Seed Reserves in Soils of Successional Stages of Pinyon Woodlands

Abstract

Seedlings emerging from soil, duff and litter samples from four microsites (duff, transition, interspace, shrub) and four successional stages (grass-forb, shrub-tree, tree-shrub, tree) were recorded for 6 weeks. Seed reserves in soils decreased in number and species diversity from early to late successional stages. Of seedlings emerging, 890% were annuals. The shrub microsite from the shrub-tree stage had the highest total and perennial seed reserves. As shrub cover decreased, the transition microsite between the duff and interspace became the most important source of total seed reserves, and the duff microsite became the most important source of perennial seed reserves. INTRODUCTION The temporal and spatial variation in seed reserves in soils is an important component in modeling succession (Major and Pyott, 1966; Livingston and Allessio, 1968; Kellman, 1970). Previous studies found that seed reserves decline from early to late stages of succession (Oosting and Humphreys, 1940; Olmsted and Curtis, 1947; Quick, 1956; Livingston and Allessio, 1968). In southwestern shruband grasslands, seed reserves are concentrated under or around shrubs and grasses, with few seeds in the barren area in between (Knipe and Springfield, 1972; Nelson and Chew, 1977). Seed reserves also vary with soil depth, with most of the reserves concentrated in the top 2-3 cm of soil (Floyd, 1966; Wesson and Wareing, 1969; Childs and Goodall, 1973; Strickler and Edgerton, 1976). The objective of this study was to determine variation in amounts and species of seed reserves in the soils from various stages of pinyon woodland succession. The most commonly identified stages of pinyon-juniper succession are: (1) annual grass-annual forb stage; (2) perennial grass-perennial forb stage; (3) shrub-perennial grass-perennial forb stage; (4) shrub-perennial grass-perennial forb-young tree stage; (5) mature treeshrub stage, and (6) climax tree stage (Arnold et al., 1964; Erdman, 1970; Barney and Frischknecht, 1974; Stager, 1977; Tausch and Tueller, 1977). In addition, distribution of seed reserves among seedbed components of the woodlands was examined. This seedbed consists of a mosaic of shrub litter, pinyon duff, bare ground (interspace) and transition microsites. SITE DESCRIPTION The study was conducted in April 1979 on the N-facing foothill slopes of the Sweetwater Mountains, 0.5 km SE of Walker, California (T. 8 N., R. 23 E., Sec. 27). The study area has a slope of 15 % and an elevation of approximately 1890 m. Average annual precipitation is 250 mm and mean annual temperature is 9.8 C. The soils are well-drained typic torripsamments of granitic parent material. The area is characterized by variations in understory and pinyon (Pinus monophylla) cover resulting from a combination of past use of wood products, livestock grazing, wildfires and disease or insect attacks. Within a recent burn and adjacent woodland area, four sites in different successional stages were identified as grass-forb, shrub-tree, tree-shrub and tree. The grassforb site is located on the 1973 Rock Creek burn. The burn was aerial-seeded in the 1Stationed at the Renewable Resources Center, University of Nevada, Reno 89512.