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

Abstract:

Arbuscular mycorrhizae are important for growth and survival of tropical trees. We studied the community of arbuscular mycorrhizal fungi in a tropical mountain rain forest and in neighbouring reforestation plots in the area of Reserva Biológica San Francisco (South Ecuador). The arbuscular mycorrhizal fungi were analysed with molecular methods sequencing part of the 18S rDNA. The sequences were classified as Operational Taxonomic Units (OTUs). We found high fungal species richness with OTUs belonging to Glomerales, Diversisporales and Archaeosporales. Despite intensive sampling, the rarefaction curves are still unsaturated for the pristine forest and the reforestation plots. The communities consisted of few frequent and many rare species. No specific interactions are recognizable. The plant individuals are associated with one to ten arbuscular mycorrhizal fungi and mostly with one to four. The fungal compositions associated with single plant individuals show a great variability and variety within one plant species. Planted and naturally occurring plants show high similarities in their fungal communities. Pristine forest and reforestation plots showed similar richness, similar diversity and a significantly nested structure of plant-AMF community. The results indicate that small-scale fragmentation presently found in this area has not destroyed the natural AMF community, at least yet. Thus, the regeneration potential of natural forest vegetation at the tested sites is not inhibited by a lack of appropriate mycobionts.

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.

Abstract:

Increased nitrogen (N) depositions expected in the future endanger the diversity and stability of ecosystems primarily limited by N, but also often co-limited by other nutrients like phosphorus (P). In this context a nutrient manipulation experiment (NUMEX) was set up in a tropical montane rainforest in southern Ecuador, an area identified as biodiversity hotspot. We examined impacts of elevated N and P availability on arbuscular mycorrhizal fungi (AMF), a group of obligate biotrophic plant symbionts with an important role in soil nutrient cycles. We tested the hypothesis that increased nutrient availability will reduce AMF abundance, reduce species richness and shift the AMF community toward lineages previously shown to be favored by fertilized conditions. NUMEX was designed as a full factorial randomized block design. Soil cores were taken after 2 years of nutrient additions in plots located at 2000 m above sea level. Roots were extracted and intraradical AMF abundance determined microscopically; the AMF community was analyzed by 454-pyrosequencing targeting the large subunit rDNA. We identified 74 operational taxonomic units (OTUs) with a large proportion of Diversisporales. N additions provoked a significant decrease in intraradical abundance, whereas AMF richness was reduced significantly by N and P additions, with the strongest effect in the combined treatment (39% fewer OTUs), mainly influencing rare species. We identified a differential effect on phylogenetic groups, with Diversisporales richness mainly reduced by N additions in contrast to Glomerales highly significantly affected solely by P. Regarding AMF community structure, we observed a compositional shift when analyzing presence/absence data following P additions. In conclusion, N and P additions in this ecosystem affect AMF abundance, but especially AMF species richness; these changes might influence plant community composition and productivity and by that various ecosystem processes.

Abstract:

Studies in temperate systems provide evidence that the abundance of arbuscular mycorrhizal fungal (AMF) depends on soil nutrient availability, which is mainly explained in the context of resource stoichiometry and differential plant biomass allocation. We applied this concept to an understudied ecosystem – tropical montane forest – analyzing root and AMF abundance along an elevational gradient with decreasing nutrient availability, combined with responses to nitrogen (N) versus phosphorus (P) additions. At three sites from 1000 to 3000 m above sea-level we analyzed fine root length, AMF root colonization as well as extraradical AMF biomass (neutral lipid fatty acid 16:1?5, hyphal length and spore counts) in a nutrient manipulation experiment. We found a significant increase in root length as well as intra- and extraradical AMF abundance with elevation. Overall, P additions significantly increased, whereas N additions decreased AMF abundance, with differential though nonsystematic changes along the elevational gradient. Strongest effects were clearly observed at the intermediate site. These findings suggest a general dependency of roots and AMF on nutrient availability, though responses to N and P additions differed from previous studies in temperate systems. In the context of future nutrient depositions, results suggest diverging responses of AMF abundance depending on site characteristics.

Abstract:

An arbuscular mycorrhizal fungus (AMF) was isolated from a south Ecuadorian reforestation site in the vicinity of a pristine mountain rainforest ecosystem. Morphologically, it corresponded with the description of an AMF first described from Cuba as Gigaspora savannicola, before it was placed in Scutellospora. No living culture of this species has been available previously to allow detailed morphological review or DNA barcoding. After comparison of the morphology of the holotype and authenticated specimens with that of the Ecuadorian isolate, it was concluded that they are conspecific. A detailed redescription of the species is provided, including morphological characters not included in the original description, and an epitype is designated. To provide an extended DNA barcode for this AMF species, intraspecific variants of the near full-length small subunit rRNA gene (SSU) and a rDNA region comprising part of the SSU, the internal transcribed spacer (ITS) region including the 5.8S rRNA gene, and part of the large subunit rRNA gene (LSU) were sequenced. Phylotaxonomic analyses confirmed the classification in the genus Scutellospora sensu lato in a phylogenetic clade that, based on questionable evidence, was considered to circumscribe a distinct genus, Fuscutata. Recently, this genus was merged with Dentiscutata. Consequently, we move S. savannicola (=Fuscutata savannicola?=?Gigaspora savannicola) to the genus Dentiscutata (Gigasporaceae).