Silvicultural Implications in Hibrid of Eucalyptus urophylla S . T . Blake × Eucalyptus grandis Hill ex Maiden Stand

The objective of this work was to evaluate the silvicultural implications in a Eucalyptus urophylla S.T. Blake × Eucalyptus grandis Hill ex Maiden (Eucalyptus urograndis) stand 4.5 years-old, located in Alegrete RS. The trees were fractionated in the following components: root, wood, bark, branches and leaves for later estimation of nutrient content and stock. An inventory was made for the dendrometric characterization of the stand. The nutrient concentration and stock in each component were evaluated. The nutrient removal was estimated considering three biomass harvesting scenarios: wood with bark + canopy, wood with bark and wood only. The nutritional balance and the number of rotations (4.5 years) of production were determined, considering the nutrient stock in the soil, the inputs through precipitation + mineral fertilization and the outputs from the biomass harvest. The risk of yield limitation among the nutrients considering the harvest of biomass showed the following pattern: Wood with bark + canopy: Sulfur ~ Potassium > Nitrogen > Calcium > Magnesium; Wood with bark: Sulfur > Potassium > Calcium > Nitrogen > Magnesium; Wood: Sulfur > Potassium > Nitrogen > Magnesium > Calcium. Phosphorus showed a tendency of nutritional sustainability in the three harvest scenarios evaluated.


Introduction
Forestry with exotic species is present in most Brazilian biomes. The genus Eucalyptus occurs mainly in the Cerrado biome, in the southeastern and central-western regions of the country. In the Pampa biome, southwestern Rio Grande do Sul, commercial plantations for industrial purposes started to receive the largest investments since 2000, with wood production projects to serve the pulp, energy and sawn wood segments.
The Eucalyptus urophylla S.T. Blake × Eucalyptus grandis Hill ex Maiden (Eucalyptus urograndis) is characterized by good growth due to Eucalyptus grandis having a significant increase in the basic density of wood (better cellulose yield) and Eucalyptus uropohylla providing greater resistance to water deficit as well as adaptation to different forest sites, with consequent increase in yield indexes (Montanari et al., 2007).
Because eucalyptus is a fast-growing species, and consequently with high biomass production and nutrient accumulation, there are many questions about the impacts and sustainability of the stand. In this context, the study of the species' behavior in the edaphic environment and its other interactions provides elements that may contribute to increase yield gains in consonance with the use of natural resources, besides assisting in the decision making of the forester regarding the choice of the species to be implanted and the nutritional replacement for the next production cycle (Viera, Schumacher & Caldeira, 2015).
high-intense harvest (e.g., wood with bark) increases the cost with corrective fertilization and maintenance (Viera et al., 2013).
The objective of this study was to evaluate the implications of silvicultural management in a Eucalyptus urograndis stand established in the Pampa biome, by estimating: biomass and nutrients in tree components (leaves, branches, bark, wood and root), nutrient removal under different harvesting scenarios and nutritional balance.

Characterization of the Experimental Area
The experiment was carried out in a 4.5 year-old commercial stand, implanted with clonal Eucalyptus urograndis seedlings at a spacing of 3.5 m × 2.5 m, located at Fazenda Cabanha da Prata, in the municipality of Alegrete-RS (55º32′53″ W, 29º47′60″ S).
The region climate according to Köppen's classification (Alvares et al., 2014) is considered humid sub-temperate with frosts from May to August, and intense heat mainly in the months of January and February, with average temperature of the warmest month > 22 °C and average annual temperature > 18 °C. Precipitation has rainfall ranging from 1250 to 1500 mm.
The soil is classified as typical Distrophic Red Argisol (EMBRAPA, 2006). According to Pessotti (2006), this class involves deep soils, well drained, sand texture on the surface, followed by sandy clay loam texture in the deepest horizons. Chemically they are soils with medium to high values of exchangeable bases, subject to leaching of mobile nutrients such as N and K, and presents moderate retrogradation of soluble Phosphorus. According to Streck et al. (2008) the soil presents low natural fertility. The physical and chemical attributes verified in the soil analysis of the sample units are presented in Table 1.  Planting was carried out with initial density of 1150 plants per hectare. For the implantation, cultural treatments were: subsoiling in September of 2007, using subsoiler with three stems, fertilization by incorporation of 300 kg ha -1 of reactive natural phosphate (GAFSA, 12% P 2 O 5 soluble in citric acid) in the center of the planting line and 40 cm deep, followed by harrowing.

Estimation of Biomass and Nutrient Stock
The methodology used to estimate the biomass and the nutrients was adapted from Melo et al. (1995) and Neves (2000). In March 2011, when the plantation was 4.5 years old, three sample plots of 10 m × 35 m (350 m²) were randomly distributed in an area of 10 hectares, where the diameter was measured at breast height (DBH) and the height of 20% of the trees, for the dendrometric characterization of the stand. There was a mean DBH of 17.6 cm and a total height of 22.7 m. The average annual increment with bark (AAI b) observed was 47.2 m 3 ha -1 , with a density of 1110 plants ha -1 .
After DBH measurement, 3 trees were selected per sample by the mean DBH-standard deviation, mean DBH and mean DBH+standard deviation, totaling 9 trees. The selected trees were sectioned at soil level and the volume with and without bark determined through rigorous sampling according to the methodology described by Smalian (Péllico Netto & Brena, 1997), and later fractionated in the following components: leaf, branch, bark, wood and root. The total wet biomass of each component was determined directly in the field by weighing with hook scale.
For dry biomass estimation, wet samples (150 g each) of the different components were collected, one sample per tree (randomly) of leaf and branch components. Three samples of wood and bark (150 g to 500 g) were collected per tree, distributed along the commercial stand. Samples were obtained at the median positions of the sections resulting from the division into three equal parts of the trunk. The minimum diameter considered was 8 cm.
For the biomass of the roots, 3 trees (one in each plot) were selected among the 9 used for the biomass above the soil, being chosen by the mean DBH. The root system (stump and thick roots) of the trees was extracted by backhoe and manual excavation (shovels and hoes) in the useful area, considering the spacing between the trees of 3.5 m × 2.5 m (8.75 m²) and a depth of 1 m. The roots were weighed in their entirety and a sample (150 g) was collected. After weighing the samples were stored in paper bags and sent to the Forest Ecology Laboratory of the Forest Engineering Department/UFSM. The samples were dried in forced air oven at 70 ºC, and weighed in a 0.01 g precision scale after reaching constant weight.
For the chemical analysis and estimation of total nutrient stocks in the soil, samples were collected in the pits where the roots were excavated, in the layers of 0-20, 20-40 and 40-100 cm. During soil sampling for chemical analyzes, samples were also collected at the mid-points of the layers, using Kopecky volumetric rings to determine soil density. The analyzes of plant tissues and soil were followed by the methodology of Tedesco et al. (1995), recommended by the SBCS-CQFS (2016) and Miyazawa (1999).
Total tree biomass ha -1 was estimated by multiplying the sampled dry mass average of the sampled trees by the total number of individuals in each plot, extrapolating to the plots area, determining the accumulated biomass therein. The same extrapolation procedure was used to determine the accumulated biomass ha -1 .
The amount of each nutrient in the different components was obtained by multiplying nutrient concentration and dry biomass. Total amount of nutrients hectare -1 was estimated by extrapolation of the amounts observed in each plot, similar to the procedure described above. Total nutrient stocks in the soil were estimated bymultiplying soil volume, nutrient concentration and soil density obtained at the midpoint of each layer. For Nitrogen, due to its great dynamics in the soil, and because it is contained in little available forms (very stable humic fractions), only 10% was considered as available to the plants.

Estimation of Nutritional Balance and Number of Production Rotations (NPR)
The nutrient balance estimation was obtained by the difference between nutrient input via mineral fertilization + available stocks in the soil (Table 2) and the output, as a result of the nutrient removal through the biomass harvest, considering three scenarios: 1) Wood with bark + canopy, 2) Wood with bark, and 3) Wood, with the use of the whole trunk, without discarding the tree tops.

Statisti
The contra completely for its resp 3. Results

Bioma
The bioma (15%) > br   Note. Vertical letters do not differ statistically between nutrient concentrations in the different biomass components, at the 0.05 level of significance by the Tukey test.
According to Pallardy (2008), the difference in nutrient concentration among plant components is related to maturity. The more mobile nutrients of senescent tissues tend to move to regions with higher metabolic activity. Poggiani and Schumacher (2004), explain biochemical cycling is more important for the maintenance of nutrients with high mobility (N, P, K and Mg), and lower for low mobility nutrients (Ca, S) and also micronutrients.
Considering only the biomass above the soil, the leaves had the highest concentration for most of the elements. Viera (2012) notes that most nutrients tend to focus on newer plant structures, especially on leaves, where the main metabolic processes (transpiration and photosynthesis) occur. The high concentration of Ca in the bark is related to the low mobility of this element in the plant phloem and also to be a structural component of the cell membrane (Ferri, 1985).
Total storage of macro and micro-nutrients was 1232 kg ha -1 and 14.7 kg ha -1 , respectively (Table 4). Relative nutrient allocation for above-ground biomass was of: N (25%), P (2%), K (25%), Ca (34%), Mg (8%), S and B (5%), Cu (4%), Mn (86%), and Zn (5%). Allocation magnitude followed the order Ca > N ~ K > Mg > S > P, and Mn > B ~ Zn Cu (Figure 1).    Hernández et al. (2009), observed biomass production, concentration and nutrient amounts in the various components of eucalyptus trees are directly related to planting density and soil fertility. Poggiani and Schumacher (1993) emphasize the nutritional characteristics of the species and the age of harvest influence the accumulation of nutrients. The magnitude of concentration of the macronutrients presented the following order: leaf > bark > branch > root > wood. For micronutrients the order was the following: root > leaf > bark > branch > wood.

Silvicultural Implications After Harvest
Harvesting of all biomass above ground is the most aggressive scenario of nutrient removal, which is reflected in the nutritional balance and maintenance of the productive capacity of the site. It can be observed that the Phosporus presents nutritional sustainability trend in the three scenarios evaluated (Table 5).  Note. NRP = Number of Production Rotations, i = infinite rotations, suggesting nutritional sustainability of P in the production system.
The removal of the wood provided the lowest removal of nutrients from the system because although the wood presents the highest production of biomass, the highest concentration of nutrients is located in the leave, branch, bark and roots compartments. According to the intensity of the biomass harvest, the following nutrient removal gradient can be defined: i) Wood + bark + tree tops: Ca > N > K > Mg > S > P; ii) Wood + bark: Ca > K > N > Mg > S > P; iii) Wood only: K > N > Mg > Ca > S > P, and Mn > Fe > B > Zn > Cu.
The greatest differences in nutrient permanence according to the harvest intensity were observed in Ca and Mg, and it was observed these nutrients present in greater proportions in the bark component. According to Schumacher and Caldeira (2001), the harvesting of the wood with the bark enhances the nutrient removal from the forest site, especially for Ca. Considering the harvest of the wood with the bark and only the wood, similar results were found by Viera (2012) and Neves (2000), in studies with the hybrid of Eucalytpus urophylla × Eucalytpus globulus, Eucalyptus grandis, and Eucalyptus saligna. This trend of distribution was not found by Spangenberg et al. (1996) and Merino et al. (2005), studying Eucalytpus urograndis and Eucalytpus globulus, respectively.
Although there is a greater removal of the nutrients from the stands with the increase of the biomass harvest intensity, in practice, the absolute nutrient depletion does not occur. When analyzing this behavior, we observe a transition from a level of productivity in one cycle, to a lower level of productivity in the next cycle, and so on (Schumacher et al., 2013).
The most productive sites extract the highest amounts of nutrients and reach exhaustion more quickly. Therefore, the maintenance of high levels of productivity will depend on the use of fertilizers, harvesting only the wood component and conservation principles (Bizon, 2005). Therefore, in order to reestablish nutritional balance and ensure forest productivity in the next production cycles, large inputs of fertilizer capital (inputs, equipment and labor) should be used, which will directly increase the cost of the crop.