1. Forest dieback caused by drought-induced tree mortality has been observed worldwide. Forecasting which trees in which locations are vulnerable to drought-induced mortality is important to predict the consequences of drought on forest structure, biodiversity and ecosystem function. 2.In this paper, our central aim was to compile a synthesis of tree traits and associated abiotic variables that can be used to predict drought-induced mortality. 3.We reviewed the literature that specifically links drought mortality to functional traits and site conditions (i.e. edaphic variables and biotic conditions), targeting studies that show clear use of tree traits in drought analysis. We separated the review into five climatic-zones to determine global versus regionally restricted relationships between traits and mortality. 4.Our synthesis identifies a number of traits that have clear relationships with drought-induced mortality (e.g. wood density at the species level and tree size and growth at the individual level). However, the lack of direct relationships between most traits and drought-induced mortality highlights areas where future research should focus to broaden our understanding. 5.Synthesis and applications. Our synthesis highlights established relationships between traits and drought-induced mortality, presents knowledge-gaps for future research focus and suggests monitoring and research avenues for improving our understanding of drought-induced mortality. It is intended to assist ecologists and natural resource managers choose appropriate and measurable parameters for predicting local and regional scale tree mortality risk in different climatic-zones within constraints of time and funding availability

Abstract. Soil respiration is the sum of respiration processes in the soil and is a major flux in the global carbon cycle. It is usually assumed that the CO 2 efflux is equal to the soil respiration rate. Here we challenge this assumption by combining measurements of CO 2 with high-precision measurements of O 2 . These measurements were conducted on different ecosystems and soil types and included measurements of air samples taken from the soil profile of three Mediterranean sites: a temperate forest and two alpine forests. Rootfree soils from the alpine sites were also incubated in the lab. We found that the ratio between the CO 2 efflux and the O 2 influx (defined as apparent respiratory quotient, ARQ) was in the range of 0.14 to 1.23 and considerably deviated from the value of 0.9 ± 0.1 expected from the elemental composition of average plants and soil organic matter. At the Mediterranean sites, these deviations are explained as a result of CO 2 dissolution in the soil water and transformation to bicarbonate ions in these high-pH soils, as well as by carbonate mineral dissolution and precipitation processes. Thus, a correct estimate of the short-term, chamber-based biological respiratory flux in such soils can only be made by dividing the measured soil CO 2 efflux by the average (efflux-weighted) soil profile ARQ. Applying this approach to a semiarid pine forest resulted in an estimated short-term biological respiration rate that is 3.8 times higher than the chamber-measured surface CO 2 . The ARQ values often observed in the more acidic soils were unexpectedly low (< 0.7). These values probably result from the oxidation of reduced iron, which has been formed previously during times of high soil moisture and local anaerobic conditions inside soil aggregates. The results reported here provide direct quantitative evidence of a large temporal decoupling between soil-gas exchange fluxes and biological soil respiration.

Knowledge of the relationship between soil water dynamics and tree water use is critical to understanding forest response to environmental change in water‐limited ecosystems. However, the dynamics in soil water availability for tree transpiration (Tt) cannot be easily deduced from conventional measurements of soil water content (SWC), notably because Tt is influenced by soil water potential (Ψs) that, in turn, depends on soil characteristics. Using tree sap flow and water potential and deriving depth‐dependent soil water retention curves, we quantified the ‘transpirable soil water content’ (tSWC) and its seasonal and inter‐annual variations in a semi‐arid Pinus halepensis forest. The results indicated that tSWC varied in time and with soil depth. Over one growing season Tt was 57% of rain and 72% of the infiltrated SWC. In early winter, Tt was exclusively supported by soil moisture at the top 10 cm (tSWC = 11 mm), whereas in spring (tSWC > 18 mm) and throughout the dry season, source water for Tt shifted to 20–40 cm, where the maximum fine root density occurs. Simulation with the soil–plant–atmosphere water and energy transport model MuSICA supported the idea that consistent tSWC at the 20–40 cm soil layer critically depended on limited water infiltration below 40 cm, because of high water retention below this depth. Quantifying tSWC is critical to the precise estimation of the onset and termination of the growing season (when tSWC > 0) in this semi‐arid ecosystem. Copyright © 2013 John Wiley & Sons, Ltd.

Trees are increasingly exposed to hot droughts due to CO 2 -induced climate change. However, the direct role of [CO 2 ] in altering tree physiological responses to drought and heat stress remains ambiguous.Pinus halepensis (Aleppo pine) trees were grown from seed under ambient (421 ppm) or elevated (867 ppm) [CO 2 ]. The 1.5-yr-old trees, either well watered or drought treated for 1 month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10 d period. Continuous whole-tree shoot and root gasexchange measurements were supplemented by primary metabolite analysis.Elevated [CO 2 ] reduced tree water loss, reflected in lower stomatal conductance, resulting in a higher water-use efficiency throughout amplifying heat stress. Net carbon uptake declined strongly, driven by increases in respiration peaking earlier in the well-watered (31-32°C) than drought (33-34°C) treatments unaffected by growth [CO 2 ]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO 2 ] was subtle and mainly reflected in enhanced root protein stability.The impact of elevated [CO 2 ] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO 2 ] cannot counterbalance the impacts of hot drought extremes in Aleppo pine.