The growth resilience of forests to extreme drought event has become an urgent topic in global change ecology because of exacerbated water constraints upon trees' growth over the last few decades. Yet, surprisingly little is empirically known about the contribution of stand age, a key factor influencing forest structure and ecological processes, to variation in growth resilience among stands. This study revealed discrepancies in the drought resilience of forests of different stand ages by analyzing an extensive tree-ring dataset from Qinghai spruce (Picea crassifolia Kom.), a typical moisture-sensitive tree species in northwestern China. We found that older growth Qinghai spruce forest stands have higher resistance to droughts than do younger growth ones. Conversely, however, the post-drought recoveries of these older growth forest stands are lower than those of the younger growth stands. Patterns in the variation of resilience indices were consistent between two contrasting hydrological niche regions, whereas the stand age-related discrepancies in drought resilience became significantly smaller going from the wetter region to the drier region. These findings imply that, instead of a one-size-fits-all strategy, more meticulous and more targeted strategies are needed to enhance forest management and strengthen forest conservation given the experienced and projected climate trends, which feature increasing precipitation but higher extreme-drought frequency across this spruce tree's habitat and distribution in northwestern China.

Understanding the impact of management upon post-drought tree growth recovery and drought legacy effects is among the fundamental challenges hindering the improvement of forest conservation strategies in the face of increasingly frequent, longer, and intensified extreme droughts under ongoing climate change. Yet surprisingly little is known to date about how management practices can influence drought legacy effects; and previous studies of management impacts on forest resilience to drought have reached inconsistent and contentious conclusions. This study sought to tackle these pressing questions and gain insight by analyzing tree-ring datasets from non-managed and managed Qinghai spruce forests in northwestern China. The results show improved growth resilience to drought of those trees under management practices. Moreover, Qinghai spruce radial growth in non-managed forest exhibited significant legacy effects of extreme drought, whereas such legacy effects were mitigated in managed forest. Nevertheless, both the resilience augmentation and the mitigation of drought legacy effects by management were much weaker in the face of a three-year persistent drought than a single-year event. Hence, we may conclude that current management practices are advantageous and necessary for forest conservation under exacerbated drought conditions, for which strategies and measures should be better thought out and tailored to specific situations, rather than being one-size-fits-all, to better serve the goals of forest managers and conservationists.

Recently, forests in the Tianshan Mountains have shown a marked decline in growth and an increased mortality rate because of the more frequent and severe effects of extreme drought, which threatens the ecosystem services they provide. To achieve forest conservation and sustainable development benefits, it is crucial to understand the post-drought recovery trajectory of tree growth and its driving factors. In this study, we quantified the growth recovery performance of dominant tree species in the Tianshan Mountains after extreme drought events and determined the influences of climate factors on forest growth resilience using tree-ring proxy data. The results showed that post-drought moisture conditions may determine the post-drought growth recovery of trees. The post-drought growth for 1997 was higher than that for 1974, which may be attributed to the subsequent period of 1997 experiencing very high precipitation, whereas the year following the 1974 drought was dry (Stan-dardized Precipitation Evapotranspiration Index < 0). Because of the more favorable climate conditions in the post-drought period, the observed relationship between resistance and recovery in 1997 showed a closer fit to the hypothetical line of full resilience which sets resilience to a constant value of 1, allowing trees to recover fully at any given value of resistance. Trees showed lower mean values of the tree growth recovery index (RC) and average recovery rate (ARR) and higher mean values of total growth reduction (TGR) and recovery period (RE) for the drought event in 1974 than that in 1997. We distinguished the relative influence of temperature and precipitation on different drought phases using Boosted Regression Tree (BRT) model. The results showed that the climate conditions during the drought year and subsequent precipitation variation were most influential variables for tree growth recovery. Specifically, post-drought precipitation explained up to 20 % of the variance in RC, TGR, RE, and ARR. These findings deepen our understanding of the impacts of prolonged drought on tree growth, which could aid in developing forest management and conservation strategies to respond to extreme drought.

Drought-related forest growth declines are observed globally in main forest types, especially with repeatedly hot droughts. Therefore, quantifying forest resilience and identifying the factors driving resilience in response to extreme drought with the consideration of atmospheric CO2 fertilization is crucial for the accurate assessment of forest dynamics under current climate change, particularly for the widespread and climate-sensitive spruce forests in the arid Tianshan Mountains, China. Here, we explored the growth response of Schrenk spruce (Picea schrenkiana) to six extreme drought events since 1900, and investigated how tree resilience in pure stands is related to local drought intensity, cambial age (CA), and intrinsic water-use efficiency (iWUE). Specifically, we found that spruce trees had a mean resistance (Rt) value of less than 1, with iWUE contributing less to Rt variation. The results are in agreement with the drought-induced limitations on tree growth in response to increasing CO2, in spite of rising iWUE trends. However, increased iWUE has significant and positive impacts on the recovery (17%) and resilience (15%) of trees, suggesting that increased iWUE enhances the restoration of Schrenk spruce growth after extreme drought events. The growth resilience indices of Schrenk spruce showed that juvenile and adult trees exhibit different strategies to mitigate the drought influences. This study indicated that tree age, climate conditions, and variation in iWUE should be considered simultaneously in drought resilience evaluations to assess forest dynamics objectively in relation to climate change (i.e., drought) and propose appropriate forest management strategies.

Permafrost is one of the key components of terrestrial ecosystem in cold regions. In the context of climate change, few studies have investigated resilience of social ecological system (SER) from the perspective of permafrost that restricts the hydrothermal condition of alpine grassland ecosystem. In this paper, based on the structural dynamics, we developed the numerical model for the SER in the permafrost regions of the source of Yangtze and Yellow Rivers, analyzed the spatial-temporal characteristics and sensitivity of the SER, and estimated the effect of permafrost change on the SER. The results indicate that: 1) the SER has an increasing trend, especially after 1997, which is the joint effect of precipitation, temperature, NPP and ecological conservation projects; 2) the SER shows the spatial feature of high in southeast and low in northwest, which is consistent with the variation trends of high southeast and low northwest for the precipitation, temperature and NPP, and low southeast and high northwest for the altitude; 3) the high sensitive regions of SER to the permafrost change have gradually transited from the island distribution to zonal and planar distribution since 1980, moreover, the sensitive degree has gradually reduced; relatively, the sensitivity has high value in the north and south, and low value in the south and east; 4) the thickness of permafrost active layer shows a highly negative correlation with the SER. The contribution rate of permafrost change to the SER is -4.3%, that is, once the thickness of permafrost active layer increases 1 unit, the SER would decrease 0.04 units.

The important factors for the agrarian output in Bulgaria are only thermal and water probability. From the two factors, the component related to soil moisture is more limited. As well water and temperature probabilities in the agrarian output are estimated through stuns of temperatures and rainfalls or by derivative indicators (most frequently named as coefficients or indices). The heat conditions and the heat resources are specified by the continuousness of the vegetative period. Duration of vegetative season is limited for each type of plant, between the spring and autumn steady pass of air temperature across the biological minimum. For the agricultural crops in Bulgaria, the three biological minimums in 5 degrees C are taken for wheat and barley, oat, pea, and lentil; in 10 degrees C for sunflower, corn, haricot, and soybean; and in 15 degrees C for the cotton, vegetables, and other spring cultures. The cold and warm period duration are mutually related characteristics. The first period defines the number of days with the snowfall and days with the snow cover that are the basis in the formation of soil moisture reserves after the spring snow melt. Definition of the regions with temperature stress conditions during vegetative season is one of the most important parameters of agroclimatic conditions. The values indicating for the limitations are one or more periods from at least 10 consecutive days with maximal air temperature over 35 degrees C. More from the agricultures, character for the moderate continental climatic zone are developed normally under temperatures 25-28 degrees C. Temperatures over 28 degrees C are ballast slowing the growth and destroying plants due to the heat tension. The component, limiting in greatest degree growth, development and formation of yields from the agricultural crops are the conditions of moisturizing, present trough atmospheric and soil moisture. The most apparent indicator is the year sum of the rains or their sum by the periods with the average daily temperatures of over 5 and 10 degrees C. Cross correlation matrix between the meteorological elements from which evapotranspiration depends - temperature, relative air humidity, wind speed, and the vapor pressure deficit - is present. The data about the limitations, emergent from the soil moisture lack, to the base of the existing agrometeorological data are present. Values of the relation between real and potential evapotranspiration were calculated for potential vegetative period which is divided up to the two subperiods, March to June, the period of formation outputs from wintering cultures, and July to August, the period of formation outputs from the spring cultures. In the 1980s and 1990s, science led debates for and against climate change. During this time they published dozens of monographs and among them are Sir John I loughton's Global Warming: The Complete Briefing and John T. Hardy's Climate Change: Causes, Effects, and Solutions. The first of them was translated into Bulgarian by the author of this paper and published in 1996 by the academic publishing house of Prof. M. Drinov. Of course, they published numerous other studies and hundreds of articles, reports, and messages (Olmstead, Rhode, Creating abundance: biological innovation and American Agricultural Development. Cambridge University Press, 2008; Croitoru et al, Glob Planet Change 102:10-19, 2013; Rosenzweig, Hillel, Climate change and the global harvest: potential impacts of the greenhouse effect on agriculture. Oxford University Press, 1998; Georgieva, Kazandjiev, Sci Pares Ser A Agron LVI:459-467, 2013; Georgieva et al, Europa XXI 29:43-58, 2015; Kazandjiev, Peev, Prerequisites for disaster by natural weather phenomena and processes, reports first scientific-practical conference on Emergency Management and Civil Protection, Sofia, Bulgarian Academy of Sciences 10.11.2005, pp 186-193 (in Bulgarian), 2005d; Kazandjiev, Agroclimatic resources and definition of less favored areas at the beginning of XXI century in Bulgaria, Conference Global Environmental Change - Challenges to Science and Society in Southwestern Europe. CD version, 2008a; Rattan et al, Climate change and global food security, CRC, 2005; Roumenina et al, Int J Remote Sens 34(8):2888-2904, 2013; Pritchard, Amthor, Crops and enviromnental changes. Haworth Press (US), 2005; Simeonov, Georgiev, Atmos Res 57:187-199, 2001; Sivakumar et al, Natural disasters and extreme events in agriculture. Springer, 367 pp, 2005; Slavov, Relationship between climate change and desertification. Problems of land degradation and combating desertification. UN str.42-48 (in Bulgarian), 1998; Slavov, Alexandrov Drought Netw News 5(2):12-15, 1993). Today science has a lot of evidence in favor of climate change. But now science nationally and globally faces new questions: How far will climate change reach? How will the various sectors of the economy adapt to change? How will agriculture in particular adapt to climate change? What must the action plan 2030-2050 contain? The purpose of this paper is to plot a strategy for the adaptation of agriculture in Bulgaria to climatic change. This will establish the vulnerability of the main types of crops to climate change and will define criteria for extreme meteorological phenomena and processes of agro-meteorological point of view. The team will assess the risk of dangerous agriculture phenomena and combinations thereof, through probabilistic and statistical research. Also we will present indices that can be used as indicators for proof of climate change. As a result, they will identify adaptation measures by regions and types of cultures and develop a strategy for adaptation of Bulgarian agriculture to changing environmental conditions.