Bark Chemistry Research Articles

Bark morphological and chemical features are differentially correlated with disease resistance and yield in hybrid poplar taxa

AbstractIn the southeastern United States, the establishment of short‐rotation intensively cultured plantations of hybrid poplar has been hindered by its susceptibility to stem cankers. We evaluated the tradeoffs between biomass yield and disease tolerance in hybrid poplar genotypes belonging to P. deltoides × P. maximowiczii (DM), P. deltoides × P. nigra (DN), P. trichocarpa × P. maximowiczii (TM), and P. deltoides × P. deltoides (DD) taxa. We hypothesized that canker resistant genotypes will have thicker bark but bark thickness and biomass yield will be negatively correlated. After two growing seasons, the DD genotypes developed thicker bark compared to the genotypes of other taxa and bark thickness was not correlated with biomass yield in the DD genotypes (R2 = 0.002). However, in the TM, DM, and DN genotypes, bark thickness was negatively correlated with biomass yield (R2 = 0.33–0.77). Disease incidence studies revealed that the DM genotypes were most susceptible to canker whereas no disease was detected in DD genotypes. Furthermore, bark analysis conducted by Fourier transform infrared spectroscopy coupled with multivariate analysis showed that that DD genotypes to be chemically separate from the three hybrid genotypes and that bark chemistry was correlated with canker disease incidence. Taken together, these results reveal that it is possible to generate hybrid poplar genotypes with thicker bark, disease resistance, and higher biomass yields. This insight should guide further efforts to develop genetically improved hybrid poplar genotypes, both in terms of biomass yield and disease tolerance, for cultivation in the southeastern United States. Hybrid poplar cultivation in southeastern United States is hindered by its susceptibility to stem cankers. We evaluated tradeoffs between yield and canker disease resistance in various hybrid poplar genotypes. After two growing seasons, the DD genotypes showed disease resistance and developed thicker bark that was chemically distinct from the other genotypes. Bark thickness was not correlated with yield in the DD genotypes but was negatively correlated with yield in the other genotypes. These results will guide the development of hybrid poplar genotypes that are both disease resistant and high yielding for cultivation in the southeastern United States.

The Role of Secondary Metabolites and Bark Chemistry in Shaping Diversity and Abundance of Epiphytic Lichens

Diversity of secondary lichen metabolites was studied in epiphytic lichens on six phorophytes—spruce, pine, birch, alder, aspen and poplar in the Middle Urals of Russia. Atranorin, usnic, fumarprotocetraric acid, zeorin, and gyrophoric acid were found in 31, 24, 23, 18, and 14 species, respectively, of 237 taxa collected. Seventy-seven species (i.e., 32% of total species documented) contained no secondary metabolites. Spectra of secondary metabolites of fruticose and foliose lichens varied on different phorophytes, while in crustose species the strong dependence on the tree species was not detected. This is different to the pH dependence of saxicolous lichens where crustose lichens were more susceptible to the rock chemistry. The results of Canonical Correspondence Analysis reveal the affinity of species containing depsides, depsidones or usnic acid to acidic substrata and those lacking secondary metabolites or containing terpenes and antraquinones to the pH-neutral bark. We suppose that phenolic compounds and flavonoids, as chemical constituents of bark, may interact with lichen symbioses and elements in phellem, and similarly to the lichen acids shape the affinity of species to the substrata.

Additive genetic variation in Pinus radiata bark chemistry and the chemical traits associated with variation in mammalian bark stripping.

Secondary metabolites are suggested as a major mechanism explaining genetic variation in herbivory levels in Pinus radiata. The potential to incorporate these chemical traits into breeding/deployment programmes partly depends on the presence of additive genetic variation for the relevant chemical traits. In this study, near-infrared spectroscopy was used to quantify the constitutive and induced levels of 54 compounds in the bark of trees from 74 P. radiata full-sib families. The trees sampled for chemistry were protected from browsing and induced levels were obtained by subjecting half of the trees to artificial bark stripping. The treatment effect on bark chemistry was assessed along with narrow-sense heritability, the significance of non-additive genetic effects and the additive genetic correlations of compounds with bark stripping by mammalian herbivores that was observed in unprotected replicates of the field trial. The results indicated: (i) significant additive genetic variation, with low-moderate narrow-sense heritability estimates for most compounds; (ii) while significant induced effects were detected for some chemicals, no significant genetic variation in inducibility was detected; and (iii) sugars, fatty acids and a diterpenoid positively genetically correlated while a sesquiterpenoid negatively genetically correlated with bark stripping by the mammalian herbivore, the Bennett's wallaby (Macropus rufogriseus). At the onset of browsing, a trade-off with height was detected for selecting higher amounts of this sesquiterpenoid. However, overall, results showed potential to incorporate chemical traits into breeding/deployment programmes. The quantitative genetic analyses of the near infrared predicted chemical traits produced associations with mammalian bark stripping that mostly conform with those obtained using standard wet chemistry.

Fate of Antioxidative Compounds within Bark during Storage: A Case of Norway Spruce Logs.

Softwood bark is an important by-product of forest industry. Currently, bark is under-utilized and mainly directed for energy production, although it can be extracted with hot water to obtain compounds for value-added use. In Norway spruce (Picea abies [L.] Karst.) bark, condensed tannins and stilbene glycosides are among the compounds that comprise majority of the antioxidative extractives. For developing feasible production chain for softwood bark extractives, knowledge on raw material quality is critical. This study examined the fate of spruce bark tannins and stilbenes during storage treatment with two seasonal replications (i.e., during winter and summer). In the experiment, mature logs were harvested and stored outside. During six-month-storage periods, samples were periodically collected for chemical analysis from both inner and outer bark layers. Additionally, bark extractives were analyzed for antioxidative activities by FRAP, ORAC, and H2O2 scavenging assays. According to the results, stilbenes rapidly degraded during storage, whereas tannins were more stable: only 5–7% of the original stilbene amount and ca. 30–50% of the original amount of condensed tannins were found after 24-week-storage. Summer conditions led to the faster modification of bark chemistry than winter conditions. Changes in antioxidative activity were less pronounced than those of analyzed chemical compounds, indicating that the derivatives of the compounds contribute to the antioxidative activity. The results of the assays showed that, on average, ca. 27% of the original antioxidative capacity remained 24 weeks after the onset of the storage treatment, while a large variation (2–95% of the original capacity remaining) was found between assays, seasons, and bark layers. Inner bark preserved its activities longer than outer bark, and intact bark attached to timber is expected to maintain its activities longer than a debarked one. Thus, to ensure prolonged quality, no debarking before storage is suggested: outer bark protects the inner bark, and debarking enhances the degradation.

Citizen science identifies the effects of nitrogen deposition, climate and tree species on epiphytic lichens across the UK

A national citizen survey quantified the abundance of epiphytic lichens that are known to be either sensitive or tolerant to nitrogen (N) deposition. Records were collected across the UK from over 10,000 individual trees of 22 deciduous species. Mean abundance of tolerant and sensitive lichens was related to mean N deposition rates and climatic variables at a 5 km scale, and the response of lichens was compared on the three most common trees (Quercus, Fraxinus and Acer) and by assigning all 22 tree species to three bark pH groups. The abundance of N-sensitive lichens on trunks decreased with increasing total N deposition, while that of N-tolerant lichens increased. The abundance of N-sensitive lichens on trunks was reduced close to a busy road, while the abundance of N-tolerant lichens increased. The abundance of N-tolerant lichen species on trunks was lower on Quercus and other low bark pH species, but the abundance of N-sensitive lichens was similar on different tree species. Lichen abundance relationships with total N deposition did not differ between tree species or bark pH groups. The response of N-sensitive lichens to reduced nitrogen was greater than to oxidised N, and the response of N-tolerant lichens was greater to oxidised N than to reduced N. There were differences in the response of N-sensitive and N-tolerant lichens to rainfall, humidity and temperature. Relationships with N deposition and climatic variables were similar for lichen presence on twigs as for lichen abundance on trunks, but N-sensitive lichens increased, rather than decreased, on twigs of Quercus/low bark pH species. The results demonstrate the unique power of citizen science to detect and quantify the air pollution impacts over a wide geographical range, and specifically to contribute to understanding of lichen responses to different chemical forms of N deposition, local pollution sources and bark chemistry.

Determinants of occurrence of epiphytic mosses in the urban environment; a case study from Katowice city (S Poland)

Abstract Katowice is the largest city within the Upper Silesian Industrial District (S Poland). The investtigation conducted in this town earlier (1998-2000) showed limited number of epiphytic species. Recent study showed significant increase in number of localities of obligate epiphytes (mainly from the genera Orthotrichum and Ulota). They colonize mainly the edges of strongly urbanized areas bordering forest complexes and larger parks. This paper is an attempt to answer the question as currently in Katowice shape the main factors determining the occurrence of epiphytes. Crucial factors which govern epiphytes colonization processes in urban areas have been confirmed: bark chemistry (taking into account natural attributes and the influence of pollution) and air humidity. The preference of epiphytes towards inhabited phorophytes was confirmed as well. The most commonly and abundantly inhabited species were poplar and willow trees (which have a higher pH of bark).

Effects of long-term elevated CO2 treatment on the inner and outer bark chemistry of sweetgum (Liquidambar styraciflua L.) trees

Long-term exposure of sweetgum trees to elevated atmospheric CO 2 concentrations significantly shifted inner bark (phloem) and outer bark (rhytidome) chemical compositions, having implications for both defense and nutrient cycling. Changes in plant tissue chemistry due to increasing atmospheric carbon dioxide (CO2) concentrations have direct implications for tissue resistance to abiotic and biotic stress while living, and soil nutrient cycling when senesced as litter. Although the effects of elevated CO2 concentrations on tree foliar chemistry are well documented, the effects on tree bark chemistry are largely unknown. The objective of this study was to determine the effects of a long-term elevated CO2 treatment on the contents of individual elements, extractives, ash, lignin, and polysaccharide sugars of sweetgum (Liquidambar styraciflua L.) bark. Trees were harvested from sweetgum plots equipped with the Free-Air CO2 Enrichment (FACE) apparatus, receiving either elevated or ambient CO2 treatments over a 12-year period. Whole bark sections were partitioned into inner bark (phloem) and outer bark (rhytidome) samples before analysis. Principal component analysis, coupled with either Fourier transform infrared spectroscopy or pyrolysis–gas chromatography–mass spectrometry data, was also used to screen for differences. Elevated CO2 reduced the N content (0.42 vs. 0.35 %) and increased the C:N ratio (109 vs. 136 %) of the outer bark. For the inner bark, elevated CO2 increased the Mn content (470 vs. 815 mg kg−1), total extractives (13.0 vs. 15.6 %), and residual ash content (8.1 vs. 10.8 %) as compared to ambient CO2; differences were also observed for some hemicellulosic sugars, but not lignin. Shifts in bark chemistry can affect the success of herbivores and pathogens in living trees, and as litter, bark can affect the biogeochemical cycling of nutrients within the forest floor. Results demonstrate that increasing atmospheric CO2 concentrations have the potential to impact the chemistry of temperate, deciduous tree bark such as sweetgum.

New ecological and physiological dimensions of beech bark disease development in aftermath forests

The ecology of northeastern North American forests is significantly altered by the effects of beech bark disease (BBD) on American beech (Fagus grandifolia). The conventional understanding is that BBD involves two native pathogenic fungi, Neonectria ditissima and N. faginata, which are preceded by Cryptococcus fagisuga, a non-native, invasive scale insect. Described in the early 20th century, this model of disease development describes the sequence of insect colonization at the expanding front of this invasive disease complex but lacks supporting data in long-affected (aftermath) forests. A clearer understanding of factors governing BBD development in aftermath forests is needed to develop sound, targeted management strategies. We established a case-control study to determine how Neonectria infections and C. fagisuga infestations are driven by nutritional, physiological, and entomological factors of the previous year. Findings suggest that N. ditissima infection is predisposed by the native scale insect Xylococculus betulae and low bark levels of phosphorus and the phenolic isorhamnetin while C. fagisuga and low bark levels of phosphorus and the phenolic catechin predispose trees to N. faginata infection. C. fagisuga population growth was best predicted by insect density the previous year, but was variously correlated with bark chemistry factors. We conclude that the classic BBD development model does not adequately predict disease development in aftermath forests of New York, and propose separate modified models for N. ditissima and N. faginata that include distinct abiotic and biotic factors. An improved understanding of the ecological interactions among the organisms involved and the physiological basis of infection have implications for future disease development and resistance biomarkers. We suggest that BBD management in aftermath forests should avoid single-factor strategies in favor of multi-factor approaches targeting specific pathosystems.

Scale insects, decay and canker fungi in American beech

SummaryTripartite interactions among phytophagous insects, pathogens and their host plants provide insight into the role of host physiology in determining susceptibility to attack. American beech (Fagus grandifolia) often is simultaneously attacked by beech scale (Cryptococcus fagisuga), one or more Neonectria pathogens and Xylococculus betulae that can result in beech bark disease (BBD). Additionally, beech is frequently infected by heartrot‐decay fungi. Cursory observations in 2011 suggested that beech scale and Neonectria lesion densities were lower and greater, respectively, on trees with decay. In 2012, digital image analysis was used to quantify densities of these organisms on 123 beech from the Adirondack region of New York. Three groups of study trees (n = 41) were used: Inonotus glomeratus‐infected, Phellinus igniarius‐infected and non‐decay trees. Trees infected by decay pathogens supported lower densities of beech scale and higher densities of Neonectria. Densities of X. betulae did not significantly vary among decay groups. These results may be explained by decay‐induced changes in host physiology. Additional work is needed to elucidate the potential role of host bark chemistry in the BBD complex.

Corymbia phloem phenolics, tannins and terpenoids: interactions with a cerambycid borer

Plant secondary chemistry mediates the ability of herbivores to locate, accept and survive on potential host plants. We examined the relationship between attack by the cerambycid beetle Phoracantha solida and the chemistry of the secondary phloem (inner bark) of two differentially attacked plantation forestry taxa, Corymbia variegata and its hybrid with C. torelliana. We hypothesised that this differential rate of attack may have to do with differences in secondary chemistry between the taxa. We found differences in the bark chemistry of the taxa, both with respect to phenolic compounds and terpenoids. We could detect no difference between bored and non-bored C. variegata trees (the less preferred, but co-evolved host). Hybrid trees were not different in levels of total polyphenols, flavanols or terpenes according to attack status, but acetone extracts were significantly different between bored and non-bored trees. We propose that variations in the bark chemistry explain the differential attack rate between C. variegata and the hybrid hosts.

Acidic atmospheric deposition interacts with tree type and impacts the cryptogamic epiphytes in Acadia National Park, Maine, USA

We examined plot-level interactions among nitrogen and sulfur deposition, tree type, and epiphytic diversity, biomass, and abundance in Acadia National Park (ACAD), Maine, that receives pollutant deposition and is of particular concern for biological conservation. Spruce appeared to acidify incoming deposition resulting in more concentrated inputs of nitrogen and sulfur within the canopy as well as to the soil relative to maple. This resulted in more acidic and sulfur-rich substrates and reduced biomass and species richness for cryptogamic epiphytes on spruce relative to maple. Position vertically on the tree bole was more important for epiphytes on maples than on spruces because spruces were uniform in their bark chemistry. Apparent overlap between the bark chemistry of spruce and maple, particularly for samples from higher on maple boles, suggests a reduction in the area of chemically suitable substratum for epiphytes in ACAD. Because tree species vary markedly in habitat variables important to epiphytes, stands with a mix of deciduous and evergreen trees provide the widest gradient of habitats, which appears to be reflected in the highest species richness for epiphytes occurring in mixed stands. Air quality and forest health monitoring studies utilizing epiphytes as indicators should account for the complex effects of stand-level tree species composition to better detect biological effects of deposition at the landscape scale.

Small increase in substratum [corrected] pH causes the dieback of one of Europe's most common lichens, Lecanora conizaeoides.

Lecanora conizaeoides was until recently western and central Europe's most abundant epiphytic lichen species or at least one of the most common epiphytes. The species is adapted to very acidic conditions at pH values around 3 and high concentrations of SO(2) and its derivatives formed in aqueous solution, and thus spread with increasing SO(2) deposition during the 19th and 20th centuries. With the recent decrease of SO(2) emissions to nearly pre-industrial levels within 20 years, L. conizaeoides declined from most of its former range. If still present, the species is no longer the dominant epiphyte, but is occurring in small densities only. The rapid spread of the L. conizaeoides in Europe from an extremely rare species to the probably most frequent epiphytic lichen and the subsequent rapid dieback are unprecedented by any other organism. The present study aimed at identifying the magnitude of deacidification needed to cause the dieback of the lichen. The epiphytic lichen diversity and bark chemistry of montane spruce forests in the Harz Mountains, northern Germany, were studied and the results were compared with data recorded with the same methods 13-15 years ago. Lecanora conizaeoides, which was the dominant epiphyte of the study area until 15 years ago, is still found on most trees, but only with small cover values of ≤1 %. The bark pH increased by only 0·4 pH units. The data suggest that only slight deacidification of the substratum causes the breakdown of the L. conizaeoides populations. Neither competitors nor parasites of L. conizaeoides that may have profited from reduced SO(2) concentrations are likely causes of the rapid dieback of the species.

Environmental quality assessment of the drainage basin of Lake Engure using Scots pine as a bioindicator

Environmental quality assessment of the drainage basin of Lake Engure using Scots pine as a bioindicator Environmental quality assessment of the Lake Engure drainage area, which is the LT(S)ER region of the Latvian National Long-term Ecological Research network (Latvia LTER), was conducted using three bioindication methods based on Scots pine Pinus sylvestris L.: unspecific bioindication by pine needle tip necrosis, ground level ozone assessment by pine needle chlorotic mottling, and chemical analysis of pine bark. Samples were collected from 40 sites of the region in November 2010. Extent of needle tip necroses did not exceed class 4 (maximum possible value 6). The highest value of index of needle damage by ozone was Idam = 1.62 (maximum possible value 6). Multiple regression analysis of variables describing bark chemistry and needle damage in relation to distance from the sea, nearest roads and villages was performed. Pine bark acidity pH(KCl) and concentration of Mg and Fe decreased significantly (βpH(KCl) = -0.672, P < 0.001; βMg = -0.676, P < 0.001; βFe = -0.514, P < 0.001) with distance of sample site from gravel roads. Electric conductivity (EC) of pine bark and the extent of ozone damage of the first year needles Idam significantly decreased with distance from the sea (βozone = -0.507, P < 0.01; βEC = -0.453, P < 0.01). PCA of the pine bark chemistry data showed dust pollution from gravel roads to be the main factor responsible for the 33% variation of data. No statistically significant correlations were found between different bioindicator characteristics, except between first and second year ozone damage (ρ = 0.589, P = 0.01).

Epiphytic lichen diversity and its dependence on bark chemistry in the northern Mongolian dark taiga

Epiphytic lichen diversity was studied in a dark taiga forest of Pinus sibirica, Abies sibirica and Picea obovata in the western Khentey Mountains, northern Mongolia. Though most lichen species occurred on all three tree species, lichen diversity was higher on Abies and Picea than on Pinus. On branches, lichen vegetation differed less between tree species than on the trunk. The occurrence of many Parmeliaceae species with a hydrophilic surface and of many species producing the dibenzofuran usnic acid gives evidence of the low deposition of acidic pollutants in the study area. The Mn content of bark, which is known to limit at high values the abundance of epiphytic lichens in coniferous forests of Europe and North America, is apparently not controlling the spatial distribution of epiphytic lichens in the dark taiga of Mongolia. This is attributed to the dry and cold winters in Mongolia, as high Mn is especially leached from the surface of trees under moist conditions at temperatures around the freezing point, when the contact between water droplets and the tree surface is particularly intensive. Such moist and cold weather conditions are frequent in most parts of the northern coniferous forests of Europe and North America, but are rare events in the most continental parts of Asia, i.e. in Mongolia and eastern Siberia.

Spatial distribution of lichens on twigs in remote Siberian silver fir forests indicates changing atmospheric conditions

Abstract:The distribution of different ecological groups of lichens (acidophytes, ‘nitrophytes’, indifferent species) was compared on 1–24 year-old twigs of Abies sibirica sampled in the ‘pristine’ West Sayan and the polluted East Sayan Mountains (Krasnoyarsk District, South Siberia, Russia) to test their value as indicators of current pollution effects. Bark pH of twigs and bark chemistry (N, S, Ca, Mg, Al, Fe) were measured, and a preliminary estimate of emissions in the Krasnoyarsk District from livestock animal populations was calculated. In both regions, an unusually high twig bark pH and an abnormal species composition for A. sibirica canopy were found (e.g. Physcia aipolia, P. dubia, P. tenella, Phaeophyscia sp., Melanelia exasperatula and Candelariella vitellina), with P. tenella (East Sayan) and M. exasperatula (West Sayan) as dominants. The results confirm that the distribution of lichen species on Abies sibirica twigs is a valuable indicator of current changes in atmospheric conditions.

The multi-element content of the lichen Parmelia sulcata, soil, and oak bark in relation to acidification and climate

Understanding how biodiversity is influenced by changing atmospheric conditions is important for conservation, public policy and environmental health. In a recent study, an impact on two of the most abundant lichens in Europe ( Parmelia sulcata and Hypogymnia physodes) was recorded at Burnham Beeches lying 40 km west of London (UK) during unusual atmospheric conditions whilst other species remained unaffected. Bark and soil chemistry also influence lichen vitality and community composition. Correlations between element concentrations and element ratios in different samples help understand element cycling. To study this further, the multi-element content of the lichen P. sulcata and bark sampled in 2000 from 16 oak ( Quercus robur) trees at Burnham Beeches was compared with the same elements determined in 24 surface soils sampled in 2005 from beneath the same trees. Soil pH ranged from 4.1 to 6.7. Highly significant correlations ( p < 0.001) for Mn/Ca ratios in lichen versus bark, soil versus bark and soil versus lichen samples confirmed cycling via the soil–tree–lichen system. Similar Mn/Ca ratios were reported in coniferous forests where high Mn concentrations were shown to limit epiphytic lichen abundance. Soil acidification influences element bioavailability and bark chemistry in Burnham Beeches with implications for lichen diversity and health. Recovery was recorded in P. sulcata, including regeneration within monitoring quadrats. Biological systems are well known to be influenced by pollution episodes.

Rapid analysis of inner and outer bark composition of Southern Yellow Pine bark from industrial sources

Differences in bark chemistry between inner and outer bark are well known and may affect the suitability of various bark supplies for a particular application. Accordingly, there is a need for quality control protocols to assess variability and predict product yields. Southern yellow pine bark samples from two industrial sources were separated into inner and outer bark tissues and used to prepare both predetermined and random bark compositions for analysis. Near infrared (NIR) spectroscopy, coupled with multivariate analysis, was successfully used to develop models to predict relative amounts of inner bark. Application of mathematical treatments to the NIR data improved the calibration performance leading to improved predictions for the test samples. Results presented here show promise for the further development of this technique as a means to provide rapid and accurate predictions of the quality of bark obtainable from industrial sources.

Using 13C nuclear magnetic resonance spectroscopy for the study of northern hardwood tissues

Nuclear magnetic resonance (NMR) spectroscopy is a useful tool for examining the structural chemistry of natural organic matter. The use of cross-polarization and magic-angle spinning to study 13C functionality (CPMAS 13C NMR) is convenient, but not always quantitative. We used various 13C NMR techniques to examine the structural chemistry of bark and wood of sugar maple (Acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britt.). Spin counting experiments showed that 87%–97% of the 13C in the samples was observable by CPMAS 13C NMR. A comparison of CPMAS and Bloch decay experiments revealed few differences in spectral properties. Together, these results suggest that CPMAS 13C NMR is quantitative for these tissues. We observed little variation in the structural chemistry of wood, either among samples of the same species or among species. Within-species variations in bark chemistry were greater than in wood, probably because of variations in environmental conditions. However, we observed no significant differences in bark chemistry among the species. Bark and wood chemistry differed significantly, with the bark spectra displaying greater contributions from lignin, suberin, waxes, and resins. Hardwood spectra differ from softwood spectra in the aromatic C regions because of the contribution of syringyl units to hardwood lignin. Hardwood bark appears to contain less tannins than softwood bark. Together, the quantitative and qualitative features of CPMAS 13C NMR spectra are useful for studying the ecology of living and detrital wood and bark.

Distribution pattern of the epiphyteNeckera pennataon three spatial scales – importance of past landscape structure, connectivity and local conditions

We tested which factors explain the distribution pattern of the epiphytic mossNeckera pennataon three spatial scales using the framework of generalized linear models. First, we tested which factors explained its occurrence in forest stands in a 2500 ha landscape. At this scale, we also tested the effect of the historic landscape structure. We recorded its occurrence in all suitable stands. The occurrence probability increased with increasing present quantity ofAcer platanoides, and with increasing present and past quantity ofFraxinus excelsior. The probability also increased with increasing connectivity to occupied stands. However, the connectivity to stands present in 1977 (recorded from infra‐red aerial photographs) explained more of the variation. This suggests that the regional metapopulation size ofN. pennatahas decreased during the past decades, and that its present distribution pattern reflects the age of the remaining stands, and the distribution of past dispersal sources in the landscape. Second, we tested which factors explained the occurrence and abundance on individual trees in three forest stands.Neckera pennatamainly occurred onAcerandFraxinusstems. The most important variable in explaining occurrence probability was connectivity to surrounding occupied trees, which probably reflects the restricted dispersal range in this species. The abundance on occupied trees was also explained by this variable. The occurrence probability and abundance also increased with increasing tree diameter, probably reflecting the time that a tree has been available for colonization and the time since colonization, respectively. The occurrence probability and abundance furthermore decreased on strongly leaning (and deteriorating) trees. The occurrence probability increased with increasing bark roughness, probably reflecting increasing suitability regarding bark chemistry and moisture. Third, we tested its vertical distribution on occupied trees. The main distribution was below 1.6 m.

Variation in birch bark secondary chemistry between and within clones: implications for herbivory by hares

We studied the variation in bark chemistry between and within 19 European white birch (Betula pendula) clones and its implications to resistance to the mountain hare (Lepus timidus). We used one‐year‐old clonal plantlets originating from randomly selected naturally regenerated parental trees. The same clones were used in both chemical analyses and in feeding experiments. The condensed tannins were analysed by an acid butanol assay, other phenolic components by HPLC‐DAD, and triterpenoid components by HPLC‐MS. The resistance to hare was tested in open‐field feeding experiment. The main phenolic compounds in birch bark were catechin derivatives, rhododendrin, platyphylloside, and condensed tannins, and the main triterpenoids were papyriferic acid and pendulic acid. Most of the variation in the concentrations of the studied compounds was found between clones for the studied phenolics and large variation for triterpenoid components were found both between clones and among plantlets within the same clone. Hares clearly selected among the studied clones. Our results suggest that birch bark chemistry play an important role in resistance to herbivory by hare. The total triterpenoids and total flavonoid‐aglycones showed significant negative correlation with hare feeding. It seems that a genetic basis for bark chemistry and birch resistance is strong. Such a high variation in secondary chemistry both between clones and within individual clones indicates that European white birch populations have a good resistance towards variable environmental conditions and varying pressures from herbivory.