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

Abstract:

Field studies have shown a wide array of responses of vascular epiphyte diversity to human disturbance-assem- blages of disturbed habitats range from largely unchanged to severely impoverished when compared with intact forest. This variability is not well understood. We explored the hypothesis that the relative impoverishment of disturbed-habitat epiphyte assemblages is a function of local climate, by analyzing the available literature on epiphyte diversity on isolated trees as a model system. We found that assemblages of moist and moderately seasonal areas experience considerably stronger impov- erishment than those of aseasonally wet or distinctly dry areas. We argue that the integrity of the vertical microclimatic gradient is more crucial for the maintenance of epiphyte diversity in moderately seasonal forests than in distinctly dry or aseasonally wet forests.

Abstract:

Not much is known about the nitrogen (N) uptake capacity and N-form preference of tropical trees. In a replicated labelling experiment with 15N-ammonium, 15N-nitrate and dual-labelled glycine applied to saplings of six tree species from southern Ecuadorianmontane forests, we tested the hypotheses that (1) the saplings of tropical trees are capable of using organicNeven though they are forming arbuscularmycorrhizas, and (2) with increasing altitude, tree saplings increasingly prefer ammonium and glycine over nitrate due to reduced nitrification and growing humus accumulation. Three- to 5-y-old saplings of two species each from 1000, 2000 and 3000 m asl were grown in pots inside the forest at their origin and labelled with non-fertilizing amounts of the three N forms; 15N enrichment was detected 5 days after labelling in fine roots, coarse roots, shoots and leaves. The six species differed with respect to their N-form preference, but neither the abundance of ammonium and nitrate in the soil nor altitude (1000–3000 m asl) seemed to influence the preference. Two species (those with highest growth rate) preferred NH4+ over NO3?, while the other four species took up NO3? and NH4+ at similar rates when both N forms were equally available. After 13C-glycine addition, 13C was significantly accumulated in the biomass of three species (all species with exclusively AM symbionts) but a convincing proof of the uptake of intact glycine molecules by these tropical montane forest trees was not obtained.

Abstract:

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO2 concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO2] (Asat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of Asat and leaf dark respiration (RD) per leaf area remain constant with elevation. Standlevel means of Asat were 8.8, 11.3, and 7.2 lmol CO2 m-2 s-1; those of RD 0.8, 0.6, and 0.7 lmol CO2 m-2 s-1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in Asat and RD of 20?53% (n = 10?16 tree species per stand). Examining our data in the context of a pan-tropical Asat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based Asat decreases in tropical mountains by, on average, 1.3 lmol CO2 m-2 s-1 per km altitude increase (or by 0.2 lmol CO2 m-2 s-1 per K temperature decrease). The Asat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal Asat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered Asat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.

Abstract:

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH4), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH4 consumption (6.2 Tg CH4 year&#8722;1), limited data are available on CH4 exchange from tropical montane forests. We present the results of an extensive study on CH4 exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH4 sinks, with decreasing annual uptake rates from 5.9 kg CH4?C ha&#8722;1 year&#8722;1 at 1,000 m to 0.6 kg CH4?C ha&#8722;1 year&#8722;1 at 3,000 m. Topography had no effect on soil atmospheric CH4 uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH4 uptake rates, mineral nitrogen content of the mineral soil and with CO2 emissions indicated that the largest CH4 uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH4 uptake was N limited instead of inhibited by NH4 +. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH4 budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests.