Comparison of Growth , Nutrition and Soil Properties of Pure and Mixed Stands of Populus deltoides and Alnus subcordata

Concerns about decline in soil fertility and long-term productivity of fast-growing plantations have promoted interest in using nitrogen-fixing trees in mixed species plantations. Populus deltoides and Alnus subcordata were planted in five proportions (100P, 67P:33A, 50P:50A, 33P:67A, 100A) in Noor, Iran. After 7 years, the effects of species interactions on tree growth and nutrient concentration in live and senescent leaves and soil properties were assessed. Diameter at breast height and total height of individual Populus trees were positively affected by the presence of Alnus. Nitrogen concentrations in fully expanded and senescent leaves of Populus were higher in mixed plantations than monoculture plantations. The results of nutrition and nutrient return and growth indicated that mixed plantations of these two species were more productive and sustainable than their monoculture plantations. Within the framework of this experiment, it appeared that production was maximized when these two species were grown together in the relative proportions of 50% Populus and 50% Alnus.


Introduction
Poplars (Populus L. spp.) are preferred plantation species, because their fast growth is expected to meet the extensive demands of wood for poles, pulp and fuel (Kiadaliri 2003, Ghasemi 2000, Zia-bari 1993).Productivity of plantations depends strongly on soil nutrient supply and it may be malleable under the influence of management practices and species (Binkley 1997).Almost all the industrial plantations are monocultures, and questions are being raised about the sustain-ability of their growth and their effects on the site (Khanna 1997).Repeated harvesting of fastgrowing trees such as poplar plantations on short rotations may deplete site nutrients.Nitrogen losses are likely to be very important for future growth.It is, therefore, appropriate to explore new systems of plantation management in which N may be added via fixation (Khanna 1997).
Mixed plantation systems seem to be the most appropriate for providing a broader range of options, such as production, protection, biodiversity conservation, and restoration (Montagnini et al. 1995, Keenan et al. 1995, Guariguata et al. 1995and Parrotta and Knowles 1999).
Mixed plantations yield more diverse forest products than monospecific stands, helping to diminish farmer's risks in unstable markets.If planned with consideration for each species' response to mixed conditions, mixed designs can be more productive than monospecific systems.In addition, a mixture of species, each with different nutrient requirements and different nutrient cycling properties, may be overall less demanding on site nutrient than monoculture stands (Montagnini 2000).
Mixed plantations can produce more biomass per unit area because competition among individuals is reduced and the site is used integrally (Montagnini et al. 1995).The roots of different species may occupy different soil strata allowing more complete utilization of soil and water resources (Lamb and Lawrence 1993).More solar energy can be captured because different species have different light requirements and crowns are broadly distributed in the vertical plane (Guariguata et al. 1995).However, the success of the establishment of mixed forest plantations depends on plantation design and an appropriate definition of the species to be used, taking into consideration ecological and silvicultural aspects (Wormald 1992).
Concerns about decline in soil fertility and long-term productivity of fast-growing plantations have promoted interest in using nitrogenfixing trees in mixed species plantations (Rhoades and Binkley 1996).Nitrogen-fixing trees, mainly leguminous species, have been widely extolled for their soil-improving characteristics related to their production of nitrogen-rich, often rapidly decomposing leaf litter (Parrotta 1999).Although there have been some documented cases of increased productivity in mixed-species plantations in both temperate and tropical regions, the collective results of such studies have been inconclusive and show that accurate species/site matching and choice of complementary species strongly influence mixed-species plantation productivity (FAO 1992).
Experiments in some parts of world such as North America have shown enhanced growth of Populus spp.when grown as an intercrop with Alnus L. spp.(FAO 1992, Coté and Camiré 1987, Hansen and Dawson 1982, Radwan and DeBell 1988).The present study was undertaken to assess the influence of Alnus subcordata C.A.Mey and Populus deltoids Marsh.plantations on soil fertility parameters, the influence of Alnus on Populus growth and nutrient concentration of fully expanded and senescent leaves in monocultures and mixed plantations.

Site Characteristics
The study area is located at the Chamestan experiment station, in Mazandaran province, on the northern parts of Iran (36º29'N, 51º59'E).Experimental plots were located at an altitude of 100 m above sea level and with low slope (0-3%).Annual rainfall averages 803 mm, with wetter months occurring between September and February, and a dry season from April to August monthly rainfall usually averages < 40 mm for 4 months.Average daily temperatures range from 11.7 ºC in February to 29.5 ºC in August.
The soils are well-drained, and have a silty loam texture with a pH 7.6-8.1.Previously (approximately 50 years ago) this area was dominated by natural forests containing native tree species such as Quercus castaneifolia C.A.Meyer., Gleditschia caspica Desp., Carpinus betulus L., etc.The surrounding area is dominated by agricultural fields and commercial building.
Tree spacing within plantations was 4 m × 4 m and tow species were systematicly mixed within rows.

Site Preparation and Planting of Seedlings
Site preparation for all plantations and control plots consisted of disk harrowing to a depth of 10-15 cm.Containerized seedlings, 50-100 cm in height, were used for planting in April 1996.
Seedlings of both species were planted simultaneously in monocultures and mixed plantations.

Tree Survival and Growth Measurements
Diameters of trees at 1.3 m height (DBH), crown diameter, tree basal area and tree heights were measured in the central 24 m × 24 m area (subplot) of each plot, excluding the outer two tree rows, in July 2003.

Nutrition and Nutrient Return by the Leaves
Foliage samples were collected from the stands in September 2003.Leaves were collected from the bottom one-third of the tree by clipping two small twigs located on opposite sides of the crown.Six representative trees (two near the center of sub-plot and one in each corner of it) of each species were sampled for fully expanded leaves.
In addition, senescent leaves were collected from each species in each sub-plot.The Samples were dried at 70 ºC.Nitrogen was determind using the Kjeldhal, P using Spectrophotometer (by the Olsen method), and K, Ca and Mg (by ammonium acetate extraction at pH 9) was determined using Atomic absorption spectrophotometer (Bower et al. 1952).

Soils
Soils were sampled to a depth of 60 cm in all plantations and control plots in August using a 7.6 cm diameter core sampler (n = 3 cores/plot) taken at two 15 cm and a 30 cm interval.After air drying, soils were passed through a 2.0 mm (20 mesh) sieve to remove roots prior to chemical analyses.Soil pH was determined using an Orion Ionalyzer Model 901 pH meter in a 1:2.5, soil: water solution.EC (electrical conductivity) was determined using an Orion Ionalyzer Model 901 EC meter in a 1:2.5, soil: water solution.Soil organic matter was determined using the Walkley-Black method.Total N was determined using the Kjeldhal method (Bremmer 1960).Available P was determined with spectrophotometer by using Olsen method (Homer and Pratt 1961).Available K, Ca and Mg (by ammonium acetate extraction at pH 9) were determined with Atomic absorption Spectrophotometer (Bower et al. 1952).

Statistical Analyses
One-way analyses of variance (ANOVA) were used to compare tree growth, soil properties, and leaf nutrients data among experimental treatments.Tukey-HSD and Duncan tests were used to separate the means of dependent variables which were significantly affected by treatment.

Tree Survival and Growth
The survival of Populus was generally unaffected by the presence of Alnus with the exception of higher Populus survival when the two species were grown in equal proportion (50P:50A) than 67P:33A (p < .05,Duncan) (Fig. 2a).Alnus survival, in contrast, was unaffected by the presence of Populus in any proportion (p < .08,Duncan).Mixed plantations had a positive effect on the DBH of Populus when compared with the trees growing in monoculture.Populus in the 33P:67A treatment had the highest DBH.The DBH of Alnus was reduced when this species was grown in combination with Populus as compared with monocultures but the relative proportion of Populus did not further affect this result (p < .01,Tukey-HSD) (Fig. 2b).Total basal area of both species was not affected by relative proportions of the two species (p < .08,Duncan) (Fig. 2c).Within the 50P:50A treatment Populus accounted for significantly more of the total basal area than did Alnus.
Height growth of Populus was affected by the presence of Alnus (Fig. 2d).In the 67P:33A and 50P:50A treatments Populus height was higher than in the treatment 100P (p < .01,Tukey-HSD).The height growth of Alnus however, was unaffected by the presence of Populus (p < .08,Duncan).
Similarly the crown diameter of Populus was positively affect by the presence of Alnus (p < .01,Tukey-HSD) but the crown diameter of Alnus was unaffected by the presence of Populus (p < .08,Duncan) (Fig. 2e).When the two monocultures were compared, Alnus had a larger crown diameter than Populus.

Soil Properties
There were very few differences in soil properties between treatments.Soil pH, ranging from 5.78 to 7.12, and Carbon: nitrogen ratio, ranging from 7.56 to 18.09, did not show any signifi-cant difference between the treatments.Soil EC in 0-15 cm depth of monoculture Alnus and control treatments was different and no significant differences were found in deeper soil layers (p < .08,Duncan) (Table 1).Organic matter was different between 0-15 cm depth of monoculture Populus and 50P:50A treatments whereas no significant differences were found in deeper soil layers (p < .08,Duncan) (Table 1).Total nitrogen had some differences between the treatments in 0-15 cm and 15-30 cm soil layers.Total nitrogen in 0-15 cm depth was significantly higher in the control plot than in the 67P:33A treatment (p < .08,Duncan).The differences in 15-30 cm depth were stronger than 0-15 cm depth (p < .05,Duncan).It seems that the treatments that had more proportion of Alnus had lower C:N ratio than others (Table 1).Available P in 0-15 cm depth of monoculture Alnus was different (p < .08,Duncan) with monoculture Populus, 67P:33A and 33P:67A treatments whereas no significant differences were found in deeper soil layers (Table 1).Available K, ranging from 22.5 to 61.25 mg kg -1 , and available Mg, ranging from 10.00 to 16.25 mg kg -1 , did not show any significant difference between the treatments.Available Ca in 0-15 cm depth of 67P:33A and control treatments was different (p < .05,Duncan) whereas no significant differences were found in deeper soil layers (Table 1).

Nutrition and Nutrient Return by the Leaves
The presence of Alnus as N-fixing tree had strong influence on nitrogen cycling of Populus.Populus leaves (fully expanded and senescent) had significantly higher N concentration when grown in mixtures than in monocultures (p < .05,Duncan).Nitrogen return by senescent leaves of Alnus in monoculture plantations was significantly lower than the ones that grown in mixed plantations (p < .05,Duncan) (

Discussion
Because the survival of both species (Populus and Alnus) did not show in most cases any differences between monocultures and mixed plantations it can be concluded that there was little competition between the two species, as a result of their planting spacing.Parrotta (1999) found more survival in monoculture plantations of Eucalyptus robusta than mixtures with Casuarina and Leucacna.The differences between our results and those obtained by Parrotta (1999) were probably due to differences in planting space.The other reason might be the difference in growth rate and crown diameter of his target and associated species in comparison with our species.Our results were, howere, similar to those obtained by Khanna (1997) about Eucalyptus glabulus and Acacia mearnsii in monocultures and mixed plantations.
Higher Populus diameter growth was observed in treatments with less proportion of Populus.This could be due to a decrease in light competition, as the most important competition factor (Binkley 1992).Results of our work correspond with that of Khanna's work (1997) on monocultures and mixed plantations of Eucalyptus and Acacia.It was also found that N-fixation by Acacia in mixed plantations resulted in increased diameter growth of Populus (Khanna 1998).Rapid diameter growth was also found to be due to domination of the target species (Montagnini 2000).No influence of nitrogen fixing trees on diameter growth on non nitrogen fixing tree was observed by Parrotta (1999) but Binkley (1983) found contrary results with Alnus rubra having a positive effect on the diameter growth of Pesudotsuga menziesii in poor sites.
Basal area of both Populus and Alnus did not show any significant differences in monocultures compared to mixed plantations in our study.In contrast, Parrotta (1999), Khanna (1997), Montagnini (2000) found bigger basal area for target species in mixed plantations compared to monocultures.
Increased light competition in mixed plantations containing a grater proportion of Populus may explain the greater height growth of this species in these plots.Alnus did not show any differences in height growth between the monocultures and mixtures.Our results are similar to those obtained by Parrotta (1999) and Khanna (1997), in monocultures and mixed plantations of Eucalyptus and Acacia, and Hansen and Dawson (1982) in monocultures and mixed plantations of poplar species and Alnus glotinosa.In contrast to our results, Heilman (1985) did not observe greater height of target species in mixed plantations (FAO 1992).
The crown diameter of Populus was significantly smaller in monoculture plantations in comparison with that in the 50P:50A and 33P:67A treatments.It might be the result of decreasing light competition in mixed plantations with less proportion of Populus.Water and light competition in mixed plantations result in decreasing crown diameter (Fisher and Binkley 1999).These results have strong correlation with other results such as increasing diameter in the treatment with high Alnus proportions and increasing height in treatment with low Alnus proportions.
No statistically significant differences were observed in soil pH between the treatments, whereas significant reductions in soil pH have been found in half of the studies about the effect of nitrogen fixing trees (NFT) on soil (Fisher and Binkley 1999).Montagnini (2000) and Gia-rdina et al. (1995) reached the same result as we, whereas Rhoades and Binkley (1996), and Parrotta (1999) found lower soil pH in mixed plantations.Higher planting density and low age of our plantations might be the main reasons for no significant difference in soil pH.
No significant differences were observed in soil organic matter content in 15-30 cm and 30-60 cm soil layers between the treatments.The main reason for the reduction of soil organic matter in 0-15 cm depth in the 50P:50A treatment compared to that in the monoculture of Populus might be the result of increasing biological activities in the soil.Parrotta (1999) came to the same conclusion.In contrast with our results Garcia-Montiel and Binkley (1998) found that organic matter content in 0-20 cm depth of soil under NFT Albizia was higher than in soil under Eucalyptus.
It is obvious that Alnus increased soil nitrogen in 15-30 cm depth in comparsion with 67P:33A and 50P: 50A treatments.In 0-15 cm depth, soil nitrogen was highest in the control plot, which we can relate to grasses.The decrease in soil nitrogen of mixed plantations with high proportions of Populus might be the result of invading Populus roots to nodules of Alnus (Binkley 1992).Binkley (1997) and Garcia-Montiel and Binkley (1998) found that Albizia increased soil nitrogen more than Eucalyptus.Parrotta (1999) and to some extent Montagnini (2000) did not observe any significant differences in soil nitrogen between monocultures and mixed plantations.Whereas Hansen and Dawson (1982) observed that mixed plantations of Populus and Alnus glutinosa resulted in increasing soil nitrogen in comparison with their monoculture plantations (FAO 1992).
No significant differences were observed in the C:N ratio.The main reason might be the low percentage in the mixtures.Parrotta (1999) reached the same result with us in monocultures and mixed plantations of Eucalyptus and two NFTs.Few significant differences were observed between the treatments in concentrations of available Mg, K, Ca and P in soil.We can relate these differences to previous soils condition such as what Parrotta (1999) did.Montagnini (2000) came to the same results in monocultures and mixed plantations as we did.
Nitrogen concentrations in fully expanded and senescent leaves of Populus were higher in the mixed plantations than those in the monocultures of Populus.This is a good reason for the influence of mixed plantations with Alnus as an N-fixing tree on nutrition and nutrient return of our target species.The other result is less N of Alnus senescent leaves in monoculture plantations than mixed ones that shows more nitrogen return in mixed plantations.Parrotta (1999) reached to the same results and found that litterfall (leaves) of NFT and target species in mixed plantations had more N than in monocultures.In contrast to us Khanna (1997) did not observe any significant differences in fully expanded and senescent leaves of Acacia between monocultures and mixed plantations.The higher concentration of Ca in senescent leaves of Alnus in mixed plantations than monocultures showed that more Ca can return to the soil in mixed plantations.Finally we can conclude that mixed plantations of these two species are more productive and sustainable than monocultures.The introduction of one mixture as the best one is rather difficult but 50P:50A could be the most productive and sustainable.
Fig. 2. a) Survival, b) DBH, c) Basal area, d) Height and e) Crown diameter of both tree species were separately compared.The letters on different column indicate a different comparison.Mean values with the same letter within a tree species do not differ significantly with each other.

Table 1 .
Soil properties in plantations and control plots in different soil layers a with their standard deviations (below).
a Based on three composited 7.6 cm diameter core samples per plot b ANOVA results: ns = treatment effect not significant, * = p < 0.08, ** = p < 0.05, Duncan.Mean values with the same letter within the soil layer do not differ significantly with each other.

Table 2 .
Nitrogen concentrations in fully expanded and senescent leaves with their standard error in the parenthesis.