Eigenvector-mapping methods such as Moran’s eigenvector maps (MEM) are derived from a spatial weighting matrix (SWM) that describes the relations among a set of sampled sites. The specification of ...the SWM is a crucial step, but the SWM is generally chosen arbitrarily, regardless of the sampling design characteristics. Here, we compare the statistical performances of different types of SWMs (distance-based or graph-based) in contrasted realistic simulation scenarios. Then, we present an optimization method and evaluate its performances compared to the arbitrary choice of the most-widely used distance-based SWM. Results showed that the distance-based SWMs generally had lower power and accuracy than other specifications, and strongly underestimated spatial signals. The optimization method, using a correction procedure for multiple tests, had a correct type I error rate, and had higher power and accuracy than an arbitrary choice of the SWM. Nevertheless, the power decreased when too many SWMs were compared, resulting in a trade-off between the gain of accuracy and the loss of power. We advocate that future studies should optimize the choice of the SWM using a small set of appropriate candidates. R functions to implement the optimization are available in the adespatial package and are detailed in a tutorial.
In tropical rainforests, termites constitute an important part of the soil fauna biomass, and as for other soil arthropods, variations in soil composition create opportunities for niche partitioning. ...The aim of this study was twofold: first, we tested whether soil-feeding termite species differ in the foraging substrate; second, we investigated whether soil-feeding termites select their foraging sites to enhance nutrients intake. To do so, we collected termites and analysed the composition and structure of their feeding substrates. Although Anoplotermes-group members are all considered soil-feeders, our results show that some species specifically feed on abandoned termite nests and very rotten wood, and that this substrate selection is correlated with previous stable isotope analyses, suggesting that one component of niche differentiation among species is substrate selection. Our results show that the composition and structure of bare soils on which different termite species foraged do not differ, suggesting that there is no species specialization for a particular type of bare soil. Finally, the bare soil on which termites forage does not differ from random soil samples. Overall, our results suggest that few species of the Anoplotermes-group are specialized toward substrates rich in organic matter, but that the vast majority forage on soil independently of its structural and chemical composition, being ecologically equivalent for this factor.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A comprehensive understanding of Ca cycling in an ecosystem is desirable because of the role of this element in tree mineral nutrition and its status as a major base cation on the soil exchange ...complex. The determination of the origin of Ca in forests is particularly indicated in regard of important changes linked to acid inputs and intensive logging. Natural strontium isotopes are increasingly used as tracers of Ca in forest ecosystems for qualitative and quantitative assessments. Nevertheless this method is limited to relatively simple systems with two sources of nutrients. Some recent studies coupled Sr/Ca or Sr/Ba ratios to Sr isotopic measurements in order to solve more complex systems. Such method has however associated with it some uncertainties: this approach assumed that Ca, Sr and Ba behave similarly throughout the ecosystem and does not take into account the Ca biopurification processes occurring in some tree's organs which can alter element ratio. The present work focuses on two deciduous species covering large areas in Europe: European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). In order to test the similarity of behaviour between Ca, Sr and Ba, their concentrations were measured extensively in the major compartments of two forest ecosystems. In parallel, the discrimination process inside tree organs was studied in 23 stands for beech and 10 stands for oak. We found that Sr and Ca behave similarly in all soil and tree compartments. By contrast, Ba and Ca appear to have contrasting behaviours, especially in streams, soil solution and soil exchange complex (no correlations between element concentrations). Sr/Ba and Ba/Ca ratios must therefore be used with care as tracer of Ca. The Ca biopurification is absent in roots and slight in bole wood but is large in bark, twigs and leaves. The discrimination factors (DF) between wood and leaves are characteristic of the two species studied and do not change significantly as a function of the soil Ca status (acidic or calcareous soils). Therefore, strontium-calcium DF can be used as a correction factor of the Sr/Ca ratio of leaves when this ratio is used in connection with Sr isotopic ratios. This correction allows to solve systems of tree nutrition with more than two sources of Ca.
The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient ...slash-and-burn agricultural activities that created large openings, while a decline of these activities since the colonial period could explain their deficit of regeneration. To verify this hypothesis, we compared soil charcoal abundance, used as a proxy for past slash-and-burn agriculture, and tree species composition assessed on 208 rainforest 0.2 ha plots located in three areas from Southern Cameroon. Species were classified in regeneration guilds (pioneer, non-pioneer light-demanding, shade-bearer) and characterized by their wood-specific gravity, assumed to reflect light requirement. We tested the correlation between soil charcoal abundance and: (i) the relative abundance of each guild, (ii) each species and family abundance and (iii) mean wood-specific gravity. Charcoal was found in 83% of the plots, indicating frequent past forest fires. Radiocarbon dating revealed two periods of fires: "recent" charcoal were on average 300 years old (up to 860 BP, n = 16) and occurred in the uppermost 20 cm soil layer, while "ancient" charcoal were on average 1900 years old (range: 1500 to 2800 BP, n = 43, excluding one sample dated 9400 BP), and found in all soil layers. While we expected a positive correlation between the relative abundance of light-demanding species and charcoal abundance in the upper soil layer, overall there was no evidence that the current heterogeneity in tree species composition can be explained by charcoal abundance in any soil layer. The absence of signal supporting our hypothesis might result from (i) a relatively uniform impact of past slash-and-burn activities, (ii) pedoturbation processes bringing ancient charcoal to the upper soil layer, blurring the signal of centuries-old Human disturbances, or (iii) the prevalence of other environmental factors on species composition.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Initiated by the World Wildlife Fund (WWF) more than a decade ago in North Kivu, single-species plantations of Eucalyptus saligna and Grevillea robusta constitute, with other village plantations, the ...current legal source of wood-energy for the communities bordering the Virunga National Park (PNVi). This study assesses the growth and productivity of these plantations in two sites with different soil and climatic conditions to predict their production over time. The study also assesses the carbon stock and long-term CO2 fixation in the biomass of the studied plantations to deduce their contribution to climate change mitigation. Non-destructive inventories were carried out during three consecutive years in 20 E. saligna and 12 G. robusta plantations in Sake and Kirumba. Analysis of the data revealed that both species have similar diametric growth while height growth and productivity were significantly higher in the E. saligna plantations. The productivity of E. saligna was also higher in Kirumba than in Sake, while that of G. robusta was higher in Sake than in Kirumba. The differences observed were mainly related to species, silviculture, altitude and concentration of bioavailable elements in the soils. The analysis of productivity evolution over time allowed us to determine optimal rotations at 8 and 12 years, respectively, for E. saligna and G. robusta plantations. The relationships between biomass or carbon stock and tree diameter were not different between the studied species but were significantly different at the stand level. If silviculture was standardized and plantations carefully monitored, carbon stock and long-term CO2 fixation would be higher in G. robusta plantations than in E. saligna plantations. These results indicate that while for productivity reasons E. saligna is the favoured species in wood-energy plantations to quickly meet the demand of the growing and disadvantaged population living in the vicinity of PNVi, carefully monitored G. robusta plantations could be more interesting in terms of carbon credits. To simultaneously optimise wood-energy production and carbon storage in the plantations initiated in North Kivu, E. saligna and G. robusta should be planted in mixture. In addition, species and site characteristics adapted silvicultural management practices must be applied to these plantations, which are very important for the region, its population and its park. Finally, the economic profitability as well as the sustainability of the plantations should be assessed in the longer term in North Kivu.
Background and aims
Adequate cultural cycles for phytoextraction with
Noccaea caerulescens
have never been investigated, whereas they directly influence biomass production. The aim of this study was ...to investigate the effects of the season of establishment, the seeding strategy (sowing vs transplantation) and the length of the growing season on biomass production and metal accumulation of
N. caerulescens
.
Methods
A field trial was conducted on an urban wasteland contaminated with trace metals. Two populations with contrasted natural life cycles, one Cd-accumulating metallicolous (Ganges, GAN) and one non-metallicolous from Luxemburg (LUX), were compared in 6 cultural cycles.
Results
Direct sowing in the fall yielded satisfying metal uptake (260 g Cd ha
−1
with GAN and 25 kg Zn ha
−1
with LUX), though lower than those obtained with transplantation (320 g Cd ha
−1
with GAN and 45 kg Zn ha
−1
with LUX) due to less biomass production (1.6 vs 3.2 t ha
−1
). Extending the growing period from 6 to 12 months was mostly beneficial for biennial LUX plants (with 3 times higher Zn uptake), than for annual GAN plants (with 1.5 times higher Cd uptake).
Conclusions
The natural variation in life cycles – annual or biennial – of
N. caerulescens
should be taken into account when selecting cultivars for phytoextraction.
This article explores the relationship between beech (Fagus sylvatica L.) and its environment on the scale of Belgium. Radial increment is used to assess the effects of global change, particularly ...climate change, on the development and vitality of beech since the beginning of the 20th century. The average radial increment of beech on the scale of Belgium has decreased while its variability has increased. Climate has had a growing influence on growth and has become the predominant factor since the beginning of the 1990’s. The increments of all trees have become synchronised mainly due to the increase in frequency and intensity of summer heat waves and spring droughts. The results are interpreted on the basis of knowledge about the ecophysiology of beech and the consequences of current and future climate change are discussed.
Cet article s’intéresse à la relation entre le Hêtre (Fagus sylvatica L.) et son environnement à l’échelle de la Belgique. L’accroissement radial a été utilisé pour évaluer l’effet des changements globaux, et plus particulièrement du changement climatique, sur le développement et la vitalité du Hêtre depuis le début du XXe siècle. L’accroissement moyen du Hêtre à l’échelle de la Belgique a diminué et sa variabilité a augmenté. Le climat a eu une influence grandissante sur l’accroissement, devenue prépondérante à partir du début des années 1990. Les accroissements de l’ensemble des arbres se sont synchronisés, principalement sous l’effet de l’augmentation de la fréquence et de l’intensité des canicules estivales et des sécheresses printanières. Les résultats sont interprétés sur la base des connaissances sur l’écophysiologie du Hêtre et les conséquences du changement climatique en cours et futur sont discutées.
Eigenvector mapping techniques are widely used by ecologists and evolutionary biologists to describe and control for spatial and/or phylogenetic patterns in their data. The selection of an ...appropriate subset of eigenvectors is a critical step (misspecification can lead to highly biased results and interpretations), and there is no consensus yet on how to proceed. We conducted a ten‐year review of the practices of eigenvector selection and highlighted three main procedures: selecting the subset of descriptors minimising the Akaike information criterion (AIC), using a forward selection with double stopping criterion after testing the global model significance (FWD), and selecting the subset minimising the autocorrelation in the model residuals (MIR). We compared the type I error rates, statistical power, and R² estimation accuracy of these methods using simulated data. Finally, a real dataset was analysed using variation partitioning analysis to illustrate to what extent the different selection approaches affected the ecological interpretation of the results. We show that, while the FWD and MIR approaches presented a correct type I error rate and were accurate, the AIC approach displayed extreme type I error rates (100%), and strongly overestimated the R². Moreover, the AIC approach resulted in wrong ecological interpretations, as it overestimated the pure spatial fraction (and the joint spatial‐environmental fraction to a lesser extent) of the variation partitioning. Both the FWD and MIR methods performed well at broad and medium scales but had a very low power to detect fine‐scale patterns. The FWD approach selected more eigenvectors than the MIR approach but also returned more accurate R² estimates. Hence, we discourage any future use of the AIC approach, and advocate choosing between the MIR and FWD approaches depending on the objective of the study: controlling for spatial or phylogenetic autocorrelation (MIR) or describing the patterns as accurately as possible (FWD).
Questions
Studying the spatial distribution of tree species is fundamental to understanding the ecological processes governing forest dynamics. Habitat filtering and dispersal limitation are two key ...processes in the emergence of intraspecific aggregation in natural forest communities. The contributions of these ecological processes vary among species, communities, ontogenetic stages (i.e. juveniles and adults) and spatial scales. In this study, we investigated the ecological processes underlying species spatial distributions in two ontogenetic stages and determined how these processes are linked to species functional traits.
Location
A naturally established 2.4‐ha temperate forest located on a limestone hill in SW Belgium.
Methods
Trees ≥1.3 m were exhaustively censused and mapped, and environmental variables and functional traits were collected. We applied spatial point pattern modelling to explore the processes underlying species distributions. First, point process models simulating complete spatial randomness, habitat filtering and non‐habitat clustering processes were fitted to observed species point patterns. The analyses were conducted for juveniles and adults both separately and together. The most parsimonious model for each species/ontogenetic stage was selected based on three summary statistics. Secondly, we investigated the links between functional traits and the parameters of the spatial models.
Results
(a) Dispersal limitation plays a central role in the formation of intraspecific aggregation in this forest, alone or in combination with habitat filtering. (b) Distributions of juveniles are less influenced by habitat filtering than distributions of adults. (c) Dispersal differ substantially among species and interspecific differences in clustering characteristics are explained by seed mass, wood density and maximum height.
Conclusions
Dispersal limitation can be of major importance in shaping temperate forests. Coupling spatial point pattern modelling and functional traits is an efficient way to understand mechanisms of species assemblages at fine spatial scales in temperate forests.
We combine spatial point process modelling of trees with soil, topographic and functional traits data in a temperate forest. We show: (a) dispersal limitation determines the shape of fine‐scale spatial patterns; (b) an ontogenetic shift in the importance of habitat specialisation and dispersal limitation; and (c) a correlation between specific functional traits and key parameters of spatial structure.
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