Freezing tolerance in grasses along an altitudinal gradient in the Venezuelan Andes

Abstract

The tropical high Andes experience greater daily temperature oscillations compared to seasonal ones as well as a high frequency of night frost occurrence year round. Survival of organisms, under such environmental conditions, has been determined by selective forces which have evolved into adaptations including avoidance or tolerance to freezing. These adaptations have been studied in diVerent species of trees, shrubs and perennial herbs in páramo ecosystems, while they have not been considered in grasses, an important family of the páramo. In order to understand survival of Poaceae, resistance mechanisms were determined. The study was performed along an altitudinal gradient (2,500-4,200 m a.s.l.) in the páramo. Supercooling capacity and frost injury temperature were determined in nine species in order to establish cold resistance mechanisms. Grasses registered a very low supercooling capacity along the altitudinal gradient, with ice formation between ¡6 and ¡3°C. On the other hand, frost injury temperature oscillated between ¡18 and ¡7°C. Our results suggest that grasses exhibit freezing tolerance as their main cold resistance mechanism. Since grasses grow at ground level, where greatest heat loss takes place, tolerance may be related to this life form as reported for other small life forms.

Does plant height determine the freezing resistance in the páramo plants?

“…Life form co-varied with the degree of freezing resistance in the páramo plants, which is consistent with findings from other alpine regions (Taschler & Neuner 2004; see also Márquez et al 2006). The relationship cannot be explained by the altitudinal distribution (as there is no difference between the life forms) or plant height, thus an alternative explanation must be searched for.…”

“…Thus, it seems that neither of the two mechanisms imposes a constraint on the plants as to which páramo environment to inhabit. The injury temperature Lt50 tended to decline with species' altitude but the correlation was weak and not significant, consistent with Márquez et al (2006); however, there may be strong altitudinal patterns within species (Rada et al 1987). These findings support Beck's (1994) conclusion that the resistance to freezing temperatures is not the primary factor that determines the survival and altitudinal range of (the majority of) the páramo species.…”

“…Páramo plants use two different mechanisms to cope with sub-zero temperatures. They either avoid freezing of the tissues, for instance by insulation with a thick mantle of dead leaves or by transient supercooling, or they tolerate freezing by the formation of ice crystals in extracellular spaces, which prevents ice occurrence and subsequent damage in the living cells (Beck et al 1982;Squeo et al 1991;Beck 1994;Márquez et al 2006). Squeo et al (1991) observed in the páramo of Venezuela that taller plants avoided freezing by supercooling whereas plants near the ground tolerated ice formation.…”

“…Moreover, plant height was found to correlate with the temperature at which freezing injury occurred. Therefore, they suggested that the development of the resistance mechanism was related to the microclimate plants are experiencing (Squeo et al 1991;Márquez et al 2006).Whereas growth near the ground, where more extreme temperatures are encountered, would favour the evolution of frost tolerance, growth into higher strata with a milder microclimate would favour frost avoidance. In line with the microclimate hypothesis, the presence of freezing tolerance in the afro-alpine giant rosette plants Dendrosenecio and Lobelia, in contrast to freezing avoidance by Espeletia in the páramo, was also suggested to be related to the greater microclimatic extremes of the East African mountains (Beck et al 1982;Rada et al 1985;Beck 1994).…”

Changes in plant form and function across altitudinal and wetness gradients in the wetlands of the Maloti-Drakensberg, South Africa

“…Therefore, plants from the wetter parts of the wetland are likely to generally have more stress to endure during the cold season than plants from the drier parts of the wetland (especially, since the cold season in the Maloti-Drakensberg is also the dry season). The unusual distribution of C 4 taxa in habitats which routinely experience freezing temperatures, such as the high Andes, has been attributed to the evolution of freezing tolerance (Marquez et al 2006) and such taxa are hypothesized to be occupying habitats that were previously suitable for C 4 plants (Boom et al 2001). …”