Éditeur(s) : HAL CCSD Wiley
Résumé :  
1. Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability. 2. We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for >175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls. 3. Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher. 4. In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops. 5. Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands. 6. Synthesis. Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.
ISSN: 0022-0477

hal-01032100
https://hal.archives-ouvertes.fr/hal-01032100

Éditeur(s) : HAL CCSD Wiley
Résumé : 1. A recent article published by Toledo (2011b) investigates the effects of spatial variations in climate and soil, and of logging disturbance, on tree and forest growth in Bolivia. It concludes that climate is the strongest driver of tree and forest growth and that climate change may therefore have large consequences for forest productivity and carbon sequestration. However, serious methodological and conceptual discrepancies have been found that challenge these conclusions.[br/] 2. Because of an errant coding of time after logging in the regression analysis, and because floristic changes induced by logging could not be incorporated into the analysis, the effect of logging on the average diameter growth is likely to have been strongly underestimated.[br/] 3. Basal area growth was improperly calculated as basal area change, and it displayed surprisingly high values, even among unlogged plots. We hypothesize that either these plots may be actually located in secondary forests recovering from past logging, or measurement biases may have hampered the data set.[br/] 4. Regardless of climategrowth relationships established across these plots, any inference concerning the potential effects of climate change on forest growth would require a specific quantitative assessment.[br/] 5. Synthesis. It is critical to re-assess the relative weight of climate and logging disturbance as driving factors of tree and forest growth, and to find an explanation for the very high basal area increment reported among the unlogged plots. We provide specific recommendations for further analyses of this and similar data sets.
ISSN: 0022-0477

hal-01032432
https://hal.archives-ouvertes.fr/hal-01032432
DOI : 10.1111/j.1365-2745.2011.01895.x