Introduction

Enterolobium cyclocarpum (Jacq.) Griseb is a multipurpose tree that grows in tropical ecosystems such as high evergreen, medium sub-deciduous, medium sub- evergreen, and low deciduous rainforests (^{Salas-Morales et al., 2003}; ^{Pennington and Sarukhán, 2005}). Its distribution area in Mexico extends from Sinaloa to Chiapas along the Pacific coast and from Tamaulipas to the Yucatán Peninsula along the Gulf of Mexico (^{Pennington and Sarukhán, 2005}).

In its areas of distribution, this species is used mainly for the restoration of degraded sites, in agroforestry systems, and in commercial forest plantations (^{Muñoz-Flores et al., 2016}; ^{Velasco-García et al., 2019}). However, survival rates are low during the first year of establishment, being associated mainly with the origin of the germplasm (^{Muñoz et al., 2013}), poor quality of the planting site, damage due to pests, diseases, or herbivory (^{Cibrián, 2013}; ^{Velasco-García et al., 2019}) and even droughts (^{Hernández-Hernández et al., 2019}).

The exploitation of E. cyclocarpum for timber purposes has a negative impact on its natural regeneration and, consequently, increases the fragmentation of its populations (^{Olivares-Pérez et al., 2011}). For this reason, it is essential to establish plantations that will allow its sustainable use. However, there are still gaps in knowledge regarding the best practices to increase plantation survival rates and productivity (^{Basave et al., 2014}; ^{Hernández-Hernández et al., 2019}).

Nursery cultivation practices have a direct effect on plant quality and, therefore, on plantation performance (^{Grossnickle, 2012}). Likewise, the survival and growth of plants in field depends, to a certain extent, on the methods or techniques used in the planting process (^{Löf et al., 2012}). Within this context, the objective of the present study was to evaluate the effect of different nursery growth media and auxiliary treatments to mitigate water stress on the performance of E. cyclocarpum plants established in field. The hypothesis was that the type of substrate influences plant survival and growth, and that auxiliary treatments may promote a better performance of this species in forest plantations or reforestations.

Materials and Methods

Seed collection and plant production

The production of the plant was carried out in the nursery of the Los Tulillos ranch, in the municipality Tzitzio, Michoacán (Figure 1). The seeds were collected in the same locality between May and June 2019. Prior to sowing, the seeds were scarified with mechanical tweezers to homogenize germination, and subsequently sown in low-density, caliber 400, 13×25 cm polyethylene bags with four perforations. The bags were filled with three different types of substrate: 1) base mix ―peat, vermiculite and agrolite― at a ratio of 2:1:1, with 4 g L^-1 of controlled release fertilizer; 2) aerobic compost, consisting of cattle manure, and 3) forest land, obtained in an area of low deciduous forest. A total of 200 plants were produced per substrate type. Irrigation was applied manually with a watering can every other day. The cultivation period in the nursery was three months, before the plants were taken to the field.

Figure 1 Location of the planting site and location of Enterolobium cyclocarpum (Jacq.) Griseb. trees. 

Results

Ten months after planting, the substrate type had a significant effect on plant survival (Chi ² =10.62, p=0.005). Plants grown on native soil showed higher survival than those grown with the base mix and compost. At the end of the study, plants grown on topsoil achieved 76 % survival, while plants with base mix and compost had 62 and 65 %, respectively. Likewise, regardless of treatment, the relationship between the survival rate and the initial diameter of individuals was significant (Chi ² =20.51, p<0.001) (Figure 2). The survival probability ratio as a function of diameter was 1.44, indicating that a 1 mm increase in the diameter of nursery plants increases the probability of survival in the field by 1.4 times. On the other hand, the auxiliary treatment to mitigate water stress did not significantly influence survival (Chi ² =1.07, p=0.785).

Figure 2 Probability of survival of Enterolobium cyclocarpum (Jacq.) Griseb. plants ten months after establishment as a function of the initial diameter at planting and the nursery growing substrate. 

In general, the auxiliary treatment for mitigating water stress affected diameter growth significantly (F=15.17, p<0.001), unlike the type of substrate (F=2.01, p=0.135). However, the interaction of the auxiliary treatment with the substrate was significant (F=4.23, p<0.001). This interaction showed that the response to the auxiliary treatment was different for each type of substrate. Although irrigation increased diameter growth with respect to the control, its effect was significant only in plants grown with compost (Figure 3). The hydrogel treatments induced greater growth in diameter, with respect to the control, but only in those cultivated on native soil. Finally, the effect of the auxiliary treatments for mitigating water stress was not significant in plants grown on the base mixture, with respect to the control.

Figure 3 Mean diameters±SE in Enterolobium cyclocarpum (Jacq.) Griseb. plants ten months after establishment, grown in three types of substrates and planted with different auxiliary treatments for mitigating water stress. Differences between means that do not share the same letter are significant (α=0.05). 

Auxiliary treatments to mitigate water stress had a highly significant effect on plant height (F=28.83, p<0.001). On the other hand, the type of substrate did not affect height growth (F=0.97, p=0.378); however, the interaction of the auxiliary treatment with the substrate was significant (F=3.14, p=0.005).

For the three types of substrate, the irrigation treatment improved height growth significantly, compared to the control (Figure 4). In plants grown in compost, irrigation yielded superior results to those obtained with hydrogel application. The hydrogel treatments did not produce a significant increase in height in relation to the control, except for the treatment with four grams of hydrogel in the plants cultivated with loam (Figure 4).

Figure 4 Mean heights±SE in Enterolobium cyclocarpum (Jacq.) Griseb. plants ten months after establishment, grown in three types of substrates and planted with different auxiliary treatments to mitigate water stress. Differences between means that do not share the same letter are significant (α=0.05). 

Discussion

The present study demonstrated that the type of substrate with which E. cyclocarpum plants are grown in the nursery influences survival at early age of field establishment. On the other hand, the application of auxiliary treatments for water stress mainly affected growth, although their effect also depended on the type of substrate used in the nursery.

In plant production, the substrate is the element that supports the plant. The type of substrate affects growth and root architecture, and also influences the cohesion and integrity of the root ball (^{Landis, 1990}). This latter aspect is considered an important trait for the quality of nursery-grown plants (^{Pemán et al., 2017}), since proper root ball integrity or conformation has a positive effect on plant survival, particularly on sites with degraded soils (^{Grossnickle and El-Kassaby, 2016}).

In the present study, the native soil proved more favorable to the survival of E. cyclocarpum than the other substrates. In a visual inspection, better root ball conformation was observed with the native soil, which possibly improved the ability of the plants to survive. Other characteristics of local substrates, such as additional nutrient content and beneficial microorganisms, can improve plant performance (^{Jacobs and Landis, 2014}). The soil is widely used in the production of plants of other forest species, with good results in variables indicating plant quality (^{López et al., 2018}; ^{Reyes et al., 2018}). However, the physical and chemical characteristics of these substrates vary over time and between locations, which limits the standardization and continuous improvement of cultural practices in nurseries (^{Bakry et al., 2012}).

An important finding was the positive relationship between plant diameter and the probability of survival. This effect highlights the importance of considering diameter as a basic indicator of E. cyclocarpum survival in the first years of field planting. This result is consistent with studies showing that plants with larger diameters tend to survive better than those with smaller diameters (^{Orozco et al., 2010}; ^{Grossnickle, 2012}). In this regard, it has been suggested that the diameter has a direct relationship with certain characteristics of the root system, such as volume or architecture (^{Jacobs et al., 2005}). Furthermore, plants with larger diameters are likely to have more carbohydrate and nutrient reserves (^{Tsakaldimi et al., 2013}), which, in turn, influences the probability of survival in the field. In addition, plants with larger diameters have greater resistance and resilience to physical damage caused by biotic and abiotic agents.

E. cyclocarpum inhabits the dry tropical forest and is drought tolerant (^{Foroughbakhch et al., 2006}; ^{Laborde and Corrales-Ferrayola, 2012}). In this sense, it is likely that the application of some auxiliary treatment to mitigate water stress does not have a significant impact on survival rates, as observed in the present study. However, in the research documented here, it is possible that the plants were not exposed to water stress given the high rainfall at the planting site. On the other hand, in juvenile stages, E. cyclocarpum exhibits high growth rates (^{Foroughbakhch et al., 2006}; ^{Rocha et al., 2018}), and its performance increases considerably under favorable humidity and fertility conditions (^{Craven et al., 2007}; ^{Pineda-Herrera et al., 2017}). Therefore, this may explain why the positive effect of the application of the auxiliary treatment for mitigating water stress manifested in the growth rather than in the survival of the plants.

Irrigation was the auxiliary treatment that increased growth the most, compared to the control. Hydrogel application did not significantly improve plant growth. However, in plants cultivated with loam, more growth in diameter was observed with the application of hydrogel.

The main benefit of hydrogels is to increase the moisture-holding capacity of the soil or growing medium in which they are incorporated (^{Chen et al., 2004}). However, the effectiveness of these polymers will depend on multiple factors such as the type of polymer, particle size, dosage, amount of water available, application method, among others (^{Crous, 2017}). Also, the physical and chemical properties of the soil often affect the effectiveness of hydrogels (^{Agaba et al., 2010}; ^{Crous, 2017}). In this sense, the variable response of the plants to the hydrogel application could be related to the differences in the physicochemical characteristics of the substrates. However, more research is needed to study these relationships in depth.

Conclusions

The authors accept the hypothesis that the type of substrate influences the survival and growth of the Enterolobium cyclocarpum plants, and that native soil promotes better performance. Diameter maybe an indicator of survival of E. cyclocarpum at early ages after planting. Auxiliary treatment to mitigate water stress impacts growth rather than survival. However, it is advisable to consider the rainfall and humidity conditions of the planting site in order to determine the need for the application of this type of treatment. The results contribute to promoting a better performance of the study species in commercial plantations or reforestations.