Abstract
Fine roots (≤2 mm) consume a large proportion of photosynthates and thus play a key role in the global carbon cycle, but our knowledge about fine root biomass, production, and turnover across environmental gradients is insufficient, especially in tropical ecosystems. Root system studies along elevation transects can produce valuable insights into root trait-environment relationships and may help to explore the evidence for a root economics spectrum (RES) that should represent a trait syndrome with a trade-off between resource acquisitive and conservative root traits. We studied fine root biomass, necromass, production, and mean fine root lifespan (the inverse of fine root turnover) of woody plants in six natural tropical ecosystems (savanna, four tropical mountain forest types, tropical alpine heathland) on the southern slope of Mt. Kilimanjaro (Tanzania) between 900 and 4,500 m a.s.l. Fine root biomass and necromass showed a unimodal pattern along the slope with a peak in the moist upper montane forest (~2,800 m), while fine root production varied little between savanna and upper montane forest to decrease toward the alpine zone. Root:shoot ratio (fine root biomass and production related to aboveground biomass) in the tropical montane forest increased exponentially with elevation, while it decreased with precipitation and soil nitrogen availability (decreasing soil C:N ratio). Mean fine root lifespan was lowest in the ecosystems with pronounced resource limitation (savanna at low elevation, alpine heathland at high elevation) and higher in the moist and cool forest belt (~1,800–3,700 m). The variation in root traits across the elevation gradient fits better with the concept of a multi-dimensional RES, as root tissue density and specific root length showed variable relations to each other, which does not agree with a simple trade-off between acquisitive and conservative root traits. In conclusion, despite large variation in fine root biomass, production, and morphology among the different plant species and ecosystems, a general belowground shift in carbohydrate partitioning is evident from 900 to 4,500 m a.s.l., suggesting that plant growth is increasingly limited by nutrient (probably N) shortage toward higher elevations.
Highlights
Fine roots are a small but functionally important component of plant biomass, which controls the uptake of water and nutrients and influences biogeochemical cycles through rapid biomass turnover (Vogt et al, 1996; Gill and Jackson, 2000)
Fine root biomass and necromass revealed a unimodal curve with a peak in the Podocarpus forest zone (2,720–2,970 m a.s.l.) (Figure 1)
Fine root production was unaffected by elevation below 3,000 m a.s.l. but decreased higher up the slope toward the Erica forest and Helichrysum heathland
Summary
Fine roots (i.e. roots ≤2 mm in diameter) are a small but functionally important component of plant biomass, which controls the uptake of water and nutrients and influences biogeochemical cycles through rapid biomass turnover (Vogt et al, 1996; Gill and Jackson, 2000). Trees adopt different strategies of resource allocation to the fine root system, which all aim at minimizing the resource investment needed to secure nutrient and water uptake. These strategies vary with the species and functional groups, and in dependence on climatic and edaphic factors such as temperature, precipitation, nutrient availability, soil acidity, and soil bulk density (Cairns et al, 1997; Hertel and Leuschner, 2002; Finér et al, 2011). Large variation in fine root system size, mean fine root diameter, branching patterns, and fine root turnover has been found in different forest types, even when the general climate is similar (Leuschner and Hertel, 2003; Finér et al, 2011)
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