Experimental design

The trial was established in the Institute of Agricultural Sciences greenhouse at the Universidad Autónoma del Estado de Hidalgo, located at coordinates 20° 3’ 36.44” N and 98° 22’ 53.26” W. The one-year-old P. leiophylla plants were produced in a 77-cavity Styrofoam tray (170 cc per cavity). The substrate used was peat moss, perlite and vermiculite at a ratio of 3:1:1, with 6 g of Osmocote slow-release (eight months) fertilizer added per liter of mixture. Similar plants in height (25 to 30 cm) and diameter at the base (3.5 mm), disease-free, with . of the stem lignified and with fully developed fascicles and needles were selected. The treatments applied at transplanting are described in Table 1.

Individual plants were transplanted into 40 x 40 x 40 cm plastic bags containing 30.6 L of agricultural soil (23 % initial moisture, 56.5 % total porosity, 10.2 % available water retention capacity and 89.7 % aeration porosity). Plants were placed in the center of the bags and treatments were applied around the root ball. Hydrated phenolic foam was deposited so as to have the largest possible contact surface with the root ball.

Subsequently, the filling of the pots was concluded and the soil was compacted slightly, removing lumps and large stones. The pots were placed in a plastic-covered 10 x 30 m greenhouse with an internal temperature ranging from 10 °C in the coldest time to 38 °C in the hottest time in November 2013. The pots were arranged in accordance with a completely randomized experimental design. Each treatment was formed by three replicates of 20 plants, resulting in a total of 60 individuals per treatment and 240 plants in the experiment. No water was added during the evaluation period, in order to assess the moisture contribution from the phenolic foams to the plant and evaluate survival and other variables under study.

Variables and analyses

The variables studied were survival, growth in height and diameter and increase in shoot and root biomass of the plant. Survival was evaluated visually each week for three months. The individual plant was considered dead when it lost turgor in the apical bud, the characteristic color of the species changed and it showed signs of wilting in the leaves, according to the methodology proposed by ^{Barchuk and Díaz (2000)}. The increase in height was measured with a measuring tape (Trooper, Mexico) and that of diameter with a digital Vernier caliper (0.001 mm accuracy, Trooper, Mexico). Biomass was assessed using the methodology proposed by ^{Schlegel, Gayoso, and Guerra (2000)}, which is to obtain, separately, the dry weight of the shoot and root of the plants that were dying in the course of the experiment; subsequently, the values were summed to obtain the total biomass.

Survival was analyzed using the Kaplan-Meier estimator (^{Sigala, González, & Jiménez, 2015}), while height and diameter data were subjected to an analysis of covariance, using the individual value of initial height as a covariate (^{Palacios et al., 2015}). The shoot, root and total biomass data were subjected to traditional analysis of variance. Variables that showed statistically significant differences (P ≤ 0.05) were subjected to Tukey’s range test

Survival of P. leiophylla

Survival analysis, using the Kaplan-Meier estimator, showed a highly significant difference between control and the other treatments (P = 0.000008) (Figure 1). These results are similar to those reported by ^{Agaba et al. (2010)} and ^{Orikiriza et al. (2013)}, who added hydrogel as a water reservoir in other tree species and concluded that the survival time increased. Table 2 shows a comparison between these studies.

Figure 1 Survival of Pinus leiophylla plants with different water reservoir treatments. 

Figure 2 shows the survival rate of P. leiophylla under various treatments over a 12-week period. The figure shows that all the plants (100 %) of the four treatments survived during the first three weeks, so that errors during transplanting can be discarded. From the fourth week, plant survival decreased by 23 % in the control treatment and by 8 % in the phenolic foam treatments, while the treatment with hydrogel maintained 100 % of the plants until the fifth week. In this week, plant survival in the control treatment was 50 %, while the other groups recorded a rapid decline in the survival curve after the sixth week.

Figure 2 Survival of Pinus leiophylla plants with different water reservoir treatments. 

At two months after the start of the experiment, the control was statistically different (P = 0.000008) from the other treatments by having only 8 % survival, while the hydrogel had 45 % and the 231-cc and 308-cc phenolic foam treatments had 52 and 62 %, respectively (Table 3).

Survival time was extended 30 days by using a 308-c hydrated phenolic foam block. In an open field planting, there is a possibility of rain in that period, so that the plant recovers turgidity and the phenolic foam block is hydrated again. ^{Al-Humaid and Moftah (2007)} also reported that adding hydrogel increased survival to two months in Conocarpus erectus L individuals. In the present study, the 308-c phenolic foam kept 62 % of the P. leiophylla plants alive until 60 days after transplantation, while the hydrogel maintained 45 %; therefore, phenolic foam is another option to mitigate the effects of prolonged drought in the first months after transplantation.

Survival times are similar to those reported by ^{Agaba et al. (2010)} and ^{Orikiriza et al. (2013)} in other tree species (Table 2). It should be noted that they watered plants to field capacity before subjecting them to drought conditions and they applied irrigation to ensure establishment. Another study by ^{da Silva et al. (2012)} of the phenolic foam block is increased, survival in hybrid seedlings of Eucalyptus urophylla S.T. Blake and E. resinifera Sm. also increases. This is consistent with what was found in this study, since the 308-cc phenolic foam treatment showed 10 % more survival more than the 231-cc one.

Increase in height, diameter and biomass of Pinus leiophylla

The analysis of covariance showed no significant differences among treatments with respect to the increase in height at eight (P = 0.250) and 12 weeks (P = 0.135) (Table 4). ^{Chirino, Vilagrosa, and Vallejo (2011)} and ^{Maldonado-Benitez, Aldrete, López-Upton, Vaquera- Huerta, and Cetina-Alcalá (2011)} reported similar results and attributed them to drought conditions. Evaluation was made at week eight, because most of the plants of the different treatments maintained around 50 % survival, and at week 12, because almost all of the plants had died.

Regarding diameter, the analysis of covariance showed significant differences among treatments at eight (P = 0.013) and 12 weeks (P = 0.002). Table 5 shows the diameter growth of P. leiophylla plants under the four evaluated treatments. The control treatment had the smallest diameter (3.9 mm), while the increase of this variable in the phenolic foam and hydrogel treatments was similar. These results agree with those reported by ^{De la O-Quezada, Ojeda-Barrios, Hernández-Rodríguez, Sánchez-Chávez, and Martínez-Tellez (2011)}, who indicate that, under water stress conditions, walnut seedlings are mainly affected in increase of diameter.

Table 6 shows the analysis of variance of the shoot, root and total biomass with statistically significant differences (P ≤ 0.05) among the treatments. On the other hand, Figure 3 shows a graphical comparison of the biomass of P. leiophylla under the different treatments. The control treatment had the highest root biomass (0.99 g). In total biomass, the difference between the extreme values was 0.78 g, corresponding to the control and hydrogel treatments. In plants with phenolic foam, part of the roots was lost when removing the root system, because they went through the foam and it was impossible to separate them from it, an error that was not considered in the weighing. It is also important to mention that, according to ^{Cornejo and Emmingham (2003)}, the results can be attributed to the fact that the evaluations were conducted during the cold season and in a short period (12 weeks), as the increase in biomass during the cold season under greenhouse conditions is not affected by water stress.

Figure 3 Biomass of P. leiophylla under different water reservoir treatments. Bars with the same pattern and a different letter indicate statistically significant differences according to the Tukey test (P ≤ 0.05).