Aim The predictions from biogeographical models of poleward expansion of biomes under a warmer 2 × CO2 scenario might not be warranted, given the non-climatic influences on vegetation dynamics. Milder climatic conditions have occurred in northern Québec, Canada, in the 20th century. The purpose of this study was to document the early signs of a northward expansion of the boreal forest into the subarctic forest-tundra, a vast heterogeneous ecotone. Colonization of upland tundra sites by black spruce (Picea mariana (Mill.) BSP.) forming local subarctic tree lines was quantified at the biome scale. Because it was previously shown that the regenerative potential of spruce is reduced with increasing latitude, we predicted that tree line advances and recent establishment of seedlings above tree lines will also decrease northwards.
Location Black spruce regeneration patterns were surveyed across a > 300-km latitudinal transect spanning the forest-tundra of northern Québec, Canada (55°29′–58°27′ N).
Methods Elevational transects were positioned at forest–tundra interfaces in two regions from the southern forest-tundra and two regions from the northern forest-tundra, including the arctic tree line. The surroundings of stunted black spruce, forming the species limit in the shrub tundra, were also examined. Position, total height and origin (seed or layer) of all black spruce stems established in the elevational transects were determined. Dendrochronological and topographical data allowed recent subarctic tree line advances to be estimated. Age structures of spruce recently established from seed (< 2.5 m high) were constructed and compared between forest-tundra regions. Five to 20-year heat sum (growing degree-days, > 5 °C) and precipitation fluctuations were computed from regional climatic data, and compared with seedling recruitment patterns.
Results During the 20th century, all tree lines from the southern forest-tundra rose slightly through establishment of seed-origin spruce, while some tree lines in the northern forest-tundra rose through height growth of stunted spruce already established on the tundra hilltops. However, the rate of rise in tree lines did not slow down with latitude. The density of < 2.5-m spruce established by seed declined exponentially with latitude. While the majority of < 2.5-m spruce has established since the late 1970s on the southernmost tundra hilltops, the regeneration pool was mainly composed of old, suppressed individuals in the northern forest-tundra. Spruce age generally decreased with increasing elevation in the southern forest-tundra stands, therefore indicating current colonization of tundra hilltops. Although spruce reproductive success has improved over the twentieth century in the southern forest-tundra, there was hardly any evidence that recruitment of seed-origin spruce was controlled by 5- to 20-year regional climatic fluctuations, except for winter precipitation.
Main conclusions Besides the milder 20th century climate, local topographic factors appear to have influenced the rise in tree lines and recent establishment by seed. The effect of black spruce's semi-serotinous cones in trapping seeds and the difficulty of establishment on exposed, drought-prone tundra vegetation are some factors likely to explain the scarcity of significant correlations between tree establishment and climatic variables in the short term. The age data suggest impending reforestation of the southernmost tundra sites, although the development of spruce seedlings into forest might be slowed down by the harsh wind-exposure conditions.