Litter decomposition in cut and uncut western juniper woodlands

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Abstract

The expansion of western juniper (Juniperus occidentalis ssp. occidentalis Hook.) in the northern Great Basin has resulted in management efforts to reduce juniper by prescribed fire or tree cutting to restore shrub-grassland plant communities. Herbaceous succession following juniper cutting or prescribed fire has been well documented, however, impacts of these disturbances to litter and nutrient cycling is limited in these invasive semi-arid woodlands. This study evaluated the effects of cutting juniper trees on leaf litter decomposition and nitrogen (N) dynamics over a 2-year period in eastern Oregon, USA. Litter bags were used to measure juniper leaf litter decomposition and track litter carbon (C) and N fluxes in cut and uncut juniper woodland treatments. Litter mass loss was 37% greater in the cut treatment compared to the uncut treatment after 2 years. Greater litter inputs, higher litter fall quality, and micro-environmental differences were suggested to have been the main causes for the higher litter decomposition rate in the cut treatment. In both treatments, litter N was released by the second year of decomposition, though N release was greater in the uncut treatment. Retention of juniper debris on site permits storage and release of nutrients through decomposition processes which is likely important in maintaining site productivity.

Introduction

The expansion of western juniper (Juniperus occidentalis ssp. occidentalis Hook.) and other pinyon-juniper woodlands is of major ecological importance in the western United States (Miller et al., 2005; Van Auken, 2000; West, 1984). Western juniper has increased 9-fold the past 130 years and encompasses an estimated 3.2  million ha in eastern Oregon, southwestern Idaho, and along the northern border of California and Nevada. Woodland dominance reduces productivity and diversity of shrub-steppe communities (Bates et al., 2005; Miller et al., 2005; West 1984), alters the cycling and distribution of soil and litter nutrients (Bates et al., 2002; Doescher et al., 1987; Tiedemann, 1987; Tiedemann and Klemmedson, 1995), and increases soil erosion and runoff (Buckhouse and Mattison, 1980; Miller et al., 2005). The desire by land managers to restore or maintain shrub-steppe grasslands has resulted in large-scale efforts to reduce the expansion of western juniper and other conifer species throughout the western United States. Though plant community succession following juniper cutting or prescribed fire has been well documented (Bates et al., 1998, Bates et al., 2005, Bates et al., 2006; Miller et al., 2005), knowledge of other ecological processes, particularly litter and nutrient cycling is limited in invasive semi-arid woodlands. It is important to evaluate these processes as woodland expansion and associated control measures impact ecosystem carbon and nutrient dynamics (Jackson et al., 2002; Schlesinger et al., 1990).

As woodlands age, nitrogen (N), other nutrients, and carbon (C) accumulate in tree biomass, litter mats, and canopy influenced soils (Doescher et al., 1987; Klemmedson and Tiedemann, 2000; Tiedemann and Klemmedson, 1995, Tiedemann and Klemmedson, 2000). Surface litter shifts from a composition of herbaceous and shrub detritus to primarily juniper leaf litter. The buildup of litter is characteristic of juniper and other semi-arid woodland expansions indicating increased litter residence times and potentially slower release of plant available nutrients compared to sagebrush steppe grassland (Tiedemann, 1987; Young et al., 1984). Roberts and Jones (2000) measured lower N mineralization and available N fractions in soils under juniper than under grass and sagebrush in central Oregon.

Cutting of juniper woodlands is a common management practice to reduce tree dominance. Because of the limited commercial value attached to juniper in these remote locations, leaving cut trees and debris on site is a common practice and has similarities to forest harvesting which leave substantial amounts of slash. The effects of overstory removal to litter decomposition and nutrient cycling in coniferous forests and woodlands have not produced consistent results among field studies. In boreal and sub-alpine forests litter decomposition and nutrient release may not change or actually decrease following cutting (Feller et al., 2000; Gurlevik et al., 2003; Palviainen et al., 2004; Prescott et al., 2000, Prescott et al., 2003). In drier coniferous systems, which are often water limited, there is a tendency for litter decomposition rates and nutrient release to increase in response to overstory cutting (Fahey, 1983; Hart et al., 1992; Klemmedson et al., 1985). In dry conifer forests, increased decomposition may be a result of improved environmental conditions, such as increased soil and litter water content (De Santo et al., 1993). Bates et al. (2000) measured higher soil water content throughout the growing season after cutting western juniper.

In this study, we examined juniper leaf litter mass loss, litter decomposition rates, and litter C and N dynamics in cut and uncut western juniper woodland in southeast Oregon over a 2-year period. Because the cut junipers were left on site, dead trees provided a continual source of litter fall that was expected to be greater and of higher quality than uncut woodlands. Additions of fresh litter of higher quality increases decomposition rates of older litters (Dalenburg and Jager, 1981; Melillo et al., 1982; Sorensen, 1974). Thus, the decomposition of juniper leaf litter was expected to increase following cutting as a result of greater deposition of higher quality litter and a more favorable micro-environment. Nitrogen in conifer litters often increases in early stages of decomposition and may take longer than 2 years for N to be released (Klemmedson et al., 1985; Yavitt and Fahey, 1986). Therefore, we expected N to accumulate in leaf litter of both cut and uncut juniper treatments.

Section snippets

Site description and experimental design

The study was located on Steens Mountain in southeast Oregon, approximately 9.5 km south of the town of Diamond (118°37′W, 47°55′N). The site was on a west-facing slope at an elevation of 1500 m and was dominated by an 80-year-old western juniper woodland. Prior to juniper dominance, the site was a basin big sagebrush/Thurber's needlegrass association (Artemisia tridentata spp. tridentata Nutt./Stipa thurberiana Piper). Juniper canopy cover was about 24% and the density of mature trees averaged

Litter decomposition

Leaf litter mass loss was significantly greater in the cut treatment than in the uncut treatment on all measurement dates following litter bag placement (Fig. 1; Table 1). There was a significant time by treatment interaction resulting from increasing differences in remaining leaf litter mass between cut and uncut treatments during the course of the study. By the end of the study, leaf litter k values were 1.7 times more negative in the cut than the uncut treatment (Table 1). Mean residence

Litter decomposition and litter fall

After 2 years of in situ exposure leaf litter mass loss was greater in the cut juniper treatment compared to the uncut treatment. This result was expected as litter decomposition rates frequently increase following overstory cutting in dry coniferous forests (Fahey, 1983; Hart et al., 1992; Klemmedson et al., 1985). We attributed the increase in juniper leaf litter decomposition in the cut treatment to greater inputs of higher quality litter (e.g. lower C/N ratio) than the uncut treatment,

Conclusions

The results were useful in comparing short-term juniper leaf litter decomposition and N dynamics between the two treatments. However, because of the study's short-term nature and slow decomposition rates of juniper leaf litter, results were of limited value in determining impacts of the cut treatment to site fertility. A difficulty common in short-term decomposition studies is that results may not be good predictors of long-term litter mass and nutrient dynamics (Prescott, 2005). For instance,

Acknowledgments

The authors are grateful to Otley Brothers, Inc. for providing the property on which the study was conducted. Thanks are due to Stephan Hart and Steve Griffith for their comments and suggestions on an earlier draft of the manuscript.

References (53)

  • J. Bates et al.

    Understory dynamics in cut and uncut western juniper woodlands

    Journal of Range Management

    (2000)
  • B. Berg et al.

    Nitrogen immobilization in decomposing needle litter at variable carbon:nitrogen ratios

    Ecology

    (1983)
  • K.L. Bocock et al.

    The disappearance of leaf litter under different woodland conditions

    Plant and Soil

    (1957)
  • J.C. Buckhouse et al.

    Potential soil erosion of selected habitat types in the high desert region of central Oregon

    Journal of Range Management

    (1980)
  • P.L. Comaner et al.

    Decomposition of plant litter in two western North American deserts

  • J.W. Dalenburg et al.

    Priming effects of small glucose additions to 14C-labelled soil

    Soil Biology and Biochemistry

    (1981)
  • P.S. Doescher et al.

    Evaluation of soil nutrients, pH, and organic matter in rangelands dominated by western juniper

    Northwest Science

    (1987)
  • R.A. Evans et al.

    Plant succession following control of western juniper (Juniperus occidentalis) with Picloram

    Weed Science

    (1985)
  • T.J. Fahey

    Nutrient dynamics of aboveground detritus in lodgepole pine (Pinus contorta) ecosystems, southeastern Wyoming

    Ecological Monographs

    (1983)
  • M.C. Feller et al.

    Influence of forest harvesting intensity on nutrient leaching through soil in southwestern British Columbia

    Journal of Sustainable Forestry

    (2000)
  • C.L. Goodale et al.

    Uncertain sinks in the shrubs

    Nature

    (2002)
  • N. Gurlevik et al.

    The effects of vegetation control and fertilization on net nutrient release from decomposing loblolly pine needles

    Canadian Journal of Forest Research

    (2003)
  • S.C. Hart et al.

    Decomposition and nutrient dynamics of ponderosa pine needles in a Mediterranean-type climate

    Canadian Journal of Forest Research

    (1992)
  • R.B. Jackson et al.

    Ecosystem carbon loss with woody plant invasion of grasslands

    Nature

    (2002)
  • P.E. Jansson et al.

    Temporal variation in relation to simulated soil climate: long-term decomposition in a Scots pine forest

    Canadian Journal of Botany

    (1985)
  • J.O. Klemmedson et al.

    Influence of western juniper development on distribution of soil and organic layer nutrients

    Northwest Science

    (2000)
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