Nutrition of Populus deltoides plantations during maximum production

Forest Ecology and Management, 20 (1987) 25-41

25

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Nutrition of Populus deltoides Plantations During M a x i m u m Production L.E. NELSON 1, G.L. SWITZER 2 and B.G. LOCKABY 3

Departments of 'Agronomy and 2Forestry, Mississippi Agricultural and Forestry Experiment Station, Mississippi State, MS 39762 (U.S.A.) 3School of Forestry, Louisiana Tech University, Ruston, LA (U.S.A.) (Accepted 19 September 1986)

ABSTRACT Nelson, L. E., Switzer, G.L. and Lockaby, B.G., 1987. Nutrition ofPopulus deltoides plantations during maximum production. For. Ecol. Manage., 20: 25-41. Maximum annual production (19 Mg ha-' ) ofPopulus deltoides plantations in the Lower Mississippi River Valley is attained near the end of their first decade of development, which is also when the mass of the forest floor, understory and overstory canopy approach a steady state. Annual above-ground net primary production at this stage of development is partitioned into 87% overstory and 13% understory, and annual increases in the permanent tissues of the standing crop immobilize from 13 to 27% of the nutrients in flux. Above-ground nutrient pools in the standing crop (68 Mg h a " ) were about 60% of those estimated for the system at carrying capacity and, dependingon the nutrient, represented 1-10% of the system's total, the remainder being in the soil (0-120 cm). The cycling of nutrients in these rapidly accumulating systems is already well developed, and is dominated by the fluxes of the biogeochemical cycle (55-97% of the requirements) and is rapid. Thus, demands on the soilnutrient pools during maximum rates of production are relativelymodest, since steady-state levelsof the canopy have already been attained,and such demands only replace nutrients retained in the annual increments of woody tissues.

INTRODUCTION

Populus deltoides Bartr. is a widely distributed species of this ubiquitious genus of the northern hemisphere. It is found throughout the United States from 100 degrees W longitude eastward, and attains its best development in the Lower Mississippi River Valley (Fowells, 1965 ). In this locale, best development is attained on areas within 4-6 m above mean stream level. Seed dispersal strongly coincides with the fall of the river from flood stage levels in the lower valley, and this coincidence present optimum conditions for establishment in pure, even-aged stands on the bars along the river. The natural occurJournal Contribution No. 6207 of the Mississippi Agricultural and Forestry Experiment Station, Mississippi State, MS, U.S.A.

0378-1127/87/$03.50

© 1987 Elsevier Science Publishers B.V.

26 rence in even-aged stands, as well as high rates of production, made the species an early and continuing candidate for domestication through plantation culture (Bull and Muntz, 1943; Thielges and Land, 1976), as indeed its hybrids and other members of the genus have been in the old world (Anonymous, 1979; Barndoud and Bonduelle, 1979). Investigators have generally found that closely spaced I plantations of this species and some of its hybrids attain maximum rates of net primary production during the first decade of development (Leith et al., 1965; Switzer et al., 1976; Shelton et al., 1982; Bernier, 1984). The period of maximum rates corresponds to that determined for natural stands from the data provided by Williamson (1913). Rates attained during this decade approach the maximum of 20 Mg ha -1 year -1 reported for the warm temperature forest by Leith (1975). This study investigated the nutrient flux and requirements of plantations in the Lower Mississippi River Valley during the period of their maximum rates of production which, in this locale, had been indentified as occurring between the 5th and 8th years of development ( Shelton et al., 1982 ). A description of the relationship between maximum rates of production and nutrient cycling should add to an understanding of the production processes of these systems. STUDY AREA The study area is in the floodplain of the lower Mississippi River at approximately 33°45'N latitude and 91 o10'W longitude. The climate of the locale is subtropical - - humid with a mean annual precipitation of 129 cm consisting almost entirely of rain. December-January is the time of maximum precipitation, and August-September is the interval of minimum values. Winter temperatures average 9.5 ° C, summer temperatures 26.2 oC. The study plantations are located on the unprotected land between the river levees and have soils dominated by deep ( > 1 m), moderately to somewhat poorly drained silt loams with inclusions of deep and well-drained sandy loams. The silty soils belong to the fine-silty, mixed, nonacid, thermic family of Aeric Fluvaquents, and the sandy soils to the coarse-loamy, mixed, nonacid, thermic family of Typic Udifluvents. These soils occur as a complex on the locally higher units of alluvial topography with slopes ranging from 0 to 5% - - silts on the flats and sands on the intervening low ridges. The alluvial sites on which these soils occur are considered excellent for Populus deltoides, having site indices at 30 years ranging from 28 to 36 m (Broadfoot, 1976). The study used 6-and 8-year-old unthinned plantations that had been estab-

1Closelyspacedimpliesspacingsapproximating3 X3 m or 750 stems ha -1at establishment.This is in contrastto the widerspacingsof 5 X 8 m or 250 stemsha -1usedwith Euramericanhybrids in the Po River Valley (Anonymous,1979).

27 TABLE 1 Mean stand properties of the 6-and 8-year-old Populusdeltoidesplantations Plantation DBH (cm) agea 2 range

Height (m)

Stems ha -1

Basal area (m2 ha -I)

2

range

2

range

2

range

6 8

15.7 17.6

16.5-17.5 18.8-20.4

778 708

749-797 696-714

17.8 20.9

17.2-18.7 19.5-23.2

17.1 19.4

16.8-17.3 18.9-20.3

a This age is the number of completed field growing seasons. lished with about 800 stems ha 1 from cuttings produced from unselected stock from u n k n o w n local sources. Three plantations were used for each age. At these ages the plantations were two-storied, having a main canopy and a lower understory-ground flora stratum. The main canopy consisted entirely of Populus deltoides, and was continous in coverage and regular in depth ( about 8 m ) . The understory-ground flora stratum extended irregularly to heights of 2 m and was dominated by herbaceous vegetation. During these plantation ages, Koeln (1977) found that grasses (mainly Leersia virginica and Sorghum halepense ), forbs (mainly Commelina virginica, Tovara virginiana, Polygonum spp. and Boehmeria cylindrica) and minor inclusions of sedges (Carex spp.) constituted three-quarters of the standing crop of this stratum. The remainder of the standing crop was woody, e.g., Rhus radicans, Rubus trivialis, Celtis laevigata and Acer negundo. METHODS

Characterizing the system Standing crop Only the above-ground portion of the standing crop of vegetation was considered.

Overstory. Nine plots were selected for each plantation age and were inventoried in the winter of 1978. Plot size averaged 0.25 ha in the 6-year-old plantations and 0.50 ha in the 8-year-old plantations. Plot size differed in order to attain uniform stocking and density and to accommodate local variations in topography. Diameters at breast height (dbh) of all trees were measured. In addition, 60 trees representing the dbh range of each plantation age were selected for height m e a s u r e m e n t and regression equations were used to estimate total height of all trees as a function of dbh. Mean stand properties for the two plantation ages are given in Table 1. The above-ground dry weight of the overstory standing crop was based on these plot inventories and used pre-

28 diction equations developed from studies in similar plantations of this locale (Shelton et al., 1982 ).

Understory. The annual maximum and minimum above-ground standing crop of the understory for the three plantations within an age were determined by harvesting areas of 1 m 2 in August and December 1978, respectively, using a rank-set sampling technique McIntyre, 1952). In both August and December, fifteen samples were taken from each plantation for a total of 45 for each age and month. Forest floor Forest floors were sampled concurrently with the understory in August and December. The forest floor was removed from an area of 0.1 m 2 within each of the fifteen 1-m 2 areas that were harvested during understory sampling. The weights of the August and December samples were assumed to be the minimum and maximum forest floors, respectively.

Soil The soils of the plantations were sampled to two depths, the surface 3 cm (considered to be the transition between the forest floor and mineral soil), and to a depth of 120 cm. The surface 3 cm of soil was sampled with the same frequency and intensity as the forest floor. Bulk density samples of the surface 3 cm were obtained in August by taking an undisturbed core from nine randomly selected plots. Soil sampling to 120 cm was done with a vehicle-mounted hydraulic 10-cm diameter sampling tube only during August. Two sample cores were taken within each of the plantations of each age and were divided into five depths: 3-15, 15-30, 30-60, 60-90, and 90-120 cm. Bulk density samples and samples for chemical analysis were collected from each depth interval.

Precipitation Bulk precipitation and throughfall were sampled from November 1977 through October 1979 on two replicated 0.1-ha plots in the open and within the 8-year-old plantations. Ten collectors were placed at random in each replicate; placement was stratified according to canopy closure and all collectors were permanently located. From November 1977 until March 1979, the samples from each collectible event ( > 0.25 cm) were composited by replication to give two bulk precipitation and two throughfall samples. Beginning in March 1979, compositing was done on a monthly basis and was weighted according to the quantities of rainfall at each event. Events less than 0.25 cm were allowed to remain in the collectors and were included in the next collectible event. All samples were treated with thymol and refrigerated at 4 ° C when brought to the laboratory.

29

Laboratory analysis All understory and forest floor tissues were dried to a constant weight at 70 ° C. The fifteen samples from each collection date and plantation were randomly bulked by groups of three to yield five samples. Total N, P, K, Ca and Mg were determined with standard procedures (Jackson, 1958; Isaac and Kerber, 1971 ). The air-dried soil samples were analysed by standard procedures for total N and for neutral N ammonium acetate-extractable K, Ca and Mg. Available P was determined by the procedure described in Sabbe and Breland (1974). Total N, P, K, Ca and Mg in precipitation and throughfall were determined for unfiltered samples using standard procedures. SYNTHESIS

Approach The difference between the dry weight and nutrient content of the standing crops of the 6- and 8-year-old plantations was the basis for determining the accumulation of dry matter, nutrients and net primary production (NPP) during the 7th year. This approach is based on the assumptions that the differentaged plantations had and will have similar patterns of development, and that the change during the 2-year interval is linear - - a pattern which has been observed in similar plantations of this species on these soils in this locale during this interval of development (Mueller, 1976; Shelton et al., 1982 ).

Net primary production Net primary production of the overstory was determined solely from changes in the standing crop since whole-tree mortality was not observed during the study period and since such losses during this 2-year period are normally less than 1% (Krinard and Johnson, 1975). The foliar fraction of the overstory is an exception to this procedure since its contribution to 7th-year production was the replacement of that present at 6 years plus the annual increment. The net production of the understory, which is predominantly herbaceous annuals, is the average of the maximum observed standing crops of this stratum found in the 6- and 8-year-old plantations.

Determining nutrient pools Overstory. The quantities of nutrients in the aboveground components of the standing crop for the two plantation ages were determined with regression equations developed to predict nutrient content of such components from stem diameter (Shelton et al., 1982). These equations, which were developed from

30 trees on adjacent and similar sites, were applied to the plantation inventories of diameters and the results summed by component for each of the two plantation ages. The quantities present in the standing crop at 7 years are the averages of those found at 6 and 8 years.

Understory. Maximum and m i n i m u m nutrient pools in the standing crop of the understories of all plantations were determined from the concentrations and dry weights obtained in the August and December samplings. Forest floor. Forest floor pools in August and December were also estimated from the nutrient concentrations and dry weight obtained in these samplings.

Soil. Quantities of nutrients in this compartment were obtained by combining concentration data with bulk densities. The quantity of nutrients in the surface 3 cm of soil are based on August samples from both plantation ages, while the quantities in the 3-120 cm depths are based only on the samples of the 8year-old plantations.

Determining nutrient requirements Quantities of nutrients required during the 7th year are those contained in the annual N P P of each stand component in this annual interval. The amount of each nutrient in any overstory component was estimated using equations of Shelton et al. (1982). Since the understories are predominantly grasses and forbs, the annual requirements of this stratum were assumed to be those contained in the annual maximum standing crop.

Nutrient fluxes The flux of nutrients was considered for the three general cycles recognized in terrestrial systems, namely, geochemical, biogeochemical, and biochemical (Charley and Richards, 1983 ).

Geochemical cycling. The only flux of the geochemical cycle estimated was the input from bulk precipitation. This input was the annual average of the study area over space and time, and was determined from the long-term monthly means of precipitation for Vicksburg and Stoneville, MS, and Monticello, AR, and the observed monthly weighted means of nutrient concentrations in the collected precipitation.

Biogeochemical cycling. Biogeochemical cycling or transfer between vegetation and soil were considered to consist primarily of four phenomena: throughfall, litterfall, forest floor decomposition, and nutrient uptake. The nutrient flux associated with throughfall is the difference between the

31 content of this component of precipitation and the content of bulk precipitation, i.e., (content of t h r o u g h f a l l ) - ( c o n t e n t of bulk precipitation) = (throughfall flux). The annual flux of nutrients associated with the system's litter is from the foliage of the overstory and the maximum standing crop of the understory. The overstory's contribution is dominated by foliage since tree mortality did not occur. Further, foliar litter from the overstory is only about 50% of the standing crop of foliage due to weight reduction prior to its abscission (Baker and Blackmon, 1977). Litter contribution from the understory is considered to be the standing crop found in this stratum in August. Nutrient fluxes associated with forest floor decomposition were assumed to be the differences between the annual maximum (December) and minimum (August) contents. The uptake of nutrients by the overstory was assumed to equal the difference between annual requirements of the trees and the quantities involved in biochemical cycling. The understory uptake was assumed to be the content found in the standing crop during the August sampling.

Biochemical cycling. Only N and P are known to be involved in this flux between the foliage and the permanent tissues of the overstory, and the proportions reported by Baker and Blackmon {1977) are assumed to also apply to the 7year-old plantation. Nutrient behavior Characterization of nutrient behavior in the fluxes of the system's cycles are expressed in terms of selected coefficients that characterize rates, duration, and amounts, using procedures of Jenny et al. (1949), Westman (1978) and Whittaker et al. (1979). RESULTS AND DISCUSSION

Status of the system Standing crop and forest floor. The approximately 80 Mg ha -1 of dry matter present after the 7th year of development (Table 2 ) represents a mean annual accumulation of slightly more than 11 Mg hal. This mean rate of accumulation approaches the higher productions of agronomic crops in the mid-latitudes, and is characteristic of these rapidly developing systems which are assisted in their establishment by 3 years of soil cultivation to control undesirable vegetation. The overstory standing crop of 66 Mg ha 1 found at 7 years is about one-half of the carrying capacity of these sites which, for this species, has been estimated at 130-140 Mg ha 1 and is attained during the second decade ( Switzer et al., 1976). However, the foliar component of the overstory, 4.7 Mg ha -1, is approaching a steady state ( Switzer et al., 1976; Shelton et al., 1982). In addi-

32 TABLE 2 Net primary production during the 7thyearofdevelopmentofunthinnedplantations (Mgha 1) and the estimated distribution of phytomass in the standing crop ( % ) Stratum and component

Phytomass %

Overstory foliage branches stemwood stembark

Net primary production Mg ha- 1

o~

Mg ha 1

(83)

4.7 1.5 9.1 1.3 16.6

(25) (8) (47) (7) (87)

2.4

(3 )

2.4

(13)

Forest floor

11.0

(14)

System Total

79.2

(100)

19.0

(100)

4.7 13.9 40.9 6.3

(6) (17) (52) (8)

65.8

Understory

Total

tion, the mean understory of 2.4 Mg ha 1 also approximates the steady state of this stratum in these systems (Blackmon, 1977; Koeln, 1977). Values for the two plantation ages were 2.1 and 2.7 Mg ha 1. Further, a steady-state condition is also indicated for the forest floor since the average minimum value (August) for the two plantation ages was 11 Mg ha -1 (10 and 12 observed) and the maximum values (December) of both were 14 Mg ha -1. The minimum annual value of the forest floor is the same as that found in the climax forest composition of the region's uplands (Switzer et al. 1979). The distribution of the system's above-ground phytomass is dominated by the overstory (83%) in wich the accumulated woody tissues--wood, branches and bark--are the major components. The foliated portion of the system (overstory foliage and the entire understory of forbs and grasses) represents only 10% of the phytomass. Since both the foliage and understory approximate their steady-state levels, their proportion of the system's mass will decline with future development.

Net primary production. The net primary production of the plantation during the 7th year was 19 Mg ha -1 of which the overstory contributed 87% (Table 2). This estimate of production is conservative since roots, consumption by primary consumers, and mortality losses were not included. The distribution of net primary production among the sand components of the overstory is 28, 9, and 63% for foliage, branches, and combined stemwood and bark, respecti.vely. The net overstory production of 16.6 Mg ha 1 is similar to the 17 Mg ha -I reported by Shelton et al. (1982) for comparably aged plantations of this spe-

33 cies on similar sites, and approaches the annual maximum of 20 Mg ha ~ reported for the warm-temperate forest (Leith, 1975).

Litter production and decomposition. The system's litter production was 4.8 Mg ha 1 and was equally divided between contributions from the overstory and understory. Production mainly occurred between August and December when the weight of the forest floor increased by 3.0 Mg ha 1. This periodic production consisted of 2.4 Mg ha 1 of overstory foliage and 0.6 Mg ha -1 of understory tissue. The remaining 1.8 Mg ha ~ of litter was contributed by the understory from January through July. The 2.4 Mg ha ~ of foliar litter produced by the overstory represents only 51% of this stratum's foliar weight. Thus, a 49% reduction in weight occurred during abscission, a value very similar to the onehalf reported by Baker and Blackmon (1977). The decomposition rate of the system's litter, using the minimum steady state of the forest floor as a base (Jenny et al., 1949), is 0.30 with a residence time of 3.3 years. These values resemble those of other deciduous systems of the temperate region (Olsen, 1963; Gosz et al., 1976; Peterson et al., 1979). The 3.0 Mg ha ~ change in the forest floor represents a 24% change from the yearly mean, and is similar to the 27% reported by Peterson et al. (1979) for deciduous floodplain systems in Illinois.

Nutrient pools. The system's nutrient status is dominated by the content of the soil compartment (Table 3 ). This compartment contains 97, 97, 90, 98 and 99% of the measured N,P,K, Ca and Mg, respectively. Thus, the nutrient content of the aboveground organic matter is only a small portion of the system's total. The system's nutrient status is further characterized by the dominance of exchangeable cations, particularly Ca D a property strongly associated with the natural occurrence and satisfactory performance of this genus (Bernier, 1984). The system's total stock of nutrients is large and is typical of this productive forest and agricultural region. Although the data are not strictly comparable, the level of the nutrient stocks of this system are considerably greater than those found in most landscapes of the temperate regions of the northern hemisphere, e.g., see Cole and Rapp (1981). On a more local scale, a comparison of the nutrient levels of the alluvial soils of these sites with those of the uplands of the adjacent East Gulf Coastal Plain (Miller, 1967; Hinesley, 1978) also serves to illustrate the comparative size of their nutrient stock. Comparable stocks of N, P, K, Ca and Mg for the alluvial soils are respectively 1.9, 2.2, 1.3, 7.1 and 2.0 times greater than those of the region's uplands. The content and distribution of nutrients in the above-ground organic matter at this age or stage of development is generally balanced between the overstory and forest floor, the most striking exception being with K, and less so with P. The distribution of the sum of all nutrients among the forest floor, under-

34 TABLE 3 Estimated content and distribution of nutrients within unthinned Populus deltoides plantations after the 7th year of development Stratum and Component

Nutrient (kg ha -1) N

P

K

Ca

Mg

Overstory foliar woody

89 119

8 22

69 146

109 283

14 31

Total

208

30

215

392

45

Understory Forest floor Aboveground

36 200 444

7 18 55

49 50 314

22 294 708

6 37 88

Soila surface subsurface

852 12 900

75 2 110

208 2 710

2 100 42 400

342 12 200

13 752

2 185

2 918

44 500

12 542

14 196

2 240

3 232

45 208

12 630

Total

System Total

aSoil nutrients are total N, available P, and exchangeable K, Ca, and Mg.

story and overstory is 37, 7 and 56%, respectively. This compares with 14, 3 and 83% for distribution of above-ground dry matter and serves to emphasize the forest floor as a pool of cycling nutrients. The accumulation of nutrients in the overstory of the standing crop represents about 60% (54-65%) of that found at carrying capacity for natural stands on these sites {Switzer et al., 1976). However, the accumulation in the foliar fraction at 7 years is the equivalent of that found at carrying capacity (93, 10, 61,160 and 15 kg ha 1 for N, P, K, Ca and Mg, respectively). Thus, the period of high production during the accumulating phase of these systems is characterized by the rapid attainment of a fully functional, although temporal, foliar mass that is carried on a relatively small quantity of perennially accumulating support tissue. However, the foliage is a decreasingly smaller fraction of the total above-ground phytomass during the developing phase of these systems, being 36% in a 1-year-old stand, 6% in this 7-year-old stand, and 4% in a 20year-old stand at carrying capacity ( Switzer et al., 1976; Baker and Blackmon, 1977; Shelton et al., 1982).

35

Requirements and fluxes Requirements. The annual requirements for N, P, K, Ca and Mg of the plantations during the 7th year are estimated at 143, 18, 140, 179 and 24 kg ha -1, respectively (Table 4). The dominant characteristic of these requirements is that they are not proportionately divided among the system's increments in dry matter, but are concentrated in the temporal foliar tissues, i.e., the overstory foliage and herbaceous understory. Such tissues constitute 38% (7.1 Mg h a l ) of NPP but account for from 73 % (Ca) to 87 % ( N ) of the annual nutrient requirements. These comparative values are similar to those reported in other forests, e.g., Rochow (1975). Further, although the system is accumulating dry matter at its maximum rate, these foliated fractions are estimated to be approaching or have already attained the site's carrying capacity and, as previously noted, constitute a declining fraction of the standing crop. The annual requirements of the overstory are greater during this seventh year than that of the average of a mixed lot of fourteen 30-120-year-old temperature deciduous forest stands reported by Cole and Rapp (1981). The average annual requirements for their array of stands for N, P, K, Ca and Mg were 98, 7, 48, 56 and 10 kg ha -1, respectively. The larger requirements for Populus deltoides plantations are in part a reflection of their higher annual production (17 vs. 10 Mg ha-~), which in turn may also be due to differences in stand ages or stages of development. Another way of comparing the annual requirements of the 7-year-old Populus deltoides and the temperate deciduous forests of which it is a component is to express the annual requirements of macronutrients relative to that of N. The ratios for Populus deltoides are 1.0, 0.1, 0.9, 1.5, and 0.2 for N, P, K, Ca and Mg, respectively, and for the temperate deciduous forests are 1.0, 0.1, 0.5, 0.6 and 0.1 {Cole and Rapp, 1981). In this comparison the actual N requirements are very similar (107 vs. 98 kg ha 1 ) as are the P requirements. However, the K, Ca and Mg requirements of Populus deltoides are greater.

Fluxes. The relative importance of the geochemical, biochemical and biogeochemical cycles in satisfying the system's annual requirements varies by nutrient and is dominated by the fluxes of the biogeochemical cycle ( Table 4 ). The biochemical cycle is important only for N and P and constitutes 38% and 22% of their respective annual requirements ( Table 5). The fluxes within the biogeochemical cycle are estimated to contribute from 55% (N) to 97% (Ca) of the 7th-year requirements (Table 4). Although this cycle contains three fluxes, it is dominated by the flux associated with production and decomposition of the system's litter (Table 4). The significance of the remaining biogeochemical fluxes, throughfall-leaching and others, differ by nutrient, the former being more important for K and Mg, the latter more important for N and Ca, and both fluxes of equal importance for P. The dom-

36 TABLE 4 Nutrient requirements of Populus deltoides plantations for the 7th year and the quantities in the fluxes of the nutrient cycles Requirements andcycles and fluxes

Nutrient (kg ha -1) N

P

K

Ca

Mg

Requirements o[ standing crop Overstory foliage woody

89 18

8 3

69 22

109 48

14 4

Total

107

11

91

157

18

36

7

49

22

6

143

18

140

179

24

10 54

1 4

4 0

6 0

2 0

71 0 8

9 2 2

78 40 18

106 25 42

15 5 2

143

18

140

179

24

Understory Total

Cycles and fluxes Geochemical precipitation Biochemical Biogeochemical decomposition throughfall leaching othera Total

aUnidentified and unquantified sources of which the inorganic soil fraction is assumed dominant but could also include stemflow, N2 fixation, root exudation, etc. i n a n c e o f d e c o m p o s i t i o n in t h e fluxes o f t h e b i o g e o c h e m i c a l cycle reflects t h e a p p a r e n t a c h i e v e m e n t o f a s t e a d y - s t a t e c o n d i t i o n in t h e forest floor, w h i c h in t u r n reflects s t e a d y - s t a t e c o n d i t i o n s in t h e o v e r s t o r y c a n o p y a n d u n d e r s t o r y . T h e a t t a i n m e n t o f t h e s e c o n d i t i o n s at this c o m p a r a t i v e l y y o u n g age parallels TABLE5 Contribution of the three cycles to the annual nutrient requirements of Populus deltoides plantations during the 7th year Cycle

Geochemical Biochemical Biogeochemical

Percent of annual requirement N

P

K

Ca

Mg

7 38 55

6 22 72

3 0 97

3 0 97

8 0 92

37 the rate of production of these systems (estimated to have attained one-half of their carrying capacity by 7 years of development) and is also an expression of the relative 'tightness' of the system's cycling of required nutrients. In this regard, estimates of the fluxes of the geochemical cycle, which range from 3% (K and Ca) to 870 (Mg) of the requirements, indicate a minor role for this cycle. However, this estimate is restricted to that associated with bulk precipitation and, as already noted, ignores leaching losses from the soil. Further, ~he actual or real long-term role of the import and export phenomena of the geochemical cycle associated with flooding in these riparian systems has not been assessed. The accumulation of nutrients in the increment of the standing crop during the 7th year removed 13, 17, 16, 27 and 1770 of the N, P, K, Ca and Mg, respectively, from the system's total flux of nutrients. Conversely, the remainder of the nutrients continue within the fluxes of the cycles. Thus, even at maximum rates of production, a relatively large proportion of the total flux is retained within the cycles. Further, the quantity accumulated by the standing crop is very small when compared to the total stocks present in the system (Table 3 ). Moreover, the geochemical cycle has the potential of supplying 56, 33, 20, 12 and 50% of the N, P, K, Ca and Mg, respectively, that is being accumulated in the permanent tissues. Thus, less than one-half of the N - - likely the most limiting nutrient - - that is being accumulated (9 kg) must be supplied by N2 fixation or mineralization of soil organic matter. This quantity is not unreasonable for the locale of these systems. In addition, the quantity of the other nutrients in the soils is deemed adequate since they have sustained agronomic production in adjoining areas for generations without requiring nutrient amendments. Nutrient behavior

The behavior of nutrients in ecosystems can be described by a variety of coefficients (Whittaker et al., 1979). Those used here characterize the behavior in the system's dominating biogeochemical cycle (Table 6). Any of these comparisons or expressions emphasizes that the turnover rates and times are always fastest and shortest for K and slowest and longest for N. The relative activity of K and N is also emphasized by the ratio of nutrients in the two primary fluxes of the biogeochemical cycle, i.e., throughfall enrichment/litter content. This ratio, based solely on Populus deltoides tissues, yields values for N, P, K, Ca and Mg of 0.0, 0.67, 1.74, 0.33 and 0.62, respectively. The comparative inactivity of N, and also P, in the fluxes of the biogeochemical cycle is largely offset by their singular activity in the biochemical cycle (Table 4). Another expression of nutrient behavior is the amount of dry matter produced per unit of plant nutrient, sometimes called efficiencies. During the 7th growing season such ratios for nutrients in cottonwood plantations are in the order: P >/Mg > K >/N > Ca (Table 7 ). Comparable efficiencies of Pinus taeda

38 TABLE 6 Descriptive coefficients of nutrient behavior in the biogeochemical cycle of Populus deltoides plantations during the year Comparison

Nutrient N

P

K

Ca

Mg

Decomposition of system litter rate, k time (year)

0.26 3.85

0.33 3.03

0.61 1.64

0.27 3.20

0.29 3.45

Forest turnover rateb rate, k time (year)

0.29 3.45

0.30 3.33

0.45 2.22

0.32 3.13

0.39 2.56

0.50

0.61

0.84

0.73

0.83

2.00

1.64

1.19

1.37

1.20

Net primary production rate c rate, k time (year)

aJenny et al. (1949): k--(forest floor decomposition)/(minimum forest floor +forest floor decomposition) bWestman (1978 ): k = ( total litter production + throughfall ) / ( standing crop ) cWhittaker et al. (1979 ): k = (total litter production + throughfall) / ( net primary production )

p l a n t a t i o n s o f t h e s a m e age, g r o w i n g a t t h e s a m e s p a c i n g o n a good site u n d e r c o m p a r a b l e c l i m a t i c c o n d i t i o n s , are in t h e order: P>~ M g > Ca>~ K > N. W h i l e t h e o r d e r o f efficiencies of p l a n t a t i o n s o f t h e s e t w o species is n o t dissimilar, t h e a b s o l u t e efficiencies v a r y . B o t h species a t 7 y e a r s o f age p r o d u c e t h e s a m e

TABLE 7 Net primary productivity and the ratios of NPP: annual nutrient requirements of Populus deltoides and Pinus taeda plantations at comparable ages and maximum rates of NPP Species

P. deltoides P. taeda*

Age (year) 7 7 14

NPP (Mg ha -1 year ~) 16.6 8.9 21.5

Ratio NPP: annual requirement (kg kgl ) N

P

K

Ca

Mg

155 167 259

1510 1560 2260

182 456 602

106 486 670

922 1510 2070

*Unpublished data, Mississippi Agricultural and Forestry Experiment Station.

39 amount of dry matter per unit of N or P. However, Populus deltoides produces much less dry matter per unit of K, Ca or Mg than Pinus taeda, an expression of its higher requirements for these basic cations. If the efficiencies are compared at the same stage of development, i.e., at maximum N P P rather than age, the differences between the two species are much greater. The Populus deltoides plantations reach maximum annual NPP during the 7th year; for the Pinus taeda plantations this does not occur until the 14t h year. During the 14th year the order of nutrient efficiencies for Pinus taeda is unchanged from the 7th year. However, all nutrients in the Pinus taeda plantations are more efficient in the 14th than in the 7th year, and in the comparison of K, Ca and Mg efficiencies, the disparity with Populus deltoides is even greater. Thus, the early rapid growth of Populus deltoides plantations and early attainment of maximum NPP is accompanied by a relatively high demand for the basic cations. This may explain why Populus deltoides attains its maximum productivity on soils with a high base status (Bernier, 1984). CONCLUSIONS During the 7th year of development these plantations have already attained their maximum rate of NPP, and at this stage are accumulating nutrients at lower rates than during prior periods when they were accumulating steadystate levels of foliage and forest floor mass. These conditions emphasize the rapidity with which these systems accumulate nutrients and reach a stage when most of the nutrient requirements are met by cycling, particularly those that are more internal, i.e., the biochemical and biogeochemical cycles. This stage of development appears to coincide with the time when foliage has reached a steady state. Apparently the maximum demand for nutrients from the soil pool occurs during the initial period of plantation establishment when the aboveground pools, particularly those associated with the canopy, are being accumulated. Once foliar and forest floor steady-state levels are reached, cycling fluxes are at a maximum and demands on the soil nutrient pools are relatively modest since such demands are only needed to satisfy the requirement of the annual increment of permanent woody tissues. ACKNOWLEDGEMENTS The authors gratefully acknowledge the cooperation of the United States Gypsum Corporation as well as the staff of the Southern Hardwoods Laboratory, USDA Forest Service, Stoneville, MS, and especially J.B. Baker, USDA Forest Service, Monticello, AR. This study was partly funded by the Southern Forest Experiment Station, Forest Service, USDA, under Cooperative Agreement No. 19-256, and is based in part on the Ph.D. dissertation of B.G. Lockaby.

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