Introduction

Nursery cultural practices play an important role in the production of quality plant for reforestation projects; with them, morphological and physiological attributes are manipulated that regulate the development of plants in the field, in terms of survival and growth (^{Grossnickle and MacDonald, 2018}).

In Mexico, the low survival of reforestations is a persistent problem. Therefore, improving the quality of the plants for these programs through nursery practices continues to be necessary (^{Burney et al., 2015}). Most of the research made in this field is aimed at producing quality plants for temperate climates (^{Burney et al., 2015}); however, the need to generate information on cultural nursery management practices for other species, such as those of the dry tropics, which are required in various reforestation or restoration projects, is evident (^{Conafor, 2014}; ^{Riikonen and Luoranen, 2018}). The use of forest species from the dry tropics is limited by the lack of sufficient knowledge for their production in the nursery, particularly under the quality plant approach (^{Bonfil and Trejo, 2010}).

In the context of defining cultural practices to produce quality plants, it is convenient to explore those whose background with other species have shown satisfactory results. In this regard, aerial pruning or stem pruning is an example. It is argued that pruning as a cultural practice in the nursery favors the maintenance of woody species because it stimulates their growth and allows it to be directed in a certain way (^{Wade and Westerfield, 2009}). In this area, pruning has shown potential to increase the quality of some forest species. For example, Pinus greggii Engelm specimens improved their morphological quality and increased the photosynthetic rate and carbohydrate content after being pruned in 50 % of their aerial biomass (^{Cetina-Alcalá et al., 2001}, ²⁰⁰²). Another experimental support is provided by a study with seedlings of Nothofagus nervosa (Phil.) Dim. (^{Donoso et al., 2009}), whose response in the field was positive after being pruned at the nursery.

However, the proper implementation of aerial pruning depends on understanding how and when to do it, since reactions can vary between species (^{Wade and Westerfield, 2009}). For example, in Enterolobium cyclocarpum (Jacq.) Griseb. and Swietenia humillis Zucc. subject to three pruning intensities (0 %, 25 % and 50 %), aerial pruning is not recommended as a nursery practice because, according to the results provided by ^{Basave et al. (2014} and ²⁰¹⁵), the quality of the plants did not improve.

The differences consigned by the previously mentioned experimental evidence, regarding the feasibility of aerial pruning, suggest the need to evaluate its effects in a greater number of species, especially in those with high ecological and social value such as Caesalpinea coriaria (Jacq.) Willd, commonly known as cascalote. Cascalote is a multipurpose arboreal legume species of the dry tropics present in the pastures, where it is used as shade, forage, live fences and firewood (^{Olivares-Pérez et al., 2011}; ^{Palma-García and González-Rebeles, 2018}). In recent years, the importance of this type of species has been highlighted to reforest and rehabilitate grasslands degraded by extensive livestock farming through the promotion of silvopastoral systems (^{Murgueitio et al., 2011}). This aspect justifies the need to generate relevant information on the production of C. coriaria nursery plants, in relation to cultural practices that improve their quality. For this reason, the present study evaluated the effect of aerial pruning on the morphological quality of C. coriaria plants.

Materials and Methods

Results and Discussion

In general, aerial pruning did not affect the morphological quality of C. coriaria plants (p> 0.05), since only in the height of the aerial part (APA) and slenderness index (IE) variables, there were significant differences (p = 0.0001 and 0.02, respectively). The highest height (37.08 cm) was recorded in plants without pruning, while the lowest (29.06 cm) in plants pruned at 25 %. When comparing both values, a reduction of 28 % in the height of the plants stands out after receiving the 25 % pruning. In IE, the highest value also corresponded to plants without pruning, with 12.81, which was 29 % higher than the low value of 10.75 registered by plants pruned at 25 %. The aerial pruning at 50 % turned out to be the worst treatment, presenting values lower than those of the unpruned plants in most of the variables (Table 1).

In the production of forest plants in nurseries, it has been proposed that aerial or stem pruning has potential as a cultural practice to improve the quality of the plant for reforestation work (^{South, 2016}). This approach has been ratified in a study with Pinus greggii plants, in which such practice improved the morphological quality, increased the net assimilation rate and the carbohydrate content. These characteristics, together, contributed to a favorable performance of the plants in the field under a drought condition (^{Cetina-Alcalá et al., 2001}, ²⁰⁰²). Contrary to P. greggii, the C. coriaria plants of the present study did not have results that support the potential of aerial pruning as a cultural practice. The divergence between the results obtained in P. greggii and in C. coriaria suggests a differential effect of pruning between species, attributed to the different growth and branching patterns that are controlled by mechanisms of dominance and apical extention (^{Wilson, 2000}; ^{Cline and Harrington, 2007}). In this situation, the effects of hormones in apical growth could also be involved (^{Pallardy, 2008}). The responses of C. coriaria are in agreement with those of other tropical broadleaf species.

In plants of Swietenia humilis (^{Basave et al., 2015}) and Enterolobium cyclocarpum (^{Basave et al., 2014}) neither did aerial pruning as a cultural practice. In these two species, the effect that pruning had was to diminish the slenderness of the plants by reducing the relationship between the height of the aerial part and the diameter of the stem. This effect is one of the general responses of plants to pruning (^{Wade and Westerfield, 2009}) and coincides with what was observed in pruned plants of C. coriaria.

Slender plants are not desirable in the nursery; it is better if they are robust, since it is assumed that they are more vigorous; therefore, during the production of plants in the nursery, to promote this characteristic is very important (^{Orozco et al., 2010}). In several forest species, it has been shown that the greater the robustness, the better the probability of survival in the field (^{Tsakaldimi et al., 2013}). However, in the nursery, it is preferred to induce robustness is with a larger growth in diameter and not by a low ratio defined by the height of the plants, as happened in the 25 % pruned plants.

In C. coriaria, the effect of pruning was null in stimulating plant growth, as is commonly assumed (^{Wade and Westerfield, 2009}; ^{South, 2016}), so that the plants did not show an increase in stem diameter. This answer is negative in several forest species, because it is known that a thick diameter is a determining attribute in field survival under some environmental conditions (^{Grossnickle, 2012}; ^{Tsakaldimi et al., 2013}). For example, it was shown that in Prunus avium L. seedlings survival becomes higher by increasing the initial stem diameter of the nursery plants to a measure of 7-8 mm (^{Esen et al., 2012}).

The stem diameter of the C. coriaria plants did not exceed 3 mm in any of the treatments. This low value in diameter could be attributed to the fact that the species is slow growing (^{Palma and González-Rebeles, 2018}); however, the morphology of plants produced in containers is also determined by the production period in the nursery and by the effect of various cultural practices, according to conclusions derived from a study with Acacia macrostachya Reichenb plants. ex Benth and Pterocarpus erinaceus Poir. (^{Zida et al., 2008}).

In this sense, possibly the production time was not enough and the production conditions in the greenhouse were not adequate to optimize the growth of the C. coriaria plants. However, this hypothesis regarding production time is contradictory if compared, on the one hand, with the growth in diameter that Caesalpinia platyloba S. Watson, C. eriostachys Benth present in the nursery. and C. velutina Standl., whose stem diameter values vary between 2.5 and 4.4 mm at three -months old (^{Ngulube, 1989}; ^{Orozco et al., 2010}), and, on the other hand, if contrasted with the time of four to five months in which other tropical species of the same family (Fabaceae) commonly occur, such as Prosopis laevigata (Humb. et Bonpl. ex Willd.) MC Johnston (^{Prieto et al., 2012}; ^{Basave et al., 2017}) and Enterolobium cyclocarpum (^{Basave et al., 2014}). Later studies should improve the growth of C. coriaria at the nursery.

Finally, the lack of significant effects of pruning on the morphological quality of C. coriaria could be attributed to the fact that pruning may have limited photosynthesis due to the loss of leaves, as occurs in plants subject to loss of foliar biomass from herbivory (^{Nabity et al., 2009}), which causes a negative carbon balance (^{Kitajima, 2007}) that limited the ability of plants to produce enough carbohydrates to promote vigorous regrowth in the face of damage induced by pruning. In addition, this effect could have been highlighted by the fact that, before pruning, the plants did not have sufficient reserve carbohydrates, which favored greater competition for resources for growth, as discussed in the storage theory (^{Lambers et al., 2008}).

In this study, it was omitted to measure the content of reserve carbohydrates, but the previous proposals are based on its importance on the regrowth ability of plants (^{Von Fircks and Sennerby-Forsse, 1998}), a condition that was corroborated by Quercus crispula Blume plants (^{Kabeya and Sakai, 2005}). Furthermore, in other species, the regrowth capacity is determined by the same interspecific and intraspecific differences (^{Umeki et al., 2018}), according to their age, size (^{Vesk, 2006}), or nutritional status (^{Kabeya and Sakai, 2005}), which are factors that could be analyzed in greater detail in later studies of this sort.

Conclusions

Aerial pruning at the evaluated intensities of 25 % and 50 % has no potential as a cultural practice to improve the quality of the C. coriaria plant in the nursery. The effects on the plants were the reduction in height and its relationship with the diameter at the root neck, which leads to less robust plants. It is recommended to analyze the interaction of pruning with other cultural practices.