Wall structure of terminal latewood tracheids of healty and declining silver fir trees in the dinaric region, Slovenia

Fir trees (Abies alba Mill.) in a permanently monitored forest in the Di-naricregion in Slovenia respond to crown damages by distinctly reducing their ring widths. According to transmission electron microscopy (TEM) and UV-microspectrophotometry (UMSP) of cambium-adjacent latewood tracheids of affected trees, the secondary wall formation and lignification were completed by the middle of October. In samples taken at the same date from healthy looking silver firs, the S3 and the warty layer were not yet present in cambium-adjacent latewood tracheids. Additionally, their inner S2 showed lower lignin deposition, whereas the compound middle lamella, S1 and outer S2 were distinctly lignified as revealed by TEM and UMSP. It is assumed that these youngest tracheids of healthy trees will later undergo lignification or remain less lignified. From these observations we conclude that the cambial activity at breast height ends later in healthy silver fir trees as compared to declining tree