High solar radiation hinders tree regeneration above the alpine treeline in northern Ecuador

Abstract

Many tropical alpine treelines lie below their climatic potential, because of natural or anthropogenic causes. Forest extension above the treeline depends on the ability of trees to establish in the alpine environment. This ability may be limited by different factors, such as low temperatures, excess solar radiation, competition, soil properties, dispersal ability, and fires. In this paper we address the following two questions: Do trees regenerate above the present treeline, and what are the inhibiting factors for tree establishment? To answer these questions we described the spatial pattern of recent tree establishment below and above the present treeline in northern Ecuador. Also, we experimentally transplanted seedlings into the alpine vegetation (pá ramo) and the forest, and investigated the effect of shade, neighboring plants, and substrate on their survival. The number of naturally occurring tree sprouts (seedlings, saplings and ramets) was highest just outside the forest, and decreased with distance to the forest edge. However, only two species that were radiation-tolerant made up these high numbers, while other species were rare or absent in the pá ramo. In the forest, the species diversity of sprouts was high and the abundance per species was relatively low. The transplanted seedlings survived least in experimental plots without artificial shade where neighboring plants were removed. Seedling survival was highest in artificially shaded plots and in the forest. This shade-dependence of most tree species can strongly slow down forest expansion toward the potential climatic treeline. Due to the presence of radiation-tolerant species, the complete lack of forest expansion probably needs to be ascribed to fire. However, our results show that natural processes can also explain both the low position and the abruptness of tropical treelines.

Treelines—Approaches at Different Scales

“…Nevertheless, tree roots spreading into treeless sunlit gaps as well as tree seedlings would benefit from the warmer conditions, provided no other adverse factors, such as intense solar radiation loads, moisture deficiency, and wind impact outweighed the advantage of wider spacing (e.g., [68][69][70]). Thus, widely spaced trees are more exposed to climatic injuries than those within tree stands, as is reflected in an abrupt increase of climatically shaped growth forms above the closed forest.…”

Estructura, Diversidad Y Dinámica De La Vegetación en El Ecotono Bosque-Páramo: Revisión De La Evidencia en La Cordillera De Mérida

“…Este mecanismo pudiera a su vez generar un proceso de retroalimentación positiva entre la presencia de un dosel continuo y la regeneración de las leñosas (Bader et al, 2007b), que pudiera modular o retardar la velocidad de respuesta del bosque en escenarios de cambio ambiental (i.e. en respuesta al calentamiento global y/o a disturbios que eliminan la cobertura forestal continua).…”

“…La altitud y conformación del ecotono bosquepáramo en los Andes Tropicales puede ser muy variable y responder de forma compleja a la heterogeneidad espaciotemporal Andina, aún en áreas sin intervención antrópica, en función de factores como el grado de aislamiento y altitud máxima del macizo montañoso, la estacionalidad de las precipitaciones, la exposición, topografía y las características de los suelos (Monasterio y Reyes, 1980;van der Hammen y Cleef, 1986;Bader et al, 2007b;Cuello y Cleef, 2011;Ataroff y García-Nuñez, 2013). La tendencia general es que el ecotono se ubique a menores elevaciones en: a) montañas más bajas o aisladas (el llamado efecto "top", "telescopio" o "Massenerhebung"; van Steenis, 1961;Grubb, 1971); b) en vertientes más secas y con mayor estacionalidad hídrica (Ataroff, 2003); y c) en áreas sujetas a intervención antrópica (fuegos, pastoreo, agricultura), un proceso conocido como "paramización" del bosque (Cortés et al, 1999).…”

“…Esta respuesta tan dinámica a los cambios ambientales en el pasado, sugiere que la estructura y distribución del ecotono bosque-páramo pudiera cambiar también como respuesta al cambio climático global actual, con un aumento de temperatura esperado de 2-4 ºC en el norte de Sur América para el año 2100 (Rull et al, 2007). El aumento de temperatura pudiera resultar, especialmente en ausencia de disturbios como el fuego o el pastoreo, en un desplazamiento hacia arriba del bosque (o elementos leñosos dominantes del bosque) sobre el páramo (Bader et al, 2007b;Suárez y Chacón-Moreno, 2011;Llambí et al, 2013a).…”

The ecological adaptability of four typical plants during the early successional stage of a tropical rainforest

“…In low temperature conditions, excessive solar radiation leads to more serious damage to plant performances (Bader et al 2007;Vitale & Santo 2008). Under these circumstances, plants exhibit two general strategies: one physiological and the other structural or architectural.…”