**3.4 Diversity patterns**

The greates richnness and diversity were found in the intermediate successional stage (30-35 years growth). This stage presented species that are characteristic both of the mature forest and the early growth (Table 4). The nature of this intermediate state is supported by statistical differences in diversity between the early growth and mature forest (Δ = - 0.21 p = 0.01), while there were no differences between early growth and old growth (Δ = -0.56 p > 0.05), nor between old growth and mature forest (Δ = 0.35 p > 0.05).


Table 4. Number of plots, total sampled area (ha), plant density and estimated values for species richness(S), Shannon-Wiener diversity index (H'), Simpson's diversity index (D) and Fisher's alpha (α) for each successional stage.

In all three successional stages there was a high abundance of plants in early development stages and a low abundance of bigger sizes, so, according to Peter (1996), the class of structure found in the system is Type I (Figure 3). In all three successional stages, short size individuals are predominant, while in the early successional stage, no individual reaches a DBH of 200 cm.

Similarity analysis results showed that mature forest has a higher similarity with the old growth (26%). Figure 4 shows dominant species of each successional stage and shared species between habitats. Eight species were found in all three stages, while mature forest presented the highest number of exclusive species.

Structure and Floristic Composition in

**Forest stage Country Age** 

**Tropical subalpine forest** 

**Tropical upper montane forest** 

**Tropical montane cloud forest** 

**Tropical lower montane forest** 

forests in the Neotropics.

2006).

a Successional Gradient in a Cloud Forest in Chiapas, Southern Mexico 143

In contrast with Kappelle (1996) who found the highest richness and diversity in early successional stages, our results indicate that intermediate stage (old growth) has the highest richness and diversity. Nowadays, the old idea that the highest diversity is to be found in mature undisturbed systems has been challenged. The intermediate disturbance hypothesis, originally presented by Connell (1978) states that at intermediate levels of disturbance, diversity is maximized because both competitive and opportunistic species can coexist. The highest diversity at intermediate levels of disturbance that we found may support this notion. It is probable that our intermediate successional stage has spatial and temporal heterogeneity within the system, favoring the coexistence of numerous species. Denslow (1980) suggests that alpha diversity in successional stages in tropical forests is due to the high number of plantules of disturbance adapted species. According to Bazzaz (1975) the high species diversity in secondary forests may be explained by the high degree of

horizontal and vertical micro-environmental complexity in early successional stages.

Williams-Linera (1991) found for mature Cloud Forest IVI values very similar to what we report in this study for *Matudaea trinervia, Quercus peduncularis,* and *Hedyosmum mexicanum* in a study done in the same area. Old growth had a 20% similarity with early growth, which suggests a high rate of species substitution. Availability, dispersion and germination of these species may play a key role in forest recovery and restauration in early phases after a disturbance (Guariguata & Kattan, 2002; Ten Hoopen & Kappelle, 2006; Wilms & Kappelle,

We may consider our system to be healthy because all class structures in all habitats had a DBH between 0-20 cm in more than 40% of individuals. This, according to Peter (1996)

Finally, we compared our results with several other studies on successional gradients elsewhere in the neotropics (table 5). Some Costa Rica mature forest has twice the number of species than El Triunfo. However, secondary forest in Costa Rica showed similar numbers to

> **Average altitude**

**Species richness**  **Shannon** 

**Index Reference** 

Jørgensen, 1993

suggests that these structures are more stable and are considered healthy.

**Plot size (ha)** 

Primary Peru Mature 0.10 3400 9 - Young, 1992

Primary Ecuador Mature 1 3280 32 - Valencia &

Primary Mexico Mature 1 1920 54 3.13 This study Secundary Mexico 30-35 0.7 1860 56 3.48 This study Secundary Mexico 20-25 1 1820 43 2.92 This study

Primary Costa Rica Mature 0.18 1050 105 4.06 Kuzee et al, 1994 Secundary Costa Rica 11 0.19 1030 54 2.84 Kuzee et al, 1994 Secundary Costa Rica 35 0.10 950 69 3.51 Kuzee et al, 1994 Table 5. Comparison of species richness and diversity between some primary and secondary

Primary Colombia Mature 0.80 2850 33 - Bazuin et al 1993 Primary Costa Rica Mature 0.1 2975 20 3.31 Kappelle et al 1996 Secundary Costa Rica 15 0.10 2975 21 3.39 Kappelle et al 1996 Secundary Costa Rica 30 0.10 2975 20 3.14 Kappelle et al 1996

**(years)** 

Fig. 3. Diametric class structure in three successional stages in El Triunfo, Chiapas, Mexico. Black columns denote Mature Forest, gray columns Old Growth and blank columns Early Growth.

Fig. 4. Unique and shared species for each successional stage and their transitional phases between them. A=Mature forest, B=Old Growth, C=Early growth.

Fig. 3. Diametric class structure in three successional stages in El Triunfo, Chiapas, Mexico. Black columns denote Mature Forest, gray columns Old Growth and blank columns Early Growth.

0-20 21-40 41-60 61-80 81-100 >101

**DBH (cm)**

**Frequency**

Fig. 4. Unique and shared species for each successional stage and their transitional phases

between them. A=Mature forest, B=Old Growth, C=Early growth.

In contrast with Kappelle (1996) who found the highest richness and diversity in early successional stages, our results indicate that intermediate stage (old growth) has the highest richness and diversity. Nowadays, the old idea that the highest diversity is to be found in mature undisturbed systems has been challenged. The intermediate disturbance hypothesis, originally presented by Connell (1978) states that at intermediate levels of disturbance, diversity is maximized because both competitive and opportunistic species can coexist. The highest diversity at intermediate levels of disturbance that we found may support this notion. It is probable that our intermediate successional stage has spatial and temporal heterogeneity within the system, favoring the coexistence of numerous species. Denslow (1980) suggests that alpha diversity in successional stages in tropical forests is due to the high number of plantules of disturbance adapted species. According to Bazzaz (1975) the high species diversity in secondary forests may be explained by the high degree of horizontal and vertical micro-environmental complexity in early successional stages.

Williams-Linera (1991) found for mature Cloud Forest IVI values very similar to what we report in this study for *Matudaea trinervia, Quercus peduncularis,* and *Hedyosmum mexicanum* in a study done in the same area. Old growth had a 20% similarity with early growth, which suggests a high rate of species substitution. Availability, dispersion and germination of these species may play a key role in forest recovery and restauration in early phases after a disturbance (Guariguata & Kattan, 2002; Ten Hoopen & Kappelle, 2006; Wilms & Kappelle, 2006).

We may consider our system to be healthy because all class structures in all habitats had a DBH between 0-20 cm in more than 40% of individuals. This, according to Peter (1996) suggests that these structures are more stable and are considered healthy.

Finally, we compared our results with several other studies on successional gradients elsewhere in the neotropics (table 5). Some Costa Rica mature forest has twice the number of species than El Triunfo. However, secondary forest in Costa Rica showed similar numbers to


Table 5. Comparison of species richness and diversity between some primary and secondary forests in the Neotropics.

Structure and Floristic Composition in

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what we found in El Triunfo. These results suggest that secondary forest at El Triunfo could be more diverse than primary forest.

The patterns on species diversity and species replacement along a successional gradient we obtained from this study would be of great help to design sound strategies for Cloud Forest restoration. This is very important since little is known on Cloud Forest dynamics and because this habitat is considered one of the most endangered all over the world.
