The analysis of leaves of seven legume species distributed along a rainfall gradient in a seasonally dry tropical forest revealed that dry species perform conservative strategies in individuals living in higher levels of annual rainfall, for example, presenting thicker cuticles with denser and smaller stomata and narrower vessels when compared with individuals living under lower rainfall levels. In the same context, wet and indifferent species in areas with higher rainfall show less investment in security, presenting a thicker spongy parenchyma and higher leaf construction costs. The main source of photoassimilate production, the leaves in deciduous woody species with a short annual rainy season, does not seem to be able to invest in safer structures when subjected to severe conditions. The positive and negative aspects of this behavior are discussed. Identifying leaf structure, behavior, and dynamics in dry forests contributes to understanding community composition and provides additional resources for future scenario prediction studies. In addition, our study provides new data on the anatomy and ecophysiology of species of the most representative family in a seasonally dry tropical forest. Considering the great biodiversity of this environment, a gap exists in knowledge about the biology of the species that comprise it, which, with our data, we seek to reduce.
Higher abundance in lower rainfall: dry species
Cenostigma microphyllum and Bauhinia acuruana, two of the three most abundant species in lower rainfall areas (below 762 mm/year rainfall average), presented smaller stomata and narrower vessels, respectively, when living in areas with higher levels of rainfall. Peltogyne pauciflora, the second most abundant species in lower rainfall areas, adopts a different strategy; it has a wider average vessel tangential diameter. However, it is a more deciduous species, flushing almost all leaves for a longer period during the dry season. In addition, these species showed strategies to avoid the risk of hydraulic failure, even in higher rainfall plots, either with epidermal strategies to avoid dehydration or with reduced transpiration through the flushing of leaves. These behaviors make them more likely to grow in drier areas because they are more efficient in the transport and assimilation apparatus.
Bauhinia acuruana showed a contrasting response to higher rainfall, adopting conservative strategies, such as reducing vessel diameter, a trait related to water flow security (Hacke et al. 2016). In contrast, it also increased stomatal pore size, indicating a greater water loss during transpiration (Li & Liu 2016), compared with low rainfall areas. This species seems to take advantage of increased water availability to invest in conservative strategies, and once seasonality is strongly marked in the Caatinga domain, it invariably faces stress due to a lack of water. Notably, B. acuruana was more abundant in the driest sampled plots, suggesting a high tolerance under low water availability. Additionally, although there were no differences between rainfall levels, this species has a heterobaric leaf with a high density of trichomes organized in clusters, a common strategy for reducing transpiration water loss (Bosabalidis and Kofidis 2002). These results suggest that B. acuruana has an efficient water-use strategy, and additional physiological studies are necessary to better identify these mechanisms.
Higher abundance in higher rainfall: wet species
Trischidium molle, Senegalia piauhiensis, and Senegalia bahiensis were the most abundant species in wetter areas (average rainfall above 762 mm/y). T. molle was the most plastic species, with four different traits resulting from rainfall. It adopts a similar strategy in dry species, creating a safer epidermis with smaller and denser stomata and thicker cuticles. In contrast, S. bahiensis had thinner leaves and narrower vessels and sieve tubes than the Caesalpinioideae mimosoid clade species (Pityrocarpa moniliformis and Senegalia spp.) but increased LCC in wetter areas. This species used some strategies with greater assimilation efficiency when more water was available. However, these changes have rendered them less secure. Moreover, this species had fewer traits than the dry species, adding to its smaller distribution and absolute abundance, suggesting its security is limited. Senegalia bahiensis and S. piauhiensis have leaves composed of tiny leaflets but only a few tector trichomes, as well as a papillary epidermis with a fine cuticle. These attributes do not indicate a high tolerance to drought, although small leaf areas and papillary cuticles are related to water-loss strategies (Li and Liu 2016). In addition to the restricted distribution of S. bahiensis and the predilection of S. piauhiensis for areas with greater water availability, our results suggest a risk to this species facing climate change. In a niche modeling study (Yule and Santos, unpublished data), we found a niche reduction for S. bahiensis and, despite increasing fragmentation for S. piauhiensis in future climate change scenarios (PBMC 2014; Fick and Hijmans 2017), these results indicate a worrisome situation for these species.
High abundance and wide distribution: indifferent species
Pityrocarpa moniliformis was the most abundant and widely distributed species in this study. However, only one trait changed due to rainfall: the spongy parenchyma thickness (SPt) increased with increasing rainfall. This feature, added to heterobaric leaves with EBS and a thick cuticle, for example, promotes a high level of transpiration control and thus conditions to grow well in different levels of rainfall.
Some anatomical characteristics observed in the present study were previously related to the studied genus or other members of the family Leguminosae, such as the shape of the main vascular bundle and epidermal cells (Bento et al. 2020), type of stomata (Maiti et al. 2016) and glandular trichomes (Roth 1990; Marinho et al. 2016). They are phylogenetically related and may not vary in appearance depending on the environmental conditions.
However, this may vary in number, as observed in our results. Variations in stomatal and epidermal cell densities and stomatal pore size are commonly associated with the control of transpiration and CO2 uptake (Bosabalidis and Kofidis 2002; Bertolino et al. 2019). Likewise, stomatal aspects and cuticles showed differences in the four analyzed species. The cuticle is formed by waxes, polysaccharide microfibrils, and cutin, providing mechanical protection against water loss through the epidermis. In Caatinga species, it plays an important role in controlling transpiration (Figueiredo et al. 2015; Medeiros et al. 2017; Pereira et al. 2019).
The paraveinal mesophyll (PV) and extended bundle sheath (BSEs), as observed in Senegalia spp., Bauhinia acuruana, P. moniliformis, and Trischidium molle, have been previously described in legumes (Wylie 1952; Rutten et al. 2003), deciduous species (McClendon 1992) and trees and shrubs growing under high irradiance and low precipitation (Zsögön et al. 2015). These structures reduce the hydraulic resistance from the bundle sheath to the epidermis, optimizing lateral air transport in the mesophyll, rate of stomatal opening, and stomatal conductance (Buckley et al. 2011; Zsögön et al. 2015). In addition, paraveinal mesophyll promotes changes between neighboring bundles outside the hydric tissues (Weston and Cass 1973; McClendon 1992), guaranteeing faster rehydration and improved photoassimilate transport (Kevekordes et al. 1988; Zsögön et al. 2015). These tissues are formed from layers of ground meristems that are different from those that form the palisade and spongy parenchyma, performing specific roles in leaf physiology (Weston and Cass 1973; Kevekordes et al. 1988). Consequently, BSEs consist of colorless tissue and surrounding bundles and extend from one epidermis to the other, completely or not, creating compartments in the mesophyll (McClendon 1992; Rodrigues et al. 2017). B. acuruana, P. moniliformis, and T. molle are classified as heterobaric based on the presence of BSEs tissue (Rodrigues et al. 2017) as the CO2 pressure varies between different parts of the leaf (Zsögön et al. 2015). This reduces air diffusion and allows the leaf to open or close groups of stomata in different regions, and consequently, has more control over water loss.
Similarly, woody species tend to allocate more biomass to the leaves and phloem to ensure transport and resources for assimilation under drought (Kiorapostolou and Petit 2019). We believe that in these organs, in addition to hydraulic tissues, the assimilation and the epidermis play an important role in maintaining the function of the leaves, as demonstrated in experimental and modeling studies (Dayer et al. 2020). This is mainly reasonable for deciduous and sub-shrubby species, whose leaves usually have lower construction costs (Falcão et al. 2017) and can flush in the case of conductivity failure (de Lima et al. 2012). For these species, in these conditions, leaf construction cost only varies among rainfall events in S. bahiensis, and this may be explained by the predominance of habit, phenological behavior, and phylogenetic relationships: all species are deciduous woody (trees or shrubs) legumes. Additional studies on species biology are necessary to understand why S. bahiensis invests in more expensive leaves.
There was no direct relationship between low rainfall and the adoption of more conserved and secure characteristics in the leaves of five of the seven woody legumes evaluated. Studies on recurrent stress have revealed that severe events tend to reduce response capacity (Camarero et al. 2018), deplete productivity, increase water evapotranspiration loss, and lead to death (Fleta-Soriano and Munné-Bosch 2016). In contrast, moderating drought events promote a stress “memory” producing epigenetic changes and accumulating singling proteins or transcription factors, affecting persistent gene expression changes (Li and Liu 2016). Individuals, populations, and species distributed in wetter areas of the Caatinga experience seasonality with the same drought intensity as those occurring in drier areas but with a greater amplitude of water availability. We suggest that this method can recover from drought damage more effectively and quickly after stress and is a more persistent community component. However, the adoption of less conservative strategies indicates a process of acclimatization that promotes a reduction in the safety margin for the operation of species under similar conditions (Tombesi et al. 2018), increasing the risks for leaves and, consequently, for the entire plant. To better understand how these species respond to the trade-off of security versus productivity, further studies are necessary, with pluriannual sampling, including other organs, in vivo analyses, and the “omics” approach.
The abundance of the species observed in this study was related to the rainfall gradient, which is an important record of its behavior. Functional attributes related to drought sensitivity have proven to be good predictors of species distribution in tropical forests (Engelbrecht et al. 2007). Based on our results that plants subjected to lower water availability showed less secure anatomical changes, we are concerned about populations in the face of climate change predictions. Climate change intensifies the water stress due to the lack of water inherent to the Caatinga, as an increase of 1.5 ºC to 3.5°C in the average temperature is expected, accompanied by a 20–60% reduction in rainfall, still in this century, to the Northeast region of Brazil (Rodrigues-Filho et al. 2016; Torres et al. 2017). It promotes changes in vegetation dynamics, increases hydraulic insecurity, delays responses to recovery from damage, and increases the risk of mass mortality, which are related to increasingly frequent and severe drought events (Anderegg et al. 2015a; Anderegg et al. 2015b). Our results corroborate the risk of vegetation with niche loss, depletion of assimilation and growth rates, and hydraulic failure, but collaborate to understand future scenarios once vegetation responses are species-specific (Camarero et al. 2018; Kiorapostolou et al. 2020).
In conclusion, rainfall range promotes anatomical variations in Leguminosae leaves distributed along different levels in the Caatinga, mainly affecting morphological attributes, followed by epidermal and xylem tissues. Similarly, changes caused by environmental variations affect the abundance of species along the rainfall gradient. However, in contrast to what was expected for species distributed in restrictive environments, intraspecific variation did not occur when relatively more conservative characteristics with reduced water availability were adopted. Our results corroborate the risks to vegetation under future climate change scenarios as stressed species and populations may not endure even more severe conditions. Finally, it is important to know the responses of other organs to effectively understand plant strategies against stress due to lack of water, both now and in the future.