Structural leaf attributes indicate different degrees of xeromorphism: New discoveries in co-occurring species of savanna and forest formations

Abstract

The diversity of environments in the Neotropical region has different environmental filters on vegetation. The southern face of the Amazon is exposed to high rainfall seasonality and poor soils, which imposes a severe environmental filtering. Plants with structural and functional adaptations can occur in specific locations due to microenvironmental characteristics. We investigated how anatomical-leaf attributes of seven co-occurring species of savanna and forest formations respond to the environmental conditions of their phytophysiognomies using usual optical microscopy techniques and statistical models. The species have different strategies to support environmental conditions. Even under a more limiting rainfall regime, plants living in seasonal forests closely related with savannic areas (Cerradão area - CDA) are less endowed with xeromorphic characteristics, which we can attribute to the conditions of continuous canopy and soil, comparatively more clayey than savanna soils. The species associated with rocky outcrops more closely related to savannic habitats (Rupestrian Cerrado of Transition area - RCT) showed greater investment in thickness of epidermal cell walls and the total thickness of the central vein. These species, together with the species associated with rocky outcrops more closely related to forest habitats (Amazon Savanna on Rocky Outcrops area - ASR), demonstrated greater investment in cuticle, straight anticline walls, straight periclinal walls or straight and convex periclinal walls, epidermis thickness, parenchyma, secretory ducts, collenchyma and sclerenchymatous fibers. Our findings reveal species’ different responses to environmental conditions: ASR and RCT species present a likely longer (temporal) selective process with a high investment in hydraulic safety, but RCT species showed greater xeromorphism; CDA forms a less xeromorphic microenvironment. The leaf xeromorphism is not only determined by the rainfall levels or soil fertility characteristics, but also by the degree of exposure of these plants to solar radiation and local seasonality.