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Dendroecological approaches for investigating the climate-driven decline and mortality in oak forests of marginal protected areas.
Dendroecological approaches for investigating the climate-driven decline and mortality in oak forests of marginal protected areas.
Trees and shrubs form annual growth rings that potentially register changes in climate variability, representing a valuable natural archive to track their past and current performance. In addition, growth trajectories can reflect changes in vitality and productivity, although they are strongly influenced by tree ontogeny and phenological adjustments, as well as by stand density and competition.
Dendroecological analyses are a highly effective tool for conducting retrospective analyses of the phenomenon of decline, using tree growth trends. Tree rings can be used as multi-proxy archives able to provide different levels of climatic (from short-term extreme events to seasonal-annual averages) and physiological information.
In laboratory the procedures and methods are defined for the application of the dendrochronological technique and for functional wood anatomy (phenotyping) to investigate the climatic stress effects on dieback of oak species and to detect long-term responses between D and ND trees.
Through a comparative analysis within each pair, differences in growth rates between non-declining and declining trees at each study site are evaluated to reveal the onset of decline and the year of “no return”, i.e. the year from which the divergence in growth between individuals of the two vigour classes is always statistically significant.
Results from BAI trajectories indicate that the mean growth rate of the last 30 years was significantly higher in ND than in D trees.
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Through this innovative sampling design and approach, it is possible to compare growth within pairs and quantify the decrease in growth in declining individuals, thus identifying a threshold beyond which the relative decrease in growth does not seem to be justifiable solely based on age trends and could represent a phase of decline.
Increases in drought–growth coupling, overall loss of resilience, and negative growth trajectories can be considered early warning signals of forest dieback.
The presented framework is a fundamental basis for developing indices to estimate drought-induced vulnerability of trees based on radial growth trajectories and the response of growth variability to climatic drought indices.
XLA Petiole Index
Innovative physiological indices based on quantitative wood anatomy for determining responses to drought stress effects on dieback of oak species.
In studies examining drought-induced tree mortality episodes, is useful to employ a comprehensive approach, combining ecophysiological and dendro-anatomical analyses to compare non-declining (ND) and declining (D) coexisting trees.
Recent advancements in understanding the relationship between petiole xylem anatomy and leaf form and function have revealed a positive correlation between petiole vessel diameter and leaf size, both within and across species.
Leaf petioles, serving as the single entry point for water into the leaf venation system, offer a standardized basis for comparing xylem investment with downstream transpirational demands. To quantify this relationship, a novel index derived from petiole quantitative wood anatomy is used to interpret the physiological responses of decay of deciduous oak in marginal protected areas.
Specifically, a new index named the XLA_petiole index, introduces an integrative trait for characterizing leaf water transport function measured as the cross‐sectional Xylem Area (XA) at the petiole divided by the downstream leaf area (LA). XLA petiole could be correlated with theoretical petiole xylem conductivity (Ks_petiole) and strongly negatively correlated with leaf cavitation vulnerability.
To measure various functional traits of the xylem of the petioles, quantitative wood anatomy is used to calculate variables such as leaf area (LA), xylem area (Ax), hydraulic diameter (Dh), theoretical (Kh) and specific (Ks) hydraulic conductivity of the petiole xylem, leaf specific conductivity (LSC), vulnerability index (VI) and vessel frequency (VF).
Results from this approach show that the hydraulic diameter (Dh) and specific hydraulic conductivity (Ks) are significantly higher in non-declining trees than in declining trees, reflecting the more efficient hydraulic transport capacity of ND trees compared to D trees. Declining trees showed higher leaf area and xylem area than in ND trees. As a result, the XLA index is significantly higher in D trees than in ND.
This shows that ND trees are potentially more efficient at supplying water to the leaves, probably due to better stomatal control and water use efficiency, mitigating the risk of xylem embolism, which causes hydraulic failure.
The assessment of XLA petiole variation can provide evidence supporting a safety-efficiency trade-off in oak leaves, a crucial aspect of plant hydraulic strategy. In the future, the XLA index could serve as an additional variable to include in predictive and interpretative models to characterize the decline of Mediterranean oak trees.
Consequently, the results obtained using this approach represent an effective tool to monitor the health status of oak trees and predict their risk of decline.
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