Tree age determination in the widespread woody plant invader Pittosporum undulatum

Abstract

Pittosporum undulatum Ventenat (Pittosporaceae) is an invasive tree of forests and woodlands in several regions of the world. In the Azores archipelago it occupies about 30% of the forested areas and efforts to valorize its biomass are underway. Implementing sustainable forest management requires basic information on tree age and growth rate, yet this information was lacking. We used tree-ring analysis to obtain accurate age estimates and growth rates from a total of 982 live trees (1318 samples: 646 increment cores and 336 pairs of stem discs) that were sampled in São Miguel, Terceira and Graciosa islands. Pittosporum undulatum showed distinct annual growth rings with an increase in cell-wall thickness and a decrease in cell lumen of the Fibers, from early to latewood. We found trees with ages at base and at breast height ranging from 5/4 to 131/108years, respectively. Samples taken at tree base and at breast height allowed to estimate a diameter growth rate of 0.38cm.year−1, with an average of 8years needed to attain breast height, and a rotation period of 26years to reach the average diameter of 7cm presently found at the stands. Allometric equations relating tree age and diameter at breast height, tree height, basal area and biomass were found to be significant. Among the models with lowest AIC, equations predicting tree age from diameter at breast height or from biomass showed the best fit (R2=0.65 and for R2=0.70, respectively). The developed methods and the obtained results can now be used to support management of biomass valorization projects but also to better understand invasion dynamics at conservation devoted areas.