Study area

The evaluated plantations (PP and PM) are geolocated at 21°43'35.7" N; 105°06'47.1" W and 21°43'38.3" N; 105°06'46.4" W, within the Santiago Ixcuintla municipality, in the state of Nayarit (Figure 1). Its establishment dates from June 2001 and they belong to the Agroforestales Nayarita company. The area has a subhumid warm climate, with summer rains, of medium humidity [Aw₁(w)], according to the Köppen classification modified by ^{García (1987)}; the annual average temperature is 24.4 °C, the maximum occurs between April and July (^{Inegi, 2017}); with a total annual precipitation of 1 529 mm with variations between the driest and the rainiest period from 1 085 to 2 367 mm (^{Inegi, 2017}). In regard to altitude, it is located between 0 - 800 masl.

Figure1. Geolocation of 15 year pure and mixed commercial Gmelina arborea Roxb. ex Sm. plantations in the State of Nayarit, Mexico. 

In the plantations pre-establishment (use of existing vegetation, weed control and controlled burning), a physical and chemical analysis of the soils was made, from 30 collected samples distributed in zig-zag (10 m distance between holes) at a depth of 30 cm in the forest production plots. The determined texture was clay loam with an organic matter content of 2.5 % and pH between 5.7 - 6. Regarding fertility, the concentration of total N (0.07 %), Mn (0.007 %) and available Cu (0.004 %) was medium, very low in P (0.001 %) and high in K (5 %), Ca (69 %), Mg (0.9 %), available Fe (0.06). One month before planting, to compensate the deficiencies and increase the concentration of macronutrients, 2 Mg ha^-1 of agricultural lime were applied to neutralize the pH, 2 Mg ha^-1 of chicken manure, 50-120-25 fertilizers (NPK) mixed with 250 kg of diammonium phosphate (NH₄) 2HPO₄, 20 kg of potassium chloride (KCl) and 24 kg of potassium sulfate (K₂SO₄).

In the present investigation, the type of plantation with an area of 12.8 ha for PP and 20.4 ha for PM was considered as treatment; the distance between them was 5 m.

The used seedlings were 20 cm tall and 8 mm in diameter; the plantation system was the real frame at 3 × 3 m spacing between seedlings, for a density of 1 111 trees ha^-1. The number of initial trees in the PM was 455 from Pseudosamanea guachapele (Kunth) Harms (guayaquil), 356 from Cordia alliodora (Ruiz & Pav.) Oken (laurel) and 300 from G. arborea. In both the PP and the PM, partial cuts were practiced: two in the PP (5 and 10 years old) and one in the PM (7 years old). In 2016, the final density was 273 trees ha^-1 in the PP and 250, 190 and 163 trees ha^-1 corresponding to P. guachapele, C. alliodora and G. arborea in the PM.

Measurement of mensuration variables, biomass and soil collection

In the two types of evaluated plantations, three rectangular 40 × 25 m plots (1 000 m²) were distributed in random blocks. In each one, the height of all the trees was measured with a Haga hypsometer, and DBH with a Forestry Suppliers Inc., 800-647-5368 diameter tape. The basimetric area (AB) was estimated from the data of 15 trees per sampling unit, with Equation (1):

Where:

AB = Basimetric area (m²)

D = DBH (cm) (^{Arteaga and Castelán, 2008})

The volume was estimated with data from 15 trees felled in each plantation. The stem was cubed according to Smalian (^{Cancino, 2006}) and the calculation was made with Equation (2):

Where:

V = Volume of the log (m³)

A _b = Basimetric diameter (largest) of the log (cm)

A _s = Diameter at the smallest side of the log (cm)

L = Length of the solid log

L = L₂ - L₁ (m) (^{Cancino, 2006})

The annual mean increment (IMA, for its acronym in Spanish) of the measured variables was obtained from dividing their average with the age of the plantation by the following Formula (3):

Where:

IMA = Annual mean increment

DDV = Diameter, total height, basimetric area and volume (cm, m, m² y m³)

In each plantation, 15 G. arborea trees were felled, which were selected randomly according to the diameter size, the biomass of leaves, branches, stem and root was weighed with an electric hanging Rhino BAC-300 scale of 300 kg ± 100 g capacity (^{Parada et al., 2010}). In each of the plots located in the two types of plantations, nine soil samples were collected distributed in zig-zag (10 m distance between holes) at a depth of 45 cm. The measurements and collections were made in December 2016.

Laboratory procedures

The leaves, branches, stem and root of G. arborea were dried in an Terlab S.A. de C.V. oven at 70 ± 3 ºC, until achieving a constant weight; subsequently, they were ground and sieved with a number 60 mesh. The dry biomass was determined by Equation (4):

Where:

BS = Dry biomass (kg)

Pf = Fresh weight (kg)

Ps = Biomass dry weight (kg) (^{Parada et al., 2010})

The allometric equations were built based on the principles described by ^{Segura and Andrade (2008)}. Based on the dry biomass by component (n = 30), allometric equations of two inputs were generated, using multiple regression models, in which the predictor variables were DBH and H.

The physical and chemical properties of the soil were determined following the NOM-021-RECNAT-2000 standard (^{Semarnat, 2002}): texture (Bouyoucos Hydrometer), bulk density (Da) (clod method), pH in water in relation 1: 2 (Orion Star A210 potentiometer), Cation exchange capacity (CIC) (Ammonium Acetate solution), Electrical conductivity (CE) (Electrical conductivity), Organic carbon by combustion at 900 ºC (Shimadzu TOC 5050-A), Organic matter (MO), multiplying the percentage of average C by the MO (58 %), total N by combustion at 900 ºC (Leco TruSpec^TM Micro elemental analyzer) and the C / N ratio of the soil as indicated by its expression.

The differences in DBH, H, AB, V, the increments and total biomass production and by component among individuals and plantations, as well as soil physical and chemical properties among individuals and plantations, were analyzed by ANOVA. Tukey’s mean comparison test (p ≤ 0.05) was used to identify the differences between the means of each variable evaluated in the PP and PM. The normality of the means was analyzed using P-P Plot charts. SAS v9.0 statistical program (^{SAS Institute, 2009}) was used for the procedures.

Growth of Gmelina arborea in pure and mixed plantations

Figure 2 shows the statistical differences of the contrasted variables between individuals and plantations. Although H (Figure 2A; p = 0.106), V (Figure 2D; p = 0.964), IMAH (Figure 2F; p = 0.124) and IMAV (Figure 2H; p = 0.873), the average per tree of G. arborea between PP and PM was statistically similar; in PM the species was characterized by increasing its DBH by 5.8 cm tree^-1 (Figure 2B; p = 0.0001), AB by 0.04 m² tree^-1 (Figure 2C; p = 0.0001) and its IMADAP by 0.01 cm year^-1 (Figure 2G; p = 0.002). Overall, the net productivity (Figure 2E; p = 0.0001) and the IMAV ha^-1 (Figure 2; p = 0.0001) between individuals was around 21 % higher in PP, whose determining factor was the density of trees of the species by surface area.

*Means with different letters in each bar (standard error) are statistically different (Tukey, p ≤ 0.05). a) Total height (H); b) diameter at breast height (DBH); c) Basimetric area (AB); d) Volume (V); e) Volume per area (V); f) Annual average increase in total height (IMAH); g) Annual average increase in diameter (IMADAP); h) Annual average increase in volume (IMAV); i) Annual average increase in volume per area (IMAV ha^-1yr^-1).

Figure 2 Growth of Gmelina arborea Roxb. ex Sm. established in 15 year- old pure and mixed plantations in the state of Nayarit. 

It became evident that when G. arborea grew in the PM, it improved its dendrometric characteristics associated with diameter; although H, V, IMAH and IMAV were statistically similar, and, consequently, the productivity in the PP was higher, mainly due to the density of trees of the evaluated species. However, the results exceed those documented by other authors in plantations of the same age. ^{Onyekwelu et al. (2006)} in Nigeria register an H = 20.9 m tree^-1, DBH= 26.2 cm tree^-1, AB = 50.5 m² ha^-1 and V = 550.5 m³ ha^-1; and ^{Espinoza-Durán and Moya (2013)} in Costa Rica reported an H = 14.9 m tree^-1, DBH = 30.9 cm tree^-1, V = 240.1 m³ ha^-1. In Mexico, values of 14.5 m tree^-1 have been cited for H; DBH =19.3 cm tree^-1; AB = 22.5 m² ha^-1; and V =270 m³ ha^-1 (^{Martínez-Zurimendi et al., 2015}).

In regard to IMA, the productivity estimated in the present study was higher than those obtained by ^{Moya (2004)} (40 m³ ha^-1 year^-1) in Costa Rica; ^{Vanclay et al. (2008)} in the Philippines (29 m³ ha^-1 year^-1) and ^{Martínez-Zurimendi et al. (2015)} in Mexico (7.5 to 33.2 m³ ha^-1 year^-1).

Forestry practices and site conditions play an important role in the growth of G. arborea. Timely plantation management and good soil conditions benefit the dendrometric increase and productivity of the species (^{Swamy et al., 2003}; ^{Rojas et al., 2004}; ^{Martínez-Zurimendi et al., 2015}).

Allometric equations for biomass estimation in Gmelina arborea

Table 1 shows the equations generated to estimate biomass in G. arborea. Multiple regression analyzes optimized by Box-Cox transformations related DBH and H; in turn, the direct measurement of biomass also favored obtaining values of R ² ≥ 0.9 in leaves, branches, stem, root and total biomass, with a standard error of ≤ 1.

The allometric equations for biomass estimation in G. arborea had R ² values similar to those reported in India by ^{Swamy et al. (2003}): leaves R ² = 0.97, branches R ² = 0.93, stem R ² = 0.98, root R ² = 0.97 and total biomass R ² = 0.99); ^{Onyekwelu (2004)} in Nigeria: leaves R ² = 0.97, branches R ² = 0.98, stem R ² = 0.98 and total aerial biomass R ² = 0.98); and in Costa Rica, ^{Arias et al. (2011)} record values in leaves R ² = 0.97, branches R ² = 0.92, stem R ² = 0.99 and total aerial biomass R ² = 0.97.

Based on the above, the biomass measurements obtained by the direct method allowed the construction of equations with higher adjustment values than those generated from indirect measurements (^{Chaturvedi and Raghubanshi, 2015}; ^{Ruiz-Blandon et al., 2019}). The proposed expressions may be used for the estimation of biomass in G. arborea in future investigations that are carried out under similar climate and soil conditions associated with this study.

The allometric equations by tree component will allow to quantify with certainty the distribution of biomass in the tree, as well as its accumulation while it grows. On the other hand, having an equation to estimate the value of root biomass and its impact on total biomass constitutes an important contribution of the present study, since the root is rarely considered in forest inventories.

Biomass production in Gmelina arborea

The biomass production in G. arborea per tree and surface area unit was different between the plantation systems. In PM, the biomass of the leaves increased by 12.2 % (Figure 3A; p = 0.0001), 8.8 % the biomass of the leaves (Figure 3B; p = 0.002) in branches, 7.7 % (Figure 3C; p = 0.004) in stem, 10 % (Figure 3D; p = 0.0006) in root, and 7.6 % (Figure 3E; p = 0.002) in average total biomass per tree, with respect to PP. The biomass distribution showed the following order: 3 % in leaves, 8 % in branches, 69 % in stem and 20 % in the root, which indicated that the stem is the component that produced and accumulated the most biomass (by weight). Consequently, a greater number of G. arborea trees per hectare in the PP (273 trees ha^-1), gives it 17.9 % (Figure 3F; p <0.0001) more yield, than the total production in the PM (163 trees ha^-1).

* Means with different letters in each bar (standard error) are statistically different (Tukey, p ≤ 0.05). a) Dry biomass of the leaves (BSh); b) Dry biomass of the branches (BSr); c) Dry biomass of the stem (BSf); d) Dry biomass of the root (BSra); e) Total dry biomass (BST), and f) Total dry biomass per area (BST).

Figure 3 Biomass production per tree and area in 15 year- old pure and mixed plantations in State of Nayarit. 

In the present investigation, it was confirmed that when G. arborea grows in PM, it produces greater biomass per component (leaves, branches, stem, root and total biomass), compared to what happens in a PP; in spite of it, by number of trees per hectare it turns out to be the most productive PP. However, these values differ significantly from those recorded in 15-year-old plantations established in the Philippines and Nigeria at 142.1 and 233.7 Mg ha^-1, respectively (^{Sales et al., 2005}; ^{Onyekwelu et al., 2006}).

Biomass production in G. arborea plantations is usually affected by physiological aspects of the plant (adaptation), forestry management, plantation system and age (^{Rasineni et al., 2011}; ^{Verma et al., 2017}). Regardless of the type of plantation, pure or mixed, the stem of G. arborea is the component with the highest biomass production, which coincides with that documented in other studies (^{Arias et al., 2011}; ^{Cook et al., 2014}; ^{Goussanou et al., 2018}).

Poor management and arrangement of species within PM are likely to affect biomass yields in this type of system, due to the overpopulation of trees per unit area (^{Redondo-Brenes and Montagnini, 2006}). This explains the lower production in total volume of G. arborea in the PM, in contrast to that recorded in the PP. Therefore, it is important to consider a strategic forestry arrangement in the PM that allows having a greater number of trees of the species when associating it with other taxa; this will obtain better results in biomass and an increase in the productivity of the system.

Physical and chemical soil properties in pure and mixed plantations of Gmelina arborea

The soils where the G. arborea plantations were established rendered the same texture (loam-clay), bulk density of 1.1 g cm^-3 (p = 0.081), CE of 0.04 dS m^-1 (p = 0.804), concentration of total N of 0.8 mg g^-1 (p = 0.105), similar concentration of MO (around 3 %, p = 0.663), and C between 19 - 20 mg g^-1 (p = 0.849). Despite this, the best pH (6.3; p = 0.011) and C / N ratio (25; p = 0.028) were recorded in the PM; the CIC was higher in the PP soils (41.7 cmol ⁽⁺⁾ kg^-1; p = 0.040) (Table 2).

Under the conditions of the present study, the edaphic properties that determined the difference between the PP and PM soils were pH and the C / N ratio.

Investigations carried out in Nigeria and Costa Rica showed a greater growth and biomass production in PP of G. arborea, when the texture was loamy-clay, the pH between 5 - 7.5 and bulk density higher than 1 g cm^-3 (^{Nwoboshi, 2000}; ^{Onyekwelu, 2001}; ^{Rojas et al., 2004}; ^{Onyekwelu et al., 2006}; ^{Adekunle et al., 2011}). If the soil conditions are adverse, their individuals grow poorly: crooked stems, low height, highly branched and shrubby; therefore, it is recommended to plant it in deep, moist, well-drained soils without obstacles that interfere with root development (^{Rojas et al., 2004}). The above coincides with the values reported here for the PM; however, it is essential to note that pH has a positive relationship with the availability of nutrients, since it exerts a great influence on the balance of ionic change in the soil due to its effects on erosion, the mineralization of organic matter and the nutrient mobilization (^{Adekunle et al., 2011}), which supports the values obtained in the studied soils. Likewise, it is cited that the pH range that favors the vigor of G. arborea varies from 6.47 to 7.47, since in these values most of the nutrients essential for plant growth are present, especially N (^{Onyekwelu, 2001}).

A high concentration of edaphic N (natural or synthetic incorporation) guarantees the vigorous growth of G. arborea (^{Rojas et al., 2004}); however, the C / N ratio of the evaluated soils was very high. When said ratio> 11.5, it is considered that there is excess C and energy, which indicates the immobilization of N by microorganisms, which in turn prevents its use by plants (^{Gamarra et al., 2018}; ^{Salcedo-Pérez et al., 2019}). Furthermore, in this type of soil the release of nitrogen is very high, which explains the higher biomass yield recorded in the present study, possibly due to the fact that said element is concentrated in the biomass of the species, particularly in the leaves where it acquires greater preponderance.