ADVANCES IN ECOLOGICAL RESEARCH, VOLUME 15
Production, Turnover, and Nutrient Dynamics of Above- and Belowground
Detritus of World Forests
K. A. VOGT, C. C. GRIER, and D. J. VOGT
I. Introduction . . 303 11. Contents and Definitions of Data Set . . 304
111. Forest Floor Mass . . . 306 A. Forest Floor Mass Accumulation . . 306 B. Litter Decomposition . . 310
IV. Litterfall . . 313 V. Woody Litterfall . . 317
VI. N and P Dynamics: Litterfall and Forest Floor , . 318 VII. Mean Residence Times , . 326
VIII. Fine Root Mass, Production, and Nutrient Contents . . 330 IX. Fine Root Mass and Turnover as Related to Other Ecosystem Components, 339 X. Conclusions . . 345
XI. Summary . . 346 XII. Appendix I . . 348
XIII. Appendix I1 . . . 360 References , . 366
Recent years have seen a tremendous increase in the number of papers published on carbon and nutrient circulation in forest ecosystems. A few books have been published with the purpose of synthesizing data and developing some generalities on factors affecting forest productivity and nutrient circulation in different climatic regimes (Clark and Rosswall, 1981; Reichle, 1981; Cannell, 1982). These studies either listed baseline data from numerous sites throughout the world without any attempt to synthesize that data or presented broad generalizations which were de- rived from data representing just a few selected sites. Few attempts have been made to synthesize the large data pool now available in the literature and specifically examine the factors modifying and contributing to organic matter accumulation on a worldwide basis. Despite this large volume of information, the relationship of climatic factors to litter production and
Copyright 0 1986 by Academic Press, Inc. (London) Lid. All rights of reproduction i n any form reserved.
304 K . A. VOGT ET AL.
decomposition has not been examined on a global scale except by Meen- temeyer (1978a,b) and Meentemeyer et al., (1982).
Aboveground litter production has been extensively studied since it was considered to be the dominant pathway joining the living biological component to the organic matter decomposition cycle (Bray and Gorham, 1964: Meentemeyer et al., 1982). Recently, it has also become apparent that aboveground litterfall may not be the dominant pathway of carbon and nutrient circulation in some forest ecosystems. Earlier literature had suggested the importance of fine roots in ecosystem processes (Rodin and Bazilevich, 1967); however, it is not until recently that enough quantita- tive data have been collected to examine the role of roots in carbon and nutrient circulation in forests (Edwards and Harris, 1977; Persson, 1978; Vogt et al., 1983a). These authors have shown that belowground inputs from fine root turnover may contribute more to the organic matter decom- position cycle than aboveground litterfall, at least in cold temperate cli- mates.
In this article, we have synthesized and analyzed currently available data on organic matter accumulation and above- and belowground contri- butions to this pool on a global scale. These data have been used to develop generalities on the importance of both above- and belowground litter input in carbon and nutrient circulating pathways by climate, tree life form (broad-leaved, needle-leaved), and tree behavior (deciduous, evergreen). This composited data base was restricted to studies where information was available on both forest floor mass and litterfall from the same sites (Appendix I). Only fine root data obtained from actual field sampling are included in this article (Appendix 11).
11. CONTENTS AND DEFINITIONS OF DATA SET
Data were grouped in a manner similar to the classification system used by Burgess (1981) to delineate forests by climate, life form, and behavior. Climate encompassed the following zones: tropical, subtropical, mediter- ranean, warm temperate, cold temperate, and boreal, while life form was either broad-leaved or needle-leaved, and behavior was evergreen or de- ciduous. In cases where publications did not include climatic zonation, the Holdridge Life Zone system (Holdridge et. al., 1971) was consulted for classification.
Data for each site are presented in Appendices I and 11. Mean residence time of organic matter, nitrogen(N), and phosphorus(P) in the forest floor were estimated for all study sites in this article using the following equa- tions. Average time in years in which a defined piece of organic matter, N,
ABOVE- AND BELOWGROUND DETRITUS OF FORESTS 3 05
or P remains in the forest floor was estimated using the following equa- tion:
T = HIL
where T is the mean residence time of organic matter, N, or P in the forest floor (years), H i s the forest floor mass, N, or P content, and L is the mass, N, or P content of an annual litterfall (Vogt et al., 1983a). None of these estimates of mean residence time included root contributions, since the necessary data for computation were generally not available. These calculations were based on the assumption that detrital organic matter was in steady state where annual litter input equals annual forest floor decomposition. All stands included in this data set were assumed to have reached canopy closure.
Forest floor was defined to include recently fallen litterfall (L horizon) as well as decomposing organic matter (F and H horizons) above the mineral soil. In this article, litterfall mass consisted of fine aboveground litterfall mass only, while woody litterfall mass included only above- ground woody input greater than 1 cm in diameter.
The compilation of ecosystem research data obtained from studies con- ducted by many different scientists throughout the world has obvious disadvantages: for example, differences in analytical methods for a given variable and quality of technique, differences in the definition of the mea- sured variable, incomplete site descriptive information, and also the lack of uniformity in selecting variables. In some areas a certain variable was not selected for study because locally it had been assumed or shown to be less important than some other variable(s). However, that rejected vari- able might be selected as the most important one in some other ecosys- tem. When all these variables were compiled into one data set, it was generally unbalanced and, therefore, much less instructive. However, on a long-term basis, the advantages of these compilations prove to be greater than their inherent disadvantages. The more obvious benefits come not only from the data syntheses and subsequent concept extrapola- tion, but from the illumination of variables that may be important for future ecosystem analyses. This would then produce a more complete or balanced data base from which additional information could be ex- tracted with the use of even more powerful statistical analyses.
It follows then that comparison of and, hence, conclusions drawn from this geographically expansive data set are not being presented as axioms, but rather as hypotheses to be further tested with the completion of new ecosystem research in world forests.
The dependent variables selected in this article were those related to above- and belowground detritus and its production, turnover, and nutri- ent dynamics. Generally the independent variables were selected a priori
306 K . A . VOGT ET AL.
from supposition or published evidence on their ability to affect the de- pendent variables. In most cases the independent variables can be classi- fied as climatic variables. Other independent variable used in this article consisted of categories called forest types which combine tree life form and behavior with various climatic variables. These forest types were then further combined into climatic zones as additional independent vari- ables.
Statistical analyses on these data sets were conducted using the Statisti- cal Package for the Social Sciences (SPSS) software (Nie et al., 1975; Hull and Nie, 1981). SPSS was used for all the data computation, simple linear regressions, one way analysis of variance, and multiple comparisons of means using the Scheffes test. Multiple regressions were not attempted as so few cases contained the same dependent and independent variables. This again points to the need for researchers to report as much site- specific data and measure as many variables as possible for their possible inclusion into a larger data set.
Dependent variables were checked for normality and variance homoge- neity. Transformations were applied to those variables that were not nor- mal and/or where heteroscedasticity existed. In some cases a logarith- mic transformation was sufficient to satisfy the statistical models assumptions. However, in many cases a rank transformation (Conover and Iman, 1981) was utilized. Group means of the transformed data (or nontransformed if no transformation was necessary) were statistically compared using a one-way analysis of variance and Scheffes tests at P = 0.05. Each group was included in the tests if its sample size was at least three or greater. Interaction variables derived from independent variables were also used to predict certain dependent variables when they de- creased the unexplained variation of the estimation. Variables that were formed by the combination of two or more variables through some mathe- matical computation (such as N : P ratio) were calculated at the individ- ual (or case) level rather than at a higher (or group) level.
111. FOREST FLOOR MASS
A. Forest Floor Mass Accumulation Attempts have been made to generalize about the amount of organic
matter accumulation that will occur over mineral soil and the factors that modify this accumulation in a variety of forest ecosystems (Ovington, 1965: Rodin and Bazilevich, 1965: Shidei and Tsutsumi, 1962). It has been stated that (1) greater litterfall mass accumulation occurs with increasing distance from the equator (i.e., less organic matter accumulates in tropi-
ABOVE- AND BELOWGROUND DETRITUS OF FORESTS 307
cal than temeperate forests), and (2) temperate softwood forests contain about four times the amount of litterfall present in temperate hardwood forests (Ovington, 1965).
Based on data available to Rodin and Bazilevich (1967), greater forest floor accumulation was found to occur with (1) increasing distance from the tropics, and (2) softwood in contrast to hardwood forests. Rodin and Bazilevich (1967) showed forest floor accumulation decreasing from the southern taiga pine and central taiga spruce forests (22,500 kg and ha-'), to the southern and northern taiga spruce forests (17,500 and 15,000 kg ha-l respectively), broad-leaved forests (7500 kg ha-'), subtropical for- ests (5000 kg ha-I), and tropical rain forests (1000 kg ha-'). Changes in organic matter accumulation have also been related to temperature; the greater the number of months within a year with mean temperatures above O"C, the less organic matter accumulation will occur (Shidei and Tsutsumi, 1962). All the above generalities were based on smaller data sets representing fewer climatic zones than exist in the literature today. These generalities are valid on a site or regional level; however, a larger data set shows that they cannot be arbitrarily expanded to a global scale.
Mean climatic and forest floor mass data are presented for the 13 forest types in Table 1 . No significant (P > 0.05) difference in forest floor mass accumulation was observed between forests located in the tropical, sub- tropical, warm temperate, cold temperate, and boreal regions. This sug- gested that organic matter accumulation does not increase as a simple function of distance from the equator, as generalized by Ovington (1965). Forest floor mass was not correlated with latitude even when grouped by needle-leaved ( r 2 = 0.00, N = 121, P > 0.05) and broad-leaved ( r 2 = 0.06, N = 84, P < 0.05) life forms across all climatic regions.
Since climatic variables followed the range of variation expected with latitudinal changes in forest climatic zones, climatic variables were not correlated with organic matter accumulation. None of the climatic vari- ables, their transformations, or interaction terms accounted for the varia- tion in forest floor mass. If any relationship existed between forest floor mass and forest climatic region or latitude, it was masked by the high variation in forest floor mass estimates that occurred within each forest type (Table 1).
In addition, patterns of forest floor accumulation did not separate into distinct groups under broad-leaved and needle-leaved forests, as sug- gested by Ovington (1965), but by the evergreenness or deciduousness of the foliage. Tropical, subtropical, and warm temperate forests located at latitudes less than 40" had similar mean accumulations of forest floor mass when the tree behavior was either deciduous (8789, 8145, and 11,480 kg ha-') or evergreen (22,456, 22,185, 19,148, and 20,026 kg ha-') (Table 1 ) . Within these forest types, deciduous forests generally had only half the
Mean Climatic, Forest Floor Mass, and Litterfall Mass Data by Forest Type"
Temperature ("C) Forest
Minimum Maximum Precipi- floor Woody litterfall Forest monthly monthly tation mass Litterfall
Annual mean mean (mm) (kg ha I) (kg ha-l year1) (kg ha-' year-() type
Tropical broadleaf deciduous
Tropical broadleaf evergreen
Tropical broadleaf semideciduous
Subtropical broadleaf deciduous
Subtropical broadleaf evergreen
Mediterranean broadleaf evergreen
26. la 2(3)
0.3(14) 22.5 0.7(2) 12.5 2.5(2)
21 .0 (1)
28.la 0.6(7) 32.0
- 29.6 6.5(2)
2,504a 208( 16)
I ,43 1 4 1(2)
738b 213(4) 1,705
( 1 )
8,789a 2,772(5) 22,547a 4,890( 1 6) 2,170 95(2)
8,145a 3,9334) 22,185
5,890 I , I70(2) 3,333abc 1,605(4) 5,098 I ,962(2) 3,042
- 3,114a 862(9)
3,477 (1) 637 286(3)
2,902 362(2) 800
Warm temperate broadleaf
Warm temperate broadleaf
Warm temperate needle-
Cold temperate broadleaf
Cold temperate needleleaf
Cold temperate needleleaf
13.9a 0.4( 11)
13.9a 0.8(16) 5.4b 1.0(24)
10.2 (1 )
8.lb 0.3(47) 2.lc 0.5(28)
-4.0 0(2) 7.2a 0.9(12)
1 S(27) -8.5cd
- 19.9ed 3.0(7)
17.0b 0.8(28) -
l,391a,b 89( 12)
1374ab 144( 16) 87% 63(38)
1 1,480a 2,206( 12)
6,484ab 1,224(5) 4,432db
891ab 99(7) - -
I , 107ab 273(7)
1,046ab 2 10( 13) -