# Biomass Dynamics in a Fragment of Brazilian Tropical Forest (Caatinga) over Consecutive Dry Years
## 3 Results and Discussion
### Strub Layer
is a commonly reported species in floristic surveys due to its shrub habit and great capacity for regrowth and is considered the main colonizing shrub in successive DTF/CPD [7]. _Aspidosperma_ spp. is present in more preserved environments with little anthropogenic intervention [12]. _Mimosa caesalpinifolia_ Benth. and _Baulinia cheilantha_ ([PERSON].) Steud are pioneer species, common in primary and secondary formations [11], and characterized by fast growth, high regenerative capacity and resistance to drought. In the first survey (2010), with above average rainfall, the abundance of most species was slightly less in 2010 than in 2009 (Figure 4). However, it cannot be said whether there was any real significant change or simply random variation. This reduction in species abundance may be related to intra- and interspecific competition. The greatest increases in recruited individuals were seen in _Ximenia americana_ L. (37.5 ind. ha\({}^{-1}\)), _Aspidosperma_ spp. (37.5 ind. ha\({}^{-1}\)) and _Jatropha mollissima_ ([PERSON]. (25 ind. ha\({}^{-1}\)). In the survey 2015 to 2019, a period below-average rainfall, there was a reduction in the number of individuals, especially for _Croton_ spp., _Aspidosperma_ spp. and _Piptadenia stipulacea_ ([PERSON].) Duck., with a mean mortality rate of 45.0, 41.3 and 16.3 ind. ha\({}^{-1}\), respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Family**} & \multirow{2}{*}{**Species**} & \multirow{2}{*}{**Wood Density** **(g cm\({}^{-3}\))**} & \multirow{2}{*}{**Habit**} & \multicolumn{2}{c}{**Density**} \\ \cline{4-5}  & & & & **Absolute** & **Relative** \\  & & & & **(ind. ha\({}^{-1}\))** & **(\%)** \\ \hline Euphorbiaceae & _Croton_ spp. & 0.760 & Shrub & 885 & 32.1 \\ Apocynaceae & _Aspidosperm_ spp. & 0.710 & Tree & 538 & 19.5 \\ Combreaceae & _Combretum_ spp. & 0.696 & Shrub & 279 & 10.1 \\ Fabaceae & _Mimosa caesalpinifolia_ Benth. & 0.839 & Tree & 277 & 10.0 \\ Fabaceae & _Bauulinia cheilantha_ ([PERSON].) [PERSON]. & 0.865 & Shrub & 217 & 7.8 \\ Mimosaceae & _Piptadenia stipulaca_ (Benth.) [PERSON] & 0.653 & Shrub & 127 & 4.6 \\ Euphorbiaceae & _Jatropha mollissima_ ([PERSON]) Bail. & 0.270 & Shrub & 117 & 4.2 \\ Burseraceae & _Comniophora leptrophloos_ ([PERSON]) [PERSON] & 0.309 & Shrub/Tree & 94 & 3.4 \\ Olacaceae & _Ximenia americana_ L. & 0.700 & Shrub & 73 & 2.6 \\ Anacardiaceae & _Myracrodruon urundeuva_ [PERSON] & 0.845 & Tree & 50 & 1.8 \\ Fabaceae & _Caesalpinifera_ Mart. e. Tul. & 0.820 & Tree & 35 & 1.3 \\ Fabaceae & _AnacIndividuals that died during the study period had a wood density that was higher than the average for the area (0.660 g cm\({}^{-3}\)) (Table 2), suggesting that species with higher density, have less capacity to resist periods of prolonged drought. Tree species with highly dense wood may find it difficult to avoid a reduction in water potential compared to species whose wood is less dense [14]. Such a characteristic can result in smaller variations in leaf water potential over the year [30], in addition to affecting mechanical stability, inducing cavitation [14]. It is therefore important to conduct studies that relate wood density to periods of drought and rainfall. Furthermore, when climate conditions exceed the plant tolerance limits (e.g., an increase in temperature and vapor pressure) [10], hydraulic failure can result in tree mortality due to [31] xylem embolism [18,31]. Another factor possibly causing mortality is an excess of water in soils that affects water potential in both soils and plants [32]. The study area is on a Vertisol, characterized by contraction and expansion of the soil mass due to expansive clays, showing cracks in the dry periods. Due to restrictions imposed by soil physics, plants need a large amount of energy to absorb water from the soil, increasing as the soil dries [32].