EFFECTS OF NITROGEN-FERTILIZATION ON LEAF-AREA, LIGHT INTERCEPTION, AND PRODUCTIVITY OF SHORT-ROTATION POPULUS-TRICHOCARPA X POPULUS-DELTOIDES HYBRIDS

Nitrogen treatment consisted of spring applications of ammonium nitrat e at the rate of 167 kg N.ha-1.year-1 repeated for 3 years starting in the 2nd year after the planting of six Populus hybrids. No response t o fertilizer was evident either in bole growth or leaves in year 2, bu t in year 3, fertilization caused a 14% increase in leaf size (upper t erminal leaves), a 54% increase in leaf-fall mass, and a 58% increase in leaf area index (L, M2.M-2). Leaf and canopy responses were not as large in year four, i.e., a 32% increase in leaf-fall mass and a 31% i ncrease in L. Biomass response to nitrogen, reported in an earlier pap er, was a 24% increase in dry weight at the end of 4 years. This total was the result of a very large and significant dry weight response to nitrogen in year 3 (about 40%), and a small, nonsignificant response of about 8% in year 4. Leaf-fall means ranged from 3.3 Mg.ha-1.year-1 in year 3 on control plots to 5.8 Mg.ha-1.year-1 on fertilized plots i n year 4. Means for L ranged from 3.8 on control plots in year 3 to 8. 5 on fertilized plots in year 4. Plotting annual production of abovegr ound woody biomass versus L revealed a curvilinear relationship descri bed by the polynomial model y = 6.92x - 0.18x2 (r2 = 0.963), where y i s the current annual biomass production in Mg.ha-1.year-1 and x = L. C alculation of the extinction coefficient (k) in the Beer-Lambert equat ion indicated decreased efficiency of the canopies in capturing light in year 4 (k = 0.48) compared with year 3 (k = 0.63). Fertilization ha d no effect on k in either year. Efficiency of the leaf area in produc ing biomass per unit of L (E) was lower (though not significantly) on fertilized plots than on control plots (E = 5.3 vs. 5.8 Mg.ha-1.year-1 .L-1, respectively). In contrast with the indications from the k-value s, E appeared to increase in year 4 compared with year 3 (5.8 vs. 5.4 Mg.ha-1.year-1.L-1, respectively), a contradiction that could arise fr om a change in carbon allocation favoring aboveground over belowground tissues. The limited response of woody biomass to nitrogen in year 4 appears to be a consequence of the large increase in canopy density (L ) on the control plots in that year compared with fertilized plots. Th e average for L on control plots was higher in year 4 than it was on f ertilized plots in year 3. Also contributing to the limited response t o nitrogen in year 4 were lower leaf area efficiencies at the high lev els of L on the fertilized plots in year 4.