Overstorey–Understorey Interactions
Intensify After Drought-Induced
Forest Die-Off: Long-Term Effects for
Forest Structure and Composition
Timothy Thrippleton,
1
* Harald Bugmann,
1
Marc Folini,
1
and
Rebecca S. Snell
1,2
1
Department of Environmental Systems Science, Forest Ecology, Swiss Federal Institute of Technology, ETH Zurich, Univer-
sita
¨
tstrasse 16, CHN G 78, 8092 Zurich, Switzerland;
2
Department of Environmental and Plant Biology, Ohio University, Athens,
Ohio 45701, USA
A
BSTRACT
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 there-
by alter forest structure and composition. How-
ever, 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, Quer-
cus pubescens). For adult trees, the understorey
had a minor effect on growth. Under future cli-
mate change scenarios, increasing drought fre-
quency and intensity resulted in large-scale
mortality of canopy trees, which intensified the
competitive interaction between the under-
storey and tree regeneration. At the driest land-
scape 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 herba-
ceous vegetation may thus lead to a strong
underestimation of future drought impacts on
forests.
Key words: dynamic vegetation model; under-
storey layer; soil water competition; vegetation
shift; Valais; Central Alps.
Received 6 March 2017; accepted 14 August 2017;
published online 7 September 2017
Electronic supplementary material: The online version of this article
(doi:10.1007/s10021-017-0181-5) contains supplementary material,
which is available to authorized users.
Author’s Contribution T.T., R.S.S. and H.B. designed the study; T.T.,
M.F. and R.S.S. developed the submodel; T.T. conducted the simulation
experiments; T.T., H.B. and R.S.S. wrote the paper.
The URL to the model LandClim, which our publication is based on, is:
https://uwis-server102.ethz.ch/openaccess/software/view/2.
*Corresponding author; e-mail: timothy.thrippleton@usys.ethz.ch
Ecosystems (2018) 21: 723–739
DOI: 10.1007/s10021-017-0181-5
Ó 2017 Springer Science+Business Media, LLC
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