Synergy of extreme drought and shrub invasion reduce ecosystem functioning and resilience in water-limited climates

Extreme drought events and plant invasions are major drivers of global change that can critically affect ecosystem functioning and alter ecosystem-atmosphere exchange. Invaders are expanding worldwide and extreme drought events are projected to increase in frequency and intensity. However, very little is known on how these drivers may interact to affect the functioning and resilience of ecosystems to extreme events. Using a manipulative shrub removal experiment and the co-occurrence of an extreme drought event (2011/2012) in a Mediterranean woodland, we show that native shrub invasion and extreme drought synergistically reduced ecosystem transpiration and the resilience of key-stone oak tree species. Ecosystem transpiration was dominated by the water use of the invasive shrub Cistus ladanifer, which further increased after the extreme drought event. Meanwhile, the transpiration of key-stone tree species decreased, indicating a competitive advantage in favour of the invader. Our results suggest that in Mediterranean-type climates the invasion of water spending species and projected recurrent extreme drought events may synergistically cause critical drought tolerance thresholds of key-stone tree species to be surpassed, corroborating observed higher tree mortality in the invaded ecosystems. Ultimately, this may shift seasonally water limited ecosystems into less desirable alternative states dominated by water spending invasive shrubs

Species-specific differences in temporal and spatial variation in delta C-13 of plant carbon pools and dark-respired CO2 under changing environmental conditions

Dubbert M, Grieve Rascher K, Werner C. Species-specific differences in temporal and spatial variation in delta C-13 of plant carbon pools and dark-respired CO2 under changing environmental conditions. Photosynthesis Research. 2012;113(1-3):297-309.Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic delta C-13 discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in delta C-13 between carbon pools along the plant axis and in respired CO2. Little is known about the within-plant variation in delta C-13 under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in delta C-13 of water soluble organic matter (delta C-13(WSOM)) of leaves, phloem and roots, as well as dark-respired delta(CO2)-C-13 (delta C-13(res)) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs-Halimium halimifolium and Rosmarinus officinalis-and a woody invader-Acacia longifolia). Spatial patterns in delta C-13(WSOM) along the plant vertical axis and between respired delta(CO2)-C-13 and its putative substrate were clearly species specific and the most delta C-13-enriched and depleted values were found in delta C-13 of leaf dark-respired CO2 and phloem sugars, similar to-15 and similar to-33 aEuro degrees, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem delta C-13(WSOM) and delta C-13(res) varied by up to 4 aEuro degrees between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using delta C-13 signatures to trace changes in photosynthetic discrimination