Age and thinning effects on elemental composition of Pinus pinaster and Pinus radiata needles

The information about the impacts of thinning on the availability of micro-nutrients, as well as macro-nutrients other than N, P and K, is still scarce. We assessed the changes in the concentrations of 12 elements (Al, B, Ca, Cu, Fe, K, Mg, Mn, N, Na, P and Zn) with needle age (current year or 1-year-old) and three early thinning treatments in two of the most widely distributed pines in SW Europe: P. pinaster and P. radiata. Four treatments were setup in triplicate: control (C), light thinning (LT), heavy thinning (HT) and selection thinning of dominant trees (ST), with 0%, 10%, 20% and 20% of total basal area removed, respectively. Needle δ15N varied little with needle age and most thinning treatments in both species, but ST triggered an increase of N in P. pinaster needles. Needle Ca and Na increased with age, but were unaffected by treatment. Foliar K, Zn and Cu decreased with age in both species and increased with ST only in P. pinaster. Jointly considering all treatments, there was no needle age effect on Mn concentration, neither in P. radiata nor in P. pinaster, but in the latter species Mn levels increased with age in the selection thinning plots. There were significant thinning effects on Mn levels in both P. pinaster (ST>C) and P. radiata (HT > LT, ST). Foliar Fe and Al concentration increased with age in both pines; the former increased with ST only in P. pinaster while the latter was affected by thinning only in current year needles and without a clear tendency. Neither age nor treatment effects on needle Mg and B were found, while for P needle age had a significant effect only in P. pinaster.