title: Forest tree pollen dispersal via the water cycle.
abstract: OBJECTIVE
  Pine pollen (Pinus spp.), along with other atmospheric particles, is dispersed by the water cycle, but this mode of dispersal requires cloud-pollen interactions that depend on taxon-specific biological properties. In the simplest form of this dispersal, pine pollen ascends vertically to altitudes of 2 to 6 km, where a fraction is captured by mixed-phase cloud formation. Captured pollen accretes into frozen droplets, which ultimately descend as rain, snow, or hail. Whether Pinus pollen can still germinate after its exposure to high-altitude freezing is pertinent to (1) how forests adapt to climate change and (2) potential gene flow between genetically modified plantation species and their conspecific relatives. •
  
  
  METHODS
  To address this question, pollen from four Old World and two New World Pinus species were subjected to immersion freezing, a common cloud formation mode, under laboratory conditions. •
  
  
  RESULTS
  Some pollen grains immersed at -20°C for 15, 60, or 120 min in either a dehydrated or a water-saturated state were still capable of germination. After exposure, dehydrated pine pollen had higher germination (43.3%) than water-saturated pollen (7.6%). •
  
  
  CONCLUSIONS
  Pine pollen exposed to freezing during cloud formation can still germinate, raising the question of whether rain-delivered live pollen might be linked to rain-facilitated pollination. Dispersal of live pine pollen via cloud formation and the water cycle itself deserves closer study.
authors.affiliation: Forest History Society, Durham, NC 27701, USA. claire.williams@duke.edu
authors.forename: Claire G
authors.initials: CG
authors.lastname: Williams
mesh_terms: D028223:Pinus / Q000502:physiology*; D011058:Pollen / Q000737:chemistry / Q000502:physiology*; D013997:Time Factors; D014197:Trees; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water
keywords: bioprecipitation; cloud condensation nuclei; gene flow; hydrological cycle; ice nuclei; long-distance dispersal; seed plants