Abstract:
It is generally assumed that tree growth in tropical low-elevation forests is primarily limited by phosphorus
while nitrogen limitation is more prominent in tropical montane forests where temperature is
lower and the soils are poorly developed. We tested this hypothesis in mountain rainforests of South
Ecuador by investigating the growth response of tree fine roots to N, P and K fertilization in ingrowth cores
exposed at 1050 m (pre-montane) and 3060 m (upper montane) elevation. Root growth into unfertilized
ingrowth cores (control treatment) was about 10 times slower at 3060 m than at 1050 m. At 1050 m, root
growth was stimulated not only by P, but also by N and K. In contrast, N was the only element to promote
root growth at 3060 m. The N concentration in fine root biomass dropped to nearly a third between 1050
and 3060 m, those of P, K, Ca and Mg decreased as well, but to a lesser degree. According to a 15NO3
15NH4
tracer study along the slope, tree fine roots accumulated nitrate and ammonium in root biomass at similar
rates between 1050 and 3060 m, despite lower temperatures higher upslope.We conclude that the nature
of nutrient limitation of tree fine root growth changes with elevation from an apparent co-limitation by
P together with N and K at 1050 m to predominant N limitation at 3060 m, which is also reflected by low
foliar N concentrations. Increasing N limitation may have caused the high fine root biomass and root/shoot
ratio in the high elevation forest, while the capability of the roots to acquire mineral N apparently was not
affected by lower temperatures at high elevations.
