Dries Vansteenkiste

Radial growth change of temperate tree species in response to altered regional climate and air quality in the period 1901–2008

Both increasing and decreasing 20th century growth trends have been reported in forests throughout Europe, but only for few species and areas suitable modelling techniques have been used to distinguish individual tree growth (operating on a local scale) from growth change due to exogenous factors (operating on a broad geographical scale). This study relates for the first time observed growth changes, in terms of basal area increment (BAI) of dominant trees of pedunculate oak, common beech and Scots pine, in north-west European temperate lowland forests (Flanders) to climate, atmospheric CO2 and tropospheric O3 concentrations, N deposition, site quality and forest structure for more than a century (the period 1901–2008), applying mixed models. Growth change during the 20th century is observed for oak (increasing growth) and beech (increasing growth until the 1960s, growth decline afterwards), but not for pine. It was possible to relate growth change of oak and beech to climate time series and N deposition trends. Adding time series for CO2 and O3 concentration did not significantly improve model results. For oak and beech a switch from positive to negative growth response with increasing nitrogen deposition throughout time is observed. Growth increase for oak is mainly determined by the interaction between growing season temperature and soil water recharge. It is reasonable to assume that the observed growth trend for oak will continue for as long as early season water availability is not compromised. The decreasing trend in summer relative air humidity observed since the 1960s in the study area can be a main cause of recent beech BAI decrease. A further growth decline of beech can be expected, independent of site quality.

Leaf area index development in temperate oak and beech forests is driven by stand characteristics and weather conditions

Using data from 20 even-aged and homogeneous mature beech and oak study plots in Flanders (Northern Belgium), an analysis of the empirical relationships between the rates of leaf area index (LAI) change throughout the leaf development of 2008 and stand, site and meteorological variables was performed. Species-specific multiple linear regressions were fitted between the rates of LAI change and the predictors for two distinct periods from April until August. After a sharp increase in LAI following budburst, the seasonal LAI development for both species showed a marked period of stationary LAI development over all study plots. The cause for the cessation of LAI growth was assumed to be the decline of air temperature and radiation during this period. Later on, the rate of LAI development restarted similarly in every plot. The influence of weather on LAI development was high and its effects were different between species, with beech mostly affected by radiation and oak negatively related to minimal and maximal values of air temperature. Furthermore, our analysis suggested that stand structural (tree density and stand basal area for both species) and tree growth characteristics (average tree-ring width ratio for oak) variables were major drivers of the LAI development during early spring. Later during the growth period, stand variables became less predominant in affecting LAI development. Site quality variables affected LAI development to a lesser extent. The seasonal LAI development was found very similar among stands. This study adds a more accurate and comprehensive approach to the modelling of LAI development during leaf growth of two important European temperate deciduous forest species.

Intra- and inter-annual variation of Cd, Zn, Mn and Cu in foliage of poplars on contaminated soil.

The uptake of trace metals in the leaves of fast-growing woody species is a crucial factor in ecological risk assessment and in the evaluation of phytoextraction potentials. In this study, we present a long-term data series of foliar Cd, Zn, Mn and Cu concentrations in poplar (Populus trichocarpa x P. deltoides). Leaves were collected every three weeks from 2001 until 2007 on three sites, (i) a new plantation on an alluvial soil polluted by river sediments, (ii) a new plantation on an unpolluted soil and (iii) a 10-year old plantation on a polluted dredged sediment soil. In addition, tree rings were measured on the alluvial soil in order to better assess growth over the past seven years. Foliar concentrations of Cd, Zn and Mn decreased considerably with time in the new plantation on polluted soil. Concentrations of Zn and Mn decreased in the new plantation on unpolluted soil as well. The older plantation on polluted soil did not show changes in foliar concentrations for Cd, Zn or Mn. Foliar Cu concentrations slightly increased for all sites. Within one growing season, foliar concentrations of Cd, Zn, Cu and Mn increased towards the end of the season. The tree ring data of the poplars on the alluvial soil indicated a strong decrease in growth due to declining tree condition from 2005 onwards, the same year that foliar Cd and Zn concentrations markedly decreased. Lower transpiration rates probably induced a lower uptake of dissolved trace metals. It is concluded that stand health and growth rate have a strong impact on the variation of foliar trace metal concentrations over time.

High-resolution proxies for wood density variations in Terminalia superba.

BACKGROUND AND AIMS Density is a crucial variable in forest and wood science and is evaluated by a multitude of methods. Direct gravimetric methods are mostly destructive and time-consuming. Therefore, faster and semi- to non-destructive indirect methods have been developed. METHODS Profiles of wood density variations with a resolution of approx. 50 µm were derived from one-dimensional resistance drillings, two-dimensional neutron scans, and three-dimensional neutron and X-ray scans. All methods were applied on Terminalia superba Engl. & Diels, an African pioneer species which sometimes exhibits a brown heart (limba noir). KEY RESULTS The use of X-ray tomography combined with a reference material permitted direct estimates of wood density. These X-ray-derived densities overestimated gravimetrically determined densities non-significantly and showed high correlation (linear regression, R(2) = 0·995). When comparing X-ray densities with the attenuation coefficients of neutron scans and the amplitude of drilling resistance, a significant linear relation was found with the neutron attenuation coefficient (R(2) = 0·986) yet a weak relation with drilling resistance (R(2) = 0·243). When density patterns are compared, all three methods are capable of revealing the same trends. Differences are mainly due to the orientation of tree rings and the different characteristics of the indirect methods. CONCLUSIONS High-resolution X-ray computed tomography is a promising technique for research on wood cores and will be explored further on other temperate and tropical species. Further study on limba noir is necessary to reveal the causes of density variations and to determine how resistance drillings can be further refined.

End-use related physical and mechanical properties of selected fast-growing poplar hybrids (Populus trichocarpa × P. deltoides).

This study focused on physical and mechanical properties of fast-growing poplar clones in relation to potential end uses with high added value. A total of 14 trees from three different clones, all P. trichocarpa × deltoides (T×D) hybrids, were felled in a poplar plantation in Lille (Belgium): six ‘Beaupré’, four ‘Hazendans’ and four ‘Hoogvorst’. Growth rate was found to have no significant influence on the physical mechanical properties. Although the investigated clones are genetically closely related, important variations in physical and mechanical properties were observed. Specific features such as spatial distribution of tension wood and dimensional stability are the main quality factors. It was concluded that ‘Beaupré’ is suitable for a wide range of high value added applications, such as plywood or construction wood. ‘Hazendans’ and ‘Hoogvorst’ will need adapted technology in processing. Further research is needed to characterize clonally induced variation in properties and to assess adequate processing strategies for multiclonal poplar stands.RésuméCette étude porte sur les caractéristiques physiques et mécaniques du bois de clones de peupliers à croissance rapide, en fonction de l’aptitude à l’emploi. Quatorze arbres ont été étudiés, provenant d’une plantation de peuplier à Lille (Belgique), appartenant à trois hybrides différents de P. trichocarpa × deltoides, à savoir six “Beaupré”, quatre “Hazendans” et quatre “Hoogvorst”. Les caractéristiques de croissance n’ont pas affecté de manière significative les propriétés physiques et mécaniques. Bien que les clones étudiés soient génétiquement rapprochés, des variations importantes ont été constatées dans les propriétés physiques et mécaniques. Des caractéristiques spécifiques telles que la distribution spatiale du bois de tension et la stabilité dimensionnelle du bois sont des propriétés importantes affectant sur la qualité du produit final. On peut conclure que le bois de “Beaupré” est apte à la fabrication de panneaux contreplaqués et de bois de sciage. Une adaptation de la technologie de transformation sera nécessaire pour les clones ‘Hazendans’ et ‘Hoogvorst’. Des recherches approfondies seront requises afin d’évaluer la variabilité induite par l’effet clonal ainsi que pour identifier des stratégies adaptées à la transformation du bois de peuplements multiclonaux.

Influence of stand, site and meteorological variables on the maximum leaf area index of beech, oak and Scots pine

Different multiple linear regression models of maximum leaf area index (LAImax) based on stand characteristics, site quality, meteorological variables and their combinations were constructed and cross-validated for three economically important tree species in Flanders, Belgium: European beech (Fagus sylvatica L.), Pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.). The models were successfully tested on similar datasets of experimental sites across Europe. For each species, ten homogeneous and mature stands were selected, covering the species’ entire stand productivity range based on an a priori site index classification. LAImax was derived from measurements of leaf area index (LAI) made by means of hemispherical digital photography over the whole growing season (mid-April till end October 2008). Species-specific models of LAImax for beech and oak were mostly driven by management practice affecting stand characteristics and tree growth. Tree density and dominant height were main predictors for beech, while stand age and tree-ring growth were important in the oak models. Scots pine models were more affected by site quality and meteorological variables. The beech meteorological model showed very good agreement with LAI at several European sites. Scots pine’s stand model predicted well LAI across Europe. Since the species-specific models did not share common predictors, generic models of LAImax were developed for the 30 studied sites. Dominant height was found to be the best predictor in those generic models. As expected, they showed a lower predictive performance than species-specific ones.

Forest structure and soil fertility determine internal stem morphology of Pedunculate oak: a modelling approach using boosted regression trees

This study aims at the explanation of internal stem morphology of vital (co)dominant Pedunculate oak (Quercus robur L.) trees in homogeneous even-aged high-forests by the factors tree age, forest structure and site quality, using boosted regression trees as a powerful modelling technique. The study area covers the region of Flanders (Northern Belgium), which is characterised by the absence of strong topographic and climatic gradients. For 76 adult sample trees covering the entire productivity range of Pedunculate oak, morphological characteristics were derived from measurements of ring width or heartwood area on wood cores. Forest structure, soil physicochemical properties, humus quality, vegetation indices and litter nutrient contents were quantified at each sample location. Model predictive performance and generality are good. Tree age effects correspond to expected trends in age-related radial growth and heartwood portion. Even if management of oak trees in even-aged high-forests is rather similar over Flanders, forest structure is the most important factor determining ring width, followed by soil fertility. Heartwood portion is determined by soil fertility and crown structure. Effects of topsoil and humus physicochemical characteristics, litter nutrient contents and water supply mainly confirm autecological knowledge on oak. However, variables related to soil water availability are only occasionally relevant, and always of lower importance than soil fertility. The low importance of water availability in the models contradicts results from other studies, and the potential effect of confounding is discussed. The observed growth reduction at low litter N/P ratios might be indirectly linked to early litterfall.

High temperature drying of fresh sawn poplar wood in an experimental convective dryer

The special interest in Europe for the properties and the use of poplar and its hybrids, especially for the wood technological properties and their effect on the industrial processing of poplar wood, as well as the growing need for fast drying processes producing wood with a low final moisture content, led to this study. It was to be investigated whether high temperature drying (HTD) could also be applied to light hardwoods with properties similar to that of softwoods, like e.g. poplar. Poplar has some peculiar properties, e.g. large heterogenity, wet pocket formation, tension wood, juvenile wood., ..., complicating the application of HTD and increasing the risk for drying defects. Eleven different high temperature drying schedules and one low temperature drying schedule were performed in an experimental electrical drying oven on piles of small sizes of fresh poplar. For reference, five piles of fresh poplar were subjected to accelerated air-drying. Equations were obtained from two seperate multivariate regression analyses with wood characteristics for the first and typical drying-schedule parameters for the second regression, as independent variables, and a number of quantified drying defects as dependent variables for both regressions.ZusammenfassungVeranlaßt waren diese Untersuchungen einerseits durch das spezielle Interesse in Europa für die technologische Verwendung von Pappelholz, andererseits durch den wachsenden Bedarf an schnellen Trocknungsprozessen. Zu untersuchen war, inwieweit die Hochtemperaturtrocknung (HTD) auch auf leichte Laubhölzer, am Beispiel der Pappel, anwendbar ist. Pappelholz weist einige spezielle Eigenschaften auf (starke Heterogenität in Dichte und Feuchte, Zugholz, juveniles Holz etc.), welche die Anwendung der HTD schwierig machen. Elf unterschiedliche Hochtemperaturprogramme und eines bei niedriger Temperatur wurden in einem elektrisch beheizten Umlufttrockner mit frischgesägtem Pappelholz von kleiner Abmessung geprüft. Zum Vergleich wurden fünf Stapel frischen Pappelholzes einer beschleunigten Lufttrocknung unterzogen. Durch multivariate Analyse ergaben sich Regressionsgleichungen, wobei einmal die Holzeigenschaften zum andern die Trocknungsparameter als Variablen enthalten sind. Zusätzlich wurden Regressionsgleichungen für Trocknungsfehler abhängig von beiden Variablentypen ermittelt.

Composition, distribution and supposed origin of mineral inclusions in sessile oak wood - consequences for microdensitometrical analysis

SEM and light-microscopical observations, supported by chemical microanalysis with an EDXA system, revealed that light-saturated pixels observed in X-ray negatives of sessile oak (Quercus petraea Liebl.) wood were caused by inorganic deposits present inside multiseriate ray and axial parenchyma cells. Calcium oxalate crystals, silica grains and amorphous granules with varied mineral compositions have been identified. The wood strips of three out of six sampled trees contained measurable amounts of mineral inclusions which were quantified using image analysis. Based on the variations of mineral content observed between trees and within and between annual rings of the same tree, some hypotheses were formulated concerning the factors involved in the formation of inorganic deposits in oak wood. Their occurrence varies depending on the mineral concerned and seems to be controlled largely by a tree effect. The time of formation appears to coincide with a shifting of the oak wood’s functions as a result of heartwood formation processes (inter-annual scale) or changes in leaf phenology and climate (intra-annual scale). In addition, the technical consequences of their presence as well as their effects on wood density measurements through microdensitometry are discussed.RésuméDes observations à l’aide de microscopes électronique à balayage et optique, appuyées par des analyses élémentaires au moyen d’un système EDXA, ont révélé que les points-images saturés en niveau de gris, constatés dans les négatifs de radiographies de bois de chêne sessile (Quercus petraea Liebl.), étaient dus à des dépôts inorganiques présents à l’intérieur des cellules parenchymateuses des rayons ligneux multisériés et du parenchyme axial. Des cristaux d’oxalate de calcium, des grains de silice et des granules amorphes ayant des compositions minérales variées ont été identifiés. Des barrettes de trois sur six arbres échantillonnés contenaient des quantités mesurables d’inclusions minérales qui ont été quantifiées par analyse d’images. En se basant sur les variations du contenu en dépôts minéraux observées entre arbres ainsi qu’à l’intérieur d’un arbre, aux niveaux intra-et intercerne, quelques hypothèses ont été avancées concernant les facteurs potentiellement responsables de la formation de dépôts minéraux chez le chêne. Leur présence dépend du minéral concerné et semble être contrôlée par un fort effet arbre. L’apparition des minéraux coïncide apparemment avec des changements de fonctions du bois relatifs aux processus de duraminisation (échelle interannuelle) ou correspondant à des évolutions phénologiques ou climatiques (échelle intra-annuelle). Enfin, les conséquences techniques ainsi que les effets de leur présence sur des mesures de la densité du bois par analyse microdensitométrique sont discutés.