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In the context of changing climate conditions, it is crucial to understand how trees maintain resistance and resilience to and recover from drought stress and management-related disturbances, and how management (intensive/extensive) modulates tree growth responses to climate. Dendrochronological methods were used to examine radial growth rates, to reconstruct disturbance history and to evaluate growth resistance and resilience to and recovery from disturbance events identified by pointer year analysis in two coexisting Quercus species (Q. macrolepis Kotschy and Q. trojana Webb). Moreover, a dendro-ecological approach was used to quantify climate–growth relationships, including a drought index, the Standardised Precipitation Index (SPI). Q. trojana/Q. macrolepis showed major/moderate growth releases and moderate growth suppression. Q. trojana demonstrated statistically lower growth recovery than Q. macrolepis. The climatic drivers of radial growth in Q. macrolepis were May, June, September precipitation, July temperatures and the SPI accumulated since the previous year (up to 18 months); whereas Q. trojana responded significantly to May and June precipitation, July temperatures and the SPI at mid-time scales (< 8 months). Under intensive management, both species were more responsive to precipitation, temperature and short to mid-time scale SPI than during the extensive management period where they showed loss of sensitivity to spring–summer precipitation, temperature and SPI. These results are relevant to understand species vulnerability to climate and management/ human-related disturbances, and the effects of intensive/extensive management in modulating climate (drought)–growth relationships in Q. macrolepis and Q. trojana trees. [ABSTRACT FROM AUTHOR]

Improved knowledge of the time scales at which drought stress mostly influences tree growth is crucial for the early detection of forest dieback. This study aimed to evaluate the impact of climate (temperature and precipitation) on vegetation activity (normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI)) of Pinus halepensis Mill. and Pinus pinea L. mixed forest located in western Albania and to assess the drought impact (standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI)) calculated at different time scales (1-12 months) on radial growth (earlywood width, latewood width, and tree-ring width) and vegetation activity of these species. Both vegetation indices showed a negative response to August temperatures, and the EVI responded positively to September precipitation. NDVI and EVI were significantly affected by the SPI in spring and late summer. All tree-ring features in P. halepensis were positively related with EVI in August, whereas P. pinea latewood width showed a significant and positive relationship with NDVI in September. Radial growth of P. halepensis responded significantly to both drought indices in late summer and early autumn, particularly the latewood width. Contrastingly, in P. pinea, only earlywood width showed vulnerability, mostly to the summer SPEI drought indices. These results are relevant to understand the impacts of increased drought intensity and frequency on tree radial growth and vegetation activity in a region that is vulnerable to climate variability. [ABSTRACT FROM AUTHOR]

Assessing the risk, the severity and the likely evolution of droughts are key tasks for improving preparedness of regions prone to drought conditions, and mitigation of drought consequences. The access to real-time and high-quality climatic information is essential for this purpose. Different climatic databases are being developed and made available on real time by climatic research institutions, but their capability for quantifying droughts characteristics including severity, or spatio-temporal variability, is uncertain given their low spatial resolution. In this study, we assessed the capability of three databases with contrasted spatial resolution for measuring spatial and temporal variability of drought occurrence. The standardized precipitation index, calculated for each database, showed that the low resolution datasets allow an acceptable measurement of the magnitude, intensity and duration of droughts, while failing mostly in detecting the spatial patterns of the specific drought episodes. Moreover, the capability of the datasets for assessing the impacts of droughts on surface hydrology and tree growth was examined. Results confirmed the usefulness of the drought index for assessing drought impacts on water resources and forest ecosystems even when low resolution databases are used. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Abstract: We analyzed the impact of drought measured on different time-scales on radial growth of eight tree species during the period 1950–2005 growing across a wide climatic gradient encompassing semiarid Mediterranean woodlands and wet mountain forests in north-eastern Spain. A drought index (standardized precipitation index, SPI) at different time scales (1–48 months) was correlated with chronologies of ring width to determine the significant time scale at which drought affected most tree growth. The findings indicated that the impact of drought on growth varied noticeably among species and sites. Two distinct patterns were clearly observed considering spatial and temporal differences in the response of species to drought. Species growing in xeric sites (Pinus and Quercus species and Juniperus thurifera) showed the highest responses to SPI time-scales of 9–11 months while those located in mesic sites (Abies alba, Pinus sylvestris) did respond more to SPI time scales shorter than 5 months. The SPI-growth correlations were significant, although weak, up to 30 months in xeric sites while no consistent association was observed at higher time scales. Important seasonal differences were noticed in the SPI-growth associations. Species growing in xeric areas responded to spring-summer SPI while those distributed in mesic sites responded more to summer SPI. Our findings should be useful to understand forest responses to climate change, including an increasing frequency of severe droughts, and to adapt appropriate management strategies to mitigate the impact of drought on tree growth. [Copyright &y& Elsevier]

Abstract: In this study we analyzed the influence of general atmospheric circulation patterns and the frequency of weather types on the spatio-temporal variability of tree-ring growth in Pinus halepensis forests in eastern Spain. Three atmospheric circulation patterns affecting the western Mediterranean region were included in the study: the North Atlantic Oscillation (NAO), the Western Mediterranean Oscillation (WeMO) and the Mediterranean Oscillation (MO). In addition, the particular circulation pattern affecting eastern Spain was quantified using the frequency of weather types. The variability of radial growth (width) of earlywood and latewood in P. halepensis was quantified at 19 sites using dendrochronological methods. Two distinct patterns, reflecting growth variability in the northern and southern areas involved in the study, were identified for both earlywood and latewood tree-ring series. The influence of atmospheric circulation modes on tree growth resembled the spatial patterns identified, as earlywood and latewood formation in northern sites was determined by the NAO variability, whereas the WeMO dominated growth at the southern sites. Winter, summer and autumn weather types also exerted a control over tree radial growth. We conclude that both atmospheric circulation indices and weather types exert significant control on the formation of earlywood and latewood, because of their influence on precipitation patterns. The findings also suggest that wet and mild conditions during winter and the following autumn enhance P. halepensis earlywood and latewood formation, respectively. Thus, winter atmospheric patterns may indirectly influence latewood growth through direct effects on previous earlywood development driven by precipitation variability. [Copyright &y& Elsevier]