Paludification

Abstract: Introduction Major patterns of plant communities and species distribution are induced by various disturbance regimes operating at different spatial and temporal scales (Loucks 1970; White 1979; Bormann & Likens 1979 b ; Delcourt, Delcourt & Webb 1983). The development of temperate forests is controlled by canopy disturbance associated with single and multiple treefalls creating small gaps (Henry & Swan 1974; Oliver & Stephens 1977) and, occasionally, larger gaps along windstorm tracks (Canham & Loucks 1984). In contrast, fire disturbance in the boreal forest creates all sizes of canopy gap, and constitutes one of the most important community processes in vegetation development along complex environmental gradients (Heinselman 1981 b ). At the landscape level, ecological disturbances result in the spatiotemporal development of the vegetation mosaic composed of an assemblage of stand patches of different age, areal extent, and floristic composition (Pickett & White 1985). The boreal forest is one of the world's two largest forest belts; it covers more than 12 6 km 2 (Baumgartner 1979). The North American segment of the circumboreal forest, which constitutes a well-delineated biome both geographically and ecologically (Fig. 5.1), will be analyzed here with respect to fire disturbance. The boreal forest (excluding the Cordilleran region) spans more than 10° of latitude in eastern and western Canada; it is somewhat contracted south of Hudson Bay and James Bay, and in Alaska. At the continent scale, the boreal forest is a floristically poor biome (Takhtajan 1986) with only nine tree species dominating regionally or throughout the range, in coexistence with a subdued under-canopy flora in dense stands and an ubiquitous cryptogamic flora in open stands.

Abstract: Long-term forest productivity decline in boreal forests has been extensively studied in the last decades, yet its causes are still unclear. Soil conditions associated with soil organic matter accumulation are thought to be responsible for site productivity decline. The objectives of this study were to determine if paludification of boreal soils resulted in reduced forest productivity, and to identify changes in the physical and chemical properties of soils associated with reduction in productivity. We used a chronosequence of 23 black spruce stands ranging in postfire age from 50 to 2350 years and calculated three different stand productivity indices, including site index. We assessed changes in forest productivity with time using two complementary approaches: (1) by comparing productivity among the chronosequence stands and (2) by comparing the productivity of successive cohorts of trees within the same stands to determine the influence of time independently of other site factors. Charcoal stratigraphy indicates that the forest stands differ in their fire history and originated either from high- or low-severity soil burns. Both chronosequence and cohort approaches demonstrate declines in black spruce productivity of 50-80% with increased paludification, particularly during the first centuries after fire. Paludification alters bryophyte abundance and succession, increases soil moisture, reduces soil temperature and nutrient availability, and alters the vertical distribution of roots. Low-severity soil burns significantly accelerate rates of paludification and productivity decline compared with high-severity fires and ultimately reduce nutrient content in black spruce needles. The two combined approaches indicate that paludification can be driven by forest succession only, independently of site factors such as position on slope. This successional paludification contrasts with edaphic paludification, where topography and drainage primarily control the extent and rate of paludification. At the landscape scale, the fire regime (frequency and severity) controls paludification and forest productivity through its effect on soil organic layers. Implications for global carbon budgets and sustainable forestry are discussed.