Variation in Onset of Leaf Unfolding and Wood Formation in a Central African Tropical Tree Species

A diversity of phenological strategies has been reported for tropical tree species. Defoliation and seasonal dormancy of cambial activity inform us on how trees cope with water stress during the dry season, or maximize the use of resources during the rainy season. Here, we study the matching between leaf phenology (unfolding and shedding) and cambial activity for Prioria balsamifera, a key timber species in the Democratic Republic of Congo. In particular, we (i) evaluated the seasonality of cambial activity and synchrony of phenology among trees in response to climate and (ii) identified the seasonality of leaf phenology and its relation with cambial phenology. The study was conducted in the Luki Man and Biosphere Reserve, located in the Mayombe forest at the southern margin of the Congo Basin. Historic defoliation data were collected every ten days using weekly crown observations whereas recent observations involved timelapse cameras. Cambial pinning was performed on ten trees during 20 months and radius dendrometers were installed on three trees during 13 months. Tree rings were measured on cores from 13 trees and growth synchrony was evaluated. We found that P. balsamifera defoliates annually with a peak observed at the end of the dry season and the beginning of the rainy season. The new leaves unfolded shortly after shedding of the old leaves. The peak defoliation dates varied across years from September 12 to November 14 and the fraction of number of trees that defoliated at a given time was found to be negatively correlated with annual rainfall and temperature; during the dry season, when precipitation and temperatures are the lowest. Wood formation (radial growth), was found to be highly seasonal, with cambial dormancy occurring during the dry season and growth starting at the beginning of the rainy season. Individual ringwidth series did not cross date well. The within species variability of leaf phenology and cambial rhythms provides indication about resistance of the population against climatic changes

Impact of industrial logging on specific diversity and floristic composition of a tropical rainforest, case of Cotrefor-Alibuku concession forest in DRC

Abstract The industrial logging has in most of cases a huge impact on specific diversity and floristic composition of tropical rainforests, compromising their ability to play correctly their role. But it is alarming to note that studies related to the assessment of the industrial logging impacts on specific diversity and floristic composition have not yet been carried out in DRC, despite the intensity and the spread of logging in the country. The aim of this article is to analyze the specific diversity and floristic composition surveyed in different impact points of logging in Cotrefor-Alibuku concession forest, one year after the operation. Surfaces plots and itinerant method were used to collect floristic data. In total, 20 plots of size 30 m² have been surveyed in different impact points : the logging roads, the skid trails, the logging gaps and the log landings. An area not disturbed by logging was also sampled as a control point. The diversity and the wealth of flora, as well as the altitude and the intensity of logging operation of the impacts point have been analyzed. All the points of impact showed a specific richness from 1.3 to 2.4 times higher than that of the not disturbed site which shows a weak floristic similarity (less than 25%) with all points of impact. In terms of diversity, the Shannon index reveals that only the logging gaps (H’ =3.04) and the skid trails (H’ =2.95) are more diversified than the undisturbed site (H’ =2.87). On this last site, non commercial species and commercial species of Class 1 reveal the greatest proportions (22 % each one), whereas the species of the other commercial classes present significant proportions in the points of impact. Except from the logging roads, the diversity of the points of impact is inversely proportional to the intensity of logging operations. Forestry monitoring after logging seemed to be necessary in order to make profitable the positive effects and to minimize the negative effects of logging. Initially, the results found in this study can serve as a guide track for logging operations and allow forest managers and other actors to run a successful program of natural regeneration aid and to properly ensure sustainability in the management of their forests. Résumé L’exploitation industrielle du bois induit dans la plupart des cas un impact non négligeable sur la diversité spécifique et la composition floristique des forêts tropicales denses et humides compromettant leur capacité à jouer correctement leurs rôles. Cependant, il est alarmant de constater que les études dédiées à cette question sont quasiment absentes en RDC en dépit de l’ampleur de cette opération. Cet article analyse ainsi la diversité spécifique et la composition floristique relevées au niveau de différents points d’impact de l’exploitation forestière dans la concession forestière de Cotrefor-Alibuku, une année après l’opération. La collecte des données sur la flore a combiné la méthode de relevé de surface et celle dite itinérante. Les relevés de surface ont consisté à conduire des inventaires dans 20 placettes de 30 m² dans différents points d’impact de l’exploitation : les parcs à bois, les routes d’exploitation, les pistes de débardage et les trouées d’abattage. Un site témoin au sein d’une zone forestière non encore exploitée a également été pris en considération. Une analyse des données collectées sur le terrain a été effectuée autour de la diversité et richesse de la flore mais également de l’altitude et de l’intensité de l’exploitation sur les différents points d’impact. Tous les points d’impact ont affiché une richesse spécifique de 1,3 à 2,4 fois supérieure à celle du site témoin. La flore du site témoin ne présente qu’une faible similarité (moins de 25%) avec celle de tous les sites exploités. En termes de diversité, l’indice de Shannon révèle que seules les trouées d’abattage (H’=3,04) et les pistes de débardage (H’=2,95) sont plus diversifiées que le site témoin (H’=2,87). Sur ce dernier site, les espèces non commerciales et les espèces commerciales de Classe 1 ont affiché des plus grandes proportions (22 % chacune), alors que les espèces des autres classes présentent des proportions importantes sur les sites exploités. Hormis les routes, la diversité des sites exploités est inversement proportionnelle à l’intensité des opérations. Des opérations post-exploitation de suivi sylvicole s’avèrent donc nécessaires en vue de rentabiliser les effets positifs et minimiser les effets négatifs de l’exploitation industrielle du bois d’œuvre. Les résultats de cette étude peuvent servir de piste pour guider les exploitants dans la planification harmonieuse de leurs opérations en vue d’en garantir la durabilité

Height-diameter allometric equations of an emergent tree species from the Congo Basin

Reliable tree height-diameter (H-D) allometric equations are a key tool for the estimation of forest productivity and Above Ground Biomass (AGB). Most existing H-D allometric equations developed for the tropical region are based on large-scale multi-species datasets, and their use to derive information on productivity and AGB at the species level is prone to uncertainties. The single-species H-D allometric equations available are mainly focused on monocultures or stands with simple tree species mixtures and did not account for the site effects. Here we measured the height and diameter of 2,288 trees of the emergent tree species Pericopsis elata (Harms) Meeuwen in the Democratic Republic of the Congo (DRC) and in Cameroon. We first examined how accurate multispecies H-D allometric equations are in predicting the total height of P. elata. We then tested whether single-species H-D allometric equations vary between sites. We developed the first H-D allometric equation of P. elata and tested whether and how stand-level and environmental variables induce changes in H-D allometric relationship of P. elata at the regional level. We additionally evaluated whether tree-level variables are important at the local level where climate and stand development stage are expected to be less variable. We found that pantropical, regional and local H-D allometric equations significantly underestimate the total height of P. elata. The local multi-species H-D allometric equation developed for Yangambi showed the highest underestimation in all the studied sites. This result supports the need for an H-D allometric equation specific for P. elata. The species-level H-D allometric equation developed showed significant underestimations for trees from the disturbed and undisturbed forests in DRC, while overestimations were observed for similar sites in Cameroon. Using a mixed-effect H-D allometric equation, we showed that even within a single species, a substantial variation exists between sites. This variation showed to be driven by the differences in the maximum asymptotic height (Hmax) between sites. We found that P. elata trees are taller and attain higher Hmax in DRC than in Cameroon. The basal area showed to be a significant covariate accounting for the site effects at the regional-scale where climate variables showed minor effects. However, at the local-scale, none of climate or stand variables showed to be significant. Local-scale variation showed to be associated with differences in light availability, highlighting the potential of management options that shape the local environment in driving species productivity