Estados Unidos: Estado de Alaska

Given the soft-muddy sediments and frequent tides in mangrove environments (Cintron and Schaeffer-Novelli, 1984), the field survey and detailed measurements of tree traits and forest structural parameters including biomass often present huge difficulties (Komiyama et al., 2008; Proisy et al., 2007). This challenge may partly explain why structural data and biomass of large mangrove trees are rarely available. Meanwhile, tools like terrestrial laser scanning (TLS) and other remote sensing technology have offered the possibility for non-destructive and high resolution three-dimensional (3D) structure of trees with relatively low time and labour

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requirements (Dassot et al., 2011). These remotely sensed measurements have found extensive applications in various ecological studies, although it is largely unexplored in mangrove studies. In Chapter 2, I have proposed a method to capture tree morphological attributes and biomass of mangrove trees, which was tested for a wide range of tree sizes, up to very large trees. By employing a pixel-based analysis in segmented tree sections, the TLS-based biomass estimation approach presented in this thesis specifically addressed issues of irregular tree shapes or presence of the buttresses that may impact on the overall accuracy (e.g., Nogueira et al. (2006)). Accordingly, it is particularly relevant for extensive application to the large mangrove trees, which may present protuberance or buttresses at the lower parts.

Allometric models found applications in various studies concerning the structure and functioning of mangrove forests (Fromard et al., 1998; Kauffman and Donato, 2012;

Komiyama et al., 2008). Common to most available allometric models is the fact that they are mostly calibrated based on measurements from small trees for the lack of large tree data. This factor marks a potential source of error propagation, especially when such models are applied to sites dominated by large trees, like the case of French Guiana. Besides, such large trees represent keystone structures explaining variations in forest biomass distributions, stand productivity and ecosystem dynamics (Bastin et al., 2015; Slik et al., 2013). The case is true for the tall mangrove forests common in the equatorial axis (Proisy et al., 2012).

I have integrated the data so obtained via TLS measurements to extend the validity domain of allometric models of Avicennia germinans to very large trees up to ca. 125 cm diameter size. In particular, a significant shift found in the partitioning of biomass to different aboveground components between small-moderate diameter size trees and the larger individuals, marks the basis extending allometric models to large trees. Even when the biomass allometric models achieved at this stage significantly improve on the predictive power of the preceding models, it is worth to consider the fact that trees growing in different shape and allocate biomass in a divergent manner to the influence of environmental conditions. This factor establishes the need to move beyond static allometric models into a more mechanistic approach in mangrove biomass modelling. In this direction, the work of Peters et al. (2014) that proposed a single-tree model mechanistically explaining environment-induced changes in tree allometric relations and biomass allocation made the first leap, upon which I built on in my thesis to explore the implications of morphological plasticity and changing biomass partitioning for biomass distributions and forest structure in variable environmental settings (Chapter 5).

Combining structural descriptions of A. germinans trees found at both sites, we highlighted the site-specific differences in tree allometries and the implication for

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cross-regional biomass modelling (Chapter 3), thanks to the analysis of covariance (ANCOVA). Apparently, our nested ANCOVA (site-dependent) models of allometric relations differed significantly from the joint models obtained through data aggregated from across the study sites. The crown – stem diameter allometry was largely congruent to the tree height – diameter allometric relations in terms of variations, from French Guiana to the two forest classifications in Bragança. This aligns with the behaviours of mangrove trees growing in sites of varying salinity and water stress conditions, earlier documented in other mangrove regions (e.g.,

Vovides et al. (2014)). Of the two models (power function and logarithmic model) tested for the investigated tree allometries, we selected the model based on the logarithmic transformation of stem diameter. The reasons being that: 1) while power function resulted in over prediction of large tree height and crown diameter, the logarithmic model displayed a good fit for the asymptotic nature of tree growth (Feldpausch et al., 2011; Olagoke et al., 2016); 2) the logarithmic model returned much lower error estimates for trees in the lower and larger diameter size classes than the power model.

Consequently, it was relevant to investigate the extent to which the tree scale plasticity can impact on the accuracy of diameter-biomass allometric equations, when found applications at the cross-regional levels. The parameterization of biomass model that decomposes aboveground tree part into the trunk and crown components (Sensu. Ploton et al. (2016)) with the 3D architectural data of selected trees (based on laser scanning and tacheometer measurements) thus became advantageous in this respect. Indeed, local variations in tree morphology significantly impact on aboveground biomass estimates of diameter – biomass allometric models calibrated for French Guiana mangroves when tested against for sites of contrasting tree structure in Bragança, even when sites are located in the same geographical region. Further than the explanation rooted on latitudinal variation in tree biomass (Twilley et al., 1992), morphological plasticity here appears an important factor in the diameter – biomass relationship. The study concludes that regional differences in mangrove tree structure and function could be captured through better description of crown metrics could generate a plus-value in the understanding of mangrove stand dynamics across contrasting coastal environments.

Together, these tree scale allometric studies may facilitate accurate determinations of the biomass for the aboveground parts with potential benefit to the success of the coastal blue carbon projects (Howard et al., 2014) in the effort for conservation of mangrove forests.

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