Dasometric and phenotypic characterization of trees

Based on the qualitative assessment of the parameters proposed in Samaniego’s matrix ^{(Samaniego, 2002)}, the corresponding values were added in order to obtain the score per tree and to classify each tree by its phenotypic characteristics (Table 1).

Seedling assessment

Dickson’s Index (DI) was estimated using the follwing formula (^{Olivo and Buduba, 2006}):

The quality of the plants was classified as low (<0.2), medium (0.3-0.4) and high (≥0.5), based on the quality intervals established by ^{Sáenz et al. (2010)} for Dickson’s Index, in order to evaluate the plants produced in nine nurseries in the state of Jalisco.

Phenotypic and dasometric classification of trees

In terms of phenotypic assessment, J. pyriformis trees in both populations were grouped into Classes 1 and 2; this is an indication that they may be considered to be seed trees. A higher number of Class I individuals--which stand out for their height and diameter, are dominant and are part of the higher canopy--were identified in Coatepec. On the other hand, Class 2 trees are codominant and have medium phenotypic characteristics (Table 2).

Class I trees are less numerous in the population of Coacoatzintla and they are codominant, while Class 2 trees are dominant. The height and mean diameter of J. pyriformis in both populations are within the ranges of 4-25 m and 38-69 cm, respectively, cited by ^{Acosta-Hernández et al. (2011b)} for groups I and III in the population of Coacoatzintla.

Using the same methodology, ^{Acosta-Hernández et al. (2011a)} characterized 112 J. pyriformis specimens in the population of Coacoatzintla, in order to determine the level of morphological variation. The data analysis, based on Main Components (MC), evidenced three groups, which coincided with the classes proposed by ^{Samaniego (2002)}. This matrix was used by Salan (2011) to select Cedrela odorata L. seed trees and identified seven Class III and three Class II trees in the native forests of the Ecuadorian Amazon Region.

The method developed by ^{Samaniego (2002)} has been utilized to assess previously identified and selected Caesalpinia spinosa (Molina) Kuntze seed sources in order to promote their management for purposes of genetic enhancement.

In their assay of 24 J. regia offsprings, ^{Aletá et al. (2009)} point out that the branch insertion angle, branching and straightness are highly hereditary; the same phenotypic characteristics were evaluated in J. pyriformis individuals. According to these authors, the branch insertion angle and branching show no modifications through time, and therefore are reliable characteristics for the selection of seed trees. ^{Aletá et al. (2003)} mentioned that the expression of diameter and height depends on the conditions of the site where the seedlings are established, and its observation as a criterion for the selection of seed trees must therefore be taken with reservations.

Physiological quality of the seed

Seeds from Coatepec began germinating in the fourth week, but they reached a GI of 5% (GI) and a MGT of 50% one week after those from Coacoatzintla; at the end of the assay (133 days), they had a higher germinative potency (GP%) (Figure 2).

Figure 2 Germination percentage, accumulated per week, of Juglans pyriformis Liebm. seeds from the populations of Coatepec (CT) and Coacoatzintla (CC). 

The GP per tree/population was over 50% in the seeds of individuals from Coatepec; nine of these had values near or above 80%. Conversely, only the seeds of 16 trees from Coacoatzintla had a GP of 50%; nine registered values near or equal to 80% (Figure 3 A and B).

Figure 3 Germinative Potency per Juglans pyriformis Liebm. tree and per population. 

According to the established seed quality intervals (Table 3), twelve individuals with a high quality seed and eight with a medium quality seed were identified in the population of Coatepec, while in the population of Coacoatzintla twelve trees had a high quality seed, four had a medium quality seed, and another four, a low quality seed.

Most seeds from Coatepec had high values for germinative potency (GP%), mean daily germination (MDG) and germinative vigor (GV); therefore, their quality is higher than that of seeds from Coacoatzintla. The variation coefficient for GP% and GV is low, a sign that there is little variation in germination in both populations (Table 4).

The germination percentage of J. pyriformis is higher than that cited for Juglans neotropica (35%) (^{López and Piedrahita, 1998}), but lower for those registered by ^{CATIE (1999)} in fresh seeds of J. neotropica (80-90%): 100% was registered by ^{Quintero-García and Jaramillo-Villegas (2012)}, and 80%, by Gómez, (2002 quoted in ^{Quintero-García and Jaramillo-Villegas, 2012}) for the same species. It also came lower than the value reported for J. regia under nursery production conditions (80%) and with pregerminative treatments (85-95%) (^{Vahdati and Hoseini, 2005}).

The comparison between germination percentages must be taken with reservations, given that, in many of the referred works, values were obtained after the biological material was subjected to pregerminative treatments. It is noteworthy that J. pyriformis seeds (Figure 4) are cataloged as sub-orthodox, i.e., their storage periods must be short due to their high content of lipids (^{Rojas, 1995}). Nevertheless, the percentages for both populations in the present study are high without any treatment; this is attributed to the fact that the seeds were sown immediately after they were collected, and therefore their germinative vigor had not diminished due to the passage of time.

A) Fruit, B) Rotting of the fruit, C) Seeds, D) Seeds sown in tubes, and E) Germination.

Figure 4 Collection and establishment of nursery-grown Juglans pyriformis Liebm. seeds. 

Plant quality

The seedlings obtained from germinated seeds from Coatepec had a Dickson’s Index (DI) of >0.500, and were therefore classified as high quality. Tree No. 9 stands out with an index of 2.081. Seedlings from Coacoatzintla seeds attained a high and a medium quality; five trees stand out for their high plant quality, including tree No. 7, which had an index of 0.961--the highest among this population--, while the rest of the individuals (15) were classified as medium quality (Table 5).

The asessment of the plants using Dickson’s index is highly important, as it predicts the in-field survival and growth, by integrating morphological variables of height and diameter with the weight of the plant, all of which maintain a highly significant interaction; thus, the higher the index, the better the plant (^{Dickson et al. 1960}; ^{Olivo and Buduba, 2006}; ^{Rodríguez, 2008}; ^{Rueda et al. 2012}).

The records for J. pyriformis are comparable to those documented by ^{Rueda et al. (2012)} for five pine species (Pinus devoniana Lindl., P. oocarpa Schiede ex Schltdl., P. greggii Engelm. ex Parl. and P. pseudostrobus Lindl., Pinus douglasiana Martínez) and 10 rainforest species (Leucaena leucocephala (Lam.) de Wit, Enterolobium cyclocarpum (Jacq.) Griseb., Roseodendron donell-smithii (Rose) Miranda, Tabebuia rosea (Bertol) DC., Eucalyptus globulus Labill., Pithecellobium dulce (Roxb.) Benth., Prosopis juliflora (Sw.) DC., Eysenhardtia polystachya (Ortega) Sarg, Cupressus lusitanica Mill., Cedrela odorata), produced in eight forest nurseries in Jalisco under a technified production system.

The quality of the nursery-grown plants of Michoacán is comparable to that obtained for J. pyriformis. For example, P. michoacana had a DI between 0.59 and 4.1 and was rated as high; in P. greggii, P. oocarpa and P. pseudostrobus, a medium quality was obtained (ID 0.21 to 0.32) , while P. greggii had the lowest DI (0.13), rated as low quality (^{Saenz et al.,2010}).

After comparing the plant quality (DI) of the species mentioned above with the DI values determined for J. pyriformis, it may be said that, as long as it is grown in similar edaphic and climatic conditions to those of its place of origin, it will be able to survive and develop properly; this shows the importance of having a priori knowledge of the conditions into which it will be planted and of determining the quality of the plant before it leaves the nursery.

Seed tree selection

Based on the facts that seed trees must have outstanding characteristics, that the germination percentages and vigor of their seeds must be high (^{Poulsen, 2000}), and that the seedlings produced in the nursery must have morphological characteristics suitable for the plantation site in order to survive and develop properly (^{Rodríguez, 2008}), two categories were defined in the selection of J. pyriformis seed trees, under the following criteria:

Category I: Class 1 trees, with a germinative vigor or Czabator’s Index (GV ≥ 0.341) equivalent to GP% ≥ 75 and a Dickson’s Index (DI) equal to or above 0.5 per tree/population.

Category II: Class 1 or Class 2 trees, with a low or mediumgerminative value (<0.169; 0.170-0.340, respectively) and a low or medium Dickson’s Index (<0.2; 0.2-0.4, respectively).

Based on these criteria, nine potential seed trees in the population of Coatepec and two in Coacoatzintla were identified as belonging to Category I; the rest pertain to Category II (Table 6).

For the populations of Coatepec, columns 1 to 9 belong to Category I and from 10 to 20 to Category II. For those of Coacoatzintla, only columns 1 and 2 are classified into Category I and the rest into Category II.

Table 6 Category I and II Juglans pyriformis Liebm. seed trees within the populations Coatepec (CT) and Coacoatzintla (CC). 

According to the criteria of ^{Samaniego (2002)}, Class 1 J. pyriformis individuals are phenotypically outstanding and therefore may be regarded as seed trees. ^{Sanjuanbenito and Ramírez (2007)} mention that the seed collected from the outstanding trees can be destined to the establishment of commercial plantations, genetic enhancement and conservation and reforestation programs. Class 2 trees have certain outstanding phenotypic characteristics, but they exhibit high forking and a partial dominance of the main axis (^{Samaniego, 2002}; ^{Sanjuanbenito and Ramírez, 2007}); their seeds can be utilized in conservation and protection programs.

Populations localized in Coatepec and Coacoatzintla have sufficient characteristics that make them sources of seeds: they are accessible, and the trees they contain have a good phytosanitary status and include sufficient mature individuals for seed collection. The germoplasm from trees classified within Category I can be used to produce seedlings for the establishment of commercial plantations, genetic enhancement and conservation and reforestation programs. The seeds of Category II trees can be used to grow seedlings for programs for the conservation and protection of soils and ecosystems.

In order to manage them, it is important to implement a conservation program in which Juglans pyriformis may be considered as a component of the structure and functionality of the Montane Cloud Forest, not only from an economic perspective but also from the ecologic point of view. Such a program must address the local conditions and integrate the coordinated participation of the involved sectors so as to be sustainable and ensure the permanence of the species.