Mexico is estimated to have lost more than 44 million hectares of forests over the past 60 years (Velázquez, Durán, Mas, Bray, & Bocco, 2005). Faced with this situation, several reforestation programs have been put in place, but they have generally failed to achieve the desired results because the plants used die from various causes such as: zero site preparation, grazing, competition with native vegetation, diseases and fires; however, the factors with the greatest impact are drought and inappropriate planting dates (Comisión Nacional Forestal [CONAFOR], Colegio de Postgraduados, & Secretaría de Medio Ambiente y Recursos Naturales [SEMARNAT], 2008).
Climate change has altered rainfall patterns, prolonging droughts, causing plants to be exposed to increased water stress and resulting in low survival in the field (Hanjra & Qureshi, 2010). Some technologies that help provide resistance to water stress in different species of commercial interest have been developed (Barón, Barrera, Boada, & Rodríguez, 2007). One of the technologies that has drawn the most attention is the use of hydrogel, a polymer capable of absorbing up to 400 mL of water per gram dry weight (Ahmed, 2013). However, it has been shown that soil texture can affect the performance of the hydrogel (Agaba et al., 2010), and water salinity can decrease its effectiveness (Akhter et al., 2004). Moreover, other authors state that applying the polymer does not provide benefits to plantations (Farrell, Ang, & Rayner, 2013).
In this regard, open-cell phenolic foam has been proposed as an alternative, being a thermosetting synthetic resin able to save more than 40 times its own weight in water without suffering deformation (Gardziella, Pilato, & Knop, 2000). Due to the foam’s physical structure, plant roots can pass through it and make use of the trapped water. This type of foam is used in hydroponic greenhouses and its effectiveness as a substrate is widely documented (Bezerra et al., 2010; Chugh, Guha, & Rao, 2009; da Silva, Kager, de Moraes, & Gonçalves, 2012).
Pinus leiophylla Schiede ex Schltdl. & Cham. is found in transition zones between natural Pinus and Quercus forests and agricultural areas, thus fulfilling the important role of forest protection and buffering. The species is valued for its use in the production of poles, furniture and wood pulp, as well as being a source of firewood and charcoal for rural communities and industries (Musálem & Martínez, 2003).
In light of the above, the aim of this study was to evaluate the effect of adding water reservoirs at transplanting on the survival, growth in height and diameter, and biomass of P. leiophylla plants grown under simulated drought conditions in a greenhouse.
Materials and methods
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.
|T1||Control. No phenolic foam blocks||The plant was placed in a traditional way|
|T2||Hydrated phenolic foam block of 3.3 x 7 x 10 cm and 231 cc dry volume||The block was placed on one side of the plant root ball, at a depth of 7 cm below the surface|
|T3||Hydrated phenolic foam block of 4.4 x 7 x 10 cm and 308 cc dry volume||The block was placed on one side of the plant root ball, at a depth of 7 cm below the surface|
|T4||3 g of hydrated hydrogel||The hydrogel was dispersed around the root ball, at a depth of 5-7 cm|
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
Results and discussion
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.
|Authors||Species studied||Days of survival|
Agaba et al
|Orikiriza et al
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.
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).
|Treatment||Weekly survival (%)|
|Control||100.0 a||100.0 a||100.0 a||76.7 a||51.7 b||43.3 b||19.7 b||8.3 b||3.3 a||3.3 a||3.3 a||1.7 a|
|Phenolic foam (231 cc)||100.0 a||100.0 a||100.0 a||92.0 a||88.0 a||88.0 a||75.0 a||52.0 a||32.0 a||15.0 a||10.0 a||5.0 a|
|Phenolic foam (308 cc)||100.0 a||100.0 a||100.0 a||92.0 a||90.0 a||87.0 a||78.0 a||62.0 a||40.0 a||20.0 a||17.0 a||13.0 a|
|Hydrated hydrogel (3 g)||100.0 a||100.0 a||100.0 a||100.0 a||100.0 a||91.7 a||45.0 a||45.0 a||20.0 a||13.3 a||11.6 a||8.3 a|
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.
|Variable||Week after transplanting||Mean squares||Pr > F|
|Treatment (3)*||Error (235)*|
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.
|Treatment||Average diameter in week 8 (mm)||Average diameter in week 12 (mm)|
|Control||3.92 b||3.96 b|
|Phenolic foam (231 cc)||4.21 a||4.31 a|
|Phenolic foam (308 cc)||4.04 a||4.20 a|
|Hydrated hydrogel (3 g)||4.29 a||4.44 a|
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.
|Variable||Mean squares||Pr > F|
|Treatment (3)*||Error (236)*|
Adding hydrated open-cell phenolic foam, at the time of transplanting P. leiophylla plants, significantly increased survival time under simulated drought conditions in a greenhouse to 63 days, compared to 35 days for the control and 49 days for the hydrated hydrogel treatment. A significant increase in diameter of up to 0.35 mm relative to the control was also observed; differences in the variable height were negligible. This type of study provides guidelines for further research and possible application of a new water reservoir system under conditions different from those of a greenhouse to help mitigate the effects of drought, increase survival and improve the establishment of pine plants.