Impacts of land clearance by fire on spatial variation of mountain cedar pollen concentrations in Texas

Austin, Texas, is one of the worst places with respect to allergies. In winter, mountain cedar (Juniperus ashei) is producing high pollen amounts exerting the so-called cedar fever in individuals allergic to this pollen species. In this study, we evaluated to which extent the effect of prescribed fires in a semi-arid ecosystem decreases pollen concentrations at the local scale. The study was performed on patches characterized by human-induced fire disturbances and unmanaged areas (control sites) in the southwest of Austin. Pollen was sampled on four consecutive days in January 2015 using ten pollen traps installed at 1.5 m a.g.l. Microscope slides were inserted every second hour during 8 am and 6 pm. Each station was equipped with meteorological devices to account for influencing factors such as wind speed and direction. Mean cedar pollen concentrations differed across the studied days and plots and ranged between 633 (16th January) and 126,400 pollen grains/m³ (17th January). We determined diurnal patterns with peak values between 2 and 3 pm. Pollen concentrations were on average 15% lower at burnt vs. control plots. Comparing pairs of adjacent plots even revealed a reduction of up to 50%. Therefore, local land management can alter pollen concentrations drastically. A walk across previously burnt areas may trigger less severe symptoms for allergic people during the flowering period of mountain cedar. Our results add to the knowledge about individual cedar pollen exposure in heterogeneous areas and help improving mitigation strategies.