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

Abstract:

Ideas on the spatial variation of biodiversity often imply a causal link between the abundance and species richness of organisms. We investigated this ?more individuals hypothesis? using light-trapping data of three unrelated groups of moths (Arctiidae, Geometridae and Pyraloidea) from the Ecuadorian Andes. We analyzed environmental correlates of specimen densities found in different habitats, finding effects of temperature, moonlight, forest succession, elevation and season. We corrected abundance data for light-trapping artefacts, and we measured species diversity with various metrics known to be unbiased by undersampling. We found significant positive correlations between abundance and species diversity for all three taxonomic groups. We discuss implications for a general evaluation of species-energy theory as well as for a better understanding of ecological processes in montane habitats of the Andes.

Abstract:

To improve our knowledge of the influence of land-use on solute behaviour and export rates in neotropical montane catchments we investigated total organic carbon (TOC), Ca, Mg, Na, K, NO3 and SO4 concentrations during April 2007?May 2008 at different flow conditions and over time in six forested and pasture-dominated headwaters (0.7?76 km2) in Ecuador. NO3 and SO4 concentrations decreased during the study period, with a continual decrease in NO3 and an abrupt decrease in February 2008 for SO4. We attribute this to changing weather regimes connected to a weakening La Nina event. Stream Na concentration decreased in all catchments, and Mg and Ca concentration decreased in all but the forested catchments during storm flow. Under all land-uses TOC increased at high flows. The differences in solute behaviour during storm flow might be attributed to largely shallow subsurface and surface flow paths in pasture streams on the one hand, and a predominant origin of storm flow from the organic layer in the forested streams on the other hand. Nutrient export rates in the forested streams were comparable to the values found in literature for tropical streams. They amounted to 6?8 kg ha−1 y−1 for Ca, 7?8 kg ha−1 y−1 for K, 4?5 kg ha−1 y−1 for Mg, 11?14 kg ha−1 y−1 for Na, 19?22 kg ha−1 y−1 for NO3 (i.e. 4.3?5.0 kg ha−1 y−1 NO3-N) and 17 kg ha−1 y−1 for SO4. Our data contradict the assumption that nutrient export increases with the loss of forest cover. For NO3 we observed a positive correlation of export value and percentage forest cover.

Abstract:

The evaporative loss of intercepted water from the canopy constitutes an important element of the water budget of forests. Starting April 1998, incident precipitation (P), throughfall (TF), and stemflow (SF) were measured in five transects laid out in three small watersheds (~10 ha each) with lower montane rain forest at 1900?2200 m.a.s.l. in South Ecuador. Interception loss (I) was also modeled using the analytical model of Gash (1979). The storage capacity of the leaves and of the trunks and branches, as well as the direct throughfall, and stemflow fractions were determined using conventional regression approaches. In addition, apparent total evaporation (ET) was determined from the water budget for the three watersheds. Mean annual P in the first 4 years ranged between 2363 and 2592 mm among the three watersheds. Average I derived from weekly measurements of P, TF, and SF ranged between 2.0 and 3.5 mm/day (i.e. 32?50% of P). Modeled average I was similar to measured values at 2.1?3.4 mm/day (32?49% of P). We found that I constituted an important part of the average estimated watershed ET of 3.5?4.3 mm/day. The high evaporative losses are attributed to a combination of low rainfall intensities, the usual absence of fog, high canopy density, abundant epiphytes, and advected energy from lower elevations.

Abstract:

Canopy structure and throughfall (TF) were determined in three different forest types within the tropical montane rain forest belt in southern Ecuador. Heterogeneity of TF amounts and selected nutrient concentrations were compared to heterogeneity of canopy structure and tree species diversity. Canopy structure was characterized using hemispheric images and software calculating radiation beneath the canopy, mean leaf angle, and canopy openness. TF was sampled over a 1-year period (November 2001–November 2002), and analyzed for pH, electric conductivity, potassium, calcium, and magnesium. Radiation penetrating through the canopy ranged between 9.7% and 17.2% and gap fractions between 6.1% and 9.5% in the respective forests. At 71%, 85%, and 91% of incident precipitation, TF differed significantly between the three forest types, although standard deviations (SD) were high. The highest heterogeneity in TF (as represented by SD) was found for the forest type with the greatest heterogeneity in canopy structure, and vice versa. Heterogeneity of element concentrations in TF (again represented by their SD) exhibited strong correlations (r2 ¼ 0.912–0.987) with tree species diversity per forest as expressed by the Shannon–Wiener diversity index. Rates of nutrient leaching from seven tree species were determined experimentally. Amounts of elements leached differed between species, and specific patterns were observed per species. These findings suggest that higher tree diversity leads to greater complexity in leaching patterns and to greater heterogeneity in TF nutrient composition. Water storage capacities of single leaves (SL) of 11 tree species were determined experimentally and ranged from <30 ml m'2 of leaf surface area to >200 ml m'2, depending on species and leaf angle. Combining the mean forest leaf area index (5.37 m2 m'2) with the average SL (70.7 ml m'2) and an overall mean leaf angle of 46# gave a theoretical maximum water storage capacity of 0.38 mm for the foliar component of the canopy.

Abstract:

Die Interaktion von Pflanzen und Tieren trägt zur Komplexität eines Waldökosystems entscheidend bei. Die Diversität der Arten und die Vielfalt der Prozesse, die von der Zersetzung über die Bestäubung bis hin zu den Nährstoffzyklen reichen, um nur einige zu nennen, lässt diese Komplexität nur erahnen. Die Verteilung der durch die Krone erzeugten Muster des Eintrags von Licht, Wasser und Nährstoffen als Nischen bildende Parameter soll Gegenstand dieser Arbeit sein. Hierzu wurde die Nährstoffauswaschung aus den Blättern (Kap. 1.1), die Verteilung und Zusammensetzung des Bestandesniederschlags als Teile des Nährstoffzyklus (Kap. 1.2), sowie die Kronenstruktur (Kap. 1.3) und die Verteilung der Nährstoffe in den oberen Bodenschichten (Kap. 1.4) genauer untersucht. Eine Betrachtung der Auswirkungen der unterschiedlich heterogenen Eintragsmuster an Licht, Wasser und Nährstoffen für die Keimlinge im Wald wird am Ende der Arbeit diskutiert (Kap. 6.6). Die konkreten Arbeitshypothesen sind in Kapitel 3 zusammengestellt. Weiterhin wird in Kapitel 2 eine Standortbeschreibung gegeben, in Kapitel 4 die in dieser Arbeit verwendeten Methoden beschrieben und in Kapitel 5 die Ergebnisse dieser Arbeit dargestellt. Einleitend sollen die einzelnen Themenblöcke näher dargestellt werden. Die Nährstoffauswaschung aus den Blättern ist schon oft untersucht worden, wie in Kapitel 1.1 detailliert aufgeführt wird. Die ausgewaschenen Nährstoffe werden mit dem Bestandesniederschlag weitertransportiert. Auf die Bedeutung des Bestandesniederschlags für den Wasserund Nährstoffkreislauf und die Faktoren, die Auswirkungen auf den Bestandesniederschlag haben, wird in Kapitel 1.2 näher eingegangen. Ob die im Bestandesniederschlag angereicherten Nährstoffe den Waldboden in spezifischen Mustern erreichen, ist abhängig von der Verteilung des Bestandesniederschlags. Diese Verteilung sollte von der Kronenstruktur bedingt sein ebenso wie die Verteilung des Lichts. Beide Annahmen sollen in dieser Arbeit geprüft werden. Da die Untersuchung der Kronenstruktur nur einen Teil dieser Arbeit darstellt, musste eine wenig aufwendige Methode verwendet werden, mit der eine ganze Reihe von Kronenstrukturparametern erfasst werden konnte. Die hemisphärische Photographie ist eine solche Methode. Mit der Kronenstruktur im Allgemeinen und mit der Methode der hemisphärischen Photographie befasst sich das Kapitel 1.3. Es soll gezeigt werden, dass die Verteilung des Bestandesniederschlags, also von Wasser und Nährstoffen, und des Lichts von der Kronenstruktur bestimmt wird. Eine heterogene Kronenstruktur müsste also heterogene Bedingungen am Waldboden erzeugen; eine homogene Kronenstruktur homogene Bodenbedingungen. Welche Bedeutung heterogene Bodenbedingungen für das Wachstum und die Etablierung von Keimlingen hat, wird in Kapitel 1.4 basierend auf der Literatur erläutert. Schließlich wird in Kapitel 1.5 auf die verschiedenen Skalenebenen dieser Arbeit aufmerksam gemacht und als Voraussetzung für die Wahl der richtigen Methodik hergeleitet, wie hoch die Anzahl der Messwiederholungen bzw. Parallelmessungen in einem zu erwartenden sehr heterogenen System sein müsste.

Abstract:

The number of previous hydrological studies concerning water flow paths in tropical montane rainforest is small. However, due to the increasing pressure of deforestation and land use change comprehensive knowledge of these natural ecosystems is needed if sustainable land use strategies should keep negative effects of human impacts on water flow paths as low as possible. In this context, present work addresses the identification, characterisation, and modelling of water flow paths in soils of an undisturbed and landslide affected natural Andean forest ecosystem in the south of Ecuador whose deforestation rate is one of the highest in South America. In an investigation area situated in the Andes of South Ecuador, in gentler slopes and altitudes above 2100 m ASL mainly Stagnosols and Histosols with stagnic colour pattern and low to negligible rock fragment content prevail. With increasing altitude the abundance of these soils increase, while the presence of Cambisols and Regosols is most pronounced below 2100 m ASL and clearly correlated with the slope angle. Therefore, these soils were mainly encountered in steeper, particularly landslide affected sites often resembling a melange of fine soil and high contents of rock fragments. Aside the investigation of the influence of the rock fragment content on soil hydrological- and physical parameters such as the relationship between rock fragment content and saturated hydraulic conductivity of the soil, present study aims particularly to investigate flow paths of water in soils of landslide affected and unaffected hillslopes. Therefore, we employed conventional field- and laboratory methods, dye tracer experiments including an appropriate image processing technique, as well as statistical models. Results show that both rock fragment content and bulk density control significantly, but not largely the saturated hydraulic conductivity of the mineral soils. Dye tracer experiments and soil parameters document a deeper percolation in the landslide affected hillslopes than in the landslide unaffected hillslopes, where we found preferential flow in root channels with low soil matrix interaction as dominant flow mechanism. A surface near quasi impervious layer along the interface between topsoil and subsoil limits percolation of the water giving the prerequisites of a lateral shallow subsurface flow along the interface between topsoil and organic layer. This is in line with previous studies performed in the same investigation area which already proved indirectly the existence of this flow. However, in none of these studies the shallow subsurface flow was assigned to certain slope inclinations or altitudes. Due to a recently published digital soil map and the results we obtained from the landslide unaffected sites, we know that particularly in hillslopes of less than 30 ° above 2100 m ASL prerequisites are given for spatially extended shallow subsurface flow. However, even if these prerequisites are not evident for the landslide affected hillslopes, we cannot exclude the possibility of shallow subsurface flow occurrence here since soil cover of the steep terrain is relatively shallow while rainfall is high throughout the year. Therefore, and given that key parameters such as permeability of subsoil and bedrock, interception and evaporation remain unclear or were investigated exclusively such as the spatial variability of the saturated hydraulic conductivity, we conducted a series of virtual experiments in order to assess the potential occurrence of shallow subsurface flow in Cambisols below 2100 m ASL. In these experiments we also included the organic layer being highly abundant in the investigation area, whose hydraulic parameters were estimated by means of inverse numerical modelling. The virtual experiments were based on a two dimensional finite element model representing a steep forested hillslope transect of ~54 m length. Aside soil properties, evapotranspiration and interception, the model included the spatial variability of the saturated hydraulic conductivity, the pressure head and their spatial trends. The results of virtual experiment series show that a sound evidence of the key parameters aforementioned is obligate if process conceptualisation regarding shallow subsurface flow generation, but also landslide initiation, solute and matter transport is in the spotlight.

Abstract:

The diversity of geometrid moths (Lepidoptera: Geometridae) was investigated in two tropical montane cloud forests in southern Ecuador and in central Costa Rica. The study covered an elevational range from c. 1000 to 2700 m.a.s.l. in both locations. Data were analyzed for eight sites, representing four elevations in each of the two study areas. A total of 770 species (4569 specimens) were sampled in Ecuador vs. 503 in Costa Rica (7303 specimens). Sampling was incomplete and it is expected that more species will be found in both areas. Moth diversity was extremely high in Ecuador and almost invariably higher than at comparable elevations in Costa Rica. Values of Fisher?s a index ranged between 70 and 131 in Ecuador, and between 31 and 83 in Costa Rica. An analysis of rarefied species numbers led to very similar results. At a level of 390 rarefied specimens, the range was 131?179 species at the Ecuadorian sites, vs. 77?140 at the Costa Rican sites. Only 64 (5.3%) out of the total of 1209 species were common to both areas. The results of this study underline the fact that the tropical Andean mountains are one of the ?hot spots? of local diversity of geometrid moths, and that the threatened Andean cloud forests should be given the highest priority in conservation policy. Similarly, countries like Costa Rica, which are already actively engaged in nature conservation, may find additional motivation in the present results to continue their efforts.

Abstract:

The formation of nocturnal convective clouds at the eastern Andes of south Ecuador and the adjacent Peruvian Amazon basin was investigated in a numerical model study. Their formation is expected to be an interactive procedure of nocturnal downslope flows in the Andean terrain, which forms a concave drainage system in the target area. Satellite imagery were used for both the identification of a sample case with a nocturnal cold cloud appearance and for the verification of the simulated results. The cloud patterns were distinguished on the basis of IR temperatures. A comparison of the data demonstrated the occurrence of a cold cloud shield in the target area, although the modeled cluster is significantly smaller. Further analysis of the development of the convective cells confirmed the assumed underlying processes. A strong current in the lower atmosphere, presumably a drainage flow, was recognizable in association with strong moisture convergence using a cross section through the cluster. Their presence was confirmed on the basis of their characteristic features and the surface energy fluxes as the driving force for thermally induced downslope flows.

Abstract:

As part of an interdisciplinary research programme, the spatial and temporal variability of precipitation in southern Ecuador has been investigated since January 2002. The study site is located at the northern margin of the Podocarpus National Park in the vicinity of Loja, about 500 km south of Quito, at altitudes ranging from 1800 to 3200 m.a.s.l. Due to its low density, the conventional rainfall station network fails to register the highly variable spatial distribution of rain, whereas contributions by fog are not accounted at all. Hence, for the first time in a tropical montane forest setting, a weather radar was used, covering a radius of 60 km and reaching from the Amazon Basin to the coastal plains of the region. Furthermore, a network of sampling stations supplies data about the altitudinal gradient of fog and rainwater inputs. The precipitation distribution in the study area proves to be far more variable than previously thought and is strongly coupled to the orographic characteristics and the special topographical setting of the landscape. Maxima in precipitation occur especially in the eastern parts of the radar range on slopes exposed to advected moisture from the Amazon Basin, whereas the highest crests of the Andes receive less precipitation. The study area has two cloud condensation levels, occurring at 1500?2000 and 2500?3500 m.a.s.l., respectively. At 1800?2000 m.a.s.l., fog is estimated to contribute an additional input of 5% of conventionally measured rainfall, increasing to about 35% at the highest measurement station (3200 m.a.s.l.). In contrast to some other tropical mountains, there seems to be no maximum zone of water input, although the gradient remains positive up to the highest altitudes. The unusual precipitation distribution is thought to reflect the contrasting climatological influences operating in the study area.

Abstract:

As part of an interdisciplinary research program, the spatial and temporal variability of precipitation and associated nutrient inputs in southern Ecuador have been investigated since January 2002. The study site is located at the northern margin of the Podocarpus National Park in the vicinity of Loja, about 500 km south of Quito, at altitudes ranging from 1800 to 3180 m.a.s.l. Due to its low density, the conventional rainfall station network fails to register the highly variable distribution of rain whereas fog is not accounted for at all. Hence, a new measurement infrastructure had to be installed. For the first time in a tropical montane forest setting, a Weather Radar was used, covering a radius of 60 km and reaching from the Amazon Basin to the coastal plains of the region. Furthermore, a dense network of sampling stations provided data about the altitudinal gradient of fog water inputs and the chemical properties of the different precipitation types. This combined approach provided important information on the formative processes of rain events on the eastern escarpment of the Andes. Rainfall distribution proved far more variable than previously known and strongly coupled to the orographic characteristics of the landscape. Maxima occurred especially on the exposed mountain slopes in the eastern parts of the Radar range, whereas the highest crests of the Andes received less precipitation. The study area has two cloud condensation levels, occurring at 1500?2000 m and 2500?3500 m.a.s.l., respectively. Fog was estimated to provide an additional 5?35% of water to conventionally measured rainfall. As with rainfall, fog capture exhibited a marked altitudinal gradient. Precipitation samples were analyzed for their main chemical properties to calculate spatial gradients as well as temporal trends of nutrient inputs. Although average matter fluxes were fairly low, episodic events contributed relevant amounts to the overall ecosystem nutrient budget.

Abstract:

The Andes of Ecuador show an extreme heterogeneity of spatial and temporal distribution of precipitation. The existing operational network of the national weather service is not capable of reproducing these complex patterns. By using a cost-efficient rain radar and a network of high-resolution rain gauges, the real complexity of the rainfall distribution and the meteorological processes of rainfall formation can be assessed. A blendingmethod encompassing geostatistical tools allows to derive a comprehensive rainfall climatology for the study area. Precipitation is predominantly of the advective type, associated with humid air masses from the Amazon basin transported by the tropical easterlies. The typical form is light to heavy drizzle with long duration but lower rain rates. However, in contrast to former knowledge there is no single mechanism of rain formation for any given place. Several processes interact like small and large-scale convective cloud systems, local and regional valley/mountain breeze systems and terrain-lines of preferred moisture transport interact on various time scale. This leads to complex patterns of rainfall in space and time. Several types of characteristic weather situations are revealed by the study. They are characterized by specific combinations of local and regional atmospheric processes and interactions with the topographical configuration. They are modified by mesoscale and continental circulation patterns as the annual shift of pressure cells, the east Andean low-level Jet and katabatic flows.

Abstract:

The optical properties (reflectance and transmittance) of selected leaves from a tropical mountain rain-forest in southern Ecuador are determined to parametrize optical traits of plant functional types (PFT)of a state of the art land model (Community Land Model, CLM). 46 spatially dominating species are selected from 4 different forest types, the subpáramo and a succession stage of pasture areas representing ecologically predefined functional types within the study area. Measurements are conducted under a standardized experimental setup with a field spectrometer covering the radiation between 305 and 1305 nm. The results of the optical properties of all species are checked for similarity by cluster analysis and are compared to the composition of species of the predfined PFTs. Furthermore the results are compared to other studies, the default values for the globally defined PFT of tropical evergreen trees in the CLM and another forest growth model operated in the same study area. The results show that the clusters aggregated by the reflectance, transmittance or combined properties do not represent the predefined PFTs. The values of the other studies suggest a reassessment of the experimental setup for the transmittance measurements. Nevertheless, new reflectance values for the regionalized PFTs can be determined. The optical values differ from the CLM-PFT of tropical evergreen trees, and new values for the reflectance are recommended.

Abstract:

Cloud forests are one of the most species rich ecosystems in the world, but also one of the most endangered. Despite their importance they are deforested at an overwhelming rate all over the world. The scarce knowledge about these fragile systems definately is a hindrance for effective conservation management. This work therefore aims at improving the knowledge of tropical cloud forest streams and of the effects of land-use change on stream water quality.

Abstract:

Tropical montane forests are frequently located on steep slopes with pronounced differences in topographic exposure, related microclimatic conditions and hence in composition and structure of the vegetation over small distances. The objective of this work was to test the hypothesis that topographic position significantly influences soil fertility and water flow in these forests. Soil properties were determined at various topographic positions and water samples of selected ecosystem fluxes analyzed over a 1-year period for oxygen isotopes in three small, steep watersheds under lower montane forest in the Eastern Cordillera of the Andes in southern Ecuador. The soils are subject to lateral material movement (landsliding and solifluction). This, together with the pronounced variation in climatic conditions and vegetation over small distances, resulted in high heterogeneity of soil properties. The pH of the A-horizon ranged between 3.7 and 6.4; concentrations of base metals (calcium, magnesium), sulfur and phosphorus, and trace metals (manganese, zinc) showed enormous spatial variation (coefficient of variation: 358?680% over a surface area of <30 ha). The steepness of the study area and the large contrast in hydraulic conductivities of the organic layer and the mineral soil resulted in a hillslope flow regime dominated by fast lateral flow. During baseflow conditions, d18O values were similar to that of the subsoil solution, but rapidly became similar to values in the top-soil solution during rain storms. The chemical composition of stormflows resembled that of the litter leachate. Stormflow had lower pH and higher organic carbon and metal concentrations than did baseflow. It is concluded that topographic position and lateral transport of water and matter (as a consequence of the pronounced inclination) are important controls of the water and nutrient cycles of the study forest.

Abstract:

The evaporative loss of intercepted water from the canopy constitutes an important element of the water budget of forests. Starting April 1998, incident precipitation (P), throughfall (TF), and stemflow (SF) were measured in five transects laid out in three small watersheds (~10 ha each) with lower montane rain forest at 1900?2200 m.a.s.l. in South Ecuador. Interception loss (I) was also modeled using the analytical model of Gash (1979). The storage capacity of the leaves and of the trunks and branches, as well as the direct throughfall, and stemflow fractions were determined using conventional regression approaches. In addition, apparent total evaporation (ET) was determined from the water budget for the three watersheds. Mean annual P in the first 4 years ranged between 2363 and 2592 mm among the three watersheds. Average I derived from weekly measurements of P, TF, and SF ranged between 2.0 and 3.5 mm/day (i.e. 32?50% of P). Modeled average I was similar to measured values at 2.1?3.4 mm/day (32?49% of P). We found that I constituted an important part of the average estimated watershed ET of 3.5?4.3 mm/day. The high evaporative losses are attributed to a combination of low rainfall intensities, the usual absence of fog, high canopy density, abundant epiphytes, and advected energy from lower elevations.

Abstract:

We analyzed the suitability of a combined sampling approach ? consisting of visual search and branch-beating ? for quantifying tropical caterpillar communities. Surveys were conducted in the Ecuadorian montane forest zone, with two shrub species from the genus Piper serving as focal targets. We sampled 160 shrubs in the course of four experiments following a standardized sampling protocol. Subsequently each shrub was completely defoliated accompanied by an intensive leaf-by-leaf search, in an effort to extract as close to 100% of all present caterpillars as possible. We analyzed the resulting dataset with regard to completeness, taxonomical bias, and influences of daytime, complexity of shrub structure, or experience of the researcher. The standardized sampling protocol extracted between 50.6% and 71.6% of the caterpillars present on a shrub. A minor taxonomic bias of the sampling protocol was observed, but appears to be of a simple and predictable nature, and is therefore easy to account for. We did not find any significant influences of daytime. Structure and size of shrubs had a strong influence on sampling results with small and simply structured shrubs being sampled most completely, large and complex shrubs most incompletely in our dataset. Researcher experience did not appear to have an influence on the sampling efficiency or taxonomic composition of samples obtained when we compared caterpillars obtained by standardized sampling with those collected by exhaustive leaf-by-leaf search. Comparison of caterpillar sizes revealed however, that inexperienced field assistants tended to overlook large fractions of the smallest caterpillars entirely. We conclude that our standardized combined sampling approach is fairly suitable for studies concerning caterpillar communities, especially when resampling of the same shrub individuals is desired.

Abstract:

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3?N, NH4?N and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 during the passage of rain water through the ecosystem and bulk d15N values in soil to detect N transformations. Depletion of 15N in NO3 and increased NO3 fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 concentrations progressively decreased and were enriched in 15N but did not reach the d15N values of solid phase organic matter (d15N = 5.6?6.7%). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the d15N value of NO3 in litter leachate was smaller (d15N = -1.58%) than in the other quarters (d15N = -9.38 ± SE 0.46%) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3-N from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.

Abstract:

Bracken (Pteridium spec.) is one of the most wide-spread weeds, especially where fire has been used for forest clearing or maintenance of agricultural areas, in particular of pastures. Taxonomically, it is considered an aggregate that separates into a northern hemispherical and a southern, tropical complex. The taxonomic ranks of the members of these complexes is still a matter of debate. Different from the extensively studied northern bracken, the knowledge of ecology and control measures of the tropical species is still fragmentary. A research team funded by the German Research Foundation (DFG) is working in an ecosystem study in a narrow valley of the eastern range of the South Ecuadorian Andes. Part of this ecosystem are pastures, replacing the natural rain forest. These pastures are in part heavily infested by bracken and many of them have been already abandoned for this reason. Several groups of the research team work on various aspects of the bracken invasion, e.g. on ecophysiology of bracken, soils and microclimate of the study area, mycorrhiza, and on insect interactions with bracken. The main aims of my thesis were (1) identification and population structure of bracken, (2) ecology of tropical bracken with particular emphasis on its survival of bushfires, and (3) development of an effective bracken control strategy and subsequent re-pasturisation of abandoned areas. The bracken vegetation of the study area consists of mixed stands of Pteridium arachnoideum (KAULF.) MAXON and P. caudatum (L.) MAXON with a proportion of 3:2, and of a hybrid (ca. 2%). Identification was by leaf morphology, allozyme analysis, comparison of particular chloroplastic DNA sequences, and analysis of four genomic and one plastidic microsatellites. Dominance of P. arachnoideum was explained by the fact that P. caudatum, as a lowland species, reaches its upper altitudinal limit in the research area. Analysis of heterozygosity indicated a higher genetic stability of the diploid P. arachnoideum population as compared to the allotetraploid P. caudatum population. Spatial extension of the individual clones is apparently much smaller than reported for members of the northern bracken, indicating higher significance of sexual reproduction for the tropical fern in comparison to vegetative propagation by rhizome fragmentation. Four weeks after burning the natural rain forest, vigorously sprouting bracken sporophytes were observed. These developed from gametophytes, which germinated from the wind dispersed spores. Fast growth of the young sporophytes established the fern in the areas. After planting pasture grass, bracken was supported by repeated burning of the areas. In the long run, the grass was outcompeted by the fern possibly due to weakening of its vitality by burning and grazing, and the areas have been abandoned. The density of bracken fronds in a settled bracken area of our research site remained constant over years with only small deviations caused by particular weather situations. Since this balance holds also for patchy fern canopies, it is assumed that this is due to nutrient shortage of the soil. Most probably, a new leaf can only develop from the nutrients remobilized from a senescing old leaf. Two to three months after a fire, an explosive emergence of new leaves was observed at rates, which substantially exceeded those under undisturbed growth. The newly formed leaves showed an extended life-span, which was attributed to a better nutrient supply from the ash. Subsequent self-thinning reduced the density of the leaves to a stable level within two years. In a laboratory experiment, the effects of heat pulse by a simulated bushfire on the bracken rhizomes were investigated. Separated long and short shoots were heated for a short time either in a water bath or embedded in soil. Subsequent to this heat pulse, they were cultivated in original soil. Short shoots showed a significantly higher heat resistance (up to 80°C) than long shoots (up to 60°C). In addition, the short shoots showed elongation growth and an enhanced frond production, whereas long shoots were not stimulated by the heat pulse. In a bracken control experiment, thirteen control measures (cutting of the fronds, several herbicides, covering with plastic foil and combinations thereof) were applied over a time-period of 23 months. Each treatment was repeated six times and the effects were recorded monthly. Quarterly cutting of the leaves as well as treatment with a customary herbicide mixture (picloram and metsulforon methyl) were the most effective treatments resulting in a reduction of the standing biomass by 65%. Monthly records of the resprouting bracken was necessary to work out the minimum number of treatments required for a clear control effect. For the five most efficient treatments among two to four applications were necessary. However, complete eradication of bracken was not possible. For re-pasturisation, the common pasture grass Setaria sphacelata was planted on the treated areas within a long-term experiment. After nearly two years of observation, the system had stabilized with a cover of S. sphacelata of 75% and of bracken of below 40%. This result demonstrated that the competitive strength of S. sphacelata was sufficient to control bracken once weakened by control treatments. The long-term experiment and, in addition, an experiment in which a gradually bracken-infested area is subjected to controlled burning, are continued.