Study area

The study area is located in the locations known as El Tejamanil and Huiramo of the Indigenous Community of Nuevo San Juan Parangaricutiro (CINSJP), Michoacán state, between 19°24'16.7" North and 102°14'12.7" West, at an average altitude of 2 735 m. They belong to the Transverse Neovolcanic Axis, hydrological region RH: 18 (Balsas river) and sub-basin G (Cupatitzio river) (^{INAFED, 2018}).

The climate, C (m) (w) big (Köppen modified by ^{García, 1973}) is humid temperate with summer rains and <5 % winter rains.

The dominant soil is of the humic Andosol type (^{INEGI, 2009}; ^{UNDP, 2012}). The average slope in the sites is 5 %, and they were intended for agricultural use and cattle grazing. The vegetation bordering the plantation sites is made up, mainly, of mixed pine-oak forests; and its most representative conifer species are P. pseudostrobus and P. montezumae, which have great commercial value in the Sierra Purhépecha region.

Characteristics of the plant

The plant was produced in the El Durazno forest nursery of the CINSJP. The plant of P. pseudostrobus was produced in black polyethylene bag (1 + 1) of 5 × 20 cm and with a volume capacity of 393 mL, the height and diameter at the base of the initial plant was 35.81 cm and 10.01 mm respectively, for the plantation of P. montezumae, one-year (1 + 0) plants produced in polystyrene trays of 77 cavities and a volume capacity of 170 mL, of at least 4 cm in height and 4 mm in diameter were used to the base of the plant. The substrate used in black polyethylene bags was 60 % of oak land (decomposing litter) and 40 % of red soil (topure-soil andosol) and for the plant produced in polystyrene trays was a mixture of Sunshine peatmoss (40%), perlite (20 %), insulex vermiculite (20 %) and Osmocote™ (18-6-12) (10 %).

Two plantations were carried out with both species, the first in 2013 and a replica in 2015. The planting of both species was made during the beginning of the rainy season (July-August) in the CINSJP region. The plants were established with the plantation system of common stock 40 × 40 × 40 cm, with a plantation design with real frame, at a spacing of 2 m between plants and rows.

Experimental design and evaluated variables

A randomized complete block design was used in the 2013 plantation and in the 2015 replica. Five treatments were used (five planting dates) and four replications (blocks) per species. Each experimental unit was composed of 25 plants, with a total of 100 individuals per treatment and 500 per species, in a total area for both plantations of 5 200 m². The planting dates were July 1 (F1), July 15 (F2), July 30 (F3), August 15 (F4) and August 30, 2013 (F5).

The evaluated variables were the total height of the plant (cm), measured with an Apex model stalk (cm) and the diameter of the base of the plant (mm) was measured with a Neiko type digital vernier; survival (alive or dead), by direct counting.

In El Tejamanil, for each date of planting and species, three measurements were made, the first of which was at the end of its establishment and later, at one year, two years and three years of planting; in Huiramo only one evaluation was made one year after it was established.

Data analysis

With the total height and diameter at the base of the plant, an analysis of variance was carried out with the STATISTICA program, version 13 (^{Statsoft, 2017}), to determine statistical differences together with the survival data, which were transformed to natural logarithm; the planting date (F1, F2, F3, F4 and F5) was taken as the grouping variable. The mathematical model used was the following:

Where:

Y _ij = Random variable that represents the value of the response in the j ^th observation of the i ^th treatment

M = Constant that represents the average response of the Y variable

A _i and B _j = Effects of the i (i = Date: 1, 2, 3, 4 and 5) treatment and of time j (j = 1, 2 and 3 years)

A _i *B _j = Effect of the interaction of the i treatment by the j time

E _ij = Experimental error (^{Norman et al., 1996})

Simple variance analyzes were performed only when the treatment effect of the previous model was significant. The multiple comparison method used for the classification of means was that of Tukey with 95 % confidence limits.

Survival

The survival of P. pseudostrobus and P. montezumae in El Tejamanil and Huiramo showed no significant differences related to the date of planting (p≥0.05) in any period evaluated, although the first year of planting in the first site, P. pseudostrobus registered 60 % survival for the plants established in July and 65 % in the August ones, differences that kept as such during the second year with 50 % in favor of the early dates (July) and 54 % for late dates (August); finally, in the third year of evaluation, the percentage of survival was 35 % for the late and 30 % for the early. This decrease in survival was mainly due to the attack by Michoacana wild rodent (Zygogeomys trichopus Merriam, 1895), -a species subject to protection (P) according to NOM-059-Semarnat-2010-which reduced survival up to 35 % of the first two years after the plantation of both species.

^{García and Aguilar (1996)} cite that these rodents are harmful and one of the main problems of mortality in forest plantations established in the Sierra Purhépecha, Michoacán; they cause important economic losses during the first years in forest plantations and agricultural crops (^{Monroy et al., 2005}).

For the P. montezumae plantation in El Tejamanil, 70 % survival was registered at the evaluation year at early dates and 60 % at late dates; during the second year this percentage remained in favor of the plantation carried out in July with 60%; in the last evaluation, both lapses showed 45 % survival on average (Figure 1), a reduced percentage as a result of the rodent attack.

Supervivencia = Survival; Año = Year

Figure 1 Survival of Pinus pseudotrobus Lindl. and Pinus montezumae Lamb. at El Tejamanil three years after its establishment with five planting dates (mean ± standard deviation). 

In the plantation of Huiramo, survival was also 70 % for the studied species in the first year of establishment, with a higher percentage in the early dates (July), compared to the late ones (August) (Figure 2). However, no significant differences attributable to the date of planting were confirmed.

Supervivencia = Survival

Figure 2 Survival of Pinus pseudotrobus Lindl. and P. montezumae Lamb. at Huiramo one year after establishment with five planting dates (mean ± standard deviation). 

Some studies have addressed the influence of this factor on survival and growth with Pinus species, with which the results have been unequal. ^{Royo et al. (2000)} and ^{Ariza et al. (2008)} report that there are no significant survival differences when comparing different planting dates with P. halepensis Miller., with a logical tendency to decrease from an initial 100 % up to 80 % at the end of the first year.

However, these results are different from those obtained by ^{Rodríguez (2013)}, since in a trial with two species of pine identified significant differences between treatments, which were explained by the date of planting. ^{Royo et al. (2000)} and ^{Navarro et al. (2004)} have shown the importance of this variable, even considering it as having the greatest influence on the development, growth and subsequent survival of the plantation.

Diameter to the base of the plant

The differences in diameter at the base of the plant between planting dates were significant (p≥0.0001) in El Tejamanil in each year evaluated for both species. In all the measurements, the growth pattern in diameter was the same; the dates of the 1^st and July 15^th (F1 and F2) showed a growth significantly higher than in August. In the first year of measurement, the differences between the planting dates with the highest growth (July) and those with the lowest growth (August) were 3 mm in both species, which increased in the second year of evaluation (30 mm) and thus until the third when they exceeded 30 mm from one year to the next (Table 1). Only in the second, significant differences were obtained (p≤0.03) from the diameter at the base of the P. pseudostrobus plant, so the growth of the early dates of July is related to the height of the same dates for this species.

In the first year, P. montezumae also recorded statistical differences (p≤0.002) since the growth of the date of August 30^th (late date) exceeded that of July, but the trend in successive years remained in favor of the early dates (Table 1).

In Huiramo, no statistical differences were confirmed (p≥0.05) in either of the two species, but it is noted that there is a trend of greater growth in the early dates (July) with respect to August with more 4 mm in favor of the first. In both places, it is observed that the diameter growth at the base of the plant from one year to the next is 45 % more in the July specimens than in those of August. In terms of growth, the development of plants is largely determined by the evolution of climatic conditions after their establishment (^{Palacios et al., 2008}; ²⁰⁰⁹); thus, they are conditions that cannot be manipulated (^{Sigala et al., 2015}). The water supply was adequate for the plants incorporated at an early date (F1 and F2) since in the region, the rainy season starts in July.

These results coincide with those obtained by ^{Mateos et al. (2008)} when comparing two planting dates of P. halepensis; where the growth differences in height and diameter were significant (p <0.05) since the growth in diameter was greater in the plantation in November (early dates) with respect to January (late dates). ^{Rodríguez (2013)} under greenhouse conditions, points out that late sowing with P. patula and P. greggii Engelm., can reach significant differences in the same variables. However, there are contrary cases in which significant differences in the date of planting, both early and late, can reach their maximum development, provided that the climatic conditions are favorable during the first year of their establishment, as shown by the evaluations made during the three years in this case.