Leaf biomass in thinned yellow-poplar (Liriodendron tulipifera L.) stands in relation to stemwood growth and other stand characteristics

Abstract

Collections of leaf and overstory litter were made in 19 yellow-poplar stands in the Southern Appalachians during 1966, 1967, and 1968. The stands represented a broad range of stand age and site index. All stands were thinned to prescribed densities 4 to 6 years prior to leaf trapping. The quarter-acre plots were selected from 141 plots established to study growth and yield of yellow-poplar. Complete stand and growth data were available for the first 5 years of the larger study, and stand measurements were made at the end of 1966, 1967, and 1968.

Ovendry leaf biomass averaged annually 1,937 pounds per acre ranging from 1,167 to 2,530 pounds per acre, Total overstory litter production, which included leaves and all non-leaf material, averaged 2,574 pounds per acre, with a range from 1,427 to 3,351 pounds per acre.

Both leaf biomass and total overstory litter were significantly related to stand density. Density was expressed as basal area per acre for trees 4.5 inches d.b.h. and larger. Leaf and litter amounts increased linearly with increasing stand density from approximately 40 to 140 square feet per acre. Stand density accounted for 54 percent of the variation in leaf biomass and 43 percent of the variation in total production of litter. No other stand variable was significantly related to leaf and litter production.

The maximum leaf biomass for one year exceeded the minimum by only 4.2 percent. The constancy of the leaf biomass suggested that leaf mass recovery was complete before leaf sampling, or that leaf mass recovery was extremely slow. The stands seemed well buffered against changes in variable site factors.

The relationship of stemwood growth to leaf biomass and site and stand factors showed that leaf biomass accounted for 67 percent of the variation in annual stemwood growth. An additional 27 percent was accounted for by specific leaf weight and duration of leaf retention. Long leaf retention and thin leaves (low specific leaf weight) resulted in high stemwood growth rates and tended to occur on high site index plots. A regression of stemwood growth on leaf biomass and site index revealed that site index accounted for only 15 percent of the stemwood growth variation.

A leaf variable, the product of leaf area index and leaf retention time, proved to be a good estimator of annual stemwood growth. The LAI-retention variable accounted for 89 percent of the variation in stemwood growth.

Leaf efficiency (cubic feet of wood per acre per year per unit of leaf) increased with increase of leaves, but at a decreasing rate. The average leaf efficiency of 81.7 cubic feet per acre per year per 1,000 pounds of ovendry leaves was high when compared to leaf efficiency in many of the world's forest types.

Variables associated with site utilization apparently had much more influence on stemwood growth than variables associated with site quality in these even-aged stands of yellow-poplar. Site quality was relatively high, making leaf biomass and stand density the key factors influencing stemwood volume production.