Importance of pollen rain in boreal Manitoba, Canada

Abstract

Importance of pollen rain as nutrient source was investigated in the boreal forest ecosystem of west-central Manitoba, Canada. Tree and tall shrub pollen rain was sampled along the 5.5 km transect in mixed boreal forest and two regenerating jack pine stands in May and June of 1991-1993 using gravity collectors. Annual pollen averaged ca. 6850 grains/cm$\sp2$ in 1992, with jack pine contributing 67.3% and spruce 24.5% of the total. Total annual pollen deposition ranged between 16-25 kg/ha at the three sites, corresponding to nutrient inputs of 0.34-0.49 kg N/ha, 0.04-0.07 kg P/ha, and 0.11-0.20 kg K/ha. These values are small when compared to estimated rates of nutrient uptake from boreal forest. However, the combination of highly episodic, early summer deposition and very rapid nutrient turnover suggest that pollen may play an important role in supplying nutrients and promoting decomposition in the forest litter layer. The spatial and temporal patterns of pollen infestation were investigated along a gradient of boreal microhabitats. This study shows that chytrids have roles in initial colonization and degradation of resistant pollen substrate in boreal forests and making it less refractory for other organisms. Significant differences in total pollen infestation in diverse boreal forest sites were found. The results indicated that infestation levels vary with types of decaying litter types. It is concluded that competition for nitrogen and other nutrients may occur in substrates of high C/N ratio such as in the boreal forest and chytrids adapt more favourably in dry, acidic sites with needle litters which are more hostile conditions to other decomposers. Possible linkages between pollen, chytrids, ectomycorrhizal fungi and forest tree nutrient acquisition are discussed.