Background: Biotic and abiotic disturbances such as frequent wildfires and herbivory contribute to maintain trees and grasses coexistence in savanna ecosystems. In comparison to stems and leaves, exposed to fire and herbivory, the roots, protected by being belowground, are less affected by these disturbances. Therefore, indirect estimation of belowground biomass (BGB) of savanna trees from simple allometric relations based on stem measurements can lead to major biases. Aims: In this study we explored how the Leaf ontogenetic change index (LOCI), a quantitative index based on leaf heteroblastic development, can provide an accurate estimate of BGB in Cussonia arborea, a widespread species in West African humid savannas. Methodology: We examined leaf morphometrics on post-fire resprouts of 40 individuals to assess whether LOCI can inform on plant age. We then analyzed by log-level regressions the variation of LOCI in relation to plant stem volume. Subsequently, we studied the variation of BGB according to stem volume, and as a function of both stem volume and LOCI, which allowed us to evaluate the contribution of LOCI to BGB estimation. BGB was obtained destructively by digging up roots and weighing total dry mass of 25 individuals including small and large trees. Statistical analyses were done with the R software. Place and Duration of Study: Study was performed in the Lamto Scientific Reserve, Côte d’Ivoire, between May 2020 and June 2021. Results: Using the stem volume as single explanatory variable of BGB, the regression model provided an adjusted R2 of 0.71. Association of the stem volume with LOCI increased the adjusted R2 from 0.71 to 0.90. Conclusion: Combining LOCI with a measure of stem size provides better estimate of BGB in C. arborea compared to estimate based on stem size only. Since a large proportion of woody species in frequently disturbed environments exhibit an overall strategies promoting persistence, future works should evaluate how these strategies are modulated during ontogeny and can explain biomass variation over time.
Currently, tools to predict the aboveground and belowground biomass (AGB and BGB) of woody species in Guinean savannas (and the data to calibrate them) are still lacking. Multispecies allometric equations calibrated from direct measurements can provide accurate estimates of plant biomass in local ecosystems and can be used to extrapolate local estimates of carbon stocks to the biome scale. We developed multispecies models to estimate AGB and BGB of trees and multi-stemmed shrubs in a Guinean savanna of Côte dâIvoire. The five dominant species of the area were included in the study. We sampled a total of 100 trees and 90 shrubs destructively by harvesting their biometric data (basal stem diameter , total stem height , stump area , as well as total number of stems for shrubs), and then measured their dry AGB and BGB. We fitted log-log linear models to predict AGB and BGB from the biometric measurements. The most relevant model for predicting AGB in trees was fitted as follows: AGB = 0.0471 () (with AGB in kg and in g cm m). This model had a bias of 19%, while a reference model for comparison (fitted from tree measurements in a similar savanna ecosystem, Ifo et al. 2018) overestimated the AGB of trees of our test savannas by 132%. The BGB of trees was also better predicted from ÏDb2H as follows: BGB = 0.0125 () (BGB in kg and in g cm m), with 6% bias, while the reference model had about 3% bias. In shrubs, AGB and BGB were better predicted from together with the total number of stems (). The best fitted allometric equation for predicting AGB in shrubs was as follows: AGB = 0.0191 () . This model had about 1.5% bias, while the reference model overestimated the AGB of shrubs of Lamto savannas by about 79%. The equation for predicting BGB of shrubs is: BGB = 0.0228 () that overestimated the BGB of the shrubs of Lamto savannas with about 3% bias, while the reference model underestimated the BGB by about 14%. The reference model misses an important feature of fire-prone savannas, namely the strong imbalance of the BGB/AGB ratio between trees and multi-stemmed shrubs, which our models predict. The allometric equations we developed here are therefore relevant for C stocks inventories in trees and shrubs communities of Guinean savannas.DbHSSnÏDb2H0.915ÏDb2Hâ1ÏDb2H0.6899ÏDb2Hâ1ÏDb2HnÏDb2H0.6227n0.9271ÏDb2H0.7205n0.992
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