Fine root dynamics and resource uptake in a South Ecuadorian mountain rainforest as dependent on elevation

Abstract

Tropical mountain rainforests extend over large elevational gradients with diverse climatic conditions. They are characterised by a decrease of above ground biomass, and a pronounced increase of fine root biomass with increasing elevation. Forest stands at higher elevations are further characterised by thick organic layers and very moist soil conditions. The turnover of fine roots plays an important role in the cycling of carbon and nutrients in theses ecosystems. However, the knowledge about the patterns of fine root dynamics in tropical mountain rainforests is still scarce. Likewise, not much is known about water and nutrient uptake capacities of roots at different elevations.The present study was conducted in five forests stands along an elevational transect between 1050 and 3060 m asl in a mountain rainforest of South Ecuador. It aimed (1) to analyse the effect of temperature and soil conditions on fine root turnover along the elevational transect by means of minirhizotrons; (2) to assess the role of nutrient limitation and water-logging for fine root activity by means of a fertilization and throughfall exclusion experiment, respectively; as well as to examine nitrogen uptake capacity of fine roots by 15N tracer application; and (3) to investigate the dependence of root sap flow on environmental variables by means of miniature heat balance sap flow gauges, and to analyse related anatomical characteristics of root cross sections.Fine root turnover (d < 2 mm) was significantly higher in the lowermost and the uppermost stand (0.9 cm cm-1 yr-1) than in the three mid-elevation stands (0.6 cm cm-1 yr-1). The turnover of finest roots (d < 0.5 mm) was higher compared to the root cohort with d < 2 mm, and exceeded 1.0 cm cm-1 yr-1 at the lower and upper elevations of the transect. Hence fine root turnover decreased from pre-montane to mid-montane forests as would be expected from an effect of low temperature on root turnover, but it decreased further upslope despite colder temperatures. It is assumed that this non linear altitudinal trend of fine root turnover originates from an overlapping of a temperature effect with other environmental gradients in the upper part of the transect. Adverse soil conditions may reduce root longevity at high elevations, and are thus additional factors besides temperature that control root dynamics in tropical mountain forests. The fast replacement of fine roots is possibly used as an adaptive mechanism by trees to cope with limiting environmental conditions.The fertilizer study revealed highest root growth stimulation after P addition at 1050 m, and after N addition at 3060 m, thus pre-montane forests may be presumably limited by the availability of P, whereas upper montane forests are rather limited by the availability of N. The concentrations of major nutrients in fine root tissue dropped significantly along the elevational transect, with strongest reductions observed for N. This may support the assumption of a decreased nutrient availability with increasing altitude. Throughfall exclusion at 3060 m did not reduce fine root mortality in this forest stand, hence high soil moisture contents do not seem to be a main stressor leading to high fine root turnover rates in upper montane forests. The 15NO315NH4 uptake study yielded a constantly high nitrate and ammonium uptake capacity of fine roots at altitudes at 1050, 1890 and 3060 m, suggesting that fine roots in upper montane forests compensate for low nutrient supplies with high nutrient uptake capacities.Root sap flow followed marked diurnal and seasonal courses in the forest stands at 1050, 1890 and 3060 m. Sap flow decreased roughly by a factor of three between the lowermost and the uppermost forest stand. A reduced sap flow was observed when VPD was above average for several days. VPD was identified as the most influential environmental factor controlling root water uptake, but its significance decreased with increasing altitude. In contrast, the influence of temperature increased along the elevational transect, and was identified as the most influential factor determining root sap flow at 3060 m. Anatomical analyses of root cross-sections provided evidence for decreasing vessel diameters with increasing altitude. Theoretical hydraulic conductivity was found to decrease more than ten fold from 50.2 m² MPa-1 s-1 at 1050 m to 4.0 m² MPa-1 s-1 at 3060 m. It is concluded that the temperature decrease with increasing altitude acts on water uptake through several pathways, (i) on the physics of water flow in the soil-plant-atmosphere continuum (increasing viscosity of water, decreasing VPD), (ii) by reducing vessel cell growth, (iii) by restricting nutrient availability (which may lead to smaller vessels), and (iv) possibly by lowering aquaporine activity in the roots.