ANNUAL VARIATION IN NEEDLE FALL OF A LOBLOLLY-PINE STAND IN RELATION TO CLIMATE AND STAND DENSITY

A 10-year-old stand of loblolly pine (Pinus taeda L. ) was thinned to three residual basal area levels: 7.8 m2 ha-1, 12.6 m2 ha-1, and 26.6 m2 ha-1 (unthinned). Monthly temperature, rainfall and needle fall were determined for 5 consecutive years following thinning. The amount of needle fall produced each year was positively related to the amount of basal area on the plot. However, at a given basal area a wide range of needle-fall biomass was observed over the 5 year period. Much of the variation in needle-fall patterns appeared to be correlated with the droughtiness of the growing season. In loblolly pine stands needle fall represents the death of the total needle population formed in the previous year. The amount of needle fall that occurred in a given year varied with the rainfall and temperature conditions that existed in the year the foliage was formed. Average annual needle fall varied by more than 29% from year to year for the unthinned control plots. Maximum monthly needle fall occurred 2 months earlier in dry years than in wet years. These results indicate that accurate predictions of the amount of canopy needle biomass must account for both the effects of the previous year's climate on needle production and the effect of the current year's climate on needle duration. Currently, mechanistic models being used to predict annual net carbon gain, stand productivity or annual evapotranspiration do not adequately consider the interactive effects of climate and stand density on needle biomass dynamics.