## 3 Observations of Drought Impacts  
Under drought conditions, plants typically undergo C starvation or hydraulic failure [60] (Figure 2 and Table S1). Either condition can be fatal. Carbon starvation occurs when stomata close to constrain water loss, resulting in significantly lower C assimilation that will not only halt growth, but also risk insufficient reserves to sustain plant maintenance requirements. Hydraulic failure occurs when xylems become damaged or collapse, limiting a plant's ability to extract water. The timing of drought and phenology will play a key role that drives plant response [61]. For example, a drought event that coincides with the peak growth period will result in higher plant mortality than drought during a less active growth period. Another important consideration is drought intensity versus drought frequency [62]. A severe drought with a long duration will have a different impact on plant response and survival than short, frequent droughts. The length of time a plant has been exposed to drought cycles is an important driver of trait changes that increase drought tolerance. Over short time scales, stomata regulate water loss, but, over longer time scales, changes in allometry will occur to optimize hydraulic conductance [22]. Plant traits may change similarly across short and long drought intervals; for example, leaf area may decrease with decreasing precipitation, while other processes may be delayed (i.e., partitioning changes between the leaf and roots) or homeostatic (e.g., xylem vulnerability) [22]. Figure 2 shows some of the effects forests may experience under drought. There are many ecological traits that can make a system more or less tolerant to drought, thereby leading to improved water use efficiency, including control over stomatal conductance, allometric plasticity, hydraulic redistribution [63], or even long-term acclimation. To survive drought, plants may also reduce C demand, which can be achieved by leaf senescence and the down-regulation of respiration [64]. Many field studies have observed increased stomatal closure [20; 28; 65] and increased water use efficiency during droughts [20]. However, observations also found species that have less stomatal control and may continue to transpire well into a drought event [65]. This may provide an advantage during short-term droughts but not necessarily during longer, more intense drought events. The ability to moderate water uptake and growth may be absent in the tropics; [PERSON] et al. [66] found no change in the growth rate of trees in the Amazon under a long-term drought. Stomata closure will reduce photosynthesis and, over long periods, lead to lower biomass accumulation, litter production, and ultimately N mineralization in the soil [67]. Additional nutrient limitations can occur during drought because of low soil moisture levels, which act to reduce nutrient flow and diffusion in soils. These N limitations can exacerbate the effects of drought by limiting photosynthesis further. Figure 2: Effects of drought on ecosystems. Up arrows represent an increase, and down arrows represent a decrease. Four main pathways exist: changes in C availability, increases in N limitation, changes to C mobilization and transport, and changes to biomass partitioning (i.e., roots shoot). Changes in C availability and N limitation both lead to decreases in photosynthesis, which has a positive feedback on N limitation when less biomass is contributing N to litter pools. Changes in C availability and a lack of C mobilization can lead to death from carbon starvation. Several pathways result in altered species composition from shifts in nutrient availability and competition. In general, the impacts from drought occur over short timescales (¡5 years), while changes to species composition and disturbance occur over longer timescales. Since plants continue to demand C for metabolic respiration, when stomata close during drought, plants rely on reserves to meet C requirements.