Meteorological Impacts on Pollen Emission and Spread

Emission and spread of tree pollen are important processes, firstly, because pollen of several tree species act as major allergen in the human population, and secondly, because pollen dispersal is a crucial process in the life cycle of the predominantly wind-pollinated trees in temperate forests. Thus, pollen dispersal provides reproduction and gene flow, and contributes significantly to the genetic diversity within and among populations. The proposed project aims at investigating and highlighting the meteorological factors causing local pollen emission and transport in a typical Central European forest of mixed deciduous and coniferous trees, and the development of a functional model for pollen emission. We will conduct measurements of pollen concentrations of spring flowering trees and meteorological conditions in a high-temporal resolution at three levels on a 30 m high tower within a 20 m high forest canopy in the "Lehrforst Rosalia" in Eastern Austria. The instrumentation consists of a special adapted pollen collector with three sampling units, which sample pollen from all directions in the same quantity, and which allow the recording of pollen concentration in a high temporal resolution. The meteorological equipment consists of three ultrasonic anemometers and conventional temperature, humidity and radiation sensors, which measure meteorological data in the same heights as the pollen concentrations (above the canopy, within the canopy, and on the forest ground). In addition to the field campagn, pollen transport will be modelled with a Lagrangian particle model based on trajectories determined from a diagnostic wind field model fed also by the meteorological data of surrounding stations. The project will improve knowledge on: a) pollen emission with respect to different tree species common in Central Europe, b) the meteorological parameters favouring pollen emission, c) the horizontal and vertical pollen transport within and above the forest canopy, and d) the ability of a Lagrangian particle model to correctly estimate ambient pollen concentrations. The expected better understanding of pollen emission and spread of typical European forest tree species will help to develop a new generation of pollen dispersal models. Such models can be important tools for forest research and landscape management, for forecasting and monitoring of pollen allergy symptoms, and for a risk evaluation and monitoring of genetically modified trees.

Emission and dispersal of pollen from wind-pollinated trees is important for several processes: first, spring flowering trees are among the major causes of allergic diseases affecting up to 15 % of the human population, and second, pollen dispersal is the major mediator of gene flow of trees in the temperate zone safeguarding reproduction, fructification and regeneration of forests. The present project aimed at characterising the meteorological conditions during pollen emission and transport in order to develop functional models of pollen emission and spread and to better understand the spatial scale of pollen dispersal. The main results of the project are the following: For the analysis of pollen concentrations in high temporal resolutions and under turbulent conditions, a new active pollen sampler, the `PMO Pollenmonitor` has been developed and tested. The PMO sampler allows more efficient pollen sampling under turbulent conditions and the temporal resolution of the pollen counts can be decreased down to few minutes. The meteorological conditions favouring pollen emission were analysed for the three tree species Sessile oak, Norway spruce, and Scots Pine on 15, 10, and 21 days of the flowering period 2009, respectively. As the most important predictor for pollen emission the standard deviation of vertical wind speed - a measure of vertical exchange - has been identified, which showed positive correlations to pollen concentrations for all three species. Also, the water vapour pressure deficit and the relative humidity are important drivers of pollen release. Due to the high temporal resolution of the pollen concentration measures we were able to study an episodic event of beech pollen transport into the study area in detail by using hourly backwards trajectories. The results indicate that long-range transport contributed significantly to the respective pollen measures. Local pollen transport is also calculated with the Lagrangian particle dispersion model LASAT to re-construct the spatial distribution of pollen around the tower. The necessary emissions of pollen are deduced from the pollen concentration measurements by an inverse dispersion technique. These results have severe implications for the development of improved models for pollen emission and dispersal that are strongly needed for better forecasts of allergic particles and an understanding of pollen-mediated gene flow in wind-pollinated trees. The new PMO pollen sampler is an important new aerobiological tool, which should find applications not only in allergenic monitoring, but especially in the analysis and monitoring of pollen gene-flow from genetically-modified plants, for which more accurate equipment is strongly needed.