Climate factors promoting intra-annual density fluctuations in Aleppo pine (Pinus halepensis) from semiarid sites

Research was carried out on the wood formation process and the intra-annual density fluctuations (IADFs) in Mediterranean Aleppo pine (Pinus halepensis Mill.) trees from the coastal site of Guardamar, and inland Maigmo in south-eastern Spain. Samples taken at breast height of the trees were inspected to define the frequency of tree rings with latewood-like cells in earlywood (E-ring) and with earlywood-like cells in latewood (L-ring). L-rings were generally more frequent, especially on the warmer, dryer, coastal site of Guardamar. Dendrochronological techniques using tree-ring features vs. climate showed that L-rings were formed in Guardamar when August and/or September precipitation was higher than normal and in Maigmo when August precipitation was higher than normal. The formation of E-rings was promoted when winter and early spring were warmer than long term average. By studying intra-annual tree-ring formation at a cellular level, we found that at Maigmo in 2004 (MAI2004) all monitored trees presented a normal ring with normal earlywood and latewood and gradual transition between both. At Guardamar in 2005 (GUA2005) a typical L-ring was formed in the majority of monitored trees. In GUA2005 the wood formation started before February and the transition from early- to latewood occurred in late spring. In summer, the cambial cell production occurred at a very low rate, but an increased production of xylem cells took place in September when the amount of precipitation was twice as high as the long term average. During this period, a band of earlywood-like cells was formed, followed by the production of latewood-like cells that continued until the end of December. The normal ring in MAI2004 was formed because climatic factors triggering IADF did not occur (no precipitation in August 2004). After a modest cell production of cambium in September, only few latewood-like cells were produced. According to the strong agreement of results obtained from studying long-term dendrochronological series and intra-annual information on wood formation, such combined study indicates a high potential for use in explaining the environmental signals registered by a tree during different phases of wood formation.