Relationships between nitrogen uptake, leaf area, water status and growth in an 11-year-old Pinus radiata plantation in response to thinning, thinning residue, and nitrogen fertiliser

Abstract

Nitrogen and water status were manipulated experimentally by thinning, residue management and application of N and P fertiliser. The experiment was thinned in November 1988, residue treatments were installed in December 1988 and fertiliser was applied in September 1989. Basal area increment (m 2 ha −1) increased in response to thinning, the quantity of residue retained, and with the rate of N fertiliser. There was no response to P nor any N×P interaction. Nitrogen uptake in 1989–1990 was highly correlated with tree growth in thinned treatments over the next 3 years. The positive growth response to N uptake was mediated primarily through increased leaf area index (LAI). Thus LAI in 1991 was highly correlated with N uptake in 1989–1990 and with subsequent growth. Growth per unit N uptake or per unit leaf area were much lower in the unthinned treatment where predawn needle water potentials ( Ψ) were consistently more negative than the thinned treatments. Combining Ψ with N uptake or LAI in a simple linear model explained 96% and 94%, respectively of the variation in tree growth for all treatments. Annual growth increments in thinned treatments not fertilised with N were correlated with current N uptake. In N fertilised treatments, growth was highly correlated with N uptake in the year after fertiliser application but was then independent of subsequent N uptake. Foliar N concentrations in June 1990 were highly correlated with N uptake and subsequent growth in thinned treatments. However, treatment concentration differences declined with time while differences in growth remained due to differences in LAI. Consequently, the correlation between foliar N concentration and growth was substantially reduced. An index of canopy N content (LAI×foliar N concentration) was no better correlated with growth than LAI alone. Litter N concentrations (maxima, minima or mean annual weighted) in thinned treatments were correlated with tree growth in the subsequent year. Regression coefficients were initially low but increased up to a maximum of 0.90 for growth in 1990–1993. Stem growth in both thinned and unthinned treatments could be explained by combining foliar or litter N concentrations with Ψ in a simple linear relationship. Litter N concentrations could not be used to estimate N uptake.