3 search hits
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Effects of land-use changes on the properties of a Nitisol and hydrological and biogeochemical processes in different forest ecosystems at Munesa, south-eastern Ethiopia
(2004)
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Yeshanew Ashagrie
- The effects of conversion of natural forest into different exotic tree species plantations and crop cultivation were investigated at Munesa, south-eastern Ethiopia with the objectives of (i) determining changes on soil physical and chemical properties, (ii) quantifying water and nutrient fluxes under the different forest ecosystems, and (iii) assessing nutrient dynamics in water flowing through the soil under the different forest ecosystems. Soil samples were taken from the organic layer and at 0-20, 20-40, 40-70, 70-100 cm depths from the mineral soil. Rainfall and throughfall were collected using plastic funnels mounted 1 m above the ground. Soil solutions were collected with zero-tension (organic layer) and tension (mineral soil at the depth of 20, 50 and 100 cm) lysimeters. After 26 years of cultivation, surface (20 cm depth) soil structure was deteriorated and total soil organic carbon (SOC) and N contents both in bulk soil and water stable aggregates were significantly reduced. Below 21 years old Eucalyptus plantation no significant changes on the above mentioned parameters could be identified, but significant reductions in SOC, N and S concentrations associated with the sand and silt separates were evident. There were also significant reductions both in quality and quantity of particulate organic matter (POM) due to cultivation and only in quality of POM due to 21 years Eucalyptus plantation. The organic layer mass under 21 years old Pinus patula, 21 years old Eucalyptus globulus and third rotation Eucalyptus globulus (established 42 yr ago) decreased by 43%, 57% and 15%, respectively, relative to the natural forest. There were also significant reductions in the organic layer C and N stocks (9 to 60% and 25 to 68%, respectively), being highest under Pinus and lowest under third rotation Eucalyptus. In the mineral soil, to 1 m depth, there was a significant (P<0.05) reduction (16 to 20%) in SOC stock after conversion of natural forest into forest plantations. The N stocks under the 21 years old Pinus and third rotation Eucalyptus plantations were significantly reduced amounting 27 and 20% respectively, whereas 21 years old Eucalyptus had nearly an equivalent amount of N as that of the natural forest, probably due to a dense forest floor vegetation, fixing N. The changes in the organic layer and mineral soil S stocks after plantation establishment were not significant. Of the total annual rainfall (1190 mm) recorded during the monitoring period (October 2001 to September 2002), about 47% and 18% were intercepted by the canopies of Cupressus and the natural forest, and Eucalyptus, respectively. Total annual nutrients (Ca, Cl, K, Mg, Na, NH4–N, NO3–N, PO4–P, SO4–S ) deposition by rainfall was 12 kg ha–1yr–1. Throughfall K, Mg, Ca and Cl fluxes were enriched relative to rainfall, whereas Na, NO3–N, NH4–N, PO4–P and SO4–S were depleted. Total annual throughfall nutrient inputs (Ca, Cl, K, Mg, Na, NH4–N, NO3–N, PO4–P, SO4–S) were 14 kg ha–1yr–1 under Cupressus, 21 kg ha–1yr–1 under the natural forest and 24 kg ha–1yr–1 under Eucalyptus. Water passing through the different forest floors differed only in K, Mg and NO3–N concentrations, the latter two being higher under the natural forest and Eucalyptus plantation than Cupressus. Potassium was greater under Eucalyptus than the natural forest and Cupressus. Except for NH4–N in the natural forest, forest floor leachate nutrient concentrations were enriched in all forest types in relation to throughfall. Most nutrient fluxes to the mineral soil decreased in relation to throughfall fluxes, whereas NO3–N fluxes increased by over 50% in all forest types. At all soil depths, the concentrations of most nutrients in the mineral soil solution decreased relative to the concentrations in the forest floor leachate, but Mg, Na and NO3–N at all depths in Cupressus plantation and SO4–S and Na at some soil depths in the natural forest and Eucalyptus plantation had increased. The vertical trends in soil solution nutrient concentrations showed a decreasing trend with depth increments for most of the nutrients, but the concentrations of Cl and Na in all forest types and Ca, Mg and NO3–N in Cupressus increased with increasing soil depth. At 1 m soil depth, the concentrations of Ca, Mg and NO3–N in Cupressus, respectively, were 8, 7 and 23 times higher than in the natural forest and 3, 4 and 81 times higher than in Eucalyptus indicating losses by leaching. Generally, the results of this study emphasize the importance of forest type, species composition and management in affecting carbon and nutrient storage, water and nutrient fluxes and dynamics.
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Biomass and Nutrient Studies of Selected Tree Species of Natural and Plantation Forests: Implications for a Sustainable Management of the Munessa-Shashemene Forest, Ethiopia
(2004)
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Asferachew Abate
- Plantation forests with exotic tree species have been introduced to alleviate the problems of deforestation in Ethiopia. In the future, more plantation forests with fast growing species should be grown for coping with the ever-increasing demands for fuelwood and other forest products. However, it is not known whether plantation forests are sustainable or not. For the sustainability of plantation forests with exotic tree species, it is of paramount importance to thoroughly understand their ecological and social attributes through a holistic approach. For this reason, a multidisciplinary project was initiated in the Munessa-Shashemene Forest. Such an approach gives valuable information about the sustainability of plantation forests when the basic ecological features of the natural forests are compared with plantation forests. As an integral part of the multidisciplinary project, the objectives of this study are to: 1) quantify the fine roots and aboveground biomass of selected tree species in both natural and plantation forests; 2) quantify the macronutrient stocks of the fine roots and aboveground components of selected trees species in both natural and plantation forests; and 3) evaluate the implication of the changes in the biomass and macronutrient stocks for a sustainable management of forests. The study focused on four tree species, Podocarpus falcatus (Thunb.) Mirb., Podocarpaceae and Croton macrostachys Hochst. ex Del. Euphorbiaceae, were selected from a natural forest. Cupressus lusitanica Miller, Cupressaceae and Eucalyptus globulus Labill. Myrtaceae were selected from plantation forests. Root architectures of the study trees were studied by excavation. The live fine root biomass (<2 mm in diameter) of the dry and wet seasons was determined from samples collected at the distances of 1, 2 and 3 meters from the bole of the study trees. At each of the distances, root cores were taken at the depth intervals 0-10, 10-35, 35-60, 60-85 and 85-100 cm using a hand auger. Linear regression equations were used to estimate the aboveground biomass on the basis of the relation between DBH and dry weights of the aboveground plant components. Macronutrient concentrations were determined following a standard laboratory procedure. Studies on the root architecture revealed that C. lusitanica has a shallow root and is more susceptible to windthrow compared to E. globulus. With the exception of E. globulus, the dry season live fine root (LFR) biomass was higher for all trees studied. The seasonal variation in the fine root biomass was attributed to the changes in soil moisture of the study area. For all trees investigated, the mean annual LFR biomass was highest at the depth interval 0-10 cm at all distances. The favorable soil texture, pH and organic matter content at the depth interval 0-10 cm might be responsible for higher LFR biomass. The significantly higher LFR biomass of P. falcatus (1.34 kg m-2) coupled with its higher macronutrient stocks compared to C. macrostachys (0.32 kg m-2) suggest the importance of P. falcatus in the sustainability of the natural forest by transferring more macronutrients to the soil through its fine roots. Similarly, the significantly higher total LFR biomass of C. lusitanica (0.88 kg m-2) coupled with its higher macronutrient stock compared to E. globulus (0.27 kg m-2) indicated less depletion of soil nutrients by the former. The stand structure of the natural and plantation forests differed largely. In the natural forest, the density of C. macrostachys was much higher (143 ± 72 trees ha-1) than the density of P. falcatus (73 ± 39 trees ha-1). Generally, the structural change of the natural forest due to selective cutting of P. falcatus was found to have negative implications on the sustainability of the natural forest. The differences in the structure of C. lusitanica and E. globulus, despite their similar densities, resulted in a significantly lower understory ground cover by herbaceous and shrub species in the former. The effect of a poor understory growth on the floor litter thickness and thereby on nutrient capital of the soil may negatively affect the sustainability of C. lusitanica plantation. The harvesting of the stemwood of C. lusitanica and E. globulus removes a substantial amount of nutrients from the plantation sites. Furthermore, the current practice of collecting foliage, twigs and branches for firewood by the local people results in a higher depletion of nutrients. In order to make the plantation forests sustainable, the silvicultural practice in the future should consider on site conservation of foliage and bark. It is recommended that more studies on aboveground and belowground biomass, fine root turnover, and nutrient concentrations of the plantation forests should be carried out in a chronosequence in order to gain more insight on their sustainability.
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Direkte Messung und Bewertung des nebelgebundenen Eintrags von Wasser und Spurenstoffen in ein montanes Waldökosystem
(2004)
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Thomas Wrzesinsky
- Der nebelgebundene Eintrag von Wasser und Spurenstoffen kann in den Bergwäldern Mitteleuropas eine wichtige Rolle spielen. Die Quantifizierung dieses Eintrags stieß jedoch in der Vergangenheit auf messtechnische Grenzen. Nach der Entwicklung und Erprobung eines Systems aus einem Tropfenspektrometer zur schnellen Messung der Größenverteilung (40 Tropfengrößenklassen zwischen ø1,5 und 50 µm) im Nebel und einem Ultraschallanemometer zur Bestimmung des vertikalen Windes konnten an der Ökosystemmessstation „Waldstein“ von April 2001 bis März 2002 Messungen zur Nebelwasserdeposition durchgeführt werden. Zusätzlich wurden die Sichtweite und die chemische Zusammensetzung (pH, elektrische Leitfähigkeit, Na+, K+, NH4+, Mg2+, Ca2+, Cl–, NO3–, SO42– und PO43–) des Nebelwassers gemessen. Zur Sammlung von Nebelwasser wurde ein aktiver beheizbarer Nebelsammler entwickelt und parallel zu den Wasserflussmessungen eingesetzt. Die Proben wurden automatisch alle acht Stunden genommen. Die Sammelmengen betrugen im Median 249 ml und erlaubten die gewünschten chemischen Analysen. Im Untersuchungszeitraum waren 223 Nebeltage zu verzeichnen. Der Nebelanteil betrug 25,7 %. Für die Qualitätskontrolle der gemessenen Flüsse wurden die Daten auf Stationarität und Turbulenz überprüft und der Datensatz entsprechend angepasst. Die Messung der Nebeldeposition im Untersuchungszeitraum ergab einen Eintrag von 108 kg ha–1 a–1 Wasser für die turbulente Deposition und 17 kg ha–1 a–1 für den Eintrag über Sedimentation. Der turbulente Eintrag dominiert also mit ca. 86 % die Nebeldeposition. Die Summe aus beiden Eintragsarten entspricht einem Nebelniederschlag von 125 mm p. a. Eine klare Saisonalität der Nebelwasserflüsse ist erkennbar. Die höchsten Nebelniederschläge sind im Spätherbst und im Winter zu verzeichnen, monatlich bis zu 24 mm (Januar) wurden gemessen. Die geringste Nebeldeposition wurde im August mit ca. 1 mm gemessen. Die ermittelten Tropfenspektren zeigen bei der Anzahlverteilung Maxima bei 2, 6 und 9 µm sowie ein Maximum von 12 µm in der Massenverteilung. Für die Massengrößenverteilung sind Verteilungen mit Maxima bei 9, 12 und 15 µm häufig. Die gemessenen Flüssigwassergehalte lagen bei einem Median von 156 mg m–3 und erreichten Maxima von 2639 mg m–3 (5-min-Mittel). Den größten Anteil am Fluss hatte die Größenklasse von 14,5 bis 15,5 µm Tropfendurchmesser. Tropfen kleiner 7 µm wurden effektiv emittiert, die größeren deponiert. Die im Untersuchungszeitraum gefallene Menge an Regen und Schnee beträgt 1414 mm. Der Anteil des Nebels am atmosphärischen Eintrag von Wasser beträgt demzufolge etwa 8 %. Für insgesamt 253 Nebelereignisse wurden im Untersuchungszeitraum Proben gewonnen. Außerdem wurden zum Vergleich auf wöchentlicher Basis wet-only-Proben genommen. Die Konzentrationen in Nebel- und Regenwasser sind hoch variabel. Die Mediane liegen im Nebelwasser bei pH 4,14, 621 µeq l–1 für NH4+, 487 µeq l–1 für NO3– und 321 µeq l–1 für SO42–. Diese 3 Hauptionen machen ca. 87 % der Gesamtkonzentration aus. Die Konzentrationen im Nebelwasser sind deutlich gegenüber dem wet-only-Niederschlag erhöht. Die Anreicherungsfaktoren sind 18,1 (NH4+), 13,1 (NO3–) bzw. 11,5 (SO42–). Der nebelgebundene Eintrag der wichtigsten Ionen wurde aus der Konzentration und dem Nebelwasserfluss errechnet. Die eingetragenen Mengen sind 9,8 kg ha–1 für NH4+ (7,9 kg ha–1 für wet-only), 27,9 kg ha–1 für NO3– (25,1) bzw. 14,0 kg ha–1 für SO42– (15,0). Die durch feuchte oder okkulte Deposition eingetragene Menge ist für diese Ionen also im gleichen Größenbereich wie die Menge aus Regen und Schnee. Der Stickstoffeintrag beträgt insgesamt 13,9 kg N ha–1 a–1 (11,8 für wet-only). Der im Unterschungszeitraum durch den Bestandesniederschlag gemessene Eintrag von Stickstoff liegt bei 23,3 kg N ha–1 a–1. Die Differenz aus Bestandesniederschlag einerseits und wet-only und Nebel andererseits liegt mit –0,9 kg N ha–1 a–1 nahe Null. Zusätzliche Einträge sind durch die trockene Deposition (z. B. durch partikuläres Nitrat und Salpetersäure) zu erwarten. Der Umsatz von Stoffen im Kronenraum spielt dann eine wichtige Rolle in der Schließung der Ökosystembilanz für die verschiedenen Stoffe.
