Trade wind effects on vegetation are well documented. For example, they generate distinct microclimates on leeward and adjacent winward sides Vincristine clinical trial of mountains (Smith and Young, 1987), resulting in longitudinal rainshadow gradients along which the altitudinal limits of vegetation belts vary (e.g. Sklenář and Laegaard, 2003). Low precipitation levels combined with (1) the absence of water input provided by durable snowbeds and (2) well-documented water
stress due to reduction in soil depth and organic matter content at high elevation (Pérez, 1987b, Körner, 2003 and Anthelme et al., 2012) make tropical mountains more arid than their extratropical counterparts (Leuschner, 2000). This is illustrated by the occurrence of ‘alpine deserts’ on the leeward slopes of high isolated
mountains such as Mount Kilimanjaro in East Africa (Crawford, 2008), Mount Chimborazo in Ecuador (Sklenář and Laegaard, 2003), Cordillera de Merida in Venezuela (Monasterio, Belnacasan mw 1979 and Pérez, 1987a), Mount Cameroon (Letouzey, 1985), volcano Maui in Hawaii (e.g. Pérez, 2003), or in large plateaux bordered to the East by high mountain ranges, such as the Bolivian altiplano (Herzog, 1923). The relative aridity observed in TAE is likely responsible for the common occurrence of (1) scleromorphic plant types such as giant cushions, giant rosettes, and microphyllous shrubs (Ramsay and Oxley, 1997 and Leuschner, 2000) and (2) natural and man-induced fire episodes which constitute severe constraints for plant development (Smith and Young, 1987 and Luteyn, 1999). Altitudinal variation is a powerful proxy of the main drivers of the spatiotemporal dynamics of alpine ecosystems (Körner, 2007 and Nagy and Grabherr, 2009). Because of a lower latitudinal position, TAE occur at a much higher altitude than other alpine ecosystems, especially close to the equator (Körner, 2003). Consequently, TAE are exposed to lower partial pressures of atmospheric gases than most extratropical alpine systems, among which low levels of atmospheric CO2 can have a substantial effect on plant growth and biomass (Körner, 2003 and Körner, 2007). For the
same reasons, ultraviolet (UV) radiations are much stronger in TAE (Körner, 2007) and represent a supplementary physical stress for plants (Caldwell and Robberecht, GPX6 1980). It is interesting to note that some subtropical, subarctic, and subantarctic isolated islands, as well as New Zealand, share several ecological features with TAE – including similar growth forms such as giant rosettes, giant cushions, and tussock grasses (Halloy and Mark, 1996, Leuschner, 1996, Mark et al., 2000, Bannister et al., 2005 and le Roux and McGeoch, 2010). As these regions do not share necessarily the specific TAE abiotic features of inverted rainfall gradients and high altitude, it seems that the reason for such similarity in vegetation may rely on the strong oceanic influence on the local climate which buffers seasonality (Leuschner, 1996).