DYNAMICS OF PINUS-RADIATA FOLIAGE IN RELATION TO WATER AND NITROGEN STRESS .2. NEEDLE LOSS AND TEMPORAL CHANGES IN TOTAL FOLIAGE MASS

The pattern of production and fall of needles was measured over a 4 year (1983-1987) period in 10- to 14-year-old stands of Pinus radiata near Canberra, Australia which were subjected to markedly varying degrees of water and N stress. Annual needle loss (death of green needles) was estimated as annual needle fall plus the increment (or minus the decrement) in the mass of dead needles held in the crown between successive winter measurements. Monthly estimates of needle production and loss were used to calculate seasonal (3-month) changes in total live foliage mass between annual winter measurements of foliage biomass. Annual needle fall ranged from about 1.5 to 5.0 t ha-1, and although well correlated with needle loss, the two parameters differed by up to 2 t ha-1 as stands approached canopy closure. In stands closing canopy, 2-3 t ha-1 of dead needles were retained in the crown. Loss in mass of needles due to leaching and decomposition during the period between senescence and fall results in needle fall being an underestimate of the mass of needles senescing in any year. Annual needle loss was positively linearly correlated with total foliage biomass (r2=0.70) or stand basal area (r2=0.75) measured in the previous winter. The average life of needles declined from about 4 years in open stands not suffering severe water stress, to about 2 years after canopy closure or where water stress induced significant loss of foliage. Monthly needle fall varied from less than 100 to greater than 500 kg ha-1 and was positively linearly correlated with a measure of cumulative tree water stress (the water stress integral. S(PSI) during the same month. The correlation was highest (r2=0.68) for fertilised stands which were most water stressed, and declined with declining water stress. S(PSI) was useful as a guide to the timing of needle fall, but not as a predictor of total annual needle fall which was mainly determined by foliage biomass of the stand. Older (mostly more than 2 years old) needles were shed largely in response to water stress. and in most years needle fall peaked in the summer-autumn period. In wet years and in irrigated stands the peak in needle fall was delayed by 3-6 months. The periods of maximum foliage production (spring and early summer) and needle loss (late summer or later) do not coincide for P. radiata. resulting in very large changes in foliage mass during a growing season. Foliage biomass increased by about 2 (control stand) to 6.5 t ha-1 (irrigated and fertilised stands) between the minima at the end of winter and maxima at the end of summer. Reductions of 0.6-3.6 t ha-1 then occurred during the following 6 months because of needle senescence. During the 4 year study, foliage biomass (at the end of winter) increased from about 5 to 11 t ha-1 in the control stand and from 5 to 14 t ha-1 in irrigated-fertilised stands. The rate of increase declined with time. The maximum foliage mass (t ha-1) at the end of summer in 1987 was 12.5 (control), 13.0 (fertilised), 13.6 (irrigated) and 16.8-17.6 (irrigated and fertilised). Water and N availability thus interacted positively to affect both the rate of increase in foliage mass and its maximum. The changes in live foliage biomass within a year were equivalent to up to 3 units of projected leaf area index (LAI). The LAI in winter ranged from about 2 in 1983 to 5.0-6.3 in 1987, with a peak value of about 8 at the end of summer 1987 in the irrigated-fertilised stands. The surface area of live foliage estimated from foliage sampling or by measures of light transmission were positively correlated (r2=0.92). When light transmission methods are used to infer LAI, corrections are needed to allow for the mass of dead needles (up to 2-3 t ha-1) held in crowns that are closing canopy, and for the effects of branches and boles on light interception. In this study, components other than live foliage contributed 10-25% of the surface area of the forest stands.