# Biomass Dynamics in a Fragment of Brazilian Tropical Forest (Caatinga) over Consecutive Dry Years
## 3 Results and Discussion
### Biomass  
Plant density in 2009 was 3063 ind. ha\({}^{-1}\), increasing to 3175 ind. ha\({}^{-1}\) in 2010, and decreasing to 3063 ind. ha\({}^{-1}\) in 2015, 2825 ind. ha\({}^{-1}\) in 2016, 2338 ind. ha\({}^{-1}\) in 2018 and 2113 ind. ha\({}^{-1}\) in 2019 (Figure 6). This reduction in plant density is the result of a higher mortality rate than recruitment rate, seen during the study period, where there was a mean mortality of 134 ind. ha\({}^{-1}\) yr\({}^{-1}\) and recruitment of 39 ind. ha\({}^{-1}\) yr\({}^{-1}\), generating a mean deficit of 95 ind. ha\({}^{-1}\) yr\({}^{-1}\). Probably, mortality rates that are higher than those of recruitment are strongly influenced by climate seasonality and consecutive years of drought. The reduction in plant density from 2015 to 2019, compared to the period from 2009 to 2010, is related to the droughs which have occurred in the region since 2012 [33]. Trees typically reduce evapotranspiration during drought by means of adaptive mechanisms, such as stomatal closure and the seasonal loss of leaves and canopy [14]. However, the intra- and interannual variation in rainfall accentuates the influence of water stress and can lead to plant mortality, which is strongly associated with drought [31]. In forested communities, the prolonged hydric stress reduces the number of trees, increasing the probability of survival of the remaining trees [10]. The reduction of the competition among vegetable individuals allows the hydric transportation systems to re-establish themselves, thus diminishing hydric stress [17]. Years of water deficit cause various problems to the development of newly recruited individuals, including delaying their development. As their roots are shallower than those of the older trees, during the dry season, they may suffer the negative effects of water stress in the upper layers of the soil [34]. Another important characteristic is the low capacity for accumulating reserves to maintain individuals during the dry season [13]. These reserves are influenced by phenological stage, genetic load, and the severity and intensity of the water stress [16]. Following the same trend as plant density, between 2009 and 2010, an increase of 7.26 Mg ha\({}^{-1}\) was seen in the biomass stored in the above-ground woody compartment, going from 65.33 to 72.59 Mg  
Figure 5: Annual mortality proportion of individuals from the fragment of Dry Tropical Forest in the Caatinga Phytogeographic Domain (DTF/CPD). ha\({}^{-1}\) (Figure 7). In succeeding surveys, 2015 (55.20 Mg ha\({}^{-1}\)), 2016 (45.04 Mg ha\({}^{-1}\)) and 2018 (43.46 Mg ha\({}^{-1}\)), a decreasing sequence was identified for the estimated biomass. In 2019, there was an increase of 1.1 Mg ha\({}^{-1}\), for a total stock of 44.47 Mg ha\({}^{-1}\). As such, a mean increase of 1.33 Mg ha\({}^{-1}\) yr\({}^{-1}\) was seen in the above-ground woody biomass for the entire study period, with 0.92 Mg ha\({}^{-1}\) yr\({}^{-1}\) related to the increase and 0.41 Mg ha\({}^{-1}\) yr\({}^{-1}\) to recruitment. This was not enough to overcome the loss of biomass, which averaged 3.26 Mg ha\({}^{-1}\) yr\({}^{-1}\) due to the mortality of the woody individuals, giving a negative balance of 1.97 Mg ha\({}^{-1}\) yr\({}^{-1}\). Figure 6: Annual distribution of tree density, recruitment, and mortality in a fragment of dry forest in the northeast of Brazil. Figure 7: Annual distribution of above-ground woody biomass and rainfall in a fragment of dry forest, Iguatu-Ceáárá. The same lowercase letters between years do not differ by the Friedman test (\(p\leq 0.05\)). The greatest production of biomass was seen 2010, which differed statistically by the Friedman test (\(p\leq 0.05\)) from that of 2016, 2018 and 2019, but was like that of 2009 and 2015 (Figure 7). The increase in biomass from 2009 to 2010 is therefore related to the total rainfall between 2008 and 2009, which was above average, and resulted in greater water availability for the period.