Jürgen Homeier

Mapping tree density at a global scale

The global extent and distribution of forest trees is central to our understanding of the terrestrial biosphere. We provide the first spatially continuous map of forest tree density at a global scale. This map reveals that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these trees, approximately 1.30 trillion exist in tropical and subtropical forests, with 0.74 trillion in boreal regions and 0.66 trillion in temperate regions. Biome-level trends in tree density demonstrate the importance of climate and topography in controlling local tree densities at finer scales, as well as the overwhelming effect of humans across most of the world. Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization.

Tropical Andean Forests Are Highly Susceptible to Nutrient Inputs—Rapid Effects of Experimental N and P Addition to an Ecuadorian Montane Forest

Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha−1 yr−1) and P (10 kg ha−1 yr−1). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes—notably aboveground productivity—are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs.

Land-cover classification in the Andes of southern Ecuador using Landsat ETM+ data as a basis for SVAT modelling

A land‐cover classification is needed to deduce surface boundary conditions for a soil–vegetation–atmosphere transfer (SVAT) scheme that is operated by a geoecological research unit working in the Andes of southern Ecuador. Landsat Enhanced Thematic Mapper Plus (ETM+) data are used to classify distinct vegetation types in the tropical mountain forest. Besides a hard classification, a soft classification technique is applied. Dempster–Shafer evidence theory is used to analyse the quality of the spectral training sites and a modified linear spectral unmixing technique is selected to produce abundancies of the spectral endmembers. The hard classification provides very good results, with a Kappa value of 0.86. The Dempster–Shafer ambiguity underlines the good quality of the training sites and the probability guided spectral unmixing is chosen for the determination of plant functional types for the land model. A similar model run with a spatial distribution of land cover from both the hard and the soft classification processes clearly points to more realistic model results by using the land surface based on the probability guided spectral unmixing technique.

Deforestation and Forest Fragmentation in South Ecuador since the 1970s – Losing a Hotspot of Biodiversity

Deforestation and fragmentation are major components of global change; both are contributing to the rapid loss of tropical forest area with important implications for ecosystem functioning and biodiversity conservation. The forests of South Ecuador are a biological ‘hotspot’ due to their high diversity and endemism levels. We examined the deforestation and fragmentation patterns in this area of high conservation value using aerial photographs and Aster satellite scenes. The registered annual deforestation rates of 0.75% (1976–1989) and 2.86% (1989–2008) for two consecutive survey periods, the decreasing mean patch size and the increasing isolation of the forest fragments show that the area is under severe threat. Approximately 46% of South Ecuador’s original forest cover had been converted by 2008 into pastures and other anthropogenic land cover types. We found that deforestation is more intense at lower elevations (premontane evergreen forest and shrubland) and that the deforestation front currently moves in upslope direction. Improved awareness of the spatial extent, dynamics and patterns of deforestation and forest fragmentation is urgently needed in biologically diverse areas like South Ecuador.

Arbuscular endomycorrhizas are dominant in the organic soil of a neotropical montane cloud forest

The ecology and forest management of tropical montane cloud forests of the Neotropics have attracted little scientific attention so far, and understanding of the ecosystem is still fragmentary (Churchill et al . 1995). The montane forests of southern Ecuador are an outstanding biodiversity hotspot for vascular plants (Barthlott et al . 1996), including a multitude of tree species (Madsen & Ollgard 1994). However, identification of species, studies of phenology, productivity, seed production and growth of seedlings have only just begun (Homeier & Breckle 2002). None of the trees has been investigated with respect to their mycorrhizal status, although it is well established that mycorrhizal symbioses are not only important for survival of trees in nutrient-poor habitats (Read 1991) and play a key role for nutrient cycling and nutrient retention in the humus layers (Medina & Cuevas 1993, Rilling et al . 2001), but also have an important impact on the composition of plant communities (Allen et al . 1995, Kottke 2002, van der Heijden et al . 1998).

Caterpillars and Host Plant Records for 59 Species of Geometridae (Lepidoptera) from a Montane Rainforest in Southern Ecuador

Abstract During four months of field surveys at the Reserva Biológica San Francisco in the south Ecuadorian Andes, caterpillars of 59 Geometridae species were collected in a montane rainforest between 1800 and 2800m altitude and reared to adults. The resulting data on host plant affiliations of these species was collated. The preimaginal stages of 58 and adult stages of all 59 species are depicted in colour plates. Observations on morphology and behaviour are briefly described. Five species, documented for the first time in the study area by means of larval collections, had not been previously collected by intensive light-trap surveys. Together with published literature records, life-history data covers 8.6% of the 1271 geometrid species observed so far in the study area. For 50 species these are the first records of their early stages, and for another 7 the data significantly extend known host plant ranges. Most larvae were collected on shrubs or trees, but more unusual host plant affiliations, such as ferns (6 geometrid species) and lichens (3 geometrid species), were also recorded. Thirty-four percent of the caterpillars were infested by wasp or tachinid parasitoids.

Is tropical montane forest heterogeneity promoted by a resource-driven feedback cycle? Evidence from nutrient relations, herbivory and litter decomposition along a topographical gradient

Summary 1. Ridges of tropical mountains often differ strikingly from neighbouring ravines in terms of forest structure, productivity and species composition. This heterogeneity is poorly understood despite its critical role in biodiversity maintenance, carbon and nutrient budgets. 2. We examined measures of tree biomass and productivity, foliage and litter quality (nutrient concentrations, specific leaf mass, phenolics), herbivory and leaf litter decomposition in each six plots laid out in upper and lower slope position in a tropical montane moist forest in southeastern Ecuador. 3. Productivity, quality of foliage and litter as well as herbivory were significantly lower in upper slope position, and closely correlated with soil nutrient concentrations and accumulated humus. The decomposition of upper slope leaf litter (decomposition rate k) was substantially lower than in litter from lower slope forest, whereas the site of decomposition (slope position) only had a marginal effect on the decomposition rate. 4. Our results suggest that the differences in stand structure, productivity, foliar quality, herbivory and decomposition between slope positions are ultimately due to stronger nutrient limitations in upper slope forest. We propose a general conceptual model that explains origin and maintenance of contrasting forest types along topographical gradients through down-slope fluxes of nutrients and water, and a nutrient-driven positive feedback cycle.

Diversity patterns of ferns along elevational gradients in Andean tropical forests

Background: Biodiversity is perceived to decline from lowlands towards mountain peaks and away from the Equator towards the Poles, but supporting data for most groups of organisms are lacking, especially at the local scale. Aims: Elevational gradients of fern species richness in tropical forest habitats were analysed to test the hypotheses that fern species composition patterns were similar between elevational gradients, that total species richness of complete elevational gradients gradually declined due to changing climatic conditions with increasing distance from the Equator, and that the elevation of highest species richness gradually declined with increasing latitude. Methods: We used plot-based elevational fern species richness surveys compiled from recent field work and own published studies, between 18° N and 18° S along the Andean mountain range, and compared the transects of patterns of species richness and composition, and distribution of taxa. Results: Taxonomic composition was highly similar among tropical regions. Elevational richness patterns were symmetrically hump-shaped and overall richness was virtually equal along most of the tropical latitudinal gradient. In contrast, the subtropical localities at the edges of our study area, ca. 18° N and 18° S, showed strikingly different patterns from those in the tropical zone. Conclusions: Within the tropics, there appears to be no latitudinal diversity gradient of ferns at the local scale. We suggest that, in tropical habitats, species richness of ferns at the local scale is limited by the number of species that can co-occur and that the available niche space is saturated.

Mycorrhiza Networks Promote Biodiversity and Stabilize the Tropical Mountain Rain Forest Ecosystem: Perspectives for Understanding Complex Communities

To better understand the mechanisms behind maintenance of the extraordinary plant and fungal diversity in tropical mountain forests we applied, for the first time, network theory to investigate the mycobiont–plant communities. We addressed three different mycorrhizal classes, arbuscular mycorrhizae of tropical trees, mycorrhizae of terrestrial and epiphytic Orchidaceae and cavendishioid mycorrhizae among Ericaceae and Sebacinales. We found significant nestedness (NODF) for arbuscular and orchid mycorrhizal networks. In accordance to previous simulations and verifications of species-rich, mutualistic plant–animal networks, we conclude that preferential attachment of new members to already existing links integrates and maintains rare species and stabilizes our species rich assemblages.

Plant Growth Along the Altitudinal Gradient — Role of Plant Nutritional Status, Fine Root Activity, and Soil Properties

In tropical montane forests, aboveground net primary productivity (ANPP ) usually decreases with increasing altitude. Besides low photosynthesis (Kitayama and Aiba 2002) and direct impact of low temperatures on plant growth (Hoch and Korner 2003), low ANPP at high altitudes has often been attributed to nutrient limitation (Bruijnzeel et al. 1993; Bruijnzeel and Veneklaas 1998; Tanner et al. 1998). Plant growth is often correlated with nutrient availability in tropical montane forests. For example, the exceptionally high tree stature in a montane forest stand in Papua New Guinea was attributed to its nutrient rich soil parent material (Edwards and Grubb 1977). In montane forests of Jamaica (Tanner et al. 1990), Hawaii (Vitousek and Farrington 1997; Vitousek et al. 1993), and Venezuela (Tanner et al. 1992), trunk diameter growth and leaf production of several native tree species were enhanced by addition of N or P. The nutritional status of plants is governed by the amounts of chemically available nutrients in soil and the ability of fine roots for nutrient acquisition. The ability for nutrient acquisition comprises the spatial exploitation of the soil by roots and nutrient uptake activity. Chemical nutrient availability in tropical montane forests may be affected by parental substrate, weathering intensity, cation exchange capacity, the rates of litter decomposition, or extracellular phosphatase activity (Treseder and Vitousek 2001; Kitayama and Aiba 2002; Wilcke et al. 2007). Spatial nutrient availability is dependent on the exploitation of soil by roots or mycorrhizal hyphae and the mobility of the respective nutrient in soil. High abundance of mycorrhizal fungi contributes to high spatial availability of nutrients in the organic surface layer (Treseder and Vitousek 2001; Haug et al. 2004; Kottke et al. 2004). Also fine root abundance in the organic layers is generally very high (Hertel et al. 2003). Unfavourable soil conditions such as shallow mineral soils (Ostertag 2001), oxygen deficiency (Santiago 2000), low nutrient concentrations (Cavalier 1992), and low pH (Godbold et al. 2003) may cause a superficial distribution of fine roots , and may impair the physiologically based ability of roots for nutrient uptake in deeper soil layers.