| Global climate change and environmental issues pose a serious challenge to human survival and development.They do not only change the natural environment for human survival,but also have profound impacts on social and economic development.Radial growth of trees is very sensitive to climate change.Trees continuously adjust their structure and function to adapt to changes in the external environment while at the same time,they also influence regional and global climate through material circulation and energy flow.Therefore,detailed knowledge on the radial growth dynamics of trees and how they respond to climate under global change is crucial to better understand how climate change will impact forest ecosystems.It can simultaneously provide a solid scientific ground to understand tree adaptation.The Qilian Mountains is an important ecological security barrier in northwest and even the whole China.It is also,due to its special geographical location and geological characteristics,one of the most threatened regions by increasing global temperatures and changing drought conditions that can affect these fragile ecosystems.The Qilian Mountains is one of the most studied areas in China for tree-ring research,most of them focused on climate reconstructions.However,most reconstructions were primarily based on empirical relationships between tree growth and climate factors identified through statistical analysis.Consequently,there is very little in-depth information on the physiological responses of trees to changing climate,especially at the level of individual.This motivated us to investigate the cambial phenology and intra-annual tree growth dynamics to better understand tree growth–climate interactions in the main tree species of the Qilian Mountains.Here,we conducted a high-resolution monitoring of the cambial phenology and xylogenesis of three different coniferous species up to 6 years across 16sites,monitoring a total of 80 trees across the different regions of Qilian Mountains,focusing on the relationships between cambial phenology,seasonal radial growth and climate.This thesis is divided in 5 parts that address different dimensions of this relationships and explore the response of different species to environmental change.In part 1 we studied the phenological traits of cambial activity and xylogenesis of Juniperus przewalskill Kom.using microcores sampled weekly or biweekly during 6 growing seasons（2011-2016）.The results identified that the main growth period for J.przewalskill from May to August.During the growing seasons,cambial phenology was very sensitive to the external environment,showing significant differences between monitored years.Temperature played a decisive role in the onset of tree growth,with minimum temperature of 1-2°C being the threshold that determined growth onset.The end of wood formation,on the other side,was mainly controlled by water availability.During the growing seasons,early growing season precipitation,especially in May,played the most key role in growth rate and xylem cells total production.In part 2,we explored an altitudinal transect ranging from 3580 to 3980 m a.s.l,which covers most of the distribution of J.przewalskii in the central Qilian Mountains.We studied the phenological traits of cambial activity and xylem formation biweekly during 2012 and 2013.We found that a significant correlation between the onset of wood formation and elevation in 2013,with the onset advancing 8.2 days earlier for every 100m decrease in elevation.The total length of the growing season decreased from 97 to 65 days from low to high elevations.Although the end of growing season appeared minimally related to elevation during both growing seasons,the differences in the end of wood production between the two growing seasons were significant,suggesting that summer drought conditions have constricted the end of growing season across all elevations along the transect.Sensitivity analysis found a strong relationship between xylem growth and the rate and duration of wood production,with the former explaining most of growth variability.In part 3 we analyzed weekly or biweekly by taken microcores from 30 trees over the course of two extreme hydroclimatic years to study the variation in cambial phenology and xylogenesis across six sites that span the entire spatial distribution of J.przewalskii in the Qilian Mountains under the extreme conditions.The onset of xylogenesis significantly correlated with mean and minimum annual temperatures,regardless of the moisture conditions,resulting in an advance of the onset of xylogenesis of 10.1 days°C-1.In contrast,the end of xylogenesis was controlled by both maximum temperatures and drought conditions.The end of xylogenesis was very sensitive to drought conditions,advancing up to 30 days during the extreme conditions of 2013 compared with that of the wetter and cooler 2012 conditions.The total number of xylem cells was correlated with both the duration and the rate of wood production,stressing that changes in the timing of xylogenetic processes and growth rate strongly influence the future growth and performance of J.przewalskii.Our findings indicate that both warming and decreased moisture availability are strong drivers of tree growth and,thus productivity,in cold and arid regions and,possibly,the evolutionary adaptation of trees almost regardless of local conditions.In part 4 we combined microcoring data from 2 growing seasons（2012-2013）with dendrometer data of three growing seasons（2013-2015）to better understand growth phenology and seasonal growth dynamics of three main coniferous species:J.przewalskii,Pinus tabulaeformis,and Picea wilsonii over the eastern Qilian Mountains.Our results showed very different growth phenology patterns between species.P.tabulaeformis showed the earliest growth initiation,latest growth cessation and,therefore,the longest growing season duration.J.przewalskii showed an opposite growth phenology to that of P.tabulaeformis.P.wilsonii laid between the two in all growing seasons.The onset of growth for all species was mainly controlled by early spring temperature,while the end of growth seemed to depend on water availability.Stem radial increments of all species were significantly and positively correlated with precipitation and relative humidity,and negatively correlated with air temperature and vapor pressure deficit.There finding suggest that water availability play a key role in seasonal tree growth across species and environmental conditions in the area.Intra-annual stem radial growth seemed to be controlled mostly by climatic variation across tree species.This consistent response will allow us to develop a multi-species physiological basis for growth–climate interactions in semi-arid mountainous areas.Finally,in part 5 we build upon the results obtained in the previous parts to develop a process-based Vaganov-Shashkin（VS）model to simulate the regional patterns of climate-tree growth relationships over the different regions of the Qilian Mountains.Our simulated growth curves were broadly consistent with the actual tree-ring chronologies for all three regions.The VS model simulations accurately represented the effect of climatic controls on the growth of J.przewalskii and the model outputs closely matched the observed relationships between tree-ring width and climate factors as well as observational physiological behavior（i.e.available water in May and June had the largest contribution to ring formation in Qilian Juniper over the Qilian Mountains）.We found a double effect of water availability on the wood production by indirectly influence the ability of cell division and enlargement by modulating the photosynthetic rate and carbohydrate sink;but we also found that water availability might directly influence wood formation by participating in the cambial cell division and enlargement. |
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