Olivier Flores

Rate of tree carbon accumulation increases continuously with tree size

Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest carbon cycle—particularly net primary productivity and carbon storage—increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree’s total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.

Wood production response to climate change will depend critically on forest composition and structure

Established forests currently function as a major carbon sink, sequestering as woody biomass about 26% of global fossil fuel emissions. Whether forests continue to act as a global sink will depend on many factors, including the response of aboveground wood production (AWP; MgCxa0ha(-1xa0) yr(-1) ) to climate change. Here, we explore how AWP in New Zealands natural forests is likely to change. We start by statistically modelling the present-day growth of 97xa0199 individual trees within 1070 permanently marked inventory plots as a function of tree size, competitive neighbourhood and climate. We then use these growth models to identify the factors that most influence present-day AWP and to predict responses to medium-term climate change under different assumptions. We find that if the composition and structure of New Zealands forests were to remain unchanged over the next 30xa0years, then AWP would increase by 6-23%, primarily as a result of physiological responses to warmer temperatures (with no appreciable effect of changing rainfall). However, if warmth-requiring trees were able to migrate into currently cooler areas and if denser canopies were able to form, then a different AWP response is likely: forests growing in the cool mountain environments would show a 30% increase in AWP, while those in the lowland would hardly respond (on average, -3% when mean annual temperature exceeds 8.0xa0°C). We conclude that response of wood production to anthropogenic climate change is not only dependent on the physiological responses of individual trees, but is highly contingent on whether forests adjust in composition and structure.

A Functional Characterisation of a Wide Range of Cover Crop Species: Growth and Nitrogen Acquisition Rates, Leaf Traits and Ecological Strategies

Cover crops can produce ecosystem services during the fallow period, as reducing nitrate leaching and producing green manure. Crop growth rate (CGR) and crop nitrogen acquisition rate (CNR) can be used as two indicators of the ability of cover crops to produce these services in agrosystems. We used leaf functional traits to characterise the growth strategies of 36 cover crops as an approach to assess their ability to grow and acquire N rapidly. We measured specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen content (LNC) and leaf area (LA) and we evaluated their relevance to characterise CGR and CNR. Cover crop species were positioned along the Leaf Economics Spectrum (LES), the SLA-LDMC plane, and the CSR triangle of plant strategies. LA was positively correlated with CGR and CNR, while LDMC was negatively correlated with CNR. All cover crops could be classified as resource-acquisitive species from their relative position on the LES and the SLA-LDMC plane. Most cover crops were located along the Competition/Ruderality axis in the CSR triangle. In particular, Brassicaceae species were classified as very competitive, which was consistent with their high CGR and CNR. Leaf functional traits, especially LA and LDMC, allowed to differentiate some cover crops strategies related to their ability to grow and acquire N. LDMC was lower and LNC was higher in cover crop than in wild species, pointing to an efficient acquisitive syndrome in the former, corresponding to the high resource availability found in agrosystems. Combining several leaf traits explained approximately half of the CGR and CNR variances, which might be considered insufficient to precisely characterise and rank cover crop species for agronomic purposes. We hypothesised that may be the consequence of domestication process, which has reduced the range of plant strategies and modified the leaf trait syndrome in cultivated species.

Can functional classification of tropical trees predict population dynamics after disturbance

Abstract Question: How far can we simplify the floristic complexity of a tropical rainforest into functional groups in order to predict tree population dynamics after logging–induced disturbance? Location: Paracou experimental site, French Guiana. Methods: We used data from over 15 years in control and disturbed plots from a silvicultural trial started in 1984. We selected 53 common tree species assigned to five functional groups based on potential size and light requirement. For each species, we quantified: the fate, i.e. variation in population size, and dynamic processes, i.e. mortality, recruitment and growth, driving this fate. We investigated the links between dynamic processes, fate and functional groups. Results: Disturbance stimulated growth and recruitment for most species, but had a heterogeneous impact on mortality. Species fate in disturbed plots depended on recruitment and was more favourable than in control plots. The functional classification was more predictive for most separate dynamic processes than for species fate: after disturbance, significant differences were found between all functional groups for growth. Pioneer+heliophilous species showed significantly higher recruitment rates. Mortality of shade–tolerant species slightly increased and of mid–tolerant and heliophilous species decreased. Conclusions: A combination of three species classifications separately built from the growth, recruitment and mortality processes is more informative than a global classification combining the processes. Identifying the pioneer+heliophilous species on the basis of their growth rate is crucial to predict species fate after disturbance. We showed that potential growth rate could be used as a reliable indicator to identify this group. Nomenclature: Hollowell et al. (2001) for Anthophyta families A–L; http://www.mnh.si.edu/biodiversity/bdg/planthtml/index.html for other families.

Wolbachia-based population control strategy targeting Culex quinquefasciatus mosquitoes proves efficient under semi-field conditions.

In mosquitoes, the maternally inherited bacterial Wolbachia induce a form of embryonic lethality called cytoplasmic incompatibility (CI). This property can be used to reduce the density of mosquito field populations through inundative releases of incompatible males in order to sterilize females (Incompatible Insect Technique, or IIT, strategy). We have previously constructed the LR[wPip(Is)] line representing a good candidate for controlling field populations of the Culex quinquefasciatus mosquito in the islands of the south-western Indian Ocean. The main purpose of the present study was to fill the gap between laboratory experiments and field implementation, i.e. assessing mating competitiveness of these incompatible males under semi-field conditions. In a first set of experiments, we analyzed crossing relationships between LR[wPip(Is)] males and La Réunion field females collected as larvae in 19 distinct localities throughout the island. This investigation revealed total embryonic mortality, confirming the strong sterilizing capacity of LR[wPip(Is)] males. Subsequently, mating competitiveness of LR[wPip(Is)] males was assessed under semi-field conditions in the presence of field males and females from La Réunion. Confrontations were carried out in April and December using different ratios of LR[wPip(Is)] to field males. The results indicated that the LR[wPip(Is)] males successfully compete with field males in mating with field females, displaying even higher competitiveness than field males in April. Our results support the implementation of small-scale field tests in order to assess the feasibility of IIT against Cx. quinquefasciatus in the islands of southwestern Indian Ocean where this mosquito species is a proven competent vector for human pathogens.

Functional Traits Help Predict Post-Disturbance Demography of Tropical Trees

How tropical tree species respond to disturbance is a central issue of forest ecology, conservation and resource management. We define a hierarchical model to investigate how functional traits measured in control plots relate to the population change rate and to demographic rates for recruitment and mortality after disturbance by logging operations. Population change and demographic rates were quantified on a 12-year period after disturbance and related to seven functional traits measured in control plots. The model was calibrated using a Bayesian Network approach on 53 species surveyed in permanent forest plots (37.5 ha) at Paracou in French Guiana. The network analysis allowed us to highlight both direct and indirect relationships among predictive variables. Overall, 89% of interspecific variability in the population change rate after disturbance were explained by the two demographic rates, the recruitment rate being the most explicative variable. Three direct drivers explained 45% of the variability in recruitment rates, including leaf phosphorus concentration, with a positive effect, and seed size and wood density with negative effects. Mortality rates were explained by interspecific variability in maximum diameter only (25%). Wood density, leaf nitrogen concentration, maximum diameter and seed size were not explained by variables in the analysis and thus appear as independent drivers of post-disturbance demography. Relationships between functional traits and demographic parameters were consistent with results found in undisturbed forests. Functional traits measured in control conditions can thus help predict the fate of tropical tree species after disturbance. Indirect relationships also suggest how different processes interact to mediate species demographic response.

An eco-epidemiological study of Morbilli-related paramyxovirus infection in Madagascar bats reveals host-switching as the dominant macro-evolutionary mechanism

An eco-epidemiological investigation was carried out on Madagascar bat communities to better understand the evolutionary mechanisms and environmental factors that affect virus transmission among bat species in closely related members of the genus Morbillivirus, currently referred to as Unclassified Morbilli-related paramyxoviruses (UMRVs). A total of 947 bats were investigated originating from 52 capture sites (22 caves, 18 buildings, and 12 outdoor sites) distributed over different bioclimatic zones of the island. Using RT-PCR targeting the L-polymerase gene of the Paramyxoviridae family, we found that 10.5% of sampled bats were infected, representing six out of seven families and 15 out of 31 species analyzed. Univariate analysis indicates that both abiotic and biotic factors may promote viral infection. Using generalized linear modeling of UMRV infection overlaid on biotic and abiotic variables, we demonstrate that sympatric occurrence of bats is a major factor for virus transmission. Phylogenetic analyses revealed that all paramyxoviruses infecting Malagasy bats are UMRVs and showed little host specificity. Analyses using the maximum parsimony reconciliation tool CoRe-PA, indicate that host-switching, rather than co-speciation, is the dominant macro-evolutionary mechanism of UMRVs among Malagasy bats.

Functional Diversity of Subalpine Bryophyte Communities in an Oceanic Island (La Réunion)

Abstract Increasing temperature imperils worldwide tropical subalpine and alpine ecosystems with the threat of mountaintop extinctions and a subsequent loss of biodiversity and ecosystem functions. This paper provides a first assessment of functional diversity along a high climatic gradient for bryophytes, which represent a major plant compartment in these ecosystems. The present study takes place on the highlands of the Piton des Neiges volcano, the highest summit of La Réunion Island (Western Indian Ocean). We find a relatively high species richness of bryophytes in these subalpine habitats, with a peak of diversity at 2750 m for the ground-dwelling community. We report a strong linear relationship between functional diversity and species richness for epiphytes. Within the same plant group, the relationship between functional richness, based on eight traits, and elevation differed between the ground-dwelling and epiphytic communities, suggesting that different processes may structure bryophyte species assemblages along this short subalpine climatic gradient. Higher functional redundancy in ground-dwelling bryophyte assemblages indicates that this community may be more robust than epiphytic bryophytes to disturbances in these subalpine ecosystems.

Predicting the altitudinal distribution of an introduced phytophagous insect against an invasive alien plant from laboratory controlled experiments: case of Cibdela janthina (Hymenoptera:Argidae) and Rubus alceifolius (Rosaceae) in La Réunion

The sawfly Cibdela janthina (Hymenoptera: Argidae) native to Sumatra was introduced on La Réunion (France, Indian Ocean) in 2007 to control the giant bramble Rubus alceifolius (Rosaceae), one of the most invasive plants on this island. We determined the influence of temperature on the development duration and survival of C. janthina preimaginal stages in controlled conditions in order to parameterize a survival model and to relate the predicted survival with observed patterns of defoliation of the host plant at different altitudes. We adjusted the Régnière model to survival data, combined with the Lactin-2 model on development rate of the three preimaginal stages of C. janthina. This model adequately predicts the observed defoliation and the altitudinal limit of the biological control agent. Our results also show that studies on temperature-related constraints on the biology of an agent introduced for weed control should be emphasized both in the pre-release and the post-release phases of a biological control program to evaluate the potential success of the control programme.

Diversité spécifique et regroupement d'espèces arborescentes en forêt guyanaise

La forte diversite despeces arborescentes observees en forets tropicales represente a la fois une source permanente de questionnements scientifiques et une contrainte forte pour ameliorer les connaissances du fonctionnement de lecosysteme forestier. Les hypotheses avancees pour expliquer le maintien de cette diversite sont legion. Elles sopposent sur la nature deterministe ou stochastique des mecanismes maintenant cette forte diversite. En pratique et pour mieux aborder les problemes poses, cette diversite peut etre simplifiee en regroupant les especes presentant des similitudes dans leur comportement ecologique. Larticle presente quelques-unes des theories du maintien de la diversite et une synthese des approches utilisees pour regrouper les especes en illustrant chacun de ces themes par des travaux realises en Guyane.