An experimental forest ecosystem drought: Drought is affecting many of the world’ s forested ecosystems, but it has proved challenging to develop an ecosystem-level mechanistic understanding of the ways that drought affects carbon and water fluxes through forest ecosystems. Werner et al . used an experimental approach by imposing an artificial drought on an entire enclosed ecosystem: the Biosphere 2 Tropical Rainforest in Arizona (see the Perspective by Eisenhauer and Weigelt). The authors show that ecosystem-scale plant responses to drought depend on distinct plant functional groups, differing in their water-use strategies and their position in the forest canopy. The balance of these plant functional groups drives changes in carbon and water fluxes, as well as the release of volatile organic compounds into the atmosphere. —AMS

Waterlogging and flooding cause oxygen deprivation in the root system of trees. Since oxygen is essentially for mitochondrial respiration, this process cannot be maintained under anoxic conditions and must be replaced by other pathways. For the roots it is therefore a matter of survival to switch from respiration to alcoholic fermentation. Due to the low efficiency of this process to yield energy equivalents (ATP), energy and carbon metabolism of trees are usually strongly affected by oxygen deprivation, even if a rapid switch from respiration to fermentation is achieved. The roots can compensate for the low energy yield of fermentation either (1) by decreasing the demand for energy by a reduction of energy‐dependent processes such as root growth and/or nutrient uptake, or (2) by consuming more carbohydrates per unit time in order to generate sufficient energy equivalents. In the leaves of trees, flooding and waterlogging cause a decline in the rates of photosynthesis and transpiration, as well as in stomatal conductance. It is assumed that, due to reduced phloem transport, soluble sugars and starch accumulate in the leaves of flooded trees, thereby negatively affecting the sugar supply of the roots. Thus, root growth and survival is negatively affected by both changes in root internal carbon metabolism and impaired carbon allocation to the roots by phloem transport. In addition, accumulation of toxic products of fermentation in the roots, such as acetaldehyde, can further impair root metabolism. A main feature of tolerance against flooding and waterlogging of trees seems to be the steady supply of carbohydrates to the roots in order to maintain alcoholic fermentation; in addition, roots of tolerant trees seem to avoid accumulation of fermentation‐derived ethanol and acetaldehyde. From studies with flooding tolerant and non‐tolerant tree species, it is hypothesized that (1) the transport of ethanol produced in the roots under hypoxic conditions into the leaves via the transpiration stream, (2) its conversion into acetyl‐CoA in the leaves, and (3) its use in the plant's general metabolism, are mechanisms of flooding tolerance of trees.

Reduced unloading and enhanced loading of sulfate increased xylem sap sulfate during early drought, which affects the opening of the ALMT12 channel and induces

The occurrence of hot drought, i.e. low water availability and simultaneous high air temperature, represents a severe threat to ecosystems. Here, we investigated how the 2018 hot drought in Central Europe caused a tipping point in tree and ecosystem functioning in a Scots pine (Pinus sylvestris L.) forest in southwest Germany. Measurements of stress indicators, such as needle water potential, carbon assimilation and volatile organic compound (VOC) emissions, of dominant P. sylvestris trees were deployed to evaluate tree functioning during hot drought. Ecosystem impact and recovery were assessed as ecosystem carbon exchange, normalized difference vegetation index (NDVI) from satellite data and tree mortality data. During summer 2018, needle water potentials of trees dropped to minimum values of −7.5 ± 0.2 MPa, which implied severe hydraulic impairment of P. sylvestris. Likewise, carbon assimilation and VOC emissions strongly declined after mid‐July. Decreasing NDVI values from August 2018 onwards were detected, along with severe defoliation in P. sylvestris, impairing ecosystem carbon flux recovery in 2019, shifting the forest into a year‐round carbon source. A total of 47% of all monitored trees (n = 368) died by September 2020. NDVI recovered to pre‐2018 levels in 2019, likely caused by emerging broadleaved understorey species. The 2018 hot drought had severe negative impacts on P. sylvestris. The co‐occurrence of unfavourable site‐specific conditions with recurrent severe droughts resulted in accelerated mortality. Thus, the 2018 hot drought pushed the P. sylvestris stand towards its tipping point, with a subsequent vegetation shift to a broadleaf‐dominated forest.

Monoterpenes (C₁₀H₁₆) are emitted in large quantities by vegetation to the atmosphere (>100 TgC year⁻¹), where they readily react with hydroxyl radicals and ozone to form new particles and, hence, clouds, affecting the Earth’s radiative budget and, thereby, climate change^1–3. Although most monoterpenes exist in two chiral mirror-image forms termed enantiomers, these (+) and (−) forms are rarely distinguished in measurement or modelling studies^4–6. Therefore, the individual formation pathways of monoterpene enantiomers in plants and their ecological functions are poorly understood. Here we present enantiomerically separated atmospheric monoterpene and isoprene data from an enclosed tropical rainforest ecosystem in the absence of ultraviolet light and atmospheric oxidation chemistry, during a four-month controlled drought and rewetting experiment⁷. Surprisingly, the emitted enantiomers showed distinct diel emission peaks, which responded differently to progressive drying. Isotopic labelling established that vegetation emitted mainly de novo-synthesized (−)-α-pinene, whereas (+)-α-pinene was emitted from storage pools. As drought progressed, the source of (−)-α-pinene emissions shifted to storage pools, favouring cloud formation. Pre-drought mixing ratios of both α-pinene enantiomers correlated better with other monoterpenes than with each other, indicating different enzymatic controls. These results show that enantiomeric distribution is key to understanding the underlying processes driving monoterpene emissions from forest ecosystems and predicting atmospheric feedbacks in response to climate change.