Shift of Conifer Boreal Forest to Lichen–Heath Parkland Caused by Successive Stand Disturbances

Abstract

In the southern boreal forest (Qué bec, Canada), tree harvesting is a major disturbance affecting the dominant black spruce (Picea mariana) stands already suffering from naturally recurrent insect and fire disturbances. Although recovery of the spruce forest after an insect infestation or a fire is possible under current site conditions, it is less likely when both types of disturbance occur during a short period of time. The addition of yet another disturbance, such as tree harvesting, can thus have catastrophic consequences. We analyzed the impact of three successive disturbances-tree harvesting, insect infestation, and fire-on the regeneration of boreal spruce-moss forests within a period of approximately 50 years. The spruce forests were harvested in the 1940s and the 1950s. Recovery from the logging consisted of advance regeneration (spruce layers less than 1 m high that were left intact during clear-cuts), which was burned in 1991. The vegetation cover (mostly heath and lichen species) and soil conditions (acidic, nutrientpoor podzolic soils developed from coarse materials) of the postfire sites that we studied were similar. Stand structure and tree regeneration were documented from large quadrats (0.25 ha) using age, size, and tree ring data from postlogged and postfire spruce. At an early stage of development, the growing advance regeneration was damaged by insect defoliators in the late 1970s and the mid1980s, and several trees died a few years before the 1991 fire. The successive disturbances considerably reduced the number of seed-bearers, leading to the collapse of postfire regeneration and a shift to parkland. Through a successional trajectory far from the expected trend for boreal forests influenced by single disturbance, the shift resulted in the formation of divergent plant communities. The development of divergent communities at the landscape scale is generally overlooked due to their small size. They indicate, however, the weak resilience of boreal forests faced with cascading perturbations, which are likely to increase in intensively logged areas.

Current distribution of older and deciduous forests as legacies from historical use patterns in a Swedish boreal landscape (1725–2007)

“…grazing, low-intensity fires, and dimension cuttings in the past (see Section 4.1). Similar results indicating permanent or longer species and structural shifts following compounded disturbances have previously been found in boreal Sweden, Finland, and in North America although involving different species compositions (Ericsson et al, 2000;Axelsson and Östlund, 2001;Hellberg et al, 2003;Payette and Delwaide, 2003;Schulte et al, 2007;Käyhkö and Skånes, 2008;Rhemtulla et al, 2009). The present Swedish national conservation objective aims to increase the amount of older deciduous forests in boreal Sweden, implying a need to identify and manage remnant deciduous elements to maintain deciduous continuity in landscape networks.…”

Understory Vegetation Dynamics of North American Boreal Forests

“…Allelopathy by the ericaceous shrub K. angustifolia has been identified as the cause of the failure of black spruce to regenerate following harvesting, resulting in the conversion of coniferous forests to a lichenheath dominated ecosystem (Mallik, 2003). Fire and insect outbreaks after logging have also been associated with the conversion of black spruce stands in Quebec to lichen-heath parkland as a result of a reduction in seed bearing trees (Lavoie and Sirois, 1998;Dussart and Payette, 2002;Payette and Delwaide, 2003).…”

Strategic analysis of forest vulnerability to risk related to fire: an example from the coniferous boreal forest of Quebec

“…In terms of productivity, the boreal forest is characterized by a mosaic of forest stands varying widely in tree growth and tree density (from closed forest to open lichen woodland), as well as by areas that currently have no trees (Beaudoin et al 2014;Lord and Robitaille 2013). This variation in tree density can be caused by a number of factors, including site properties and disturbance history (Sirois 2000;Payette and Delwaide 2003;Gauthier et al 2010). Therefore, current productivity can be different for sites with similar biophysical characteristics (climate, surficial deposits, and drainage) due to differences in factors such as the last disturbance intensity or insufficient regeneration.…”