Overstorey–Understorey Interactions Intensify After Drought-Induced Forest Die-Off: Long-Term Effects for Forest Structure and Composition
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Severe drought events increasingly affect forests worldwide, but little is known about their long-term effects at the ecosystem level. Competition between trees and herbs (‘overstorey–understorey competition’) for soil water can reduce tree growth and regeneration success and may thereby alter forest structure and composition. However, these effects are typically ignored in modelling studies. To test the long-term impact of water competition by the herbaceous understorey on forest dynamics, we incorporated this process in the dynamic forest landscape model LandClim. Simulations were performed both with and without understorey under current and future climate scenarios (RCP4.5 and RCP8.5) in a drought-prone inner-Alpine valley in Switzerland. Under current climate, herbaceous understorey reduced tree regeneration biomass by up to 51%, particularly in drought-prone landscape positions (i.e., south-facing, low-elevation slopes), where it also caused a shift in forest composition towards drought-tolerant tree species (for example, Quercus pubescens). For adult trees, the understorey had a minor effect on growth. Under future climate change scenarios, increasing drought frequency and intensity resulted in large-scale mortality of canopy trees, which intensified the competitive interaction between the understorey and tree regeneration. At the driest landscape positions, a complete exclusion of tree regeneration and a shift towards an open, savannah-like vegetation occurred. Overall, our results demonstrate that water competition by the herbaceous understorey can cause long-lasting legacy effects on forest structure and composition across drought-prone landscapes, by affecting the vulnerable recruitment phase. Ignoring herbaceous vegetation may thus lead to a strong underestimation of future drought impacts on forests.
Keywordsdynamic vegetation model understorey layer soil water competition vegetation shift Valais Central Alps
We are grateful for the support by Dominic Michel in all IT-related questions. Denis Loustau kindly provided biological data from the study site Le Bray. We furthermore thank Maxime Cailleret, Sebastian Wolf, Arnaud Giuggiola, Nica Huber and Laura Schuler for their helpful input. Funding for Rebecca S. Snell was provided by the EU FP7 Project ‘IMPRESSIONS’, Grant No. 603416.
- Allen CD, Macalady AK, Chenchouni H, Bachelet D, McDowell N, Vennetier M, Kitzberger T, Rigling A, Breshears DD, Hogg EH, Gonzalez P, Fensham R, Zhang Z, Castro J, Demidova N, Lim JH, Allard G, Running SW, Semerci A, Cobb N. 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. For Ecol Manage 259:660–84.CrossRefGoogle Scholar
- Clark JS, Iverson L, Woodall CW, Allen CD, Bell DM, Bragg DC, D’Amato AW, Davis FW, Hersh MH, Ibanez I, Jackson ST, Matthews S, Pederson N, Peters M, Schwartz MW, Waring KM, Zimmermann NE. 2016. The impacts of increasing drought on forest dynamics, structure, and biodiversity in the United States. Glob Change Biol 22:2329–52.CrossRefGoogle Scholar
- Giuggiola A. 2016. Impact of forest management on the drought resistance of dry pine forests. Ph.D. Thesis No. 22802. ETH-Zurich. Zurich, p 214.Google Scholar
- McCarthy BC. 2003. The herbaceous layer of eastern old-growth deciduous forests. In: Gilliam FS, Roberts MR, Eds. The herbaceous layer in forests of eastern North America. New York: Oxford University Press. p 163–76.Google Scholar
- Morris LA, Moss SA, Garbett WS. 1993. Competitive interference between selected herbaceous and woody plants and Pinus taeda L. during two growing seasons following planting. For Sci 39:166–87.Google Scholar
- R Development Core Team. 2016. R: a language and environment for statistical computing. Vienna: R Foundation for Statistical Computing.Google Scholar
- Schulze ED, Beck E, Müller-Hohenstein K. 2005. Plant ecology. Berlin: Springer. p 702p.Google Scholar
- Schumacher S. 2004. The role of large-scale disturbances and climate for the dynamics of forested landscapes in the European Alps. Ph.D. Thesis No. 15573. ETH Zurich, p 141.Google Scholar
- Thrippleton T, Dolos K, Perry GLW, Groeneveld J, Reineking B. 2014. Simulating long-term vegetation dynamics using a forest landscape model: the post-Taupo succession on Mt Hauhungatahi, North Island, New Zealand. N Z J Ecol 38:26–43.Google Scholar