Nitrogen transformations in fallen tree boles and mineral soil of an old-growth forest

Ecology, June, 1999 by Stephen C. Hart

INTRODUCTION

Coarse woody debris (CWD), in the form of standing dead trees, fallen boles, large branches, and roots, is abundant in many forest ecosystems, and plays several important ecological roles within forests (Harmon et al. 1986). These roles include the reduction of soil erosion, reservoirs for nutrient and water storage, seed beds for plant establishment, and habitat for fungi, bacteria, arthropods, and a variety of vertebrates. Coarse woody debris also has been suggested to play a key role in carbon (C) flow and nutrient cycles of many forests, but few studies have explored these ecosystem functions of CWD (Harmon et al. 1994).

The low nutrient concentrations, decomposition rates, and mass of annual inputs of CWD compared to fine litterfall and root turnover have been used as arguments for discounting the significance of CWD in nutrient cycles during short-term periods in stand development (Arthur and Fahey 1990, Harmon and Chen 1991, Harmon et al. 1994). However, some investigators have suggested that, following catastrophic disturbances resulting from blowdown or fire, large amounts of nutrients from the massive new inputs of CWD may become available to the recovering forest, and that the timing of nutrient release may closely match nutrient demand (Harmon and Chen 1991). Furthermore, many undisturbed, old-growth forests have large accumulations of old and well-decayed CWD (McFee and Stone 1966, Harvey et al. 1981, Little and Ohmann 1988, Means et al. 1992, Keenan et al. 1993), and the nutrient dynamics of these materials may be [TABULAR DATA FOR TABLE 1 OMITTED] very different from nutrient dynamics of relatively young CWD (Harmon et al. 1994).

Our current knowledge of nutrient mineralization and immobilization processes in CWD of forests primarily comes from chronosequences of fallen tree bole (hereafter referred to as simply "bole") decomposition. For nitrogen (N), the nutrient often limiting productivity (Vitousek and Howarth 1991), these studies have shown contrasting patterns of net N dynamics during bole decomposition. Some bole chronosequence studies have shown net N immobilization fairly consistently throughout the decay process (Grier 1978, Foster and Lang 1982); however, other studies have shown little net N dynamics in boles during the early stages of decay (first 30 to 100 yr), followed by a period of net N release (Lambert et al. 1980, Means et al. 1992) or net N immobilization (Sollins et al. 1987). These indirect measurements of net N flux from boles may be misleading, because errors in estimates of N stores over long periods of time can obscure short-term N dynamics. For example, chronosequence studies frequently fail to consider fragmentation and fungal sporocarp production, both of which may transfer substantial quantities of relatively N-rich organic matter to the forest floor (Harmon et al. 1994).

In many old-growth forests of the U.S. Pacific Northwest (PNW), boles may occupy [greater than]20% of the area of the forest floor (Harmon et al. 1986). Well-decayed boles, typically [greater than]75 yr old (Sollins et al. 1987, Means et al. 1992), may occupy [greater than]5% of the forest floor area. Tree fine roots proliferate within these well-decayed boles (Vogt et al. 1995), suggesting that these boles may be important sites of water or nutrient acquisition. Plant water uptake from well-decayed boles may be significant during the summer dry period in this region, because these boles contain substantial quantities of available water relative to the rest of the soil during this period (Hope and Li 1997). However, in these N-limited forest soils (Velazquez-Martinez et al. 1992), high fine-root density also might indicate that well-decayed boles are an important source of plant-available N.

I conducted in-field incubations to determine the net rates of N mineralization and nitrification in well-decayed boles and compared these rates to adjacent surface mineral soils in an old-growth forest of the PNW. Laboratory estimates of available C and N also were conducted to elucidate the potential factors controlling net N transformations in boles and mineral soil under field conditions. Finally, my results are interpreted in the context of the importance of well-decayed CWD in N cycles of PNW forest ecosystems.

MATERIALS AND METHODS

Study site

The study was conducted at an elevation between 900 and 950 m within Reference Stand 3, an old-growth forest stand (oldest age class 550 yr) located within the H. J. Andrews Experimental Forest in the central Oregon Cascade mountains (44 [degrees] 14 [minutes] N, 122 [degrees] 11 [minutes] w). The mixed-age stand has a uniform westerly slope of about 25%, and is dominated in the overstory by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.), with scattered western red cedar (Thuja plicata Donn ex D. Donn). The climate in this region is characterized by mild, wet winters and warm, dry summers. The stand received 1935 mm of precipitation during the one-year study period (19 October 1990 to 19 October 1991). Mean daily air-temperature during this period was 7.0 [degrees] C, and mean daily soil temperature (at 0.10-m mineral soil depth) was 6.5 [degrees] C. Mean daily air and soil temperatures during the "winter" incubation period (19 October 1990 to 4 June 1991) were 2.8 and 3.8 [degrees] C, respectively; mean daily air and soil temperatures during the summer incubation period (4 June to 19 October 1991) were 13.6 and 10.8 [degrees] C, respectively (see Field Methods).