Christoph Leuschner

BIODIVERSITY INDICATOR GROUPS OF TROPICAL LAND-USE SYSTEMS: COMPARING PLANTS, BIRDS, AND INSECTS

Tropical landscapes are dominated by land-use systems, but their contribution to the conservation of biodiversity is largely unknown. Since changes in biodiversity in response to human impact are known to differ widely among taxonomic groups and guilds, there is a need for multidisciplinary collaboration of plant, vertebrate, and invertebrate experts. We used inventories of trees, understory plants, birds (subdivided into endemics, insectivores, frugivores/nectar feeders), butterflies (endemics, fruit feeders), and dung bee- tles in Sulawesi (Indonesia) to characterize a gradient from near-primary to secondary forests, agroforestry systems, and annual crops. As expected, overall species richness tended to decrease within this gradient of increasing habitat modification, but, in contrast to pre- vious studies, we found the species richness between most taxonomic groups to be signif- icantly correlated (36 out of 38 pairwise comparisons). However, on average only 48% of the variance could be explained (within the five main groups), and only a few taxonomic groups/guilds turned out to be good predictors for others: for example, trees for fruit- and nectar-feeding birds (88% explanation) and fruit-feeding butterflies (83%), endemic birds for endemic butterflies (72%), and frugivorous/nectar-feeding birds for fruit-feeding but- terflies (67%). Although biodiversity of land-use systems showed taxonomic group- and guild-specific differences, most groups were affected in a similar way by habitat modifi- cation. Near-primary forest sites proved to be of principal importance for conservation; however, land-use systems such as secondary forests (for understory plants, birds, and butterflies) and agroforestry systems (for butterflies) supported relatively high numbers of species and might play a significant role for biodiversity conservation in tropical landscapes.

The impact of agricultural intensification and land-use change on the European arable flora

The impact of crop management and agricultural land use on the threat status of plants adapted to arable habitats was analysed using data from Red Lists of vascular plants assessed by national experts from 29 European countries. There was a positive relationship between national wheat yields and the numbers of rare, threatened or recently extinct arable plant species in each country. Variance in the relative proportions of species in different threat categories was significantly explained using a combination of fertilizer and herbicide use, with a greater percentage of the variance partitioned to fertilizers. Specialist species adapted to individual crops, such as flax, are among the most threatened. These species have declined across Europe in response to a reduction in the area grown for the crops on which they rely. The increased use of agro-chemicals, especially in central and northwestern Europe, has selected against a larger group of species adapted to habitats with intermediate fertility. There is an urgent need to implement successful conservation strategies to arrest the decline of this functionally distinct and increasingly threatened component of the European flora.

Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year−1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m−2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g m−2) was found in the driest stand with 520 mm year−1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.

A comparison of four different fine root production estimates with ecosystem carbon balance data in a Fagus-Quercus mixed forest

The controversy on how to measure fine root production of forests (P) most accurately continues. We applied four different approaches to determine annual rates of P in an old-growth temperate Fagus sylvatica–Quercus petraea stand: sequential soil coring with minimum–maximum calculation, sequential coring with compartmental flow calculation, the ingrowth core method, and a recently developed root chamber method for measuring the growth of individual fine roots in situ. The results of the four destructive approaches differed by an order of magnitude and, thus, are likely to introduce large errors in estimating P. The highest annual rates of P were obtained from the sequential coring approach with compartmental flow calculation, intermediate rates by sequential coring with minimum–maximum calculation, and low ones by both the root growth chamber and ingrowth core approaches. A carbon budget for the stand was set up based on a model of annual net carbon gain by the canopy and measurements on carbon sink strength (annual leaf, branch and stem growth). The budget implied that a maximum of 27% of the net carbon gain was available for allocation to fine root growth. When compared to the carbon budget data, the sequential coring/compartmental flow approach overestimated annual fine root production substantially; whereas the ingrowth core and root growth chamber approaches grossly underestimated P rates. With an overestimation of about 25% the sequential coring/minimum–maximum approach demonstrated the best agreement with the carbon budget data. It is concluded that the most reliable estimate of P in this temperate forest will be obtained by applying the sequential coring/minimum–maximum approach, conducted with a large number of replicate samples taken on a few dates per season, in conjunction with direct root growth observation by minirhizotrons.

Above- and below-ground litter production in three tropical montane forests in southern Ecuador

Litter production from above-ground (leaves, twigs, fruits, flowers) and below-ground (roots) plant organs is an important component of the cycling of carbon and nutrients in forests. Tropical montane forests possess\ comparatively large quantities of fine-root biomass, suggesting that litter production by dying fine roots may represent a major component of total litter production. In a comparative study in three tropical montane forests of southern Ecuador at 1890, 2380 and 3060 m elevation, we measured leaf-fall by litter trapping and fine-root litter production by sequential soil coring and fine-root biomass and necromass analysis for about 1 y with the objectives (1) to quantify annual above- and below-ground litter production, and (2) to investigate elevational differences in litter production. Leaf litter mass decreased to less than a third (862 to 263 g m -2 y -1 ) with increasing elevation (1890 m to 3060 m), whereas fine-root litter production increased by a factor of about four (506 to 2084 g m -2 y -1 ). Thus, the ratio of leaf to fine-root litter shifted by an order of magnitude in favour of fine-root litter production between 1890 to 3060 m. Fine-root litter production was not synchronized with leaf litterfall and was seasonal only at 3060 m with mortality peaks in the drier and the wetter periods. We conclude that dying fine roots represent a very important fraction of total litterfall in tropical montane forests that can exceed the quantity of leaf litter. At 3060 m, the largest part ofthe organic material on top of the soil must originate from dying fine roots but. not from fallen leaves.

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.

Are marginal beech (Fagus sylvatica L.) provenances a source for drought tolerant ecotypes

Due to the expected increases of number and intensity of summer droughts in Central Europe the identification of drought tolerant ecotypes becomes more important in future forestry. A common garden experiment with seedlings of Fagus sylvatica provenances from the center (Germany) and eastern margin (Poland) of the species’ distribution range was conducted. Responses of morphological, physiological, chemical and growth parameters to three drought treatments were analyzed. Relative growth rates of the marginal provenance were lower as compared to the central provenance. The marginal seedlings showed a tendency to higher total biomasses because of higher seed masses. In both provenances drought decreased biomass production and root/shoot ratio which was lower in the central provenance. A lower specific root area of the marginal provenance indicated a better adaptation to low xylem water potentials. Under moderate drought, lower leaf δ13C signatures may indicate lower stomatal limitation (or a reduced rate of CO2 assimilation) in the marginal provenance. We conclude that marginal beech provenances may exhibit a better drought adaptation.

Leaf water status and stem xylem flux in relation to soil drought in five temperate broad-leaved tree species with contrasting water use strategies

Abstract• Five temperate broad-leaved tree species were compared with respect to their water consumption strategies under ample and restricted water supply. We measured synchronously leaf conductance (gL) in the sun canopy, xylem sap flux (Js) and leaf water potential (predawn, ψpd and noon, ψnoon) in adult trees in a mixed stand and related them to the fluctuations in vapor pressure deficit (D) and soil moisture.• Maximum gL was particularly high in F. excelsior, C. betulus and T. cordata and revealed a higher D sensitivity. ψpd remained constantly high in A. pseudoplatanus, C. betulus and F. excelsior, but decreased in T. cordata and F. sylvatica with decreasing soil moisture.• Jsddecreased linearly with decreasing soil matrix potential in all species except for F. excelsior. Apparent hydraulic conductance in the soil-to-leaf flow path (Lc) was higher in A. pseudoplatanus than in the other species.• F. sylvatica maintained a low maximum gL and reduced Jsd markedly upon drought, but faced severe decreases in ψpd and ψnoon. F. excelsior represents an opposite strategy with high maximum gL and stable ψpd.• The species drought sensitivity increases in the sequence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica.Résumé• Les stratégies de consommation d’eau de cinq espèces d’arbres feuillus tempérés ont été comparées sous approvisionnement en eau suffisant ou limité. De façon synchrone nous avons mesuré la conductance hydraulique des feuilles (gL) dans la partie du couvert exposée au soleil, le flux de sève xylémique (Js) et le potentiel hydrique foliaire (potentiel de base (ψpd) et potentiel minimum (ψnoon)) d’arbres adultes en peuplement mixte et nous les avons reliés aux fluctuations du déficit de pression de vapeur (D) et à l’humidité du sol.• gL maximum était particulièrement élevée chez F. excelsior, C. betulus et T. cordata et a révélé une plus grande sensibilité à D. ψpd est resté constamment élevé chez A. pseudoplatanus, C. betulus et F. excelsior, mais a diminué chez T. cordata et F. sylvatica lorsque l’humidité du sol diminuait.• Jsd a diminué linéairement avec le potentiel matriciel du sol pour toutes les espèces excepté F. excelsior. La conductivité hydraulique apparente du trajet sol-feuille (Lc) était plus élevée chez A. pseudoplatanus que dans les autres espèces.• F. sylvatica a maintenu une faible gL maximum et a réduit sensiblement Jsd face à la sécheresse, mais a connu de graves diminutions de ψpd et ψnoon. F. excelsior présentait une stratégie opposée avec une gL maximum élevée et un ψpd stable.• La sensibilité des espèces à la sécheresse augmente selon la séquence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica.

Spatial and temporal patterns of fine root abundance in a mixed oak-beech forest

Abstract Spatial distribution and seasonal fluctuation of fine root density (mass per unit soil volume) and abundance (mass or surface area per unit ground surface area) were investigated by the sequential coring technique in a 100–220 year old mixed Fagus sylvatica-Quercus petraea stand on acidic sandy soil in northwest Germany. The fine root systems of the two co-existing species overlapped completely with beech roots being twice as abundant as oak roots. Since Fagus and Quercus occupied equivalent parts of the canopy volume, oak appeared to be under-represented in the below-ground space. There was evidence for some degree of below-ground niche partitioning between the species in both the vertical and the horizontal direction. Oak fine roots were found to be more superficially distributed than beech roots in the organic layers, indicating a vertical stratification of the root systems of the two species. In the forest floor, fine roots were more abundant in the vicinity of aok stems where thicker organic layers occurred. However, this distribution pattern was not a consequence of a greater abundance of oak roots close to their parent stem, but was due to a higher frequency of beech roots here.

Tropical Andean Forests Are Highly Susceptible to Nutrient Inputs—Rapid Effects of Experimental N and P Addition to an Ecuadorian Montane Forest

Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha−1 yr−1) and P (10 kg ha−1 yr−1). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes—notably aboveground productivity—are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs.