Impacts of forest fragmentation on the reproductive success of white spruce (Picea glauca)

Abstract

The fragmentation of forests into small, isolated remnants may reduce pollen quantity and quality in natural plant populations. The reproductive success of white spruce ( Picea glauca (Moench) Voss) was assessed in a landscape fragmented by agriculture in northern Ontario, Canada. We sampled a total of 23 stands and 104 white spruce trees from three different stand size classes. Each sampled stand was separated by 250–3000 m from the nearest neighbouring stand. Reproductive success, measured as the number of filled seeds per cone, increased with stand size. The total number of seeds per cone, a measure that includes both filled and aborted seeds, also increased with stand size, suggesting that pollen receipt limits the number of seeds in a cone. The proportion of empty seeds (postzygotic abortions) was highest in the two smallest stand size classes, suggesting that inbreeding levels were also highest in these stands. We detected no difference in germination success, seedling growth, and growth of trees up to 10 years from seeds produced by trees from different stand size classes. These results suggest that inbred individuals are largely eliminated during the seed development stage. We estimated that a threshold population size of 180 trees is needed to reduce the negative effects of pollen limitation and inbreeding and maintain seed yields observed in large contiguous stands.