Some aspects of the reproductive biology of Arceuthobium americanum in Manitoba
Gilbert, Jeannie Anne
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Dwarf misletoe, Arceuthobium americanum (Viscaceae), a damaging parasite of jack pine, Pinus banksiana, was studied at Grand Beach Provincial Park, north of Winnipeg, Manitoba. In previous phenological studies of Arceuthobium spp. an important role for microclimatic variables has often been implied. However, past records of these variables have been too sporadic and unreliable to draw firm conclusions. Consequently, it was appropriate to reexamine some aspects of the reporductive biology of Arceuthobium americanum in conjunction with the use of dataloggers to monitor temperature, relative humidity, windspeed and direction, and rainfall. Atmospheric pollen loads were recorded on a volumetric spore trap and flower movements by time-lapse photography. The male flowers opened 6 April 1987 after a mild winter but 18 April 1985, after a winter in which February temperatures were lower than average. Nectar with a mean sugar concentration of approximately 30%, was always apparent on male flowers under conditions of high humidity. On the other hand, large, highly concentrated nectar droplets were seen on the female flowers only in the hot, dry spring of 1987. Although readings of 60-65% sugar were obtained, few measurements could be taken on account of the high concentration. Anther opening in response to rising temperatures and falling relative humidity, and closing under the reverse conditions, was observed for the first time in Arceuthobium. The mechanism effectively retains pollen during unfavourable conditions, and further, helps to explain the fluctuations in atmospheric pollen loads observed in earlier studies. The high levels of activity of large Diptera, evident in the hot spring weather of 1987 and the measurable air-borne pollen concentrations provide evidence that pollination is effected by both insects and wind. Germination of the pollen was examined for the first time in Arceuthobium. The percentage germination fluctuated with changes in the temperature and generally increased as the season progressed. The over-winter survival of maturing fruits was 88% and 94% in 1985/6 and 1986/7, respectively. Losses of approximately 30% of the fruits occurred over the summer, and a further decline in numbers was attributed to the high incidence of the fungal hyperparasites, Wallrothiella arceuthobii and Colletotrichum gloeosporioides in addition to the hot, dry spring of 1987. Seed dispersal started 10 days earlier in 1987 than in 1986 as a result of the warm, early start to the growing season. Peak daily dispersal occurred between 0900 and 1100 CST and was associated with rising temperatures. Seeds were dispersed for distances up to 18 m, but there was no linear relationship between the logarithm of the seeds dispersed and distance, as reported for other Arceuthobium spp. While explosive seed discharge accounts for local spread and intensification, small mammals may carry seed into uninfected parts of the forest and birds are potential vectors of long- and medium-range seed dispersal, depending on their migration habits. No seeds were found on any of the 193 mammals examined, but over 5% of the birds mist-netted in each year carried seed. In addition to gray jays and juncos, a brown creeper, a red-breasted nuthatch and a Swainson's thrush were recorded for the first time carrying seed. During seed dispersal 41 of 73 naturally deposited seeds were washed from needles. Three of these later slid on to host twig tissue, whereas 7 of 27 that had originally landed on twigs were lost. In the same interval 3 of 25 seeds artifically placed on twigs of various ages were lost. After fall rains, over-winter retention of the surviving seeds was high, but losses occurred during the summer. The frequency of production of radicles was 88% in 1986, a spring characterised by persistent rain and high humidities, but only approximately 50% in the dry conditions in 1987. When seeds were examined in September few radicles had even penetrated the bark of the jack pine, an essential preliminary step in the infection process.