Evolutionary trends in safety factors against wind-induced stem failure

We explore the hypothesis that the safety factor against wind-induced stem failure remained high during early land plant evolution despite an evolutio nary increase in height with concomitant increases in wind-induced drag for ces, bending stresses, and moments. This hypothesis was examined for 17 Pal eozoic plant species assuming that each (1) existed in a densely packed com munity of conspecifics with equivalent height, (2) coped with the same wind profile (where ambient wind speed decreased toward ground level), but (3) had different within-canopy wind speeds depending on plant height and gener al morphology. Drag forces, stresses, and moments were computed, and a safe ty factor was calculated for each taxon using the quotient of its stem-tiss ue breaking Stress and maximum wind-induced bending stress. The highest factors of safety were calculated among the most ancient rhynio phyte and zosterophyllophyte species examined (e.g., Rhynia and Gasslingia) , and, on average, decreased among the taller and geologically younger spec ies. The tallest species examined (e.g., Archaeopteris and Diaphorodendron) had safety factors equal to or higher than those of some of their presumed ancestors (e.g., Psilophyton and Leclercqia). These trends were statistica lly more robust among rhyniophytes and their presumed descendants. Even though the results comply with the hypothesis, numerous limitations of our protocol exist (e.g., the requirement for reliable whole-plant reconst ructions). These are discussed in terms of our theory Nonetheless. we belie ve our theory and protocol afford a reasonable opportunity to explore the e ffects of wind on early plant evolution.