Litter decomposition in cut and uncut western juniper woodlands☆
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.
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2018, Forest Ecology and ManagementCitation Excerpt :Intact aspen stands have been noted to support lower soil pH and lower amounts of salts, lime, and sulfate, and greater amounts of magnesium, iron, manganese, and copper than aspen woodlands dominated by juniper (Wall et al., 2001). Aspen produces higher amounts of litter annually than conifers, and aspen litter decomposes at a faster rate than conifer litter, especially western juniper (Bartos and DeByle, 1981; Bates et al., 2007). Native species dynamics varied among the treatments.
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2017, Forest Ecology and ManagementCitation Excerpt :Microbial N immobilization tends to occur with inputs of litter with high C/N ratios (Davidson et al., 1992), which is characteristic of juniper leaf litter (Bates et al., 2007). In treated woodlands and forests it may require 2 to 3 years before litter N is released into soils (Bates et al., 2007; Hart et al., 1992; Klemmedson et al., 1985; Yavitt and Fahey, 1986). The later increase in inorganic N levels in CUT debris soils may result from lower plant N uptake because herbaceous cover was lower in this treatment zone (both sites) than debris zones of the other treatments and the interspace (Bates and Davies, 2016).
Soil water dynamics and water use in a western juniper (Juniperus occidentalis) woodland
2014, Journal of Arid EnvironmentsCitation Excerpt :Once juniper was removed, water use in the understory increased to about 70–80% of the seasonal water use estimated for juniper stands, while total cover increased from 22% to 27%. Other studies report similar positive effects of juniper removal on herbaceous cover and production rates (Vaitkus and Eddleman, 1987; Bates et al., 2005, 2007) as well as prolonged seasonal moisture availability which, in turn, enhanced fall herbaceous growth (Bates et al., 2000; DeBoodt, 2008). Eddleman (2002) found that removing juniper competition had a positive long-term effect on shrubs.
Nutrient dynamics during decomposition of the residues from a sown legume or ruderal plant cover in an olive oil orchard
2014, Agriculture, Ecosystems and EnvironmentCitation Excerpt :The increases in N concentration may have been driven by mechanisms such as microbial immobilization of N (Koeing and Cochran, 1994), fixation, absorption of atmospheric ammonia, leaching, dust, green litter, fungal translocation and/or immobilization (Melillo et al., 1982). When N is a limiting factor during litter decomposition, microbes and fungi immobilize N, even from the surrounding litter substrates (Bates et al., 2007). In our case, N immobilization was found only for cover crop residues which were incubated on the soil surface, but not in the buried residues.
Fire in a sub-humid woodland: The balance of carbon sequestration and habitat conservation
2012, Forest Ecology and ManagementCitation Excerpt :Higher N in the mature oak-dominated woodlands is likely due to annual contribution of low-lignin litter from these deciduous oak species. In contrast, high lignin content in juniper leaves may have increased litter residence time (Bates et al., 2007; Tiedemann and Klemmedson, 1995), which decreases N availability similar to other juniper-dominated sites (Roberts and Jones, 2000). Soils C and N are also dependent upon fire intensity.
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The Eastern Oregon Agricultural Research Center, including the Burns and Union Stations, is jointly funded by the Oregon Agricultural Experiment Station and USDA-Agricultural Research Service.