Herb-layer diversity in deciduous forests: Raised by tree richness or beaten by beech?

Where natural production capacity permits, modern silvicultural management in Central Europe frequently aims at the development of mixed broadleaved stands, instead of pure European beech (Fagus sylvatica) stands. It is crucial to study the effects of these tree-layer diversity variations on herb-layer vegetation, since herb-layer vegetation contributes significantly to ecosystem functioning in forests. In Hainich National Park (Thuringia, Germany), we conducted observational research in deciduous stands to investigate whether herb-layer diversity was related to canopy-layer diversity, and to ascertain possible causal mechanisms. We found that herb-layer vegetation of deciduous forest stands rich in canopy species appeared to be more diverse than herb-layer vegetation of beech-dominated stands. We surmise that herbaceous understorey diversity was indirectly influenced by canopy tree species through the medium of the altered environmental factors soil pH and litter layer thickness. Apparently, lower beech proportion had a more profound effect than the number of secondary tree species. There were no correlations between herb-layer diversity and light transmissibility of the canopy layer, indicating that the light factor was not crucial for herb-layer diversity. At least for the Hainich research sites, our results indicated that small-scale light and soil heterogeneity is insignificant for herb-layer diversity. We found several herb-layer species whose occurrence was particularly correlated with tree-layer diversity and environmental factors. Remarkably, all species positively correlated with soil pH were important for the phytosociological classification of the research sites. Beech-dominated research sites showed high tree-layer volumes, whereas research sites with high tree-layer diversity tended to feature lower tree-layer volumes. These findings could be the result of differing former silvicultural systems and varying soil clay contents affecting tree species composition. In contrast, herb-layer biomass was positively correlated with tree-layer diversity. Herb-layer productivity might be promoted in more diverse research sites by increased nutrient supply and base saturation. It is also possible that greater beech proportion interfered with herb-layer productivity. However, herb-layer biomass was also positively correlated with herb-layer diversity. Hence, our study hints that positive diversity-functioning relationships might occur in the herb-layer of the deciduous forest under investigation.     

Testing dependence between growth and needle litterfall in Scots pine--a case study in northern Finland

Both drought and fungal disease increase needle litterfall of Scots pine (Pinus sylvestris L.) trees, but most factors causing annual variation in needle litterfall are poorly understood. We hypothesized that radial growth and weather conditions favorable to growth correlate positively with needle litterfall with a lag equal to the number of needle cohorts (here being 5-6). We studied the time series of needle litterfall, stem increment, pollen cone litter and daily weather conditions in a Scots pine stand over 43 years (1961-2004). The cross-correlations of standardized time series were estimated with various lags. Model predictions of annual needle litterfall were tested against independent data. Changes in annual growth and needle litterfall correlated with lags of 0 and 4 years. The best predictors for needle litterfall were May to mid July temperature sum with a lag of 4 years, May rainfall with a lag of 2 years and September temperature with a lag of 6 years. Pollen cone litter correlated negatively with needle litterfall with a lag of 2 years. The study provided empirical evidence that needle litterfall of Scots pine in northern Finland is influenced by needle production and needle mass development that occurred 4 to 6 years earlier.     

Rockfall and snow avalanche impacts leave different anatomical signatures in tree rings of juvenile Larix decidua

Rockfall and snow avalanche events often cause injury to European larch (Larix decidua Mill.) trees, giving rise to the formation of callus tissue and tangential rows of traumatic resin ducts (TRDs). We analyzed and quantified anatomical reactions of juvenile trees injured before the start of the growing season by snow avalanches (15 trees, 324 cross sections) or rockfalls (18 trees, 270 cross sections). Traumatic resin ducts were observed in the growth ring formed following injury in 94.3% of the rockfall samples and 87.3% of the snow avalanche samples. Traumatic resin ducts were formed at the beginning of the new annual ring around wounds caused by rockfalls. In contrast, in trees injured by snow avalanches, TRDs were not formed until after the formation of several rows of early earlywood (EE) tracheids (mean +/- SD = 4.19 +/- 2.56 rows). The dimensions of the EE tracheids observed in the snow avalanche samples were greatly reduced in the tissues bordering the wound, with radial width reaching an average of only 50% and lumen cross-sectional area an average of only 46% of pre-event values. It is therefore possible to differentiate injuries due to past snow avalanches from injuries due to rockfall based on anatomical growth reactions in the tissues bordering scars.     

Partitioning of Hg Between Solid and Dissolved Organic Matter in the Humus Layer of Boreal Forests

The mobility of mercury (Hg) deposited on soils controls the concentration and toxicity of Hg within soils and in nearby streams and lakes, but has rarely been quantified under field conditions. We studied the in situ partitioning of Hg in the organic top layer (mor) of podsols at two boreal forest sites differing in Hg deposition and climatic regime (S. and N. Sweden, with pollution declining to the north). Soil solution leaching from the mor layer was repeatedly sampled using zero-tension lysimeters over 2 years, partly in parallel with tension lysimeters. Concentrations of Hg and dissolved organic carbon (DOC) were higher while pH was lower at the southern site (means ± SD: Hg?=?44?±?15 ng L^?1, DOC?=?63.0?±?31.3 mg L^?1, pH?=?4.05?±?0.53) than at the northern site (Hg?=?22?±?6 ng L^?1, DOC?=?41.8?±?12.1 mg L^?1, pH?=?4.28?±?0.43). There was a positive correlation over time between dissolved Hg and DOC at both sites, even though the DOC concentration peaked during autumn at both sites, while the Hg concentration remained more constant. This correlation is consistent with the expected strong association of Hg with organic matter and supports the use of Hg/C ratios in assessments of Hg mobility. In the solid phase of the overlying O_f layer, both Hg concentrations and Hg/C ratios were higher at the southern site (means ± SD: 0.34?±?0.06 μg g^?1 dw and 0.76?±?0.14 μg g^?1 C, respectively) than at the northern site (0.31?±?0.05 μg g^?1 dw and 0.70?±?0.12 μg g^?1 C, respectively). However, concentrations in the solid phase differed less than might be expected from the difference in current atmospheric input, suggesting that the fraction of natural Hg is still substantial. At both sites, Hg/C ratios in the upper half of the mor layer were only about two thirds of those in the lower half, suggesting that the recent decrease in anthropogenic Hg deposition onto the soil is offset by a natural downward enrichment of Hg due to soil decomposition or other processes. Most interestingly, comparison with soil leachate showed that the average Hg/C ratios in the dissolved phase of the mor layers at both sites did not differ from the average Hg/C ratios in the overlying solid organic matter. These results indicate a simple mobilisation with negligible fractionation, despite differences in Hg deposition patterns, soil chemistry and climatic regimes. Such a straight-forward linkage between Hg and organic matter greatly facilitates the parameterisation of watershed models for assessing the biogeochemical fate, toxic effect and critical level of atmospheric Hg input to forest soils.     

Silver birch and climate change: variable growth and carbon allocation responses to elevated concentrations of carbon dioxide and ozone

We studied the effects of elevated concentrations of carbon dioxide ([CO2]) and ozone ([O3]) on growth, biomass allocation and leaf area of field-grown O3-tolerant (Clone 4) and O3-sensitive clones (Clone 80) of European silver birch (Betula pendula Roth) trees during 1999-2001. Seven-year-old trees of Clones 4 and 80 growing outside in open-top chambers were exposed for 3 years to the following treatments: outside control (OC); chamber control (CC); 2 x ambient [CO2] (EC); 2 x ambient [O3] (EO); and 2 x ambient [CO2] + 2 x ambient [O3] (EC+EO). When the results for the two clones were analyzed together, elevated [CO2] increased tree growth and biomass, but had no effect on biomass allocation. Total leaf area increased and leaf abscission was delayed in response to elevated [CO2]. Elevated [O3] decreased dry mass of roots and branches and mean leaf size and induced earlier leaf abscission in the autumn; otherwise, the effects of elevated [O3] were small across the clones. However, there were significant interactions between elevated [CO2] and elevated [O3]. When results for the clones were analyzed separately, stem diameter, volume growth and total biomass of Clone 80 were increased by elevated [CO2] and the stimulatory effects of elevated [CO2] on stem volume growth and total leaf area increased during the 3-year study. Clone 80 was unaffected by elevated [O3]. In Clone 4, elevated [O3] decreased root and branch biomass by 38 and 29%, respectively, whereas this clone showed few responses to elevated [CO2]. Elevated [CO2] significantly increased total leaf area in Clone 80 only, which may partly explain the smaller growth responses to elevated [CO2] of Clone 4 compared with Clone 80. Although we observed responses to elevated [O3], the responses to the EC+EO and EC treatments were similar, indicating that the trees only responded to elevated [O3] under ambient [CO2] conditions, perhaps reflecting a greater quantity of carbohydrates available for detoxification and repair in elevated [CO2].     

Large magnetic anisotropy of a single atomic spin embedded in a surface molecular network

Magnetic anisotropy allows magnets to maintain their direction of magnetization over time. Using a scanning tunneling microscope to observe spin excitations, we determined the orientation and strength of the anisotropies of individual iron and manganese atoms on a thin layer of copper nitride. The relative intensities of the inelastic tunneling processes are consistent with dipolar interactions, as seen for inelastic neutron scattering. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network in the copper nitride. These structures, which provide atom-by-atom accessibility via local probes, have the potential for engineering anisotropies large enough to produce stable magnetization at low temperatures for a single atomic spin.     

Germination, genetics, and growth of an ancient date seed

An ancient date seed (Phoenix dactylifera L.) excavated from Masada and radiocarbon-dated to the first century Common Era was germinated. Climatic conditions at the Dead Sea may have contributed to the longevity of this oldest, directly dated, viable seed. Growth and development of the seedling over 26 months was compatible with normal date seedlings propagated from modern seeds. Preliminary molecular characterization demonstrated high levels of genetic variation in comparison to modern, elite date cultivars currently growing in Israel. As a representative of an extinct date palm population, this seedling can provide insights into the historic date culture of the Dead Sea region. It also has importance for seed banking and conservation and may be of relevance to modern date palm cultivation.     

The herbicide binding niche of photosystem II-a model

The Q_B binding niche of photosystem II is also the binding site for many different herbicides. In order to understand the mode of binding of the herbicides, a 3-dimensional model of the binding niche was constructed. The model was based upon a comparison of the known structure of the Q_B binding niche in purple bacteria with sequence and mutant data of the D-1 protein of photosystem II. Plastoquinone builds up hydrogen bonds to phenylalanine 265 backbone amide nitrogen, to serine 264 hydroxyl, and to histidine 215 delta-1 nitrogen. In addition to these hydrogen bond donors and acceptors, herbicides can build up hydrogen bonds to backbone carbonyl of alamine 263 and to serine 268 hydroxyl. This is supported by binding data of inhibitors in Chlamydomonas reinhardtii chloroplasts of wild type and of five D-1 protein mutants (Ser264 Ala, Ala251 Val, Phe255Tyr, Val2191le, Leu275Phe).     

Effects of surfactants on water permeability of isolated plant cuticles and on the composition of their cuticular waxes

Treating the outer surfaces of isolated cuticles of Seville orange (Citrus aurantium L.) and pear (Pyrus communis L. cv. Bartlett) leaves with a number of nonionic (polyoxyethylene) surfactants increased their permeability to water by factors ranging from 4.1 to 14.7 and from 7.2 to 152.4, respectively. However, sodium dodecylsulphate, an anionic surfactant, had little effect on water permeability. In both species the major aliphatic constituents were n-alkanes, 1-alkanols and n-alkyl esters. None of the surfactant treatments altered the amounts or composition of waxes in the cuticles used for transport measurements. The reasons for the apparent absence of solubilization or dissolution of cuticular waxes by nonionic surfactants are discussed and a hypothesis is presented for the mechanism by which activator adjuvants may increase the permeability of plant cuticles.     

Weak relationships between landscape characteristics and multiple facets of stream macroinvertebrate biodiversity in a boreal drainage basin

Variability in biodiversity is often assessed based on species richness, and this adherence to a single index has been typical in studies of ecology, biogeography, and conservation in the past two decades. More recent studies have suggested that species richness alone is insufficient as a measure of biodiversity, mainly because it is not necessarily correlated with other measures of biodiversity. We examined (1) if nine indices embracing species diversity, functional diversity, and taxonomic distinctness of stream macroinvertebrate assemblages show congruent patterns, and (2) if these indices show similar relationships to landscape characteristics. Not all indices varied similarly and were thus not significantly correlated. There were three principal components that effectively described variation in the correlation structure of the nine indices. These three components were: (1) diversity and evenness indices, (2) two indices of taxonomic distinctness, and (3) species richness and functional richness. Four of the nine biodiversity indices examined showed no significant relationships to landscape-catchment characteristics, and even the significant correlations between the remaining five indices and explanatory variables were rather weak. However, species richness showed a rather strong quadratic relationship to catchment area. Our study provided a number of suggestions for future biodiversity studies at the landscape scale. First, given that different indices describe different components of biodiversity and are not strongly correlated, multiple indices should be considered in any study describing stream biodiversity. Second, despite the study was restricted to near-pristine streams, all indices showed considerable variation. Thus, this natural variability should be accounted for prior to the examination of anthropogenic effects on stream biodiversity. Third, landscape-catchment variables may have only limited value in explaining variability in biodiversity indices, at least in regions with no strong anthropogenic gradients in land-use.