Ananias Francisco Dias Júnior · Ana Lúcia Martins Sodero Pincelli ·Ana Paula Câmara da Silva · Álison Moreira da Silva · Natália Dias de Souza ·Mario Tommasiello Filho · José Otávio Brito

Abstract As forests in neotropical regions, particularly in developing countries, are devastated, interventions to restore biodiversity and its ecological functions are needed. Rural producers have thus been encouraged to grow trees for wood production as an economic activity. The objective of this study was to quantify the increment of wood density of four forest species from dif ferent successional classes of a mixed system of restoration in a neotropical forest in Brazil. Tree discs were sampled at breast height and analyzed radially by X-ray densitometry to obtain apparent density and basic density. Three trees each of a species from the pioneer, early and late secondary successional stages and of the dominant species in the climax community. The radial prof iles indicated increasing density from the pith to bark of trunks, except for some variations due to wood defects and growth. Average density was 0.576 g cm −3 , 0.655 g cm −3 , 0.706 g cm −3 and 0.775 g cm −3 , respectively, for Peltophorum dubium, Schinus terebinthifolius, Cariniana estrellensis and Hymenaea courbaril. Radial prof iles indicated higher amplitudes in the apparent densities for slow-growing species. X-ray densitometry generated parameters such as minimum, average and maximum densities, and radial density variations. These parameters are important for understanding the ecological functional role of successional classes of the Atlantic Forest from the Neotropical region.

Keywords X-ray densitometry · Timber species of neotropical forest · Successional classes of forest species ·Maintenance of forest restoration

Introduction

To restore the biodiversity and ecological functions of devastated forests and large degraded areas in tropical regions,especially in developing countries, interventions are urgently needed (Rodrigues et al. 2009; Slik et al. 2015). The need is even more urgent for neotropical forests that support most of the tree species on the planet (Slik et al. 2015 ; Beech et al.2017; Amazonas et al. 2018). In biomes where ecological functioning has been compromised and much of the native biodiversity is now seriously threatened, ecological restoration is needed on a large scale, mainly to restore the ecological services it provides (Engel and Parrota 2001; Paula et al. 2004; Rodrigues et al. 2011; Sos Mata Atlântica 2015).These are minimum actions necessary for the recovery and permanent preservation of legally protected reserves that are primarily rural areas. Including species with wood potential in restoration processes through mixed plantations and allowing silvicultural management are important for valuation for commercial purposes and for socioeconomic and socioenvironmental aspects (Gandolf i et al. 1995; Amazonas et al.2018). This process enables the perpetuation of native neotropical wood species dominated by traditional silviculture based onEucalyptus andPinus(Kelty 2006; Amazonas et al.2018) and supports the producer as the landscape is recomposed in his/her rural area, producing wood to generate f inancial resources and maintain the plantations (Amazonas et al.2018; Brancalion et al. 2012; Lamb 2005).

Commercial mixed plantations generally comprise two to four species and are often preferred because they can produce a larger quantity or higher quality of wood (or biomass) than monocultures can (Kelty 2006). The demand for wood products coincides with a growing international demand for forest and landscape restoration and for forests that can be used to achieve multiple objectives (FAO 2016; Brancalion and Chazdon 2017; Chazdon et al. 2017). The properties of wood enable its use for multiple products and purposes. However, we need to understand the interaction between dif ferent successional ecological classes and their ef fect on wood properties.Wood density, in particular, is an ef ficient, widely studied and used genetic variable for characterizing the quality of wood to determine its best use (Ferreira and Tomazello Filho 2009;Lobão et al. 2010). Although several factors inf luence wood density, it can be considered as a universal index for evaluating the wood due to the ease of determination and the excellent relation to wood use (Tomazello Filho et al. 2005; Santos and Sansígolo 2007; Sette Júnior et al. 2009). This property is complex, resulting from the combination of anatomical,physical and chemical characters and can vary signif icantly with age, source, place of origin, spacing, growth rate, between genera and species, and even between trees of the same species(Souza et al. 1986; Sette Júnior et al. 2009).

Wood density is primary property that limits the ultimate use of a wood product. In some cases, when shorter cutting cycles for power generation, f irewood and posts, high densities are increasingly required. Knowledge of forest species with potential for use in forest restoration systems that can simultaneously contribute to the ecological and economic success of an ecosystem and provide wood for suitable use, has been the constant goal for mixed planting systems to restore and maintain the system by cutting some species in a shorter period of time. Toward this goal, we thus analyzed here the increase in wood density of four forest species from dif ferent successional classes from a mixed forest restoration system from the neotropical region.

Material and methods

Experimental area

Trees of 20 pioneer and non-pioneer native species constitute an experimental plantation established in 2004 in plots(12 rows × 11 plants = 132 plants), with 3 m × 2 m spacing,in a legal reserve area of the Experimental Station of Forestry Sciences of Anhembi, São Paulo State, Brazil. This forest plantation integrates the project “Models of restoration of degraded areas of the Atlantic Forest aiming at carbon sequestration”. The plantation area is located at 22° 40′S latitude and 48° 10′ W longitude, at 455 m a.s.l., in sandy,deep soil (Stape et al. 2006).The climate of the region is classified as mesothermal Cwa (Köppen), with hot, rainy summers and moderately cold, dry winters. The average annual temperature is 23.0 °C, annual average rainfall is 1100 mm, with 20 mm of annual water def icit in the dry months (April to September).During 2014–2015, an extreme climatic event was recorded,with a reduction in rainfall, and the water balance was calculated using the method of Thornthwaite and Mather ( 1955)to be 300 mm for available soil water capacity for the type of soil in the experimental area (Fig. 1).

Experimental methods

One species from each succession groups was selected (Schinus terebinthifolius,Peltophorum dubium,Cariniana estrellensisandHymenaea courbaril, Table 1), and in September of 2015, three trees of each species were felled. We then removed discs from the wood (10 cm thick) at breast height(DBH, 1.30 m from the ground level),

Fig. 1 Monthly water balance from January 2014 to December 2015.DEF (− 1) is water def icit; EXC is water surplus

Table 1 Forest species, family and succession groups

Table 2 Dendrometric information of felled trees

These species had previously been inventoried, so that the individuals selected and sampled for this study were the ones that most approached the average classes of each species of the dendrometric data presented in Table 2.

Basic and apparent wood density of the four tree species

Radial test specimens (2 × 1 cm, width and height) were cut from wood samples of the trunk of the four species and conditioned in an acclimatization chamber (20 °C, 50% RH and 12% f inal moisture) for the determination of the basic density and apparent density (ABNT 1997, 2003) (Eqs. 1 and 2). By comparing both parameters, we can identify the dif ferences and characterize the species, observing the variations of the same material submitted to dif ferent moisture contents.

whereBdis basic density (g cm −3 );amis anhydrous mass(g);vsat is saturated volume (cm 3 );Ad is apparent density(g cm −3 );m12% andv12% are mass and volume of the wood measured at 12% moisture.

The radial microdensity prof ile of the wood (X-ray densitometry) consisted of scanning the radial cross-section through collimated X-ray beam (80 μm, distance between points) in a QTRS-01X device (Quintek Measurement Systems), which allowed obtaining the value of the average, maximum and minimum densities of the growth rings(QMS 1999; Tomazello Filho et al. 2008; Castro et al.2014). In the determination of the trees of the species with largest wood/volume, the normality, homoscedasticity and analysis of variance tests were applied, with the Tukey test(95% probability) to compare the averages. All analyses were processed in R software (R Core Team 2014).

Results and discussion

Basic wood density of the trees

C. estrellensis, a species classif ied as late secondary, had the lowest basic density of wood. The other species, from the other successional groups studied, had values between 0.44 g cm −3 (S. terebinthifolius) and 0.500 g cm −3 (H.courbaril) (Fig. 2). Late secondary species, compared to climax species, have the fastest growth, which favors an increase in wood and reduces the basic density (Swaine and Whitmore 1988 ). Reduced availability of edaphoclimatic resources for plants as a result of interactions among dif ferent species and their respective classes interferes in wood development (Piotto 2008). Ecophysiological characteristics, such as growth velocity and shade tolerance,used to distinguish successional groups are also important determinants for an increase in density (Chazdon 2008).

The highest basic density was detected in the wood ofH. courbaril, a climax species, characteristic of mature forest. This species, the last to be established within an ecological system, has slow growth, which promotes maturation and lignif ication of cell walls, and as a result,increase in wood density. In spite of this, the species classif ied as early did not dif fer statistically fromH. courbaril,indicating that the species factor is strong and should be considered in the density analysis because it is a property with high genetic heritability. The four species investigated had densities considered medium, making them suitable for the wood uses within a rural property.

Fig. 2 Basic and apparent density of the wood of the trees of dif ferent successional stages. Equal letters do not dif fer between species for each variable analyzed by the Tukey test at 95% probability level. S.,Schinus; P., peltophorum; C., Cariniana; H., Hymenaea

Apparent wood density of the trees

Species with fast growth (pioneers) had a greater increase in density than in the very slow-growing climax species(Fig. 2).

In summary, species in order of lowest to highest apparent densities were (1)P. dubium(0.50 g cm −3 ), classif ied as fast-growing early secondary species; (2)S. terebinthifolius(0.63 g cm −3 ), classif ied as a very fast-growing pioneer species, with a high light requirement for its establishment; (3)C. estrellensis: (0.78 g cm −3 ), classif ied as slow-growing late secondary species, which requires varying amounts of light as a function of planting arrangement; and, (4)H. courbaril:(0.98 g cm −3 ), classif ied as a slow-growing, climax species,with a strong light requirement as an adult.

The species of slower growth had greater increments of density (C. estrellensisandH. courbaril). When considering a mixed forest system for ecological restoration, it is important to integrate fast-growing species that form and recompose the landscape with slower-growing species to enable the removal of wood with higher basic density after a certain time. The benef its of such population dynamics involve both restoration and economic return, where the use of higher-density species enables wood use for live fences,support posts, f irewood, among others. However, ecological, silvicultural and phytosociological aspects should be observed previously for large-scale implementation. Apparent density depends on the climatic conditions under which it was determined because it considers the moisture content of the material, but it is a variable that is strongly related to the basic density (Rezende et al. 1988; Padilha and Marco Júnior 2018), a genetic variable considered as the main attribute for wood use. In general, apparent density values are higher than those of basic density as they include the volume of wood voids (pores). In this study, this relation was described by a fourth-order asymptotic curve (Fig. 3).

Table 3 shows a considerable variation in the basic wood density of the species studied, as a function of variation in age and planting arrangement. We observed a tendency corresponding to that found in the relation of the basic and apparent wood density of the trees (Fig. 3), in which the climax species has a higher wood density, followed in decreasing order by the late secondary, early secondary and pioneer species.

Radial prof ile of microdensity and apparent density of wood

The faster growing speciesS. terebinthifolius(pioneer) andP. dubium(early secondary) had closer values for apparent densities, while values for slower growing species had wider ranges and higher values. The grouped species, pioneer and early secondary, had typical pith, with lower apparent density (Fig. 4), as verif ied in the radial prof iles.

Fig. 3 Relationship between the basic and apparent wood density of trees of dif ferent successional stages

Table 3 Basic density and apparent density of the forest species of the dif ferent successional stages

We detected an increase in the apparent density in the pith to bark direction in fast-growing species. Radial prof iles of apparent density of the wood are consistent with the values reported in the literature and characterized by lower density near the pith, increasing toward the bark (Amaral and Tomazello Filho 1998; Tomazello Filho et al. 2008; Sette et al.2009). InH. courbaril, there was a trend of higher density values near the pith, which is associated with the presence of nodes. The qualitative analysis showed that the wood of this species has a less variation in density in the prof ile,highlighted by the similarity in gray tones (lighter gray corresponds to higher density, darker gray to lower density),although the total radial density is higher (Fig. 4). Albuquerque et al. ( 2016) describedH. courbarilas having good dendrochronological potential due to the abrupt decrease in density at the ring boundary because of the parenchyma,but in the mixed planting used here, the microdensity prof ile showed greater homogeneity.

Regions of the density prof ile characterized by higher values of apparent density are related to the formation of f ibrous zones in the wood, where there is a predominance of thick wall f ibers, smaller diameter of the lumen, and vessels of smaller diameter and frequency, typical of latewood(Ferreira and Tomazello Filho 2009). The formation of growth bands of higher and lower density wood is probably a result of the seasonality of the trunk activity of the trees,due to climatic variations, mainly related to the seasons with higher and lower availability of water, temperature and photoperiod. The greater alternation of values throughout the section, regardless of the studied species, is possibly due to seasonal variations, typical of neotropical regions. The ef fect of climate variations on annual growth rings could be evaluated using X-ray densitometry. According to Tomazello Filho et al. ( 2001), trees show a reaction to environmental variables that af fect their numerous physiological processes,such as respiration, sap f low, transpiration, rate of cell division among other factors, af fecting cambial activity and wood anatomy.

The mean apparent wood density of the four forest species with potential for ecological restoration and economic maintenance dif fered signif icantly, making it possible to group them using Tukey’s test (p< 0.05) into three classes:(1) lower density,P. dubium; (2) intermediate density,S.terebinthifolius; and (3) higher density,C. estrellensisandH. courbaril(Table 3). Trees with the highest apparent density usually have a higher percentage of latewood, mainly in adult wood, resulting in an increase in their mean apparent density due to the greater attenuation of X-rays. The trees with the lowest apparent density of the wood have higher percentage of early wood in the juvenile wood, reducing the average value of the apparent density (Castro et al. 2014).

The minimum wood density and maximum wood density of the four species studied ranged from 0.34 (P. dubium) to 0.99 g cm −3 (H. courbaril). However, the highest amplitudes were observed for slow-growing species. The density of the wood encompasses dif ferent components of the xylem; therefore, many variables can inf luence the density values (Roque and Tomazello Filho 2007). The maximum apparent density values (Table 4) are higher than that of tree species of more humid forests (Pagotto et al. 2017). For the other two climax species,Swietenia macrophyllaandCedrela odorataof the Amazon Forest and Atlantic Forest,respectively, wood density was 0.81 g cm −3 and 0.54 g cm −3(Alvarado et al. 2010; Anholetto 2013).

Possibly the variations in wood density were caused by climate changes and water availability (Balducci et al. 2014;Oliver et al. 2015). We observed density variations over time in all four species. InCariniana estrellensis, there was an abrupt decrease near the pith. Thus, variations during tree growth may also inf luence the density through the radial section of the wood (Hietz et al. 2013). Generally, the proportion of juvenile wood, anatomically characterized by ahigher proportion of early wood, with thin cell walls and larger lumens, decreases with tree aging, as wood density increases, a relatively frequent pattern of radial variation in native woods (Fearnside 1997a, b; Knapic et al. 2007).

Fig. 4 Radial variation of apparent tree density of Schinus terebinthifolius, Peltophorum dubium; Cariniana estrellensis and Hymenaea courbaril.Red circles show regions with greater density heterogeneity

Anatomical features such as the marginal parenchyma band also interfere with the wood densitometry prof ile, as verif ied inHymenaea courbaril, in which the thin parenchyma band was characterized by a density of 0.80 g cm −3 ,delimited by a region of higher density of 1.10 g cm −3(Albuquerque et al. 2016). Another factor that may inf luence wood density variation is the core formation, that is, the deposition of extractives in the wood closest to the pith (Parolin 2002). The high density of the internal wood may ref lect the investment in wood protection against rot, although it adds little to mechanical resistance (Hietz et al. 2013). Therefore,X-ray densitometry can also provide relevant information on the intra- and inter-annual density patterns of neotropical forest species and their functional signif icance within each successional class.

Table 4 Average, minimum (min) and maximum (max) values of the apparent density obtained by X-ray densitometry of neotropical species used in forest restoration

Conclusions

We observed higher amplitudes of apparent density in the slow-growing species and the pattern of increase in the density from pith to bark was maintained for the species planted in the mixed forest restoration system.Cariniana estrellensisandHymenaea courbaril, late secondary and climax species,respectively, had the highest density. Using X-ray densitometry, we determined characteristics such as minimum, average and maximum densities, and radial density variations,important for understanding the ecological functional role of successional classes of the neotropical forest.

AcknowledgementWe thank Petrobrás for f inancing the implementation project and the Institute for Research and Forest Studies (IPEF)for the assistance in obtaining study material.