Net primary production and nutrient cycling in replicated stands of Eucalyptus saligna and Albizia facaltaria

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1 Forest Ecology and Management 112 (1998) 79±85 Net primary production and nutrient cycling in replicated stands of Eucalyptus saligna and Albizia facaltaria Dan Binkley a,*, Michael G. Ryan b a Department of Forest Sciences, Graduate Degree Program in Ecology, and Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO 80523, USA b USDA Forest Service, Rocky Mountain Research Station, 240 W. Prospect, Ft. Collins, CO 80523, USA Received 18 December 1997; accepted 30 April 1998 Abstract Production and nutrient cycling budgets were estimated at three locations within 1 km on the island of Hawaii, for replicated plantations of Eucalyptus saligna Sm. and nitrogen- xing Albizia facaltaria (L.) Fosberg (ˆParaserianthes facaltaria (L.) Nielson)). At the age of 16 years, the aboveground biomass of Eucalyptus averaged 323 Mg/ha, about 50% more than the 216 Mg/ha of Albizia biomass. Net primary production (NPP) was about 40 Mg ha 1 year 1 for both species. Eucalyptus allocated 45% of NPP to stem production, compared with 34% of Albizia (pˆ0.02). Conversely, Eucalyptus allocated less production belowground (29% of NPP) than did Albizia (41% of NPP, p<0.01). Litterfall mass did not differ between species, but differences in litterfall nutrient concentrations led to greater litterfall cycling of N and P for Albizia than for Eucalyptus (141 vs. 105 kg N ha 1 ; 6.2 vs. 4.8 kg P ha 1 year 1 ). The rate of N cycling in the aboveground-increment plus litterfall did not differ signi cantly between species. Lower soil P supply under Albizia may be partially responsible for the high ratio of belowground:aboveground production for Albizia. The mean annual increment (MAI) of aboveground biomass of Eucalyptus for 16 years was 20.2 Mg ha 1 year 1, which is not different from the annual increment of 19.3 Mg ha 1 year 1 between the age of 14 and 16 years. The MAI for Albizia (13.5 Mg ha 1 year 1 ) also matched the annual increment (13.9 Mg ha 1 year 1 ) from ages 14±16. The sustained high productivity of these stands may warrant longer rotation periods than currently recommended, especially on fertile soils or in silvicultural systems with high rates of fertilization. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen xation; Tropical forest productivity; Ecosystem production 1. Introduction Tropical forest plantations are among the most productive ecosystems in the world, with growth rates often exceeding 40 Mg ha 1 year 1 of aboveground net primary production, and 15 Mg ha 1 year 1 of *Corresponding author. wood production (Lugo et al., 1988; Evans, 1992; Binkley et al., 1997). High rates of production depend strongly on species selection, nutrient supply, and the ef ciency of nutrient use. Nutrient limitations are common in tropical plantations, particularly for nitrogen (N) and phosphorus (P) (Evans, 1992; Binkley et al., 1997). The productivity and nutrient cycling pattern of forests change substantially with forest /98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S (98)

2 80 D. Binkley, M.G. Ryan / Forest Ecology and Management 112 (1998) 79±85 age (reviewed by Ryan et al., 1997). Current stem production typically peaks between 2 and 6 years of age (Lugo et al., 1988); (Whitesell et al., 1992, Binkley et al., 1997, DeBell et al., 1997). The patterns of production over time may differ between species, resulting in part from different effects of tree species on nutrient availability over time. On the wet side of the island of Hawaii, replicated plantations of Eucalyptus saligna (Sm.) and nitrogen- xing Albizia falcataria (L.) Fosberg [ˆParaserianthes falcataria (L.) Nielsen]) showed that current annual aboveground increment peaked at about 28 Mg ha 1 year 1 for both species between 2 and 4 years of age (DeBell et al., 1997). At age 6, the Albizia plots cycled 3 timed more N and 2 times more P in aboveground litterfall than in the Eucalyptus plots (Binkley et al., 1992). By age 10, Albizia plots had accumulated about 15±35% more biomass than pure plots of Eucalyptus (depending on the amount of extra fertilization applied to Eucalyptus). In the present study, we took advantage of these monoculture plots plus additional plots at two nearby sites to develop a well-replicated, descriptive case study of the productivity and nutrient cycling in stand of Eucalyptus and N- xing Albizia at the relatively `old' age of 14±16 years. 2. Methods Eucalyptus saligna is native to fertile valleys in northern New South Wales and southern Queensland in Australia, with annual rainfall of more than 1000 mm and occasional frost (FAO, 1981). Albizia facaltaria is native to New Guinea, Irian Java, and some surrounding island, in locations with more than 1000 mm year 1 of rain with no prolonged dry periods or frosts (MacDicken, 1994). Three study sites are located on the northeast coast of Hawaii, about 20 km north of Hilo ( N, W) at 480 elevation with little annual variation in temperature (mean annual temperature 218C) and evenly distributed rainfall of >4000 mm year 1 (Binkley et al., 1992). The slopes are moderate (<15%)), with deep (>2 m), acidic (ph 4.7 in water) soils of the Kaiwiki series of thixotrophic isotropic Typic Hydrudands. The three sites are within 1 km of each other, and were cropped with sugar cane for more than 50 years, with the last harvest in 1980 (Site 1) or 1960 (Sites 2 and 3). At each of the three sites, BioEnergy Development Corporation planted both species in pure plots with a completely randomized block design, with 4 blocks per site (a total of 12 plots for Eucalyptus and 12 plots for Albizia). At Site 1, a mixed species trial was established in The soil was plowed and new vegetation was herbicided prior to the planting of pure and mixed stands of Eucalyptus and Albizia (DeBell et al., 1989; 1997; Binkley et al., 1992). The pure Albizia plots received a total of 80 kg N ha 1 (as ammonium nitrate) and 6 kg P ha 1 (as triple superphosphate), and the pure Eucalyptus plots received 370 kg N ha 1 and 108 kg P ha 1. Tree spacing was 2 m2 m in 30 m30 m plots (pure Eucalyptus) or15m30 m plots (pure Albizia). Site 2 and 3 are 1 km north of Site 1. In 1981, the vegetation that developed during the fallow period was cleared, the soil plowed, and the sprouting vegetation herbicided prior to planting (in 1982) of pure stand in a species trial (tree spacing m, plot size 1218 m). All the plots received a total of 100 kg N/ha, and 50 kg P/ha. Information on soil characteristics is available in Rhoades and Binkley, 1995; Garcia-Montiel and Binkley, 1998, and Binkley et al. in review. The soil under Eucalyptus contained 6 Mg N/ha in the 0±20 cm depth, compared with 7 Mg N/a under Albizia. The supply of N was several-fold greater under Albizia (Garcia-Montiel and Binkley, 1998), whereas the availability of P was 20±50% lower under Albizia (Binkley, 1997). In October 1997, we measured the diameters of all the trees in a 48row subplot in the center of each plot. The height of the tallest tree in each plot was measured (with a clinometer), and the heights of the other trees were approximated relative to the tallest tree. Total aboveground biomass at each period was estimated using equations developed locally for these species. The data used for previously published biomass equations (Whitesell et al., 1992) were combined with additional data from destructively sampled trees at age 10 (T. Cole, D. DeBell, unpublished) and age 17 (S. Resh, J. Kaye, pers. commun.) to develop new biomass equations: Eucalyptus biomassˆ0.016 * diameter 2.145* height (r 2ˆ0.997, nˆ32)

3 D. Binkley, M.G. Ryan / Forest Ecology and Management 112 (1998) 79±85 81 Albizia biomassˆ0.036 * diameter 1.783* height (r 2ˆ0.985, nˆ36) where total aboveground biomass is in kg, diameter (at 1.4 m height) is in cm, and height is in m. The largest Eucalyptus sampled for biomass determination was 50 cm dbh, and the largest Albizia was 36 cm dbh. Plot biomass was estimated by summing the mass of the trees within each subplot, and then plot biomass values were estrapolated to a hectare basis. Annual aboveground production was estimated from the measured change in diameter of 12 trees per subplot that had been measured in October 1994 and October For these 12 trees, we calculated annual increment directly as the difference in biomass divided by 3 years. For the remaining trees on the subplot, we established a relationship between 1997 biomass and annual increment (r 2ˆ0.95, p<0.001 for Eucalyptus; r 2ˆ0.81, p<0.001 for Albizia), and used this relationship to estimate the increment on the other trees in each subplot. Some increment may have occurred on suppressed trees that did not survive this 3 year period, but the omission of this unmeasured increment should be very small relative to the growth of surviving, dominant trees. Nutrient content of biomass was estimated as the nutrient concentration of boles times the total stand biomass. These values underestimate the true content, as twigs and leaves have higher concentrations, but no separate biomass estimates for these components were available. Nutrient concentration of boles was determined by taking cores (from the bark to the pith) from the largest tree in each plot. 300 mg subsamples were digested in 15 ml of concentrated H 2 SO 4 (with Na 2 SO 4 added to raise the boiling point), and boiled for 4 h after clearing to insure complete digestion. Replicate samples were within 5±10%, and standard wood samples were within 10% of reported values for nitrogen. The increment of nutrients is aboveground biomass was calculated as the biomass increment times the bolewood concentration. Non-woody litterfall biomass was examined by placing 5 littertraps (0.125 m 2 ) in each plot, with monthly collections for 5 months in 1996 (January, February, March, April and June). The litter was composited within plots for each sampling period, dried and weighed. The samples were then composited across sampling periods for determining element concentrations (as above the wood). The annual rate of litterfall was calculated by assuming the 5-month average represented 5/12 the annual litterfall. Soil-surface CO 2 ef ux was measured on 3 dates (January, April, and May 1996) using a PP Systems portable IRGA (EGM-1, PP Systems, Haverhill, MA). The PP Systems IRGA measures soil-surface CO 2 ef ux using a closed system (Field et al., 1991), with a chamber volume of 1.17 l enclosing an area of 78.5 cm 2. Samples were taken at 6±8 locations along a transect through each plot at each sampling date. Annual soil-surface CO 2 ef ux was estimated as the average of the 3 sampling dates ± a reasonable assumption, given the small seasonal variation in temperature, rainfall, and soil-surface CO 2 ef ux at a nearby Eucalyptus plantation (MG Ryan, unpublished data). Conservation of matter dictates that total belowground C allocation equals the annual soil-surface CO 2 ef ux minus annual litterfall plus annual increment for coarse roots (see Raich and Nadelhoffer, 1989), as long as changes in C storage are minor and ux rates of soil-surface CO 2 and litterfall are accurate. We estimated total belowground C allocation using this method, and converted total belowground C allocation into production by assuming that 50% of the ef ux resulted from respiration of roots and mycorrhizae, that 50% went for dry matter production (after Ryan, 1991), and that roots are 50% C. From Eucalyptus plantations in this area, we know that mineral soil C accrues at rates of <1 Mg ha 1 year 1 (Bashkin and Binkley, 1998), and forest oor biomass accrues at rates of 0.5± 1.0 Mg ha 1 year 1 (Zou et al., 1995). Inclusion of these uxes might increase our estimates of total belowground C allocation by 5±10%. The original plantations were completely randomized with 4 blocks at each site. Combining all three sites into one analysis no longer provides a completely randomized design, so `site' was used as the main effect, `blocks' were used as plot effects, and specieswithin-blocks were a split-plot effect. The error term for the main effect was plot-within-site sum of squares. The error for the species (ˆ split plot) effect and the site X species interaction was the sums of squares for the species X plot-within-site. Statistical analyses were computed with SYSTAT version 7.0 (SYSTAT, 1997), with a p value of 0.10.

4 82 D. Binkley, M.G. Ryan / Forest Ecology and Management 112 (1998) 79±85 3. Results and discussion At age 16, Eucalyptus plots had accumulated 323 Mg/ha of aboveground biomass, about 50% more than Albizia (216 Mg/ha; Table 1). The mean annual increment (MAI, based on aboveground biomass) was 20.3 Mg ha 1 year 1 for Eucalyptus, and 13.5 Mg ha 1 year 1 for Albizia. The annual aboveground increment between ages 14 and 16 (19.3 Mg ha 1 year 1 Eucalyptus, 13.9 Mg ha 1 year 1 Albizia, pˆ0.10; Table 2) were very close to the MAI for each species, indicating the plantations have sustained high growth rates well past the recommended rotation age of about 5±8 years (Whitesell et al., 1992 but see Phillips et al., 1997). Concentrations of nutrients in boles differed between the species, so the nutrient content of the aboveground biomass was not simply proportional to the biomass differences. The Albizia plots contained about three times as much N as the Eucalyptus plots, but only 60% as much P (Table 1). Total NPP averaged about 40 Mg ha 1 for both species (pˆ0.54; Table 2, Fig. 1). The allocation of production differed substantially between species (p<0.01). Eucalyptus allocated 45% of NPP to stem increment and 29% belowground, whereas Albizia for allocated 34% to stem increment and 41% belowground. Belowground NPP increased in Eucalyptus plots as total NPP increased, but belowground NPP in Albizia stands was more constant and showed little trend with NPP. In Eucalyptus plots, belowground NPP increased across plots by 26% of the increase in NPP (pˆ0.06, nˆ1). Belowground NPP in Albizia tended to increase at a rate of 13% of the increase in NPP across plots, but the trend was not signi cant (pˆ0.36, nˆ12). Earlier data on stand biomass and production are available only for the least-productive Site 1. DeBell et al. (1997) reported greater Albizia biomass (200 Mg ha 1 ) than Eucalyptus biomass (169 Mg/ ha; DeBell et al., 1997) at age 10 at Site 1. Our corresponding values at age 16 were 220 Mg ha 1 for Eucalyptus and 190 Mg ha 1 for Albizia (data not shown by site). DeBell et al. (1997) used the equations developed for trees up to 4 years of age, and we used equations based on these trees plus trees sampled at age 10 and 16. Using the same equations as DeBell et al. (1997) drops our estimated Eucalyptus biomass at Site 1 to 200 Mg ha 1 (a 10% decrease), and increases our Albizia biomass to 200 Mg ha 1 (a 5% increase). Regardless of the use of different equations, these comparisons show a substantial net biomass increment for Eucalyptus at Site 1, and no (or perhaps negative) net increment for Albizia. The net annual increment at Site 1 peaked between years 2 and 4 for Eucalyptus at 27.9 Mg ha 1 year 1, and MAI culminated at age 6 at 22.4 Mg ha 1 year 1. From age 10±16, the Eucalyptus plots at Site 1 had a net increment of 10.9 Mg ha 1 year 1, about 20% below the 16 year MAI for this site of 13.8 Mg ha 1 year 1. The stem production at this poor site appears to have Fig. 1. Estimated production budget for age 14±16 years; total NPP does not differ between species ( pˆ0.54)but production of stems and roots differ by species ( pˆ0.10), as does the proportional allocation to roots and stems by species ( pˆ0.02). Table 1 Aboveground biomass and nutrient content at age 16 (kg/ha). Standard deviation and p value are in parentheses Species Biomass N P K Ca Mg Eucalyptus (89000, <0.01) 134 (45, <0.01) 28 (10, <0.01) 170 (60, 0.95) 295 (100, 0.10) 31 (11, 0.55) Albizia (109400, <0.01) 323 (115, <0.01) 16 (6, <0.01) 169 (57, 0.95) 244 (87, 0.10) 29 (10, 0.55) No effects of site of speciessite interactions were significant (p>0.1).

5 D. Binkley, M.G. Ryan / Forest Ecology and Management 112 (1998) 79±85 83 Table 2 Net primary production and nutrient content of biomass increment and litterfall at age 14±16 years (kg ha 1 ) Species Component Biomass N P K Ca Mg Eucalyptus Stem production (9000, 0.10) 8 (4, <0.01) 1.7 (0.8, 0.02) 10.2 (4.7, 0.71) 18 (8, 0.56) 1.9 (0.9, 0.94) Albizia Stem production (8000, 0.10) 21 (12, <0.01) 1.0 (0.6, 0.02) 10.9 (6.2, 0.71) 16 (9, 0.56) 1.9 (1.1, 0.94) Eucalyptus Litterfall (2100, 0.16) 105 (37, 0.02) 4.8 (1.2, 0.14) 14.2 (5.4, 0.06) 348 (111, 0.21) 24.3 (5.0, 0.63) Albizia Litterfall 9500(1800, 0.16) 141 (42, 0.02) 6.2 (3.0, 0.14) 17.8 (8.8, 0.06) 284 (120, 0.21) 25.5 (7.7, 0.63) Eucalyptus Aboveground NPP, nutrient use (10 300, 0.06) 113 (39, <0.01) 6.5 (1.5, 0.35) 24.4 (9.3,0.11) 365 (112, 0.21) 26.2 (5.6, 0.60) Albizia Aboveground NPP, nutrient use (8200, 0.06) 162 (41, <0.01) 7.2 (12.6, 0.35) 28.8 (12.6, 0.11) 300 (123, 0.21) 27.4 (7.8, 0.60) Eucalyptus Belowground NPP (5700, 0.10) Albizia Belowground NPP (3900, 0.10) Eucalyptus Total NPP (12 300, 0.54) Total NPP (12 300, 0.54) Standard deviation and p values in parentheses. No effects of site or species site ineractions were significant.

6 84 D. Binkley, M.G. Ryan / Forest Ecology and Management 112 (1998) 79±85 declined more substantially than at the more fertile Sites 2 and 3. The net increment for Albizia plots at Site 1 also peaked between years 2 and 4 at 28.6 Mg ha 1 year 1, with culmination of MAI at age 6 at 22 Mg ha 1 year 1, declining to 9.4 Mg ha 1 year 1 at age 14±16. The death of some large Albizia trees probably accounted for the lack of net increment between age 10 and 16, despite substantial stem production. Eucalyptus mortality was con ned to small, severely suppressed trees with little effect on stand net increment. Nitrogen in non-woody litterfall was about 40% greater under Albizia than Eucalyptus, a much smaller amount than the 3±fold difference at age 6 (for Site 1, Binkley et al., 1992). The total N use for aboveground production (at age 15) was also about 40% greater for Albizia than Eucalyptus. The narrowing of the differences between species over time resulted from a substantial increase in litterfall N in Eucalyptus (from 30 to 80 kg N ha 1 year 1 ) and a decrease in Albizia (from 180 to 120 kg N ha 1 year 1 ). We speculate that the Eucalyptus plots may have gained N from Albizia litter falling from adjacent plots and perhaps also from ne roots `mining' Albizia plots. Indeed, this increase in N supply and use may explain how the Eucalyptus plots have maintained a current annual increment at age 14 to 16 which matches the MAI. Larger plantations of Eucalyptus without the in uence of Albizia as a source of N may require fertilization to sustain high productivity in older stands (Binkley et al., 1997). At age 6, the Eucalyptus plots (at Site 1) were returning less than half as much P (3 kg P ha 1 year 1 3 for Site 1) in non-woody litterfall as the Albizia plots (Binkley et al., 1992). By age 15, the rates no longer differed: 4.6 kg P ha 1 year 1 Eucalyptus, 4.4 kg P ha 1 year 1 for Albizia at Site. 1. Across all the three sites at the age of 15 years, the annual incorporation of P into aboveground tissues was similar between species, which was surprising given that the availability of P was substantially lower under Albizia (based on bioassays with each species, with on-site resin bags, and laboratory fractionation; Binkley and Giardina, 1997; Binkley et al. in review). The relatively high rate of P uptake by Albizia despite lower P supply could result in part from greater belowground C investment in roots and mycorrhizae; we estimated a 30% greater belowground allocation under Albizia ( pˆ0.10). The high uptake may also have resulted from substantial differences in the organisms and processes that affect P uptake by trees, including the soil communities. Albizia soils had signi cantly less fungi and more bacteria (Garcia-Montiel and Binkley, 1998) and earthworms (Zou, 1993) than in Eucalyptus soils, as well as higher rates of phosphatase activity (Zou et al., 1995). The contribution of each of these potentially important factors to the tree-available supply to P remains unclear. The overall picture of these plantations is one of continued high NPP and stem increment for both species, except of Site 1 where current Albizia increment was less than half of the MAI. Net primary production did not differ between species; the substantially greater stem production of Eucalyptus resulted from differences in allocation rather than differences in ecosystem production. The sustained high growth rates of both species may warrant extension of optimal rotation periods from 5±8 years (Whitesell et al., 1992) to 15±20 years (Phillips et al., 1997) for these fast-growing species, especially: on more fertile sites; in systems with greater inputs of fertilizer; and where logs can be used for sawn products rather than for pulp or energy production. Acknowledgements We thank the former BioEnergy Development Corporation, a subsidiary of C. Brewer and Company, for establishing these plantation, and for permission to sample them; the assistance of T. Schubert and T. Crabb has been essential to this project to our related work. R. Senock, B. Ewers, M. Bashkin, C. Giardina, and D. Garcia-Montiel helped substantially with the eld work. We also thank J.R. zumbrunnen of the Statistical Laboratory at Colorado State University for consulting services on the statistical design and analysis. This project was funded by NSF DEB and DEB , and by Mclntire-Stennis appropriation to Colorado State University. References Bashkin, M., Binkley, D., Changes in soil carbon following afforestation in Hawaii. Ecology 79, 828±833.

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