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

Abstract

Background: Constraints on tree regeneration have been described from several temperate treeline sites, however data from tropical mountains are few. Aim: We test the hypothesis that high altitude treelines are caused by constraints on tree regeneration. We report the results of recent studies on climatic variables and on the reproduction of Erica spp. in east Africa, and compare those with similar studies on treeline stands of Polylepis spp. in Ecuador and Bolivia. Methods: We established altitudinal and horizontal transects comprising sampling plots from forests to surrounding grasslands. Data on microclimate, vegetation structure and reproductive activity were collected in the field, and supplemented by tests on germination requirements in the field in permanent exclosures and in the laboratory. Results: Our measurements demonstrated that the number and severity of frost events and potential water stress were higher near the soil surface (0.5 m) than in free air (2.0 m). Seeds of Erica trimera did not germinate, as the necessary climatic requirements (high diurnal temperature range, but no frost) were rarely fulfilled in the field. Most studied species were capable of clonal growth, and the relative importance of vegetative persistence tended to increase with altitude. These effects were independent of the intensity of land use. Conclusions:The harsh microclimate near the soil surface at tropical treelines may be one reason why recruitment from seed is limited for treeline-forming species. This is important for restoration or conservation schemes as well as predictions of treeline dynamics under global change.

Winter crop growth at low temperature may hold the answer for alpine treeline formation

“…However, aerodynamic decoupling also has a price during clear nights, when *Email: ch.koerner@unibas.ch ground vegetation undergoes stronger radiative cooling (down to 7 K below ambient) than do upright structures (e.g. Squeo et al 1991;Jordan and Smith 1994;Wesche et al 2008). In addition, small herbaceous growth forms have more developmental flexibility, shorter life cycles and shorter leaf life span and thus, can use shorter, climatically more favourable periods during the year than evergreen treeline trees.…”

Tending for Cattle: Traditional Fire Management in Ethiopian Montane Heathlands

“…However, the occasional death of Erica lignotubers we observed after fires that had burnt towards the end of the prolonged drought in 2008 shows that there is indeed a potential for type conversion towards grassland since seed recolonization of Erica spp. is very poor (Wesche et al 2008). It is not known for how long these heathlands have been subjected to anthropogenic fire, but it is likely that they have been burnt on short rotation as long as people have kept cattle there.…”

“…There is a short dry season which normally lasts two to three months (December-January), but small amounts of precipitation sometime fall during the dry season. The dry season is characterized by more pronounced diurnal temperature fluctuations and common night frosts (Wesche et al 2008). Inter-annual variation in rainfall is very large, and there are recurring inter-decadal droughts (Seleshi and Demaree 1995).…”

Allometry and effects of extreme elevation on growth velocity of the Andean tree Polylepis tarapacana Philippi (Rosaceae)

“…Decreasing height and stem diameter with increasing elevation have been reported for several species including P. australis , P. pepei (Hertel and Wesche 2008), P. tarapacana (Hoch and Körner 2005) in addition to P. incana and P. pauta (Cierjacks et al 2007). The reduction in above-ground biomass has been attributed to the effect of two major causes on growth: (1) an effect produced by a decrease in temperatures (Cierjacks et al 2007;Hoch and Körner 2005;Wesche et al 2008) and (2) reduced photosynthetic activity leading to a reduction in carbon gain (Hertel and Wesche 2008).…”