Decline of an ecotone forest: 50 years of demography in the southern boreal forest

Joseph D Birch,George H Laroi,Justine Karst,E H Hogg,Suzanne W Simard,Rick Pelletier,James A Lutz

doi:10.1002/ecs2.2698

Joseph D Birch, George H Laroi + Show 5 more

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https://doi.org/10.1002/ecs2.2698

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Abstract

AbstractVariation in tree recruitment, mortality, and growth can alter forest community composition and structure. Because tree recruitment and mortality events are generally infrequent, long‐time scales are needed to confirm trends in forests. We performed a 50‐yr demographic census of a forest plot located on the southern edge of the Canadian boreal forest, a region currently experiencing forest die‐back in response to direct and indirect effects of recent severe droughts. Here, we show that over the last 30 yr biomass, basal area, growth, and recruitment have decreased along with a precipitous rise in mortality across the dominant tree species. The stand experienced periods of drought in combination with multiple outbreaks of forest tent caterpillar (Malacosoma disstria) and bark beetles. These insect disturbances interacted to increase mortality rates within the stand and decrease stand density. The interaction of endogenous and exogenous factors may shift forests in this region onto novel successional trajectories with the possibility of changes in regional vegetation type.

Highlights

Forests change through mortality, recruitment of new stems, and growth
Our study provides an unparalleled reference on trends in forest demography in the region to which subsequent studies can be compared
Peng et al (2011) reported that mortality rates are increasing across the Canadian boreal forest with Picea increasing by 3.29% yrÀ1 and Populus increasing by 2.84% yrÀ1

Summary

Introduction

These processes underlie stand demography and are critical for predicting the responses to future climates (Kobe 1996, Dietze and Moorcroft 2011, Chen and Luo 2015). Tree mortality rates have increased in forests across North America (van Mantgem et al 2009, Peng et al 2011) with some regions demonstrating concomitant reductions in growth and recruitment rates (Barber et al 2000, Chen and Luo 2015, Hogg et al 2017). Long-term studies are critical for predicting forest responses to future climates and disturbance regimes. Widespread increases in tree mortality have been reported, for Populus tremuloides Michx. in the aspen parkland ecozone that forms an ecotone between the

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