Characterization of coarse woody debris across a 100 year chronosequence of upland oak–hickory forests

Forest Ecology and Management 149 (2001) 153±161

Characterization of coarse woody debris across a 100 year chronosequence of upland oak±hickory forests Travis W. Idola,*, Rebecca A. Figlerb,1, Phillip E. Popea, Felix Ponder Jr.c a

Department of Forestry and Natural Resources, Purdue University, 1159 Forestry Building, West Lafayette, IN 47907, USA Department of Natural Resources and Environmental Science, Purdue University, Lily Building, Room 3±440, West Lafayette, IN 47907, USA c USDA Forest Service, North Central Research Station, 208 Foster Hall, Chestnut Street, Lincoln University, Jefferson City, MO 65102, USA b

Received 27 February 2000; accepted 11 June 2000

Abstract In most forest ecosystems, the total amount of coarse woody debris and its distribution into decay classes change over time from harvest to old growth stages. The relationship of decomposition classes to substrate quality is important to determine the contribution of woody debris to ecosystem nutrient cycling and forest development. The two objectives of this study were: (1) to determine if down dead wood (DDW) nutrient content varied with decomposition class or forest stand age; (2) to determine if DDW decomposition classes were related to indicators of substrate quality. Volume, mass, and indicators of substrate quality, such as N content and lignin:N ratio, were determined for woody debris from several decomposition classes in upland hardwood forest stands of different ages in southern Indiana, USA. Results showed a large decrease in volume and mass of DDW from recently harvested to mature stands. The dominant decomposition class shifted from Class II to Classes III and IV with increasing stand age. No Class I woody debris was found within any of the study plots. Nutrient concentration (N, S, and P) and carbohydrate fractions (soluble, hemicellulose, cellulose, and lignin) all varied signi®cantly among certain decomposition classes, but N and P concentration and the C:N ratio were the best indicators of decomposition class. Patterns of P retention in decomposition classes suggested a strong potential for immobilization of this nutrient in woody debris. Based on substrate quality groupings, there were three distinguishable decomposition classes: Classes II and III, Class IV, and Class V. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Coarse woody debris; 100 year chronosequence; Oak±hickory forests; C:N ratio

1. Introduction Measurements of soil nutrient pools (Kaczmarek et al., 1995; Clinton et al., 1996) and returns of nutrients from leaf fall (Gholz et al., 1985; Taylor * Corresponding author. Present address: Department of Land, Air, and Water Resources, University of California-Davis, One Shields Ave., Davis, CA 95616 USA. Tel.: ‡1-530-754-5734. E-mail address: [email protected] (T.W. Idol). 1 Present address: Depaul College of Law, 25 E Jackson Blvd., Chicago, IL 60604, USA.

et al., 1989) and ®ne root turnover (McClaugherty et al., 1982; Joslin and Henderson, 1987) have been made in numerous studies of ecosystem nutrient cycling. The contribution of coarse woody debris (CWD) to nutrient cycling has received less attention. Coarse woody debris is generally de®ned as dead woody material with a diameter of 10 cm or greater. This includes a range of woody debris from fallen logs and branches to standing dead trees and stumps. As a subset of this material, down dead wood (DDW) is considered to be those branches, logs, and stumps that

0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 3 6 - 3

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Table 1 Classification scheme for down dead wood decomposition stage, taken from Sollins (1982) Character

Class I

Class II

Class III

Class IV

Class V

Bark Structural integrity Branches

Intact Sound All twigs present

Mostly intact Sapwood rotting Larger twigs present

Mostly absent Heartwood sound Larger branches present

Absent Heartwood rotten Branch stubs present

Absent Heartwood rotten Absent

are in contact with the soil and may have the most immediate effect on nutrient cycling processes. There are visual evaluations of the state of decay of CWD or DDW that are used by academic researchers (Muller and Liu, 1991; Jenkins and Parker, 1997) and government personnel (Thomas, 1979; Shi¯ey et al., 1995). These systems use cues that can be recognized in the ®eld, e.g., bark slippage, decreases in wood density, increases in log fragmentation, and loss or decay of branches. A common classi®cation scheme that has been used for both conifer and deciduous forests of the US is listed in Table 1. Although these types of classi®cation systems are useful, they are qualitative assessments and are subject to interpretation by the investigator. Linking these ®eld-oriented evaluations of CWD decay stage to actual rates of decomposition and patterns of nutrient immobilization (net uptake) or mineralization (net release) requires an assessment of the factors that affect these processes. One of the important factors for decomposition and nutrient dynamics is litter ``substrate quality''. Although the de®nition of this term is variable, in general it is used to indicate the relative energy and nutrient content of different litter substrates. High substrate quality leads to rapid decay and nutrient release and vice versa. Although energy and nutrient content tend to co-vary in different litter types, this is not always the case. For example, glucose has a high energy but low nutrient content; thus, it tends to be decomposed rather quickly but releases few nutrients to the inorganic soil pool. Conversely, humi®ed soil organic matter tends to have a low energy but high nutrient content; thus, it tends to be decomposed rather slowly, but its degradation often results in the net release of inorganic nutrients like N and P to the available soil pool. Indicators of substrate quality include nutrient concentration (Vesterdal, 1999), C:N ratio (Hunt et al., 1988), the proportion of structural

versus non-structural carbohydrates (Aber et al., 1990), and the lignin:N ratio (Meentemeyer, 1978). The amount and distribution of DDW into different decomposition classes change with forest stand development (McCarthy and Bailey, 1994; Jenkins and Parker, 1997). Logging slash, usually classi®ed as Classes I or II DDW, dominates recently harvested stands (McCarthy and Bailey, 1994), while Class III DDW seems to dominate in forest stands varying in age from 10 to 100 years (Jenkins and Parker, 1997). It is important to investigate the change with stand age in DDW nutrient pools, as well as mass and volume. Understanding the nutrient dynamics of DDW as a stand develops from harvest through maturity or an old growth stage will allow a more accurate assessment of the role of DDW in nutrient cycling and forest regeneration. Several studies have attempted to characterize the nutrient content or decay rate of CWD and DDW. Density, C:N ratio, and lignin:N ratio have been correlated with DDW decay rate in some studies (Abbott and Crossley, 1982; MacMillan, 1988). Lambert et al. (1980) and Brown et al. (1995) found that N was immobilized during the early stages of CWD decomposition, i.e., the total N content of the CWD increased despite losses in mass. Other elements, such as Ca, K, and Mg were mineralized throughout the decomposition process, i.e., the total content of these elements in the CWD declined over time. The mineralization patterns of P in CWD were somewhat more complicated, with a period of immobilization, or net gain in CWD P content, occurring after several years of mineralization, or net decline in CWD P content (Lambert et al., 1980). Bark and woody tissues were analyzed separately in some of these studies, and results from these two fractions differed widely. Very few of these studies, however, have speci®cally linked measurements of CWD substrate quality (such as nutrient concentration or carbohydrate fractionation)

T.W. Idol et al. / Forest Ecology and Management 149 (2001) 153±161

or decay rates with ®eld classi®cation systems (Lambert et al., 1980). The two objectives of our research were: (1) to assess the volume, mass, and nutrient content of DDW in different decomposition classes in upland oak± hickory forests of different ages; (2) to determine which indicators of substrate quality are best able to distinguish the different DDW decomposition classes. Initial hypotheses were: (1) the dominant DDW decomposition class will shift from Class II to Class III with increasing stand age; (2) the nutrient concentration and nutrient content of inner and outer woody tissues will differ for Classes II and III DDW; (3) the C:N and lignin:N ratios will be the best indicators of DDW decomposition class. 2. Materials and methods 2.1. Study site descriptions This study was conducted at the Southern Indiana Purdue Agricultural Center in Dubois County, Indiana. The dominant soil series on the shoulder and sideslope positions of this area are Gilpin silt loam (Typic Hapludult) and Wellston silt loam (Ultic Hapludalf) (Soil Survey Staff, 1996). The mean annual temperature is 128C, and the mean annual precipitation

155

is 1170 mm. The ecological land-type phase of the study area is Quercus alba±Acer saccharum Parthenocissus dry-mesic ridge (USDA, 1995). Three forest stands historically dominated by oak (Quercus) and hickory (Carya) species were chosen for this study. The ®rst was a 65±75-year-old stand that was commercially harvested in 1996 (1-year-old stand). Complete removal of the remaining overstory trees (stems 4 cm or greater diameter at breast height (DBH)) was accomplished through mechanical removal and herbicide application to cut stumps. The second stand was clear-cut harvested in 1966 (31-year-old stand). The third is a mature stand dominated by white oak (Q. alba) in the overstory and has not been intensively harvested for the last 80±100 years, although the stand was probably selectively thinned on different occasions prior to the 1950s. Vegetation structure, composition, and abundance for the three stands are given in Table 2. For the stand harvested in 1996, pre-harvest vegetation data are provided. 2.2. Field sampling Three circular plots measuring 500 m2 were established in the 1- and 80±100-year-old stands. In order to maintain uniformity in the physiographic and soil characteristics of the sampling locations, only two plots were established in 31-year-old stand. In the fall

Table 2 Vegetation inventory for upland hardwood forests in southern Indiana Stand age (years)

1c

31

Saplingsa Species

Stems/ha

Species

Stems/ha (m2/ha)

A. saccharum (sugar maple) Nyssa sylvatica (black gum)

2370 554

All species

3780

Q. alba (white oak) Q. rubra (red oak) A. saccharum Carya glabra (pignut hickory) All species

71 (15.9) 40 (12.6) 123 (5.6) 40 (4.7) 384 (47.9)

A. saccharum Prunus serotina (black cherry) Sassafras albidum (sassafras) All species

515 300 143 1430

Q. alba A. saccharum All species

103 (19.0) 253 (3.9) 445 (27.4)

A. saccharum Asimina triloba (pawpaw) All species

100

a

741 546 1950

A. saccharum

740

All species

770

Saplings are 2.5±9.9 cm DBH. Overstory trees are 10 cm or greater DBH. c Data for stand age 1 are based on a pre-harvest inventory. b

Overstoryb

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of 1997, all DDW within the plots were sampled according to the protocol of Jenkins and Parker (1997). The length and mid-point diameter of all DDW at least 10 cm in diameter were sampled. Each piece of DDW was evaluated and assigned a decomposition class according to the methods of Sollins (1982). The criteria for this classi®cation scheme are listed in Table 1. The length and mid-point diameter of the DDW were used to calculate DDW volume, using the equation for the volume of a cylinder pr2l. 2.3. Laboratory methods Two cross sections from one log from Classes II±V were taken from each plot for laboratory analysis. No Class I DDW was found within any of the study plots. For Classes II and III material, we choose logs that were approximately 15±25 cm in diameter. For Classes IV and V DDW, poor log integrity precluded the collection of a true cross section. Instead, 3±5 broken pieces of DDW from each of these classes, each piece approximately 10 cm in length and 5 cm in diameter, were taken from each plot for analysis. Each cross section or piece of DDW was cut into approximately 3 cm diameter pieces and dried in a convection oven at 658C to constant weight (approximately 1 week). Within each cross section of Classes II and III DDW the outer wood (bark plus sapwood) was separated from the inner wood (heartwood). The thickness of the outer wood varied between 2 and 3 cm for all cross sections. The density of DDW was determined using the soil clod bulk density method (Blake and Hartge, 1986). DDW density was then multiplied by DDW volume to calculate DDW mass. For Classes II and III DDW, both the inner and outer wood densities were used in the calculations. An average thickness of the outer wood of 2.5 cm was used to determine the proportion of total log volume that was outer versus inner wood. DDW samples were ground in a Wiley mill until they passed through a 1 mm diameter mesh screen. The ground samples were re-dried at 658C for at least 24 h. Total C, N, and S content of DDW were determined using a LECO CNS 2000 elemental analyzer. Total P content was determined using the phosphomolybdate blue colorimetric procedure (Olsen and Sommers, 1982) after digestion of the material in

perchloric acid and hydrogen peroxide at approximately 2008C. Soluble organic material, hemicellulose, lignin, and cellulose were determined for DDW samples using a standard sequential ®ber digestion (Goering and Van Soest, 1970) in which the same sample is sequentially digested in different solutions in order to digest different organic fractions. Soluble carbohydrates were digested in a neutral detergent solution of sodium lauryl sulfate, EDTA, and sodium phosphate dibasic. Hemicellulose was digested in an acid detergent solution of hexadecyl trimethyl ammonium bromide and sulfuric acid. Lignin was digested in a saturated potassium permanganate solution. Cellulose content was assumed to be the remaining insoluble organic fraction and was determined by dry-ashing the remaining material. Concentrations of both elements and carbohydrate fractions were multiplied by the estimated DDW mass to determine total nutrient pools of DDW in the different decomposition classes. For Classes II and III, the inner heartwood and outer sapwood plus bark fractions were analyzed separately. 2.4. Statistics The design of this experiment was a randomized complete block. Individual decomposition classes were designated as different treatment levels. Stand age was the block variable. Values were averaged across plots within stand age for the comparisons. The data were checked for approximation to a normal distribution and for equality of variances using the Box±Cox procedure (Box et al., 1978). Because of the wide range in mean values for many of the variables, obtaining equal variances among the different treatments necessitated certain transformations, which are listed below. Volume (m3/ha), mass (Mg/ha), elemental C (Mg/ ha), and elemental N, S, and P (kg/ha) content of the four DDW decomposition classes in the three stands were compared using the ANOVA procedure in SAS (SAS Institute, 1989). The data were log-transformed prior to analysis. Duncan's multiple range test (aˆ0.05) was used to identify signi®cant differences among treatment levels. The percent C, N, S, P, soluble, hemicellulose, lignin, and cellulose and the calculated C:N and lignin:N ratios of the different DDW classes averaged

T.W. Idol et al. / Forest Ecology and Management 149 (2001) 153±161

157

across stand ages were also compared using the ANOVA procedure in SAS. Percent N, S, P, and soluble carbohydrates were arcsine-transformed prior to analysis. Duncan's multiple range test (aˆ0.05) was used to identify signi®cant differences among treatment means. Equality of variances could not be achieved through transformation of the data on percent lignin and cellulose or the C:N and lignin:N ratios. A t-test assuming unequal variances with aˆ0.05 was used to compare decomposition classes for these variables. 3. Results

Fig. 1. Down dead wood volume by decomposition class across a chronosequence of upland hardwood forests in southern Indiana, ($) stands measured in this study. All others taken from Jenkins and Parker (1997).

3.1. Volume, mass, and C content The volume and distribution of DDW in the different decomposition classes is illustrated in Fig. 1, along with the results from Jenkins and Parker (1997), who estimated the volume and decomposition class distribution of DDW in numerous hardwood forest stands in southern Indiana. The total volume of DDW at each stand age in our study was greater than the estimates from their study. There were also differences in the distribution of DDW into the different decomposition

classes. The proportion of total DDW volume in Class II in the 1-year-old stand was greater in our study than in their study. Also, the proportion of Class IV DDW in all age stands of our study was greater than in their study. Conversely, the proportion of total DDW volume in Class III was less in our study than in theirs. Table 3 contains the results of the comparisons of volume, mass, and C content of the different

Table 3 Total volume, mass, and nutrient content of down dead wood by decay class across a 100-year chronosequence of upland oak±hickory forests in southern Indianaa Age (years)

Decay class

Volume (m3/ha)

Mass (Mg/ha)

C (Mg/ha)

N (kg/ha)

S (kg/ha)

P (kg/ha)

1 1 1 1

II III IV V

90.8 a 38.3 bc 17.8 bc 1.2 c

85.2 a 35.3 b 15.7 b 1.0 b

42.8 a 18.2 bc 7.5 bc 0.5 c

80.0 a 42.4 abc 27.9 abc 5.5 c

26.4 a 17.5 ab 14.6 abc 7.4 bcd

4.7 1.8 4.9 4.6

148.1

137.2

69.0

155.8

66.0

16.0

0.6 c 27.1 bc 15.9 bc 0.5 c

0.6 b 24.8 b 14.0 b 0.4 b

0.3 c 12.7 bc 6.7 bc 0.2 c

1.4 c 60.9 ab 51.7 abc 5.1 c

0.2 8.9 7.3 0.6

0.1 1.7 4.6 0.3

44.2

39.8

19.9

119.1

17.0

6.7

0.7 c 41.7 b 22.3 bc 1.0 c

0.7 b 37.9 b 19.6 b 0.8 b

0.3 c 19.5 b 9.4 bc 0.4 c

1.6 c 72.8 a 72.5 a 9.7 bc

0.2 d 11.7 bcd 10.3 bcd 1.1 cd

0.1 1.7 6.5 0.5

65.7

59.0

29.7

156.5

23.3

8.8

Total 31 31 31 31

II III IV V

Total 100 100 100 100

II III IV V

Total a

d bcd bcd cd

abc bc ab abc c bc abc bc c bc a bc

Log-transformed data obtained by using Duncan's multiple range test with aˆ0.05. Data within columns followed by the same letters do not differ significantly.

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T.W. Idol et al. / Forest Ecology and Management 149 (2001) 153±161

Table 4 Indicators of down dead wood decomposition class across a 100-year chronosequence of upland oak±hickory forestsa Variableb

II in

III in

II out

III out

IV

V

N S P C Soluble Hemicellulose Lignin Cellulose C:N Lignin:N

0.101 c 0.022 d 0.003 e 51.22 ab 4.64 c 26.92 a 23.12 a 44.6 bc 514 a 232 a

0.126 c 0.034 cd 0.002 e 51.90 a 12.08 b 24.08 ab 18.68 ab 53.09 a 417 b 146 b

0.307 b 0.035 cd 0.021 c 49.84 b 14.01 b 23.00 b 14.25 b 49.3 ab 180 cd 43.4 d

0.318 b 0.049 bc 0.009 d 50.93 ab 12.76 b 25.29 ab 14.88 ab 47.46 ab 224 c 59.7 cd

0.373 b 0.066 b 0.030 b 47.92 c 12.21 b 22.21 b 22.62 a 39.05 bc 132 d 61.3 c

1.246 a 0.150 a 0.071 a 46.55 c 22.66 a 16.19 c 22.68 a 32.92 c 40.2 e 19.2 e

a

Values within the same row followed by the same letter do not differ significantly. See Section 2.4 for description of statistical analyses. All values are in percentages, except C:N and lignin:N. These are unitless ratios based on percentage comparisons.

b

decomposition classes among the different stand ages. The log-transformed comparisons show that volume and mass in Class II DDW in the 31- and 80±100-year-old stands and Class V DDW in all age stands were signi®cantly lower than all other classes. Volume and mass of Classes III and IV DDW are generally not signi®cantly different, but Class II DDW in the 1-year-old stand was signi®cantly greater than Class IV DDW in all age stands. 3.2. N, S, and P content Similar to volume, mass, and C content, the N and S content of Class II DDW in the 31- and 80±100year-old stands and Class V DDW in all the stands were signi®cantly lower than Classes III and IV DDW (Table 3). Despite a signi®cant difference in mass between Class II DDW in the 1-year-old stand and Class IV DDW in all the stands, there were no signi®cant differences in the N content between these classes. The S content of Class II DDW in the 1-year-old stand was signi®cantly greater than in Class IV DDW in the 31- and 80±100-year-old stands. P content showed an even more complex distribution among the decomposition classes and stand ages (Table 3). Within each age stand, there was an increase in P content from Class III to Class IV DDW and a decrease from Class IV to Class V DDW. Changes in P content with decomposition class were signi®cant only in the 80±100-year-old stand, however.

3.3. Indicators of decomposition class Table 4 lists the concentrations of the elements N, S, P, and C and the concentration of the soluble, hemicellulose, lignin, and cellulose carbohydrate fractions. Also listed are the C:N and the lignin:N ratios. There were signi®cant differences by decomposition class for all the measured element concentrations, carbohydrate concentrations, and for the C:N and lignin:N ratios. In general, the inner woody tissue of Classes II and III DDW were indistinguishable by element or carbohydrate concentration or by the C:N or lignin:N ratios. Likewise, the outer woody tissue of Classes II and III DDW were generally indistinguishable. The inner and outer woody debris, however, were generally signi®cantly different from each other. Within these classes, outer woody tissue had a signi®cantly higher N and P concentration and a lower C:N and lignin:N ratio than inner woody tissue. When only the inner woody tissues from Classes II and III are considered, then the N, S, and P concentration of DDW increased in the order Class IIˆIIIIII>IV>V. 4. Discussion 4.1. Volume, mass, and nutrient content The volume, mass, and distribution of DDW into decomposition classes in this study were generally

T.W. Idol et al. / Forest Ecology and Management 149 (2001) 153±161

consistent with that found in other studies in the Central Hardwood Region. The trend of decreasing DDW volume with increasing stand age is supported by the work of Jenkins and Parker (1997), which was also conducted in southern Indiana hardwood forests. They found that Class III DDW dominated the volume of total DDW in stands of different ages. Although our study included stands in the same geographical region as those of Jenkins and Parker (1997), our results suggest a greater volume and mass of Class II DDW in recently harvested stands and a greater volume and mass of Class IV DDW in all age stands. Although Jenkins and Parker (1997) did not study stands younger than 8 years after harvest, they hypothesized that Class III material would dominate stands less than 12 years of age. McCarthy and Bailey (1994) assessed the CWD volume and mass of forest stands in the Central Appalachians that ranged in age from clearcut to old growth. As in our study, they found that Class II CWD dominated the clear-cut stand. They also found that Classes IV and V CWD was more abundant in older forest stands than is suggested by Jenkins and Parker (1997). The DDW N and S content followed a similar trend as mass and volume; however, there was no signi®cant difference in N and S content between Class II DDW in the 1-year-old stand and Class IV DDW in any of the stands. The same pattern exists between Classes III and IV. Although the mean mass of DDW in Class III is approximately twice that in Class IV, there is only a 16% decrease in N and S content from Class III to Class IV. This suggests some immobilization of N and S during the early stages of DDW decomposition. Brown et al. (1995) found net N immobilization in small diameter (10±15 cm) logs after 5 years of decomposition in a temperate Australian forest. Lambert et al. (1980) found no net loss of N from decomposing balsam ®r (Abies balsamea [L.] Mill.) logs during the ®rst 30 years of decomposition, after which N loss rates closely followed mass loss rates. These differences in N dynamics by age and decomposition class suggest that there is a delay in N mineralization from DDW for several decades. Very little is known about patterns of S mineralization and immobilization from DDW. The increase in the P content of DDW from Class III to Class IV within each age stand suggests a strong immobilization potential for this nutrient. The role of

159

P in litter decomposition is variable depending on the type of litter studied. Results from Blair (1988) and Kaczmarek et al. (1998) show that the total P content of deciduous leaf litter tends to increase for at least 2 years, despite signi®cant losses in litter mass. This phenomenon of net P immobilization has also been seen in decomposing conifer needle litter on P-limited soils (Gholz et al., 1985). Brown et al. (1995) found that P was mineralized from small diameter DDW over a 5-year period, but P release rates were less than for Ca, Mg, and K. Lambert et al. (1980) found that P was released from decomposing balsam ®r boles for the ®rst 10±15 years, followed by a 20-year period of possible P immobilization. They hypothesized that the initial drop in P content was due to the sloughing of bark and outer woody tissues. Our study suggests a much more signi®cant immobilization of P with perhaps little initial mineralization, except for that due to leaching of soluble components. Thus, in these forest stands, the majority of the N, P, and S contained in freshly fallen DDW is probably unavailable for use by the regenerating vegetation for several decades. 4.2. Indicators of decomposition class Although all the substrate quality indicators measured in this study were able to distinguish at least some of the different decomposition classes from each other, the better ones were N and P concentration and the C:N and lignin:N ratios. For these indicators, the inner and outer woody tissues of Classes II and III DDW were signi®cantly different. Also, there were signi®cant differences between Class V, Class IV, and the inner woody tissues of Classes II and III DDW. The signi®cant differences between the inner and outer woody tissues of Classes II and III DDW validate our decision to analyze these components separately. The substrate quality of these two components of DDW most likely translates into different decomposition rates and nutrient dynamics (Lambert et al., 1980; Brown et al., 1995). This may explain why Classes III and IV DDW are so prevalent in stands of different ages. The transition from Class II to Class III includes the decomposition of small branches, bark, and the outer woody tissues. The transition from Class III to Class IV is characterized by signi®cant decay of the inner woody tissues (Table 2). The differential decay rates of these components means that DDW

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within a stand spends much more time in transition between Classes III and IV than between other classes. Except for situations where a large proportion of the overstory has recently been felled due to logging or some other major disturbance, DDW in forest stands of varying age will be comprised mainly of Classes III and IV material. The visual characteristics used to de®ne decomposition classes (Sollins, 1982) that best corresponded to the substrate quality indicators in this study were log integrity and bark slippage. The inner and outer woody tissues decompose at different rates due to differences in substrate quality. Class I DDW was not present in the plots investigated in this study, but this material is characterized by a lack of any evident decomposition. Classes II and III DDW have varying levels of outer woody tissue decomposition (indicated by bark loss or slippage and woody tissue fragmentation) but relatively little inner woody tissue decomposition. In this study, no consistent differences in substrate quality were evident between Classes II and III DDW. Class IV DDW is characterized by a signi®cant and relatively uniform state of decay throughout the entire log. Class V DDW is unique in that almost all the decomposable material has been utilized, leaving a porous, highly fragmented remnant of the former log, often intimately mixed with the ®ne litter layer and/or the mineral soil. Based on the use of these two major decay stage indicators (bark slippage and log integrity) and the substrate quality analysis, we propose the combination of Classes II and III DDW into a single class for the purposes of nutrient budgeting or nutrient cycling studies in these upland hardwood forests. Classes IV and V DDW would remain separate. Class I DDW was not an important component of DDW in this study, even though the majority of DDW in the 1year-old stand was logging slash from the previous year's harvest. Thus, we do not know if the visual differences between Classes I and II DDW correspond to differences in substrate quality and represent separate decay stages. 4.3. Study limitations The major limitation to the present study is the relatively few number of forest stands sampled for DDW volume and its distribution into decomposition

classes (three stands with 2±3 plots per stand). This can lead to a poor ability to detect differences in DDW volume and mass by stand age or class (Shi¯ey and Schlesinger, 1994). However, the volume of DDW and its distribution into decomposition classes found in the present study agreed well with results from more comprehensive ®eld studies (McCarthy and Bailey, 1994; Jenkins and Parker, 1997). The purpose of the present study was not to duplicate the efforts of more comprehensive ®eld studies. Rather, the objective was to investigate how DDW nutrient content and concentration was related to decomposition class at different stages of forest development. This information can then be used to more fully understand the role of DDW in nutrient cycling and forest regeneration. 5. Conclusions The volume, mass, and distribution of DDW in the different aged stands in this study followed the same general trends as those found in Jenkins and Parker (1997) and McCarthy and Bailey (1994). Class II DDW (as logging slash) was most abundant in the recently harvested stand, while Classes III and IV DDW dominated the 30- and 80±100-year-old stands. N and S contents among Classes III and IV material were generally similar, but increasing P concentrations from Class III to Class IV DDW suggest a strong immobilization of P in the DDW of these stands. The best substrate quality indicators of DDW decomposition class were the C:N and lignin:N ratios. These indicators best corresponded to visual differences in bark slippage and log integrity and suggest that Classes II and III DDW should be combined into a single class. Acknowledgements We wish to thank Dr. Michael Jenkins for his technical assistance with ®eld techniques associated with measurement of down dead wood volume and decomposition class. Mr. Ronald Rathfon, Ms. Kristen Holzbaur, and Mrs. Judith Pope were invaluable assistants with plot selection and ®eld data collection. Mrs. Sherry Fulk-Bringman and Dr. Keith Johnston provided technical support in the laboratory. Finally, we

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