Does water stress, nutrient limitation, or H-toxicity explain the differential stature among Heath Forest types in Central Kalimantan, Indonesia?

To investigate the causes of the reduced stature of heath forest compared to lowland evergreen rain forest (LERF), the quantity and quality of small litterfall (LF), the standing crop of litter on the forest floor (LSC), and the annual rates of litter decay were determined over a period of 12 months in three contrasting lowland rain forest types in Central Kalimantan, Indonesia. In addition, a litterbag experiment monitored the mass loss of leaves from three dominant tree species in two heath forests (HF) of contrasting stature. Soil water and shallow groundwater dynamics in the two HFs were monitored as well. LF in the LERF was higher compared to both tall heath forest (THF) and relatively stunted heath forest (SHF), but did not differ between the two HFs. Stand-level nutrient-use efficiencies for nitrogen and phosphorus were greatest for the SHF, followed by the THF and the LERF, respectively. The observed differences in nutrient-use efficiency between the two HFs did not result in different LF totals, LSC or decomposition rates and hence cannot explain the difference in HF stature. Neither could phenolic concentrations in leaf LF, which were very similar for the two HFs. Top-soil moisture levels were consistently higher in the SHF compared to the THF and never reached wilting point in either forest type whereas shallow groundwater levels in the SHF were both closer to the surface and more persistent than in the THF. Thus, severe water stress is not thought to be a factor of importance determining HF stature. Rather, considering the much lower pH of the topsoil in the SHF compared to the THF it is hypothesized that different degrees of H-toxicity to fine roots may ultimately prove responsible for the contrast in HF stature. © 2012 Springer Science+Business Media B.V.