Of Thermophilization Rates & Forest Microclimates

The thermophilization of vegetation is the process by which plant communities are increasingly dominated by species from warmer biogeographic affinities (i.e. from lower latitudes or elevations) and thus better adapted to a locally warming climate (Gottfried at al. 2012). The rate at which thermophilization is happening within plant communities can be assessed thanks to permanent or semi-permanent vegetation plots monitored at least two times and sometimes more often or even on a regular basis (every year, every five year, etc.) to generate more detailed time series of climate change as felt by plant communities.

Recently, we published a study assessing the thermophilization rate of understorey plant communities across European temperate forests and testing whether or not forest microclimate dynamics, through changes in canopy cover over time, could explain the observed variation (Zellweger et al. 2020a). We found that changes in forest microclimate over the last decades matter for driving community responses to climate change. Closed canopies, following stand densification, buffer against the effects of macroclimatic change through their cooling effect, slowing down the thermophilization rate of understorey plant communities. In contrast, open canopies, following disturbances, tend to accelerate it, which supports former findings from a Californian study (Stevens et al. 2015). Few months after our work was published in Science, making the cover of the journal issue (see picture below), two technical comments (TCs) discussed our findings.

Science Cover, Volume 368, Issue 6492. Photo Credit: Julian Stratenschulte.

The first TC from Bertrand et al. (2020) reanalysed our dataset to precise the relative impact of macroclimate warming on the thermophilization rate of understorey plant communities. This is an interesting complementary analysis but the main conclusions and the main message remain more or less the same (Zellweger et al. 2020b). The debate in this first discussion is more or less a semantic one as we, on both sides, are studying and agreeing on the same thing, namely the impact of climate warming on the thermophilization rate of understorey plant communities. Although we did not assess the relative effect of macroclimate warming on the thermophilization rate of understorey plant communities, as Bertrand et al. (2020) did, this effect is indirectly incorporated in our analysis as part of what we call microclimate warming. Indeed microclimate warming mostly happens because of macroclimate warming, albeit it can be amplified by canopy openings following disturbances or it can be reduced by canopy closings following stand densification. The reason why we did not deconstruct microclimate warming into (1) macroclimate warming plus (2) the temporal fluctuations in the offset component of canopy cover changes (1+2) is that we are taking a plant-eye view (Lenoir 2020; Vandvik et al. 2020) on the impact of climate change (bottom-up) while Bertrand et al. (2020) take a top-down approach with the main emphasis on macroclimate warming. But in the end, the results and interpretations remain exactly the same. We are basically concluding on the same thing. It is just a matter of perspective and view point on how to analyse the impact of climate change on understorey plant communities.

As for the second TC from Schall & Heinrichs (2020), it is more a critical than a complementary comment. Schall & Heinrichs (2020) first criticized the fact that our metric of community temperature index or “CTI” does not show a 1:1 relationship with the actual temperature as measured in degree Celsius (T°C) and thus cannot accurately reflect true thermophilization. Of course CTI does not equal T°C as we measure it with sensors and we never claimed otherwise in our work (Zellweger et al. 2020c). Using CTI to predict accurate T°C values would not be appropriate but analysing the change in CTI values is not an issue because we focus on the relative difference and not on the actual T°C value. In fact, as physicians, we should express this difference over time in degree Kelvin and not in degree Celcius, to avoid such misunderstanding. Whatever, the point is that looking at CTI changes over time is not a problem, even if CTI itself does not show a 1:1 relationship with T°C. Still the relationship, as shown by Schall & Heinrichs (2020), is very linear. So, we do not think our approach is problematic to assess the rate of thermophilization over time. Finally, Schall & Heinrichs (2020) disagree with our suggestions that canopy openings, through management decisions, can create adverse thermal conditions for many forest species. Yet, large canopy openings as currently implemented by dynamic sylvicultural practices in Europe imply huge amounts of wood extraction that will dramatically change microclimatic conditions near the ground. Although plant species richness may momentaneously increase and enhance a rapid thermophilization of the community following the disturbance (see Stevens et al. 2015), it will also induce the loss of forest specialists in the long run. If light-demanding and generalist plants can easily germinate from the soil seed bank just after a canopy opening and thus can rapidly thermophilize the community, it takes much more time for forest specialists to come back after canopy closure. That is why we argue for more parsimonious thinning and logging practices to avoid large and brutal openings in forest stands as these have detrimental effects on the forest microclimate and thus on the successs of tree regeneration. For some dryad tree species, successful regeneration needs a sufficient amount of mother trees in the neighbourhood to maintain a healthy forest microclimate and ensure a successful regeneration: microclimate is thus integrative of the regeneration niche.

In the end, this debate clearly highlights the increased attention on forest microclimate ecology and the need for more microclimatic data from forest systems to better understand forest microclimate dynamics and how it interferes with the biotic response of forest-dwelling species to macroclimate warming.