Performance of Peltophorum dubium under intraspecific tree competition and cardinal directions as possibility for integrated livestock-forestry systems

Peltophorum dubium can be an alternative of forestry species to be included in integrated crop-livestock-forestry systems in tropical region. The aim of this research was to evaluate the performance of P. dubium under intraspecific tree competition and cardinal directions to measure the possibility of introduction in integrated livestock-forestry systems. The experiment was performed in a Nelder Wheel competition, which is used in forestry research to test tree competition. The experimental design was factorial (10×2×5) with 12 replicates. The treatments were accomplished by ten tree competitions (159, 201, 255, 322, 408, 516, 653, 827, 1,046 and 1,324 trees ha -1 ), two cardinal directions (North-South and East-West) and five times after planting (12, 18, 24, 30 and 36 months). The highest tree competition (1,342 trees ha -1 ) in 36 months after planting, promoted increase in the cylindrical volume of the tree in both North-South and East-West cardinal directions. Nevertheless, the cardinal direction East-West was profitable to increase tree height, diameter at breast height (DBH), cylindrical volume per tree and cylindrical volume per hectare. The DBH measured in 24 months after planting was 5.0 cm, which is considered the minimum DBH to introduce livestock into the integrated system without significant damage on trees. Regarding these preliminary results in the first three years, P. dubium was promising as Brazilian native species to be inserted in integrated livestock-forestry system or single forestry as an option for Eucalyptus spp.


INTRODUCTION
Sustainable agricultural systems have increased through the last decade.The integrated crop-livestock-forestry systems (ICLFS) have pointed out as a quite promisor production system to be used to recover degraded pasture in Brazilian Cerrado (Almeida et al., 2013).ICLFS is defined as integrated production system that enables to have three components (crop, livestock and forestry) at the same area, with at least two components at the same time.Nevertheless, forestry is always in the area and determines the cycle of the system.ICLFS allows increase in the diversity of products at the same production area.In this ICLFS, trees species might be well defined because of its impact on crop, pasture and livestock combined in integrated systems of production.
The benefits of trees in integrated systems for animal grazing is related to better microclimate and animal thermal comfort (Karvatte et al., 2016).However, the trees can result in negative effects on grain crops yields in tree crop zone (Nasielski et al., 2015) and pasture in case of higher tree competition (Burner and Brauer, 2003).As reported by Franchini et al. (2014), decrease in soybean grain yield can be related to the age of the trees, which was not observed significant effects in the first two year of Eucalyptus species age; nonetheless, the decrease of soybean grain yield can achieve 27% after four years of Eucalyptus spp.establishment.
Nevertheless, the most common ICLF systems use Eucalyptus spp. as tree species because of faster growth and enables to introduce livestock sooner than native forest species; besides Eucalyptus spp., multiple use and high commercial value (Grossman, 2015).Even with many positive feature of Eucalyptus spp., the possibility to have native forestry species, as Peltophorum dubium, might be considers.Monocrop of Eucalyptus spp. is not profitable for balanced ecosystems; this way is quite relevant to have other forestry species as the alternative to maximize the diversity of the ecologic system.
As possibility of native species, P. dubium shows some features that have been pointed as a profitable species to introduce in ICLS with some positive features as narrow crown area (Matos et al., 2015), which increases the incidence of sunlight in crops and pasture cultivated under the trees.Soybean, rice and corn as crop components and Brachiaria species as pasture are profitable to be inserted in ICLFS.P. dubium (Spreng) is a leguminosae found in tropical seasonal semi-deciduous forest in Brazil (Lisi et al., 2008).Moreover, P. dubium can be found in Atlantic forest of Brazil and can reach 20 m of height and 90 cm of trunk diameter at breast height (DBH).
As reported by Lima et al. (2015), P. dubium is very adaptable in different regions, which may imply in different response with the environment conditions.To insert the P. dubium in an integrated crop-livestockforestry system is quite important to know the growth Heid et al. 3579 rate, because tree height and DBH determine the moment to introduce the livestock into the integrated system of production.Faster growth shown by Eucalyptus spp.makes this species widely used in tropical climate in integrated systems (Grossman, 2015).However, the P. dubium shows higher wood basic density (0.65 g cm -3 ) in ten-years-old (Vivian et al., 2010) in comparison to Eucalyptus grandis (0.52 g cm -3 ), at the same age (Githiomi and Kariuki, 2010).
The preferable destination of wood in ICLS is for timber, as veneer wood and sawmills (Almeida et al., 2013), which turn P. dubium as a profitable option due to its higher wood basic density.To obtain high quality of wood from integrated system is necessary to conduct the trees with pruning and find the profitable plant density and cardinal directions to improve wood quality and growth.
The tree competition has been pointed as the variable that affects some dendrometric features.Matos et al. (2015) observed decreasing in cylindrical volume of trees in higher P. dubium competition (1,324 trees ha -1 ), on the other hand, lower tree competition results in less volumetric trunk production per hectare (Folkard et al., 2012).The ideal tree competition might be found for P. dubium to improve the recommendation as native species to be chosen as forest component in ICLS.The aim of this research was to evaluate the performance of P. dubium under intraspecific tree competition and cardinal directions to be possible inference of some dendrometric parameters to assess the viability of inserting the native species in integrated livestockforestry systems.

Location of the experiment
The experiment was carried out from November 2010 to November 2013 on the experimental field of Brazilian Agricultural Research Corporation (Embrapa Western Agriculture), followed by geographic coordinates, 22°33'07'' S, 55°38'37'' W, and average altitude 496 m, the experimental area belongs to the municipality of Ponta Porã, state of Mato Grosso do Sul, Brazil.The weather condition is classified as Aw Köppen-Geiger (Fietz, 2008), with rainy summer and dry winter.The average rainfall and temperature in the region of the experimental is shown in Figure 1.

Soil physical and chemical properties
The experimental site topography was under 5.0% of slope.The soil of the study area was classified as dystroferric Red Latosol, according to Santos et al. (2013), the landscape originally covered *Corresponding author: ademar.serra@embrapa.br.

Implementation of the experiment
The whole area of the experiment was 1.33 hectares, defining according to the Nelder Wheel design.The soil tillage was carried out with heavy harrow and leveling harrow, followed by the planting of 528 P. dubium seedlings with height of 20 cm in November, 2010.This seedling height is defined as the ideal one for field planting.In order to supply the nutritional requirement of P. dubium, each seedling was fertilized with 150 g of N-P-K (6-30-6 + 1% B + 0.5% Zn + 0.5% Cu), at the same day of planting.The application of fertilizer was established in two portions of 75 g; both of them were put in 15 cm apart from the stem of the seedling.
The topdressing fertilization was applied 30 days after the planting.In this occasion, 120 g of the fertilizer formulated as 20-0-20 (N-P-K) was used.The location of the fertilizer was in the area around the tree, being applied at the end of the crown projection.The arrangement of the trees followed the design proposed by Nelder (1962) (Figure 2).The trees distances and angles were determined according to the Equations 1, 2 and 3 (Namkoong, 1965): where rn is the radial distance to the last trees, r0 is the radial distance to the first tree in each ray, An means the area of tree in each ray,  is the angles between adjacent rays, and  is the constant that determines the rate of change in space growth.
The Nelder Wheel competition was defined by 22 concentric circles, with distance from the center ranging between 19.60 m (r0) and 67.51 m (r21).The decrease rate was 12.5% in the trees densities from (r0) to (r21), resulting in the increase of 6.066% in the distance for each new circle, represented by =1.06066.Both inner and outer circles were considered borders.The angle  between the Nelder Wheel rays was of 15°, resulting in 24 rays and 24 seedlings implanted in each ray (Figure 2).The ray number one was located in the direction of the north.This arrangement allowed the evaluation in the experimental circles from r3 to r21 of the trees densities as shown in Table 2.

Measurement of dendrometric variables of P. dubium
The dendrometric variables were measured at 12, 18, 24, 30 and 36 months after seedling planting.In all trees, the circumference at 1.3 m of height (C1.3), the tree height (TH) and cylindrical volume per tree (CVT) were measured.In order to obtain these measurements, the tape-measure and graduated scale were used.Based on these measurements earlier, the diameter of trunk (DBH1.3m= C1.3/pi), the transversal area of trunk (g= pi/( D1,3 2 /4) and the cylindrical volume (CV= g × TH) were defined.The cylindrical volume per hectare (CVH) was defined with the relation of CVT and tree densities evaluated.

Statistical analysis
The variables evaluated in the experiment were submitted to the analysis of variance (ANOVA) by the F-test.The response surface was adjusted in case of significant interaction (p<0.01) between trees competition and time after planting.The simple Pearson's correlation matrix of dependent variable was performed to obtain the degree of relationship between them.In case of significant correlation (p<0.05), the strength was defined as Table 3, according to Hinkle et al. (2003).These statistical analyses were carried out with the software SPSS for Windows, version 11.0.0 (SPSS Inc., Chicago, IL, EUA).

P. dubium tree competition impact on tree height
The dendrometric parameters of P. dubium changed with the treatments applied (Table 4).The tree height (TH) and diameter at breast height (DBH) were affected (p<0.01) by tree competitions, time after planting and the     interaction between tree competitions vs. cardinal directions (Table 4).The tree height of 2.62 m (12 months after planting) was compatible for what was obtained by Oliveira et al. (2009), who obtained the range of 1.5 to 3.5 m at 12 months after planting for native species including P. dubium.On the other hand, tree height was 29.1% higher than the average found by Matos et al. (2015) for P. dubium in the same region of studying in 12 months after planting.As reported by Carvalho (1994), P. dubium showed high adaptive plasticity, resulting in different behavior among the same location of plantation.The height of P. dubium showed faster growth between 12 and 24 months after planting and slow between 24 and 36 months.The absence of growth between 24 and 30 months may be related to the freezing occurred in winter season (Figure 1), which may have compromised the growth during this period; anyway, the growth in summer season after fall-winter season was not as fast as the initial growth which comprised between 12 and 24 months.

Size of correlation
The cardinal directions affected the tree height (Figure 3B).The cardinal direction East-West showed improvement in tree height in comparison with North-South (Figure 3B).Tree competition of 1047.6 trees ha -1 was necessary to achieve the highest tree height in cardinal direction North-South, occasioning in 4.88 m of tree height, and cardinal direction East-West of tree rows required 1143.78 trees ha -1 to achieve the highest tree height (5.24 m).The row directions of East-West promoted increase of 6.87% of P. dubium height, which is possibly related to improvement of sunlight use efficiency.The East-West cardinal orientation of the tree row is quite important to have higher light incidence, temperature and canopy openness (Camargo et al., 2011).

Diameter at breast height under tree competition and cardinal directions
The diameter at breast height (DBH) of P. dubium was affected (p<0.01) by tree competition, time after planting and the interaction between tree competition vs. cardinal directions (Table 4).DBH of 5.0 cm was achieved in 24 months after planting (Figure 3A), which may be considered a profitable time for grazed animal introduction in the integrated crop-livestock-forestry systems.In order TH: Tree height (m); DBH: diameter at breast height (cm); CVT: cylindrical volume per tree (m 3 ); CVT: cylindrical volume per hectare (m 3 ha -1 ).
to introduce cattle, DBH needs to be above 5.0 cm to avoid significant damage caused on tree trunk in DBH lower than 5.0 cm (Sanchez-Velasquez and Pineda-Lopez, 2010).Even with this recommendation, the improvement in researches related to P. dubium associated to stocking rate, cattle weight and damage caused on tree through different DBH with the introduction of livestock might be investigated.
The tree competition (1144 trees ha -1 ) in East-West cardinal direction showed higher DBH (5.24 cm) than North-South cardinal direction, which showed 4.88 cm of DBH in 1,040 trees ha -1 (Figure 3B).The results showed by P. dubium in relation to DBH were not expected because in lower tree competition, DBH decreased, however, these results can be associated with other factors as wind and lower tree competition that affected negatively the growth in diameter and tree height.
The cardinal directions North-South and East-West of the tree rows did not affect the correlation between DBH and tree height (Figure 3C and D).In both cardinal directions, the P. dubium showed ratio of tree height and diameter at breast height close to 1:1 (0.97:1); this way DBH of 5.0 cm is going to reach a height of 5.0 m, which is crucial to determine the livestock initiation into the integrated livestock-forestry system.
The greatest challenge in introducing P. dubium or other Brazilian native species in integrated livestockforestry systems is the slow growth in comparison to Eucalyptus spp.Tree height and DBH are traits that most affect the time of livestock introduction in the production systems.But, in comparison to Eucalyptus spp., the results showed by P. dubium are promisor due to the DBH achieved 5.0 cm at 24 months after planting (Figure 3A).Nevertheless, this recommendation of 5.0 cm of DBH was established by Eucalyptus spp.and not for P. dubium.This way, it is quite important in further research to evaluate the possibility to introduce livestock with other DBH of P. dubium to know if higher wood density of P. dubium in comparison to Eucalyptus spp.can be a positive point to reduce the introduction time of livestock in the integrated livestock-forestry systems.
Based on the equation adjusted for time after planting and DBH, in a scenario with the possibility to introduce livestock with ≥5.0 cm DBH, the animal grazing might be introduced in 24 months after tree planting, which is not much longer than Eucalyptus spp.

Cylindrical volume of P. dubium under tree competition
In respect to cylindrical volume per tree (CVT), significant effects (p<0.01) were obtained by tree competition, time after planting, cardinal directions of the rows and their interactions (Table 4).The cardinal direction East-West resulted in higher CVT in tree competition above 408 trees ha -1 , these results showed very high positive correlation with DBH and TH in both cardinal directions (Table 5).In cardinal direction East-West, the CVT was 0.014 and 0.013 m 3 for North-South, resulting in 6.27% of CVT higher in cardinal direction East-West (Figure 4A).The cardinal direction East-South can have higher use efficiency of sunlight (Camargo et al., 2011), occasioning in higher carbon dioxide assimilation and consequently higher CVT.
Based on significant (p<0.01)interactions between time after planting vs. tree competition, the response surface was adjusted and showed in higher tree competition increase in CVT (Figure 4B).The highest CVT (0.0248 m 3 tree) was obtained in the extreme values of tree competition (1324 trees ha -1 ) and time after planting (36 months after planting) (Figure 4B).The highest tree competition did not affect negatively the CVT, which is quite important to use P. dubium in ICLS due to the absence of decreasing in higher tree competition and the possibility to increase the cylindrical volume per hectare.Usually, the increase in tree competition decreases CVT, which depletes the capacity of increasing the number of trees per hectare without decreasing CVT.In the case of Eucalyptus spp., the increase in tree competition reduced CVT due to higher tree competition (Ferreira et al., 2016), as well as observed for P. dubium (Matos et al., 2015).

Interactions of tree competition vs. cardinal directions on cylindrical volume per hectare
The interaction of times after planting vs. tree competition was observed by cylindrical volume per hectare (CVH) in both cardinal directions (Table 3).The CVT was higher in 1,324 trees ha -1 in both cardinal directions (Figure 5A and  B).The initial development of the P. dubium until 36 months after planting showed this species quite interesting to be introduced in integrated crop-livestockforestry systems or single forestry, due to its flexibility to adapt under variable tree competition without negative effect on cylindrical volume produced.
The expected is decreasing CVT and CVH under higher tree competition, due to reduction of light use efficiency (Nelson et al., 2016), soil nutrients competition (Dong et al., 2016), and water limitation (Pezzopane et al., 2015).Nevertheless, this absence of limitation in higher tree density showed by P. dubium indicates this native Brazilian species as a quite promising native forestry species for woody production in higher tree density.Furthermore, the highest wood density (0.75 g cm -3 ) for mature trees of P. dubium in comparison to Eucalyptus ) as forestry component in integrated system may be a decision point to be evaluated to choose which species to use.Nevertheless, the woody density depends on the tree maturity and is expected increase through time (Richter, 2015).The possibility to increase the number of trees per hectare without decrease in CVT which is quite preferably due to the opportunity of increasing economic gain with the timber.

Conclusion
The P. dubium showed potential to be inserted in integrated livestock-forestry system due to the absence of negative effect in tree competition on dendrometric tree parameters.The diameter at breast height and tree height showed very high positive correlation; the increase in tree height resulted in increase in DBH in a proportion of 1.0 cm (DBH) for 1.0 m of tree height.
The highest tree competition of 1,342 trees ha -1 , in 36 months after planting, promoted increase in cylindrical volume per tree in both cardinal directions North-South and East-West.Nevertheless, the cardinal direction East-West was profitable to increase tree height, DBH, and cylindrical volume per tree.
DBH measured in 24 months after planting was 5.0 cm, which is considered the minimum DBH to introduce livestock into the integrated system without significant damage on trees.Concerning these preliminary results in the first three years, P. dubium was promising as Brazilian native species to be inserted in an integrated crop-livestock-forestry system or single forestry as an option for Eucalyptus spp.

Figure 1 .
Abbreviations: DBH, Diameter at breast height; TH, tree height.Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License

Figure 2 .
Figure 2. Scheme of Nelder Wheel to define the tree competition of P. dubium, and the image of the experiment on the field.

Figure 3 .
Figure 3. Diameter at breast height (DBH) of Peltophorum dubium.(A) Effects of time after planting on DBH; (B) Effects of tree competition on DBH; (C) correlation between tree height vs DBH in North-South cardinal direction; (D) correlation between tree height vs DBH in East-West cardinal direction.

Figure 4 .
Figure 4. Cylindrical timber volume per tree (CVT) of Peltophorum dubium.(A) CVT in function of cardinal directions and tree competition; (B) CVT surface response in function of tree competition and time after planting (months).

Figure 5 .
Figure 5. Cylindrical volume per hectare (CVH) of Peltophorum dubium.(A) CVH surface response in function of tree competition vs. time after planting in North-South cardinal direction; (B) CVH surface response in function of tree competition vs. time after planting in East-West cardinal direction.

Table 1 .
Some initial soil physical and chemical properties from the experimental area.

Table 2 .
Trees competition in each circle of the Nelder Wheel competition.

Table 3 .
The rule for interpreting the size of Person's correlation coefficients.

Table 4 .
Summary of analyses of variance (ANOVA) for tree height (TH), diameter at breast height (DBH), cylindrical timber volume per tree (CTV), and cylindrical volume per hectare (CVH) of Peltophorum dubium.
ns, **, * no significant effects, significant at level of 1 and 5% by F-value, respectively.Df: Degree of freedom.