The climatic debt is growing in the understorey of temperate forests: Stand characteristics matter

Abstract

Aim Climate warming reshuffles biological assemblages towards less cold‐adapted but more warm‐adapted species, a process coined thermophilization. However, the velocity at which this process is happening generally lags behind the velocity of climate change, generating a climatic debt the temporal dynamics of which remain misunderstood. Relying on high‐resolution time series of vegetation data from a long‐term monitoring network of permanent forest plots, we aim at quantifying the temporal dynamics – up to a yearly resolution – of the climatic debt in the understorey of temperate forests before identifying the key determinants that modulate it. Location France. Time period 1995–2017. Taxa studied Vascular plants. Methods We used the community temperature index (CTI) to produce a time series of understorey plant community thermophilization, which we subsequently compared to a time series of mean annual temperature changes over the same period and for the same sites. The direction and magnitude of the difference (i.e., the climatic debt) was finally analysed using linear mixed‐effect models to assess the relative contributions of abiotic and biotic determinants, including forest stand characteristics. Results We found a significant increase in CTI values over time (0.08–0.09 °C/decade), whereas the velocity of mean annual temperature changes was three times higher over the same period (0.22–0.28 °C/decade). Hence, the climatic debt increased over time and was greater in forest stands with higher basal area or older trees as well as under warmer macroclimate. By contrast, a greater frequency of anthropogenic disturbances decreased the climatic debt, while natural disturbances and herbivory had no impact. Conclusions Although often overlooked in understanding the climatic debt of forest biodiversity, changes in forest stand characteristics may modulate the climatic debt by locally modifying microclimatic conditions. Notably, the buffering effect of the upper canopy layer implies microclimate dynamics that may provide more time for understorey plant communities to locally adapt.

Unveil the unseen: Using LiDAR to capture time‐lag dynamics in the herbaceous layer of European temperate forests

“…increasing dominance of warm-adapted species; Gottfried et al, 2012); biotic homogenization (Staude et al, 2020); and even species extinction (Panetta et al, 2018). Yet, the velocity at which these biotic responses happen is generally lower than the velocity at which the macroclimate is warming (Bertrand et al, 2011;Dullinger et al, 2012;Rumpf et al, 2019;Vitasse et al, 2021), leading to disequilibrium or lagging dynamics (Alexander et al, 2018;Svenning & Sandel, 2013) sometimes also referred as the (macro)climatic debt in the scientific literature (Bertrand et al, 2016;Devictor et al, 2012;Richard et al, 2021). Likewise, delayed recovery of plant species richness and composition in response to the decreased inputs of atmospheric pollutants, after the peak in Europe during the 1970s for sulphur and during the 1980s for nitrogen, have been reported for both forest and grassland habitats (Riofrío-Dillon et al, 2012;Storkey et al, 2015).…”

“…Lagging dynamics in response to macroclimate warming and the reduction in atmospheric deposition, among other macroenvironmental change drivers, are especially pronounced within the herbaceous layer of temperate deciduous forests (Bertrand et al, 2011(Bertrand et al, , 2016De Frenne et al, 2013;Richard et al, 2021;Riofrío-Dillon et al, 2012;van Dobben & de Vries, 2017), which is the most biodiversity-rich vegetation layer in these ecosystems (Gilliam, 2007;Landuyt et al, 2019). In terms of biotic responses to macroclimate warming, thermophilization rates are ranging from 0.01 to 0.05°C per decade within understorey plant communities of temperate forests, which is several orders of magnitude lower (cf.…”

“…greater lags) than the rates observed in other groups such as trees (0.11°C per decade), bumblebees (0.14°C per decade), freshwater invertebrates (up to 0.22°C per decade) or marine fishes and invertebrates (up to 0.38°C per decade) [see table S5 in Richard et al, 2021, and references therein, for a more exhaustive description]. For comparative purposes, mean annual temperature increased at a rate of 0.23°C per decade between 1995 and 2015 in France (Richard et al, 2021). As for the recovery time following the reduction in atmospheric deposition in Europe, lagging effects seem also more pronounced in the herbaceous layer of forests than in grassland communities (Schmitz et al, 2019;Storkey et al, 2015;van Dobben & de Vries, 2017).…”

“…Such forest microclimate dynamics driven by changes in canopy cover could explain part of what seems to be a delayed biotic response to macroclimate warming, meaning that the so-called macroclimatic debt involves microclimatic processes Zellweger et al, 2020). Accordingly, Richard et al (2021) have recently demonstrated that lags in community thermophilization are accumulating over time in the herbaceous layer of denser and older forest stands in France, while anthropogenic and natural disturbances generating canopy gaps above the herbaceous layer tended to reduce these lags. Hence, stand characteristics are important determinants of time-lag dynamics observed in the herbaceous layer of temperate forests in response to macroclimate warming (Brice et al, 2019;Richard et al, 2021).…”

“…Accordingly, Richard et al (2021) have recently demonstrated that lags in community thermophilization are accumulating over time in the herbaceous layer of denser and older forest stands in France, while anthropogenic and natural disturbances generating canopy gaps above the herbaceous layer tended to reduce these lags. Hence, stand characteristics are important determinants of time-lag dynamics observed in the herbaceous layer of temperate forests in response to macroclimate warming (Brice et al, 2019;Richard et al, 2021). Besides, changes in stand characteristics over time interact with long-term environmental changes through complex historical trajectories of forest management practices and natural disturbances (e.g.…”