Recruitment of white spruce (Picea glauca), one of the most widely distributed conifer species in the boreal forest, is reportedly declining in areas of western Canada where there is limited water availability. The adverse effects of moisture limitation on performance of white spruce seedlings may be compounded by above- and below-ground competition from neighbouring understory plants. To test this hypothesis, we conducted a competition-removal experiment on white spruce seedlings in a water-limited island forest landscape within the Canadian Prairies. In 2016, we measured the height and diameter at root collar of 572 natural seedlings. In 2017, we excavated a 20-30 cm deep, 75-cm radius trench around 55 of these seedlings to remove below-ground competition, and manually cleared all vegetation within a 75-cm radius of another 105 seedlings to remove above-ground competition. We took hemispherical photographs above each seedling to calculate canopy openness, and re-measured the height and diameter of all seedlings in 2017 and 2018 to calculate pre- and post-treatment growth.
Results/Conclusions
There was a pronounced difference in precipitation between years, with spring-summer precipitation 58% higher in 2016 and 62% lower in 2017 than average levels. Seedlings growing under shade had a greater decrease in growth in the dry year than seedlings growing under a more open canopy. Neither competition treatment affected the growth of seedlings under shade compared to control seedlings. Removal of below-ground competition had a positive effect on seedlings located under an open canopy, but removal of above-ground competition did not. These results indicate that local competition did not compound the stress caused by low-moisture, low-light conditions, but that below-ground competition did reduce seedling performance in high-light areas in a dry year. The interactions between light, moisture, and competition that we report have implications for white spruce regeneration in water-limited areas, especially as inter-annual variation in precipitation increases in a changing climate.