Nathalie Soethe

Nutrient availability at different altitudes in a tropical montane forest in Ecuador

We measured macronutrient concentrations in soils and leaves of trees, shrubs and herbs at 1900, 2400 and 3000 m in an Ecuadorian tropical montane forest. Foliar N, P, S and K concentrations in trees were highest at 1900 m (21.7, 2.2, 1.9 and 10.0 mg g −1 ). At 2400 and 3000 m, foliar concentrations of N, P, S and K were similar to nutrient concentrations in tropical trees with apparent nutrient deficiency, as presented in literature. Unlike foliar nutrient concentrations, the amounts of plant-available nutrients in mineral soil were not affected by altitude or increased significantly with increasing altitude. High C:N ratios (25:1 at 2400 m and 34:1 at 3000 m) and C:P ratios (605:1 at 2400 m and 620:1 at 3000 m) in the soil organic layer suggested slow mineralization of plant litter and thus, a low availability of N and P at high altitudes. Foliar N:P ratios were significantly higher at 2400 m (11.3:1) than at 3000 m (8.3:1), indicating that at high altitudes, N supply was more critical than P supply. In conclusion, the access of plants to several nutrients, most likely N, P, S and K, decreased markedly with increasing altitude in this tropical montane forest.

Altitudinal Changes in Stand Structure and Biomass Allocation of Tropical Mountain Forests in Relation to Microclimate and Soil Chemistry

In tropical montane forests, the decline of tree size with increasing elevation is a well recognized phenomenon (Lieberman et al. 1996; Raich et al. 1997). The decrease aligns with a continuous species shift from lowland forests, to lower, middle and upper montane forests (Gentry et al. 1995). Leaf area index (LAI) also decreases with elevation from lowland to upper montane forest (Kitayama and Aiba 2002). With respect to other structural and functional parameters such as plant biomass and productivity, however, only very limited data exist from tropical montane forests. Altitudinal changes in aboveground biomass and productivity were studied in transects in Malaysia (Kitayama and Aiba 2002), Hawaii (Raich et al. 1997), Puerto Rico (Weaver and Murphy 1990) and Jamaica (Tanner 1980), some of them covering only a few hundred meters of altitudinal distance. The data base is even more limited if belowground biomass is considered: for example, a combined assessment of aboveand belowground biomass in neotropical montane forests has been conducted in not more than 16 different stands so far, and only exceptionally included altitudinal transects. A better understanding of the causes of tree size reduction with elevation in tropical mountains is closely linked to information on altitudinal changes in biomass, carbon allocation and productivity of montane forests. Although numerous hypotheses focusing on climatic or edaphic constraints of tree growth have been formulated in order to explain this phenomenon (e.g. Bruijnzeel and Proctor 1995; Flenley 1995), all of them are eventually linked to carbon gain and allocation of the trees and their control by the environment. Thus, tree biomass and productivity data (see Chapter 17 in this volume) are of paramount importance. In this chapter, we present detailed aboveand belowground biomass data of an altitudinal transect study in the Ecuadorian Andes. Study aim was to analyze altitudinal changes in forest biomass and tree root/shoot ratio , and to relate them to possible underlying climatic and edaphic factors.

Carbon and nutrient stocks in roots of forests at different altitudes in the Ecuadorian Andes

Carbon and nutrient stocks in below-ground biomass have rarely been investigated in tropical montane forests. In the present study, the amounts of carbon, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium in root biomass were determined by soil coring and nutrient analysis in forests at three altitudes (1900, 2400 and 3000 m) in the Ecuadorian Andes. Root biomass increased markedly from 2.8 kg m −2 at 1900 m and 4.0 kg m −2 at 2400 to 6.8 kg m −2 at 3000 m. The contribution of coarse roots (> 2 mm in diameter) to total root biomass increased from about 70% at 1900 m to about 80% at higher altitudes. In fine roots (≤ 2 mm in diameter), concentrations of nutrients except calcium markedly decreased with altitude. Therefore, the nutrient stocks in fine roots were similar at 1900 m and 3000 m for nitrogen and sulphur, and were even lower at higher altitudes for phosphorus, potassium and magnesium. In coarse roots of Graffenrieda emarginata concentrations of nutrients were substantially lower than in fine roots, and were little affected by altitude. The data suggest that the importance of coarse roots for long-term carbon and nutrient accumulation in total plant biomass increases with increasing altitude.

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.

The Carbon Balance of Tropical Mountain Forests Along an Altitudinal Transect

Not much is known about the role of tropical mountain forests in the global carbon cycle. This chapter summarises a decade of research on C pools and C fluxes in Andean mountain forests of the San Francisco region along an elevation transect from 1,000 m to 3,000 m a.s.l. based on measurements in 5 (3) intensively studied stands at five elevations and supplementary data collected in additional 54 forest plots at three elevations covering different topographic positions at these altitudes. With ecosystem C pools in the range of 320–370 Mg C ha−1, these forests store equally large, or even larger, amounts of C than neotropical lowland forests, despite the decrease in aboveground biomass with elevation. Gross and net primary production (NPP) and net ecosystem production all decrease largely with elevation while fine root production seems to increase. Our results show that tropical mountain forests are playing an important, yet underestimated, role as C stores.

Biomass and productivity of fine and coarse roots in five tropical mountain forests stands along an altitudinal transect in southern Ecuador

Background: Data on below-ground production of tropical montane forests along elevation gradients are scarce. Aims: To determine fine, coarse and large root biomass and productivity along a 2000 m elevation transect. Methods: In five south Ecuadorian mountain forests along a transect from 1000 to 3000 m above sea level, fine (< 2 mm diameter), coarse (2–50 mm) and large root biomass (> 50 mm) were analysed by soil coring and excavation of soil pits. Fine root production was estimated synchronously by three different approaches (sequential soil coring, the ingrowth core method, and the mini-rhizotron technique). Coarse and large root production was estimated by recording diameter increment using dendrometer tapes. Results: Fine root biomass increased four-fold between 1000 and 3000 m; coarse and large root biomass doubled. The three approaches for estimating fine root production yielded highly divergent results, with the mini-rhizotron approach giving the most reliable data, and indicating a significant increase in fine root production with elevation. Conclusions: Our results indicate a marked carbon allocation shift from above- to below-ground towards higher elevations, which is probably a consequence of increasing nutrient limitation of tree growth with increasing elevation.