ALBERT

Description: Seasonal and inter-annual dynamics of growth, non-structural carbohydrates (NSC) and carbon isotope composition ( 13 C) of NSC were studied in a beech forest of Central Italy over a 2-year period characterized by different environmental conditions. The net C assimilated by forest trees was mainly used to sustain growth early in the season and to accumulate storage carbohydrates in trunk and root wood in the later part of the season, before leaf shedding. Growth and NSC concentration dynamics were only slightly affected by the reduced soil water content (SWC) during the drier year. Conversely, the carbon isotope analysis on NSC revealed seasonal and inter-annual variations of photosynthetic and post-carboxylation fractionation processes, with a significant increase in 13 C of wood and leaf soluble sugars in the drier summer year than in the wetter one. The highly significant correlation between 13 C of leaf soluble sugars and SWC suggests a decrease of the canopy C isotope discrimination and, hence, an increased water-use efficiency with decreasing soil water availability. This may be a relevant trait for maintaining an acceptable plant water status and a relatively high C sink capacity during dry seasonal periods. Our results suggest a short- to medium-term homeostatic response of the Collelongo beech stand to variations in water availability and solar radiation, indicating that this Mediterranean forest was able to adjust carbon–water balance in order to prevent C depletion and to sustain plant growth and reserve accumulation during relatively dry seasons.

Description: Carbon isotope composition ( 13 C) and carbohydrate content of phloem sap and fine roots were measured in a Mediterranean beech ( Fagus sylvatica L.) forest throughout the growing season to study seasonal changes of source–sink relationships. Seasonal variations of 13 C and content of phloem sap sugars, collected during the daylight period, reflected the changes in soil and plant water status. The correlation between 13 C and content of phloem sap sugars, collected from plants belonging to different social classes, was significantly positive only during the driest month of July. In this month, 13 C of phloem sap sugars was inversely related to the increment of trunk radial growth and positively related to 13 C of fine roots. We conclude that the relationship between 13 C and the amount of phloem sap sugars is affected by a combination of causes, such as sink strength, tree social class, changes in phloem anatomy and transport capacity, and phloem loading of sugars to restore sieve tube turgor following the reduced plant water potential under drought conditions. However, 13 C and sugar composition of fine roots suggested that phloem transport of leaf sucrose to this belowground component was not impaired by mild drought and that sucrose was in a large part allocated towards fine roots in July, depending on tree social class. Hence, fine roots could represent a functional carbon sink during the dry seasonal periods, when transport and use of assimilates in other sink tissues are reduced. These results indicate a strict link between above- and belowground processes and highlight a rapid response of this Mediterranean forest to changes in environmental drivers to regulate source–sink relationships and carbon sink capacity.