Long-Term Steady-State Dry Boreal Forest in the Face of Disturbance

Christopher Carcaillet,Yves Bergeron,Vincent Robin,Mireille Desponts

doi:10.1007/s10021-019-00455-w

Abstract

We used bioproxies from paleosoils buried within two aeolian dunes to test hypotheses concerning the origin of dry sandy boreal forests in Canada. These forests are dominated today by Pinus banksiana Lamb. One hypothesis is that too frequent Holocene stand-replacing fires would have transformed the original vegetation through extirpation of susceptible species to fire in water stress habitat. Alternatively, the ecosystem would have not changed since the dunes stabilized enough to support forest establishment. The vegetation composition and richness were determined by identification of charcoal and macroremains and radiocarbon dating for the chronology. Both sites revealed a similar history covering 6400 years. Half of the charcoal layers were less than 2500 years old in both sites, indicating that they had been subjected to the same fire history. Data indicated a stable plant composition and richness, although the percentage of Pinus decreased slightly over 4000 years (decreasing rate 1% per century). The fungus Cenococcum geophilum was consistently present, with a stochastic abundance. The vegetation grew under natural fire conditions and soil dryness since 6000 years. The ecosystem was probably not stressed by late-Holocene fires or climate changes, as the multi-millennial steady state reveals a resistant and resilient ecosystem.

Highlights

In boreal regions, the long-term compositional origin of forests on dry soils has important conservation implications because of industrial tree exploitation and other global changes that can threaten these boreal ‘snow’ forests (Moen and others 2014; Gauthier and others 2015)
If we can determine the origin of such fire-prone ecosystems, that is, progressive transformation versus initial spontaneous emergence, we may understand the ecosystem linkage between community and disturbance and, in particular, whether a change in disturbance regime could result in a transformation of community or whether a steady state would be maintained for a long time whatever the disturbance regimes
We hypothesized that the initial postglacial ecosystem was not appropriate for stand-replacing fires, and that frequent fires have progressively altered the ecosystem by limiting or increasing its functionalities (Figure 1D, E), for instance, the plant recruitment due to effects on seed bed properties or on seed productivity of trees or by progressive decrease in biogeochemical mechanisms (McLauchlan and others 2014), promoting the most fire-adaptable species over species less resilient or resistant

Summary

Introduction

The long-term compositional origin of forests on dry soils has important conservation implications (for example, regarding biodiversity, carbon, soil erosion) because of industrial tree exploitation and other global changes that can threaten these boreal ‘snow’ forests (Moen and others 2014; Gauthier and others 2015). If we can determine the origin of such fire-prone ecosystems, that is, progressive transformation versus initial spontaneous emergence, we may understand the ecosystem linkage between community and disturbance and, in particular, whether a change in disturbance regime could result in a transformation of community or whether a steady state would be maintained for a long time whatever the disturbance regimes. These hypotheses question the resilience of the system. We hypothesized that the ecosystem was functionally resilient to frequent fires as soon as the habitat was colonized after deglaciation without intermediate states (Figure 1A–C), whatever climatic changes and soil maturation processes that drive the main natural chronic or rapid environmental changes in boreal habitats

Methods

Results

Discussion

Conclusion