Search

Prescribed burning is an important management tool in many parts of the world. While natural fires generally occur during the driest and warmest period of the year, prescribed burning is often timed out-of-season, when there is higher soil moisture and lower biomass combustibility. However, fire season may influence seedling recruitment after fire, e.g. through the effect of seed hydration status on fire tolerance. In non-fire-prone temperate regions, anthropogenic fire may occur exclusively in periods outside the growing season with higher soil moisture, which may have negative consequences on seedling recruitment. Fire tolerance of moist and dry seeds of 16 temperate European herbaceous species belonging to four families was assessed using heat treatment of 100 °C for 5 min and subsequent germination trials. Moist seeds of Asteraceae, Poaceae and Brassicaceae had a predominantly negative reaction to the heat treatment, while those of Fabaceae tolerated it or germination was even enhanced. The reaction of dry seeds was completely different, with positive responses in three species of the Fabaceae and fire tolerance in species of other families. Our results point out that hydration status may significantly influence the post-fire germination of seeds. Dry seeds were found to tolerate high heat, while moist seeds were harmed in more than half of the species. This implies that if prescribed burning is applied in temperate grasslands of Europe, it should be timed to dry periods of the dormant season in order to protect seeds from negative effects of fire. [ABSTRACT FROM AUTHOR]

Owing to its wide European distribution and drought sensitivity, beech (Fagus sylvatica L.) has captured the attention of researchers in the context of climate change. Contrary to central European beech ecosystems, little is known about the nitrogen (N) balance of beech at its south-eastern European distribution limit. Here we studied the seasonal variation of key N-related parameters during the period 2003-2004, and tested whether climatic differences between the years affected the N balance of beech in north-western Greece. The warmer and drier climate of 2003 was characterised by lower soil N availability, compared to 2004. The pool of total soluble non-protein nitrogen (TSNN) was more sensitive than total N to interannual differences in growth conditions, as reflected particularly in the TSNN concentrations, and in the composition of phloem and xylem sap in early spring. In 2003, the N demands of expanding leaves were covered mainly by stored N remobilisation, as indicated by elevated TSNN levels in phloem and xylem sap, and by increased loading of arginine and glutamine, major N-storage compounds, in these N-exchange and transport pools. By contrast, the more favourable growth conditions of 2004 supported new growth also by current root N uptake, as indicated by increased asparagine levels in phloem and xylem sap. [ABSTRACT FROM AUTHOR]

Long-term nitrogen deposition into forest ecosystems has turned many forests in Central Europe and North America from N-limited to N-saturated systems, with consequences for climate as well as air and groundwater quality. However, complete quantification of processes that convert the N deposited and contributed to ecosystem N cycling is scarce. In this study, we provide the first complete quantification of external and internal N fluxes in an old-growth spruce forest, the Höglwald, Bavaria, Germany, exposed to high chronic N deposition. In this forest, N cycling is dominated by high rates of mineralisation of soil organic matter, nitrification and immobilisation of ammonium and nitrate into microbial biomass. The amount of ammonium available is sufficient to cover the entire N demand of the spruce trees. The data demonstrate the existence of a highly dynamic internal N cycle within the soil, driven by growth and death of the microbial biomass, which turns over approximately seven times each year. Although input and output fluxes are of high environmental significance, they are low compared to the internal fluxes mediated by microbial activity. [ABSTRACT FROM AUTHOR]

• Increasing atmospheric concentrations of phytotoxic ozone (O(3) ) can constrain growth and carbon sink strength of forest trees, potentially exacerbating global radiative forcing. Despite progress in the conceptual understanding of the impact of O(3) on plants, it is still difficult to detect response patterns at the leaf level. • Here, we employed principal component analysis (PCA) to analyse a database containing physiological leaf-level parameters of 60-yr-old Fagus sylvatica (European beech) trees. Data were collected over two climatically contrasting years under ambient and twice-ambient O(3) regimes in a free-air forest environment. • The first principal component (PC1) of the PCA was consistently responsive to O(3) and crown position within the trees over both years. Only a few of the original parameters showed an O(3) effect. PC1 was related to parameters indicative of oxidative stress signalling and changes in carbohydrate metabolism. PC1 correlated with cumulative O(3) uptake over preceding days. • PC1 represents an O(3) -responsive multivariate pattern detectable in the absence of consistently measurable O(3) effects on individual leaf-level parameters. An underlying effect of O(3) on physiological processes is indicated, providing experimental confirmation of theoretical O(3) response patterns suggested previously., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

The influence of free-air ozone (O(3)) fumigation on the levels of gene transcripts and compounds of defence and signalling were analysed in leaves of adult beech trees from the "Kranzberg Forest" research site in 2003 and 2004. This includes the precursor of the stress hormone ethylene, ACC (1-aminocyclopropane-1-carboxylic acid), conjugated salicylic acid, lignin content as well as of the expression level of genes connected with oxidative stress and stress signalling. At this site mature beech trees were exposed to an enhanced O(3) regime by a free-air O(3) canopy exposure system. Levels of conjugated ACC and conjugated salicylic acid in leaves were increased under O (3) fumigation whereas lignin content was only slightly enhanced. Quantitative real-time RT-PCR (qRT-PCR) was performed on transcripts of genes connected with lignin, salicylic acid, and ethylene formation, the shikimate pathway, abscisic acid biosynthesis as well as with the antioxidative system. Genes which showed O(3)-dependent increases included FSCOMT (caffeic-acid O-methyltransferase) connected with lignin formation, the stress response genes FSACS2 (ACC synthase) and FSPR1 (PR10 - pathogenesis-related protein), as well as FSNCED1 (9-cis-epoxicarotenoid dioxygenase), the rate-limiting enzyme of the ABA synthesis. For FSNCED1 expression level, a significant O(3) effect was found with an 8-fold (sun) and 7-fold (shade) induction in July 2003 and a 3-fold and 2.5-fold induction in July 2004. While the observed effects were not continuous, elevated O(3) is concluded to have the potential to disrupt the defence and signalling system.

This study investigated the influence of climate on the carbon isotopic composition (sigma13C) and oxygen isotopic enrichment (delta18O) above the source water of different organic matter pools in European beech. In July and September 2002, sigma13C and delta18O were determined in phloem carbohydrates and in bulk foliage of adult beech trees along a transect from central Germany to southern France, where beech reaches its southernmost distributional limit. The data were related to meteorological and physiological parameters. The climate along the transect stretches from temperate [subcontinental (SC)] to submediterranean (SM). Both sigma13Cleaf and delta18Oleaf were representative of site-specific long-term environmental conditions. sigma13C of leaves collected in September was indicative of stomatal conductance, vapour pressure deficit (VPD) and radiation availability of the current growing season. delta18O was mainly correlated to mean growing season relative humidity (RH) and VPD. In contrast to the leaves, sigma13Cphloem varied considerably between July and September and was well correlated with canopy stomatal conductance (Gs) in a 2 d integral prior to phloem sampling. The relationship between sigma13C and delta18O in both leaves and phloem sap points, however, to a combined influence of stomatal conductance and photosynthetic capacity on the variation of sigma13C along the transect. delta18Ophloem could be described by applying a model that included 18O fractionation associated with water exchange between the leaf and the atmosphere and with the production of organic matter. Hence, isotope signatures can be used as effective tools to assess the water balance of beech, and thus, help predict the effects of climatic change on one of the ecologically and economically most important tree species in Central Europe.

To assess the physiological performance of drought-sensitive European beech ( Fagus sylvatica L.) under the dry Mediterranean climate prevailing at its southeastern distribution limit in Europe, we analyzed seasonal changes in carbon, nitrogen and water balance of naturally grown adult trees. We determined the foliar C and N contents, delta13C and delta18O signatures, total soluble non-protein nitrogen compounds (TSNN) in xylem, leaves, and phloem, as well as leaf water potential and photosynthetic quantum yield in northern Greece during 2003. Tissue sampling was performed in May, July, and September, while field measurements were conducted regularly. Climatic conditions for the 2003 growing season fall within the typical range of the studied area. The N- and C-related parameters displayed distinct seasonal courses. TSNN was highest in May in all tissues, and asparagine (Asn) was then the most abundant compound. Thereafter, TSNN decreased significantly in all tissues and both its concentration and composition remained constant in July and September. In both months, glutamate (Glu) prevailed in leaves, gamma-aminobutyric acid (GABA) in phloem exudates from twigs and trunks, and arginine (Arg) in the xylem sap, where loading with amino acids was rather low during that period, amounting to only 0.8 micromol N ml-1 in September. Highest total foliar N and C contents were detected in May, and the elevated abundance of nutrients as well as an increased foliar delta13C signature at the beginning of the growing season is attributed to remobilization processes. The signatures of delta18O, quantum yield and leaf water potentials varied only slightly throughout the growing season. Although summer precipitation at the study site was considerably lower compared to what is usual for typical central European beech forests, no intensive drought responses of the physiological apparatus were detected in the studied beech trees. This suggests efficient internal regulation mechanisms, constantly ensuring a favourable physiological status under the relatively dry Mediterranean climate.

Whilst forest policy promotes cultivation and regeneration of beech dominated forest ecosystems, beech itself is a highly drought sensitive tree species likely to suffer from the climatic conditions prognosticated for the current century. Taking advantage of model ecosystems with cool-moist and warm-dry local climate, the latter assumed to be representative for future climatic conditions, the effects of climate and silvicultural treatment (different thinning regimes) on water status, nitrogen balance and growth parameters of adult beech trees and beech regeneration in the understorey were assessed. In addition, validation experiments with beech seedlings were carried out under controlled conditions, mainly in order to assess the effect of drought on the competitive abilities of beech. As measures of water availability xylem flow, shoot water potential, stomatal conductance as well as delta (13)C and delta (18)O in different tissues (leaves, phloem, wood) were analysed. For the assessment of nitrogen balance we determined the uptake of inorganic nitrogen by the roots as well as total N content and soluble N compounds in different tissues of adult and young trees. Retrospective and current analysis of delta (13)C, growth and meteorological parameters revealed that beech growing under warm-dry climatic conditions were impaired in growth and water balance during periods with low rain-fall. Thinning affected water, N balance and growth mostly of young beech, but in a different way under different local climatic conditions. Under cool, moist conditions, representative for the current climatic and edaphic conditions in beech forests of Central Europe, thinning improves nutrient and water status consistent to published literature and long-term experience of forest practitioners. However, beech regeneration was impaired as a result of thinning at higher temperatures and under reduced water availability, as expected in future climate.