Nutritional Effects of Seed Fall during Mast Years in Boreal Forest

Several hypotheses have been advanced to explain why masting (i.e. episodic. synchronized seed production) occurs in coniferous trees, as well as aspects of its evolution and importance for reproductive success. However, the nutritional effects of the large number of seeds that simultaneously fall to the ground, die and decompose in the soil during mast years have not previously been considered. Norway spruce (Picea abies) has a typical masting behavior and may add 5-10 kg N ha -1 yr -1 through seed fall in mast years in the boreal zone of Northern Sweden. We performed a field experiment in a strongly nitrogen-limited forest in N Sweden where pre-killed Norway spruce seeds were added to plots at an amount comparable to 7 kg N ha -1 yr - '. Seeds were added to an understory vegetation dominated by ericaceous shrubs and two different bottom layer components, Pleurozium schreberi and Cladina spp. Seedlings of Scots pine (Pinus sylvestris) were then planted in the same plots to evaluate growth response to and availability of nutrients from the dead Norway spruce seeds that were added to the two plant communities. Seedlings of Scots pine grew larger and acquired more nitrogen when dead seeds were added in plots dominated by Cladina spp. However, in plots dominated by P. schreberi, uptake of nutrients from dead seeds by Scots pine was blocked by P. schreberi except in treatments where the P. schreberi moss carpet was experimentally disturbed. Analysis of nutrients in field- and bottom-layer vegetation showed that P. schreberi acquired a large proportion (80%) of N released from dead seeds while the ericaceous dwarf shrub Vaccinium vitis-idaea and the Cladina spp. macrolichens gained much less nitrogen. The rates of nutrient release and decomposition of dead Norway spruce seeds were compared with Norway spruce needle litter in a litter bag experiment. Dead Norway spruce seeds decomposed at much higher rates than needle litter, and 65-75% of the seed nitrogen content was lost during the first growing season. We hypothesize that, if germination conditions (type of bottom-layer vegetation) allow tree seedlings to emerge, their survival, growth and nutrition may be enhanced by the nutrient flush from dead seeds and germlings in seed mast years and that this may aid in establishment of a new seedling cohort. Nutrient pulses from dead seeds during mast years may help to explain the strongly pulsed natural tree regeneration patterns that have been found in mixed late successional coniferous forests.