Higher susceptibility of beech to drought in comparison to oak

The expected increase in drought severity and frequency as a result of anthropogenic climate change leads to concerns about the ability of native tree species to cope with these changes. To determine the susceptibility of Fagus sylvatica (European beech) and Quercus robur (pedunculate oak) – the two dominant deciduous tree species in Central Europe – to drought, we quantified the climate sensitivity and drought-response of radial growth for both species using an array of dendroecological methods. Tree-ring data were collected from a site east of Coburg, Bavaria which had shown pronounced stress-symptoms (early leaf coloration) during the record drought of 2018. Climate-growth relationships were used to establish the sensitivity of radial growth to multiple climatic variables. The impact of specific drought events on tree growth was quantified using tolerance indices. In addition, we employed a Principal Component Gradient Analysis (PCGA) and remote sensing data (MODIS Normalized Difference Vegetation Index (NDVI)) to delineate the species specific drought responses. Using these methods we were able to show a clear difference in drought susceptibility between beech and oak. Beech displayed a higher sensitivity to temperature and the standardized precipitation evapotranspiration index (SPEI) and showed lower resistance and resilience to drought events than oak. In particular, beech was unable to fully recover from the 2003 drought, after which it expressed a stark growth decline, i.e. drought legacies, which was not observed for oak. The PCGA revealed a clear differentiation in the grouping of drought responses between beech and oak, supporting the findings of the climate-growth analysis and the tolerance indices. Correlations of NDVI and ring-width indices (RWI) indicated that under normal climatic conditions NDVI variability is linked to the start of the growing season. This is in contrast to drought years, such as 2003, where summer NDVI mirrored the drought response of beech and oak. These results reveal beech to have both a higher sensitivity to summer temperature and SPEI and a higher susceptibility to drought events. Although, in the past high plasticity and adaptability to drought have been attributed to both beech and oak, our study assigns beech a higher risk than oak to suffer from anticipated increases in drought frequency and intensity as a consequence of climate change.     

Quantifying massive outbreaks of the defoliator moth Ormiscodes amphimone in deciduous Nothofagus‐dominated southern forests using remote sensing time series analysis

Forest insect outbreaks are one of the major biotic disturbances on natural and artificial landscapes. Although abundant literature of insect outbreaks exists in the Northern Hemisphere, studies for the Southern Hemisphere are rare. Recently, massive outbreaks of the native moth Ormiscodes amphimone (Fabricius) (Lepidoptera: Hemileucinae) have been reported in the southern cone of South America. These O. amphimone outbreaks have defoliated large areas of temperate forests, raising great concern among local inhabitants, but yet the spatio‐temporal patterns of these events have not been evaluated. Here, we quantify the extension of the massive O. amphimone outbreaks occurred in the Aysén region (southern Chile) in the period 2000–2015 using a novel remote sensing approach and field data. Remote sensing detections were strongly in agreement with field observations and showed that massive outbreaks of O. amphimone are among the largest biotic disturbances in the forests of the Southern Hemisphere. Considering only field‐confirmed outbreaks, the defoliated area reached 164,000 hectares in total between 2000 and 2015. The estimation of the spatial impact of O. amphimone, and its recurrence, represents the first step for the search of management alternatives of this massive disturbance.     

Changes in the community of ectomycorrhizal fungi and increased fine root number under adult beech trees chronically fumigated with double ambient ozone concentration

Forest soils are an important but under-studied part of forest ecosystems. The effects of O(3) on below-ground processes in a mature forest have only received limited attention so far. In our study, we have analysed the community of ectomycorrhizal fungi and beech fine root dynamics over two growing seasons (2003 - 2004) in a 70-year old mixed spruce-beech forest stand, in which two groups of five adult beech trees were either fumigated by 2 x ambient ozone concentration or used as control. The main difference between previous studies and our approach was that previous studies were performed on seedlings in pot experiments or in closed or open top chambers, and not IN SITU, in a mature forest stand. Although beech is a relatively unresponsive species to tropospheric O(3), we found a pronounced effect of 2 x O(3) on the number of vital ectomycorrhizal root tips and non-turgescent fine roots. Both categories of roots were significantly increased when compared to controls in two consecutive years at each sampling event. The number of types of ectomycorrhizae and species richness increased in 2004, but not in the extremely dry year 2003. We hypothesised that the observed changes might be an expression of a transitional state in below-ground succession of niches caused by an O(3) induced effect on carbon allocation to roots and the rhizosphere. We have detected changes in ectomycorrhizal species level, however Shannon-Weavers species diversity index and percentage of types of ectomycorrhizae did not change significantly in any sampling year thus indicating our results cannot be unequivocally explained by summer drought in year 2003 or by O(3) exposure alone.     

Ecological consequences of shelter sharing by leaf-tying caterpillars

A wide variety of insect herbivores construct and inhabit leaf shelters (ties, rolls, folds, and webs). Shelter construction can lead to a high rate of secondary occupation by other arthropods, including other species of constructors. The consequences for the inhabitants of secondarily occupying these shelters are currently unknown. In this study, we conducted field experiments to examine the fitness consequences (survival and attack by natural enemies) for caterpillars that (i) occupy a shelter with conspecifics vs. occur singly; and (ii) establish a new shelter vs. colonize a pre‐existing one. In addition, we conducted factorial laboratory experiments to test the hypothesis that caterpillars sharing shelters with conspecifics might have reduced construction costs (a potential benefit of shelter‐sharing or secondary occupation). Larvae of Psilocorsis quercicella Clemens (Lepidoptera: Oecophoridae) placed in white oak [Quercus alba L. (Fagaceae)] leaf ties alone or in groups of three had equal likelihood of survival from natural enemies. This same caterpillar species, however, had a higher disappearance rate when placed in pre‐existing leaf ties than when placed in newly formed ones, suggesting a potential cost of secondary colonization. A similar experiment with a closely related species [Psilocorsis cryptolechiella (Chambers)], however, failed to detect a cost of secondarily occupying shelters made on beech, Fagus grandifolia Ehrh. (Fagaceae). In the laboratory experiment, we found no evidence of shelter‐sharing benefits; rather larvae reared in shelters in groups of three had lower pupal mass (and thus lower potential fecundity) than larvae reared singly, suggesting a cost of shelter sharing. Moreover, groups of larvae forced to repeatedly construct new shelters tended to have reduced survival relative to the other treatment, suggesting that energetic constraints are more likely to reduce fitness when larvae cohabit shelters. Taken together, these results indicate that the common phenomenon of shelter sharing by leaf‐tying caterpillars has either neutral or negative effects for the occupants. The fact that these leaf‐tying caterpillars actually share shelters may simply reflect limited availability of oviposition sites.     

Physiological responses of beech and sessile oak in a natural mixed stand during a dry summer

Responses of CO2 assimilation and stomatal conductance to decreasing leaf water potential, and to environmental factors, were analysed in a mixed natural stand of sessile oak (Quercus petraea ssp. medwediewii) and beech (Fagus svlvatica L.) in Greece during the exceptionally dry summer of 1998. Seasonal courses of leaf water potential were similar for both species, whereas mean net photosynthesis and stomatal conductance were always higher in sessile oak than in beech. The relationship between net photosynthesis and stomatal conductance was strong for both species. Sessile oak had high rates of photosynthesis even under very low leaf water potentials and high air temperatures, whereas the photosynthetic rate of beech decreased at low water potentials. Diurnal patterns were similar in both species but sessile oak had higher rates of CO2 assimilation than beech. Our results indicate that sessile oak is more tolerant of drought than beech, due, in part, to its maintenance of photosynthesis at low water potential.     

Four years continuous record of CH4-exchange between the atmosphere and untreated and limed soil of a N-saturated spruce and beech forest ecosystem in Germany

In order to gain information about seasonal and interannual variations of CH₄-fluxes at a spruce control site, a limed spruce site and a beech site of the Höglwald Forest, Bavaria, Germany, complete annual cycles of CH₄-exchange between the soil and the atmosphere with 2-hourly resolution were followed for 4 consecutive years. The ranges of CH₄ fluxes observed for the different sites were: +12.4 to –69.4 g CH₄ m^–2 h^–1 (spruce control site), +11.7 to –51.4 g CH₄ m^–2 h^–1 (limed spruce site), and –4.4 to –167.3 g CH₄ m^–2 h^–1 (beech site). Lowest rates of atmospheric CH₄-uptake or even a weak net-emission of CH₄ by the soils were observed during winter/spring times, whereas highest rates of CH₄-uptake were always found in summer/spring. Over the entire observation period of 4 years, mean CH₄-uptake rates were –1.82 kg CH₄-C ha^–1 yr^–1 at the spruce control site, –1.31 kg CH₄-C ha^–1 yr^–1 at the limed spruce site, and –4.84 kg CH₄-C ha^–1 yr^–1 at the beech site. The results obtained in this study demonstrate that in view of the huge interannual variations in CH₄-fluxes of approx. 1 kg CH₄-C ha^–1 yr^–1, multiple year measurements of CH₄-fluxes are necessary to accurately characterize the sink strength of temperate forest for atmospheric CH₄. By comparison of CH₄-fluxes measured at the spruce control site and the limed spruce site, a significant negative effect of forest floor liming on CH₄-uptake could be demonstrated. Compared to the spruce stand, the beech stand showed on average approx. 3 times higher rates of atmospheric CH₄-uptake, most likely due to pronounced differences between both sites with regard to the organic layer structure and bulk density of the mineral soil. Regression analysis between CH₄-fluxes and environmental parameters revealed that at all sites the dominating factors regulating temporal variations of CH₄ fluxes were soil moisture and soil temperature. Field measurements of CH₄ concentrations in the soil profile and laboratory measurements of CH₄-oxidation and CH₄-production activity on soil samples taken from different soil depths showed that the CH₄-flux at the Höglwald Forest sites is the net-result of simultaneous occurring production and consumption of CH₄ within the soil. Highest CH₄-oxidation activity was found in the uppermost centimeters of the mineral soil, whereas highest potential CH₄-production activity was found in the organic layer.     

Micropropagation ofFagus sylvatica L.

Summary An in vitro shoot multiplication system was established from juvenileFagus sylvatica L. tissues, and plantlets were regenerated. Embryonic axes were excised from beech seeds and germinated in vitro on media supplemented with 6-benzyladenine (BA) to obtain plantlets with axillary shoots. Shoot multiplication was maintained by sequential subculture of axillary shoot tips and basal segments on Woody Plant Medium supplemented with 0.5 mg/liter BA+2 mg/liter zeatin+0.2 mg/liter naphthaleneacetic acid (NAA). The effeciency of shoot multiplication clearly depended on the kind of explant used. Transfer to fresh medium every 2 wk during the 6-wk multiplication cycle improved multiplication rates. In the rooting stage, an initial 7-day dark period significantly improved rooting capacity and accelerated the emergence of roots on auxin-treated shoots. Adventitious buds were induced on the intact hypocotyls of the whole plantlets derived from the initial embryonic axis explants, especially on those cultured on medium with 1 mg/liter BA. Cotyledon and hypocotyl segments isolated from seedlings grown in vitro from embryos also exhibited capacity for adventitious bud formation, especially when cultured on media supplemented with 0.5 mg/liter BA + 0.1 mg/liter NAA.