Soil water dynamics and litter production in eucalypt and native vegetation in southeastern Brazil

FORESTRY SCIENCE

Soil water dynamics and litter production in eucalypt and native vegetation in southeastern Brazil

Paulo Henrique Muller da SilvaI, ^*; Fabio Poggiani^II; Walter de Paula Lima^{I, II}; Paulo Leonel Libardi^III

^IForest Science and Research Institute, Av. Pádua Dias, 11, C.P. 530 - 13400-970 - Piracicaba, SP - Brazil

^IIUniversity of São Paulo/ESALQ, Forest Sciences Dept., Av. Pádua Dias, 11, C.P. 09 - 13418-900 - Piracicaba, SP - Brazil

^IIIUniversity of São Paulo/ESALQ, Dept. of Biosystems Engineering, Av. Pádua Dias, 11, C.P. 09 - 13418-900 - Piracicaba, SP - Brazil

ABSTRACT

High productivity of eucalypt plantations is the result of advances in research that have led to gradual improvements in intensive silvicultural technology. High productivity notwithstanding, eucalypt plantations remain the focus of environmental concerns. Our study aimed to compare the soil water regime, litter fall and nutrients dynamics either in a fragment of native forest or in an adjacent stand of growing eucalypt. We took field measurements during the first three years of eucalypt plantation in a sandy soil in the southeastern region of Brazil. Soil moisture and internal drainage were higher during the early stages of growth of the eucalypt stand, as compared with native vegetation. However, one and a half years after planting, available soil water was similar in both vegetations. Higher water availability under the eucalypt stand during the first year occurs because of silvicultural operations (soil preparation and weed control) and the small size of eucalypt trees; these factors increase water infiltration and decrease transpiration. Total leaf fall, over the study period, was similar for both ecosystems; however, differences were observed in the winter and early spring of 2010. The transfer of nutrients to soil by leaf fall was similar except for N and S, which was higher in native vegetation. Nitrogen concentration in the soil solution was higher in native vegetation, but K was higher under the eucalypt stand, mainly to a depth of up to 0.2 m.

Keywords: Initial growth, soil water flow, soil solution, nutrient cycling

Introduction

The widespread occurrence of eucalypt in natural areas in Australia has generated different genotypes (species and origin) that have high productivity for commercial use in several regions of the world. Furthermore, over the past few decades, the development of effective silvicultural techniques (Pallett and Sale, 2004) have contributed to doubling average eucalypt productivity in Brazil, increasing from 20 m³ ha ^{- 1} year ^{- 1} in 1970 to more than 40 m³ ha ^{- 1} year ^{- 1} currently (Gonçalves et al., 2013). Brazilian silviculture has shown advances over the years with regard to economic, environmental and social concerns. Changes have been observed at all stages of wood production, from nursery to harvesting of the forest (Stape et al., 2001; Dias Junior et al., 2005).

Many researchers have brought competitive advantages to commercial eucalypt plantations through improvements in productivity. But there are also several studies that have contributed to appropriate environmental management of eucalypts stands. The studies are related to invasiveness (Silva et al., 2011; Gordon et al., 2012), flammability (Gill and Zylstra, 2005), nutrient cycling (Laclau et al., 2005) and water consumption (Lima et al., 1990; Almeida and Soares, 2003).

The most important concerns in eucalypt plantations are probably related to water and nutrition, which are both strongly linked to the productivity of the forest stand. The productivity of eucalypts in tropical regions is most likely constrained by water supply, and water affects the efficiency of nutrient uptake (Stape et al., 2004).

Our study monitored and compared eucalypt stands during the first three years of growth (half of the commercial rotation in Brazil) and seasonally dry native forest vegetation. We evaluated soil water dynamics at depths of 0.2 and 0.9 m, N and K concentration in the soil solution, and litter fall nutrient dynamics.

Materials and Methods

The experiment was set up in Jan 2009 in Anhembi, in the state of São Paulo, Brazil (22º47' S; 48º09' E; 500 m a.s.l.). The climate of the region is hot with rainy summers and moderately cool, dry winters, in between the Cwa and Aw groups (Köppen classification). The mean annual air temperature is 21.8 °C, and mean annual rainfall is 1,240 mm, with mild to moderate water stress during winter. The soil is a deep (<10m) Ferralic Arenosol- AR (FAO) or Quartzipsamment (USDA), and the slopes are gently rugged (5 - 10 %) with low natural fertility (Table 1). The soil is 90 % sand with a very homogeneous profile (Table 2). The surface horizon contains organic matter in the first 10 or 15 cm. The A horizon is followed directly by C horizon, as the high sand content does not favor the formation of B horizons. The soil analyzes were performed based on Sparks et al. (1996) formulations.

Eucalypts stand: The site had been previously cultivated with a commercial plantation of a Eucalyptus grandis. After seven years, in Aug 2008, the area was cut down and cleared. In Jan 2009, a new stand was established using a hybrid of Eucalyptus urophylla and E. grandis (C041H) with 1,333 trees ha ^{- 1}. Silvicultural practices included herbicide application for weed control (glyphosate in total area), soil preparation (subsoiling to 0.45 m deep), leaf-cutting ant control (manual application of baits), planting and fertilizer application. The fertilization was performed manually, simulating commercial application procedures, applying 80 kg of N, 32 kg of P 105 kg of K per hectare. The N and K were top-dressed four times (at 3, 6, 12 and 18 months). Total height and diameter at breast height (DBH) of the eucalypts were measured at 12, 24 and 36 months after planting to estimate the stem volume. The stem volume in our study (Table 3) is representative of eucalypt productivity in this type of soil and climatic conditions in southeastern Brazil (Silva et al., 2011).

Native vegetation: The vegetation comprised an early-successional forest remnant of Seasonally Dry Forest (with hyper-abundant and light-dependent lianas) inserted in an ecotone between Cerrado (Neotropical Savannah) and Seasonal Semideciduous Atlantic Forest. The canopy of native vegetation has an approximate height of 5 m and almost all treetops are covered by hyper-abundant, light-dependent lianas, most of them belonging to the Bignoniaceae and Sapindaceae families. Just a few trees, like Aspidosperma polyneuron and Anandenanthera spp. from Apocynaceae and Leguminosae families respectively, are taller reaching about 15 m. The soil forest is colonized by invasive grasses, such as Urocloa (Brachiaria) spp., mainly in forest clearings and borders. The eucalypt stand and native vegetation are separated by a narrow rural road (6 m apart). Plot selection was restricted to the experimental area with the same soil and topography. We used three replications for both vegetation types.

Soil moisture was measured by tensiometers, which allowed for the calculation of internal drainage (soil water flow) using the Darcy - Buckingham equation.

in which: qz = soil water flow density; K(θ) = soil hydraulic conductivity as a water content function; = soil water potential (m day ^{- 1}).

The K(θ) and at 0.2 and 0.9 m soil depth were calculated from weekly readings of tensiometers installed at depths of 0.15, 0.25, 0.85 and 0.95 m. The tensiometer readings were monitored for 36 months (Feb 2009 to Jan 2012). To calculate the soil water flux density, we determined the soil water retention curve to obtain the soil water content from tensiometer readings and then the soil hydraulic conductivity K from the equation of K as a function of volumetric water content obtained at both depths (0.2 and 0.9 m) by the instantaneous profile method (Ghiberto et al., 2011).

In the determination of soil water retention curve, ten undisturbed soil samples of 100 cm³ were collected at depths of 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 m. The samples were submitted to tensions from 0 to 90 kPa using porous plate funnels (funnels of Haines) and from 100 to 1,000 kPa using a porous plate air pressure chamber (Richards chamber).

For the chemical analysis, we collected soil solution samples monthly, when the soil moisture content allowed, using solution extractors (porous cup). Extractors were installed at a depth of 0.2 and 0.9 m, and the samples comprised three sub-samples collected in each replication of the experiment at both depths. Soil solution samples were collected to compare the N and K concentration. To extract the soil solution, we used a pump to suck the air from the extractor, generating a pressure of at least -80 kPa relative to atmospheric pressure in the extractor. Preservative (thymol-2-isopropyl-5-methyl-phenol) was added to the soil solution samples, which were stored at 8 °C. The analyses were performed using flow injection analysis (Laclau et al., 2010).

Leaf fall and nutrient return to the soil were assessed for 24 months from the 12^th to the 36^th month after eucalypt planting. The leaf fall was collected monthly using three leaf-traps (0.25 m² each) in each replication, giving a total of nine per vegetation type. The samples were dried, weighed and chemically analyzed following procedures described by Laclau et al. (2009). Composite samples were prepared for the chemical analysis to enable the concentrations of N, P, K, Ca, Mg and S to be determined. We estimated the transfer of nutrients from the trees' canopy to the soil through leaf fall by multiplying the dry matter by the nutrient concentration (Laclau et al., 2010).

These measurements were analyzed using a t-test to identify whether the average result from the eucalypts stand and native vegetation differed. Analyzes were performed for every period that measurements were evaluated.

Results and Discussion

Fertilization of eucalypts produced proper growth similar to that observed in commercial plantations in Brazil with adequate silvicultural practices in place (Stape et al., 2010), including genotypes with good adaptative qualities, soil preparation, weed, pest and disease control and mineral fertilization (Gonçalves et al., 2013). Under the eucalypt stand the soil had higher pH, P, Ca and Mg and lower OM, S and K content than in native vegetation (Table 1). Such differences can be attributed to the effect of previous mineral fertilizations in agricultural crops cultivated in the same area where the treatment with eucalypts was allocated in this experiment. Fertilization is necessary to maintain eucalypt productivity and nutrient stocks in infertile tropical soils over successive rotations (Laclau et al., 2010). Silva et al. (2013) found that doses of fertilizers applied to eucalypt affected nutrient dynamics differently. Fertilization increases the nutrient concentration in the leaves, the nutrient cycling and the growth of the eucalypts stand.

Water dynamics in the soil and soil solution

During the early stage of eucalypt growth, the water content in the soil under the eucalypt stand was higher than under the remnant native vegetation. However, when the plantation was one and a half years old, this difference in available soil water was no longer observed (Figure 1). The water content in the soil followed the monthly precipitation. The largest difference (p < 0.05) in internal drainage was found in the first year after planting (Table 4), when 22 % of the rainfall drained below 0.9 m deep at the eucalypt stand. One and a half years after the eucalypts planting, internal drainage was similar for both vegetations (Figure 2). In the second year, less than 10 % of the rainfall drained below the 0.9 m depth for both vegetations.

Up to one and a half years after planting, the eucalypt stand had higher water content in the soil than the native forest remnant (Table 4). Higher soil water content during the first year occurs due to the establishment process of the eucalypt stand: soil preparation, weed control and the small size of young eucalypts. These factors increased water infiltration, changed the soil bioporosity (Shipitalo and Protz, 1987) and decreased transpiration (Newton and Preest, 1988). However, this difference soon disappeared and soil water content became similar in both types of vegetation, due to greater expansion of the roots and shoots of eucalypts that start the competition among the eucalypts trees, when canopies closed.

High drainage could be attributed to rainfall distribution throughout the year. In our study the rainfall was 444 mm in Jan 2011 (Figure 1), which corresponded to more than 30 % of the total annual rainfall and resuled in high drainage in both vegetations. The drainage is a combination of distribution, gross rainfall and throughfall (after leaf interception) that has been studied for a long time in forests (Helvey and Patric, 1965). In our study, high drainage from Jan to Mar 2011 was observed in both vegetations because of the cumulative rainfall during this period, even at the stage when the eucalypt roots had already explored a considerable soil volume (Laclau et al., 2013).

The high density of lianas in native vegetation reduces sources of soil water, mainly at the beginning of the dry season, and sap flow is similar in lianas and trees (Andrade et al., 2005; Tobin et al.; 2012). Lianas can grow approximately seven times more in height than the trees, during the dry season, though only twice as much during the wet season. Over time, the advantage in the dry season may allow the lianas to increase in abundance in seasonal forests (Schnitzer, 2005). In our study, the native vegetation with high liana infestation had the same availability of soil water as that in the eucalypt stand between 18 and 36 months after the eucalypt planting. In order to correctly apply the eucalypt zoning, differences in water drainage during the rotation must be taken into account (Laclau et al., 2005; Lima et al., 2012).

After one year of eucalypts planting, N concentration in the soil solution was higher under native vegetation than under the eucalypts stand. In the first two years after planting, higher concentration of K was found under the eucalypts stand at 0.2 m than under native vegetation (Table 5).

N and K concentrations in the soil solution in our study were higher than those found by Laclau et al. (2003) in eucalypt plantation on sandy soil in a savannah. Although other factors may have contributed to the results in our study, the amount of fertilizers was higher than found in Laclau et al. (2003).

The difference in N and K concentrations in the soil solution observed in our study (Table 5) between the two types of vegetation reflects the higher OM content in the soil under native vegetation and the presence of leguminous plant species. The high K concentration in the soil solution under the eucalypt stand was a direct effect of the mineral fertilization in the first year after eucalypt planting.

Nutrient cycling

The annual leaf fall from Jan 2010 to Dec 2011 was similar in native vegetation and eucalypts (≈ 6,500 kg year ^{- 1}), but differed in the months of July and Sept 2010 (Figure 3). Leaf fall concentration and flow from the canopy to ground of N and S were higher in native vegetation than in the eucalypts stand (Table 6). During the dry season, many trees lose their leaves as a strategy to overcome the typical water deficit and re-sprout at the beginning of the rainy season.

The annual leaf litter fall production of the two vegetation types was similar to the results found by Silva et al. (2011) under similar climatic conditions. Few factors affect the litter fall and the most important are tree genotypes and climatic variations (Parrotta, 1999). Climatic conditions played an important role in litter fall production. In both vegetation types, high litter fall production was observed during the dry season between 2010 and 2011(Figure 3) and the smallest litter fall production was in Jan and Oct 2011, when high rainfall was observed.

The N flow through the litter fall was greater in the native vegetation than in the eucalypt stand. Laclau et al. (2005) studied nutrient cycling in the Congo (4^o08' S; 12^o08' E; 80 m) comparing a clonal stand of eucalypts with an adjacent savannah ecosystem and found that leguminous plant species in the native vegetation are responsible for substantial N uptake by symbiotic fixation. In commercial stands, however, legumes were eliminated during weed control applications. Nitrogenous contents should be monitored during eucalypt stand rotations, because N presents a negative budget in eucalypts stands (Silva et al., 2009).

Acknowledgments

We thank the Conpacel-Suzano company for its support and the laboratory coordinators for their assistance. We also thank our colleagues Pedro Henrique Santin Brancalion (ESALQ/USP), Otávio Camargo Campoe (IPEF), the reviewers and editor for their contribution.

Received October 03, 2013

Accepted March 28, 2014

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Publication Dates

• Publication in this collection
  01 Oct 2014
• Date of issue
  Oct 2014

History

• Received
  03 Oct 2013
• Accepted
  28 Mar 2014