Tropical montane forests in the Andes are subjected to deforestation and subsequent transformation into pastures. Abandoned pastures are frequently reforested by planting monoculture timber plantations, resulting in reduced aboveground diversity and changes in soil characteristics compared to primary forests. In this study, we evaluated differences in soil properties (litter layer thickness, pH, water content and C-to-N ratio) between degraded primary montane forest and monoculture pine (Pinus patula) and alder (Alnus acuminata) plantations and their effect on density, diversity and community structure of litter and soil living mesofauna, with focus on oribatid mites (Acariformes). The study was performed in a montane region in the southern Ecuadorian Andes (2000-2600 m a.s.l.). C-to-N ratios in the litter and upper 5 cm soil layer were higher in pine plantations, while other soil characteristics were similar between vegetation types. Surprisingly, microbial biomass and density of soil mesofauna in the litter layer did not differ between vegetation types, while density and species richness of oribatid mites were higher in pine plantations. Community structure of oribatid mites differed between vegetation types with only few species overlapping. The results indicate that quality and diversity of litter were not the major factors regulating the mesofauna community. Instead, soil animals benefit from increased habitat structure in thicker litter layers and potentially increased availability of root-derived resources. Overall, the results suggest that from a soil animal perspective, monoculture plantations are less detrimental than commonly assumed and enrichment of abandoned plantations with native tree species may help to restore tropical montane forests.

Twelve study sites were selected in the Loja valley between 2400 and 2600 m a.s.l.: (1) four pine and (2) four alder plantations, and (3) four degraded forests (Figure 1). In each forest fragment/plantation, one experimental plot of 24 x 24 m was established. Within each plot four samples were taken at randomly selected points, resulting in a total of 48 sampling points for each investigated parameter. We used a soil corer 5 cm in diameter. Each core was divided into O horizon (litter layer) and Ah horizon (5 cm of mineral soil underneath the litter layer), which were analysed separately for all parameters, except Oribatida species numbers, which were only assessed in the litter layer.

For measuring carbon (C) and nitrogen (N) concentrations, an aliquot of both the litter and mineral soil was milled to powder;  3-4 mg were analysed for C and N concentrations using an elemental analyser (Vario EL III, elementar, Hanau, Germany) (Butenschoen et al., 2014; Marian, Sandmann, Krashevska, Maraun, & Scheu, 2017). Water content in both layers was measured from aliquots dried at 60°C for two days.

Samples for microbial parameters were placed in plastic bags, stored at 4°C and transferred to Germany. Microbial basal respiration (BR) and substrate-induced respiration (SIR) were determined by measuring O₂ consumption using a computer controlled O₂ microcompensation apparatus (Scheu, 1992). Prior to measurement, an aliquot of leaf litter was cut into pieces < 0.5 cm² and the soil was sieved through 2 mm mesh and mixed thoroughly. Before measuring, the samples were rested for 4 days at room temperature to avoid including increased basal respiration caused by cutting of leaf litter and homogenization. Moist samples equivalent to 0.2 (litter) and 2 (soil) g dry weight were supplemented with glucose equivalent to 80 (litter) and 8 (soil) mg g⁻¹ dry weight, and BR was measured for 24 h with readings taken every hour. Microbial biomass (C_mic) was calculated from the maximum initial respiratory response (MIRR; μl O₂ g⁻¹ dry weight h⁻¹) as C_mic = 38 × MIRR (Anderson & Domsch, 1978; Beck et al., 1997; Joergensen & Scheu, 1999).

Mobile soil animals were extracted by heat within 24 h after sampling (Kempson, Lloyd, & Ghelardi, 1963). Extracted animals were stored in 70% ethanol and transferred to Germany. Most animal groups were determined to order, suborder or cohort level using Schaefer (2010) except for adult oribatid mites, which were determined to species level or sorted into morphospecies.

ReadMe: Abbreviations and explanations for the dataset

Environmental data and fauna

forest_type: samples were taken in three forest types.

1. alder: Monoculture plantations of Andean alder (Alnus acuminata Kunth) plantations
2. pine: Monoculture plantations of pine (Pinus patula Schiede & Deppe)
3. degraded_forest: remnants of degraded primary tropical montane rainforest

forest.ID: Identification number assigned to each forest fragment used as a random factor during statistical analysis.

sampling_plot: each sampling plot within each forest fragment was assigned a number between one and four

layer: layers into which the sampled soil cores were separated (O horizon (litter layer) and Ah horizon (5 cm of mineral soil underneath the litter layer))

water.cont: given as percentage of total fresh weight

Ctotal: total percentage of carbon (C) in the soil/litter samples, given in percentage [%]

Ntotal: total percentage of nitrogen (N) in the soil/litter samples, given in percentage [%]

C/N ratio: ratio of carbon to nitrogen in the soil/litter samples

BR: Microbial basal respiration [µg O2*h-1*g soil dw-1]

Cmic: Microbial biomass [µg Cmic*g soil dw-1]

total_fauna: Abundance of all movable soil fauna (individuals per soil core)

Oribatida, Collembola, Mesostigmata: abundance of the respective soil fauna group (individuals per soil core)

Oribatida_species

forest_type: samples were taken in three forest types.

4. alder: Monoculture plantations of Andean alder (Alnus acuminata Kunth) plantations
5. pine: Monoculture plantations of pine (Pinus patula Schiede & Deppe)
6. degraded_forest: remnants of degraded primary tropical montane rainforest

forest.ID: Identification number assigned to each forest fragment used as a random factor during statistical analysis.

sampling_plot: each sampling plot within each forest fragment was assigned a number between one and four

Nr.: Number for internal classification system of morphospecies. Not relevant for data analysis

Sex: Reproductive mode of the respective oribatid mite species

1. parth: parthenogenetic reproduction
2. sex: sexual reproduction