Mineral control of organic carbon mineralization in a range of temperate conifer forest soils

Coupled climate–ecosystem models predict significant alteration of temperate forest biome distribution in response to climate warming. Temperate forest biomes contain approximately 10% of global soil carbon (C) stocks and therefore any change in their distribution may have significant impacts on terrestrial C budgets. Using the Sierra Nevada as a model system for temperate forest soils, we examined the effects of temperature and soil mineralogy on soil C mineralization. We incubated soils from three conifer biomes dominated by ponderosa pine (PP), white fir (WF), and red fir (RF) tree species, on granite (GR), basalt (BS), and andesite (AN) parent materials, at three temperatures (12.5°C, 7.5°C, 5.0°C). AN soils were dominated by noncrystalline materials (allophane, Al‐humus complexes), GR soils by crystalline minerals (kaolinite, vermiculite), and BS soils by a mix of crystalline and noncrystalline materials. Soil C mineralization (ranging from 1.9 to 34.6 [mg C (g soil C)^?1] or 0.1 to 2.3 [mg C (g soil)^?1]) differed significantly between parent materials in all biomes with a general pattern of ANδ¹³C values of respired CO₂ suggest greater decomposition of recalcitrant soil C compounds with increasing temperature, indicating a shift in primary C source utilization with temperature. Our results demonstrate that soil mineralogy moderates soil C mineralization and that soil C response to temperature includes shifts in decomposition rates, mineralizable pool size, and primary C source utilization.     

Litter type and soil minerals control temperate forest soil carbon response to climate change

Temperate forest soil organic carbon (C) represents a significant pool of terrestrial C that may be released to the atmosphere as CO₂ with predicted changes in climate. To address potential feedbacks between climate change and terrestrial C turnover, we quantified forest soil C response to litter type and temperature change as a function of soil parent material. We collected soils from three conifer forests dominated by ponderosa pine (PP; Pinus ponderosa Laws.); white fir [WF; Abies concolor (Gord. and Glend.) Lindl.]; and red fir (RF; Abies magnifica A. Murr.) from each of three parent materials, granite (GR), basalt (BS), and andesite (AN) in the Sierra Nevada of California. Field soils were incubated at their mean annual soil temperature (MAST), with addition of native ¹³C‐labeled litter to characterize soil C mineralization under native climate conditions. Further, we incubated WF soils at PP MAST with ¹³C‐labeled PP litter, and RF soils at WF MAST with ¹³C‐labeled WF litter to simulate a migration of MAST and litter type, and associated change in litter quality, up‐elevation in response to predicted climate warming. Results indicated that total CO₂ and percent of CO₂ derived from soil C varied significantly by parent material, following the pattern of GR>BS>AN. Regression analyses indicated interactive control of C mineralization by litter type and soil minerals. Soils with high short‐range‐order (SRO) mineral content exhibited little response to varying litter type, whereas PP litter enriched in acid‐soluble components promoted a substantial increase of extant soil C mineralization in soils of low SRO mineral content. Climate change conditions increased soil C mineralization greater than 200% in WF forest soils. In contrast, little to no change in soil C mineralization was noted for the RF forest soils, suggesting an ecosystem‐specific climate change response. The climate change response varied by parent material, where AN soils exhibited minimal change and GR and BS soils mineralized substantially greater soil C. This study corroborates the varied response in soil C mineralization by parent material and highlights how the soil mineral assemblage and litter type may interact to control conifer forest soil C response to climate change.     

Bird richness and composition along an agricultural gradient in New Guinea: The influence of land use,habitat heterogeneity and proximity to intact forest

Abstract We examined variation in bird species richness, abundance and guild composition along an agricultural gradient in New Guinea, and looked for any additive influence of habitat heterogeneity on these variables. The study was based on a grid of survey plots, six plots wide and 24 plots long with the long axis running from a settlement 2.4 km through active and abandoned agricultural plots towards a large area of forest. Each circular survey plot (25 m radius) was assigned to a broad habitat type, ten habitat measures taken, and birds counted for 1 h in each plot. Principal component analysis (PCA) habitat axis 1 described an axis of decreasing forest alteration (larger trees, greater tree densities, fuller canopy) that was positively correlated with distance from the settlement. Bird richness and abundance were highest at intermediate disturbance levels (plots with mid‐range axis 1 scores). Proportions of insectivores and frugivores increased with decreasing forest alteration, while proportions of nectarivores decreased. We calculated three measures of habitat heterogeneity by comparing each plot's PCA score to those of eight neighbouring plots (50–110 m away). These measures reflected how different the plot was to its neighbours, how variable the habitat was around the plot, and the degree to which the plot bordered less disturbed forest. We related these measures to plot bird variable scores independently, and to residuals following regressions of bird scores against PCA scores. Heterogeneity measures had no significant influence on abundance or richness measures, but there were greater proportions of frugivores in plots showing a given degree of habitat alteration if they bordered more pristine habitat. While we readily identified differences in bird communities along the agricultural gradient, the influences of habitat heterogeneity were not striking for birds at this fine scale.     

Effects of antagonistic coevolution on parasite‐mediated host coexistence

Parasites can promote diversity by mediating coexistence between a poorer and superior competitor, if the superior competitor is more susceptible to parasitism. However, hosts and parasites frequently undergo antagonistic coevolution. This process may result in the accumulation of pleiotropic fitness costs associated with host resistance, and could breakdown coexistence. We experimentally investigated parasite‐mediated coexistence of two genotypes of the bacterium Pseudomonas fluorescens, where one genotype underwent coevolution with a parasite (a virulent bacteriophage), whereas the other genotype was resistant to the evolving phages at all time points, but a poorer competitor. In the absence of phages, the resistant genotype was rapidly driven extinct in all populations. In the presence of the phages, the resistant genotype persisted in four of six populations and eventually reached higher frequencies than the sensitive genotype. The coevolving genotype showed a reduction in the growth rate, consistent with a cost of resistance, which may be responsible for a decline in its relative fitness. These results demonstrate that the stability of parasite‐mediated coexistence of resistant and susceptible species or genotypes is likely to be affected if parasites and susceptible hosts coevolve.     

Assessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analyses

Soil is the largest reservoir of organic carbon (C) in the terrestrial biosphere and soil C has a relatively long mean residence time. Rising atmospheric carbon dioxide (CO₂) concentrations generally increase plant growth and C input to soil, suggesting that soil might help mitigate atmospheric CO₂ rise and global warming. But to what extent mitigation will occur is unclear. The large size of the soil C pool not only makes it a potential buffer against rising atmospheric CO₂, but also makes it difficult to measure changes amid the existing background. Meta‐analysis is one tool that can overcome the limited power of single studies. Four recent meta‐analyses addressed this issue but reached somewhat different conclusions about the effect of elevated CO₂ on soil C accumulation, especially regarding the role of nitrogen (N) inputs. Here, we assess the extent of differences between these conclusions and propose a new analysis of the data. The four meta‐analyses included different studies, derived different effect size estimates from common studies, used different weighting functions and metrics of effect size, and used different approaches to address nonindependence of effect sizes. Although all factors influenced the mean effect size estimates and subsequent inferences, the approach to independence had the largest influence. We recommend that meta‐analysts critically assess and report choices about effect size metrics and weighting functions, and criteria for study selection and independence. Such decisions need to be justified carefully because they affect the basis for inference. Our new analysis, with a combined data set, confirms that the effect of elevated CO₂ on net soil C accumulation increases with the addition of N fertilizers. Although the effect at low N inputs was not significant, statistical power to detect biogeochemically important effect sizes at low N is limited, even with meta‐analysis, suggesting the continued need for long‐term experiments.     

A test for Allee effects in the self‐incompatible wasp‐pollinated milkweed Gomphocarpus physocarpus

It has been suggested that plants that are good colonizers will generally have either an ability to self‐fertilize or a generalist pollination system. This prediction is based on the idea that these reproductive traits should confer resistance to Allee effects in founder populations and was tested using Gomphocarpus physocarpus (Asclepiadoideae: Apocynaceae), a species native to South Africa that is invasive in other parts of the world. We found no significant relationships between the size of G. physocarpus populations and various measures of pollination success (pollen deposition, pollen removal and pollen transfer efficiency) and fruit set. A breeding system experiment showed that plants in a South African population are genetically self‐incompatible and thus obligate outcrossers. Outcrossing is further enhanced by mechanical reconfiguration of removed pollinaria before the pollinia can be deposited. Self‐pollination is reduced when such reconfiguration exceeds the average duration of pollinator visits to a plant. Observations suggest that a wide variety of wasp species in the genera Belonogaster and Polistes (Vespidae) are the primary pollinators. We conclude that efficient pollination of plants in small founding populations, resulting from their generalist wasp‐pollination system, contributes in part to the colonizing success of G. physocarpus. The presence of similar wasps in other parts of the world has evidently facilitated the expansion of the range of this milkweed.     

Population Growth and Demography of Common Loons in the Northern United States

ABSTRACT We used recent developments in theoretical population ecology to construct basic models of common loon (Gavia immer) demography and population dynamics. We parameterized these models using existing survival estimates and data from long-term monitoring of loon productivity and abundance. Our models include deterministic, 2-stage, density-independent matrix models, yielding population growth-rate estimates (λ) of 0.99 and 1.01 for intensively studied populations in our Wisconsin, USA, and New Hampshire, USA, study areas, respectively. Perturbation analysis of these models indicated that estimated growth rate is extremely sensitive to adult survival, as expected for this long-lived species. Also, we examined 20 years of count data for the 2 areas and evaluated support for a set of count-based models of population growth. We detected no temporal trend in Wisconsin, which would be consistent with fluctuation around an average equilibrium state but could also result from data limitations. For New Hampshire, the model set included varying formulations of density dependence and partitioning of stochasticity that were enabled by the annual sampling resolution. The best model for New Hampshire included density regulation of population growth and, along with the demographic analyses for both areas, provided insight into the possible importance of breeding habitat availability and the abundance of nonbreeding adults. Based on these results, we recommend that conservation organizations include nonbreeder abundance in common loon monitoring efforts and that additional emphasis be placed on identifying and managing human influences on adult loon survival.     

The ecological differentiation of asexual lineages of cotton aphids: alate behaviour, sensory physiology, and differential host associations

The cotton aphid ( Aphis gossypii Glover) is a cosmopolitan pest species consisting of various host plant-associated lineages that appear to have different patterns of insecticide resistance, electrophoretic banding, and reproductive output across alternative host species. Debate continues as to whether these differences represent intraspecific variation or indicate a complex of cryptic or sibling species. We studied the host plant-related ecology of two asexual lineages of A. gossypii associated with the families Malvaceae and Cucurbitaceae, respectively, in particular the host plant finding and acceptance processes of the alate forms. The two lineages proved to be differentiated across host plants in terms of: (1) their landing and settling behaviour, nymphal production rate, and subsequent colony establishment (i.e. nymphal survival and development) and (2) their sensory abilities in relation to host plant-derived odours. Furthermore, genetic structuring according to host plant type was detected using the gene for elongation factor 1-α. The magnitude of the genetic differentiation across lineages was low, perhaps indicating a relatively recent divergence. The results demonstrate: (1) the presence of (at least) two cryptic species within the current definition of A. gossypii , as anticipated in previous studies, and (2) the practicalities of using a multidisciplinary approach to understanding the 'limits' of asexual species. If sexuals can be generated from each of these lineages, this conclusion will be open to further testing.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 503–519.     

Self-assembling diblock copolymers of poly[N-(2-hydroxypropyl)methacrylamide] and a beta-sheet peptide

The self-assembly of hybrid diblock copolymers composed of poly(HPMA) and beta-sheet peptide P11 (CH(3)CO-QQRFQWQFEQQ-NH(2)) blocks was investigated. Copolymers were synthesized via thiol-maleimide coupling reaction, by conjugation of semitelechelic poly(HPMA)-SH with maleimide-modified beta-sheet peptide. As expected, CD and CR binding studies showed that the peptide block imposed its beta-sheet structural arrangement on the structure of diblock copolymers. TEM and AFM proved that peptide and these copolymers had the ability to self-assemble into fibrils.