The effects of gentle topographic variation on dispersal kernels of inertial particles

Abstract

Seed dispersal kernels of wind‐dispersed species imprint the initial spatial template over which later demographic processes such as establishment or re‐colonization operate from. A major knowledge gap in seed dispersal modeling by wind is the role of complex topography in modifying the kernel shape when referenced to the flat‐world case. How complex topography modifies the dispersal kernels of inertial particles such as seeds is explored here via novel flume experiments. A train of gentle cosine hills and a canopy composed of densely arrayed rods with a roughness density resembling the leaf area density profile of hardwood species at maximum leaf area were used to represent a canopy‐hill system. Spherical inertial particles having a coefficient of variation in terminal velocity commensurate with values reported from field studies were employed as model seeds. It was demonstrated that the seed dispersal kernel maintained its canonical ‘Wald’ form as derived from simplified turbulent dispersion theories for flat terrain and vertically uniform flow field. Seeds released near the canopy top and from the top of the hill have a dispersal kernel mean distance that was about 35% times larger than its counterpart for releases from the bottom of the hill. Moreover, the probability of occurrence of long‐distance dispersal (LDD) events, defined here as seeds traversing a longitudinal distance exceeding 10 times the canopy height, were one order of magnitude larger for seeds released near the canopy top and from the hill top when compared to their counterpart released from the hill bottom.