Introduction

Red cedar (Cedrela odorata L.) is one of the most valuable and widely distributed tropical timber species in Latin America and the Caribbean. Its importance as producer of precious wood, encompasses economic, ecological and social aspects (^{Mesén, 2006}). Therefore, it is widely used in reforestation plantations in the Mexican tropics.

Most reforestations are oriented to projects of productive reconversion in abandoned land with a history of agricultural use (^{Ramírez et al., 2008}). However, many of them have not had the expected success, due to the high mortality and the poor growth of the plants. These problems are associated with a number of factors, such as the vulnerability of the species to the attack of Hypsipyla grandella (Zeller, 1848) and severe soil degradation at plantation sites (^{Calixto et al., 2015}). However, the impact of each factor has been greater due to the use of low quality plant, which does not meet the morphological and physiological characteristics appropriate to the conditions of the destination sites (^{Conafor, 2012}).

At the nursery, the accomplishment of these characteristics is related to the implementation of diverse cultural practices (^{Rodríguez, 2008}). The choice of one or more depends on their effectiveness in increasing plant quality of the species to be produced (^{Jacobs and Wilkinson, 2009}).

In plants of different forest species produced in nursery, both tropical and temperate-cold, the volume of the container or the addition of hydrogel to the substrate have shown a predominant influence on the modification of some of its properties. In several of them, as the volume of the container is larger, its morphological aspects of quality are better, such as stem height and diameter, as well as the balance between aerial biomass formation and radical biomass (^{Ferraz and Engel, 2011}, ^{Lisboa et al., 2012}, ^{Abreu et al., 2014}). In others, with the use of hydrogel not only a shape improvement has been recorded (^{Orikiriza et al., 2009}; ^{Maldonado et al., 2011}), but also of physiological order, which refers to the nutritional status as one of the main attributes affected (^{Chirino et al., 2011}; ^{Bernardi et al., 2012}), a benefit that depends on the dose applied. It is likely that in C. odorata the above factors have a similar effect, so it is important to study them in detail.

Based on the above, this work analyzes the influence of the volume of the container and the addition of hydrogel to the substrate on the morphology, nutritional status and field performance of C. odorata seedlings. The study poses three hypotheses: 1) the larger the container volume, the morphological attributes of C. odorata seedlings are better; 2) the addition of hydrogel to the substrate also favors the morphology and nutritional status of the seedlings; and 3) nursery seedlings with the most outstanding quality attributes (morphological and nutritional status) have high rates of survival and growth in the field.

Materials and Methods

Statistical analysis of field data

The analysis of survival was carried out through the Log-Rank test from the survival curves built with the Kaplan-Meier method, which defines the function as:

Where=

S (t) = Survival function

P = Death probability of a seedling

T = Undefined life span of a seedling longer than the time of the study

t = Defined life span of a seedling during the time of the study

The LIFETEST procedure of SAS 9.2 version (^{SAS, 2009}) was followed.

During the second assessment period there was a high mortality rate that affected complete treatments; therefore, only the values of the absolute growth rates (height and diameter) of the first period were subjected to an analysis of variance by using the ANOVA procedure in SAS 9.2 version (^{SAS, 2009}). Prior to this, the assumptions of normality and homogeneity of variances were validated. The hypothesis test was based on a significance level of α = 0.05. The means were compared by pairs using the Tukey test (α = 0.05).

Results

In nursery, the influence of the container volume on the modification of seedling morphology was associated with the addition of hydrogel to the substrate (Table 1).

Table 1 Statistical significance for quality morphological indicators in Cedrela odorata L. seedlings produced in containers of different volumes and with different doses of hydrogel in the substrate. 

* P<0.05; ** P<0.001; *** P<0.0001; NS = Non significant; 1 = Height of the aerial part; 2 = Diameter of the root neck; 3 = Aerial dry weight; 4 = Root dry weight; 5 = Aerial/root dry weight ratio; 6 = Robustness index; 7 = Dickson quality index.

Although there were statistical similarities between T1 and T3 seedlings, those of T3 showed the best values in most quality morphological variables (except for R: PSA/PSR) (Table 2). In regard to the seedlings of the control treatment (T4), those of T3 showed a growth 19. 67 % higher in height and 24.23 % in diameter (Table 2). In addition, the T3 individuals formed 2.23 times more aerial biomass and 1.71 more radical biomass compared to the control treatment (Table 2). IR, in which low values indicate better quality, showed a 5 % difference between T3 and T4 seedlings (Table 2). Finally, in the case of ICD, where higher values are better, there were differences greater than 40 % among the mentioned treatments (Table 2).

Table 2 Average values of the quality morphology variables assessed in Cedrela odorata L. seedlings produced in containers of different volume and with different hydrogel doses in the susbstrate 

1 = Height of the aerial part (average values in cm ± 0.74 standard error); 2 = Diameter of the root neck (average values in mm ± 0.14 standard error); 3 = Aerial dry weight (average values in g ± 0.09 standard error); 4 = Root dry weight (average values in g ± 0.04 standard error); 5 = Aerial/root dry weight ratio (average values ± 0.20 standard error); 6 = Robustness index (average values ± 0.16 standard error); 7 = Dickson quality index (average values ± 0. 02 standard error). Means with different letter within the same column are statistically different (Tukey α = 0.05).

Nutritional condition

Regardless of the corresponding effect of the addition of hydrogel, the seedlings with the greatest amount of nutritional reserves were produced in the 500 mL containers (Table 3).

Table 3 Absolute and relative values of the nutritional condition of nitrogen, phosphorous and potassium of Cedrela odorata L. plants at the end of the production period at the nursery. 

Also, according to the nomograms of vectors, and taking the nutritional status of the plants of T4 in nitrogen, those of T1 showed a dilution effect, those of T2 of luxury consumption, those of T3 of sufficiency and those of T5 and T6 of depletion (Figure 1A). A similar effect to that obtained in nitrogen was observed in phosphorus, although in this case the T2 no longer showed a luxury consumption but of dilution (Figure 1B). In regard to potassium, the plants of T1, T2, T3 and T6 had in common effects of luxury consumption; nevertheless, this response was more representative in the first three treatments. The T5 specimens had a dilution effect, which was different from that of the other nutrients (Figure 1C).

Figure 1 Vector nomograms of the nutritional condition of nitrogen (N), phosphorous (P) and potassium (K) of Cedrela odorata L. seedlings at the end of the production period at the nursery 

Survival and growth in the field

At 17 months of planting, the average overall survival was 15 %. During the first two months, mortality was higher than 25 % (Table 4, Figure 2). The Log-Rank test showed highly significant differences among the treatments evaluated (X² = 21.7, P = 0.0006). The highest survival rate was obtained in T1 plants (50 %), although there were no significant differences between those belonging to T2 and T3. There was a 100 % mortality in the T4 plants, even though this treatment differed only from T1 (Table 4). Although T2 and T3 had low survival at the end of the evaluation period, the treatments with the highest survival rate were over 80 % in the first six months (Figure 2).

Table 4 Estimated field survival of Cedrela odorata L. seedlings produced in containers of different volume and different doses of hydrogel at the nursery, according to the Kaplan-Meier method. 

T1 = 500 mL black polyethylene bag + without hydrogel; T2 = 500 ml polyethylene black bag + 2 g of hydrogel; T3 = 500 ml polyethylene black bag + 4 g of hydrogel; T4 = 380 mL tube containers + without hydrogel; T5 = 380 mL tube containers + 2 g of hydrogel; T6 = 380 mL tube containers + 4 g of hydrogel. *Different letters indicate statistical differences through the Log-Rank test.

Figure 2 Estimated survival function [S (t)] for the different nursery treatments evaluated at the Cedrela odorata L. plantation. 

During the growth period evaluated in the field, there were highly significant differences between treatments (P≥0.0001) both in height and in diameter. In both cases, T1 seedlings recorded the highest absolute growth rates. When comparing the values of this treatment against those of T4 (control) in height and diameter, differences were 200 % and 86 % higher, respectively (Figure 3A, B).

Figure 3 Growth in field (during the first six months of planting) in height (A) and diameter (B) of of Cedrela odorata L. seedlings produced in containers of different volume and doses of hydrogel at the nursery 

Discussion

At the nursery, the morphology and nutritional status of C. odorata seedlings are improved with the use of 500 mL containers and the addition of 4 g of hydrogel per liter of substrate (Table 2). This response is attributed to the conditions generated in the growth medium, since both factors make it up. The growth medium positively affects plant growth when it provides water, nutrients and oxygen at appropriate levels.

In that context, the interaction between the 500 mL container (polyethylene bag) and the 4 g dose of hydrogel (T3), suggests a positive synergistic response that favored, in the first instance, the radical growth. The root biomass formation of the seedlings of this treatment was 1.71 times greater than that of the seedlings of the control treatment (Table 2). From this, it is likely, as shown by Orikiriza et al., that as they had a good root growth, the seedling of T3 showed a greater water absorption capacity as well as of nutriments and oxigen, which favores a good development of the rest of their structures (Table 2) and an adequate nutritional status (Figure 1A, B, C). This hypothesis is supported by studies whose context is similar to that of this paper. For example, ^{Annapurna et al. (2004)}, ^{Ferraz and Engel (2011)}, ^{Lisboa et al., (2012)} and ^{Abreu et al. (2014)} report a better growth and greater formation of aerial and radical biomass in seedlings of different forest species, as the greater the volume of the container with which they are produced at the nursery. Also, the incorporation of hydrogel to the substrate shows a beneficial effect on the growth and biomass formation of seedlings of other species in works developed by ^{Orikiriza et al. (2009)}, ^{Maldonado et al. (2011)}, ^{Chirino et al. (2011)}, ^{Bernardi et al. (2012)} and ^{Navroski et al. (2015)}.

Although in plant growth there are effects associated with the type of packaging (in relation to the type of material, shape, color, etc.) as recorded in the works of ^{Wightman et al. (2001)} and ^{Tauer and Cole (2009)}, the responses obtained by volume are attributed to the greater space available to plants to grow in radical biomass, since the packaging is a physical barrier that restricts growth

On the other hand, the benefits of the hydrogel coincide with the modification of the physical properties of the substrate, especially to the increase of the capacity of water retention readily available, a condition that influences in a positive way in the water state of the plants, which has been verified by the laboratory and field assays of ^{Koupai et al. (2008)} and ^{Narjary et al. (2012)}.

In this study, neither the levels of the amount of water available in the substrate nor the water status of the seedlings were measured; however, the assumption that T3 seedlings have shown the best responses due to the increase in the water retention capacity readily available due to the addition of hydrogel (since they had more material by the volume of the bag), is related to the nutritional condition represented in the nomograms of vectors (Figure 1A, B, C). Unlike the seedlings of the other treatments, whose nutritions varied (Table 3, Figure 1A, B, C), the T3 seedlings presented a luxury consumption in N, P and K, despite the fact that the rate of fertilizer addition was the same for all treatments. It is inferred that the efficiency of this input was improved by the availability of water, which is consistent with well-known relationships in plants between water and nutrient absorption (^{Jones, 2005}).

Conversely, in the field, the volume of the container and the addition of hydrogel affect the performance of the seedlings, but the most direct effect on its quality is associated with the first factor. During the initial six months of planting, the T1, T2 and T3 treatments recorded a better survival compared to T4, T5 and T6; but at 17 months, only T1 kept this behavior (Table 4, Figure 2) which is contrating to what was expected of T3, which at the time of planting, were morphologically and nutritionally better individuals. However, in statistical or operational terms with which reforestation success is evaluated (^{Conafor, 2012}), the C. odorata survival in the field was not favorable for the whole set of treatments, since there was an overall average mortality of 85 %.

Field observations indicate that the soil was poorly drained (due to its clay texture, its field capacity of 34.5 %, saturation point of 64.4 %, water conductivity of 0.80 cm h^-1 and apparent density of 1.18 g cm^-3) and the presence of phytophagous insects on the site, as the main reasons of high mortality. Both causes correspond to the factors to which C. odorata is vulnerable in a forest plantation scheme (^{Calixto et al., 2015}). Although the two conditions affected all seedlings, their characteristics determined the severity of the impact of each; in this regard, a differential effect was evidenced between treatment groups.

The seedlings of the treatments involving the 500 mL polyethylene bag (T1, T2 and T3) were killed mainly by attacks of grasshoppers and Hypsipyla grandella, whereas those developed in the 380 mL tube (T4, T5 and T6) died mostly by flooding. The susceptibility to herbivory in the plants of the first group of treatments, is based on the hypothesis of plant vigor according to reviews by ^{Baraza et al. (2007)}. Its healthy look and the nutritional quality of its tissues, apparently favored a focus of insects, as ^{Medinaceli et al. (2004)} experimentally proved it. On the other hand, the loss of seedlings of the second group is attributed to the alterations created in the soil by flooding and the low capacity of the seedlings to tolerate stress due to their lower morphological and physiological characteristics (^{Pardos, 2004}) (Table 2).

Contrary to the reactions derived from the nursery stage and similar to the survival response, the best absolute growths in height and diameter did not correspond to seedlings with the best quality characteristics (T3), but to those of T1 (Figure 3). In addition to the positive effect of the volume of the container where they were produced, whose responses coincide with those obtained in other studies (^{Aphalo and Rikala, 2003}; ^{Prieto et al., 2007}), the superior field growth of T1 seedlings is linked to lack of hydrogels in its root ball. Probably due to the poor drainage conditions in the soil of the plantation site, the seedlings retained less water compared to those with hydrogel, which reduced the waterlogging impacts described by ^{Pardos (2004)}. Being less affected by this condition, the better availability of oxygen and nutrients (which did not have the other seedlings with the lowest growth), allowed them a more outstanding performance. The limitation exerted by the hydrogel contrasts with the generally recognized benefits of this material, which is consistent because the hydrogel effectiveness has been analyzed mostly in drought contexts (^{Chirino et al., 2011}, ^{Orikiriza et al., 2013}).

These findings have practical implications for reforestation with C. odorata. Experimental evidence suggests avoiding the use of hydrogels in poor drainage soils, and instead producing plants in containers whose volume is sufficient to produce a vigorous and competitive plant with high rates of growth that enable them to rapidly evade vulnerability phases on the field; however, this recommendation should be subject to a cost-benefit analysis, such as that carried out by ^{Puértolas et al. (2012)}.

Conclusions

There was a positive effect on the morphological quality and nutritional status of C. odorata seedlings at the nursery, the greater the volume of the container with which they are produced and the addition of hydrogel to the substrate, in doses of 4 g per liter. The volume of the container and the addition of hydrogel affected the performance of the seedlings in the field, but the most direct effect on their quality is associated with the first factor.