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14 Establishment, Survival and Growth of Tree Seedlings Under Successional Montane Oak Forests in Chiapas, Mexico N. Ramírez-Marcial, A. Camacho-Cruz, M. González-Espinosa, and F. López-Barrera

14.1 Introduction Tropical montane forests are considered globally under threatened status due to their high rates of deforestation (Webster 1995; Brown and Kappelle 2001; Bubb et al. 2004; Kappelle 2004). The current land-use pattern in many montane forests in southern Mexico has created a variety of successional habitats where relatively high plant diversity levels persist. However, it appears that thresholds exist in the intensity and frequency of human disturbance to these habitats. Such thresholds impede successful establishment of seedlings of original tree species (Ramírez-Marcial 2003). Forest recovery under these circumstances may be limited by the availability of seeds, high seed and seedling predation, and competition (Holl et al. 2000; see Chap. 13). A gradual elimination of reproductive adults of understorey broad-leaved tree species has been documented in many of these successional habitats (Ramírez-Marcial et al. 2001; Quintana-Ascencio et al. 2004). Absence of reproductive adults of understorey trees results in scarce or null establishment of their seedlings in large, open, disturbed forest areas (Camacho-Cruz et al. 2000), suggesting that in the short term their regeneration may be seriously affected. Therefore, forest restoration and rehabilitation practices may be regarded as valuable options (Ramírez-Marcial et al. 2005). If forest recovery is seriously hindered by a limited availability of seeds, restoration could be achieved by artificial introduction of juveniles that can later accelerate natural regeneration. Information about tree growth, survival, and regeneration along successional and light gradients is urgently needed. Seedlings are considered a crucial life stage of tree regeneration, and their populations are highly dynamic, providing opportunities to gather meaningful data in relatively short time (Turner 2001). It is well known that seedling Ecological Studies, Vol. 185 M. Kappelle (Ed.) Ecology and Conservation of Neotropical Montane Oak Forests © Springer-Verlag Berlin Heidelberg 2006

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mortality tends to decrease with age, but may be determined by the species response to environmental conditions and disturbance regimes. Variation in the intensity, frequency, and spatial distribution of disturbance represents one of the main forces determining patterns of regeneration along successional gradients (Guariguata and Ostertag 2001), including factors such as drought (Poorter and Hayashida-Oliver 2000; Pearson et al. 2003), wind and fire (Bellingham et al. 1996), and biotic interactions such as competition or depredation (Osunkoya et al. 1992; Pearson et al. 2003). Yet, in less disturbed conditions, seedling survival and growth may be determined by different pressures and other physical constraints (Ramírez-Marcial 2003). In this chapter, we provide information on natural recruitment and establishment of native tree species in different seral stages of the montane pineoak forests of Chiapas. We include data on survival and growth (relative growth rates with respect to height and basal stem diameter) of seedlings and saplings of 54 native tree species planted under field conditions (successional habitats) and greenhouse controlled experiments (shade houses), collected over several years. Seedlings and saplings are grouped based on taxonomical and morphological traits that they share under similar environmental conditions, resulting in four ecological groups: conifers, oaks, shade-intolerant, and shade-tolerant broad-leaved trees. Determining whether or not different species may have similar performance in contrasting environments (successional habitats) is important to build a model on possible mechanisms for their coexistence, with implications for conservation and restoration of the Neotropical montane forest (McDonald et al. 2003).

14.2 Montane Pine-Oak Forest in Chiapas Montane pine-oak forest (MPOF) is included within the Tropical Montane Rain Forest (Breedlove 1981) or Tropical Montane Cloud Forest (Hamilton et al. 1995; Kappelle 2004). In Chiapas, it includes several plant associations such as the pine-oak forest, pine-oak-Liquidambar forest, and oak forest (see Chap. 16). Typical landscapes in the highlands of Chiapas include sloping lands between valleys and a karstic plateau; forested patches persist but are frequently poor in diversity of tree species (Ochoa-Gaona et al. 2004). Many areas of MPOF are secondary growth stands, surrounded by agricultural fields, grazing lands, and more recently, some dispersed human settlements (Ochoa-Gaona and González-Espinosa 2000). The high population growth in the highlands during the second half of the last century is considered to indicate that the region is being devastated by a rapid and irreversible process of deforestation. In the MPOF in Chiapas, tree richness has been estimated in 170–190 species, including 35–40 canopy and understorey trees (González-Espinosa et

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al. 2005). Quercus and Pinus represent two coexisting and ecologically distinct genera of canopy trees, and changes in their dominance may promote significant changes in the understorey composition of shade-intolerant and shadetolerant broad-leaved trees. Richness and density of naturally established understorey tree seedlings may decrease strongly when pines dominate the forest canopy, in contrast to the higher diversity observed in oak-dominated stands (González-Espinosa et al. 1991; Ramírez-Marcial et al. 2001; GalindoJaimes et al. 2002).

14.3 Ecological Niche and Performance of Seedlings The ecological niche generally describes the integrated tolerances and requirements of an organism in the habitats in which it lives, grows, and reproduces (Townsend et al. 2003). Although the niche has been considered a concept, rather than a site as such, it is very useful when attempting to explain the performance of many tree species along ecological gradients (Grubb 1977; Prinzing et al. 2002; Pearson et al. 2003). Performance of tree seedlings in tropical forests has been commonly evaluated using light gradients within gap-phase dynamics (Augspurger 1984; Turner 1990; Dalling et al. 1999). Species differences in performance along light gradients contribute to the maintenance of forest species diversity (Wright 2002). The spatial patterns of light availability within forest stands are likely to influence stand level regeneration patterns of woody species. In lowland tropical forests, natural disturbances such as treefall gaps are considered fine-scale disturbances necessary for the regeneration of canopy species (Denslow 1987; Popma and Bongers 1988). Increment in light levels at the forest floor after gap formation may increase the probability of establishment of some pioneer trees (Turner 1990; Davies 2001). However, gap partitioning among pioneer tree species arises directly from morphological and biochemical specialization to particular light gap environments, and in turn may result from a trade-off between seedling growth and survival (Kitajima 1994; Dalling et al. 1999; Kobe 1999; Koslowski and Pallardy 2002).

14.4 Survival and Growth of Tree Seedlings 14.4.1 Naturally Established Seedlings Natural establishment of tree seedlings in some MPOF in Chiapas may be highly variable in species richness (10–40 species) and seedling density

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(0.8–6.4 ind m–2; González-Espinosa et al. 1991; Camacho-Cruz et al. 2000; Galindo-Jaimes et al. 2002). By contrast, in pine-oak-Liquidambar forests (POLF) under human disturbance, seedling species richness may decrease from 26 to four tree species, and density varies from 0.5 to 2.6 seedlings m–2; in highly disturbed forests, only seedlings of pines and oaks have been recorded. After 3 years of evaluation of an old-growth Quercus-Podocarpus forest, survival of naturally established seedlings decreased to 40 %, whereas in POLF and pine forest this proportion was 64 % and 67 %, respectively (Ramírez-Marcial 2003).

14.4.2 Transplanted Seedlings All study sites were located in the municipalities of San Cristóbal de Las Casas, Huistán, and Pueblo Nuevo Solistahuacán, covering an area of 3,220 km2 in the central and northern highland regions of Chiapas. Climate is temperate sub-humid with summer rains. Site characteristics include a wide range of elevations (1,720–2,500 m) on relatively poor soils and steep slopes (Table 14.1). There is a different number of successional habitats at each study site, due to variations in area. We used the following four contrasting successional habitats: open areas (OA, mostly grasslands), shrublands (SHR), earlysuccessional forests (ESF), and mid-successional forests (MSF; Table 14.1). In many of these habitats, a variable number of seedlings and saplings (25–300) of different species (4–25) were planted, with time reaching a total of 40 species, corresponding to 7,183 individuals (Table 14.1).

14.4.3 Greenhouse Experiment We conducted a greenhouse experiment using 33–285 seedlings of 42 native tree species. Only 27 species used under greenhouse conditions had at least one replicated habitat under field conditions (Table 14.2). All plants were produced in a nursery (Ramírez-Marcial et al. 2003). Black houses were used to create four shade treatments, i.e., 0, 50, 75, and 90 % shade, representing mean levels of light recorded in previous studies in the successional habitats OA, SHR, ESF, and MSF, respectively (Quintana-Ascencio et al. 1992, 2004; Ramírez-Marcial et al. 1996). Seedlings of each species (total of 3,881 individuals) were randomly placed in black houses in order to have four replicates for each shade treatment (except in the non-shaded treatment, where only three replicates were used). Seedlings were homogeneously watered in order to avoid desiccation, and periodically rotated. Survival and relative growth rates were evaluated after one year.

2,200 2,250 2,300 2,380 1,720

2,500 2,130 2,400 2,120

S1 (600) S2 (5,000) S3 (3,200) S4 (1,600) S5 (7,200)

S6 (1,200) S7 (3,200) S8 (400) GH (800)

1,300 1,200 1,400 1,200

1,200 1,200 1,200 1,100 1,700

Mean annual rainfall (mm)

13 13 14 16

13 13 13 15 15

Mean temperature (°C)

Steep Steep Flat–steep Flat

Flat Flat–steep Flat–steep Flat Steep

Slope

Luvisol-rendzines Luvisol-rendzines Luvisol-rendzines Cambisol Luvisol, lithosols and rendzines Cambisol Lithosols Luvisol-rendzines Forest soil mixture

Soil type

OA, ESF, MSF OA ESF, MSF 0, 1, 2, 3

ESF, MSF ESF MSF OA, SHR OA

Successional habitats

7 25 9 42

9 11 5 4 16

Number of species

96 34 48 12

48 48 27 12 34

Study period (months)

– 164 108 214 486

Conifers (n=6) Oaks (n=11) Shade-intolerant broad-leaved trees (n=18) Shade-tolerant broad-leaved trees (n=19) Total (n=54) – 192 270 339 801

Study sites S1 S2

Species groups (with n species) – 1,470 – – 1,470

S3

640 640 – – 1,280

S4

379 25 245 328 977

S5

398 – 83 246 727

S6

94 140 493 393 1,120

S7

– 107 72 143 322

S8 720 1,130 846 1,185 3,881

GH

2,231 3,868 2,117 2,848 11,064

Total

Table 14.2. Initial number of seedlings of 54 native tree species within four ecological groups used in experiments conducted in eight different successional habitats and in greenhouse controlled conditions (GH) in the northern and central highlands of Chiapas, Mexico (see Table 14.1 for details on study sites)

Mean elevation (m)

Study site (area in m2)

Table 14.1. Description of the study sites. Successional habitats: OA open area, SHR shrubland, ESF early-successional forest, MSF mid-successional forest. Light availability under greenhouse (GH) conditions was simulated by shade treatment: 0 non-shaded, 1 50 %, 2 75 %, and 3 90 %

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14.4.4 Species Grouping An a priori classification of species (including 40 in successional habitats, and 42 under greenhouse conditions) resulted in the following four groups (Table 14.2): conifers (four Pinus spp. in addition to Abies guatemalensis and Podocarpus matudai), oaks (11 Quercus species), shade-intolerant broadleaved trees (18 species), and shade-tolerant broad-leaved trees (19 species). The latter two groups are defined on basis of structural data reported in González-Espinosa et al. (1991, 2005; Chap. 16). This classification is useful to understand the effects of light variation associated with canopy cover along successional gradients in MPOF in Chiapas. Since our design is evidently nonbalanced, having unequal numbers of species, different seedling ages, and different periods of evaluation, we present an exploratory analysis considering the overall response of the four groups of species, rather than individual species responses in each successional habitat or shade treatment. Mean seedling survival, and relative growth rates of height and basal stem diameter were obtained for all individual species, and these data then combined for each of the four ecological groups.

14.4.5 Natural vs. Greenhouse Survival Under field conditions, the survival of seedlings of the four ecological groups showed high variation among successional habitats (Fig. 14.1). The conifer group had highest survival in the SHR (no plants of broad-leaved trees were available for this habitat), followed by the OA, and ESF habitats (50 and 67 %, respectively). Only 24 % of the conifer species planted in the MSF survived after 96 months (Quintana-Ascencio et al. 2004). As a group, oaks presented the highest survival (67–84 %) across all successional habitats. Both groups of broad-leaved trees, shade-intolerant and shade-tolerant species, showed higher survival under ESF and MSF (42–50 and 70–72 %, respectively). Less than 32 % of the shade-intolerant species, and less than 13 % of the shade-tolerant species survived in OA habitats (Fig. 14.1). By contrast, the mean survival of seedlings under greenhouse conditions showed different trends. In general, after a year of evaluation, the survival of the four groups of species was higher with increasing shade density (Fig. 14.1). In the four shade treatments (0, 50, 75, and 90 %), mean survival was 64, 93, 98, and 93 % (conifers), 82, 95, 96, and 95 % (oaks), 50, 68, 83, and 84 % (shade-intolerant broad-leaved trees), and 51, 86, 89, and 96 % (shade-tolerant broad-leaved trees), respectively.

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a) Field Conifers

Oaks

Intolerant

Tolerant

Survival (%)

100 75 50 25 0

N=

5

2

OA

SHR

3

3

ESF MSF

4

5

OA

SHR

5

6

ESF MSF

8

OA

0

SHR

10

4

ESF MSF

13

OA

0

SHR

12

8

ESF MSF

Successional habitats

b) Greenhouse Conifers

Oaks

Intolerant

Tolerant

Survival (%)

100 75 50 25 0

N=

3

3

3

3

Non-shade 50% 75% 90%

11

11

11

11

Non-shade 50% 75% 90%

15

15

15

14

Non-shade 50% 75% 90%

13

13

13

13

Non-shade 50% 75% 90%

Shade treatment

Fig. 14.1a, b. Box plot (median indicated by open circle+upper and lower quartiles; crosses are extreme values) for seedling survival in four ecological groups: conifers, oaks, and shade-intolerant and shade-tolerant broad-leaved trees under field (a), and greenhouse (b) conditions. Successional habitats are: OA open areas, SHR shrubland, ESF early-successional forest, MSF mid-successional forest. Shade treatments under greenhouse conditions roughly represent mean values of canopy cover of each corresponding successional habitat. N Number of species included within each group

14.4.6 Relative Growth Rates Seedlings of the four ecological groups seem to grow similarly in the four successional habitats. Conifers showed the highest growth in OA and SHR habitats; their lowest value was recorded in MSF. Oaks and the shade-tolerant broad-leaved seedlings grew more in the ESF, whereas the shade-intolerant broad-leaved seedlings did not show any trend; still, the variation within this group was less pronounced in the MSF (Fig. 14.2). Under greenhouse conditions, seedlings of all ecological groups increased in height under intermedi-

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N. Ramírez-Marcial et al. a) Field Conifers

Oaks

Intolerant

Tolerant

0.71 0.55 0.38 0.21 0.05 N=

4

2

OA

SHR

3

3

ESF MSF

3

5

OA

SHR

5

6

ESF MSF

7

OA

0

SHR

9

4

ESF MSF

10

0

OA

SHR

8

8

ESF MSF

Successional habitats

b) Greenhouse Conifers

Oaks

Intolerant

Tolerant

3.04 2.14 1.24 0.34 -0.55

N=

3

3

3

3

Non-shade 50% 75% 90%

11

11

11

11

Non-shade 50% 75% 90%

9

14

15

13

Non-shade 50% 75% 90%

12

12

13

13

Non-shade 50% 75% 90%

Shade treatment

Fig. 14. 2a, b. Box plot for the relative growth rate of stem height (RGRheight, cm cm–1 year–1) of tree seedlings of four ecological groups (conifers, oaks, shade-intolerant broad-leaved trees, and shade-tolerant broad-leaved trees) under field (a) and greenhouse (b) conditions. Successional habitats and shade treatments as in Fig. 14.1

ate levels of shade (50 and 75 % shade, Fig. 14.2). Oaks and conifers showed an increase in basal diameter in the OA and SHR habitats, the latter being where conifers had their greatest increase in basal diameter (Fig. 14.3). Under greenhouse conditions, the diameter growth rate of conifers and oaks was relatively low and similar in all shade treatments, whereas shade-intolerant and shadetolerant broad-leaved trees increased their diameter under intermediate shade (50 %; Fig. 14.3). Among the four ecological groups evaluated in the greenhouse, conifers presented the lowest diameter increases – probably because the seedlings were smaller than those of the other groups. It is well known that seedlings of conifers grow less efficiently than those of angiosperms, at least during the initial stages of growth (Bond 1989).

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a) Field Conifers

Oaks

Intolerant

Tolerant

1.051 0.790 0.529 0.269 0.008

N=

4

2

OA

SHR

3

3

ESF MSF

3

5

OA

SHR

5

6

ESF MSF

7

0

OA

SHR

9

4

ESF MSF

10

0

OA

SHR

9

8

ESF MSF

Successional habitats

b) Greenhouse Conifers

Oaks

Intolerant

Tolerant

2.244 1.624 1.004 0.384 -0.237

N=

3

3

3

3

Non-shade 50% 75% 90%

11

11

11

11

Non-shade 50% 75% 90%

9

14

15

13

Non-shade 50% 75% 90%

12

12

13

13

Non-shade 50% 75% 90%

Shade treatment

Fig. 14.3a, b. Box plot for the relative growth rate of basal stem diameter (RGRdiameter, cm cm–1 year–1) of tree seedlings of four ecological groups (conifers, oaks, shade-intolerant broad-leaved trees, and shade-tolerant broad-leaved trees) under field (a) and greenhouse (b) conditions. Successional habitats and shade treatments as in Fig. 14.1

14.5 Conservation and Restoration Implications Many of the ecological attributes of a community that are lost under the impacts of natural perturbations can eventually be recovered after disturbance (Brown and Lugo 1994). However, due to the high intensity and frequency of human disturbances in the MPOF of Chiapas, natural recovery can be slow. The cumulative effect of this type of disturbance tends to favor the regeneration of some pioneer species such as pines, oaks, Alnus acuminata, Arbutus xalapensis, Buddleja cordata, and Prunus serotina, which showed good performance in open areas or shrublands. However, shade-tolerant species such as Clethra pachecoana, Cleyera theaeoides, Oreopanax xalapensis, Prunus rhamnoides, Psychotria galeottiana, Styrax magnus, Symplocos

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limoncillo, and Zanthoxylum melanostictum had poor development in these open habitats. Diurnal patterns of solar radiation, temperature, and relative humidity display more extreme variations in open pine forests than in closedcanopy old-growth oak forests. Summer temperatures at forest-floor level can rise to 45 °C, and winter temperature decreases may cause frost. Such extreme temperature fluctuations, and additional water deficits occurring during the dry season represent a set of major constraints that reduce seedling performance (Ramírez-Marcial 2003; Ramírez-Marcial et al. 2005). Restoration of native plant communities requires identifying those factors that favor or impede natural colonization by native tree species (Denslow 1996). In the study region, successful colonization of native trees in early seral stages is limited not only by physical conditions such as light intensity and related temperature (e.g., McCormick 1996), but also by arrival of the propagules. Therefore, a viable strategy for MPOF recovery in Chiapas seems to be through the reintroduction of seedlings in appropriate successional habitats. From a biological viewpoint, there could be two routes for forest restoration: (1) the suppression of disturbance in forested areas, which would promote natural regeneration, and (2) the establishment of enrichment planting in poor-species forested areas. Based on this and previous evidence, we consider feasible the reintroduction of pines and oaks, and some other shade-intolerant broad-leaved species in open and shrubland habitats, and of seedlings of shade-tolerant broad-leaved species in pinelands or other successional forests with intermediate light levels under the canopies.

14.6 Conclusions Changes in floristic composition and forest structure in the study region have promoted the gradual elimination of reproductive individuals of many broadleaved species. As a consequence, only low numbers of naturally recruited seedlings of these species are observed in the remaining secondary forests, which may lead to their local extinction. Our classification of plant functional types (sensu Denslow 1996) based on a combination of taxonomical and ecological traits will help us to understand the interaction between physical factors (mostly light conditions) and successional gradients in MPOF in Chiapas. Grouping of species is useful to identify which species show similar responses to specific environmental conditions. Comprehensive management of montane forests in Chiapas requires recognition of yet wider and more detailed sets of the species’ ecological attributes, in addition to social, economic, and cultural issues involved in local restoration programs.

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Acknowledgements We thank Liliana Mascarúa-López, Magala Alcázar-Gómez, Alfonso Luna-Gómez, Pedro Girón-Hernández, and Miguel Martínez-Icó for their assistance in the field, greenhouse, and nursery. Duncan Golicher, Pedro F. Quintana-Ascencio, and Luis Galindo-Jaimes provided useful comments that improved the manuscript. This study was supported by the European Commission (BIOCORES Project, INCO Framework Programme 5, contract no. ICA4-CT-2001-10095), CONACYT (grant no. 020395 to NRM), COCYTECH (Project FOMIX-CHIS-2002-C01-4640), SEMARNAT (Project 2002-01-C01-048) and federal subsidies to ECOSUR.

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