than in soil from either pinyon–juniper or pinyon zones. Levels of EM colonization were also reduced in seedlings grown in juniper-zone soil. Preliminary analyses indicate that EM community composition varied among sites. These results are important because recent droughts have caused massive mortality of mature pinyons resulting in a shift towards juniper-dominated stands. Lack of EM inoculum in these stands could reduce the ability of pinyon seedlings to re-colonize sites of high pinyon mortality, leading to long-term vegetation shifts.     

檀香与不同豆科植物寄生关系的研究

为了解檀香(Santalum album L.)与不同豆科植物的寄生关系,选取11种具有重要经济价值的豆科植物和檀香一起种植,对檀香叶片的光合特征和株高、地径以及寄主植物根系的吸器大小进行了测定。结果表明,檀香的吸器大小、数量在不同寄主之间存在很大差异,不同寄主对檀香的株高、地径也有显著差异。檀香的优良寄主有美洲合欢(Calliandra haematocephala)、苏木(Caesalpinia sappan)、台湾相思(Acacia confusa)、龙牙花(Erythrina corallodendron);一般的寄主植物有马占相思(Acacia mangium)、降香黄檀(Dalbergia odorifera)、黄槐(Cassia surattensis);不适宜的寄主植物有凤凰木(Delonix regia)、海南红豆(Ormosia pinnata)、银合欢(Leucaena leucocephala)、洋紫荆(Bauhinia blakeana)。同时,檀香的株高、地径及光合特征和吸器数量密切相关。因此,檀香种植时应选择适宜的优良寄主植物。     

Genetic and soil-nutrient effects on the abundance of herbivores on willow

The effects of soil-nutrient environment, plant genotype, and the interaction between the two on the resistance of the willow, Salix sericea, to insect species in a diverse herbivore community was measured. We found that soil-nutrient environment influenced plant growth and the abundance of most herbivores of S. sericea. However, environmental effects on herbivore abundance were often modified by plant genetics; the abundance of four of seven herbivores exhibited significant genotypeby-environment interaction effects. Pure genotype effects were mostly small and non-significant. The effects of fertilization differed among herbivores. Several herbivores were more abundant on fertilized plants, one was less abundant, and the abundance of others did not change. We found that feeding guild was a poor predictor of herbivore response. Finally we found significant phenotypic and genetic correlations among growth rate, internode length, and the abundances of several herbivores.     

Cell hypertrophy in stems ofPinus halepensis infested byMatsucoccus josephi

Summary Structural and ultrastructural changes occurring in the living cells of young plants ofPinus halepensis Mill. artificially infected withMatsucoccus josephi Bodenheimer et Harpaz, were studied. The study revealed that, opposite the places of attachment of the insects to the stem, groups of enlarged cells occurred in the cortex and often also in the phloem and cambial zones. In the enlarged cortical parenchyma cells there was a pronounced increase in the volume of the cytoplasm. The nuclei were large and lobed. The ER appeared in two forms, in very long cisternae arranged in parallel, and in short variously oriented tubules. The plastids were without grana, contained a tubular network and many plastoglobuli. Mitochondria and dictyosomes were relatively numerous. The function of the hypertrophic cells is discussed.     

Ha-DEF1, a sunflower defensin,induces cell death in <Emphasis Type="Italic">Orobanche</Emphasis> parasitic plants

Plant defensins are small basic peptides of 5–10 kDa and most of them exhibit antifungal activity. In a sunflower resistant to broomrape, among the three defensin encoding cDNA identified, SF18, SD2 and HaDef1, only HaDef1 presented a preferential root expression pattern and was induced upon infection by the root parasitic plant Orobanche cumana. The amino acid sequence deduced from HaDef1 coding sequence was composed of an endoplasmic reticulum signal sequence of 28 amino acids, a standard defensin domain of 50 amino-acid residues and an unusual C-terminal domain of 30 amino acids with a net positive charge. A 5.8 kDa recombinant mature Ha-DEF1 corresponding to the defensin domain was produced in Escherichia coli and was purified by means of a two-step chromatography procedure, Immobilized Metal Affinity Chromatography (IMAC) and Ion Exchange Chromatography. Investigation of in vitro antifungal activity of Ha-DEF1 showed a strong inhibition on Saccharomyces cerevisiae growth linked to a membrane permeabilization, and a morphogenetic activity on Alternaria brassicicola germ tube development, as already reported for some other plant defensins. Bioassays also revealed that Ha-DEF1 rapidly induced browning symptoms at the radicle apex of Orobanche seedlings but not of another parasitic plant, Striga hermonthica, nor of Arabidopsis thaliana. FDA vital staining showed that these browning areas corresponded to dead cells. These results demonstrate for the first time a lethal effect of defensins on plant cells. The potent mode of action of defensin in Orobanche cell death and the possible involvement in sunflower resistance are discussed.     

Response of a leaf beetle to two food plants,only one of which provides a sequestrable defensive chemical

Oreina elongata is a chemically defended leaf beetle. If its food plant contains pyrrolizidine alkaloids, all life stages of the beetle sequester them. However, one of the two known host-plant genera does not contain these alkaloids. In this paper we compare the adult feeding preference and larval performance of two populations, one feeding on Adenostyles alliariae (which contains alkaloids) and one on Cirsium spinosissimum (devoid of alkaloids). Adults of the population living on C. spinosissimum preferred the alkaloid-containing A. alliariae, while adults of the population feeding on A. alliariae showed no preference for either plant. On the other hand, larval growth of both populations is better on C. spinosissimum, without alkaloids. This is especially so in the population that never naturally encounters pyrrolizidine alkaloids; the population living on A. alliariae is apparently better adapted to its host's secondary compounds. The data are discussed in terms of cost of defense and trade-offs between growth and defense.     

Morphological and chemical components of resistance to pod borer, Helicoverpa armigera in wild relatives of pigeonpea

Host plant resistance is an important component for minimizing the losses due to the pod borer, Helicoverpa armigera, which is the most devastating pest of pigeonpea. An understanding of different morphological and biochemical components of resistance is essential for developing strategies to breed for resistance to insect pests. Therefore, we studied the morphological and biochemical components associated with expression of resistance to H. armigera in wild relatives of pigeonpea to identify accessions with a diverse combination of characteristics associated with resistance to this pest. Among the wild relatives, oviposition non-preference was an important component of resistance in Cajanus scarabaeoides, while heavy egg-laying was recorded on C. cajanifolius (ICPW 28) and Rhynchosia bracteata (ICPW 214). Accessions belonging to R. aurea, C. scarabaeoides, C. sericeus, C. acutifolius, and Flemingia bracteata showed high levels of resistance to H. armigera, while C. cajanifolius was as susceptible as the susceptible check, ICPL 87. Glandular trichomes (type A) on the calyxes and pods were associated with susceptibility to H. armigera, while the non-glandular trichomes (trichome type C and D) were associated with resistance to this insect. Expression of resistance to H. armigera was also associated with low amounts of sugars and high amounts of tannins and polyphenols. Accessions of wild relatives of pigeonpea with non-glandular trichomes (type C and D) or low densities of glandular trichomes (type A), and high amounts of polyphenols and tannins may be used in wide hybridization to develop pigeonpea cultivars with resistance to H. armigera. Handling editor: Robert Glinwood     

The mycorrhizal fungus<Emphasis Type="Italic"> Tricholoma matsutake</Emphasis> stimulates<Emphasis Type="Italic"> Pinus densiflora</Emphasis> seedling growth in vitro

While it has been suggested that Matsutake mycorrhizae might not be functional and that Matsutake may behave as a saprobic fungus in soil or even have some pathogenic activity on seedlings, we investigated the consequences of Matsutake inoculation on Pinus densiflora growth. Seventy-five days after inoculation, hyphae were anchored on short roots and well-developed Hartig net palmettis were observed. Compared to both control treatments—seedlings treated with distilled water and seedlings treated with autoclaved mycelium—inoculation significantly stimulated seedling total dry weight by 70.9% and 98.0%, respectively. These findings attest that some type of symbiotic relationship must be functional and favour host growth, ruling out claims of pathogenicity under the sterile conditions used here.     

Effect of the excretory-secretory products of some marine invertebrates on byssus production of the blue mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> (Bivalvia: Mytilidae)

Under laboratory conditions, we investigated byssus production in the blue mussel Mytilus edulis, as affected by the excretory-secretory products (ESPs) of the mussel itself and some marine invertebrates: the predatory starfish Asterias rubens, and organisms competing with mussels in White Sea fouling communities—a bivalve Hiatella arctica, the solitary ascidians Styela rustica and Molgula citrine, and a sponge Halichondria panicea. The number of attachment disks produced by a mussel per day and the thickness of byssal threads were estimated. Excretory-secretory products of H. arctica and M. citrine had no effect on the number of attachment disks, while ESPs of S. rustica, H. panacea and A. rubens stimulated mussels to produce attachment plaques. The activity of the mussel was slightly increased at low levels of its own ESPs in seawater. The thickness of byssal threads decreased with an increase in the ESPs of mussels in seawater, but it increased in experiments with the ESPs of any other species tested.     

芭蕉芋叶片结构特征及其与抗旱性的关系

运用石蜡切片法和指甲油印迹法,对3个芭蕉芋品种的叶片解剖结构——叶片厚度、叶肉厚度、上表皮厚度、下表皮厚度、气孔密度、气孔长度及宽度、长细胞长度及宽度、短细胞长度及宽度等10项相关指标进行测定,分析芭蕉芋抗旱性与叶片结构之间的关系。结果表明:(1)芭蕉芋的表皮结构与抗旱性有一定的关系,表现为抗旱性强的品种气孔密度大,且长、短细胞体积小、排列紧密。(2)芭蕉芋叶片、叶肉、上下表皮厚度与抗旱性关系在不同品种间差异不显著。(3)芭蕉芋叶肉及上、下表皮占叶片横切面的比例与抗旱性存在显著相关关系。(4)芭蕉芋叶片保水能力与叶肉比例呈极显著负相关关系,与上表皮比例呈极显著正相关关系。     

Seasonal variations in moisture use in a piñon–juniper woodland

In water-limited environments of the intermountain region of North America, summer precipitation may play a role in the structure and function of aridland communities and ecosystems. This study examined the potential reliance on summer precipitation of two widespread, coexisting woody species in the southwestern United States, Pinus edulis Englmn. (Colorado piñon) and Juniperus osteosperma (Torr) Little (Utah juniper). The current distributions of P. edulis and J. osteosperma are highly suggestive of different dependencies on summer rainfall. We hypothesized that P. edulis was dependent on summer precipitation, utilizing summer precipitation even during extremely dry summers, whereas J. osteosperma was not dependent, using summer precipitation only when amounts were above some minimum threshold. Using sap flux and stable isotopic methods to assess seasonal water sources and water use efficiency, we examined the response of these two species to seasonal variations in moisture at a site located near the northern limits of the North American monsoon. Both sap flux and isotopic results indicated that P. edulis was responsive to summer rain, while J. osteosperma was not. Following summer rain events, sap flux density increased in P. edulis for several days, but not in J. osteosperma. Isotopic evidence indicated that P. edulis took up summer-derived moisture to a greater extent than J. osteosperma. Values of the natural abundance stable isotope ratio of carbon of leaf soluble carbohydrates increased over the summer for P. edulis, indicative of assimilation at higher water use efficiency, but were invariant for J. osteosperma. Our results supported the hypothesis that P. edulis and J. osteosperma are differentially sensitive to summer precipitation and are discussed in the light of potential changes in the seasonality of precipitation associated with climate change.