El sotobosque en la composición y diversidad de áreas bajo manejo forestal en Santa Catarina Ixtepeji, Oaxaca

Lizbeth Luna-Bautista¹, Patricia Hernández-de la Rosa¹, Alejandro Velázquez-Martínez¹*, Armando Gómez-Guerrero¹, Miguel Acosta-Mireles²

¹ Colegio de Postgraduados, Campus Montecillo. km 36.5 Carretera México-Texcoco. C. P. 56230. Montecillo, Texcoco, Edo. de México. MÉXICO. Correo-e: alejvela@colpos.mx, tel.: (595) 952 0200 ext. 1470 (*Autor para correspondencia).

² Instituto Nacional de Investigaciones Forestales y Agropecuarias. Campo Experimental Valle de México. Carretera Los Reyes-Texcoco km 13.5. C. P. 56250. Coatlinchán, Texcoco, Edo. de México. MÉXICO.

ABSTRACT

In the present study, the effect of silvicultural practices on richness, composition and diversity of tree species, herbaceous and shrub species in a forest community of Santa Catarina Ixtepeji, Oaxaca was evaluated. To this end, the following silvicultural treatments were evaluated: selective cutting (1998), light thinning (2011) and seed tree cutting (1998 and 2011). Alpha and beta diversity indices of tree communities (shrub and herbaceous) were estimated, and also the Importance Value index (IVI) of the tree layer. The results showed that the herbaceous component is the most diverse in both stands with and without silvicultural management, followed by the shrub component. According to the IVI, Pinus oaxacana Mirov was the most important ecological species in all treatments evaluated, including unmanaged forest. The results indicate that logging modifies richness, diversity and composition of the tree strata (shrub and herbaceous), these two tree strata are the largest contributors to diversity. Therefore it is important to assess the understory, because it helps giving a better explanation of the total plant diversity of the forest.

Keywords: Silvicultural treatments, understory vegetation, stand structure, biodiversity indices.

RESUMEN

En el presente estudio se investigó el efecto de las prácticas silvícolas sobre la riqueza, composición y diversidad de las especies arbóreas, herbáceas y arbustivas en un bosque de la comunidad de Santa Catarina Ixtepeji, Oaxaca. Para tal fin, los siguientes tratamientos silvícolas fueron evaluados: corta de selección 1998, aclareo ligero 2011 y árboles padre 1998 y 2011. Los índices de diversidad alfa y beta de las comunidades arbórea, arbustiva y herbácea se estimaron, así como el índice de valor de importancia (IVI) del estrato arbóreo. Los resultados mostraron que el componente herbáceo es el más diverso tanto en rodales bajo manejo silvícola como sin manejo, seguido del componente arbustivo. De acuerdo con el IVI, la especie de mayor importancia ecológica fue Pinus oaxacana Mirov. en todos los tratamientos evaluados, incluyendo el bosque sin manejo. Los resultados indican que el aprovechamiento forestal modifica la riqueza, diversidad y composición de los estratos arbóreo, arbustivo y herbáceo, siendo los dos últimos estratos los que más contribuyen a la diversidad. Por lo anterior resulta importante evaluar el sotobosque, ya que ayuda a dar una mejor explicación de la diversidad vegetal total del bosque.

Palabras clave: Tratamiento silvícola, vegetación del sotobosque, estructura del rodal, índices de biodiversidad.

INTRODUCTION

Species richness and diversity of each of the strata forming a forest under silvicultural management have become topics of interest, because of the need to know the most important components of the ecosystem from an ecological, economic, social and cultural perspective. The adoption of new paradigms on forest management at international level, promote the need to employ practices imitating natural succession dynamics and enhancing biodiversity, specially flora and fauna (Ares, Berryman, & Puettmann, 2009; Puettmann, 2011). Silvicultural treatments used to control species composition, growth and development of a forest are essentially simulations of natural disturbances (Fujimori, 2001; Smith, Larson, Kelty, & Ashton, 1997) modifying structure, composition, diversity and function of the stand (Ares et al., 2009; Fujimori, 2001; Oliver & Larson, 1996). Due to the above, assessing forest ecosystems through ecological indicators such as structure and diversity, focus on the analysis of the relationship among species in a population (Jiménez, Aguirre, & Kramer, 2001) and the effect of both natural and human-induced disturbances.

In Mexico studies have been performed reporting the response of tree species to silvicultural treatments, evaluating tree structure differences in time and space (Alanís-Rodríguez et al., 2013; Castellanos et al., 2008; Leyva-López, Velázquez-Martínez, & Ángeles-Pérez, 2010; Návar-Cháidez & González-Elizando, 2009; Solís et al., 2006), and studies documenting the response to a disturbance such as forest fires (Alanís-Rodríguez et al., 2010). However, studies addressing the response of understory silvicultural practices such as regeneration cutting and thinning are scarce, even knowing that the composition of tree structure can influence the diversity of understory (Barbier, Gosselin, & Balandier 2008) and that it can intervene in changing the environment for the establishment of natural regeneration (Kuehne & Puetmman, 2008). The application of thinning leads to high levels of biodiversity and richness of tree species under logging (Ares, Neill, & Puettmann, 2010; Berger, Puettmann & McKenna, 2012; Burton, Ares, Mulford, Deanna, & Puettmann, 2013).

Understory is a key component of forest ecosystems because it provides habitat for wildlife and contributes to nutrient cycling, resulting in the maintenance of productive capacity in managed forests (Ampooter, Beaten, Koricheva, Vanhellemont, & Verheyen, 2014; Berger & Puettmann, 2000; Davis & Puettmann, 2009). This study considers that the understory (shrubs and herbs) is an integral component of the structure of managed forest. The general hypothesis is that the composition and species richness and alpha and beta diversity indices in a managed ecosystem differ between silvicultural treatments and application time.

Therefore, the objectives of this study were 1) to obtain richness, species composition and diversity indices (alpha and beta) of the tree layer and understory in forests managed under different silvicultural treatments and application times and 2) determine whether there are differences in the variables analyzed in a managed forest (silviculture and application time), when compared with unmanaged forest.

MATERIALS AND METHODS

Location of the study area

The study area is located in the communal lands of Santa Catarina Ixtepeji, Oaxaca (Figure 1) between the geographical coordinates 17° 26' N and 96° 34' W, and average altitude of 1,920 m (Zacarías-Eslava & Del Castillo, 2010). The climate is temperate subhumid with summer rains and average annual temperatures of 17.6 °C. The type of soil is silty medium textured humic acrisol (Ah) (Zacarías-Eslava & Del Castillo, 2010; Vázquez & Givnish, 1998). Pine-oak and oak forest are the predominant vegetation; Pinus oaxacana Mirov., P. teocote Schiede ex Schltdl., P. leiophylla Schiede ex Schltdl. & Cham., Quercus crassifolia Humb. & Bonpl., Q. castanea Née and Q. rugosa Née are among the most common species (Zacarías-Eslava & Del Castillo, 2010).

Forest management in the community of Santa Catarina Ixtepeji began 45 years ago. During the first 25 years, management was under Mexican method of irregular forest management (MMOBI) with selective cutting, which influenced determinedly the composition and structure of forests. In the past 20 years, the system of conservation and forestry development (SICODESI) has been used for regular stands with a rotation period of 60 years (liberation cutting, pre-thinning, four thinning and regeneration cutting) and for irregular stands with cutting cycles of10 years and selective tree cutting as main method of regeneration. In both cases it is considered that a stand with regeneration of commercial species (predominating oak and other hardwoods) has not been generated, especially due to low intensity in the application of silvicultural treatments that have created favorable conditions for the species.

Sampling sites and measurement of variables

Cutting areas correspond to annualities 1998-1999 and 2010-2011. Stands with regeneration cutting with seed tree cutting, selective cutting and light thinning were chosen in these areas. Nearby areas with similar characteristics in composition, altitude, slope and aspect were selected in order to compare the effect of these treatments on structure, composition, richness and diversity of species in unmanaged stands. Table 1 reports the number of sites sampled by silvicultural treatment and annuality. A sampling intensity of 5 % for each annuality and silvicultural treatment was established. Sampling sites were chosen randomly from a grid of dots superimposed on an image of the work area. Nested circular sites were established. At the first place (400 m², 11.28 m radius) the total height of all trees with greater normal diameter of 2.5 cm was measured and the respective species was recorded. 200 m² (7.97 m radius) sites were established in a concentric manner and the coverage percentage of shrub species and their height were measured; finally, four sites (1 m²) were placed within such areas, these sites were located systematically at 7 m from the center of each site (400 m²) in the four free zones and in the direction of clockwise. At 1 m² sites, the coverage percentage of each herbaceous species was estimated and the frequency was recorded. All species not identified in the field were collected for subsequent taxonomic identification in the herbarium at the Colegio de Postgraduados.

Data analysis

In order to know if the sampling intensity per silvicultural treatment and annuality caught the total of richness present on the site, species-area curves were created using the program Species diversity and richness 4.1 (Seaby & Henderson, 2006). Analyses and comparisons were generated for managed and unmanaged forest (silvicultural treatment and annuality). Tree stratum, shrub and herbaceous were analyzed separately in each condition, considering the last two as main components of the understory. The floristic composition at the study sites was determined for each layer at family, genus and species level.

The importance value index (IVI) was calculated only for tree species based on density, coverage and relative frequency according to Magurran (2004). Simpson and Fisher's alpha indices, both considered robust, were calculated to define the alpha diversity (Moreno, 2001). The first is influenced by the most abundant species in the sample and is less sensitive to the richness of species; the second, besides not being influenced by the sample size, is less affected by the less abundant species, allows comparison between sites and different sample sizes as long as it has more than 1,000 observations. The beta diversity or index of similarity between communities was obtained with Jaccard and Sorensen indices using the program Species diversity and richness 4.1 (Seaby & Henderson, 2006). Table 2 shows alpha and beta diversity indices used in this study. Subsequently a nonparametric statistical analysis was performed with the Kruskal-Wallis test (Wheater & Cook, 2005), and a Wilcoxon rank sum under the null hypothesis of equal silvicultural treatments and life forms.

RESULTS AND DISCUSSION

Species richness and composition

Litsea glauscences Kunth is a species recorded in the NOM-059 (Secretaría del Medio Ambiente y Recursos Naturales [SEMARNAT], 2010), which had the highest density in unmanaged forests. Moreover, two exotic herbaceous species (Geranium rutundifolium L. and Acmella papposa [Hemsl.] R. K. Jansen) and two endemic herbaceous from the state of Oaxaca (Matudanthus nanus [M. Martens & Galeotti] D. R. Hunt and Schoenocaulon oaxacense [Frame] Zomlefer & Judd) were found in managed forests.

Pinus and Quercus had higher IVI. P. oaxacana had values of 34 % for unmanaged forests and 58 % for light thinning (2011), while Quercus sp. had10 % for unmanaged forests and up to 15.9 % for seed tree cutting (1998). This coincides with the information provided by Leyva-López et al. (2010), Alanís-Rodríguez et al. (2010) and Alanís-Rodríguez et al. (2013) who found that in temperate forests, after applying silvicultural treatments, the genera with higher IVI were Pinus sp. and Quercus sp. which represent the biggest ecological weight of the ecosystem. Louman, Quirós, and Nilsson (2001) found that, generally, there is smaller proportion of dominant species in stands with greater diversity and the permanence of the few species in the stand can be influenced by silvicultural practices.

Life forms

Figure 3 shows there is greater density from trees and shrubs, regardless of the silvicultural treatment and age of application; however, density is even higher in unmanaged forest. Changes in tree density and composition influence the increase or decrease of richness and abundance of understory species (Ampoorter et al., 2014; Ares et al., 2010). Diversity and greater vertical stratification when advancing natural succession in the absence of disturbances (Smith et al., 1997) is promoted in unmanaged forests. In areas with high conservation value and low tree diversity stand out the shrub species as the most representative (García, Tapias, Fernández, Vázquez, & Salvador, 2010).

Santa Catarina Ixtepeji reports that shrub species in the areas studied are dominant at lower altitudes around 2,000 m, due to weather conditions, while trees dominate when the altitudinal gradient increases (Zacarías-Eslava & Del Castillo, 2010). According to Alanís-Rodríguez et al. (2010), life forms change with respect to climate, altitude and type of ecosystem that is being evaluated.

In the study area is evident that the history of forest management with the application of different silvicultural treatments, not always with the right intensity, has led to mixed composition for the upper canopy (Leyva-López et al., 2010), and this in turn influences the presence of life forms of temperate forests (Ampoorter et al., 2014).

Alpha diversity

Simpson diversity indices and Fisher's alpha indices reported in Table 3 indicate that regardless of the silvicultural treatment and annuality, there is greater diversity in herbaceous plants and in some cases the shrub component exceeds the tree layer. This confirms the importance of high diversity represented by the understory in forest ecosystems and the significance of incorporating it when evaluating the diversity in managed forests, even if the contribution of the understory to the total biomass may not be as important (Ampoorter et al., 2014; Ares et al., 2010; Berger & Puetmman, 2000; Elliott & Knoepp, 2005).

Specially, unmanaged forest has the highest diversity index of herbs and a significant contribution from the bushes. Theoretically, greatest diversity of life forms and further stratification of the vertical structure is promoted, when there is no logging (Davis & Puettmann, 2009), this reflects, in many cases, a high diversity.

Beta diversity

The similarity of species between the evaluated treatments increases with time of application of silvicultural treatment. Table 4 shows the floristic similarity indices for silvicultural treatment and annuality. The highest values of Jaccard and Sorensen indices are presented between the selective cutting and seed tree cutting in 1998. As the forest develops, a larger number of species is seen due to the stand opening and to the both, biotic and abiotic conditions that are modified when silvicultural practices are applied (Alanís-Rodríguez et al., 2010; Ares et al., 2009; Berger & Puettmann, 2000).

The unmanaged forest had the lowest similarity values, indicating that a significant percentage of species is exclusive to this condition and, therefore, beta diversity increases, although the species richness is relatively similar (Table 4). Leyva-López et al. (2010) and Alanís-Rodríguez et al. (2013) reported the above and say the number of species decreases when silvicultural treatments are performed; however, the species richness of forest remains.

CONCLUSIONS

Alpha diversity indices were higher in herbaceous and shrubs for all silvicultural treatments and years of application. This indicates the importance of incorporating the understory in the evaluation of plant diversity in managed ecosystems. Beta diversity indices indicate that the similarity increases over the years in the silvicultural treatments evaluated; however, the unmanaged forest has low similarity values when compared with other silviculture treatments. The explanation of composition, richness and diversity of managed forest by including understory components is complex, since they are determined by multiple biotic and abiotic factors.

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