Records of annual tree ring width are often used to understand how tree growth responds to climatic changes and ecological disturbance. However, data is rarely collected to assess the associated response of the xylem anatomy. Measured traits of the xylem can enhance our understanding of tree response to stress by indicating the functional response of the water transport system. This allows us to evaluate the impact of environmental stress on the tree’s physiology along with its growth. The objective of this study is to examine the relationships between tree ring anatomical variables, drought stress, and insect defoliation events. With this analysis, we may increase our understanding of the interaction and potential trade-offs between tree growth and hydraulic safety.
In this study, cores of aspen (Populus tremuloides) were collected from a stand in Ministik, Alberta. This stand is composed of mature trees which have experienced multiple drought and insect defoliation events over their lifespans. Micro-sections of the tree cores were imaged for analysis of xylem structural characters in the years surrounding these environmental stresses. Anatomical measurements per annual ring in the analysis included: xylem vessel diameter, vessel contact fraction, vessel lumen fraction, and fiber lumen fraction.
Preliminary results indicate that mean vessel diameter is negatively correlated with vessel contact fraction in aspen tree rings. This suggests that smaller vessels may have more water pathway redundancy to facilitate transport. Additionally, the vessel lumen fraction is strongly correlated with ring width, indicating a lack of latewood formation in years of low growth.
Insect defoliation events resulted in an increase in fiber lumen fraction during the years of defoliation. The thin-walled fibers produced in these years may compromise the structural integrity of the ring and lead to increased risk of embolism formation in the xylem vessels. Fiber lumen fraction in drought years was unchanged or slightly decreased.