Influence of the abiotic environment on Permian woods from northwestern Gondwana

Abstract

Fluctuations in cell densities are common in the secondary xylem of gymnosperms from the lower Permian of Maranhão, northeastern Brazil, but they lack any distinct annual growth ring. Although the region was under a subtropical arid climate, sedimentary sequences of the Pedra de Fogo Formation indicate that plants inhabited the shores of large lakes, ensuring sufficient moisture for continuous growth of diverse flora. Therefore, the presence of acyclical tree rings would be due to oscillations in the base level of adjacent lakes, occasioned by short periods of drought or flood. Even so, cell-to-cell variation of tracheid diameters was quantified through mean sensitivity, showing that environmental conditions are similar in the two paleobotanical sites analyzed. However, gymnosperms whose piths have canals, ducts, septa or lacunae exhibit a higher mean sensitivity, showing that such plants inhabited areas of greater water stress or have increased sensitivity to environmental disturbances. The effect of defoliation and biotic interactions is also presumed to have affected the cambial activity. Localized and concentric shear bands are features identified in several samples, indicating that the pressure of sediments on the woods resulted in non-axial plastic deformation during the burial.