Grégoire Vincent

Rubber agroforests in a changing landscape : analysis of land use/cover trajectories in Bungo district, Indonesia

ABSTRACT Land cover has changed dramatically in Sumatra Island, Indonesia over the last decades. Rampant deforestation has drawn a lot of attention due to the potential global impact of the associated carbon stock loss on climate warming and the erosion of biodiversity. The various land uses which replace natural forest are not equally benign to the environment. Rubber agroforests (jungle rubber) are extensive traditional cropping systems. They have been singled out by previous studies as the best land use option for biodiversity conservation once forest is cleared, while allowing farmers to make a living from the deforested land. But how sustainable are complex agroforestry systems themselves? Are they not just a transient stage in the overall process of land use intensification? We studied land cover change in the Bungo district, in Jambi, Sumatra (Indonesia), a 4,550 km2 area. Large forest tracks have been cleared since the early seventies and replaced by rubber plantations, oil palm plantations and other agricultural land-uses. Landsat images taken between 1973 and 2005 were used to quantify the trends of land cover changes in the area. during that period forest cover fell from more than 75% to 30%. Simultaneously monoculture plantations increased from 3% to over 40%, while rubber agroforests, decreased from 15% to 11%. Strikingly most of the rubber agroforests present in 2005 where absent in 1973 while most of the rubber agroforests present in 1973 had been replaced by more intensive agricultural systems by 2005. Rubber agroforests are now the ultimate reservoir of the original lowland forest biodiversity since natural forest has almost completely disappeared from the peneplain. They are however under growing pressure themselves and have incurred an accelerated conversion rate to more intensive agricultural systems in the period 2002–2005.

Crown allometries are less responsive than stem allometry to tree size and habitat variations in an Indian monsoon forest

While theoretical allometric models postulate universal scaling exponents, empirical relationships between tree dimensions show marked variability that reflects changes in the biomass allocation pattern. As growth of the various tree compartments may be controlled by different functions, it is hypothesized that they may respond differently to factors of variation, resulting in variable tree morphologies and potentially in trade-offs between allometric relationships. We explore the variability of tree stem and crown allometries using a dataset of 1,729 trees located in an undisturbed wet evergreen forest of the Western Ghats, India. We specifically test whether species adult stature, terrain slope, tree size and crown light exposure affect the relationships between stem diameter and stem height (stem allometry), and between stem diameter and crown width, crown area and crown volume (crown allometries). Results show that both stem and crown allometries are subject to variations in relation to both endogenous (tree size, species adult stature) and exogenous (terrain slope, crown light exposure) factors. Stem allometry appears to be more affected by these factors than are crown allometries, including the stem diameter–crown volume relationship, which proved to be particularly stable. Our results support the idea that height is a prevailing adjustment factor for a tree facing variable growth (notably light) conditions, while stem diameter–crown volume allometry responds more to internal metabolic constraints. We ultimately discuss the various sources of variability in the stem and crown allometries of tropical trees that likely play an important role in forest community dynamics.

The relative importance of dispersal limitation and habitat preference in shaping spatial distribution of saplings in a tropical moist forest: a case study along a combination of hydromorphic and canopy disturbance gradients

Abstract• IntroductionVarious processes contribute to shaping the local assemblage of species in hyperdiverse tropical forest. The relative contribution of environmental factors and dispersal limitation in determining the spatial distribution of saplings at local scale is unclear.• MethodsWe examined two types of environmental factors: (a) soil type reflecting drainage regime and (b) past logging damages reflecting light regime in a neotropical moist forest site. We used a logistic model to predict presence or absence of a given species in a network of elementary small plots.• ResultsThe effect of mapped environmental factors and a spatial correlation term were jointly estimated providing a direct measure of the relative role of habitat specialisation and dispersal limitation. At community level, dispersal limitation was the most important determinant of species absence at local scale. The two environmental factors examined played a balanced role. Different species however showed different degrees of dispersal limitation and habitat specialisation. The distribution of a large proportion of species—the majority of the most abundant species—was significantly affected by at least one environmental factor. We provide a ranking of 49 species sensitive to canopy disturbance (from shade specialist to pioneer) and 41 species affected by seasonal flooding (either positively or negatively).RésuméDivers processus gouvernent la composition floristique stationnelle en forêt tropicale hyperdiverse. La contribution relative des facteurs environnementaux et de la dispersion limitée n’est pas clairement établie. Nous examinons dans cette étude deux types de contraintes environnementales: le régime de drainage d’une part et l’impact des perturbations d’une exploitation forestière passée d’autre part. Nous utilisons un modèle logistique pour prédire la présence ou l’absence d’une espèce dans un réseau de placettes de 25m2. L’effet de facteurs environnementaux et d’un terme de corrélation spatiale sont estimés conjointement fournissant ainsi pour chaque espèce une estimation du poids relatif de l’affinité avec chacune des caractéristiques environnementales locale et de la limitation de dispersion. A l’échelle de la communauté la limitation de dispersion se révèle être le facteur prépondérant. Les deux variables environnementales semblent jouer un rôle d’importance comparable. Le poids des facteurs varie très nettement entre espèces. La distribution spatiale d’une grande proportion d’espèces —la majorité des espèces les plus abondantes — est sensible à au moins un des deux facteurs environnementaux étudiés. Nous proposons un classement de 49 espèces en termes de sensibilité à l’ouverture de la canopée (depuis les plus sciaphiles jusqu’aux espèces pionnières) et un classement de 41 espèces en termes de sensibilité (attraction ou répulsion) à l’engorgement saisonnier des sols.

Canopy height model characteristics derived from airbone laser scanning and its effectiveness in discriminating various tropical moist forest types

Mapping tropical forests to a sufficient level of spatial resolution and structural detail is a prerequisite for their rational management, which however remains a largely unmet challenge. We explore the degree to which a forest canopy height model (CHM) derived from airborne laser scanning (ALS) can discriminate between five forest types of similar height but varying structure or composition. We systematically compare various textural features (Haralick, Fourier transform-based, and wavelet-based features) and various classification procedures (linear discriminant analysis (LDA), random forest(RF), and support vector machine (SVM)) applied to two sizes of sampling units (64 m × 64 m and 32 m × 32 m). Simple height distribution statistics achieve at best 70% classification accuracy in our sample set comprising 120 sampling units of 64 m × 64 m. Using w avelet-based features, this accuracy increases to 79% but drops by 10% with smaller sampling units (32 m × 32 m). Classifier performance depends on the texture feature set used, but SVM and RF tend to perform better than LDA. High discrimination rates between forests types of similar height indicate that the ALS-derived CHM provides information suitable for mapping of tropical forest types. Wavelet-based texture features coupled with a SVM classifier was found to be the most promising combination of methods. Ancillary data derived from laser scans and notably topography could be used jointly for an improved segmentation scheme.

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

Key messageAcross five biogeographic areas, DBH-CA allometry was characterized by inter-site homogeneity and intra-site heterogeneity, whereas the reverse was observed for DBH-H allometry.AbstractTree 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.

ARE PERMANENT RUBBER AGROFORESTS AN ALTERNATIVE TO ROTATIONAL RUBBER CULTIVATION? AN AGRO-ECOLOGICAL PERSPECTIVE

ABSTRACT Many rubber smallholdings in Indonesia have developed into rubber agroforests as a result of the extensive management of the plantation. The resulting complex multi-species agroforests have a number of environmentally beneficial characteristics including a high level of natural biodiversity. Most environmental benefits would be significantly enhanced if these systems were not taken periodically through a new cycle of slash-and-burn, as normally happens when latex yield drops to uneconomic levels. This paper explores, from an agroecological perspective, the potential for such cyclical systems to be developed into permanent agroforests providing sustained latex yield over a longer time frame without a slash-and-burn intervention. Evidence is provided from direct observations, interviews with farmers and the results of specific agronomic experiments. Enrichment planting of seedling or grafted-clonal rubber plants into existing rubber agroforests resulted in low growth rates as a result of shading from canopy trees and probably below-ground competition. Below-ground competition also probably continued to limit rubber growth at the sapling and pole stage within agroforests. High investment has to be made in physical protection to prevent mortality of planted rubber in agroforests due to wild pig damage. However, direct grafting of clonal buds onto naturally regenerated rubber seedlings within agroforests provides a potential technical alternative. It is concluded that, though technically possible, such development towards permanent forest cover implies a significant change in management strategy and is unlikely to develop spontaneously on a wide scale in the study area in Jambi Province, Indonesia.

IS FLEXIBILITY OR PRODUCTIVITY THE KEY TO PERSISTENCE OF RUBBER AGROFORESTS IN JAMBI PROVINCE (SUMATRA)

ABSTRACT Rubber production and yield components (tree density, number of trees tapped per day, number of tapping days per year) were monitored in 35 plots of rubber agroforests in Jambi province (Sumatra) for one year. The results showed a great variability in yield components both between plots and over time. The overall average production of dry rubber per tree per tapping day was 17g, which is about half of the production per tree per tapping day that can be attained in well managed plantations of selected rubber clones. Productivity per ha and per year appeared to be further reduced by the low number of rubber trees per ha (c. 50% of the trees above 10 cm dbh are not rubber trees), the selective tapping of only a fraction of the population of rubber trees of tappable size and the low number of tapping days per year. Low average tapping frequency (110 days) was related to low prices prevailing during the study period. Tapping frequency appeared to be further influenced by a number of case specific factors including the degree of dependence of farmers on latex as a source of income, labor availability (for the plot owner) and presence or absence of other work opportunities (for the share-tapper). As a result of this opportunistic exploitation, the actual annual production per unit area in rubber agroforests appears to be often below maximum potential production of the plot. It is argued that the high flexibility of this extensive rubber cultivation system compensates for its low productivity and largely determines its wide adoption. It is further suggested that the opportunistic exploitation of extensively managed rubber agroforests may fulfil an important social function as it appears to serve as a reservoir of paid labour.

Equatorial Forests Display Distinct Trends in Phenological Variation: A Time-Series Analysis of Vegetation Index Data from Three Continents

Recent studies have questioned the applicability of satellite-derived vegetation indices (VIs) for evaluating phenological variation in tropical forests, due to potential artifacts caused by the bidirectional reflectance distribution function (BRDF). For nadir-normalized data, BRDF will be driven principally by intraannual variation in solar elevation. Where areas lying on the same latitude are under similar solar elevation “regimes,” if the observed variation in VIs is indeed driven by BRDF, then different regions at the same latitude should display identical VI variations. That hypothesis was tested by comparing VI data for tropical evergreen forests in three zones north of the equator (the Guianas, central Africa, and northern Borneo). Enhanced vegetation index, the fraction of green vegetation cover, and leaf area index (LAI) from MODIS and SPOT VEGETATION ultimately showed that VI trends for the regions differ greatly. The trend for Borneos forests is generally flat over the 12 years studied, while data for the Guianas and central Africa both exhibit strong but distinct seasonal patterns. Correlation analyses indicate that the VI trends between zones are neither strongly correlated to each other nor to variation in solar elevation (except in central Africa), suggesting that the observed variation in the VIs is not driven by BRDF. In contrast, regression analysis indicated that for the Guianas and central Africa, VI variation was most explained by variation in environmental factors, but not atmospheric effects, suggesting seasonally driven phenology.