DYNAMICS OF PINUS-RADIATA FOLIAGE IN RELATION TO WATER AND NITROGEN STRESS .1. NEEDLE PRODUCTION AND PROPERTIES

Measurements of the following Parameters were made over a 4 year period in 10- to 14-year-old stands of Pinus radiata subjected to markedly different degrees of water and nitrogen (N) stress: needle length, weight and specific leaf area (every 2 weeks), foliage biomass production (annually), pre-dawn needle water potential (every 2 weeks) and needle litter N concentration (monthly). Increments in needle length were a useful estimate of increments in needle weight for any given forest treatment and year because there was no consistent variation in weight per unit length of needles as they developed during the growing season. However, for well-illuminated needles, the ratio of weight per unit of needle length showed a large (approximately two-fold) variation attributable to treatment and year of foliage elongation. The ratio was loosely positively correlated with needle length (or the favourability of growing conditions) on non-irrigated plots, and appeared to result largely from increases in needle thickness rather than density. Final needle length, ranging between 40 and 160 mm, depended mainly on the amount of water and N stress experienced by trees during the growing season. The majority (greater than 90%) of needle extension occurred during a 4 month period (October-January) in spring and summer and the pattern of needle growth was affected only by water availability. Needle extension rates were negatively linearly correlated with the water stress integral (S(PSI), a temporal integration of the effects of both water and N availability on needle water potentials) for monthly periods during the growing season. Needle extension was most sensitive' to the S(PSI) in mid-spring (October/November) when needles were elongating rapidly and still less than one-half grown. About 80% of the variation in annual foliage production (3.2-8.5 t ha-1) could be explained in terms of both (a) S(PSI) during the previous summer (when primordia were initiated), and (b) the water and N status of trees concurrent with needle extension. Final needle length and total foliage biomass production in the same year were poorly correlated. The specific leaf area (SLA, all sides) ranged from 10 to 17 m2 kg-1 and was greater for needles formed under low light. Irrigation or fertilisation had only an indirect effect on SLA by hastening canopy closure.