Date of earlywood-latewood transition in provenances and families of loblolly pine, and its relationship to growth phenology and juvenile wood specific gravity

When grown together in plantations, fast-growing southern and coastal sources of loblolly pine (Pinus taeda L.) often have lower wood specific gravity than northern and inland sources. This study investigated whether this phenomenon could be explained by a later transition to latewood, associated with a longer period of height growth, of the fast-growing sources. Seven to nine open-pollinated families, from each of four provenances, were grown at two locations in southwest Georgia. Tree cambia were wounded with a needle during summer and fall of the fifth and sixth growing seasons (1993 and 1994). The wounding was done to leave a mark in the xylem used later to determine whether earlywood or latewood was being produced at the time of wounding. Provenances were significantly different for the date of transition in 1994, with 22 days between the earliest and the latest. For most families, latewood transition followed height growth cessation in 1993, but preceded it in 1994. The date of latewood transition had a strong positive correlation (family mean basis across provenances) with the date of height growth cessation and a moderate negative correlation with specific gravity. Juvenile wood specific gravity had a weak (nonsignificant) negative correlation with annual height increment and a stronger negative correlation, significant in 1993, with diameter increment. Correlations within provenances were weak or close to zero. This study provided evidence for an association (especially at the provenance level) between a later cessation of height growth, a later transition to latewood, and lower specific gravity in 5- and 6-year-old trees.