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

Abstract:

The “hierarchy of factors” hypothesis states that decomposition rates are controlled primarily by climatic, followed by biological and soil variables. Tropical montane forests (TMF) are globally important ecosystems, yet there have been limited efforts to provide a biome-scale characterization of litter decomposition. We designed a common litter decomposition experiment replicated in 23 tropical montane sites across the Americas, Asia, and Africa and combined these results with a previous study of 23 sites in tropical lowland forests (TLF). Specifically, we investigated (1) spatial heterogeneity in decomposition, (2) the relative importance of biological factors that affect leaf and wood decomposition in TMF, and (3) the role of climate in determining leaf litter decomposition rates within and across the TMF and TLF biomes. Litterbags of two mesh sizes containing Laurus nobilis leaves or birchwood popsicle sticks were spatially dispersed and incubated in TMF sites, for 3 and 7 months on the soil surface and at 10–15 cm depth. The within-site replication demonstrated spatial variability in mass loss. Within TMF, litter type was the predominant biological factor influencing decomposition (leaves > wood), with mesh and burial effects playing a minor role. When comparing across TMF and TLF, climate was the predominant control over decomposition,but the Yasso07 global model (based on mean annual temperature and precipitation) only modestly predicted decomposition rate. Differences in controlling factors between biomes suggest that TMF, with their high rates of carbon storage, must be explicitly considered when developing theory and models to elucidate carbon cycling rates in the tropics.

Abstract:

In nutrient-poor tropical forests litter decomposition is essential for the supply of nutrients to plants. This process is in turn controlled by nutrient availability. Beside nitrogen (N) and phosphorus (P) other elements may co-limitate litter decomposition. Studies revealed a relationship between sodium (Na) shortage and reduced litter decomposition which can decrease the mineralization of stored carbon (C). Investigations of element fluxes of a Na-poor tropical montane rainforest in southern Ecuador have shown low atmospheric deposition rates since 1998. Additionally, Na was retained in different parts of the ecosystem. A study found that the Na-retention in the canopy was related to the Na demand of the microorganisms in the phyllosphere. Because the Na budget of the organic layer was also positive, it was assumed that decomposers may experience a lack of Na due to low atmospheric deposition. Thus, one main goal was to investigate whether Na fertilization accelerates litter decomposition. Based on the assumption of a higher Na demand of soil fauna than of microorganisms, it was also tested whether the stimulation of soil organisms by Na is limited to the soil fauna. To test these hypotheses, a combined litter decomposition and Na fertilization experiment was conducted in an evergreen premontane rainforest and a montane rainforest in south Ecuador. Fresh litter was incubated in the field and fertilized weekly with different Na concentrations which were similar to the natural deposition rates. Litterbags with different mesh sizes allowed to study the effects of Na fertilization on microbes and mesofauna separately. Additionally, a bait lamina experiment was installed in the organic layer to study the uptake of Na fertilized baits by soil organisms. Differences of the bait loss between the fertilized and the control group should indicate a potential stimulation of the soil organisms by a Na-addition. The statistical analysis confirmed the stimulation of litter decomposition by Na additions. The comparison of both sites revealed a slightly stronger effect of Na additions in the premontane forest than in the montane forest. The accelerated litter decomposition was related to an increased activity of the soil fauna. However, a significant stimulation of microorganisms by Na additions could not be detected. The bait lamina experiment confirmed the results of the litter decomposition experiment and showed a marginally significant increase of the activity of soil organisms by Na fertilization. Sodium can stimulate the decomposition of the organic layer in the studied premontane and montane rainforest, thus affecting ecosystem functioning. Na supply is of higher importance in the ecosystem than usually assumed and its relevance for the C turnover in the organic layer may increase if the atmospheric Na deposition remains on the present low level.

Abstract:

Recent studies raise the hypothesis that Na shortage restricts decomposition and affects the carbon cycle in tropical forests. When Na concentrations in soils are low and the stands are far off-coast, they do not receive substantial Na inputs from the atmosphere. Since terrestrial plants have low concentrations of Na, which is not considered as an essential element, the demand of soil fauna may not be covered. Yet, in contrast to animals, little is known of Na demands of fungi and phyllosphere microorganisms. We present results from a study on Na limitation in a montane forest ecosystem in South Ecuador, which is located on the eastern cordillera of the Andes. We tested the hypotheses that (1) the study area is characterized by low Na concentrations because of low deposition rates with incident precipitation (wind directions mainly from the Amazonian Basin), (2) decomposition processes are limited by fauna and fungal Na restrictions and (3) Na is retained in the canopy because of Na limitation of microorganisms in phyllosphere. Since 1998, we measure Na fluxes in rainfall, throughfall, stemflow, litter leachate, litterfall and organic layer in a microcatchment under an undisturbed lower montane rainforest. Results reveal comparatively low Na concentrations in the ecosystem and similar Na concentrations in throughfall and stemflow. Since Na fluxes are lower with throughfall than with incident rainfall, we conclude that Na is retained in the canopy. To explore the role of the phyllosphere in Na retention we sampled leaves covered by phyllosphere microorganisms and leaves without phyllosphere cover from several tree species, which were sprayed with a NaCl solution containing 0.5 mg L-1 Na, corresponding to the Na concentration in incident rainfall in our study area. Additionally, responses of litter decomposition to Na additions and the involved interaction of soil fungi and fauna were tested in a litterbag experiment at two sites (1000 and 2000 m a.s.l.). Results revealed enhanced decomposition rates following Na additions, though only in the presence of soil fauna. These results might have future ecosystem implications, since our time series showed that total Na deposition decreased within the past 15 years from ca. 40 kg ha-1 a-1 to 10 kg ha-1 a-1, suggesting a potential role of Na in regulating ecosystem processes.

Abstract:

Tropical montane ecosystems of the Andes are critically threatened by a rapid land-use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico-chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico-chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land-use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf-shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land-use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems.

Abstract:

Previous research from the tropics indicates that AMF may be well adapted to organic soils and even represent the dominant mycorrhizal form, though the extraradical part of the symbiosis was omitted as in most other tropical studies. Our study aims at characterizing the extraradical part of arbuscular mycorrhizal fungi (AMF) in a highly organic tropical montane forest soil in Southern Ecuador. Based on recent studies on the interaction of AM fungal hyphae and litter we hypothesized that within the organic layer AM hyphae grow in close contact with decomposing material. To test this idea, AM fungal hyphal distribution in the organic layer was determined by directly staining roots and decomposing leaves and extracting hyphae from the remaining particulate organic material. AM and non-AM fungal hyphae were analyzed, as well as root colonization patterns. Our results showed that AMF indeed represented the dominant mycorrhizal form with an average root colonization of 43%. The extraradical AM hyphal length ranged from 2 to 34 m g?1 soil with a mean of 10.4 m g?1 soil (equals 3.1 m cm?3 soil), and therefore exceeded root length about 13-fold. As hypothesized, 29% of AM extraradical hyphae were closely attached to decomposing leaves. These hyphae were mainly located at the leaf surface, though in some parts leaf veins and inner leaf tissues were colonized. More than half of AM hyphal biomass was detected on the root surface, a pattern potentially driven by the predominant Paris-type AMF. Non-AM fungal hyphae colonized decomposing material to a significantly greater extent, though hyphal length attached to roots was equal. This study supports the adaptation of AMF to highly organic soils in the tropics and the existence of a widespread extraradical mycelium, which is not readily detectable by standard methods. The close association with decomposing leaves most likely improves direct nutrient uptake from decomposed material and points to a potential indirect contribution of AMF to the decomposition process.