Individual study: The effect of grazer exclusion on dwarf pine Pinus culminicola seedling survival and growth at Cerro El Potosí protected natural area, Nuevo León, Mexico
Jiménez J., Jurado E., Aguirre O. & Estrada E. (2005) Effect of grazing on restoration of endemic dwarf Pine (Pinus culminicola Andresen et Beaman) populations in northeastern Mexico. Restoration Ecology, 13, 103-107
Dwarf pine Pinus culminicola is endemic to northeastern Mexico where it grows only on four high peaks in the Sierra Madre Oriental. It is under pressure from grazing, wildfires and human activities such as mining and road development. An experiment was established to examine the effect of exclusion of cattle, small mammals, and planting altitude on the success of dwarf pine seedling establishment.
Study area: The study site is located around the summit of the Cerro El Potosí protected natural area in the Sierra Madre Oriental range, Nuevo León, Mexico. The mountain top is exposed to high winds for most of the year. Snow may fall several times during winter, persisting for a few days on the north and northeast slopes. Currently, only 30 ha of fragmented exists at Cerro El Potosí, and these include many old trees with low seed production which are subject to cattle grazing.
Experimental design: Dwarf pine planting plots were established in December 1997 at three elevations/ vegetation types within the natural distribution range of the species:
1) high elevation (3,500 m) Pinus culminicola matorral
2) intermediate elevation (3,400 m) P.culminicola–P.hartwegii forest
3) lower elevation (3,300 m) P.hartwegii–P.culminicola forest
At each elevation three square 625 m² plots were established for planting 2-year-old dwarf pine nursery-grown seedlings. There were three treatments: 1) small mammal plus cattle exclusion (chicken wire exclosure); 2) cattle exclusion (barbed wire exclosure); and 3) no exclosure. For each treatment at each elevation, 110 seedlings were planted (equivalent to a density of approximately 2 seedlings/10 m². At the time of planting, seedlings were approximately 10 cm tall and 0.5 cm in stem diameter.
Monitoring: All planted seedlings were labeled so they could be distinguished from any naturally occurring seedlings. Seedlings were monitored annually from November 1998 to November 2001. Plant survival was determined and stem diameter and total height were measured.
Seedling survival: Dwarf pine seedling mortality was high in all treatments, with only about 50% of seedlings surviving 4 years after planting in the plots which excluded cattle and small mammals. Survival was much lower where no exclosure was used with non surviving beyond the third year after planting, these seedlings succumbing to grazing and trampling. Where small mammals were not excluded, seedlings were found to be nibbled and occasionally had broken stems. Similar damage was also present, rarely, in plots excluding cattle and small mammals. This was attributed to parrots (common in the area) or small mammals that may have climbed over or burrowed under the chicken wire fence.
In general, elevation did not account for differences in seedling survival with patterns found for exclosures consistent at all elevations. However, in 1999 seedling mortality was consistently greater at the highest elevation where the surrounding vegetation, perhaps as a result of a very harsh winter.
Seedling growth: Average stem diameter for all seedlings in 1998 was 5.09 mm, increasing slightly in 1999 (5.15 mm) and 2000 (5.66 mm). By 2001 stem diameter had increased to 8.07 mm. There were no differences in stem diameter attributable to elevation or access to cattle and small mammals. Seedlings doubled their height between 1997 and 2001. The final average seedling height was 12 cm, but after 4 years this was considered poor. There was a tendency for seedlings to be taller at the lowest elevation (3,300 m).
Conclusions: Dwarf pine seedling survival was approximately 50% in after 4 years for seedlings protected from cattle and small mammals, whereas no unprotected seedlings survived at all. Mortality was similar at all sites in spite of differences in environments and plant communities present at different altitudes. After 4 years, surviving seedlings were still very small and thus susceptible to trampling by cattle and grazing by both cattle and small mammals. Elevation in general did not account for variation in survival.
As there were no naturally occurring seedlings found, it appears that replanting will be necessary to ensure the survival of dwarf pine at Cerro El Potosí . In addition to protection of natural populations of dwarf pine from wildfires, reforestation programmes ensuring protection for seedlings from cattle, and perhaps also parrots and rodents, should be implemented.
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