J. Sevink

Humus form development and hillslope runoff, and the effects of fire and management, under Mediterranean forest in NE-Spain

Study of a series of plots on acid to intermediate rocks under well preserved mediterranean type forests in NE Spain showed that soils have well developed mor or moder type humus forms. Mor type humus forms were observed in soils with a shallow lithic contact or an abrupt textural change, and consist of an ectorganic layer (L, Fq, H) abruptly overlying a generally water repellent Ae or E horizon. Moder type humus forms, with a gradual transition between the ectorganic layer (L, Fa and the Ah horizon, were found in soils with more favourable rooting and soil moisture conditions, indicating that these conditions have a strong control over humus form development. Quantities of organic matter (ectorganic layer + Ah/Ae) ranged from about 5 kg/m2 in mor to about 10 kg/m2 in moder humus forms, the difference being due to the presence of a well developed Ah horizon in the latter. Rainfall simulator experiments showed that slopes with mor type humus forms are likely to produce hillslope runoff during summer rain storms in spite of the relatively high storage capacity of the ectorganic layer. This is due to the hydrophobicity of the mineral top soil (if dry), which hampers infiltration. Furthermore, during the wet season the soils commonly have a perched water table, inducing saturated overlandflow. Slopes with moder type humus forms, on the contrary, are very unlikely to produce any hillslope runoff, because of the high storage and infiltration capacity of the soils. On slopes with mor type humus forms, traditional management (cutting of the undertsory) leads to erosion of the ectorganic layer and thus to a reduction of its storage capacity. Forest fires have similar effects, since they lead to the (partial) destruction of the ectorganic layer and destroy the existing vegetation. As evidenced by the rainfall simulation experiments, in both cases hillslope runoff will strongly increase, inducing erosion of the mineral soil, in particular after fire. On slopes with moder type profiles the effects of fire and traditional management will be far more limited, because of the high storage and infiltration capacity of these soils, and hillslope runoff is not likely to increase, if occurring at all.

Impact of pine plantations on soils and vegetation in the Ecuadorian high Andes

Abstract A comparative study on the impact of pine plantations on soil and vegetation development was conducted in the Ecuadorian Andes (3000–4000 m). Pine plantations of different ages under different types of management were compared with extensively grazed páramo grassland (the most common former land use) and natural forest (the formerly dominant vegetation in much of the life zone). No general impact of plantations was found, although some tendencies were identified that show that soils under pine plantations are drier and less organic. Moreover, the vegetation under pine plantations was similar to páramo grassland, though some examples of regeneration of Andean woody species were observed, as well as examples of plantations where understory was completely lacking. We concluded that the impact of pine plantations cannot be generalized but should be evaluated case by case while care is taken in implementing plantations until more knowledge is obtained about the effects on the ecosystem as a whole, especially considering their ecological importance.

Upper forest line reconstruction in a deforested area in northern Ecuador based on pollen and vegetation analysis

Pollen analysis of a 150-cm-long core from Pantano de Pecho (0°20′S, 78°37′W) at 3870 m altitude in the Ecuadorian paramo documents altitudinal migrations and the composition of the upper forest line prior to deforestation. Four successive radiocarbon dates of 293 ± 41 14 C y BP, 498 ± 40 14 C y BP, 626 ± 33 14 C y BP, and 729 ± 44 14 C y BP show that the record includes the last c. 730 radiocarbon y, corresponding to the last c. 660 calendar years (cal y BP). The natural upper forest line was at a minimum altitude of 3400-3500 m between c. AD 1290 to 1315 (zone 1), from c. AD 1315 to 1350 at 3500-3600 m (zone 2), from c. AD 1350 to 1640 at 3600-3700 m (zone 3), from c. AD 1640 to c. 1765 at 3750 m (zone 4), and from c. AD 1765 to the present at 3700-3650 m (zone 5). The most important taxa were Alnus, Hedyosmum, Miconia, other Melastomataceae, Gunnera and Solanaceae. Since c. AD 1350 Podocarpus was continuously present with low abundance, but possibly not close to the upper forest line. Rarer elements of the upper montane forest were Dodonaea, Myrsine, Weinmannia, Myrica, Myrtaceae, Sapium, Juglans, Piper, Euphorbiaceae and Rubiaceae. Human disturbance and deforestation are shown by the presence of Rumex, Spermacoce pollen and charcoal particles. We surveyed the vegetation composition from isolated forest patches located between 3650 m and 4300 m. TWINSPAN analysis indicates forest patches up to 3950 m have a similar floristic composition to closed forest below the upper forest line. We argue that this apparent similarity does not necessarily mean that the slopes between 3750 and 3950 m were covered by closed forest in the past.

Temporal and vertical changes in the humus form profile during a primary succession of Pinus sylvestris

The development of the humus form profile during a primary succession of Pinus sylvestris has been studied along chronosequences on dunes and in blow-outs. Attention was given to vertical variation within the humus form and how this changes during profile development. The mor-type ectorganic profile features marked vertical gradients of several soil attributes, while its constituting horizons show no or only small changes of chemical properties during succession. These changes in particular involve increasing calcium and nitrogen concentrations in the organic matter. After an initial high rate of organic matter accumulation in the successive organic horizons, these rates are strongly reduced, suggesting the attainment of a dynamic equilibrium within the time span of the chronosequences on dunes and blow-outs. Blow-outs differ from dunes in the sense that they have a lower amount of organic matter and a higher F/H ratio. This different ratio likely relates to microclimatic conditions less conducive to decomposition.An attempt is made to explain the vertical trends in terms of processes affecting the characteristics of the organic horizons. Main conclusions are that the development of the ectorganic profile results from a combined effect of decay dynamics, rhizosphere processes and atmospheric deposition, which cannot be unentangled quantitatively with the data available. Furthermore, the distinction between F and H horizons has morphological rather than chemical or ecological relevance, as major vertical changes occur within the F horizon.

Soil development in the coastal dunes and its relation to climate

Freely drained soils in humid climates are marked by rather rapid leaching, acidification and, eventually, podzolisation, because of of the high permeability and low acid buffering capacity of the sands. In cooler climates podzols may develop within a few centuries, whereas in warmer or less humid climates podzols require several milennia or longer periods to form. In dry climates soils contain little organic matter. Clay and soluble soil components accumulate and soil salinity may be high due to salt spray.If drainage is poor, organic matter tends to accumulate and in cooler climates peat formation becomes prominent. Soil salinity increases with increasing aridity.Effects of climatic changes can only be predicted in qualitative terms and concern changes in the pedogenic trends and in the rates of the processes described. It is suggested to execute comparative studies of well-dated soils in different climatic zones in order to quantify these effects.

Litterflow chemistry and nutrient uptake from the forest floor in northwest Amazonian forest ecosystems

Samples of the fraction of net rainfall passing through the forest floor collected at monthly intervals in four pristine forests in Colombian Amazonia, during the period between 1995–1997 were analysed for solute concentrations to estimate the element fluxes from the forest floor into the mineral soil and root nutrient uptake from these forest floors. Results were compared with inputs by throughfall, stemflow, litterfall and fine root decay. Element concentrations were tested for their relationship with litterflow amounts, rainfall intensity and length of the antecedent dry period and differences in element fluxes between ecosystems were assessed. Concentrations of elements in litterflow followed a similar pattern as those in throughfall, which indicates that element outputs from the forest floor are strongly related to those inputs in throughfall. In the forests studied, the average concentrations of elements as K, Mg, orthoP and the pH of the litterflow decreased relative to that in throughfall in most events, while the concentration of elements such as dissolved organic carbon, H, SO4 and Si increased in litterflow from these forests. Element concentrations in litterflow showed a poor correlation with variables such as litterflow amounts, rainfall intensity and antecedent dry period, except for K which showed a significant correlation (p>0.95) with analysed variables in all forests. Outputs were significantly different between forests (p>0.95); these fluxes, which particularly concerned cations, being the largest in the flood plain, while for anions outputs increased from the flood plain to the sedimentary plain. After adding the nutrient contributed by litter decomposition and fine root decay, the net outputs of main elements from the forest floors were still smaller than inputs by net precipitation (throughfall + stemflow) indicating that the litter layers clearly acted as a sink for most nutrients. Accordingly, the element balances confirm that the forest floors acted as a sink for nutrients coming in by throughfall, stemflow, litterfall and fine root decomposition. P, Mg and N appeared to be the most limiting nutrients and the forests studied efficiently recycled these nutrients.

Identification, distribution and characteristics of erosion sensitive areas in three different Central Andean ecosystems

Abstract Interactions between erosion processes and soil and vegetation were studied in the three main ecosystems (i.e. sub-Andean valley, high Andean plateau and inter-Andean valley) in the southern Bolivian Andes. Erosion features such as badlands, dunes and stripped bedrock are widespread and reflect different scales of erosion in time, space and magnitude. The extent of active erosion features has been attributed mainly to land use change but geology and climate are also factors. The principle objective of this study was to characterise Erosion Sensitive Areas (ESAs) or physiographical units that are sensitive to erosion. Soil and vegetation properties provided insight into the relative importance of grazing as opposed to geology and climate in explaining the current extent of active erosion features. Erosion features, soil surface characteristics, biomass and chemical and physical soil properties from 36 sites distributed over 15 representative physiographical units were determined. In the areas that are most sensitive to erosion, the biomass of palatable species, soil organic matter (OM) content and available water (AW) are small and grazing density is low or moderate. The areas with high grazing density have large biomass of palatable species and large organic matter and available water contents. The distribution of the ESAs coincides with a semi-arid climate, low biomass of short grasses and very erodible parent material. Current reductions in grazing intensity do not seem to have resulted in less erosion. This suggests that past grazing history has altered the sensitivity to erosion, and the resilience of certain areas, to such an extent that erosion rates are high despite current low grazing intensity. It also suggests that contemporary degradation processes are not driven by current grazing densities but by climatic and geomorphological factors.

Drift sands, lakes and soils: the multiphase Holocene history of the Laarder Wasmeren area near Hilversum, The Netherlands

A unique complex of multiphased Holocene drift sands and paleosols, with at least two lacustrine phases, was discovered during a major sanitation project in the Laarder Wasmeren area near Hilversum, the Netherlands. The complex could be studied in detail, highly facilitated by the excellent and large-scale exposure of the various deposits and soils. OSL dating was used to establish ages of the phases, where possible differentiating between time of deposition and time of burial by taking into account the potential effects of bioturbation. Pollen analysis served to reconstruct the vegetation during the various phases. A first minor phase of aeolian activity already started before approx. 5,000 BC, followed around 4,000 BC by a second phase and a rather massive third phase around 3,000 BC. After a long phase of soil formation, the latest, massive drift sand phase started around the 14th to 15th century. It clearly represents the classic drift sand phase that started in the Late Middle Ages in the Netherlands. Sand drifting followed on soil forming phases during which the vegetation became increasingly dominated by ericaceous plants and culminated in heathlands. The first three aeolian phases and associated heathlands are much older than generally assumed for heathland and drift sand to occur in the Netherlands. Moreover, podzolisation was found to have started very early, true podzols already occurring before 4,000 BC. Around 3,000 BC groundwater in the area reached a maximum altitude of about 230 cm +NAP, resulting in local open water in the area. This rise is probably linked to the development of the Dutch coastal area, where at that time peat accumulated and drainage was poor, inducing a rise of the groundwater level in ‘het Gooi’. This groundwater level fell later on, to never reach this altitude again. The Groot Wasmeer was formed by local stagnation on a slowly permeable podzol and already reached a level of 320-325 cm +NAP by 400 BC, which more or less equals its 20th century level. The results demonstrate that earlier concepts on the occurrence and age of aeolian phases, podzols and heathland vegetations in the Netherlands are far too schematic, and that early, pre-agricultural cultures may already have had an impact on the stability of fragile cover sand landscapes, e.g. through burning. Results are in line with those from several contemporary studies on early prehistoric cultures and their impact in river dune areas in the Central and Eastern Netherlands.

The relevance of humus forms for land degradation in mediterranean mountainous areas.

In the Gavarras (NE Spain), a large number of plots on respective schists, leucogranite and granodiorite was studied for their soils and vegetation. Results were used to check conclusions from earlier studies of Mediterranean forest soils (mostly shallow Regosols and Cambisols) on such acidic to intermediate rocks. They confirmed that the humus form depends on catenary position and lithology, and that aggregate stability and infiltration characteristics of the upper mineral soil horizon relate to humus form type. Aggregate stability of the topsoil was found to be relatively high in mor and mull type humus forms, but differences with moder type humus forms were not statistically significant. Differences in aggregate stability are attributed to the presence of stable humus-clay-iron complexes in mulls and to high fungal activity and organic matter content of mors. Low infiltration rates were only encountered in topsoils with mor type humus form, in line with results from the earlier studies. In deeper soil horizons with low organic matter content, aggregate stability will be largely related to soil reaction and base saturation. On leucogranite and granodiorite, these were found to vary strongly, most probably largely due to local differences in fast acid neutralizing capacity (ANC f ). These local differences are primarily attributed to differences in the mineralogical composition and texture of the soil material, connected with differences in lithology and/or brought about by erosion, colluviation and soil formation. Consonant with earlier studies, it is concluded that the susceptibility of these forest soils to erosion largely depends on properties of the upper mineral soil horizon, which are controlled by or related with humus form development. General trends in the latter are clear and can be used to predict this susceptibility. In the case of land degradation, which implies a more severe erosion, deeper soil horizons are also involved. Spatial variability in properties of these horizons, relevant for degradation, is considerable and not clearly related to humus form development. Accordingly, the abovementioned trends in humus form development cannot be used to predict the susceptibility to land degradation.

Pedogenesis by clay dissolution on acid, low-grade metamorphic rocks under mediterranean forests in southern Tuscany (Italy)

Pedogenesis on acid, low-grade metamorphic rocks under well-developed forests was studied in the meso-mediterranean climatic zone. To that purpose three representative soil profiles on phyllite and sandstone under sclerophyllous oak, pine and heathland were selected. The major soil forming processes consisted of transformation of part of the K-mica (muscovite) to (hydroxy-interlayered) vermiculite and smectite, impoverishment both in major elements (A1, K, Mg and Fe) and fine particles, and redistribution of sesquioxides as organic complexes. These processes indicate clay hydrolysis under acid conditions and subsequent removal of solutes by lateral subsurface flow. The resulting impoverishment of the soil profiles could be mainly attributed to hydrolysis under acid conditions brought about by slow decomposition of organic matter. The proposed mode of pedogenesis deviates strongly from the current concept of fersiallitisation, and is probably restricted to acid parent materials in mediterranean climates where lateral removal of soluble weathering products is ensured throughout the largest part of the year.