Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

Growth limitation induced by Al toxicity is believed to commonly occur in tropical forests, although a direct proof is frequently lacking. To test for the general assumption of Al toxicity, Al, Ca, and Mg concentrations in precipitation, throughfall, stemflow, organic layer leachate, mineral soil solutions, stream water, and the leaves of 17 native tree species were analyzed. We calculated Al fluxes and modeled Al speciation in the litter leachate and mineral soil solutions. We assessed potential Al toxicity based on soil base saturation, Al concentrations, Ca:Al and Mg:Al molar ratios and Al speciation in soil solution as well as Al concentrations and Ca:Al andMg:Al molar ratios in tree leaves. High Al fluxes in litterfall (8.77±1.3 to 14.2±1.9 kg ha?1 yr?1, mean ± SE) indicated a high Al circulation through the ecosystem. The fraction of exchangeable and potentially plant-available Al in mineral soils was high, being a likely reason for a low root length density in the mineral soil. However, Al concentrations in all solutions were consistently below critical values and Ca:Al molar and the Ca2+:Alinorganic molar ratios in the organic layer leachate and soil solutions were above 1, the suggested threshold for Al toxicity. Except for two Al-accumulating and one non-accumulating tree species, the Ca:Al molar ratios in tree leaves were above the Al toxicity threshold of 12.5. Our results demonstrate high Al cycling through the vegetation partly because of the presence of some Al accumulator plants. However, there was little indication of an Al toxicity risk in soil and of acute Al toxicity in plants likely reflecting that tree species are well adapted to the environmental conditions at our study site and thus hardly prone to Al toxicity.

Abstract:

Aims We determined the reasons why in nutrient solution increasing Al concentrations>300 ?M inhibited shoot biomass production of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson while 300 ?M Al stimulated root biomass production of Tabebuia chrysantha. Methods Nutrient concentrations in plant tissue after a hydroponic growth experiment were determined. Results Increasing Al concentrations significantly decreased Mg concentrations in leaves. Phosphorus concentrations in roots of C. odorata and T. chrysantha were significantly highest in the treatment with 300 ?M Al and correlated significantly with root biomass. Conclusions Shoot biomass production was likely inhibited by reduced Mg uptake, impairing photosynthesis. The stimulation of root growth at low Al concentrations can be possibly attributed to improved P uptake.

Abstract:

Aims: In acid tropical forest soils (pH <5.5)increased mobility of aluminum might limit aboveground productivity. Therefore, we evaluated Al phytotoxicity of three native tree species of tropical montane forests in southern Ecuador. Methods: An hydroponic dose-response experiment was conducted. Seedlings of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha(Jacq.) G. Nicholson were treated with 0, 300, 600, 1200, and 2400 ?M Al and an organic layer leachate. Dose-response curves were generated for root and shoot morphologic properties to determine effective concentrations (EC). Results: Shoot biomass and healthy leaf area decreased by 44 % to 83 % at 2400 ?M Al, root biomass did not respond (C. odorata), declined by 51 % (H. americanus), or was stimulated at low Al concentrations of 300 ?M (T. chrysantha). EC10 (i.e. reduction 10 %) values of Al for total biomass were 315 ?M (C. odorata), 219 ?M (H. americanus), and 368 ?M (T. chrysantha). Helicarpus americanus, a fast growing pioneer tree species, was most sensitive to Al toxicity. Negative effects were strongest if plants grew in organic layer leachate, indicating limitation of plant growth by nutrient scarcity rather than Al toxicity. Conclusions: Al toxicity occurred at Al concentrations far above those in native organic layer leachate.