Frank M. Thomas

Leaf litter decomposition in temperate deciduous forest stands with a decreasing fraction of beech (Fagus sylvatica)

We hypothesised that the decomposition rates of leaf litter will increase along a gradient of decreasing fraction of the European beech (Fagus sylvatica) and increasing tree species diversity in the generally beech-dominated Central European temperate deciduous forests due to an increase in litter quality. We studied the decomposition of leaf litter including its lignin fraction in monospecific (pure beech) stands and in stands with up to five tree genera (Acer spp., Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Tilia spp.) using a litterbag approach. Litter and lignin decomposition was more rapid in stand-representative litter from multispecific stands than in litter from pure beech stands. Except for beech litter, the decomposition rates of species-specific tree litter did not differ significantly among the stand types, but were most rapid in Fraxinus excelsior and slowest in beech in an interspecific comparison. Pairwise comparisons of the decomposition of beech litter with litter of the other tree species (except for Acerplatanoides) revealed a “home field advantage” of up to 20% (more rapid litter decomposition in stands with a high fraction of its own species than in stands with a different tree species composition). Decomposition of stand-representative litter mixtures displayed additive characteristics, not significantly more rapid than predicted by the decomposition of litter from the individual tree species. Leaf litter decomposition rates were positively correlated with the initial N and Ca concentrations of the litter, and negatively with the initial C:N, C:P and lignin:N ratios. The results support our hypothesis that the overall decomposition rates are mainly influenced by the chemical composition of the individual litter species. Thus, the fraction of individual tree species in the species composition seems to be more important for the litter decomposition rates than tree species diversity itself.

Productivity of temperate broad-leaved forest stands differing in tree species diversity

Abstract• Understanding the effects of tree species diversity on biomass and production of forests is fundamental for carbon sequestration strategies, particularly in the perspective of the current climate change. However, the diversity-productivity relationship in old-growth forests is not well understood.• We quantified biomass and above-ground production in nine forest stands with increasing tree species diversity from monocultures of beech to stands consisting of up to five deciduous tree species (Fagus sylvatica, Fraxinus excelsior, Tilia spp., Carpinus betulus, Acer spp.) to examine (a) if mixed stands are more productive than monospecific stands, (b) how tree species differ in the productivity of stem wood, leaves and fruits, and (c) if beech productivity increases with tree diversity due to lower intraspecific competition and complementary resource use.• Total above-ground biomass and wood production decreased with increasing tree species diversity. In Fagus and Fraxinus, the basal area-related wood productivity exceeded those of the co-occurring tree species, while Tilia had the highest leaf productivity. Fagus trees showed no elevated production per basal area in the mixed stands.• We found no evidence of complementary resource use associated with biomass production. We conclude that above-ground productivity of old-growth temperate deciduous forests depend more on tree species-specific traits than on tree diversity itself.Résumé• Comprendre les effets de la diversité des espèces d’arbres sur la biomasse et la production des forêts est fondamental pour les stratégies de piégeage du carbone, en particulier dans la perspective des changements climatiques actuels. Cependant, dans les vieilles forêts la relation diversité—productivité n’est pas bien comprise.• Nous avons quantifié la biomasse et la production hors-sol de neuf peuplements forestiers à diversité croissante, allant de monocultures de hêtre jusqu’à des peuplements constitués d’un maximum de cinq espèces d’arbres à feuilles caduques (Fagus sylvatica, Fraxinus excelsior, Tilia spp., Carpinus betulus, Acer spp.) afin d’examiner (a) si les peuplements mélangés sont plus productifs que les peuplements monospécifiques, (b) comment diverses espèces d’arbre diffèrent pour la productivité du bois de la tige, des feuilles et des fruits, et (c) si la productivité du hêtre augmente avec la diversité en relation avec une baisse de la compétition intra-spécifique et une complémentarité dans l’utilisation de ressources.• La biomasse aérienne totale et la production de bois ont diminué quand la diversité des espèces d’arbres augmentait. Pour Fagus et Fraxinus, la productivité en bois estimée d’après la surface terrière dépassait celle des autres espèces, tandis que Tilia avait la plus forte productivité en feuilles. Dans les peuplements mixtes, les hêtres ne présentaient pas de gain de productivité ramenée à la surface terrière.• Nous n’avons trouvé aucune preuve d’une complémentarité de l’utilisation de ressources liées à la production de biomasse. Nous concluons que la productivité des parties aériennes des vieux peuplements feuillus tempérés, dépend davantage de traits spécifiques des différentes espèces que de la diversité en espèces.

Morphological and physiological reactions of young deciduous trees (Quercus robur L., Q. petraea [Matt.] Liebl., Fagus sylvatica L.) to waterlogging.

One-year-old seedlings of Quercus robur L., Q. petraea (Matt.) Liebl. and Fagus sylvatica L. were cultivated in lysimeters and subjected to waterlogging for 17 weeks, interrupted by a five-week drainage period during summer. The growth of Q. robur was less affected by waterlogging than that of Q. petraea and Fagus. Waterlogging resulted in the formation of adventitious roots in Q. robur and Q. petraea, but not in Fagus. In contrast to Fagus, Q. robur and, to a lesser extent, Q. petraea were able to generate roots even below the water table. The hydraulic conductance of the excised root systems, the stomatal conductance and, in Fagus, the leaf water potential and the leaf-mass related hydraulic conductance were decreased by waterlogging. The decrease in the hydraulic conductance was largest in Fagus, and smallest in Q. robur. The roots of Fagus responded to anaerobic conditions with an increase in ethanol concentration. The measurements of nitrate reductase activities in roots and leaves provided no indications of a persistent contribution of NO3− metabolism to the alleviation of waterlogging-induced stress. It is concluded that Q. robur and, to a lesser extent, Q. petraea can tolerate waterlogging periods better than Fagus due to a different pattern of root formation, and to a better adjustment of leaf biomass production to the hydraulic conductivity of the root system.

Effects of defoliation and drought stress on biomass partitioning and water relations of Quercus robur and Quercus petraea

Summary To test the isolated and combined effects of drought and defoliation on biomass partitioning and water relations, four-year-old saplings of Quercus robur L. and Q. petraea (Matt.) Liebl. were investigated under controlled conditions. It was hypothesized that defoliation leads to a reduction of late wood and that, as a consequence, subsequent drought stress will more rapidly lead to desiccation damage due to the loss of reserves in water transport capacity. Alternatively, defoliation could have a negative effect on water relations via a growth reduction of the fine-root system, thereby diminishing the trees capacity for water uptake. Four treatments were applied to each species: (1) control C, (2) defoliation in two consecutive years De, (3) drought stress in the second year Dr, (4) a combination of (2) and (3) DeDr. Drought-stressed trees showed a decrease of predawn and mid-day water potentials and of maximum stomatal conductance (gsm). Defoliated trees exhibited a higher gsm compared to non-defoliated trees even under drought-stress conditions. At the end of the drought period the daily sum of transpiration per unit leaf area (Ed) was only reduced in Dr, but not in DeDr trees, and was even increased in De trees of Q. robur. Fine-root mass, leaf mass, leaf area and specific leaf area decreased due to defoliation. Drought stress also led to a reduction of leaf mass in Q. petraea as well as to reduced leaf/fine-root ratio (Dr and DeDr trees) in both species. Although a reduction of late wood was found in DeDr trees of Q. robur and in the interannual comparison of De and DeDr twigs of Q. petraea, no restrictions of transpiration or hydraulic conductance could be observed in both species. Therefore, this finding supports the alternative hypothesis, i.e. a possible impairment of the trees water relation caused by a reduction of the fine-root biomass due to defoliation. A combination of both stress factors impairs the morphological acclimatization to drought, i.e. an increased formation of fine-root biomass, and, partly, the physiological avoidance of dehydration, i.e. a reduction in stomatal conductance. Therefore, it is concluded that, in contrast to an isolated occurence of the stressors, their combined action depending on the time of occurrence of the stressors will ultimately result in tree damage due to desiccation. Unter kontrollierten Bedingungen wurden die Auswirkungen von Trockenheit und Entlaubung einzeln und in Kombination auf die Biomassenpartitionierung und den Wasserhaushalt vierjahriger Pflanzen von Quercus robur L. and Q. petraea (Matt.) Liebl. untersucht. Es wurde angenommen, dass Entlaubung in einer Reduktion des Spatholzes resultiert, und infolgedessen anschliesender Trockenstress schneller zu Trocknisschaden aufgrund fehlender Wassertransportreserven fuhrt. Alternativ kann Entlaubung einen negativen Effekt auf den Wasserhaushalt des Baumes uber eine Reduktion des Feinwurzelsystems ausuben, was zu einer Verringerung der Wasseraufnahmekapazitat fuhrt. Bei beiden Arten wurden vier Varianten untersucht: (1) Kontrolle C, (2) Entlaubung in zwei aufeinanderfolgenden Jahren De, (3) Trockenstress im zweiten Jahr Dr, (4) eine Kombination aus (2) und (3) DeDr. Trockengestresste Baume zeigten eine Abnahme der Predawn- und nachmittaglichen Wasserpotentiale als auch der maximalen stomataren Leitfahigkeit (gsm). Entlaubte Baume wiesen auch unter Trockenstressbedingungen eine hohere gsm auf als nicht-entlaubte Baume. Gegen Ende der Trockenperiode war die Tagestranspirationssumme pro Blattflacheneinheit (Ed) nur bei Dr, nicht aber bei DeDr, reduziert und bei De von Q. robur sogar erhoht. Feinwurzelmasse, Blattbiomasse, Blattflache und die spezifische Blattflache nahmen bei entlaubten Baumen ab. Trockenstress fuhrte zudem zu einer Reduktion der Blattflache bei Q. petraea als auch zu einem reduzierten Blatt/Feinwurzelverhaltnis (De und DeDr) bei beiden Arten. Obwohl DeDr-Baume von Q. robur eine Reduktion des Spatholzes aufwiesen und dies auch in dem Vergleich zwischen den Jahren bei De und DeDr bei Q. petraea der Fall war, wurden keine signifikanten Beeintrachtigungen der Transpiration und der hydraulischen Leitfahigkeit festgestellt. Dies unterstutzt die Alternativhypothese, d.h. eine mogliche Beeintrachtigung des Wasserhaushaltes durch eine entlaubungsbedingte Reduktion der Feinwurzelbiomasse. Eine Kombination beider Faktoren beeintrachtigt die morphologische Akklimatisation an Trockenheit, die normalerweise durch eine Erhohung der Feinwurzelbiomasse erreicht wird, und zum Teil auch die physiologische Verhinderung von Wasserverlusten, die sonst aus einer reduzierten Stomataoffnungsweite resultiert. Es wird geschlossen, dass, im Gegensatz zum isolierten Auftreten der Stressoren, eine Kombination beider Faktoren, je nach dem Zeitpunkt ihres Auftretens, letztendlich zu Trocknisschaden fuhren wird.

Growth and water relations of four deciduous tree species (Fagus sylvatica L., Quercus petraea [Matt.] Liebl., Q. pubescens Willd., Sorbus aria [L.] Cr.) occurring at Central-European tree-line sites on shallow calcareous soils: physiological reactions of seedlings to severe drought.

Summary The reactions of growth and water relations to drought were tested in young plants of four deciduous tree species ( Fagus sylvatica L., Quercus petraea [M att .] L iebl ., Q. pubescens W illd ., Sorbus aria [L.] C r .) which occur at drought-caused tree-line sites in Central Europe on shallow calcareous soils. Two-year-old seedlings were planted into lysimeters and subjected to two periods (eleven and ten weeks, respectively) of severe drought, interrupted by thorough rewatering after the first drought period. Control plants were always well watered. Drought stress responses of these plants were investigated by pV analysis, and by measurements of leaf conductance and actual water potentials. Additionally, desiccation experiments were performed with excised shoots of some of the trees. Drought-stressed F. sylvatica exhibited the highest rates of visible drought damage. However, this species was able to regain a high relative leaf water content (RWC) quickly after rewatering. S. aria, which can grow even on extremely exposed sites, was drought-adapted as shown by the following: its high leaf mass per unit area; its high leaf tissue elasticity; its high relative water content of saturated leaves (WC sat ); and the low relative water content of its symplasm, which facilitates large changes in osmotic pressure in spite of low dry-mass related solute concentrations. High δ 13 C ratios (deviation of the leafs 13 C fraction from a standard) indicated a high long-term water-use efficiency by this species. These features allow an effective gas exchange even at low leaf water potentials. In contrast to F. sylvatica and S. aria , the Quercus species exhibited adaptability to drought, which was evident due to significant morpho-anatomical differences between the drought-stressed and control trees. In Q. petraea , adaptability was achieved by reductions in the leaf/root ratio, and in the number of second flushes and buds. In Q. pubescens , adaptability was found at the leaf level; this is indicated by larger differences between the osmotic pressures at full turgor and at turgor loss, in higher WC sat , and in a decrease in the RWC at turgor loss. These alterations are obviously due to changes in the leaf tissue properties. In none of the species was osmotic adjustment found as an adaptation to drought stress.

Tree rooting patterns and soil water relations of healthy and damaged stands of mature oak (Quercus robur L. and Quercus petraea (Matt.) Liebl.)

At three sites in northwestern Germany, which represent the centres of the present oak damage, root distribution and biomass beneath healthy and damaged trees of mature pedunculate oak (Quercus robur L.; Neuenburg site) and sessile oak (Q. petraea [Matt.] Liebl.; Lappwald and Sprakensehl sites) were investigated, and soil texture, bulk density, duration of waterlogging periods and the water available in the mineral soil were determined. For Neuenburg and Sprakensehl, the available soil water was related to leaf water parameters determined in a separate investigation. At the clayey and hydromorphic sites of Neuenburg and Lappwald, the measurements were performed in each one healthy and one damaged part of the site, which differed in the number of oaks with crown damage. In the damaged stand of Neuenburg, the clay content of the subsoil was higher than in the healthy stand, and the soil water availability was reduced especially in dry periods. Compared to healthy oaks of the healthy stand, the density of finest plus fine roots as well as the biomasses of finest roots were lower beneath damaged oaks of the damaged stand. With decreasing relative available soil water (actually available water in relation to water available at the saturation state), the relative leaf water content decreased in damaged, but not in healthy oaks. At Lappwald, similar differences in soil water availability between the healthy and the damaged stand were found, but had no effect on the distribution or biomass of the roots. At the sandy site (Sprakensehl), the available soil water decreased drastically during a dry period, and predawn leaf water potentials of both healthy and damaged oaks declined with decreasing relative available soil water. However, the damaged oaks were not inferior to the healthy ones with respect to root density and biomass. It is concluded that, in the damaged stand of Neuenburg, the high clay content of the subsoil, which results in prolonged periods of waterlogging, in sharp changes from waterlogging to drought and decreased water availability in dry periods, is the reason for the reduced biomass and density of roots of the pedunculate oak. Thus, in northwestern Germany, unfavourable soil water relations are considered as a factor contributing to crown damage of pedunculate oak at hydromorphic sites, but not to damage of sessile oak.

Nitrate reduction in leaves and roots of young pedunculate oaks (Quercus robur) growing on different nitrate concentrations

Abstract Against the background of high rates of nitrogen (N) input into forest ecosystems and, in part, high nitrate (NO 3 − ) concentrations in the soil solutions, NO 3 − reduction activity and N accumulation in leaves and roots of young pedunculate oaks (Quercus robur) were investigated. Seedlings with unrestricted root growth, and 2-year-old saplings with cut root-stocks, were grown hydroponically at different forms and concentrations of N. The nitrate reductase activity (NRA) of the leaves and fine roots was measured in vivo without addition (NRA H 2 O ) or with addition of exogenous NO 3 − (NRA H 2 O ) to the incubation assay. The amounts of reduced NO 3 − as calculated with the NRA H 2 O and NRA KNO 3 were compared with the uptake of 15 NO 3 − , and with root-to-shoot translocation of NO 3 − as determined by NO 3 − concentrations of the xylem sap and transpiration rates. Compared with the NRA H 2 O , the NRA KNO 3 was higher by a factor of approximately 10 in the current year’s fine roots, and by a factor of about 60 in the leaves. In only one case did increased NO 3 − concentrations of the nutrient solution result in an increase in NRA. In some cases, NRA was diminished in the presence of ammonium (NH 4 + ) in the root medium. The quantities of reduced NO 3 − as calculated on the basis of NRA H 2 O agreed with the amounts of 15 N accumulated in roots and leaves, and with the amounts of NO 3 − translocated from the roots to the shoots. The contribution of the leaves to the total plant’s NO 3 − reduction as computed on the basis of NRA H 2 O was 1–17% in the seedlings, but up to 86% in the saplings; here, it correlated significantly with the leaf/root ratios on a fresh-weight basis. Compared with the leaf/root ratios, the form and concentration of the supplied N had a much lower impact on the share of the leaves in NO 3 − reduction; and did not affect the foliar N concentrations. In the leaves as well as in the roots, the concentrations of soluble NO 3 − were very low ( 3 − N g dry weight −1 ). The results show that young pedunculate oaks have a low affinity for NO 3 − N, even in the case of high NO 3 − supply.

Recent advances in cause-effect research on oak decline in Europe

Abstract 2 This review attempts to assess the causal factors and their interrelations that cause decline of widespread European oak 3 (Quercus) species. In particular, it provides an overview of 4 recent advances in cause-effect research since 2000. The first recorded outbreak of oak 5 decline took place in 1739–1748, and was followed by several other decline episodes, most 6 of them occurring in the 20th century. The mortality rates differ substantially among the 7 European regions and decline episodes; however, the decline of entire stands is a rare 8 event. The principal causes of oak decline are the following site-unspecific factors: severe (in 9 particular, repeated) defoliation by insect larvae, especially in combination with infection by powdery mildew, and weather extremes (summer drought,10 winter frost, spring frost); and the 11 following site-specific factors: the root pathogen Phytophthora quercina (which is virulent in 12 soils with a pH[CaCl 2 ] value  3.5 and certain fractions of exchangeable calcium), and a

Differential response of two Central-European oak species to single and combined stress factors

We studied the effects of repeated defoliation and drought, which are considered to be most effective in triggering oak decline, on the Central-European oak species Quercus robur L. and Quercus petraea [Matt.] Liebl. Three-year-old saplings were subjected to artificial defoliation in two consecutive years and to drought stress, applied singly or in combination. Differences in the morphological, anatomical and physiological responses to these stress factors among treatments and between species were tested in a 3-year experiment. Drought stress lowered the pre-dawn and the afternoon leaf-water potentials in both species, irrespective of additional defoliation. The relative water content of the leaves was unaffected by drought stress; but, in Q. robur, it decreased in trees subjected to defoliation and to the combination of drought stress and defoliation. When repeated defoliation and drought stress were applied within the same growing season, the leaf-to-air difference in the partial pressure of water vapour (Δw) explained most of the variation in the daily integral of the stomatal conductance to water vapour (gs day sum) in Q. petraea; however, in Q. robur, the hydraulic conductance of the twigs was the most powerful predictor variable. In the same year, the actual hydraulic conductance was correlated with gs day sum in Q. robur, but not in Q. petraea. The studied parameters of gas exchange and biomass production revealed that Q. robur recovered more rapidly from stress than did Q. petraea. We hypothesize that this is due to Q. robur‘s lower responsiveness to the environmental variable Δw and its higher responsiveness to internal hydraulic conductance compared to Q. petraea.

The nitrogen status of oak stands in northern Germany and its role in oak decline

The supply of excess nitrogen is considered to be a predisposing factor to the present decline of oak in northern Germany. Therefore, an investigation was started on the nitrogen status of various oak stands affected by decline. In the summer of 1992, samples of healthy leaves from both healthy and damaged oaks were taken from 18 stands of adult pedunculate and sessile oaks. In the coastal plain, where the bulk N deposition is rather high (> 15 kg ha-1 a-1), soil solutions from two adjacent stands of pedunculate oak differing in the degree of crown damage were sampled. In the winter 1992/93, the frost hardiness of bark tissue sampled from sessile oaks differing in leaf N content was determined.