Abstract
Drought stress has induced dieback episodes affecting many forest types and tree species worldwide. However, there is scarce information regarding drought-triggered growth decline and canopy dieback in Mediterranean deciduous oaks. These species face summer drought but have to form new foliage every spring which can make them vulnerable to hotter and drier conditions during that season. Here, we investigated two stands dominated by Quercus frainetto Ten. and Quercus canariensis Willd. and situated in southern Italy and Spain, respectively, showing drought-induced dieback since the 2000s. We analyzed how radial growth and its responses to climate differed between non-declining (ND) and declining (D) trees, showing different crown defoliation and coexisting in each stand by: (i) characterizing growth variability and its responsiveness to climate and drought through time, and (ii) simulating growth responses to soil moisture and temperature thresholds using the Vaganov–Shashkin VS-lite model. Our results show how growth responsiveness to climate and drought was higher in D trees for both oak species. Growth has become increasingly limited by warmer-drier climate and decreasing soil moisture availability since the 1990s. These conditions preceded growth drops in D trees indicating they were more vulnerable to warming and aridification trends. Extremely warm and dry conditions during the early growing season trigger dieback. Changes in the seasonal timing of water limitations caused contrasting effects on long-term growth trends of D trees after the 1980s in Q. frainetto and during the 1990s in Q. canariensis. Using growth models allows identifying early-warning signals of vulnerability, which can be compared with shifts in the growth responses to warmer and drier conditions. Our approach facilitates establishing drought-vulnerability thresholds by combining growth models with field records of dieback.
Highlights
IntroductionDrought stress due to climate warming and higher precipitation variability has been considered among the main abiotic causes of forest dieback in the Mediterranean regions of southern Europe [1,2].Climate warming can influence forests by modifying the start (e.g., earlier bud bursting) and Forests 2020, 11, 714; doi:10.3390/f11070714 www.mdpi.com/journal/forestsForests 2020, 11, 714 duration of the growing season (e.g., delayed leaf fall), and by amplifying drought stress rising evapotranspiration rates [3,4,5]
Drought stress due to climate warming and higher precipitation variability has been considered among the main abiotic causes of forest dieback in the Mediterranean regions of southern Europe [1,2].Climate warming can influence forests by modifying the start and Forests 2020, 11, 714; doi:10.3390/f11070714 www.mdpi.com/journal/forestsForests 2020, 11, 714 duration of the growing season, and by amplifying drought stress rising evapotranspiration rates [3,4,5]
Our study suggests that drier and warmer conditions will cause a growth reduction (Figure 3) in deciduous oak species growing in seasonally drought-prone Mediterranean regions because of increasingly warmer and drier winter to spring conditions [15,18,25]
Summary
Drought stress due to climate warming and higher precipitation variability has been considered among the main abiotic causes of forest dieback in the Mediterranean regions of southern Europe [1,2].Climate warming can influence forests by modifying the start (e.g., earlier bud bursting) and Forests 2020, 11, 714; doi:10.3390/f11070714 www.mdpi.com/journal/forestsForests 2020, 11, 714 duration of the growing season (e.g., delayed leaf fall), and by amplifying drought stress rising evapotranspiration rates [3,4,5]. Under forecasted longer and more intense hotter droughts, Mediterranean oaks, and specially anisohydric ring-porous, winter-deciduous species, are prone to showing an increasing risk of dieback and mortality due to hydraulic failure or carbon starvation [19]. In these species, drought impairs their photosynthetic activity, reducing their primary and secondary growth and triggering leaf shedding [20,21]. Little is known about the long-term responses to climate and related vulnerability thresholds in co-occurring trees of Mediterranean deciduous oak species showing different canopy dieback intensity after drought
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