Tree-line ecology and change
A shift in forest limits northwards and up-slope is a projected consequence of climate warming that could have important effects on the highlands ecosystems. There are many unanswered questions about what will happen to wildlife species, some of them already at-risk, that depend on this ecosystem should it be displaced by forest in the future.
A first logical step towards investigating potential changes to alpine treeline is to study factors that affect seedling establishment of treeline trees. This is both the most vulnerable stage of a tree’s life, and, of-course, the necessary start-point for tree-line migration. For this reason, experimental manipulations are being done with black spruce (Picea mariana) seeds and seedlings.The aim of these experiments is to better understand the relative importance of temperature, disturbance, and herbivory on seedling establishment. Plots have been set up at open canopy forest, tree-island, and alpine-heath vegetation sites.
Premilinary observations and findings: Long-lived Trees above the Forest Limit. In Gros Morne National Park, the Mealy Mountains and Red Wine Mountains, we have found isolated erect 200+ yr old White Spruce growing above the present altitudinal limit of the closed forest in a zone where Black Spruce and Balsam Fir grow only in krummholz (near-prostrate, shrublike) form. These White Spruce are either relicts of a past period of more favourable growing conditions or reflect locally anomalous conditions. In ponds above present tree-line in the Red Wine Mountains we have found 4,000 year-old sections of large fir and spruce trees. Such accumulating evidence confirms that forest boundaries are very liable and there is potential for the forest to advance to higher elevations under suitable climatic regimes.
Constraints on present tree-line. Our experiments indicate that Black Spruce seeds can germinate and seedlings can survive along an elevational gradient, even in alpine areas where there are presently no adult trees. Transplant experiments have demonstrated that conifer and deciduous seedlings can survive above present treeline. Soil conditions (chemistry, moisture, development) do not appear to be an obstacle to the expansion of forest species into higher elevations. Upslope movement of treeline appears to occur by infilling between tree islands, which act as foci for expansion. Herbivory by rodents and slugs may slow any expansion, as they cause significant seedling mortality.
In the central Mealy Mountains , the present altitudinal limit of spruce and fir is estimated at about 600 m a.s.l.
