Herbaceous Understorey: An Overlooked Player in Forest Landscape Dynamics?

Abstract

Dense herbaceous understorey layers can impact tree regeneration and thereby affect forest succession. However, the implications of this interaction on large spatial and temporal scales are not well understood. To analyse the role of overstorey-understorey interactions for forest dynamics, we implemented an understorey layer (composed of the plant functional types grasses, forbs, ferns, herbs and shrubs) in the forest landscape model LandClim, focusing on competition for light as the main mode of interaction. The model was used to simulate post-disturbance dynamics over an elevational gradient of 560-2800 m a.s.l. in Central Europe. Simulation results showed strong impacts of the herbaceous understorey on tree regeneration within the first decades, but generally little effect on late-successional forests, i.e. not providing any evidence for 'arrested' succession. The results also demonstrated varying overstorey-understorey interactions across the landscape: strongest effects were found at low to mid elevations of the study landscapes, where tree establishment was substantially delayed. At high elevations, tree growth and establishment were more limited by low temperatures, and the effect of light competition from the understorey was negligible. Although the inclusion of large windthrow disturbances increased the biomass of herbaceous understorey across the landscape, this had only a small impact on the overstorey due to the presence of advance regeneration of trees. Overall, our results demonstrate that the herbaceous understorey can have a significant impact for forest landscape dynamics through light competition, and that non-woody plants should not be neglected in forest modelling.

Herbaceous competition and browsing may induce arrested succession in central European forests

“…Herbaceous understorey vegetation has recently been included in LandClim and is represented as a single PFT here (Thrippleton et al., ). This PFT is represented in the model as a single cohort in each raster cell, with PFT‐specific variables describing maximum biomass, growth rate and bioclimatic limitations.…”

“…Furthermore, biomass of the herbaceous understorey determines the amount of water removed from the topsoil, thus increasing the drought experienced during the regeneration stage (Thrippleton, Bugmann, Folini, & Snell, ). Overstorey trees affect herbaceous biomass by decreasing light availability in the understorey, thus leading to minimal herbaceous biomass under a dense canopy (Thrippleton et al., ). Additional information on the interaction between herbaceous understorey and trees is given in Appendix and Thrippleton et al.…”

“…The performance of LandClim has previously been evaluated at various spatio‐temporal scales by confronting model outputs with palaeoecological, chronosequence and forest inventory data (e.g. Elkin, Giuggiola, Rigling, & Bugmann, ; Elkin, Reineking, Bigler, & Bugmann, ; Henne, Elkin, Reineking, & Bugmann, ; Thrippleton et al., ), where it consistently reproduced empirical patterns of forest dynamics and vegetation composition. Furthermore, the processes of large‐scale disturbances, browsing and understorey competition have been thoroughly tested in previous studies (Elkin et al., ; Henne et al., ; Schumacher et al., ; Temperli et al., ; Thrippleton et al., , ), making LandClim highly suitable to address the processes and spatial scales that are relevant for this study.…”

“…Since temperature and precipitation strongly covary in the study landscapes, elevation was used as a proxy for climatic conditions in the following analyses (see Table ). The length of the climate records for each landscape varied from 30 to 70 years (see Schumacher et al., for Dischma; Thrippleton et al., for Feldberg; Snell et al., for Jura; Thrippleton et al., , for Valais), and were randomly resampled with replacement for each simulation to generate 1,000‐year time series.…”

“…fire, wind‐throw and bark beetle infestations, Henne et al., ; Temperli, Bugmann, & Elkin, ), but none of these accounted for interactions between trees and herbaceous vegetation. Thrippleton, Bugmann, Kramer‐Priewasser, and Snell () demonstrated the importance of herbaceous competition on long‐term forest dynamics and species composition using a simulation approach, but did not account for browsing and anthropogenic overstorey disturbances such as timber harvest, or their combined effects.…”