Severe drought events increasingly affect forests worldwide, but little is known about their longterm 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.
Diurnal patterns of carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) net fluxes were studied in a permanently submerged, Carex rostrata dominated Swiss alpine fen. Fluxes were measured in August and September using static chambers. Methane was emitted at all six time points analysed during each diurnal cycle with little variation between day, night or twilight (8.3 ± 0.8 mg m −2 h −1 in August). The fen was a nighttime CO 2 source (138 ± 46 mg m −2 h −1 ) and generally a daytime CO 2 sink. Uptake of CO 2 varied with light intensity (98 ± 57 to 391 ± 43 mg m −2 h −1 in August), but light saturation of photosynthesis was apparent at low irradiation levels. Emission or uptake of N 2 O was not discerned. In September, CH 4 and CO 2 fluxes were generally lower compared with August, compliant with less green C. rostrata biomass. Diurnal changes in subsurface properties were minor with little influence on the diurnal patterns. Temperature dependency of CH 4 and CO 2 emissions was apparent under controlled conditions using C. rostrata monoliths. In August, the fen showed highest photosynthetic rates and was a net carbon sink (50 mmol m −2 day −1 ), but high CH 4 emissions resulted in the fen being a net source of 250 mmol CO 2 equivalents m −2 day −1 . Keywords Methane . Carbon dioxide . Wetlands . Carex rostrata . Flux chambers * Josef Zeyer
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