Accounting for small-size tree biomass is critical to improve total stand biomass estimates of secondary tropical forests, and is essential to quantify their vital role in mitigating climate change. However, owing to the scarcity of equations available for small-size trees, their contribution to total biomass is unknown. The objective of this study was to generate allometric equations to estimate total biomass of 22 tree species ≤ 10 cm in diameter at breast height (DBH), in the Yucatan peninsula, Mexico, by using two methods. First, the additive approach involved the development of biomass equations by tree component (stem, branch and foliage) with simultaneous fit. In the tree-level approach, total tree biomass equations were fit for multi-species and wood density groups. Further, we compared the performance of total tree biomass equations that we generated with multi-species equations of previous studies. Data of total and by tree component biomass were fitted from eight non-linear models as a function of DBH, total height (H) and wood density (ρ). Results showed that two models, identified as model I and II, best fitted our data. Model I has the form AGB = β0 (ρ•DBH 2 •H)β1 + ε and model II: AGB = exp(-β0) (DBH 2 •H)β1 + ε, where AGB is biomass (kg). Both models explained between 53% and 95% of the total observed variance in biomass, by tree-structural component and total tree biomass. The variance of total tree biomass explained by fit models related to wood density group was 96%-97%. Compared foreign equations showed between 30% and 45% mean error in total biomass estimation compared to 0.05%-0.36% error showed by equations developed in this study. At the local level, the biomass contribution of small trees based on foreign models was between 24.38 and 29.51 Mg ha -1 , and model I was 35.97 Mg ha -1 . Thus, from 6.5 up to 11.59 Mg ha -1 could be excluded when using foreign equations, which account for about 21.8% of the total stand biomass. Local equations provided more accurate biomass estimates with the inclusion of ρ and H as predictors variables and proved to be better than foreign equations. Therefore, our equations are suitable to improve the accuracy estimates of carbon forest stocks in the secondary forests of the Yucatan peninsula.

<p>Secondary tropical forests are largely dominated by small-diameter trees. Their contribution to the total stand productivity may be determinant for ecosystem functioning. However, the biomass contained in small-size trees is frequently ignored because they are rarely included in forest inventories, as well as due to the lack of biomass equations for such trees. The aim of this study was to generate specific biomass equations by tree structural components and total-tree biomass for 22 species (diameter at breast height-DBH ≤ 10 cm diameter) structurally important of secondary tropical forests in southeast Yucatán peninsula. The biomass equations were generated using two fitting approaches. First, using the additive approach where the biomass equations by structural tree components (stem, branches and foliage) were simultaneously fitted. Second, the tree-level approach where the total-tree biomass was independently fitted. We used DBH, total height (TH) and wood density (WD) as predictor variables during the fitting process. The performance of each total-tree biomass equation was compared to generalized biomass equations developed in other tropical forests. The variance explained of observed biomass across tree species were stem, 64% to 99%; branch, 24% to 95%; foliage, 35% to 94% for equations by structural tree components; while the explained variance for total tree biomass equations was between 94 to 99 %. The inclusion of TH and WD as predictor variables in the equation structure, significantly improved the goodness of fit statistics. These are the first equations developed for small-size tree species in secondary forests of Yucatán peninsula. We observed that generalized equations developed for other tropical forests, in some cases overestimated up to ~ 44.2% and in other cases underestimated up to ~ 48.4% the mean total-tree biomass. When we applied our equations to forest inventory data, they showed a better predicting performance than generalized equations. Therefore, they are reliable for estimating biomass of small-size tree species in similar forest types. Our equations could be used for biomass-carbon estimation in tropical secondary forests composed by high densities of small-size tree species; therefore, they are relevant within climate change contexts.</p>

<p><strong>Background. </strong>The mangrove forest is a mixture of tree and shrub species which inhabit the coastal areas of the tropics and subtropics, around estuaries and coastal lagoons, near the mouths of rivers, with another different capacities than the tolerance of salinity. Despite its ecological and social importance, it is one of the ecosystems most threatened by changes in land use and climatic variations. <strong>Objective.</strong> Evaluate the increase in total height and basal diameter in the dry and rainy period in <em>Avicennia germinans</em> L. seedlings, from the nursery and from natural regeneration. <strong>Methodology.</strong> In ten sampling units (SU) the total height (At), basal diameter (Db) and the survival rate (%) of the reforested individuals were recorded. In each SU, three Sub-SU (SSU) were installed where only the first two attributes were recorded for natural repopulation. The measurements were carried out in three stages during one year (M1, M2 and M3). <strong>Results</strong>. The mean annual increase in reforestation was 1.14 cm in Db and 58.8 cm in At. SU 4 was the one that reported the greatest increase in Db (1.87 cm) and At (132.6 cm), while the lowest values were presented in SU 9. The growth relationship between Db and At was statistically significant (p ≤ 0.05). Reforestation survival decreased from 73.2% to 52.7% during the evaluation period. The highest growth in Db, At and survival were in the SU´s where the water flow is greater and with a canopy gap is larger. Natural regeneration had a lower mean annual increase in both Db (0.51 cm) and At (37.2 cm). <strong>Implications.</strong> This novel information provides the basis for the planning, management and conservation of <em>A. germinans</em> in the southeast of Mexico. <strong>Conclusion</strong>. The time of measurement and the ecological differences between sampling units significantly influence the dasometric increase of <em>A. germinans</em>. The reforested individuals show a greater annual increase in Db and At compared to those of natural regeneration.</p>

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