Climate change and the lasting legacy of old vegetation plots

Figure 1. The eastern flank of Mont St-Joseph, in Parc national du Mont Mégantic, Québec. This photo was taken in spring, while deciduous trees (mostly sugar maple) were just leafing out (light green at low elevation). The dark green at high elevation is boreal forest, dominated by spruce and fir.

By Mark Vellend

People have not yet figured out how to travel in time, so in order to study historical ecological changes we need to take advantage of whatever opportunities present themselves. Several sources of data can provide windows into the past state of plant communities, the best-known of which is fossil pollen in lake sediments, which has permitted the reconstruction of plant distributions and communities going back thousands of years. But the spatial and temporal resolution of such paleoecological data is relatively coarse, leaving a gap in our knowledge of changes over periods of time ranging from decades to a century or so – sometimes referred to as ‘the invisible present’ (Magnuson 1990).

Over the past 10 years or so, many ecologists have sought to rectify this situation by revisiting vegetation plots surveyed decades ago. In our study, we did exactly this in a stunning protected area called Parc national du Mont Mégantic, in southern Québec, Canada (Fig. 1 and 2). The approach, however, comes with some important challenges. Are the original raw data available? Can we re-locate the old plots? Can we ensure comparable timing and effort involved in surveys? In Québec, there is a strong tradition of vegetation quantification and classification, and sadly, several ‘giants’ of the field have passed away in recent years, including Pierre Dansereau, André Bouchard, and Miroslav Grandtner. However, to the great benefit of science, these and other botanists often left behind raw data (Fig. 3), high-resolution maps of plot locations (Fig. 4), and detailed descriptions of methods. They also trained many ecologists active today, one of whom – Colette Ansseau – was of great help to us in understanding the details of the vegetation survey conducted 40+ years ago by Mirsolav Grandtner and one of his students, Gilles Marcotte (Marcotte and Grandtner 1974). We owe a great debt to these pioneering plant ecologists.

Figure 2. A small sample of understory plants observed in our study. From left to right: Viola selkirkii, Cornus canadensis, Erythronium americanum, Trillium erectum.




Figure 3. A portion of the raw data reported in the original survey study (Marcotte and Grandtner 1974).




Figure 4. A small section of the map showing the locations of plots in the original study (Marcotte and Grandtner 1974).

For her MSc research in 2012, Josée Savage and an assistant Valérie Massé (with a little help from me) re-located and re-surveyed 48 plots, covering the elevational gradient from temperature deciduous forest at low elevation to boreal forest at high elevation. Southern Québec has seen marked warming since the 1970s, so we were interested in testing for predicted ecological consequences of this warming trend. However, despite a clear hypothesis and predictions, I was really uncertain as to whether we’d pick up a signal of the expected changes. But there they were: species distributions are clearly moving upslope, and with faster compositional changes at low than high elevation, the gradient appears to be ‘shrinking’ in ecological terms. Low and high elevation communities are not as distinct as they once were.

As seen in some other studies, species distributions are moving upslope far slower (~ 35 m over 40 years) than are the actual temperature isotherms (> 200 m over 40 years), begging the question as to why. Perhaps these long-lived perennial species, most of which have seeds that disperse over short distances, will simply require more time to march up the hill. Or maybe factors other than climate are systematically preventing this from happening. We have some evidence for one species – the dominant tree, sugar maple – that both soil conditions and seed predators can slow down uphill range expansion (Brown and Vellend 2014), but we have no idea whether this applies to other species. Hopefully many decades from now our data will become ‘legacy’ data for use by another student who will find out.

Brown, C. D. and Vellend, M. 2014. Non-climatic constraints on upper elevational plant range expansion under climate change. – Proc. R. Soc. Lond. B 281: 20141779.

Magnuson, J.J. 1990. Long-term ecological research and the invisible present. - BioScience 40: 495-501.

Marcotte, G. and Grandtner, M. M. 1974. Étude écologique de la végétation forestière du Mont Mégantic. - Gouvernement du Québec.

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