Closing a gap in tropical forest biomass estimation: taking crown mass variation into account in pantropical allometries

Accurately monitoring tropical forest carbon stocks is a challenge that remains outstanding. Allometric models that consider tree diameter, height and wood density as predictors are currently used in most tropical forest carbon studies. In particular, a pantropical biomass model has been widely used for approximately a decade, and its most recent version will certainly constitute a reference model in the coming years. However, this reference model shows a systematic bias towards the largest trees. Because large trees are key drivers of forest carbon stocks and dynamics, understanding the origin and the consequences of this bias is of utmost concern. In this study, we compiled a unique tree mass data set of 673 trees destructively sampled in five tropical countries (101 trees > 100 cm in diameter) and an original data set of 130 forest plots (1 ha) from central Africa to quantify the prediction error of biomass allometric models at the individual and plot levels when explicitly taking crown mass variations into account or not doing so. We first showed that the proportion of crown to total tree aboveground biomass is highly variable among trees, ranging from 3 to 88 %. This proportion was constant on average for trees  1 Mg) and reduced the range of plot-level error (in %) from [−23; 16] to [0; 10]. The disproportionally higher allocation of large trees to crown mass may thus explain the bias observed recently in the reference pantropical model. This bias leads to far-from-negligible, but often overlooked, systematic errors at the plot level and may be easily corrected by taking a crown mass proxy for the largest trees in a stand into account, thus suggesting that the accuracy of forest carbon estimates can be significantly improved at a minimal cost.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Copernicus Publications on behalf of the European Geosciences Union. The published article can be found at: http://www.biogeosciences.net

Land use has little influence on the soil seed bank in a central African moist forest

peer reviewedRegeneration of tropical forest occurs naturally and mainly through early secondary succession by soil seed banks. Here, we analyzed seed bank variation in density anddiversity along a degradation gradient. We also explored the relationship (similarity incomposition) between the seed and seedling banks and the overstory vegetation. The investigated forests were located at Dja Fauna Reserve and its surroundings, south-easternCameroon. We sampled 33 one-haforest plots scattered in a protected area,in a logging concession and in community forests. Within each plot, we inventoriedthe seed bank in the litter and in four successive soil layers of 5 cm depth interval, andthe standing vegetation. Seed density significantly varied with sampling depth but not between land use types. A total of 181 morphospecies were identified. Trees species accounted for 46% of seeds with pioneer species dispersed by animals dominating the seed bank. Species diversity and composition strongly varied with the sampling depth; the litter seed bank was more similar in composition to the soil seed bank than to the seedling bank, sapling, and forest stands. We highlighted the presence of a dense and diverse seed bank in each land use type. We also showed the differentiation of each sampling depth in terms of species composition, thus revealing their complementarity in the dynamics of natural regeneration. The dominance of pioneers in the seed bank and weak relationship with the overstory are universal for tropical forests and logically follow from forest succession after disturbance

Land use has little influence on the soil seed bank in a central African moist forest

International audienceRegeneration of tropical forest occurs naturally and mainly through early secondary succession by soil seed banks. Here, we analyzed seed bank variation in density and diversity along a degradation gradient. We also explored the relationship (similarity in composition) between the seed and seedling banks and the overstory vegetation. The investigated forests were located at Dja Fauna Reserve and its surroundings, south-eastern Cameroon. We sampled 33 one-ha forest plots scattered in a protected area, in a logging concession and in community forests. Within each plot, we inventoried the seed bank in the litter and in four successive soil layers of 5 cm depth interval, and the standing vegetation. Seed density significantly varied with sampling depth but not between land use types. A total of 181 morphospecies were identified. Trees species accounted for 46% of seeds with pioneer species dispersed by animals dominating the seed bank. Species diversity and composition strongly varied with the sampling depth; the litter seed bank was more similar in composition to the soil seed bank than to the seedling bank, sapling, and forest stands. We highlighted the presence of a dense and diverse seed bank in each land use type. We also showed the differentiation of each sampling depth in terms of species composition, thus revealing their complementarity in the dynamics of natural regeneration. The dominance of pioneers in the seed bank and weak relationship with the overstory are universal for tropical forests and logically follow from forest succession after disturbance. Abstract in French is available with online material

Seed dispersal by duikers in selectively logged rainforests: Overlooked dispersal of an important animal community

peer reviewedSeed dispersal by frugivores is a key process which is necessary for the regeneration of plant communities. The decrease in the abundance of fruit-eating mammals due to anthropogenic pressures impacts on the proper functioning of forest ecosystems. In this study, for the first time we evaluate seed dispersal by duikers (Bovidae, frugivores and the most hunted animals of the rainforests of central Africa: Congo basin), using germination trials of seeds contained in their dung and rumen. The influence of selective timber harvesting on this dispersal was also examined by comparing duiker community occupancy between a logged and an unlogged site. Our results revealed a community of four duiker species Cephalophus silvicultor, Philantomba congica and two species of ‘red ’ duiker (C. callipygus and C. castaneus). A total of 79 plant species (5481 seedlings) were found in the dungs of all duikers and a total of 37 species (5225 seedlings) were observed in the rumens of red duikers and P. congica. Phyllanthus sp. and Musanga cecropioides were the dominant taxa in dung and rumen contents respectively. Taxa of high commercial value such as Erythrophleum suaveolens, Milicia excelsa and Nauclea diderrichii were also observed. The lack of a clear difference between the plant communities dispersed by the duiker species (assessed by β -diversity) showed that they play redundancy rather than complementary dispersal roles. Duikers play an underestimated role in the restoration of environments disturbed by logging or shifting cultivation as most of the species observed are pioneer colonizing species. Furthermore duiker communities appear to be affected little by selective logging.15. Life on lan

Additive influences of soil and climate gradients drive tree community composition of Central African rainforests

International audienceAim: Examining tree species-environment association can offer insight into the drivers of vegetation patterns and key information of practical relevance to forest management. Here, we aim to quantify the contribution of climate and soil gradients to variation in Central African tree species composition (abundance and occurrence). Location: Tropical rainforests of southern and eastern Cameroon. Methods: We established 82 1-ha permanent plots across seven localities and censused all trees ≥ 10 cm in diameter, representing a total of 37,733 trees and 455 species. In 60 of those plots, we measured ten soil variables describing texture and nutrients levels and extracted ten bioclimatic variables from global-gridded climate databases. We synthesized the main environmental gradients by conducting principal component analyses on climate and soil data respectively. We performed unconstrained and constrained non-symmetric correspondence analyses to account for the individual and joint contributions of climate and soil on species abundance and occurrence. Results: Climate and soil contributed similarly to variances of species abundance and occurrence (12–15 % variance for climate vs. 11–12 % variance for soil). Climate influence mostly concerns some abundant species, while some of the less abundant species were mainly driven by soil. Fractions of species variances accounted for by climate and soil show strong correlation when assessed from species occurrence and abundance data. Conclusion: Variation in occurrence and abundance of tropical forest trees can be partly shaped by both climate and soil gradients in Cameroon, which emphasizes the importance to jointly consider soil and climate in species distribution modeling. Less abundant species may express environmental influence differently than abundant species and convey complementary information about community assemblage. Though showing congruent patterns here, species abundance and occurrence reflect different interacting community processes and both should be examined to better understand vegetation patterns

Contrasted allometries between stem diameter, crown area, and tree height in five tropical biogeographic areas

International audienceTree crowns play a central role in stand dynamics. Remotely sensed canopy images have been shown to allow inferring stand structure and biomass which suggests that allometric scaling between stems and crowns may be tight, although insufficiently investigated to date. Here, we report the first broad-scale assessment of stem vs. crown scaling exponents using measurements of bole diameter (DBH), total height (H), and crown area (CA) made on 4148 trees belonging to 538 species in five biogeographic areas across the wet tropics. Allometries were fitted with power functions using ordinary least-squares regressions on log-transformed data. The inter-site variability and intra-site (sub-canopy vs. canopy trees) variability of the allometries were evaluated by comparing the scaling exponents. Our results indicated that, in contrast to both DBH-H and H-CA allometries, DBH-CA allometry shows no significant inter-site variation. This fairly invariant scaling calls for increased effort in documenting crown sizes as part of tree morphology. Stability in DBH-CA allometry, indeed, suggests that some universal constraints are sufficiently pervasive to restrict the exponent variation to a narrow range. In addition, our results point to inverse changes in the scaling exponent of the DBH-CA vs. DBH-H allometries when shifting from sub-canopy to canopy trees, suggesting a change in carbon allocation when a tree reaches direct light. These results pave the way for further advances in our understanding of niche partitioning in tree species, tropical forest dynamics, and to estimate AGB in tropical forests from remotely sensed images

A map of African humid tropical forest aboveground biomass derived from management inventories

International audienceForest biomass is key in Earth carbon cycle and climate system, and thus under intense scrutiny in the context of international climate change mitigation initiatives (e.g. REDD+). In tropical forests, the spatial distribution of aboveground biomass (AGB) remains, however, highly uncertain. There is increasing recognition that progress is strongly limited by the lack of field observations over large and remote areas. Here, we introduce the Congo basin Forests AGB (CoFor-AGB) dataset that contains AGB estimations and associated uncertainty for 59,857 1-km pixels aggregated from nearly 100,000 ha of in situ forest management inventories for the 2000 – early 2010s period in five central African countries. A comprehensive error propagation scheme suggests that the uncertainty on AGB estimations derived from c. 0.5-ha inventory plots (8.6–15.0%) is only moderately higher than the error obtained from scientific sampling plots (8.3%). CoFor-AGB provides the first large scale view of forest AGB spatial variation from field data in central Africa, the second largest continuous tropical forest domain of the world

Data from: Closing a gap in tropical forest biomass estimation: taking crown mass variation into account in pantropical allometries

Accurately monitoring tropical forest carbon stocks is an outstanding challenge. Allometric models that consider tree diameter, height and wood density as predictors are currently used in most tropical forest carbon studies. In particular, a pantropical biomass model has been widely used for approximately a decade, and its most recent version will certainly constitute a reference in the coming years. However, this reference model shows a systematic bias for the largest trees. Because large trees are key drivers of forest carbon stocks and dynamics, understanding the origin and the consequences of this bias is of utmost concern. In this study, we compiled a unique tree mass dataset on 673 trees measured in five tropical countries (101 trees > 100 cm in diameter) and an original dataset of 130 forest plots (1 ha) from central Africa to quantify the error of biomass allometric models at the individual and plot levels when explicitly accounting or not accounting for crown mass variations. We first showed that the proportion of crown to total tree aboveground biomass is highly variable among trees, ranging from 3 to 88 %. This proportion was constant on average for trees 1 Mg) and reduced the range of plot-level error from −23–16 to 0–10 %. The disproportionally higher allocation of large trees to crown mass may thus explain the bias observed recently in the reference pantropical model. This bias leads to far-from-negligible, but often overlooked, systematic errors at the plot level and may be easily corrected by accounting for a crown mass proxy for the largest trees in a stand, thus suggesting that the accuracy of forest carbon estimates can be significantly improved at a minimal cost

PlotonClosingGapTropicalSupplement.pdf

Accurately monitoring tropical forest carbon stocks is a challenge that remains outstanding. Allometric models that consider tree diameter, height and wood density as predictors are currently used in most tropical forest carbon studies. In particular, a pantropical biomass model has been widely used for approximately a decade, and its most recent version will certainly constitute a reference model in the coming years. However, this reference model shows a systematic bias towards the largest trees. Because large trees are key drivers of forest carbon stocks and dynamics, understanding the origin and the consequences of this bias is of utmost concern. In this study, we compiled a unique tree mass data set of 673 trees destructively sampled in five tropical countries (101 trees > 100 cm in diameter) and an original data set of 130 forest plots (1 ha) from central Africa to quantify the prediction error of biomass allometric models at the individual and plot levels when explicitly taking crown mass variations into account or not doing so. We first showed that the proportion of crown to total tree aboveground biomass is highly variable among trees, ranging from 3 to 88 %. This proportion was constant on average for trees  1 Mg) and reduced the range of plot-level error (in %) from [−23; 16] to [0; 10]. The disproportionally higher allocation of large trees to crown mass may thus explain the bias observed recently in the reference pantropical model. This bias leads to far-from-negligible, but often overlooked, systematic errors at the plot level and may be easily corrected by taking a crown mass proxy for the largest trees in a stand into account, thus suggesting that the accuracy of forest carbon estimates can be significantly improved at a minimal cost