APAR is a better predictor than LUE of the stem growth differences found between Loblolly pine grown in the United State and Brazil

Abstract

Pinus taeda is a species native to southeastern regions of the United State (USA), but plantations with the highest productivity are found in the southern region of Brazil (BRA). The objective of our study was to determine the higher (Light Use Efficiency) LUE and absorbed photosynthetically active radiation (APAR) of P. taeda genotypes with different spacings in order to evaluate potential strategies that could increase the productivity of intensively managed stands in Brazil and the United States. The experiment was designed as a split-split-plot, and evaluations included 81 trees in BRA and 63 trees in the USA, two genotypes (clone C3 vs. open pollination (OP)) and two spacings (BRA – 2.4 × 6.8 m, 613 trees/ha; USA – 3.66 × 4.42 m, 618 trees/ha) and narrow spacing (BRA – 2.4 × 2.2 m, 1894 trees/ha; USA – 3.66 × 1.47 m, 1853 trees/ha). Individual tree calculations of LUE were made at both locations for each tree by dividing its current annual stem increment (WNPPi) by APAR estimated from MAESTRA, a process-based model. The parameterization of this model included the use of forest inventory data, meteorological data, crown characteristics (average crown radius, height, diameter, and leaf area), leaf area density distribution, leaf transmittance, and leaf and soil reflectance. Results showed that trees growing in Brazil had greater leaf area (32%) and stem biomass growth (5%) and absorbed more light (49%) than trees in the USA. The genotype C3 was more efficient than OP in light use at both locations. Narrow spacing resulted in higher LUE values (0,8 g MJ^-1). APAR explain 80% and 65% of WNPP in BRA and USA, respective. Our results suggest that LUE explained the growth differences between sites, genotypes, and spacings, while APAR provided a better differentiation of WNPP between the sites.