Forecasted increase drought frequency and severity may drive worldwide declines in forest productivity. Species-level responses to a drier world are likely to be influenced by their functional traits. Here, we analyse forest resilience to drought using an extensive network of tree-ring width data and satellite imagery. We compiled proxies of forest growth and productivity (TRWi, absolutely dated ring-width indices; NDVI, Normalized Difference Vegetation Index) for 11 tree species and 502 forests in Spain corresponding to Mediterranean, temperate, and continental biomes. Four different components of forest resilience to drought were calculated based on TRWi and NDVI data before, during, and after four major droughts (1986, 1994-1995, 1999, and 2005), and pointed out that TRWi data were more sensitive metrics of forest resilience to drought than NDVI data. Resilience was related to both drought severity and forest composition. Evergreen gymnosperms dominating semi-arid Mediterranean forests showed the lowest resistance to drought, but higher recovery than deciduous angiosperms dominating humid temperate forests. Moreover, semi-arid gymnosperm forests presented a negative temporal trend in the resistance to drought, but this pattern was absent in continental and temperate forests. Although gymnosperms in dry Mediterranean forests showed a faster recovery after drought, their recovery potential could be constrained if droughts become more frequent. Conversely, angiosperms and gymnosperms inhabiting temperate and continental sites might have problems to recover after more intense droughts since they resist drought but are less able to recover afterwards.
This review examines the role of the atmospheric evaporative demand (AED) in drought. AED is a complex concept and here we discuss possible AED definitions, the subsequent metrics to measure and estimate AED, and the different physical drivers that control it. The complex influence of AED on meteorological, environmental/agricultural and hydrological droughts is discussed, stressing the important spatial differences related to the climatological conditions. Likewise, AED influence on drought has implications regarding how different drought metrics consider AED in their attempts to quantify drought severity. Throughout the article, we assess literature findings with respect to: (a) recent drought trends and future projections; (b) the several uncertainties related to data availability; (c) the sensitivity of current drought metrics to AED; and (d) possible roles that both the radiative and physiological effects of increasing atmospheric CO 2 concentrations may play as we progress into the future.All these issues preclude identifying a simple effect of the AED on drought severity.Rather it calls for different evaluations of drought impacts and trends under future climate scenarios, considering the complex feedbacks governing the climate system. This article is categorized under: Paleoclimates and Current Trends > Earth System Behavior K E Y W O R D S atmospheric evaporative demand,
16In this study, we analyzed the influence of El Niño -Southern Oscillation (ENSO) on the spatio-17 temporal variability of droughts in Ecuador for a 48-year period (1965-2012). Droughts were quantified 18 from 22 high-quality and homogenized time series of precipitation and air temperature by means of the 19 Standardized Precipitation Evapotranspiration Index (SPEI). In addition, the propagation of two different 20 ENSO indices (El Niño 3.4 and El Niño 1+2 indices) and other atmospheric circulation processes (e.g., 21vertical velocity) on different time-scales of drought severity were investigated. The results showed a 22 very complex influence of ENSO on drought behavior across Ecuador, with two regional patterns in the 23 evolution of droughts: (i) the Andean chain with no changes in drought severity, and (ii) the Western 24 plains with less severe and frequent droughts. We also detected that drought variability in the Andes 25 mountains is explained by the El Niño 3.4 index (sea surface temperature [SST] anomalies in the central 26 Pacific), whereas the Western plains are much more driven by El Niño 1+2 index (SST anomalies in the 27 eastern Pacific). Moreover, it was also observed that El Niño and La Niña phases enhance droughts in the 28Andes and Western plains regions, respectively. The results of this work could be crucial for predicting 29 and monitoring drought variability and intensity in Ecuador. 30
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