Water relations: Soil fertility, and plant nutrient composition of a pygmy oak ecosystem

Abstract

The water and nutrient relations of Buzzard's Roost, an unusual pygmy oak ecosystem in southwest Missouri, USA, were studied in 1976 and 1977 in an attempt both to characterize the ecophysiology of the individuals and find the causative agents for the existence of the pygmy forest. The stunted trees were mostly blackjack oak (Quercus marilandica) with some northern red oak (Q. rubra) and a few black oak (Q. velutina). In addition to being stunted, the trees were gnarled and twisted and had miniature leaves and acorns. Results of soil analysis showed the pygmy forest soil to be very acidic (pHw = 4.6), with very low levels of Ca and Mg (0.25 and 0.12 meq/100 g, respectively) and very high levels of Al (600 ppm). Adjacent non-pygmy forest soils did not display these characteristics. Foliar analysis of blackjack, northern red, and black oak showed Ca and Mg to be much lower in pygmy foliage than in non-pygmy foliage, at three times during the growing season. Diurnal and seasonal patterns of xylem pressure potential, leaf conductance, and soil moisture content illustrated the development of very severe tree water deficits at Buzzard's Roost. Predawn and midday xylem pressure potentials declined to as low as -3610 and -4200 kPa, respectively, accompanied by complete daytime stomatal closure. However, water stresses in nearby non-pygmy oaks were greater than at two of three pygmy oak sites, implying that water stress was not the major factor or causative agent in the stunting process. The evidence suggests that the very low nutrient levels in the soil, especially of Ca and Mg, plus the very high levels of Al, may be deficient (and/or toxic) for normal tree growth and development, and possibly responsible for the existence of the pygmy oak forest. Also, the xeric site characteristics add another severe stress to this ecosystem.