Effects of stand development and weather on monthly leaf biomass dynamics of a loblolly pine ( Pinus taeda L.) stand

Abstract

Annual leaf biomass production, monthly needle accretion and monthly needlefall were measured in an 11-to 17-year-old thinned stand of loblolly pine. Initial thinning levels were 7.8 m 2 ha −1, 12.6 m 2 ha −1, and 25.5 m 2 ha −1 (unthinned). A light thinning was done again at Age 14. Annual variations in annual leaf biomass production and monthly variations in monthly needle accretion and needlefall were related to measured stand and weather variables. Age variations in annual leaf biomass production occurred over the 6 year study period. The variation in annual leaf biomass production was best quantified as a quadratic function of stand basal area and average weighted temperature for the months of June, July, August and September. Although stand basal area was the major determinant of annual leaf biomass production, an increase in average temperature from 24.5 to 26.5°C resulted in a 27% reduction in annual leaf biomass production. This was translated to an approximate reduction of 7.3 m 2 ha −1 year −1 of stemwood. Monthly needle accretion varied little between years or with stand density. Thus, a single normalized logistic function was suitable for describing monthly needle accretion for all 6 years. Monthly needlefall was variable from year to year. Variation in needlefall was low for a period of 7 months (January 16–August 15). During this period monthly needlefall averaged from 3 to 8% of the previous year's annual leaf biomass production at the beginning of the phenological year. Variation in this 7 month period was not consistently related to stand density or any of the weather variables considered in this study. Monthly needlefall from August 16 to January 15 was extremely variable. This variability was not related to stand density. The weather variable that explained most of the monthly variation in needlefall during this period was the average rain-potential evapotranspiration determined for the 2 months preceding a monthly needlefall event. Peak needlefall was found to occur 2 months earlier in a drought year than in a year when rain-potential evapotranspiration was high.