Cryptic species and population genetic structure of Plasmopara viticola in São Paulo State,Brazil

Downy mildew (Plasmopara viticola) is one of the most important diseases in grape-growing areas worldwide, including Brazil. To examine pathogen population biology and structure, P. viticola was sampled during the 2015/16 growing season from 516 lesions on nine grape cultivars in 11 locations in subtropical areas of São Paulo State, Brazil. For identification of cryptic species, a subsample of 130 isolates was subjected to cleaved amplified polymorphic sequence (CAPS) analysis, and for 91 of these isolates the ITS1 region was sequenced. These analyses suggest that the population of P. viticola in São Paulo State consists of a single cryptic species, P. viticola clade aestivalis. Seven microsatellite markers were used to determine the genetic structure of all 516 P. viticola isolates, identifying 23 alleles and 55 multilocus genotypes (MLGs). Among these MLGs, 34.5% were clonal and represented 93% of the isolates sampled. Four dominant genotypes were present in at least five different locations, corresponding to 65.7% of the isolates sampled. Genotypic diversity (Ĝ = 0.21–0.89) and clonal fraction (0.58–0.96) varied among locations (populations). Most populations showed significant deviation from Hardy–Weinberg expectations; in addition, excess of heterozygosity was verified for many loci. However, principal coordinate analysis revealed no clusters among locations and no significant isolation by distance was found, suggesting high levels of migration. The results indicate that downy mildew epidemics result from multiple clonal infections caused by a few genotypes of P. viticola, and reproduction of P. viticola in São Paulo State is predominantly asexual.     

Temporal disease dynamics and relative importance of sexual and asexual reproduction of grape downy mildew (Plasmopara viticola) in an isolated vineyard in the North Georgia Mountains,USA

The North Georgia Mountains are the southernmost region along the United States East Coast where European wine grapes (Vitis vinifera) are grown commercially. Epidemics of downy mildew, caused by Plasmopara viticola, are frequent and severe, but little is known about the epidemiology and population biology of the pathogen in this region. Disease monitoring in an experimental vineyard from 2015 to 2017 indicated that times of disease onset and progress rates were highly variable across years and cultivars. Oospores were observed microscopically, and simulation with a process-based model indicated presence of conditions favourable for oospore germination in the spring and early summer each year. A total of 409 P. viticola isolates collected over three  years were genotyped with seven microsatellite markers, revealing very high genotypic diversity, which when combined with the observation of oospores is indicative of a sexually reproducing population. Among the 409 isolates, 225 multilocus genotypes (MLGs) were identified, of which 164 were detected only once and 61 were repeated (clonal). Eight MLGs (represented by 28 isolates) were detected across years, suggesting the possibility of asexual overwintering of P. viticola in this region. Across sampling dates, the percentage of isolates belonging to nonrepeated (unique) MLGs ranged from 27.3% to 63.2%. Even towards the end of the annual epidemic, the percentage of isolates in nonrepeated MLGs was still relatively high, around 30%. These MLGs may have originated from oospores germinating late during the growing season, although incomplete sampling at earlier dates and contribution by immigration cannot be fully excluded.     

Nitrogen uptake from 15N-enriched fertilizer by four tree crops in an Amazonian agroforest

Mixed tree cropping systems have been proposed for sustainable nutrient management in the humid tropics. Yet, the nutrient interactions between intercropped trees have not been addressed sufficiently. In the present study we compare the temporal and spatial patterns of the uptake of applied ¹⁵N by four different tree crops in a mixed tree cropping system on a Xanthic Ferralsol in central Amazônia, Brazil, during one year. Most of the N uptake occurred during the first two weeks. Very little N was recovered by peach palm (Bactris gasipaes), more by cupuassu (Theobroma grandiflorum) and annatto (Bixa orellana) and most by Brazil nut (Bertholletia excelsa). Due to tree pruning the total accumulation of applied ¹⁵N in the above-ground biomass of annatto decreased throughout the year. It remained constant in cupuassu and peach palm and increased in Brazil nut. Brazil nut showed an extensive root activity and took up more fertilizer N applied to neighboring trees than from the one applied under its own canopy in contrast to the other three tree crops. Therefore, trees with wide-spread root systems may not need to receive N fertilizer directly but can take up N applied to other trees in the mixed cropping system. This means that such trees may effectively decrease N leaching when intercropped with trees that have dormant periods or places with low N uptake, but also exert considerable resource competition.This revised version was published online in November 2005 with corrections to the Cover Date.     

Predicting tree regeneration in <Emphasis Type="Italic">Picea abies</Emphasis> snag stands

A bark beetle (Ips typographus) infestation caused the death of almost all Norway spruce (Picea abies) trees in a mountain forest in the Swiss Alps. We developed a tree regeneration model, ‘RegSnag’ (=REGeneration in a SNAG stand), to project the future amount and height of tree regeneration in these snag stands. The model combines a height-class structured tree module with a microsite-based module of snag decay and ground-vegetation succession. Microsite-specific rates of germination, mortality and height growth were modelled for four tree species (Picea abies, Sorbus aucuparia, Acer pseudoplatanus and Betula pendula) in eight height classes (from seedlings to saplings 5 m tall) and on 26 microsite types (e.g. moss, grass). Model tests with independent field data from 8 years after the Picea die-back demonstrated that microsites had a considerable effect on the development of tree regeneration on both the montane and the subalpine level. With microsite-specific parameters, the height and frequency of Picea in each microsite could be simulated more accurately than without considering microsite effects (e.g. bias of 8 vs. 119 saplings ha⁻¹ on the montane level). Results of simulations 40 years into the future suggest that about 330–930 Picea saplings per ha out of those that germinated in 1994 and 1996 will reach a height of 5 m within 30–35 years after Picea die-back. This is due to differences in seed inflow and browsing intensities. Picea and not Betula or Sorbus trees will replace the current herbaceous vegetation in these snag stands.     

Climate change and nature conservation in Central European forests: A review of consequences, concepts and challenges

With a predicted rise in average global surface temperature at an unprecedented rate, as well as changes in precipitation and disturbance regimes, climate change will bring forth new challenges for nature conservation in forest ecosystems. Species and habitats to be protected will be affected as well as related concepts and area specific objectives. Climate change impacts are likely to be aggravated by other anthropogenic stresses such as fragmentation, deposition or habitat destruction. To be reliable and effective, current objectives and guidelines of forest conservation need to be reassessed and improved. Our study analyses possible impacts of climate change on forests and identifies key future challenges for nature conservation in forests and ecosystem research. We reviewed 130 papers on climate change impacts on forest ecosystems and species published between 1995 and 2010. The geographical focus of the study is Central Europe. Papers were analysed accounting for direct and indirect impacts of gradual changes as well as stochastic disturbance events in forest ecosystems and their possible consequences for nature conservation.Even though broader aspects of nature conservation (protected areas, biodiversity) are frequently mentioned, little attention is given to forest-specific nature conservation. Particular aspects are insufficiently represented, such as the influence of climate change on different forest succession stages, the development of dead wood volume and quality, responses of secondary broadleaved species, azonal or extrazonal forests as well as ancient woodlands or remnants of historical silvicultural systems. Challenges arise in the context of great uncertainties about future developments. Nature conservation concepts and objectives in forests need to be adapted either within a permanent evaluation process or through the inclusion of further changes a priori, even if they are to some extent unpredictable. In some cases adaptation measures within nature conservation (e.g. adjusting protected areas) may conflict with interests of other stakeholders. Further research, particularly on interrelations between different impacts and the adaptive capacity of current forest ecosystems, associated species and existing genotypes is urgently needed. The scale and complexity of the task at hand calls for the establishment and further strengthening of international research networks.     

Weather and human impacts on forest fires: 100 years of fire history in two climatic regions of Switzerland

Understanding the factors driving past fire regimes is crucial in the context of global change as a basis for predicting future changes. In this study, we aimed to identify the impact of climate and human activities on fire occurrence in the most fire-prone regions of Switzerland. We considered forest fires, land use and meteorological data over the period 1904-2008 in the neighboring mountain cantons (states) of Valais and Ticino, which are characterized by distinct climatic regimes.The presence/absence of fire ignitions was analyzed using the Nesterov ignition index (as a proxy for fire weather), road density (for ignition sources), livestock density (for biomass removal), and change in forest area (for fire-prone abandoned agricultural areas).We found that fire weather played a key role in fire occurrence in both regions. Road and livestock densities had similar influences in the two cantons. However, while the increase in forest area was well correlated with fire occurrence in Ticino, no such correlation was evident in Valais, probably because land abandonment and forest cover change have been less extensive there. Our findings emphasize the non-linear nature of the relationships between fire occurrence and anthropogenic drivers, as we found thresholds above which road density was no longer correlated with fire occurrence. This implies that the projected future increase and spatial concentration of the human population may not result in a further increase in fire risk in intermediately to densely populated areas in both cantons.The driving factors behind fire activity differ slightly in the two cantons, in particular with increasing forest area enhancing fire occurrence in Ticino but not in Valais. These differences should be taken into account when assessing future fire risk, especially in Valais where the potential for an increase in the fire-prone area is still high. Fires are likely to become more frequent in a warmer climate, but future fire activity may develop differently in the two cantons. This should be taken into account when planning optimized fire prevention measures. This case study should help to better understand fire activity in highly populated regions where fire activity is still moderate but might markedly increase under a projected more fire-prone climate.     

Räumliche Spannungsmessungen mit dem Stress State Transducer (SST) in ungesättigten aggregierten Böden - theoretische Betrachtungen und erste Ergebnisse

Stress measurements in undisturbed unsaturated soils with a Stress State Transducer (SST) - theory and first results A method to quantify the spatial stress distribution will be introduced and first results will be discussed. This method allows the detailed analysis of principal and shear stresses as well as the determination of the direction angle of principal stresses and the octahedral shear stress angle. The described Stress State Transducer (SST) is composed of six single strain gage sensors that enable the accurate and reproducable recording of stresses in six directions in a wide load range. Their data form the base for calculation of spatial stress distribution. Some first results show that in a luvisol derived from loess wheeling at a wheel load of 4.0 Mg induces high shear stresses in a depth of 30 cm. This probably causes plastic soil deformation.     

Effect of soil compaction on hydraulic properties of two loess soils in China

Soil compaction affects hydraulic properties, and thus can lead to soil degradation and other adverse effects on environmental quality. This study evaluates the effects of three levels of compaction on the hydraulic properties of two silty loam soils from the Loess Plateau, China. Undisturbed soil cores were collected from the surface (0–5 cm) and subsurface (10–15 cm) layers at sites in Mizhi and Heyang in Shaanxi Province. The three levels of soil compaction were set by increasing soil bulk density by 0% (C0), 10% (C1) and 20% (C2) through compression and hammering in the laboratory. Soil water retention curves were then determined, and both saturated hydraulic conductivity (K_s) and unsaturated hydraulic conductivity were estimated for all of the samples using standard suction apparatus, a constant head method and the hot-air method, respectively. The high level of compaction (C2) significantly changed the water retention curves of both the surface and subsurface layers of the Heyang soil, and both levels of compaction (C1 and C2) changed the curves of the two layers from the Mizhi site. However, the effects of compaction on the two soils were only pronounced below water tensions of 100 kPa. Saturated hydraulic conductivities (K_s) were significantly reduced by the highest compaction level for both sampled layers of the Heyang soil, but no difference was observed in this respect between the C0 and C1 treatments. K_s values decreased with increasing soil compaction for both layers of the Mizhi soil. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range, and the values obtained were two to five orders of magnitude higher for the Mizhi soil than for the Heyang soil. The results indicate that soil compaction could strongly influence, in different ways, the hydraulic properties of the two soils.