Do we need livestock grazing to promote Polylepis australis tree recruitment in the Central Argentinean Mountains?

Abstract

South American Polylepis mountain forests are endangered due to centuries of logging, fire clearance, and intensive livestock grazing. Polylepis australis (Rosaceae) dominates the tree canopy in the Central Argentinean Mountains, where livestock have been excluded or reduced to very low densities over wide areas. Lowered plant and bird diversity in these abandoned grazing areas suggest that some livestock grazing may be beneficial for conservation purposes. Our study aimed to determine whether this may also be true for forest regeneration. In 300 plots of 1 m 2 distributed in subregions of high to moderate grazing pressure as well as grazing exclusion, we recorded the presence of P. australis seedlings and grazing indicators. Topographical parameters as well as vegetation and soil characteristics were also assessed. Seedling frequencies were highest in the region with moderate grazing, intermediate with grazing exclusion, and the lowest with high grazing pressures. Logistic regression models revealed that seedling occurrence was highest close to seeder trees, with intermediate litter coverage and at intermediate altitudes above sea level. Since grazing ultimately affects seeder tree occurrence through browsing and reduces litter cover and predominates in higher areas, we conclude that grazing is an important driving factor for P. australis regeneration. We confirm previous studies determining high grazing pressure to be detrimental for P. australis regeneration, but also provide data suggesting that its complete exclusion may not be recommendable either. Management should aim to alternate between a moderate grazing intensity to promote seedling recruitment, with subsequent exclusion to accelerate the growth of the established seedlings.

Differential seedling regeneration patterns across forest–grassland ecotones in two tropical treeline species (Polylepis spp.)

“…), although studies of seedling density found that seedlings reduced to nil within 10 m of trees (Zimmerman et al . ). Cierjacks et al .…”

Genetic structure and diversity ofPolylepis australis(Rosaceae) tree populations from central Argentina: Implications for forest conservation

“…A high amount of gene flow was also assumed by SCHMIDT-LEBUHN et al (2006b) for the wind pollinated Polylepis rugulosa and Polylepis tarapacana. We rejected seed dispersal as a mechanism maintaining P. australis polymorphism as their wind dispersed seeds mostly travel short distances of less than 6 m (ZIMMERMANN et al, 2009;TORRES et al, 2008).…”

Range‐wide genetic structure and diversity of the endemic tree line species Polylepis australis (Rosaceae) in Argentina

“…suggest that cold-tolerant conifer forests, in response to the Quaternary glaciations and interglacial periods, retreated to the north and to lower elevations during cooler climates and migrated to higher elevations during warmer periods ( Quiroga and Premoli, 2007 ;. Fruits are single-seeded nutlets that, at least in the southern distributional range, are produced annually and dispersed short distances by wind (up to 6 m, Torres et al, 2008 ;Zimmermann et al, 2009 ). Johnst., several Nothofagus species in temperate South America ( Marchelli et al, 1998 ;Premoli et al, 2000 ;Bekessy et al, 2002 ;Mathiasen and Premoli, 2010 ), and for • Premise of the study: Knowledge on the range-wide distribution of genetic structure and diversity is required to facilitate the understanding of historical tree migration and for predicting responses to current climate change.…”

“…Previous studies on the mating system of P. australis revealed high pollen viability and longevity, effective pollen fl ow, and that selfi ng is hampered by protogyny ( Seltmann et al, 2007 ;2009 ). Fruits are single-seeded nutlets that, at least in the southern distributional range, are produced annually and dispersed short distances by wind (up to 6 m, Torres et al, 2008 ;Zimmermann et al, 2009 ).…”