# Tropical Dry Forest Diversity, Climatic Response, and Resilience in a Changing Climate
## 3 Tropical Dry Forest Response to Variations in Climate
### Response to Variation in Precipitation  
Successful stage and flowering regimes are also affected by water availability with corresponding increases in precipitation and the magnitude of flowering and pollen production [88]. Flowering timing is also highly related to the onset of the rainy season in TDFs [88]. Successful stages have a mixed relationship with differences in precipitation. Early successional stages with limited water availability recruit more drought tolerant species but have limited regrowth and sprouting, leading to a slower recovery rate [66; 91; 92]. Proximity to old growth areas also moderates recovery rates, along with the seed dispersal mechanisms (mostly controlled by wind) and species recruitment derived from nearby neighbors [40]. Precipitation regimes or water availability do not significantly change the rate at which species diversity increases in areas, nor the species evenness between different sites [40]; however, sites in Brazil did find that the higher precipitation can offset the damages that anthropogenic alterations cause to species diversity [77]. Additionally, evapotranspiration rates, combined with water deficits, drive tree species assemblages in the Mata Atlantica, TDFs, and Caatinga biomes in Brazil [81].  
While species diversity may not be significantly related to precipitation, biomass and water availability are positively correlated. In Bolivia, for example, TDF gross primary productivity (GPP) anomalies have historically (from 1980-2009) been correlated to precipitation anomalies (\(R^{2}=0.8\)) and increases in the dry season by one day could decrease GPP by up to 1.2% [93]. On a global scale, there is a significant decrease in AGB when plots have a longer dry season, with an average reduction of 40 Mg/ha per additional month in the dry season, although this is highly variable by plot (Figure 4) [28].  
Disturbed TDFs recover quickly, retaining their maximum biomass, species richness, and structural composition within 30-50 years, though their maximum biomass remains smaller than primary forests [13; 28; 40; 67]; however, biomass differentiation can be seen earlier in wetter sites [67]. After twenty years of recovery, secondary forest sites that have a higher annual rainfall have higher biomass, although this saturates above 2500 mm/year or in tropical wet forests [65; 67] (Figure 5). The combination of stand age and mean annual precipitation in TDFs are significantly related to AGB with a logarithmic function [28]. As stands get older, their biomass increases and at 80 years of age, sites that receive \(<\)1000 mm per year average 100 Mg/ha, ones that have \(<\)1500 mm/year reach 150 Mg/ha, and ones that have between 1500-2000 mm/year can average \(\sim\)225 Mg/ha of biomass [28]. Linearly related to annual biomass growth is the climatic water deficit [65]. Combined with rainfall seasonality and the increases in dry season length having a correlative 1.2%/day decrease in biomass, the water availability in secondary tropical forests accounts for 59% of the biomass variation after 20 years of recovery, making it one of the most critical variables [65; 93]. For mature forests, an increase in mean annual precipitation of 500 mm can increase the maximum biomass by 94 Mg/ha [65].  
Figure 4: The relationship between the above-ground biomass (AGB) and the length of the dry season with plots from around the globe. The figure is reprinted from Forest Ecology and Management, 276, [PERSON] et al. Aboveground biomass in mature and secondary seasonally dry tropical forests: A literature review and global synthesis. _Forest Ecology and Management_ 2012. page 91, with permission from Elsevier.