Experimental native tree seedling establishment for the restoration of a Mexican cloud forest

Summary

In Mexico, most remaining montane cloud forests have been highly fragmented and degraded as the result of human activities. Restoration has been proposed for several areas but progress has been constrained by a lack of information on how best to establish seedlings of native tree species. An experiment was therefore conducted in central east Mexico to assess the influence of different environmental factors on seedling survival and growth of four native cloud forest tree species (Fagus grandifolia var. mexicana, Carpinus caroliniana, Symplocos coccinea, and Quercus acutifolia).

Study area: The  trials were undertaken in three tropical montane cloud forest (bosque mesófilo de montaña) fragments (1,340-1,500 m altitude) in the Sierra Madre Oriental, Veracruz, Mexico (Rancho Viejo, Xolostla, and Las Cañadas). All three fragments are crossed by paths used by local people, who occasionally cut trees or gather fuelwood. The three sites had similar climatic conditions and soil characteristics.

Seedlings production: Four species were selected, Fagus grandifolia (güichin, acailite), Carpinus caroliniana (pepinque), Symplocos coccinea (limoncillo) and Quercus acutifolia (encino). Seeds were germinated on moist filter paper in the laboratory. All seedlings were transplanted into black plastic bags (12 × 20 cm) filled with a soil potting compost mix. Seedlings were raised in a greenhouse (at Botanical Garden of the Instituto de Ecología, Xalapa) for 6 months before transplanting. Average seedling height at transplanting was 10 cm for Fagus, 80 cm for Quercus, and 20–25 cm for Carpinus and Symplocos.

Seedling establishment: At each site, seedlings were transplanted into eight 10 × 16 m plots. Four were positioned randomly within the forest in a 1 ha area at least 30 m from the forest edge, the remaining four plots were located outside the forest 5 m from its border. Forty seedlings (10 of each species) were transplanted into each plot spaced 2 m apart. All seedlings (960 seedlings, 320 per site) were numbered with metal tags.

Seedling growth: Seedling height, stem basal diameter, and number of leaves were assessed for each seedling 1 month after transplanting. Seedling survival, cause of mortality (e.g., vertebrate or invertebrate predation, and desiccation), height, diameter, and number of leaves were subsequently monitored every 3 months for a period of 14 months. Diameter was measured with a digital caliper at the stem base and stem height was measured with a ruler, and all the leaves in each plant were counted. Survivorship was estimated as the proportion of seedlings surviving at each time interval.

Microclimate: Microclimatic data were measured (one day each month at 12:00 h) at five random sampling points in each 1-ha area of forest, and at five outside the forest in the plots. The variables measured were air and soil temperature, relative humidity, gravimetric soil water content, canopy openness and photosynthetically active radiation (PAR).

Seedling survival: Highly significant differences were recorded in seedling survival and growth among sites, environments and species. Quercus survival was highest, which uniquely among the four species had similar survival inside (91%) and outside (92%) the forest (see Table 1, attached). Survival of the other species was higher inside the forest: Carpinus (78% in; 34% out); Fagus (70in; 29 out); and Symplocos (53% in; 28% out).

The most important cause of mortality outside the forest was desiccation, although significant seedling grazing damage by cows was observed In Rancho Viejo, before the fences were installed. In Las Cañadas, crickets (Orthoptera) caused high mortality outside the forest in Carpinus, Fagus, and Symplocos. Survival of all species was relatively lower in Las Cañadas, particularly outside the forest, even for Quercus (83% outside the Las Cañadas forest fragment vs. 95–98% in the other sites).

Seedling growth: In contrast to survival, growth rates of all four species were higher outside the forest. Growth varied considerably among sites, treatments and species. At Xolostla, inside and outside the forest, all the species grew most rapidly and produced more leaves.

Seedlings growing outside the forest grew faster and produced more leaves than those growing inside the forest. Average growth in height was: Quercus (0.17 inside; 0.40 cm cm/yr outside the forest); Symplocos (0.44 inside; 0.48 cm/yr outside (excluding Xolostla where Symplocos displayed its highest growth rate). Carpinus (0.41; 1.50 cm/yr) and Fagus (0.69; 0.75 cm/yr) grew more rapidly outside than inside the forest. After 14 months, Quercus (1.51 m) and Carpinus (1.26 m), reached the highest average heights, whereas Symplocos and Fagus reached only 0.35 cm (except at Xolostla outside the forest, where Symplocos reached 1 m). It should be remembered that when transplanted, the size of Quercus seedlings was larger than that of the other species.

Leaf production was mostly higher outside the forest for all of the species, and was influenced by the site. Quercus and Carpinus had the highest leaf production (more than 200 leaves per seedling outside the forest) whereas Symplocos and Fagus produced no more than 50 leaves per individual, except at Xolostla (outside the forest), where Symplocos produced more than 300 leaves. The exceptions were Quercus in Rancho Viejo where seedlings grew less rapidly outside the canopy because of cattle grazing at the beginning of the experiment, and Symplocos and Fagus in Las Cañadas, where seedlings grew more rapidly and produced more leaves in the forest.

Conclusions: Results indicate that all four species can be established successfully both within forest fragments and in adjoining agricultural areas. Species–site matching may be required if optimum rates of growth and survival are to be obtained.


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