On maize pollen transport in the atmospheric boundary layer

A literature survey shows that recent experiments on pollen deposition and observations made on outcrossing rates at the landscape scale provide contradictory results. On the one hand most experiments on pollen dispersal, performed using ground-based systems with pollen traps located at various distances downwind from a source, have shown a rapid decrease in pollen deposition with distance. On the other hand significant outcrossing rates have been observed in several occasions at distances beyond the apparent range of pollen dispersal. This apparent contradiction can be solved by realising that measurements currently performed on pollen deposition can only provide information on short-range dispersal within the surface boundary layer, thereby excluding larger-scale mechanisms associated with convective acticity. In order to better characterise pollen dispersal, the presence and viability of pollen in the convective boundary layer (CBL) has been investigated. For this purpose a series of flights were performed from 2002 to 2005 with a light aircraft over a flat region consisting in a mozaic of maize fields and maritime pine stands in South-West France. Air samples were collected during the pollination period of maize, at several altitudes within and above the CBL (up to 1800 m). The sampling device allowed pollen grains to be collected by impaction on Petri dishes, and further counted and analysed for viability. Pollen grains were consistently found at all heights, with a vertical variation in concentration typical of the structure of a CBL, i.e. with little variation within the mixed layer. The average concentration in the CBL is closely related to the proportion of surface occupied by maize fields. Simulations performed with a simple CBL model provide results compatible with measured concentrations and source strengths. Pollen viability smoothly decreases with height but remains significant throughout the boundary layer. Separate chamber measurements show that the temperature and humidity conditions encountered by pollen in the CBL are much more favourable for its survival than close to the ground. To complete the picture, outcrossings have been observed on isolated maize plots set up for the purpose of estimating the level of genetic “pollution” due to these large-scale transport mechanisms. These findings provide a rational explanation for the existence of regional background concentrations in viable pollen, that may account for the fact that observations of outcrossings form a source field often show fat-tailed distributions.