Effects of ungulate browsing on recruitment of aspen and rowan: a demographic approach

Aspen (Populus tremula L.) and rowan (Sorbus aucuparia L.) are important for biodiversity conservation and aesthetic reasons. Intensified levels of ungulate browsing the last decades have raised concerns over the recruitment of aspen and rowan in boreal forests. Demographic data are vital to understand and quantify how the structure of tree populations are affected over time by browsing and other factors but such data are rare. In a five-year study, we quantified vital demographic rates for co-occurring aspen and rowan using exclosures in a productive forest area in eastern-central Sweden. Mean annual browsing rate on small (<1 m) aspen and rowan stems was 20 vs. 38%, whereas the order in browsing intensity was reversed for stems of medium (1–2 m) size (28 vs. 14%). Browsing had a clear negative effect on transition rate from small to medium size class in both species, but had a much smaller impact on transition rate from medium to large (2–3 m) size class. Residence time for small stems and regeneration rate was much higher in aspen than in rowan, but browsing did not affect these dynamics. Transition to ungulate safe height (>3 m) was reduced with a factor of two in rowan and four in aspen by browsing.     

Application of percolation modelling on end-grain water absorption in aspen (Populus tremula L.)

Abstract

The aim of this study was to improve our understanding of the capillary uptake of water in aspen sapwood and heartwood, which may help to explain the variable performance of aspen used outdoors. The study uses a percolation model developed for softwoods to examine capillary liquid absorption in aspen and compares predicted results with ones obtained experimentally using computed tomographic scanning. The study shows that there is an equilibrium saturation level where new flow paths are being found at the same rate as old ones are being blocked. This is seen as a plateau where the water content maintains a relatively constant height within the material. This makes it possible to model the capillary behaviour of aspen sapwood. In heartwood, however, the uptake of moisture seems to be mainly restricted to bound water, except for a short region in the sample wet end. This absorption is thus governed mainly by bound water diffusion. This improved understanding of the capillary behaviour of aspen may contribute to a future market expansion where aspen may be utilized to a greater extent outdoors and above ground.     

Parasitoids of Saperda populnea (L.) (Coleoptera: Cerambycidae) on aspen (Populus tremula L.) in Bulgaria

The parasitoids of Saperda populnea (L.) (Col.: Cerambycidae) were studied in Bulgaria during the period 1997?–?2001. Galls with pest larvae were collected from young aspen trees at eight locations (Sofia, Kokaliane, Plana, Churek, Gorni Lom, Gintzi, Dolno Kamartzi and Klisura) and examined under laboratory conditions. Four species were reared from 1118 galls containing overwintering pest larvae: Iphiaulax impostor (Scop.) (Hym.: Braconidae), Dolichomitus populneus (Ratz.), Schreineria populnea (Gir.) (Hym.: Ichneumonidae) and Billaea irrorata (Meig.) (Dipt.: Tachinidae). All of the parasitoids were solitary. Schreineria populnea was found as a new parasitoid of the host in Bulgaria. B. irrorata emerged mainly from late-stage S. populnea larvae; the remaining species from both early- and mid-stage larvae. In the parasitoid complex of S. populnea, the most numerous were B. irrorata (59.8?%) and D. populneus (29.4?%). The percentage of host larval parasitism varied from 2.4 to 33.3, with an average of 9.7?%. B. irrorata was the most important in reducing the pest number. The average mortality of S. populnea caused by the tachinid was 5.8?%, and the highest mortality observed in individual study was 20.2?%.     

Fungal decomposition of sapwood and heartwood of European aspen,Populus tremula L.

In contrast to the woodrotting fungi which can only attack Populus tremula after felling, Phellinus tremulae successfully invades the central part of the living tree. Possible causes for this difference were investigated. It was found that sterile wood samples were decomposed more rapidly by saprophytic fungi than by parasitic fungi. With both types of fungi, heartwood was more resistant to decay than sapwood. It is postulated that living sapwood inhibits infection by most woodrotting fungi, but that P. tremulae can successfully overcome this barrier. This postulate cannot be confirmed by experiments using sterilized wood samples.     

On the reproduction of aspen (Populus tremula L.) with emphasis on its suckering ability

Numerical data are presented that support the proposal mentioned as early as the mid‐19th century that aspen roots may remain alive for a long period after the disappearance of parent trees. The mechanism that makes this possible is the ability of this species to form suckers continuously. In stands the bulk of the suckers live a few years and are then replaced by new ones. If conditions allow some suckers may develop into big trees even in old stands. The conclusions are based on material obtained in experiments comprising chemical control of aspen, including different application methods, herbicides, doses and in some cases control treatment (cleaning).     

European aspen (Populus tremula L.): a review with particular reference to Scotland II. Values, silviculture and utilization

Interest in European aspen (Populus tremula L.) has increasedin recent years as a result of efforts to restore and extendnative woodland, and to diversify the range of species usedin forestry plantations. Aspen is a straightforward tree tomanage; it is of considerable ecological value and capable ofproducing utilizable timber. Planting of aspen in native woodlandis currently limited by a shortage of plants of Scottish origin.Propagation from seed, whilst possible, has not been practised,and recent efforts to produce stock have concentrated on vegetativemeans. The use of aspen as a component of plantation forestrycould be increased on appropriate sites; it is an excellentamenity species and its timber is currently acceptable as chipwoodand could complement the production of poplar timber in thelowlands. However, on the basis of limited data, growth ratesand stem form of aspen in Scotland appear to be rather poorerthan on the European continent where aspen is a significanttimber producing species. More would also need to be known aboutits susceptibility to disease before its timber production potentialcould be evaluated properly.     

European aspen (Populus tremula L.): a review with particular reference to Scotland I. Distribution, ecology and genetic variation

European aspen (Populus tremula L.) has attracted relativelylittle research attention in Britain. Interest in the specieshas increased in recent years because of efforts to restoreand extend native woodland, of which aspen is an infrequentbut important component species. This paper draws together informationfrom British, European and Russian literature on the ecologyand genetics of aspen. The species is distributed throughoutBritain in many different types of woodland. It is particularlycommon in the Scottish Highlands and Islands where it occurson a diverse range of sites from sea cliffs to near the treeline.It is associated with a distinct insect fauna, several speciesof which are considered endangered. Information is lacking onseveral fundamental aspects of the ecology and genetics of aspenin Scotland including seed production, growth rates, geneticvariation and its conservation value and status in native forestecosystems. Some useful information on these topics is presentedfrom studies of aspen elsewhere in Northern Europe, particularlyNorway.     

Sexual Reproduction of European Aspen (Populus tremula L.) at Prescribed Burned Site: The Effects of Moisture Conditions

Aspen (Populus tremula) is capable of reproducing both sexually and asexually from root suckers, the latter being more common. Seedlings are rare even though potential seed production is enormous. The aim of this study was to find out how different moisture conditions affect emergence and survival of aspen seedlings on burned soil. We set up a sowing experiment on mineral soil and on humus. The sowing was done at 240 microsites (θ7 cm) in 15 blocks (1×1 m). Treatments were sowing time, watering and sowing shelter. A total of 1463 seedlings emerged on mineral soil and 26 on humus. Watering, covering, their interaction, block and sowing time had significant effects on the number of seedlings. The largest numbers of seedlings were recorded at microsites that were watered and covered. The survival of seedlings was 20% during the growing season and 45% over winter. We conclude that seedbed conditions and moisture are the most essential factors for sexual reproduction of aspen. Short-term weather conditions have an important effect on seedling emergence and are likely to greatly affect field studies.     

Productivity and thinning effects in hybrid aspen (Populus tremula L. x P. tremuloides Michx.) stands in southern Sweden

The productivity of hybrid aspen stands in southern Sweden,originating from clone selections performed during the 1980s,was investigated. Thirteen former research and demonstrationsites were included. Repeated measurements of tree diametersand heights were taken, from which yields were estimated. Theresults indicate that the mean annual increment, obtained withoutany artificial addition of fertilizers or irrigation, will exceed20 m³ of stem wood ha^–1 a^–1 during a 20–25-yearrotation period. This corresponds to a total average woody biomassproduction, including branches, of over 8 tonnes dry matterha^–1 a^–1. These production levels show that hybridaspen is a competitive alternative for short rotation forestryin Sweden, and that the clone selection programme has greatlyenhanced productivity, compared with published growth ratesof the material previously used. For comparison, two standsthat regenerated via root suckers from material used in thebreeding programme up to the 1960s were included. They showedhigher growth than the 13 planted stands, mainly due to higherstem density and faster height development. This indicates thateven further increases in hybrid aspen yield are possible instands derived from suckers of the most recently selected clones.Thinning studies, including three different weights of thinning,were carried out on five of the sites. Five years after thestart of the treatments, stands given no thinning showed significantlyhigher current annual increment, but significantly smaller meanstem diameter than stands in which thinning was applied. However,the faster diameter development (and thus higher volumes ofvaluable wood assortments) associated with heavier thinningsmay compensate economically under current market conditionsfor the loss in total volume production.     

The decay pattern of Phellinus tremulae (Bond.) Bond,et Borisov in Populus tremula L.

The distribution of Phellinus tremulae in stems of Populus tremula was studied visually, macroscopically as well as microscopically, and by means of isolation of the fungus in pure culture. Evidence suggests that P. tremulae occurs alone in most of the tree. Microorganisms associated with P. tremulae in the wood are suggested to be independent invaders or secondary invaders. A dark reaction zone was formed in certain parts of the wood and consisted microscopically of melanized hyphes and brown material in the ray cells. An inner wood, formed as a result of the death of nearby brandies, is apparently the main part infected by P. tremulae, and its outer border may basically limit the spread of the fungus in the trunk. The formation of “heartwood” in aspen is discussed, and a suggested outline shown for the infection and decay processes caused by the fungus.     

Effects of timing,duration, and intensity of simulated browsing on Scots pine growth and stem quality

This study examines a large number of structural and growth-related responses of young Scots pine in relation to simulated browsing from large herbivores. We combined different modes of intensity, timing, and duration of clipping for nine consecutive years from 2000 to 2008. Data on tree responses were collected yearly during this period, as well as in 2011, and were analyzed using ordination techniques and generalized linear models. A key result demonstrated is the serious effects of clipping on the youngest pines. Mortality was high when clipping started the first year, indicating that the trees are vulnerable at that stage, but after that increasing intensity and duration of clipping were the main causes of mortality. Among the different treatment factors, intensity of clipping had the largest influence on altering the final growth responses, some of the quality responses, and mortality. Timing of clipping was the most important factor for explaining the occurrence of the majority of responses related to stem structure. Duration of clipping had the largest effect on the responses that included repeated-measures data, such as height development and number of stems. We discuss the long-term consequences of browsing for the trees and the implications for forest management.     

The occurrence of Phellinus tremulae (Bond.) Bond. and Borisov as a primary parasite in Populus tremula L.

Phellinus tremulae was inoculated on Populus tremula. It spread in the sapwood and inner wood of a living tree without other organisms being present. Plating experiments with wood meal from different parts of a sound tree showed that there were very few, if any, propagules of micro organisms in the sound wood, neither were there any organisms found in front of the decay of P. tremulae at the top of the decay column. These results support the theory that P. tremulae is a primary parasite in aspen. Decay fungi grown on fresh and autoclaved wood of aspen had very different growth rates. Most of the fungi tested grew best on autoclaved sapvvood. The difference in growth rate on fresh and autoclaved heartwood was lcss significant, suggesting that there is a barrier in the living wood which is destroyed by autoclaving. P. tremulae was thc only fungus that grew best on fresh inner wood.     

Dependence of the water vapor diffusion coefficient of aspen (Populus spec.) on moisture content

Summary The dependence of the diffusion coefficient of water in aspen (Populus sp.) on moisture content over the range of 0 to 18 percent moisture content at 43 °C was determined. The method requires a series of adsorption experiments and application of a numerical method for solving the diffusion equation with a moisture-content-dependent diffusion coefficient. The diffusion coefficient was found to increase exponentially with moisture content. The numerical method includes variable boundary conditions, as defined by the surface emission coefficient, so that the effect of surface equilibrium moisture content can be examined. The experimental moisture content-time curves can be predicted by the numerical method and showed an average deviation of 0.2 percent moisture content with the largest deviation being 0.7 percent moisture content. Practical implications and uses of the information derived from the numerical method are also discussed.This research was partially funded under the USDA Competitive Grant program     

Genotypic variation in growth and physiological responses of Finnish hybrid aspen (Populus tremuloides x P. tremula) to elevated tropospheric ozone concentration

Saplings of six Finnish hybrid aspen (Populus tremuloides Michx. x P. tremula L.) clones were exposed to 0, 50, 100 and 150 ppb ozone (O3) for 32 days in a chamber experiment to determine differences in O3 sensitivity among genotypes. Based on the chamber experiment, three clones with intermediate sensitivity to O3 were selected for a free-air O3 enrichment experiment in which plants were exposed for 2 months to either ambient air (control) or air containing 1.3 x the ambient O3 concentration. We measured stem height and radial growth, number of leaves, dry mass and relative growth rate of leaves, stem and roots, visible leaf injuries, net photosynthesis and stomatal conductance of the clones. There was high clonal variation in susceptibility to O3 in the chamber experiment, indicated by foliar injuries and differential reductions in growth and net photosynthesis. In the free-air O3 enrichment experiment, ozone caused a shift in resource allocation toward stem height growth, thereby altering the shoot to root balance. In both experiments, low O3 concentrations tended to stimulate growth of most clones, whereas 100 and 150 ppb O3 in the chamber experiment impaired growth of most clones. However, growth of the most O3-tolerant clone was not significantly affected by any O3 treatment.     

Modelling mortality of Scots pine (Pinus sylvestris L.) plantations in the northwest of Spain

Mortality is an important element of growth and yield models, especially if only low intensity silvicultural treatments are carried out. The objective of the present study was to develop a model for predicting tree number decline in planted even-aged stands of Scots pine (Pinus sylvestris L.) in Galicia (northwestern Spain). The model was constructed using data from two inventories of a trial network involving 68 permanent plots located in unthinned stands, or stands thinned lightly from below. Two alternatives were tested. In one alternative, a two-step modelling strategy was applied. First, a binary response function predicting the survival probability of all the trees in the stand was constructed, and an equation for reduction in tree number was developed, using only data where death had occurred over the period analyzed. Three different approaches were then used to compare the application of the above-mentioned functions together. In the other alternative, a mortality function for directly predicting the reduction in tree number was fitted, including all plots (with and without occurrence of mortality). Both alternatives provided similar results, showed logical behavior, and performed satisfactorily in evaluation tests. However, in choosing the best strategy for inclusion in a stand-level simulator, the use of the second alternative is recommended because it possesses the path invariance property required in a mortality model.     

Crookedness of pedunculate oak (Quercus robur L.) growing under a canopy of Scots pine (Pinus sylvestris L.)

Natural regeneration of pedunculate oak growing under a canopy of Scots may be used for conversion into a mixed or predominantly broadleaved forest. From an economic perspective it is desirable that the oaks have straight stems. In this paper we present a case study for analysing oak stem curvature and its causes. The study was conducted in a mature pine stand with understorey oaks of age ca. 60 years in Poland. The oaks were classified into two main groups as either straight or crooked. The following variables were measured or assessed for the oaks: (1) stem curvature (crookedness), total tree height, diameter at breast height (DBH) and some tree crown characteristics, (2) the growth of DBH and height in each year of the life of each tree and (3) the competitive pressure of surrounding trees. Durable stem curvatures were formed at the culmination of the height increment. The first 20 years of the life of the oak were crucial in terms of stem quality. The annual height increment was larger in the years when the curvature was formed than in adjacent years. The factors that caused crooked stems were the same for straight and crooked oaks, but the magnitude of the response was different. When planning the conversion of Scots pine into oak, full overhead light should be provided as early as possible, but not later than 20 years since the establishment of the regeneration.     

Studies on the epiphytic bacteria on Populus tremula L. and Populus alba L. at the time of optimal and minimal cambium activity and infection by Venturia

The participation of epiphytic bacteria in the groups of bacteria dominating on the shoots and root collars of two year aspen and white poplar differs both in quantity and quality and depends on the chemical composition of the diffusion exudates related to cambium activity. This indicates that epiphytic bacteria can utilize carbon sources in the exudates and transform lignin precursors.     

Surface quality sensing of trembling aspen (Populus tremuloides Michx.) veneer products by near infrared spectroscopy

Abstract

The potential of near infrared spectroscopy (NIRS) to estimate wood moisture content (MC), surface energy characteristics and adhesive bond strength were evaluated on aspen (Populus tremuloides Michx.) veneer subjected to different drying times. For samples dried progressively at 103°C, the best MC prediction model was for the total MC range (0–100%) with an R² value of 0.68. However, exposure at 180°C produced surface colour changes, and the CIE L*a*b* colour parameters measuring colour changes were better estimated using the 400–900 nm spectral data than the 1100–2400 nm spectral data. Increased exposure time at 180°C resulted in lower wettability and, hence, larger contact angles, especially when ethylene glycol was used as probe solvent. Lap shear strength tests on veneers showed that adhesion by phenol formaldehyde resins was impaired by the high temperature exposure; however, the lap shear strength test had high variability so there was not always a clear relationship between contact angle and lap shear strength test.     

连栽对江汉平原杨树人工林生长量的影响

笔者研究了连栽对江汉平原杨树人工林林分生长量的影响,结果表明：连栽条件下,前4年对杨树生长影响并不明显,但从第5～9年,第2代林较第1代林胸径减少0．70％～12．05％,树高减少7．28%～20．41％,材积减少17．78％～27．83％,单株生物量减少7．26％～32．42%;但通过施肥、间作等合理的抚育措施可以有效减小连栽对林分生长量的负面影响。