Impact of sowing, canopy cover and litter on seedling dynamics of two Polylepis species at upper tree lines in central Ecuador

Abstract

Failure of reproduction is hypothesized as being a main reason for the formation of upper tree lines but there is, as yet, little supportive evidence of such. This study investigates the effects of experimental sowing, canopy cover and litter depth on seedling emergence and survival of Polylepis incana and Polylepis pauta at the upper distribution limit of the species in the Páramo de Papallacta, central Ecuador. We established 18 study plots located in the interior, at the edge and at the exterior of closed forests at the upper tree line and analysed seedling dynamics for 1 y following experimental sowing with four different litter treatments. For both species, seedling numbers were significantly higher in the forest interior than outside, and higher in the treatments with the litter layer removed. In P. pauta, sowing significantly increased seedling number, which was more pronounced within and at the edge of the forest than outside. In contrast, there was no impact of sowing on seedling emergence in P. incana. First-year seedling survival and mean growth rate per month were low in both species. Log-linear models did not reveal significant interactions between survival and canopy cover or litter removal. Our data show that seedling recruitment is limited at the upper tree line – presumably due to extreme climatic conditions – which indicates the importance of constraints in sexual regeneration for tree-line formation in central Ecuador.

Genetic insights into the historical distribution of Polylepis pauta (Rosaceae) in the northeastern Cordillera Oriental of Ecuador

“…The absence of such structure in populations of mature individuals is consistent with theoretical predictions for tree populations which are characterized by generally low genetic structure (Hamrick andGodt 1989, 1996;Nybom 2004). High seedling mortality (observed in Polylepis incana, P. pauta, [Cierjacks et al 2007], and P. australis [Enrico et al 2004]) and delayed reproductive maturity in trees could contribute to the attenuation of structure over time (Austerlitz et al 2000;Le Corre and Kramer 2003). No correlation was found between geographic distance and genetic differentiation, and clustering of populations on the basis of genetic affinity often revealed higher genetic similarity between populations in different basins, which may have been caused by greater gene exchange between river basins.…”

Tropical moist Polylepis stands at the treeline in East Bolivia: the effect of elevation on stand microclimate, above- and below-ground structure, and regeneration

“…This was attributed to differences in seed rain, but also to constraints in seedling survival under open conditions. Seeding experiments on Polylepis forests at the treeline in Ecuador showed that microclimatic conditions hamper seedling emergence in open grassland at the treeline compared to the forest edge or its interior (Cierjacks et al 2007). This was related to the special microclimatic conditions near the soil surface: like in our study, mean air and soil temperatures near the ground were slightly cooler inside the uppermost forest, while air temperature in the grassland outside the treeline forest showed markedly higher maximum and lower minimum temperatures compared to the forest conditions.…”

Recruitment of trees at tropical alpine treelines:Ericain Africa versusPolylepisin South America

“…Sowing experiments performed inside exclosures demonstrated that seedling emergence of both Polylepis species was < 1% of the sown seeds (Cierjacks et al 2007a). This is clearly lower than the 2-6% germination rate under controlled conditions in the laboratory (A. Cierjacks, unpublished data, see also Brandbyge and Holm-Nielsen 1992).…”