A novel mathematical procedure to interpret the stem radius variation in olive trees

Stem radius variations result from the fluctuation of environmental factors, mostly temperature trend and water availability, in turn affecting plant water balance, and plant growth. High-resolution analysis of stem radius variation provides insights into the temporal patterns in radial growth and water balance, and their relationship with environmental variables. To test the causal effects of temporal climate fluctuation on stem radius variation, a mathematical procedure was applied to normalize and synchronize radial fluctuations and environmental parameters, whose baseline is largely unexplored. Stem radius variations were continuously monitored during two consecutive years in four saplings field-grown olive tree cultivars (Canino, Cipressino, Leccino, Maurino) in an experimental farm in central Italy, between November 2004 and October 2006, using automatic high-resolution point dendrometers. A derivative analysis approach applied on point dendrometer records was conveniently used to describe stem radius variation and to distinguish the timing of transition from the dormant winter state to the active growth stage and till the slow expansion phase. Stem diameter patterns showed intense shrinkage events suddenly after air temperature drop below 0 °C during winter. The onset of radial growth was delimited by the occurrence of rehydration (beginning of transpiration cycles) and increase of air temperature (end of cold cycles). The course of the growing season was described by patterns of air temperature, reference evapotranspiration, cumulative degree-days, vapour pressure deficit and soil moisture deficit, and correlated to patterns in stem radius cycles. Three phases of stem radius variation were evidenced through the seasonal course: induction signal, growth period, and slow expansion. This approach provides new and objective insights on shrinkage–swelling phenomena in Mediterranean environments, related to dehydration and hydration cycles, which are difficult to detect with empirical treatment of stem radius variation records. The ability to switch quickly between dormancy to growth would enable the olive tree to restart physiological processes and to cope with erratic climatic conditions of the Mediterranean region.