High altitude C₄ grasslands in the northern Andes: relicts from glacial conditions?

Abstract

The altitudinal vegetation distribution in the northern Andes during glacial time differed from the present-day conditions as a result of temperature and precipitation change. New evidence indicate that as a response to a reduced atmospheric partial CO₂ pressure (pCO₂), the competitive balance between C₃ and C₄ plants have changed. Effects may have remained virtually undetected in pollen records, but can be observed using a stable carbon isotope analysis. Vegetation dominated by C₄ taxa, belonging to the families Cyperaceae (e.g. Bulbostylis and Cyperus) and Poaceae (e.g. Muhlenbergia, Paspalum and Sporobolus), may have been able to replace for a significant part the modern type C₃ taxa (e.g. species belonging to Carex, Rhynchospora, Aciachne, Agrostis, Calamagrostis, and Chusquea). Impact of reduced glacial atmospheric pCO₂ levels and lower glacial temperatures on the composition and the elevational distribution of the vegetation types is discussed. The present high Andean vegetation communities may differ from the glacial equivalents (non-modern analogue situation). We identified dry Sporobolus lasiophyllus tussock grassland and Arcytophyllum nitidum dwarfshrub paramo as the possible relict communities from glacial time. The effect on previous estimates of paleo-temperatures is estimated to be small.

Introduction

Reconstructions of climate change during the Quaternary of tropical South America are mainly based on vegetation change documented in pollen records. Records from tropical mountains are of particular interest for paleoclimate reconstructions as vegetation belts migrate along altitudinal gradients during glacial–interglacial cycles. Stable carbon isotopic analyses (δ¹³C) of sedimentary organic matter have been applied to cores from tropical mountain ecosystems. These studies have provided a new proxy of environmental change, which gives a special role to C₄ plants and partial atmospheric CO₂ pressure (pCO₂) (Street-Perrott, 1994, Street-Perrott et al., 1997, Ficken et al., 1998, Boom and Marchant, 2001). Earlier papers dealing with δ¹³C of tropical records use aridity to explain expansion of C₄ grasslands (Aucour and Hillaire-Marcel, 1994, Giresse et al., 1994, Quade et al., 1989, Sukumar et al., 1993, Talbot and Johannesen, 1992).

Among vascular plants, three different major photosynthetic routes can be distinguished: C₃, C₄ and CAM photosynthesis. The most common among present-day plants is the C₃ pathway. The C₄ pathway enables plants a higher water use efficiency (WUE) and a more effective CO₂ uptake, because they use a CO₂ concentrating mechanism (Leegood, 1999). At present, many C₄ plants can be found among the monocot families Cyperaceae (sedges) and Poaceae (grasses). According to Watson and Dallwitz (1992), 407 out of 799 Poaceae genera contain C₄ species, most of them are characteristic elements of tropical to temperate areas with abundant warm-season precipitation. WUE of CAM plants is even better than that of C₄ plants, and thus many of them can be found in hot and arid environments, where other plants cannot grow. Some CAM plants are important components of the high Andean vegetation, such as Bromeliaceae (e.g. Puya sp.). However, CAM photosynthesis can also be found in aquatic plants and epiphytes. The occurrence of CAM in Isoetes, an important plant from Andean lakes, may even point to a very remote origin in the evolution of CAM (Keeley, 1998), while the C₄ pathway is believed to have evolved during the mid-Cretaceous (C₄ grasses: Brown and Smith, 1992; C₄ plants: Bocherens et al., 1993). However, little is known about their behavior in response to Quaternary climate change, neither do they make a significant contribution to the pollen based vegetation reconstructions for tropical South America.

Most of the modern C₄ Poaceae are found at places where, during the growing season, average monthly temperature exceed 22°C and rainfall is more than 25 mm/month (Ehleringer, 1997, Collatz et al., 1998). By using established models for C₃ and C₄ photosynthesis, a relationship that describes the competitive balance between the two was proposed (Ehleringer, 1997, Collatz et al., 1998). The authors defined the point where both types of photosynthesis are equally effective in assimilating carbon, in terms of a crossover point (or crossover temperature), which is a function of pCO₂. Whenever one type is more effective than the other, that type wins the competition (Fig. 1). Thus under low pCO₂, C₄ grasses will be able to dominate over the C₃ species; even in colder areas where, under current pCO₂ conditions, C₄ grasses are only of minor importance compared to C₃ species. Lowered atmospheric pCO₂ levels in the Miocene are believed to have resulted in the appearance of C₄ dominated grasslands (Cerling et al., 1993, Cerling et al., 1997, Ehleringer, 1997, Latorre et al., 1997, Quade and Cerling, 1995, Quade et al., 1995).

Independent from the previous model, the C₄ plants will also have an advantage over the C₃ plants under low atmospheric CO₂ concentrations. To maintain a constant influx of CO₂, plants will produce more stomata per surface unit (Beerling and Chaloner, 1992, Beerling and Chaloner, 1994, Beerling et al., 1995, Wagner, 1998, Wagner et al., 1999). This will cause increased water loss through evaporation and resulting in water stress. Altered atmospheric pCO₂ even influences pure C₃ plant communities. Jolly and Haxeltine (1997) used a BIOME-3 modeling experiment (Haxeltine and Prentice, 1996) at the site of Kashiru bog in tropical East Africa to show that the montane forest at 2240 m elevation can be completely replaced by ericaceous scrub vegetation by lowering only the pCO₂ to last glacial maximum (LGM) values while maintaining a constant temperature.

Gas analysis of air bubbles from the Vostok ice core, covering the last 420,000 years, shows that CO₂ levels have varied significantly during the late Quaternary (Barnola et al., 1987, Petit et al., 1999), showing a significant drop of atmospheric CO₂ concentrations from 280 ppmV during interglacial time (modern pre-industrial value) to 180 ppmV during the LGM. This pattern is consistent with at least three older glaciations recorded in the Vostok ice core.

Until recently, lower temperatures during the glaciations have been taken as the main cause for the lower altitudinal position of the upper forest line in the northern Andes (e.g. Van der Hammen and González, 1960; Hooghiemstra, 1984; Van der Hammen and Cleef, 1986). The objective of this paper is to identify the impact of a significant drop in the concentration of atmospheric carbondioxide on the altitudinal vegetation distribution in the Colombian Andes, the C₄–C₃ grass crossover in particular. With stable carbon isotopic analysis, a Colombian high-altitude record is investigated on the presence of C₄ plants in the past. We aim to provide a sketch of potential plant communities that could have occurred under glacial low atmospheric pCO₂ and which do not occur under Holocene conditions and finally we suggest the presence of a possible relict of these vegetation types.