Structural characterization of the vegetation facilitates forest management decision-making and prescription of silvicultural practices appropriate to the landowner´s purposes. In the ejido Nopalillo, municipality of Epazoyucan, located southeast of Hidalgo, Mexico, forest harvest is practiced in three associations dominated by Pinus montezumae Lamb., Pinus pseudostrobus Lindl., and Pinus patula Schl. et Cham., respectively. In addition, the ejido has an area dominated by Pinus-Quercus used as a conservation area. To characterize these four conditions, information from 79 sampling plots was analyzed, in which the arboreal, shrub and herbaceous strata were evaluated. Structural analysis of the vegetation was based on the Importance Value Index, Margalef, Simpson and Sorensen diversity indexes, as well as the vertical and horizontal structure in each association. The Pinus-Quercus area presented the largest basal area (28 m2/ha), normalized diameter (DBH) and average height (20.5 cm and 10.7 m), respectively, the highest proportion of large trees (DBH>60 cm) and species composition different from the other associations. The greatest total diversity was found in the areas under timber harvesting, and the highest tree diversity in the conservation area. The results can be used as a reference to identify the effect of silvicultural practices, they are also the basis for the development of strategies for forest management, conservation, and use. La caracterización estructural de la vegetación facilita la toma de decisiones de manejo forestal y la prescripción de prácticas silvícolas adecuadas a los propósitos del propietario. En el ejido Nopalillo, municipio de Epazoyucan, al sureste de Hidalgo, México, se realiza aprovechamiento forestal maderable en tres asociaciones arbóreas, cada una dominada por Pinus montezumae Lamb., Pinus pseudostrobus Lindl. y Pinus patula Schltdl. et Cham., respectivamente; además, el ejido cuenta con una superficie dominada por Pinus-Quercus que ha sido destinada a la conservación. Con el objetivo de valorar composición, diversidad y estructura de estas asociaciones, se analizó información de los estratos arbóreo, arbustivo y herbáceo de 79 sitios de muestreo. Se calcularon índices de valor de importancia, de diversidad de Margalef, de Simpson y de Sorensen y se describen las estructuras vertical y horizontal de la vegetación. La asociación Pinus-Quercus presentó la mayor área basal (28 m2/ha), diámetro normal y altura promedio (20.5 cm y 10.7 m, respectivamente), la mayor proporción de árboles de dimensiones grandes (diámetro normal > 60 cm) y una composición de especies diferente a las otras asociaciones. La mayor riqueza de especies se registró en las áreas bajo aprovechamiento maderable, pero la diversidad arbórea fue superior en el bosque de Pinus-Quercus. Estos resultados se proponen como referentes para la identificación de los efectos de las prácticas silvícolas sobre la vegetación y para el desarrollo de estrategias de manejo, conservación y uso del bosque.

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·DBH2·H)β1 + ε and model II: AGB = exp(-β0)(DBH2·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.

La disponibilidad de luz en los bosques es uno de los principales factores que influyen en el establecimiento y desarrollo de especies vegetales bajo el dosel forestal. A fin de caracterizar el ambiente lumínico bajo el dosel de Abies religiosa (Kunth) Schltdl. et Cham., así como la estructura del dosel y su relación con la diversidad vegetal a nivel del sotobosque, se establecieron sitios de muestreo en los cuales se colectó información para evaluar atributos del rodal como área basal, densidad y diámetro cuadrático. Además, se determinaron, mediante fotografías digitales hemisféricas, los parámetros estructurales del dosel, índice de área foliar, fracción de apertura del dosel e índice de transmisión de luz; los parámetros lumínicos bajo el dosel: radiación directa y difusa, frecuencia, distribución de haces de luz, parches de luz, claros de luz y claros; duración media acumulada diaria de haces de luz y duración promedio de un haz de luz. Se determinó la relación existente entre las variables de estudio mediante regresión lineal simple y de asociación de estas a partir del coeficiente de correlación de Pearson. Los resultaron mostraron relaciones significativas entre parámetros estructurales del dosel (p < 0.001) y parámetros lumínicos; mientras que las relaciones de parámetros de estructura del rodal y parámetros lumínicos (p < 0.05) tuvieron una menor correlación (r ≈ 0.50). La fracción de apertura del dosel fue el mejor predictor para el índice de área foliar, radiación directa y difusa disponible en el sotobosque, así como para el índice de transmisión de luz del dosel. La variabilidad del régimen de luz bajo dosel fue mejor explicada por los parámetros de la estructura que por las características estructurales del rodal.

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.