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Above-ground net primary productivity and nitrogen mineralization in a mixed mesophytic forest of eastern Kentucky
Forest Ecology and Management, 59 (1993) 53-62
53
Elsevier Science Publishers B.V., Amsterdam
Above-ground net primary productivity and nitrogen mineralization in a mixed mesophytic forest of eastern Kentucky Yan Liu and Robert N. Muller Department of Forestry, University of Kentucky, Lexington, KY40546-0073, USA (Accepted 25 November 1992)
ABSTRACT Liu, Y. and Muller, R.N., 1993. Above-ground net primary productivity and nitrogen mineralization in a mixed mesophytic forest of eastern Kentucky. For. Ecol. Manage., 59: 53-62. Above-ground net primary productivity (ANPP) and potential nitrogen (N) mineralization were measured in four forest types along a soil fertility gradient in eastern Kentucky to test the hypothesis that ANPP in the mixed mesophytic forest region is related to both species composition and mineralizable N. N mineralization differed significantly among the four forest types, with the lowest values occurring on sites containing oak forest and the highest rates occurring on sites containing a mixed mesophytic forest. However, ANPP did not differ among forest types, and was unrelated to mineralizable N. ANPP averaged 6306 kg ha-~ year-l, of which 56% was woody production and 44% was above-ground litter production. The poor relationship between ANPP and site fertility may reflect differential patterns of nutrient use efficiency among the deciduous species which make up the vegetation gradients of eastern Kentucky.
INTRODUCTION
Numerous field studies have demonstrated that the productivity of temperate forests is closely related to soil fertility (Buckman, 1962; Farr, 1967; Duvigneaud, 1971; Viereck et al., 1983 ). Frequently, the relationship is most strongly expressed between above-ground production and nitrogen (N) mineralization. Rates of net N mineralization presumably control the rate at which N becomes available to plants. Using a variety of mineralization assays, several authors have reported that the rate of net N mineralization may predict forest above-ground net productivity reasonably well (Powers, 1980; Cole and Rapp, 1981; Pastor et at., 1984; Nadelhoffer et al., 1985 ). Generally, net N mineralization varies with species composition and edaphic properties (Pastor et al., 1984; Aber et al., 1985; McClaugherty et al., Correspondence to: Yan Liu, Department of Forestry, University of Kentucky, Lexington, KY 40546-0073, USA. ~This study (No. 90-8-130) is connected with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director. © 1993 Elsevier Science Publishers B.V. All rights reserved 0378-1127/93/$06.00
54
YAN LIU AND R.N. MULLER
1985; Nadelhoffer et al., 1985; Montagnini et al., 1989; Zak and Pregitzer, 1990). In the deciduous forests of eastern Kentucky, patterns of overstory species composition are closely associated with a gradient of surface soil fertility, including such variables as soil pH, base saturation, extractable bases and total N (Muller, 1982; R.N. Muller and P.J. Kalisz, unpublished data, 1985 ). Given these relationships among species composition, N mineralization and forest productivity, this study was initiated to test the hypothesis that above-ground net primary productivity of eastern Kentucky's forests is directly related to species composition and to surface soil fertility. STUDY AREA
This study was conducted in Improvement Hollow on the University of Kentucky's Robinson Forest in southeastern Kentucky (37 ° 27' N, 83 ° 8' W). Located near the center of the mixed mesophytic forest region (Braun, 1950 ), Robinson Forest contains a 70-year-old second-growth forest which developed from natural regeneration following intense cutting which left few individual trees standing. Because of vigorous stump sprouting, the species composition of second-growth stands on this portion of the Cumberland Plateau closely resembles that of the original forest (Muller, 1990). Soils in the study area are developed in residuum and colluvium derived from horizontally interbedded sandstone, siltstone, shale and coal of the Breathitt Formation, Lower and Middle Pennsylvanian Series (McDowell et al., 1981 ). These are well-drained stony sandy loams to silt loams similar to the Typic Haplumbrepts (Cutshin), Typic Dystrochrepts (Dekalb) and Typic Hapludults (Gilpin, Shelocta) mapped in adjacent counties (Hayes, 1982 ). Several studies have been conducted in Improvement Hollow to investigate vegetation pattern, site conditions and forest growth (R.N. Muller and P.J. Kalisz, unpublished data, 1985; Kalisz et al., 1987; Liu, 1990; Liu and Muller, 1992 ). The present vegetation of the study site was analyzed in a survey that included all stems > 2.5 cm DBH on 90 0.05-ha sample plots. Ordination of the 90 sample plots by detrended correspondence analysis (DCA-Cornell Ecology Program 40; Hill and Gauch, 1980) described four forest types: ( 1 ) mixed mesophytic forest (mid-slopes of east aspect); (2) hickory (upper slopes of east aspect ); ( 3 ) beech (lower slopes ); (4) oak (upper slopes and ridgetops ) (R.N. Muller and P.J. Kalisz, unpublished data, 1985). Basal area (stems > 2.5 cm DBH) was greatest in mixed mesophytic plots (29.5 m 2 ha -1 ) and least in oak plots (23.6 m 2 ha-1 ) , while density was least in mixed mesophytic plots ( 1296 stems h a - ~) and greatest in oak plots ( 2422 stems h a - 1). This pattern of species composition and stand structure is common in southeastern Kentucky (Muller, 1982 ), and is frequently associated with a fertility gradient in the A horizon. Along the gradient from mixed mesophytic to oak sites, soil pH declined from 6.0 to 4.6, total N declined from 0.52% to 0.13%,
PRIMARY PRODUCTIVITY AND NITROGEN MINERALIZATION IN A MIXED MESOPHYTIC FOREST
55
TABLE 1 Major species composition of four forest types, and soil fertility status of each type at A horizon in Improvement Hollow, Robinson Forest (R.N. Muller and P.J. Kalisz, unpublished data, 1985 ). Major species are those whose combined relative basal area (BA) accounts for > 75% within each forest type. Relative basal area of each major species is given in parentheses Forest type
Major species of each type and their Fertility status relative BA pH Ca TotalN P (cmol (%) (/agg-1) kg - l )
Beech
Mixed mesophytic
Hickory
Oak
K OM (cmol (%) kg - I )
Fagus grandifolia Ehrh. Liriodendron tulipifera L. Tilia L. spp.
(57%) ( 17% 6.0 (7%)
5.0
0.52
7.5
0.4
8.5
Liriodendron tulipifera L. Tifia L. spp. Acer saccharum Marsh. Quercus rubra L. Magnolia acuminata L.
(28%) (22%) (10%) 5.6 (8%) (8%)
3.9
0.27
9.4
0.3
8.6
Carya Nutt. spp. Quercus rubra L. Nyssa sylvatica Marsh. Quercus velutina Lam. Acer rubrum L.
(32%) (17%) 5.4 (10%) (8%)
3.0
0.26
7.4
0.4
6.9
0.5
0.13
5.0
0.13
6.1
(6%)
Quercus coccinea Muenchh. (31%) Quercus prinus L. (19%) Quercus velutina Lam. (11%) 4.6 Acer rubrum L. (8%) Quercus alba L. (7%)
and extractable Ca 2÷ declined from 5.0 to 0.5 cmol kg -~ (P.J. Kalisz and R.N. Muller, unpublished data, 1985 ). Other extractable nutrient elements showed similar trends (Table 1 ). METHODS
Above-groundproductivity Thirty-one 0.05-ha circular sample plots were randomly selected from the original 90 permanent 0.05-ha vegetation survey plots in Improvement Hollow to measure above-ground production. These 31 sample plots represented the four forest types within the watershed: beech (8 plots); mixed mesophytic forest (7); hickory (8); oak (8). At each sample point, two concentric cir-
56
YAN LIU AND R.N. MULLER
cular sample plots were established. The larger one (0.05 ha) was used for trees with DBH >_ l0 cm, and the smaller (0.005 ha) was used for trees 5-10 cm DBH. Diameters of all sample plots were adjusted according to the slope to maintain a fixed area on a horizontal plane. Above-ground net primary productivity (ANPP) of woody stems >_5 cm DBH was estimated by adding the biomass of annual above-ground fine litterfall to average annual woody biomass increment. Litterfall was sampled in 1985-1986 with one litter trap (0.22 m 2 ) located in the center of each sample plot. Litterfall was collected on a weekly or biweekly basis from 15 September 1985 to 7 April 1986. Annual woody production was calculated as the average annual increment in total woody biomass during a 5-year interval from the beginning of the growing season in 1983 through 1987. In March 1988, the DBH was measured and one increment core was collected from each tree within the 31 sample plots. The increment core was collected from a randomly determined position around the circumference of each tree. At an above-ground height of 1.3 m. Radial growth for the previous five growing seasons (1983-1987) was measured to the nearest 0.01 m m with the aid of a microscope equipped with a motor-driven, electronically calibrated stage. Above-ground biomass of the 34 species included in the study was estimated from regression equations relating biomass to DBH obtained from the literature. Generalized allometric regressions of Pastor et al. ( 1983 ) were used to estimate the biomass of northern red oak, red maple and sugar maple. For other major tree species, regressions of Brenneman et al. ( 1978 ), Clark et al. (1980), Phillips ( 1981 ), Tritton and Hornbeck ( 1982 ) and Boerner and Kost (1986) were used. Regressions for l0 minor species, which together accounted for less than 10% of total biomass, could not be obtained from the literature. For these species, the general equation for hardwoods of Tritton and Hornbeck ( 1982 ) was used to determine total above-ground biomass.
Nitrogen mineralization Potential N mineralization of the surface mineral soil was determined by an anaerobic incubation technique which measures the rate of conversion of organically-bound N to NH3" -N (Powers, 1980; Keeney, 1982). This measure provides an index of N mineralization potential under optimal conditions. In each of the 31 plots, eight samples of the mineral soil were collected at random around a 6 m diameter circle centered on the plot center in May 1988. The forest floor was removed to expose the A-horizon, and soil samples were taken with a 2. l cm diameter soil tube to a depth of 5 cm. A-horizon depth is extremely variable throughout this landscape, and the 5 cm sampling depth ensured that we consistently sampled the A horizon only. The eight soil samples from each plot were composited and sieved following air-drying to
PRIMARYPRODUCTIVITYAND NITROGEN MINERALIZATIONIN A MIXEDMESOPHYTICFOREST
57
remove material > 2 mm. Each composited sample was homogenized, and two 5 g (oven-dry equivalent) subsamples were incubated in 12.5 ml of water at 40°C for 7 days. Following incubation, the samples were filtered and the concentration of NH4+ -N in the filtrate was determined colorimetrically. Potential N mineralization is reported as mg N mineralized per kg dry soil per day.
Data analysis Because the data were not normally distributed and variances were not homogeneous, the Wilcoxon and Kruskal-Wallis nonparametric tests were used to determine significant differences in ANPP and potential N mineralization among forest types. Spearman's nonparametric rank-order correlation of the 31 sample plots was used to determine the significance of correlations between measured variables and species composition as determined by plot position along the first axis of the DCA ordination (R.N. Muller and P.J. Kalisz, unpublished data, 1985 ). Statistical significance was determined at a probability level of P < 0.05. However, actual P-values are reported as a measure of the strength of the statistical relationships in this study. RESULTS
Nitrogen mineralization Potential N mineralization of the A-horizon, as determined by anaerobic incubation, varied from 3 to 47 mg kg-1 day-1 among the 31 sample plots. The average rates of N mineralization were significantly different among the four forest types ( P < 0.001; Table 2). The mixed mesophytic type had the TABLE2 Potential N mineralization rates, mean annual fine litter, woody and above-ground net primary productivity (ANPP) ( +one standard error of the mean) in the four forest types in Improvement Hollow, Robinson Forest, eastern Kentucky. Values followed by the same letter are not significantly different ( P < 0.05 ). Sample sizes are indicated in parentheses Forest type
Potential N Litter production Woody production mineralization (kgha - l year - l ) (kgha - l year - l ) (mgkg - l day - t )
Mixed mesophytic (7) 3 5 + 2 a Hickory (8) 31+4ab Beech (8) 24+2b Oak (8) 10+4c
2646+255 3368+ 123 2206+ 180 2908+235
Average (31 )
2786_+ 205
25 + 3
abe c a b
ANPP (kgha -~ year - j )
3953+602 a 3724_+876 a 3032_+299 a 3424+530a
6599+481 a 7091 + 3 9 9 a 5239_+367 a 6332+684 a
3519 + 466
6306 + 511
58
YAN L1U AND R.N. MULLER
A 50 0
A
40
A
A O A
E
30
0
A
0
P,
0
0
__N
20 0
c
z
•
10 4,
0
I
I
I
I
!
100
200
300
400
500
First Axis Score
B 9500 A
8000
0
0
A 0
1-¢6 O
0 O• 0
6500
A
DO. Z
•
A
•
A
0 0
•
5000
3500 0
A
I
i
i
f
i
10
20
30
40
50
N Mineralization ( m g l k g / d a y )
Fig. 1. (A) Relationship between potential N mineralization rate and forest species composition in Improvement Hollow, Robinson Forest, eastern Kentucky. First axis score describes the change in species composition among sample plots and is the position of each sample plot along the first axis of ordination of 90 original sample plots by the detrended correspondence analysis (R.N. Muller and P.J. Kalisz, unpublished data, 1985). N mineralization and first axis score were significantly correlated (Spearman's rank correlation, P < 0.01, r 2= 0.49). (B) Relationship between above-ground net primary production and potential nitrogen mineralization rate in Improvement Hollow. The relationship between these variables was not significant (P> 0.05 ). Forest types are designated as: i , beech; A, mixed mesophytic; O, hickory; 0, oak.
PRIMARY PRODUCTIVITY AND NITROGEN MINERALIZATION IN A MIXED MESOPHYTIC FOREST
59
highest mineralization rate, which differed significantly from both beech and oak types. In contrast, N mineralization of the oak forest type was significantly lower than rates of all three other types. Also, potential N mineralization was significantly correlated with changes in species composition as determined by plot position along the first axis of the DCA ordination (P< 0.01, r 2=0.49; Fig. 1 (A)).
Above-ground productivity and soil fertility Wood production of this forest averaged 3519 kg ha- 1 year- l and litter production averaged 2786 kg ha-1 year-~ over all sample plots (Table 2). Wood production did not differ among the four forest types (P> 0.5 ). However, litter production was highest in the hickory type and lowest in the beech type. Above-ground net primary productivity ranged from 5239 kg ha -1 year-1 to 7091 kg ha-~ year-1 among the four forest types, and averaged 6303 kg ha-~ year-~ (Table 2). However, ANPP did not differ significantly among the four vegetation types (P> 0.08). Spearman's rank-order correlation between the DCA first axis score and ANPP also indicated that ANPP was not correlated with the pattern of species composition (P> 0.3). Woody increment, litter production and ANPP were not correlated with potential N mineralization (P>0.05; Fig. I(B)). Similarly, these variables showed no relation to any other measurable character of soil fertility in the A horizon including pH, base saturation, ~xtractable bases and total N (P> 0.05; P.J. Kalisz and R.N. Muller, unpublished data, 1985 ). DISCUSSION Above-ground net primary productivity of the forest types studied in Improvement Hollow were in the lower range of ANPP reported by other studies in nearby areas and similar forest types (DeAngelis et al., 1981; Edwards et al., 1989 ). The low rate of ANPP appears to reflect the lower than average rainfall which affected eastern Kentucky from 1980 through 1988 (Liu, 1990). During this period average precipitation was 12% below normal, and ANPP is estimated to have been reduced by 18% (Liu and Muller, 1992 ). We found no statistically significant difference in above-ground net production among the four forest types, and no correlation between ANPP and our measures of surface mineral soil fertility. These results do not support our original hypothesis that productivity is related to forest species composition and surface soil fertility. They also are not consistent with the conclusions of other studies (Nadelhoffer et al. 1983, 1985; Pastor et al., 1984) which have found a close relationship between ANPP of forests and N mineralization. The above studies have contained a species gradient with a significant coniferous component, and others have also reported a strong relationship be-
60
YAN LIU AND R.N. MULLER
tween forest production and nutrient availability in forests with coniferous components (Zak et al., 1989 ). The absence of coniferous forest types in our study m a y account for the poor relationship between ANPP and either forest type or site quality. Studies comparing coniferous and deciduous forests on similar sites have shown lower production in coniferous stands (Edwards et al., 1989 ). Also, production in deciduous forests has been shown to be more independent of nitrogen and phosphorus availability than in coniferous forests (Cole and Rapp, 1981 ). On oak sites with low N mineralization, above-ground net production was equivalent to that o f more fertile sites d o m i n a t e d by mixed mesophytic and hickory types (Fig. 1 ( B ) ) . One mechanism which m a y account for our results is differential nutrient use efficiency among the four forest groups. Oaks and other species from poor sites exhibit greater resorption o f nutrients from senescent ephemeral tissues than do species from more fertile sites (Eickmeier, 1982; Killingbeck, 1984, 1985 ). This may enable those species to reach higher levels of production than would be expected on nutrient-poor soils. If this is so, species-specific patterns of nutrient use efficiency m a y play an important role in the dynamics o f forest production in deciduous forests.
REFERENCES Aber, J.D., Melillo,J.M., Nadelhoffer,K.J., McClaugherty,C.A. and Pastor, J., 1985. Fine root turnover in forest ecosystems in relation to quantity and form of nitrogen availability: a comparison of two methods. Oecologia,66:317-321. Boerner, R.E.J. and Kost, J.A., 1986. Biomassequations for floweringdogwood,Cornusflorida. Castanea, 51: 153-155. Braun, E.L., 1950. Deciduous Forests of North America. Hafner, New York. Brenneman, B.B., Frederick, D.J., Gardner, W.E., Schoenhofen,L.H. and Marsh, P.L., 1978. Biomass of species and stands of West Virginia hardwoods. In: P.E. Pope (Editor), Proc. Central Hardwood Forest ConferenceII, Purdue University, West LaFayette, IN, pp. 159178. Buckman, R.E., 1962. Growth and yield of red pine in Minnesota. USDA Forest Service Technical Bulletin 1272. Clark, A., Phillips, D.R. and Hitchcock, H.C., 1980. Predicted weightsand volumesof southern red oak trees on the highland rim in Tennessee. USDA Forest Service Research Paper SE208. Cole, D.W. and Rapp, M., 1981. Elementalcyclingin forest ecosystems.In: D.E. Reichle (Editor), Dynamic Properties of Forest Ecosystems.CambridgeUniversity Press, New York, pp. 341-409. DeAngelis, D.L., Gardner, R.H. and Shugart, H.H., 1981. Productivity of forest ecosystems studies during IBP: The woodlandsdata set. In: D.E. Reichle (Editor), Dynamic Properties of Forest Ecosystems.Cambridge University Press, New York, pp. 567-672. Duvigneaud, P., 1971. Productivity of Forest Ecosystems.UNESCO, Paris. Edwards, N.T., Johnson, D.W., McLaughlin, S.B. and Harris, W.F., 1989. Carbon dynamics and productivity. In: D.W. Johnson and R.I. Van (Editors), Analysis of Biogeochemical CyclingProcesses in Walker Branch Watershed. Springer, New York, pp. 197-232.
PRIMARYPRODUCTIVITYANDNITROGENMINERALIZATIONIN A MIXEDMESOPHYTICFOREST
61
Eickmeier, W.G., 1982. Fall phosphorus resorption by Quercus prinus L. and Acer saccharum Marsh. in central Tennessee. Am. Midl. Nat., 107: 196-198. Farr, W.A., 1967. Growth and yield of well-stocked white spruce stands in Alaska. USDA Forest Service Research paper PNW-53. Hayes, R.A., 1982. Soil survey of Leslie and Perry Counties, Kentucky. U.S. Government Printing Office, Washington, DC. Hill, M.O. and Gauch, H.G., 1980. Detrended correspondence analysis, an improved ordination technique. Vegetatio, 42: 47-58. Kalisz, P.J., Zimmerman, R.W. and Muller, R.N., 1987. Root density, abundance, and distribution in the mixed mesophytic forest of eastern Kentucky. Soil Sci. Soc. Am. J., 51: 220225. Keeney, D.R., 1982. Nitrogen-availability indices, pp. 711-733. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. 2nd edn. American Society of Agronomy, Madison, WI. Killingbeck, K.T., 1984. Nitrogen and phosphorus resorption dynamics of five tree species in a Kansas gallery forest. Am. Midl. Nat., 111: 155-164. Killingbeck, K.T., 1985. Autumnal resorption and accretion of trace metals in gallery forest trees. Ecology, 66: 283-286. Liu, Y., 1990. Growth and productivity of eastern Kentucky's forests. M.S. Thesis, University of Kentucky. Liu, Y. and Muller, R.N., 1992. The effect of drought and frost on radial growth of overstory and understory stems in a deciduous forest. Am. Midl. Nat., 129:19-25. McClaugherty, C.A., Pastor, J., Aber, J.D. and Melillo, J.M., 1985. Forest litter decomposition in relation to soil nitrogen dynamics and litter quality. Ecology, 66: 266-275. McDowell, R.C., Grabowski, Jr., G.J. and Moore, S.L., 1981. Geologic map of Kentucky. U.S. Geological Survey, Reston, VA. Montagnini, F., Haines, B. and Swank, W., 1989. Factors controlling nitrification in soils of early successional and oak/hickory forests in the southern Appalachians. For. Ecol. Manage., 26: 77-94. Muller, R.N., 1982. Vegetation patterns in the mixed mesophytic forest of eastern Kentucky. Ecology, 63:1901 - 1917. Muller, R.N., 1990. Forest regeneration following harvest in the central hardwood forests of Kentucky: implications for succession. Trans. K. Acad. Sci., 51: 36-42. Nadelhoffer, K.J., Aber, J.D. and Melillo, J.M., 1983. Leaf litter production and soil organic matter dynamics along a nitrogen availability gradient in southern Wisconsin (USA). Can. J. For. Res., 13: 12-21. Nadelhoffer, K.J., Aber, J.D. and Melillo, J.M., 1985. Fine roots, net primary production, and soil nitrogen availability: A new hypothesis. Ecology, 66: 1377-1390. Pastor, J., Aber, J.D. and Melillo, J.M., 1983. Biomass prediction using generalized allometric regressions for some northeast tree species. For. Ecol. Manage., 7: 265-275. Pastor, J., Aber, J.D. and McClaugherty, C.A., 1984. Aboveground production and N and P cycling along a nitrogen mineralization gradient on Blackhawk Island, Wisconsin. Ecology, 65: 256-268. Phillips, D.R., 1981. Predicted total-tree biomass of understory hardwoods. USDA Forest Service Research Paper SE-223. Powers, R.F., 1980. Mineralizable soil nitrogen as an index of nitrogen availability to forest trees. Soil Sci. Soc. Am. J., 44: 1314-1320. Tritton, L.M. and Hornbeck, J.W., 1982. Biomass equations for major tree species of the Northeast. USDA Forest Service General Technical Report NE-69.
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YAN LIU AND R.N. MULLER
Viereck, L.A., Dyrness, C.T., Cleve, K.V. and Foote, M.J., 1983. Vegetation, soils, and forest productivity in selected forest types in interior Alaska. Can. J. For. Res., 13: 703-720. Zak, D.R. and Pregitzer, K.S., 1990. Spatial and temporal variability of nitrogen cycling in northern lower Michigan. For. Sci., 36: 367-380. Zak, D.R., Host, G.E. and Pregitzer, K.S., 1989. Regional variability in nitrogen mineralization, nitrification, and overstory biomass in northern lower Michigan. Can. J. For. Res., 19: 1521-1526.
53
Elsevier Science Publishers B.V., Amsterdam
Above-ground net primary productivity and nitrogen mineralization in a mixed mesophytic forest of eastern Kentucky Yan Liu and Robert N. Muller Department of Forestry, University of Kentucky, Lexington, KY40546-0073, USA (Accepted 25 November 1992)
ABSTRACT Liu, Y. and Muller, R.N., 1993. Above-ground net primary productivity and nitrogen mineralization in a mixed mesophytic forest of eastern Kentucky. For. Ecol. Manage., 59: 53-62. Above-ground net primary productivity (ANPP) and potential nitrogen (N) mineralization were measured in four forest types along a soil fertility gradient in eastern Kentucky to test the hypothesis that ANPP in the mixed mesophytic forest region is related to both species composition and mineralizable N. N mineralization differed significantly among the four forest types, with the lowest values occurring on sites containing oak forest and the highest rates occurring on sites containing a mixed mesophytic forest. However, ANPP did not differ among forest types, and was unrelated to mineralizable N. ANPP averaged 6306 kg ha-~ year-l, of which 56% was woody production and 44% was above-ground litter production. The poor relationship between ANPP and site fertility may reflect differential patterns of nutrient use efficiency among the deciduous species which make up the vegetation gradients of eastern Kentucky.
INTRODUCTION
Numerous field studies have demonstrated that the productivity of temperate forests is closely related to soil fertility (Buckman, 1962; Farr, 1967; Duvigneaud, 1971; Viereck et al., 1983 ). Frequently, the relationship is most strongly expressed between above-ground production and nitrogen (N) mineralization. Rates of net N mineralization presumably control the rate at which N becomes available to plants. Using a variety of mineralization assays, several authors have reported that the rate of net N mineralization may predict forest above-ground net productivity reasonably well (Powers, 1980; Cole and Rapp, 1981; Pastor et at., 1984; Nadelhoffer et al., 1985 ). Generally, net N mineralization varies with species composition and edaphic properties (Pastor et al., 1984; Aber et al., 1985; McClaugherty et al., Correspondence to: Yan Liu, Department of Forestry, University of Kentucky, Lexington, KY 40546-0073, USA. ~This study (No. 90-8-130) is connected with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director. © 1993 Elsevier Science Publishers B.V. All rights reserved 0378-1127/93/$06.00
54
YAN LIU AND R.N. MULLER
1985; Nadelhoffer et al., 1985; Montagnini et al., 1989; Zak and Pregitzer, 1990). In the deciduous forests of eastern Kentucky, patterns of overstory species composition are closely associated with a gradient of surface soil fertility, including such variables as soil pH, base saturation, extractable bases and total N (Muller, 1982; R.N. Muller and P.J. Kalisz, unpublished data, 1985 ). Given these relationships among species composition, N mineralization and forest productivity, this study was initiated to test the hypothesis that above-ground net primary productivity of eastern Kentucky's forests is directly related to species composition and to surface soil fertility. STUDY AREA
This study was conducted in Improvement Hollow on the University of Kentucky's Robinson Forest in southeastern Kentucky (37 ° 27' N, 83 ° 8' W). Located near the center of the mixed mesophytic forest region (Braun, 1950 ), Robinson Forest contains a 70-year-old second-growth forest which developed from natural regeneration following intense cutting which left few individual trees standing. Because of vigorous stump sprouting, the species composition of second-growth stands on this portion of the Cumberland Plateau closely resembles that of the original forest (Muller, 1990). Soils in the study area are developed in residuum and colluvium derived from horizontally interbedded sandstone, siltstone, shale and coal of the Breathitt Formation, Lower and Middle Pennsylvanian Series (McDowell et al., 1981 ). These are well-drained stony sandy loams to silt loams similar to the Typic Haplumbrepts (Cutshin), Typic Dystrochrepts (Dekalb) and Typic Hapludults (Gilpin, Shelocta) mapped in adjacent counties (Hayes, 1982 ). Several studies have been conducted in Improvement Hollow to investigate vegetation pattern, site conditions and forest growth (R.N. Muller and P.J. Kalisz, unpublished data, 1985; Kalisz et al., 1987; Liu, 1990; Liu and Muller, 1992 ). The present vegetation of the study site was analyzed in a survey that included all stems > 2.5 cm DBH on 90 0.05-ha sample plots. Ordination of the 90 sample plots by detrended correspondence analysis (DCA-Cornell Ecology Program 40; Hill and Gauch, 1980) described four forest types: ( 1 ) mixed mesophytic forest (mid-slopes of east aspect); (2) hickory (upper slopes of east aspect ); ( 3 ) beech (lower slopes ); (4) oak (upper slopes and ridgetops ) (R.N. Muller and P.J. Kalisz, unpublished data, 1985). Basal area (stems > 2.5 cm DBH) was greatest in mixed mesophytic plots (29.5 m 2 ha -1 ) and least in oak plots (23.6 m 2 ha-1 ) , while density was least in mixed mesophytic plots ( 1296 stems h a - ~) and greatest in oak plots ( 2422 stems h a - 1). This pattern of species composition and stand structure is common in southeastern Kentucky (Muller, 1982 ), and is frequently associated with a fertility gradient in the A horizon. Along the gradient from mixed mesophytic to oak sites, soil pH declined from 6.0 to 4.6, total N declined from 0.52% to 0.13%,
PRIMARY PRODUCTIVITY AND NITROGEN MINERALIZATION IN A MIXED MESOPHYTIC FOREST
55
TABLE 1 Major species composition of four forest types, and soil fertility status of each type at A horizon in Improvement Hollow, Robinson Forest (R.N. Muller and P.J. Kalisz, unpublished data, 1985 ). Major species are those whose combined relative basal area (BA) accounts for > 75% within each forest type. Relative basal area of each major species is given in parentheses Forest type
Major species of each type and their Fertility status relative BA pH Ca TotalN P (cmol (%) (/agg-1) kg - l )
Beech
Mixed mesophytic
Hickory
Oak
K OM (cmol (%) kg - I )
Fagus grandifolia Ehrh. Liriodendron tulipifera L. Tilia L. spp.
(57%) ( 17% 6.0 (7%)
5.0
0.52
7.5
0.4
8.5
Liriodendron tulipifera L. Tifia L. spp. Acer saccharum Marsh. Quercus rubra L. Magnolia acuminata L.
(28%) (22%) (10%) 5.6 (8%) (8%)
3.9
0.27
9.4
0.3
8.6
Carya Nutt. spp. Quercus rubra L. Nyssa sylvatica Marsh. Quercus velutina Lam. Acer rubrum L.
(32%) (17%) 5.4 (10%) (8%)
3.0
0.26
7.4
0.4
6.9
0.5
0.13
5.0
0.13
6.1
(6%)
Quercus coccinea Muenchh. (31%) Quercus prinus L. (19%) Quercus velutina Lam. (11%) 4.6 Acer rubrum L. (8%) Quercus alba L. (7%)
and extractable Ca 2÷ declined from 5.0 to 0.5 cmol kg -~ (P.J. Kalisz and R.N. Muller, unpublished data, 1985 ). Other extractable nutrient elements showed similar trends (Table 1 ). METHODS
Above-groundproductivity Thirty-one 0.05-ha circular sample plots were randomly selected from the original 90 permanent 0.05-ha vegetation survey plots in Improvement Hollow to measure above-ground production. These 31 sample plots represented the four forest types within the watershed: beech (8 plots); mixed mesophytic forest (7); hickory (8); oak (8). At each sample point, two concentric cir-
56
YAN LIU AND R.N. MULLER
cular sample plots were established. The larger one (0.05 ha) was used for trees with DBH >_ l0 cm, and the smaller (0.005 ha) was used for trees 5-10 cm DBH. Diameters of all sample plots were adjusted according to the slope to maintain a fixed area on a horizontal plane. Above-ground net primary productivity (ANPP) of woody stems >_5 cm DBH was estimated by adding the biomass of annual above-ground fine litterfall to average annual woody biomass increment. Litterfall was sampled in 1985-1986 with one litter trap (0.22 m 2 ) located in the center of each sample plot. Litterfall was collected on a weekly or biweekly basis from 15 September 1985 to 7 April 1986. Annual woody production was calculated as the average annual increment in total woody biomass during a 5-year interval from the beginning of the growing season in 1983 through 1987. In March 1988, the DBH was measured and one increment core was collected from each tree within the 31 sample plots. The increment core was collected from a randomly determined position around the circumference of each tree. At an above-ground height of 1.3 m. Radial growth for the previous five growing seasons (1983-1987) was measured to the nearest 0.01 m m with the aid of a microscope equipped with a motor-driven, electronically calibrated stage. Above-ground biomass of the 34 species included in the study was estimated from regression equations relating biomass to DBH obtained from the literature. Generalized allometric regressions of Pastor et al. ( 1983 ) were used to estimate the biomass of northern red oak, red maple and sugar maple. For other major tree species, regressions of Brenneman et al. ( 1978 ), Clark et al. (1980), Phillips ( 1981 ), Tritton and Hornbeck ( 1982 ) and Boerner and Kost (1986) were used. Regressions for l0 minor species, which together accounted for less than 10% of total biomass, could not be obtained from the literature. For these species, the general equation for hardwoods of Tritton and Hornbeck ( 1982 ) was used to determine total above-ground biomass.
Nitrogen mineralization Potential N mineralization of the surface mineral soil was determined by an anaerobic incubation technique which measures the rate of conversion of organically-bound N to NH3" -N (Powers, 1980; Keeney, 1982). This measure provides an index of N mineralization potential under optimal conditions. In each of the 31 plots, eight samples of the mineral soil were collected at random around a 6 m diameter circle centered on the plot center in May 1988. The forest floor was removed to expose the A-horizon, and soil samples were taken with a 2. l cm diameter soil tube to a depth of 5 cm. A-horizon depth is extremely variable throughout this landscape, and the 5 cm sampling depth ensured that we consistently sampled the A horizon only. The eight soil samples from each plot were composited and sieved following air-drying to
PRIMARYPRODUCTIVITYAND NITROGEN MINERALIZATIONIN A MIXEDMESOPHYTICFOREST
57
remove material > 2 mm. Each composited sample was homogenized, and two 5 g (oven-dry equivalent) subsamples were incubated in 12.5 ml of water at 40°C for 7 days. Following incubation, the samples were filtered and the concentration of NH4+ -N in the filtrate was determined colorimetrically. Potential N mineralization is reported as mg N mineralized per kg dry soil per day.
Data analysis Because the data were not normally distributed and variances were not homogeneous, the Wilcoxon and Kruskal-Wallis nonparametric tests were used to determine significant differences in ANPP and potential N mineralization among forest types. Spearman's nonparametric rank-order correlation of the 31 sample plots was used to determine the significance of correlations between measured variables and species composition as determined by plot position along the first axis of the DCA ordination (R.N. Muller and P.J. Kalisz, unpublished data, 1985 ). Statistical significance was determined at a probability level of P < 0.05. However, actual P-values are reported as a measure of the strength of the statistical relationships in this study. RESULTS
Nitrogen mineralization Potential N mineralization of the A-horizon, as determined by anaerobic incubation, varied from 3 to 47 mg kg-1 day-1 among the 31 sample plots. The average rates of N mineralization were significantly different among the four forest types ( P < 0.001; Table 2). The mixed mesophytic type had the TABLE2 Potential N mineralization rates, mean annual fine litter, woody and above-ground net primary productivity (ANPP) ( +one standard error of the mean) in the four forest types in Improvement Hollow, Robinson Forest, eastern Kentucky. Values followed by the same letter are not significantly different ( P < 0.05 ). Sample sizes are indicated in parentheses Forest type
Potential N Litter production Woody production mineralization (kgha - l year - l ) (kgha - l year - l ) (mgkg - l day - t )
Mixed mesophytic (7) 3 5 + 2 a Hickory (8) 31+4ab Beech (8) 24+2b Oak (8) 10+4c
2646+255 3368+ 123 2206+ 180 2908+235
Average (31 )
2786_+ 205
25 + 3
abe c a b
ANPP (kgha -~ year - j )
3953+602 a 3724_+876 a 3032_+299 a 3424+530a
6599+481 a 7091 + 3 9 9 a 5239_+367 a 6332+684 a
3519 + 466
6306 + 511
58
YAN L1U AND R.N. MULLER
A 50 0
A
40
A
A O A
E
30
0
A
0
P,
0
0
__N
20 0
c
z
•
10 4,
0
I
I
I
I
!
100
200
300
400
500
First Axis Score
B 9500 A
8000
0
0
A 0
1-¢6 O
0 O• 0
6500
A
DO. Z
•
A
•
A
0 0
•
5000
3500 0
A
I
i
i
f
i
10
20
30
40
50
N Mineralization ( m g l k g / d a y )
Fig. 1. (A) Relationship between potential N mineralization rate and forest species composition in Improvement Hollow, Robinson Forest, eastern Kentucky. First axis score describes the change in species composition among sample plots and is the position of each sample plot along the first axis of ordination of 90 original sample plots by the detrended correspondence analysis (R.N. Muller and P.J. Kalisz, unpublished data, 1985). N mineralization and first axis score were significantly correlated (Spearman's rank correlation, P < 0.01, r 2= 0.49). (B) Relationship between above-ground net primary production and potential nitrogen mineralization rate in Improvement Hollow. The relationship between these variables was not significant (P> 0.05 ). Forest types are designated as: i , beech; A, mixed mesophytic; O, hickory; 0, oak.
PRIMARY PRODUCTIVITY AND NITROGEN MINERALIZATION IN A MIXED MESOPHYTIC FOREST
59
highest mineralization rate, which differed significantly from both beech and oak types. In contrast, N mineralization of the oak forest type was significantly lower than rates of all three other types. Also, potential N mineralization was significantly correlated with changes in species composition as determined by plot position along the first axis of the DCA ordination (P< 0.01, r 2=0.49; Fig. 1 (A)).
Above-ground productivity and soil fertility Wood production of this forest averaged 3519 kg ha- 1 year- l and litter production averaged 2786 kg ha-1 year-~ over all sample plots (Table 2). Wood production did not differ among the four forest types (P> 0.5 ). However, litter production was highest in the hickory type and lowest in the beech type. Above-ground net primary productivity ranged from 5239 kg ha -1 year-1 to 7091 kg ha-~ year-1 among the four forest types, and averaged 6303 kg ha-~ year-~ (Table 2). However, ANPP did not differ significantly among the four vegetation types (P> 0.08). Spearman's rank-order correlation between the DCA first axis score and ANPP also indicated that ANPP was not correlated with the pattern of species composition (P> 0.3). Woody increment, litter production and ANPP were not correlated with potential N mineralization (P>0.05; Fig. I(B)). Similarly, these variables showed no relation to any other measurable character of soil fertility in the A horizon including pH, base saturation, ~xtractable bases and total N (P> 0.05; P.J. Kalisz and R.N. Muller, unpublished data, 1985 ). DISCUSSION Above-ground net primary productivity of the forest types studied in Improvement Hollow were in the lower range of ANPP reported by other studies in nearby areas and similar forest types (DeAngelis et al., 1981; Edwards et al., 1989 ). The low rate of ANPP appears to reflect the lower than average rainfall which affected eastern Kentucky from 1980 through 1988 (Liu, 1990). During this period average precipitation was 12% below normal, and ANPP is estimated to have been reduced by 18% (Liu and Muller, 1992 ). We found no statistically significant difference in above-ground net production among the four forest types, and no correlation between ANPP and our measures of surface mineral soil fertility. These results do not support our original hypothesis that productivity is related to forest species composition and surface soil fertility. They also are not consistent with the conclusions of other studies (Nadelhoffer et al. 1983, 1985; Pastor et al., 1984) which have found a close relationship between ANPP of forests and N mineralization. The above studies have contained a species gradient with a significant coniferous component, and others have also reported a strong relationship be-
60
YAN LIU AND R.N. MULLER
tween forest production and nutrient availability in forests with coniferous components (Zak et al., 1989 ). The absence of coniferous forest types in our study m a y account for the poor relationship between ANPP and either forest type or site quality. Studies comparing coniferous and deciduous forests on similar sites have shown lower production in coniferous stands (Edwards et al., 1989 ). Also, production in deciduous forests has been shown to be more independent of nitrogen and phosphorus availability than in coniferous forests (Cole and Rapp, 1981 ). On oak sites with low N mineralization, above-ground net production was equivalent to that o f more fertile sites d o m i n a t e d by mixed mesophytic and hickory types (Fig. 1 ( B ) ) . One mechanism which m a y account for our results is differential nutrient use efficiency among the four forest groups. Oaks and other species from poor sites exhibit greater resorption o f nutrients from senescent ephemeral tissues than do species from more fertile sites (Eickmeier, 1982; Killingbeck, 1984, 1985 ). This may enable those species to reach higher levels of production than would be expected on nutrient-poor soils. If this is so, species-specific patterns of nutrient use efficiency m a y play an important role in the dynamics o f forest production in deciduous forests.
REFERENCES Aber, J.D., Melillo,J.M., Nadelhoffer,K.J., McClaugherty,C.A. and Pastor, J., 1985. Fine root turnover in forest ecosystems in relation to quantity and form of nitrogen availability: a comparison of two methods. Oecologia,66:317-321. Boerner, R.E.J. and Kost, J.A., 1986. Biomassequations for floweringdogwood,Cornusflorida. Castanea, 51: 153-155. Braun, E.L., 1950. Deciduous Forests of North America. Hafner, New York. Brenneman, B.B., Frederick, D.J., Gardner, W.E., Schoenhofen,L.H. and Marsh, P.L., 1978. Biomass of species and stands of West Virginia hardwoods. In: P.E. Pope (Editor), Proc. Central Hardwood Forest ConferenceII, Purdue University, West LaFayette, IN, pp. 159178. Buckman, R.E., 1962. Growth and yield of red pine in Minnesota. USDA Forest Service Technical Bulletin 1272. Clark, A., Phillips, D.R. and Hitchcock, H.C., 1980. Predicted weightsand volumesof southern red oak trees on the highland rim in Tennessee. USDA Forest Service Research Paper SE208. Cole, D.W. and Rapp, M., 1981. Elementalcyclingin forest ecosystems.In: D.E. Reichle (Editor), Dynamic Properties of Forest Ecosystems.CambridgeUniversity Press, New York, pp. 341-409. DeAngelis, D.L., Gardner, R.H. and Shugart, H.H., 1981. Productivity of forest ecosystems studies during IBP: The woodlandsdata set. In: D.E. Reichle (Editor), Dynamic Properties of Forest Ecosystems.Cambridge University Press, New York, pp. 567-672. Duvigneaud, P., 1971. Productivity of Forest Ecosystems.UNESCO, Paris. Edwards, N.T., Johnson, D.W., McLaughlin, S.B. and Harris, W.F., 1989. Carbon dynamics and productivity. In: D.W. Johnson and R.I. Van (Editors), Analysis of Biogeochemical CyclingProcesses in Walker Branch Watershed. Springer, New York, pp. 197-232.
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Eickmeier, W.G., 1982. Fall phosphorus resorption by Quercus prinus L. and Acer saccharum Marsh. in central Tennessee. Am. Midl. Nat., 107: 196-198. Farr, W.A., 1967. Growth and yield of well-stocked white spruce stands in Alaska. USDA Forest Service Research paper PNW-53. Hayes, R.A., 1982. Soil survey of Leslie and Perry Counties, Kentucky. U.S. Government Printing Office, Washington, DC. Hill, M.O. and Gauch, H.G., 1980. Detrended correspondence analysis, an improved ordination technique. Vegetatio, 42: 47-58. Kalisz, P.J., Zimmerman, R.W. and Muller, R.N., 1987. Root density, abundance, and distribution in the mixed mesophytic forest of eastern Kentucky. Soil Sci. Soc. Am. J., 51: 220225. Keeney, D.R., 1982. Nitrogen-availability indices, pp. 711-733. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. 2nd edn. American Society of Agronomy, Madison, WI. Killingbeck, K.T., 1984. Nitrogen and phosphorus resorption dynamics of five tree species in a Kansas gallery forest. Am. Midl. Nat., 111: 155-164. Killingbeck, K.T., 1985. Autumnal resorption and accretion of trace metals in gallery forest trees. Ecology, 66: 283-286. Liu, Y., 1990. Growth and productivity of eastern Kentucky's forests. M.S. Thesis, University of Kentucky. Liu, Y. and Muller, R.N., 1992. The effect of drought and frost on radial growth of overstory and understory stems in a deciduous forest. Am. Midl. Nat., 129:19-25. McClaugherty, C.A., Pastor, J., Aber, J.D. and Melillo, J.M., 1985. Forest litter decomposition in relation to soil nitrogen dynamics and litter quality. Ecology, 66: 266-275. McDowell, R.C., Grabowski, Jr., G.J. and Moore, S.L., 1981. Geologic map of Kentucky. U.S. Geological Survey, Reston, VA. Montagnini, F., Haines, B. and Swank, W., 1989. Factors controlling nitrification in soils of early successional and oak/hickory forests in the southern Appalachians. For. Ecol. Manage., 26: 77-94. Muller, R.N., 1982. Vegetation patterns in the mixed mesophytic forest of eastern Kentucky. Ecology, 63:1901 - 1917. Muller, R.N., 1990. Forest regeneration following harvest in the central hardwood forests of Kentucky: implications for succession. Trans. K. Acad. Sci., 51: 36-42. Nadelhoffer, K.J., Aber, J.D. and Melillo, J.M., 1983. Leaf litter production and soil organic matter dynamics along a nitrogen availability gradient in southern Wisconsin (USA). Can. J. For. Res., 13: 12-21. Nadelhoffer, K.J., Aber, J.D. and Melillo, J.M., 1985. Fine roots, net primary production, and soil nitrogen availability: A new hypothesis. Ecology, 66: 1377-1390. Pastor, J., Aber, J.D. and Melillo, J.M., 1983. Biomass prediction using generalized allometric regressions for some northeast tree species. For. Ecol. Manage., 7: 265-275. Pastor, J., Aber, J.D. and McClaugherty, C.A., 1984. Aboveground production and N and P cycling along a nitrogen mineralization gradient on Blackhawk Island, Wisconsin. Ecology, 65: 256-268. Phillips, D.R., 1981. Predicted total-tree biomass of understory hardwoods. USDA Forest Service Research Paper SE-223. Powers, R.F., 1980. Mineralizable soil nitrogen as an index of nitrogen availability to forest trees. Soil Sci. Soc. Am. J., 44: 1314-1320. Tritton, L.M. and Hornbeck, J.W., 1982. Biomass equations for major tree species of the Northeast. USDA Forest Service General Technical Report NE-69.
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Viereck, L.A., Dyrness, C.T., Cleve, K.V. and Foote, M.J., 1983. Vegetation, soils, and forest productivity in selected forest types in interior Alaska. Can. J. For. Res., 13: 703-720. Zak, D.R. and Pregitzer, K.S., 1990. Spatial and temporal variability of nitrogen cycling in northern lower Michigan. For. Sci., 36: 367-380. Zak, D.R., Host, G.E. and Pregitzer, K.S., 1989. Regional variability in nitrogen mineralization, nitrification, and overstory biomass in northern lower Michigan. Can. J. For. Res., 19: 1521-1526.