# Tropical Dry Forest Diversity, Climatic Response, and Resilience in a Changing Climate
## 2 Components of Tropical Dry Forests
### Forest Structure  
Forest structure is highly dependent on forest age and the level of disturbance in both primary and secondary forests. In general, the measurable structure in a forest can be broken up into stem density, basal area, number of canopy levels, and in the case of tropical dry forests, liana infestation [15].  
Stem density in TDFs is highly variable across the continent and between plots. In Brazil, early stages have the lowest stem density and late-stage the highest, with small diameter trees most prevalent in the intermediate stages [19; 35]. In Bolivia, stem density is most variable in the late-stage [17], while these young forests have a greater abundance of small stems, as well as a low canopy cover [17], which allows for higher herb and grass abundance in early successional stages. North of the equator, in Mexico, the density of stems with the diameter at breast height (DBH) between 0.10-0.30 m varied by plot, but typically decreased with stand age. Furthermore, the density of stems >0.30 m increased asymptotically with stand age. Stem density is also impacted by the underlying geomorphology, with ridgelines and low slopes having a higher stem density in Mexican sites compared to regions with intermediate slopes [48]. In Venezuela, stem density is the lowest compared to all other biomes, except the plains, and the basal area is third smallest above only the plains (i.e., Llanos) and the mid-elevation Andean forest [49].  
The basal area and DBH of successional stages also varies across the continent. As the successional stage increases, the basal area of plants increases in the Southern Hemisphere [17; 19]. In the Northern Hemisphere, a local-level analysis found that the Yucatan has a smaller basal area compared to other TDF sites across Mesoamerica and that secondary forests never recovered a basal area comparable to the primary mature forests [39]. Slopes impact the basal area of tree stems with the total basal area increasing from 20.84 m\({}^{3}\)/ha to 27.06 m\({}^{3}\)/ha from the ridgeline to the lower slopes, respectively, in Chamela, Mexico [48].  
Liana infestations, more common in disturbed areas, also have a spatial structure component, and impact the resulting forest structure by negatively affecting the basal area and DBH in infested forests [50; 51]. Lianas in Bolivia can cover between 8%-15% of stands [17], although the density of vines correlates to the level of disturbance in a patch [52]. The higher abundance of TDF lianas compared to wet and moist forests is thought to be due to liana advantages in the dry season, including a faster green-up, slower senescence, the heightened solar radiation in arid forest zones, and the greater availability of this radiation throughout the year [51; 53; 54; 55]. Additionally, liana presence is highly correlated to elevation, with sharp declines in liana presence above 1000 masl [51; 56].  
Overall, late-stage forests in Yucatan, Mexico have the smallest basal area, followed closely by Brazil and Venezuela, while both Chamela, Mexico, and Bolivia have the largest basal area [17; 19; 39; 48; 49]. Instead of a latitudinal gradient, studies appear to find that areas closer to the Pacific have a smaller overall basal area. Stem density did not show a consistent pattern across the continent, varying widely between successional stages and individual plots [17; 19; 35; 48; 49]. Liana infestations follow a different pattern entirely, more closely associated with disturbance, rainfall, and elevation.