The number of days on which increment occurs is the primary determinant of annual ring width in Callitris intratropica

KEY MESSAGE : The number of days on which a measureable increment occurred, and the average rate of stem growth, rather than the overall duration of the wet season, were the main determinants of ring width in young Callitris intratropica trees. These effects were amplified by competition. Dendroclimatology of tropical tree species is an important tool for understanding past climatic variability at low latitudes where long-term weather records are often absent. Despite the growing number of published tropical tree-ring chronologies, however, still little is known of the factors that control annual ring formation in tropical tree species. In this paper we used an endemic Australian conifer, Callitris intratropica, to study the intra-annual dynamics of seasonal growth and xylem formation, and the effects of environmental conditions and competition, on growth ring formation. We combined high-resolution growth and climate monitoring (every 15� min for 2� years) with less frequent cambial sampling. Trees exhibited marked reductions in growth during certain periods within the rainy season when rainfall was not as regular and VPD was high. Overall, we found that ring width was most influenced by the number of days when increment occurred; regardless of how early the growing season began or ended, and by the rates of tracheid production. The effect of competition was also important. Trees growing in dense groves had narrower annual rings (4.6� mm) than trees that were growing in the open (6.7� mm), due to less active cambia, slower rates of xylem production and expansion and more increment days, although the overall growing season duration was also shorter in grove trees