Mascha Jacob

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.

Rapid recovery of stem increment in Norway spruce at reduced SO2 levels in the Harz Mountains, Germany

Tree-ring width of Picea abies was studied along an altitudinal gradient in the Harz Mountains, Germany, in an area heavily affected by SO(2)-related forest decline in the second half of the 20th century. Spruce trees of exposed high-elevation forests had earlier been shown to have reduced radial growth at high atmospheric SO(2) levels. After the recent reduction of the SO(2) load due to clean air acts, we tested the hypothesis that stem growth recovered rapidly from the SO(2) impact. Our results from two formerly damaged high-elevation spruce stands support this hypothesis suggesting that the former SO(2)-related spruce decline was primarily due to foliar damage and not to soil acidification, as the deacidification of the (still acidic) soil would cause a slow growth response. Increasing temperatures and deposited N accumulated in the topsoil are likely additional growth-promoting factors of spruce at high elevations after the shortfall of SO(2) pollution.

Small increase in sub-stratum pH causes the dieback of one of Europe's most common lichens, Lecanora conizaeoides

BACKGROUNDS AND AIMS Lecanora conizaeoides was until recently western and central Europes 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. METHODS 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. KEY RESULTS 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. CONCLUSIONS 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.

Chemical properties of decaying wood in an old-growth spruce forest and effects on soil chemistry

Live trees influence the nutrient status of the soil by the interception of substances from the atmosphere, the uptake of nutrients from the soil, and the deposition of litter. In an unmanaged old-growth spruce mountain forest on acidic soil, we analyzed how the death and decay of spruce trees affects the acidity and element concentrations of the soil, tree bark (or outermost stemwood) and stemflow. Key study objective was to examine whether the element release from decaying deadwood significantly increases the available soil nutrient stocks in the senescence phase of coniferous forests. Bark and stemflow chemistry responded to the death and decay of the trees with lowered acidity and reduced nutrient concentrations, which was attributed to the gradual loss of the intercepting canopy surface. Bark and stemflow concentrations of base cations (K, Ca, Mg) showed a transient peak in the course of wood decay. Published evidence suggests that the variability in bark and stemflow chemistry detected across the sequence of wood decay stages was sufficient to shape the epiphytic lichen and bryophyte communities. The death and decay of spruce trees also resulted in elevated base saturation near standing deadwood. Downed deadwood had a negligible effect on soil chemistry, among others due to slow decomposition in the studied cold mountain forest. Soil acidity was not significantly affected by deadwood. The release of base cations from standing deadwood to the soil suggests that sparing part of the trees in managed forests from logging could counteract nutrient depletion through timber harvesting.