Inter-annual and seasonal variability of radial growth, wood density and carbon isotope ratios in tree rings of beech (Fagus sylvatica) growing in Germany and Italy

We investigated the variability of tree-ring width, wood density and ¹³C/¹²C in beech tree rings (Fagus sylvatica L.), and analyzed the influence of climatic variables and carbohydrate storage on these parameters. Wood cores were taken from dominant beech trees in three stands in Germany and Italy. We used densitometry to obtain density profiles of tree rings and laser-ablation-combustion-GC-IRMS to estimate carbon isotope composition (δ ¹³C) of wood. The sensitivity of ring width, wood density and δ ¹³C to climatic variables differed; with tree-ring width responding to environmental conditions (temperature or precipitation) during the first half of a growing season and maximum density correlated with temperatures in the second part of a growing season (July–September). δ ¹³C variations indicate re-allocation and storage processes and effects of drought during the main growing season. About 20% of inter-annual variation of tree-ring width was explained by the tree-ring width of the previous year. This was confirmed by δ ¹³C of wood which showed a contribution of stored carbohydrates to growth in spring and a storage effect that competes with growth in autumn. Only mid-season δ ¹³C of wood was related to concurrent assimilation and climate. The comparison of seasonal changes in tree-ring maximum wood density and isotope composition revealed that an increasing seasonal water deficit changes the relationship between density and ¹³C composition from a negative relation in years with optimal moisture to a positive relationship in years with strong water deficit. The climate signal, however, is over-ridden by effects of stand density and crown structure (e.g., by forest management). There was an unexpected high variability in mid season δ ¹³C values of wood between individual trees (−31 to −24‰) which was attributed to competition between dominant trees as indicated by crown area, and microclimatological variations within the canopy. Maximum wood density showed less variation (930–990 g cm⁻³). The relationship between seasonal changes in tree-ring structure and ¹³C composition can be used to study carbon storage and re-allocation, which is important for improving models of tree-ring growth and carbon isotope fractionation. About 20–30% of the tree-ring is affected by storage processes. The effects of storage on tree-ring width and the effects of forest structure put an additional uncertainty on using tree rings of broad leaved trees for climate reconstruction.     

WHY DO PARASITIC CUCKOOS HAVE SMALL BRAINS? INSIGHTS FROM EVOLUTIONARY SEQUENCE ANALYSES

Brain size is under many opposing selection pressures. Estimating their relative influence and reconstructing the brain's evolutionary history have, however, proved difficult. Here, we confirm the suggestion that the brain of brood parasitic cuckoos is smaller in relation to their body weight than that of nonparasitic cuckoo species. Two hypotheses explaining reductions in brain size are tested, using phylogenetically controlled correlations and evolutionary pathway analyses. In a novel approach, the pathway models are combined to build the most likely evolutionary sequence of trait changes correlating with changes in brain size. Brain size changed before brood parasitism, followed by a shift toward less-productive habitats and an increase in migration. This sequence shows that brain size was not reduced as a consequence of a loss of cognitive skills related to chick provisioning, and it offers no support for the hypothesis that an increase in energetic demands or a reduction in energy availability selected for a reduction of brain size. Instead, the sequence suggests that the reduction in energetic demands due to the smaller brain size and parasitic breeding strategy may have enabled parasitic cuckoos to colonize new niches.     

The Effect of Biological Soil Crusts on Throughput of Rainwater and N into Lake Michigan Sand Dune Soils

Biological soil crusts composed of cyanobacteria, green algae, bryophytes, and lichens colonize soils in arid and semiarid ecosystems worldwide and are responsible for significant N input to the soils of these ecosystems. Soil crusts also colonize active sand dunes in more humid regions, but studies of structure and function of such sand dune crusts are lacking. We identified the cyanobacterial, algal, and bryophytic constituents and N production and leachates of biological soil crusts that colonize beach dunes at the Indiana Dunes National Lakeshore along southern Lake Michigan in Indiana, USA. To determine the role of these crusts in this system, we conducted a greenhouse experiment in which intact soil cores with biological crusts were subjected to artificial rainfall over a full growing season. The volume and N content of leachate from the cores were quantified in relation to degree of crust development, taxonomic composition, rainfall volume and intensity, light intensity, and the presence of plant litter. Net N throughput significantly exceeded N inputs to cores in rainwater. Net N outputs from crusts to subsurface soil ranged from 0. 01 to 0.19 g NH ₄ ⁺ -N m⁻² yr⁻¹ and 0.01 to 0.61 g NO ₃ ^– N m⁻² yr⁻¹. Thus, total inorganic N inputs associated with biological soil crusts ranged from 0.02 g N m⁻² yr⁻¹ to 0.8 g N m⁻² yr⁻¹. High volume (≥2 cm) rainfall resulted in more N leaching than low volume events, and plant litter added over the surface of crusted soil cores significantly increased the amount of N in leachate. Exploratory path analysis revealed direct and indirect linkages among environmental factors, crust development, and crust composition in regulating the throughput of H₂O and N from these intact soil cores. Biological soil crusts at this site, combined with other properties of the soil surface, substantially increase N inputs to this water- and nutrient-limited sand dune ecosystem.     

Mechanical Restoration of California Mixed‐Conifer Forests: Does it Matter Which Trees Are Cut?

The montane ecosystems of northern California have been subjected to repeated manipulation and active fire suppression for over a century, resulting in changes in community structure that contribute to increased wildfire hazard. Ecosystem restoration via reduction of stand density for wildfire hazard mitigation has received substantial attention in recent years; however, many ecological questions remain unanswered. This study compares belowground effects of two alternative forest thinning treatments designed to restore the large, old tree component of late‐seral structure, one of which focuses on restoring Pinus ponderosa dominance (Pine‐preference) and the other of which promotes development of large trees regardless of species (Size‐preference). We evaluated forest floor and soil chemical and microbial parameters in six experimental thinning treatment units of 40 ha each in the Klamath National Forest of northern California 5–6 years after thinning. Inorganic N availability, soil organic C content, phenol oxidase activity, and forest floor C:N ratio were greater in the Size‐preference treatment, whereas forest floor N and soil pH were greater in the Pine‐preference treatment. Our results indicate that these two thinning strategies produce differences in the soil environment that has the potential to affect growth rates of trees that remain, as well as the growth and survivorship of newly established seedlings. Thus, which species/individuals are removed during structural restoration of these mixed‐conifer forests matters both to the belowground components of the ecosystem today and the vegetation and productivity of the ecosystem in future decades.     

Lipid raft localization of EGFR alters the response of cancer cells to the EGFR tyrosine kinase inhibitor gefitinib

Epidermal growth factor receptor (EGFR) is overexpressed in many cancer types including ~30% of breast cancers. Several small molecule tyrosine kinase inhibitors (TKIs) targeting EGFR have shown clinical efficacy in lung and colon cancers, but no benefit has been noted in breast cancer. Thirteen EGFR expressing breast cancer cell lines were analyzed for response to EGFR TKIs. Seven were found to be EGFR TKI resistant; while shRNA knockdown of EGFR determined that four of these cell lines retained the requirement of EGFR protein expression for growth. Interestingly, EGFR localized to plasma membrane lipid rafts in all four of these EGFR TKI-resistant cell lines, as determined by biochemical raft isolation and immunofluorescence. When lipid rafts were depleted of cholesterol using lovastatin, all four cell lines were sensitized to EGFR TKIs. In fact, the effects of the cholesterol biosynthesis inhibitors and gefitinib were synergistic. While gefitinib effectively abrogated phosphorylation of Akt- and mitogen-activated protein kinase in an EGFR TKI-sensitive cell line, phosphorylation of Akt persisted in two EGFR TKI-resistant cell lines, however, this phosphorylation was abrogated by lovastatin treatment. Thus, we have shown that lipid raft localization of EGFR correlates with resistance to EGFR TKI-induced growth inhibition and pharmacological depletion of cholesterol from lipid rafts decreases this resistance in breast cancer cell lines. Furthermore, we have presented evidence to suggest that when EGFR localizes to lipid rafts, these rafts provide a platform to facilitate activation of Akt signaling in the absence of EGFR kinase activity.     

Arbuscular mycorrhizal development in three crucifers

To determine the developmental patterns of arbuscular mycorrhizae (AM) in three crucifers (Brassicaceae) of differing life histories, we inoculated seedlings of the annual Capsella bursa-pastoris, biennial Hesperis matronalis, and the perennial Matthiola incana with Glomus intraradices. The plants were grown either alone or in a matrix of living roots of the mycotrophic grass Sorghum sudanense. The percent root length colonized was greatest in C. bursa-pastoris and least in H. matronalis. Colonization was greater in plants grown in the grass matrix than in plants grown alone, and colonization in grass matrix-grown plants continued to increase over the 90-day growth period, whereas colonization leveled off or decreased near the end of the growth period in crucifers grown alone. No arbuscules were observed in crucifer roots at any time, which suggests that AM in these crucifers is nonfunctional. Furthermore, the increase in colonization only in pots with both crucifers and active mycotrophic roots suggests that AM development in crucifer roots is primarily the consequence of progressive root senescence in the crucifer and continued inoculum spread from the mycotrophic plant.     

Foliar Nutrient Dynamics and Resorption in Naturalized Lonicera maackii (Caprifoliaceae) Populations in Ohio,USA

Foliar phosphorus (P) and nitrogen (N) concentrations and nutrient resorption in the forest understory shrub Lonicera maackii (Rupr.) Maxim (Caprifoliaceae) were measured along contiguous topographic gradients in two southwestern Ohio forests during 1992–1994. Mean summer foliar P varied significantly among topographic positions (but not sites or years), with uplands having greatest P concentrations and bottomlands exhibiting the lowest. Unlike for P, the mean summer foliar N concentrations varied little among sites, topographic positions, and years. Mean absolute and proportional P resorption ranged from 0.48 mg/dm² (33.7%) in slope positions to 0.80 mg/dm² (53.1%) in bottomland positions. Repeated-measures analysis of variance (RMANOVA) for P resorption indicated significant topographic and year effects, a site × year and a site × year × topographic interaction. Mean absolute and proportional foliar N resorption ranged from 6.82 mg/dm² (30.7%) in bottomlands to 8.41 mg/dm² (37.3%) in slope positions. RMANOVA indicated a significant topographic effect for both absolute and proportional N resorption and a significant year effect for absolute N resorption. These significant year effects for P and N stemmed from lowest resorption of nutrients in 1993. The results for P resorption support the hypothesis that foliar resorption is greater in forested sites with lower P fertility. However, resorption rates for N did not support the hypothesis clearly, as slopes with intermediate N availability had greater N resorption rates than did N-rich bottomlands.     

Spatial patterns of mycorrhizal infectiveness of soils long a successional chronosequence

 This study quantified intersite variation and spatial pattern in arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) infectivity of soils among six sites constituting a successional chronosequence in southwestern Ohio, USA. The study sites included an active agricultural field (chronic disturbance), a site which had been stripped of its surface soil (pseudo-stripmine, acute disturbance), 5- and 10-year-old fields, a 25- to 30-year-old prairie restoration, and an undisturbed, mature forest. AM infectivity was lower in the agricultural field, successional fields, and prairie than in the mature forest, but there was no clear correlation between time since disturbance and the overall level of AM infectivity. Spatial structure in AM infectivity decreased with time since disturbance. In the pseudo-stripmine site and active soybean field, semivariance analysis attributed 44–50% of the total variance in AM infectivity among samples to spatial structure, whereas spatial dependancy accounted for only 18% of total variance in the mature forest. Kriging of AM infectiveness demonstrated small, isolated areas in the disturbed plots that were devoid of AM infectiveness, whereas the kriged AM maps of the other four sites showed AM infectiveness to become progressively more homogeneous. ECM infectiveness was lacking from 35–50% of the samples from the disturbed sites, and both overall ECM infectiveness and ECM diversity increased with time since disturbance. Approximately 44% of the variance in ECM infectiveness was related to spatial structure in the two disturbed sites, and large areas entirely devoid of ECM infectivity were present on the kriged ECM maps for these sites. There was less spatial structure in ECM in the old fields and prairie and very little in the mature forest. The results of this study emphasize the need to explicitly evaluate spatial heterogeneity in mycorrhizal infectivity in studies of the role of mycorrhizae in succession. Accepted: 4 August 1995