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Individual study: Effects of seed provenance and planting site on Polylepis australis seedling survival and growth in degraded woodland at Los Gigantes, Córdoba, Argentina

Published source details

Renison D., Cingolani A.M., Suarez R., Menoyo E., Coutsiers C. Sobral A. & Hensen I. (2005) The restoration of degraded mountain woodlands: effects of seed provenance and microsite characteristics on Polylepis australis Seedling survival and growth in central Argentina. Restoration Ecology, 13, 129-137

Summary

In the mountains of Córdoba, Argentina, Polylepis australis (known locally as queñoa or tabaquillo) woodlands are highly fragmented due to human activities, being subject to extensive burning and livestock grazing, which also hampers regeneration. In an attempt to achieve successful reforestation, P.australis seedling survival, growth and the development was compared for two seed provenances and different planting sites.

Study area: The study was undertaken in the Córdoba mountains of central Argentina (1,200–2,884 m) which support the southernmost stands of Polylepis australis woodland. P.australis grows in either a shrubby or tree habit.

Seed provenance and seedling production: Seeds were collected in January–February 1998 from two woodland areas: 40 trees in degraded patches in "Los Gigantes"; and 40 trees in woodland 30 km to the southeast, within Quebrada del Condorito National Park. Seeds were sown in May 1998, and planted out in an exclosure at the start of the summer (November–December) when 6–7 months old.

Planting and microsite characteristics: The reforestation area was in part of the Los Gigantes with a general southerly aspect. Livestock was excluded in September 1998 when a 10 ha exclosure was erected. However, about one-third of the soil in the exclosure had been lost due to erosion. After exclusion, short grasses and forbs began to be replaced by large tussocks, and some shrubby species. Natural regeneration of P.australis was poor, comprising a few seedlings already present but which before livestock exclusion were stunted due to browsing.

Using metal pins 655 microsites at least 3 m apart were marked. During November and December 1998 (the beginning of the growing season), seedlings were planted beside the pins in a random manner with respect to provenance, watered and initial height measured. Seedlings were not watered or weeded subsequently. Every winter (June–August 1999–2003) seedling survival was recorded and shoot height measured in a subsample of seedlings. In 2003 the number of basal stems and canopy width were also measured.

For all 655 seedlings, microsite characteristics (i.e. sun incidence, soil depth, substrate type, vegetation cover, bare soil, slope inclination and aspect) were measured in a 30 cm radius around the seedling.

Effects of microsite and provenance: Seedling survival over five seasons averaged 70%; survival was similar between seedling microsite and seed provenance. Mortality was highest in the first season (19%), then subsequently much lower (5, 4, 4, and 2%) in the next four seasons.

Planted seedlings averaged 3.9 cm (± 0.1) in height. Growth averaged 34.6 cm (± 1.2) over the five growing seasons (n = 339 seedlings surviving), and was fairly similar between seasons, ranging from 5.4 to a 7.9 cm. Total growth differed significantly both between seed provenance and microsite types. Seedlings from seeds originating from the National Park woodland grew on average 8 cm more than those from the degraded woodland. Growth was positively associated with increased vegetation cover and decreased with site degradation.

Shrubbiness indices (basal stems and canopy width) differed significantly between provenances and microsites. The number of stems was greatest for seedlings whose provenance was the National Park woodland.

Conclusions: Seedlings produced from seeds collected in the well-preserved woodland grew taller and showed a higher tendency for development of shrubby habit than those produced from seeds collected in a degraded woodland. Seedlings planted in more degraded microsites (exposed soil or rock due to past grazing pressure) grew less and developed a more shrubby habit than those planted in better preserved microsites. These results suggest that restoration of degraded P.australis woodland is possible where livestock access is restriced and restoration success may be enhanced by careful selection of seed provenance and planting areas.


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