# Tropical Dry Forest Diversity, Climatic Response, and Resilience in a Changing Climate
## 2 Components of Tropical Dry Forests  
### Forest Dynamics  
Forest dynamics describe how forests grow, change, and react to environmental pressures, with key components of tree mortality, growth, and recruitment [30; 31]. Anthropogenic influences, especially agricultural activities, are the primary cause of tropical dry forest alteration. Land abandonment leads to the growth of secondary TDFs interspersed with primary old growth forests as documented in Costa Rica [32; 33]. The methodology that is most appropriate to study and group successional stages is disputed, with long-term studies, palynology, chronosequences, and stand reconstruction all suggested as best practices [9; 34]. Chronosequences, or grouping stages into early, intermediate, and late-stage based mainly on time since abandonment, is one of the most common study methods but has also garnered criticism because it assumes a consistent history between different locations [9; 35; 36; 37]. Despite the criticism of chronosequence analysis of successional stages, the replication it provides and its ability to capture TDF heterogeneity make it the most feasible procedure for studying its rapidly changing dynamics [9]. The structure of the forests, both horizontally and vertically, also defines successional stages. In early-stage succession, there is a matrix of seedlings, herbs, grasses, and sparse woody vegetation [14; 17]. As succession progresses, there is a higher proportion of trees, shrubs, and bromeliads, the increased presence of lianas, and canopy coverage of 63%-80% [17; 38]. The intermediate stage typically has the highest net primary productivity and species richness, as well as the highest density of lianas [17; 39; 40]. Late stage forests, however, are typically considered to have the highest species evenness [9; 19; 40] (Figure 3). Later successional stages also exhibit reduced regeneration capabilities as there is less sapling recruitment in stands \(>\)15 years old, resulting in slower recovery from events and slower species turnover [17; 39]. Recruitment rates are highest in early stage forests, although climate moderates these rates [17; 38; 39]. In Mexican sites, there is reduced recruitment and sapling development in wetter years thought to be due to reduced flowering and seed production in nearby stands compared to dry years [38; 41; 42]. This trend is also replicated in shrubs, which is problematic for the recruitment of and subsequent biomass growth of pioneer species in newly abandoned pastures [38; 43; 44]. Figure 3: Species richness and the number of stems by successional stage reproduced from Minas Gerais, Brazil as found in [PERSON] et al. (**a**) [19] and from Santa Rosa National Park, Costa Rica, based on data from [PERSON] et al. (**b**) [45]. Intermediate stage TDFs have the highest species richness of the secondary TDFs. The ecotone consists of a combination of primary TDF and savanna land. The previous land use of secondary TDFs and individual site characteristics are impactful for the resulting species recruitment rate, diversity, and mortality [40]. Plots with a history of use as cultivated lands had a significantly lower species evenness and slower recruitment rates when compared to plots with a history of being used as unmanaged pasture land [40]. In Costa Rica, pasture sites had a much lower rate of species recruitment compared to low-intensity use plots, and site-specific conditions, such as proximity to seed sources were highly impactful for plot structure and diversity [37]. Seed sources from proximal old growth forests also tend to drive the eventual species makeup of the secondary forest [40; 46]. In all Costa Rican successional stages, the most abundant species are dispersed by either wind or by animals, particularly birds [13; 47]. Additionally, many species are unique to a single successional stage, irrespective of latitude. In Brazil, only 13.5% of species were common across all successional stages, with 43.2% of species unique to only one successional stage [19].