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 nitrate 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 to fertilizer was evident either in bole growth or leaves in year 2, but in year 3, fertilization caused a 14% increase in leaf size (upper terminal 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% increase in L. Biomass response to nitrogen, reported in an earlier paper, 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 in 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 aboveground woody biomass versus L revealed a curvilinear relationship described by the polynomial model y = 6.92x - 0.18x(2) (r(2) = 0.963), where y is the current annual biomass production in Mg.ha-1.year-1 and x = L. Calculation of the extinction coefficient (k) in the Beer-Lambert equation indicated decreased efficiency of the canopies in capturing light in year 4 (k = 0.48) compared with year 3 (k = 0.63). Fertilization had no effect on k in either year. Efficiency of the leaf area in producing 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-values, 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 from 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. The average for L on control plots was higher in year 4 than it was on fertilized plots in year 3. Also contributing to the limited response to nitrogen in year 4 were lower leaf area efficiencies at the high levels of L on the fertilized plots in year 4.