Global climate change has been seen to result in marked impacts on forest ecosystems such as accelerated tree mortality worldwide due to incidental hydraulic failure caused by intensified and more frequent occurrence of extreme drought and heat-waves. However, it is well understood how the tree hydrological strategies would adjust to environmental variability brough about by climate changes. Here we investigated the hydraulic adjustment as a mechanism of acclimation to different climate conditions along an altitudinal gradient in Faxon fir (Abies fargesii var. faxoniana) ― a tree species that plays a key role in conservation of wildlife and maintenance of ecosystem services in subalpine forests. The hydraulic traits and selective morphological and physiological variables were measured seasonally along an altitudinal gradient from 2,800 to 3,600 m a.s.l. We found that the native percentage loss of conductivity (PLC) increased with altitude across the seasonal measurements. Both the native sapwood-specific hydraulic conductivity (Ks) and native leaf-specific hydraulic conductivity (Kl) significantly decreased with altitude for measurements in July and October, coinciding with the timing for peak growth and pre-dormancy, respectively. The morphological traits varied toward more conservative tree hydrological strategies with increases in altitude, exhibiting trade-offs with hydraulic traits. The total non-structural carbohydrates in both needle (NSCNeedle) and branch (NSCBranch) as well as photosynthetic capacity of current-year leaves played variable roles in maintaining the integrity of the hydraulic functioning and shaping the hydraulic adjustment under prevailing environmental conditions. Our findings indicate that Faxon fir possesses some degree of hydraulic adaptability to water limitation imposed by climate fluctuations in subalpine region through morphological and physiological modifications.
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Soil organic carbon (SOC) is a large reservoir of terrestrial carbon (C); it consists of different fractions of varying complexity and stability. Partitioning SOC into different pools of decomposability help better predict the trend of changes in SOC dynamics under climate change. Information on how physical fractions and chemical structures of SOC are related to climate and vegetation types is essential for spatial modelling of SOC processes and responses to global change factors.
Soil samples were collected from multiple representative forest sites of three contrasting climatic zones (i.e. cool temperate,warm temperate,and subtropical) in eastern China. Measurements were made on SOC contents and physical fractions of the 0-20cm soil layer,and the chemical composition of SOC of the 0-5cm soil layer,along with measurements and compilation of the basic site and forest stand variables. The long-term effects of temperature,litter inputs,soil characteristics and vegetation type on the SOC contents and factions were examined by means of "space for time substitution" approach and statistical analysis.
Mean annual temperature (MAT) varied from 2.1℃ at the cool temperate sites to 20.8℃ at the subtropical sites. Total SOC of the 0-20cm soil layer decreased with increasing MAT,ranging from 89.2g·kg-1 in cool temperate forests to 57.7g·kg-1 in subtropical forests,at an average rate of 1.87% reduction in SOC with a 1℃ increase in MAT. With increasing MAT,the proportions of aromatic C and phenolic C displayed a tendency of decreases,whereas the proportion of alkyl C and A/O-A value (the ratio of alkyl C to the sum of O-alkyl C and acetal C) displayed a tendency of increases. Overall,there were no significant changes with MAT and forest type in either the physical fractions or the chemical composition. Based on the relationship between the SOC content and MAT,we estimate that SOC in the top 20 soil layer of forests potentially contribute 6.58-26.3 Pg C globally to the atmosphere if global MAT increases by 1℃-4℃ by the end of the twenty-first century,with nearly half of which (cf. 2.87-11.5 Pg C) occurring in the 0-5cm mineral soils.
Forest topsoil SOC content decreased and became chemically more recalcitrant with increasing MAT,without apparent changes in the physical fractions of SOC.
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