Forest understory biomass heterogeneity is "moisture complex" or associated litter the cause?

Understory biomass heterogeneity underPlatanus occidentalis L.,Quercus alba L.,Celtis occidentalis Pursh., andUlmus americana L. was studied. Soil pH, Ca, Mg, Mn, Cu, NH4 nitrogen, total N, and soil moisture were significantly different under all dominant species. The pattern of soil moisture levels under different tree species was not consistent from one sampling period to the next. The aboveground biomass of understory vegetation also varied significantly under different tree species and was not correlated with variation in any of the soil properties. Maximum understory biomass gain (340%) from May to September occurred under oak trees, where soil moisture and most nutrients were the lowest. On the other hand, sycamore and hackberry had continuous release of allelochemics and the smallest understory biomass gain (103%) during the growing season, even though soil under both species had more moisture and nutrients than the soils under white oak. Eliminating all the factors studied leads to the conclusion that organic substances released in the immediate environment of dominant trees and their litter influenced soils and associated herb growth. In each dominant niche system, understory species may have to develop their own system ("microniche") to minimize the stresses of competition and allelochemics.