Vegetation and Landscape Dynamics in Eastern Taranaki Hill Country

<p>An ecological study of hill country landscapes in eastern Taranaki, New Zealand, was undertaken as part of a project concerning the implications of long-term soil mantle changes for sustainable land use. The study was undertaken in a 417 km2 area comprising uplifted and steeply dissected soft Tertiary sediments with a predominantly sandstone lithology. Rapid European settlement in the 1890s modified the natural vegetation cover greatly, so that most remaining forest in the study area occurs in patches surrounded by a matrix of pastoral land. Vegetational and successional patterns and environmental variation : The pattern of woody vegetation was investigated by extensive reconnaissance sampling incorporating semi-quantitative analysis of canopy cover, followed by intensive, environmentally stratified sampling. The vegetation was classified on a structural and floristic basis into 19 units of forest, treeland, scrub and shrubland. The effect of environmental variation on vegetation composition was investigated by reciprocal averaging ordination (DECORANA). The first ordination axis was correlated to vegetation structure and canopy height and was interpreted as a complex disturbance gradient relating to time since disturbance. The second and third axes were related to soil fertility and topographical gradients. Forest plots were dominated by Beilschmiedia tawa and Weinmannia racemosa and had basal area values of up to >250 m2/ha. Basal area, stem and seedling density varied greatly between vegetation structural groups. Regeneration of woody vegetation following various types of disturbance: The disturbance regime was comprehensively documented. Main factors of natural disturbance are landslide erosion and windthrow; main factors of cultural disturbance are direct clearance by felling and burning, and introduced animals. A chronology is presented of successional pathways for about 400 years following major disturbance. Succession proceeds through shrubland and scrub stages dominated by treeferns, Leptospermwn scoparium or other broadleaved woody shrubs, through treeland, to broadleaved forest dominated firstly by W. racemosa or Knightia excelsa, then by B. tawa. Podocarp trees are generally only prominent after a long period of uninterrupted succession. Seedling recruitment, mortality and growth were monitored for 2 years. Seedling dynamics varied considerably between and within sampling plots, some of which contained small exclosures that excluded possums and goats. The effects of introduced animals on seedling recruitment and vegetation growth is strongly modified by microtopography. Most dominant species showed continuous regeneration at the scale of the whole study area, despite local discontinuities. This pattern was consistent with a model of interrpted gap-phase regeneration, which may be widely applicable to New Zealand lowland forests. The vegetation turnover time is in the order of 150-250 years, a period consistent with comparable temperate forest ecosystems. The successional pathway is primarily dependent on topography, previous site history and location and area of disturbance. The existence of residual-soils on landslide scars, variations in plant propagule supply, and rapid loss of soil from steep slopes cleared for agriculture, all suggest that a rigid distinction between primary and secondary succession in the study area is not appropriate. Hillslope processes underlying vegetation and landscape change: Hillslope processes were studied in five 0.1 ha plots in which slope profiles were measured, vegetation and microtopography mapped in detail, vegetation age assessed and soil properties investigated. Ground surface age was assessed as an interpretation of the above data. Mean surface age was c. 450 years, but some swales had a surface age of several thousand years. There was a significant correlation between surface age and soil depth, soil depth increase being faster and continuing for much longer under forest than under pasture. Observations were made of near-surface erosion processes such as soil creep. A model of hillslope erosion is outlined, involving periodic evacuation of swales by landslides and refilling of swales by near-surface erosion. Evidence of past environments supports a fluvial origin for swales in an early Ohakean (glacial maximum) or pre-Ohakean period of high erosion. A concluding synthesis of vegetation, topography and soils emphasises the importance of selecting appropriate temporal and spatial scales at which to study landscape processes.</p>

Vegetation and Landscape Dynamics in Eastern Taranaki Hill Country

<p>An ecological study of hill country landscapes in eastern Taranaki, New Zealand, was undertaken as part of a project concerning the implications of long-term soil mantle changes for sustainable land use. The study was undertaken in a 417 km2 area comprising uplifted and steeply dissected soft Tertiary sediments with a predominantly sandstone lithology. Rapid European settlement in the 1890s modified the natural vegetation cover greatly, so that most remaining forest in the study area occurs in patches surrounded by a matrix of pastoral land. Vegetational and successional patterns and environmental variation : The pattern of woody vegetation was investigated by extensive reconnaissance sampling incorporating semi-quantitative analysis of canopy cover, followed by intensive, environmentally stratified sampling. The vegetation was classified on a structural and floristic basis into 19 units of forest, treeland, scrub and shrubland. The effect of environmental variation on vegetation composition was investigated by reciprocal averaging ordination (DECORANA). The first ordination axis was correlated to vegetation structure and canopy height and was interpreted as a complex disturbance gradient relating to time since disturbance. The second and third axes were related to soil fertility and topographical gradients. Forest plots were dominated by Beilschmiedia tawa and Weinmannia racemosa and had basal area values of up to >250 m2/ha. Basal area, stem and seedling density varied greatly between vegetation structural groups. Regeneration of woody vegetation following various types of disturbance: The disturbance regime was comprehensively documented. Main factors of natural disturbance are landslide erosion and windthrow; main factors of cultural disturbance are direct clearance by felling and burning, and introduced animals. A chronology is presented of successional pathways for about 400 years following major disturbance. Succession proceeds through shrubland and scrub stages dominated by treeferns, Leptospermwn scoparium or other broadleaved woody shrubs, through treeland, to broadleaved forest dominated firstly by W. racemosa or Knightia excelsa, then by B. tawa. Podocarp trees are generally only prominent after a long period of uninterrupted succession. Seedling recruitment, mortality and growth were monitored for 2 years. Seedling dynamics varied considerably between and within sampling plots, some of which contained small exclosures that excluded possums and goats. The effects of introduced animals on seedling recruitment and vegetation growth is strongly modified by microtopography. Most dominant species showed continuous regeneration at the scale of the whole study area, despite local discontinuities. This pattern was consistent with a model of interrpted gap-phase regeneration, which may be widely applicable to New Zealand lowland forests. The vegetation turnover time is in the order of 150-250 years, a period consistent with comparable temperate forest ecosystems. The successional pathway is primarily dependent on topography, previous site history and location and area of disturbance. The existence of residual-soils on landslide scars, variations in plant propagule supply, and rapid loss of soil from steep slopes cleared for agriculture, all suggest that a rigid distinction between primary and secondary succession in the study area is not appropriate. Hillslope processes underlying vegetation and landscape change: Hillslope processes were studied in five 0.1 ha plots in which slope profiles were measured, vegetation and microtopography mapped in detail, vegetation age assessed and soil properties investigated. Ground surface age was assessed as an interpretation of the above data. Mean surface age was c. 450 years, but some swales had a surface age of several thousand years. There was a significant correlation between surface age and soil depth, soil depth increase being faster and continuing for much longer under forest than under pasture. Observations were made of near-surface erosion processes such as soil creep. A model of hillslope erosion is outlined, involving periodic evacuation of swales by landslides and refilling of swales by near-surface erosion. Evidence of past environments supports a fluvial origin for swales in an early Ohakean (glacial maximum) or pre-Ohakean period of high erosion. A concluding synthesis of vegetation, topography and soils emphasises the importance of selecting appropriate temporal and spatial scales at which to study landscape processes.</p>

Multi-sensor approach towards understanding debris-flow activity in the Lattenbach catchment, Austria

&lt;p&gt;Debris flows (DFs) pose a severe risk to Alpine communities and infrastructure. The Lattenbach catchment (basin area 5,3 km&amp;#178;, relief 2134 m) in Tyrol, Austria, is an example for an active DF-site with several DFs occurring per year. To improve our understanding of the DF-process cascade in this catchment, we raise the questions: where does the sediment originate, are hillslope processes the drivers for DF-activity, and how is the relationship of &lt;span&gt;&lt;span&gt;rainfall amount&lt;/span&gt;&lt;/span&gt; and DF-magnitude?&lt;/p&gt;&lt;p&gt;We employ an approach that makes use of the data richness of this study site: High resolution ALS and TLS terrain models and aerial photographs are considered to locate significant elevation differences. Furthermore, we performed an in-detail UAV-based surveying campaign of the active channel reaches for the 2019 and 2020 DF-season. Additionally, we use datasets captured by a DF monitoring station (discharge, volume, timing, precipitation) at the catchment outlet to assess triggering rainfall as well as DF-frequency and magnitudes.&lt;/p&gt;&lt;p&gt;We find that in the last fifteen years up to three events occurred annually. A single location, where all DFs originate from, is not detectable, indicating a variety of sediment sources is relevant for DF-initiation, including bank failures and channel incision, partly driven by deep-seated landslides that continuously feed the channel with sediment. Between the years 2005 and 2018 the DF-volumes recorded at the catchment outlet varied between about 5.000 m&amp;#179; (small) and 46.000 m&amp;#179; (large). A first analysis suggests that there is a prevailing &amp;#8220;background noise&amp;#8220; pattern of relatively small DF-events that happen regularly during every DF-season. We hypothesize that rare, very large events represent a tipping point in the catchment system, which leads to a period of increased large-scale DF-activity over following seasons.&lt;/p&gt;

Connecting hillslope and runoff generation processes in the Ethiopian Highlands: The Ene-Chilala watershed

Effective watershed planning requires an understanding of the hydrology. In the humid tropical monsoon climates and especially in volcanic highland regions such as the Ethiopian Highlands, the understanding of watershed processes is incomplete. The objective is to better understand the hydrology of the volcanic regions in the humid highlands by linking the hillslope processes with the discharge at the outlet. The Ene-Chilala watershed was selected for this study. The infiltration rate, piezometric water levels and discharge from two nested sub watersheds and at the watershed outlet were measured during a four-year period. Infiltration rates on the hillsides exceeded the rainfall intensity most of the time. The excess rain recharged a perched hillside aquifer. Water flowed through the perched aquifer as interflow to rivers and outlet. In addition, saturation excess overland flow was generated in the valley bottoms. Perched water tables heights were predicted by summing up the recharge over the travel time from the watershed divide. Travel times ranged from a few days for piezometers close to the divide to 40 days near the outlet. River discharge was simulated by adding the interflow from the upland to overland flow from the saturated valley bottom lands. Overland flow accounted only for one-fourth of the total flow. There was good agreement between predicted and observed discharge during the rain phase therefore the hillslope hydrologically processes were successfully linked with the discharge at the outlet.

Controls on the Dynamics of Rare Earth Elements During Subtropical Hillslope Processes and Formation of Regolith-Hosted Deposits

Subtropical weathering of granitic catchments in South China has led to the formation of numerous giant regolith-hosted rare earth element (REE) deposits that currently account for more than 15% of global REE production and more than 95% of global heavy REE (HREE) production. Understanding the controls on mobilization and redistribution of the REEs during subtropical weathering in these granitic catchments is crucial for efficient exploration for this type of deposit in the world. As exemplified by the Bankeng light REE (LREE) deposit in South China, the key factors controlling the mobilization and redistribution of the REEs, especially the easily exchangeable REEs, are soil pH and primary REE mineralogy. The nature of the primary REE minerals, apatite, monazite-(Ce), and subordinate bastnäsite-(Ce), parisite-(Ce), and xenotime-(Y) places an important control on the behavior of the REEs during incipient weathering. Dissolution of these minerals is slow during incipient weathering, and, therefore, enrichment in REEs in this stage results largely from the removal of major elements during the decomposition of albite, K-feldspar, and biotite. Dissolution of the primary REE minerals higher in the profile liberates the REEs, which are then transported to locations where the soil pH abruptly increases due to water-regolith interaction, such as the pedolith-saprolite interface, and adsorption on kaolinite-group minerals efficiently fixes the REEs in regolith. Geomorphologically, the Bankeng deposit, like most of the other regolith-hosted REE deposits in South China, is located on concave-convex hillslopes, where erosion is prevalent at the ridgetop and decreases in intensity downslope. Results of this study show that strong erosion, coupled with intense chemical weathering at the ridgetop, is responsible for the enrichment in REEs by releasing the REEs, especially the LREEs, from their primary sources and supplying kaolinite and halloysite needed for the REE adsorption by decomposing albite, K-feldspar, and biotite. Decomposition of these major rock-forming minerals also leads to an enrichment of the REEs through the removal of components. The HREEs are lost preferentially to the groundwater and transported downslope, resulting in the enrichment of these elements in the lower part of the weathering crust at the footslope. Significant lateral Ce transport is also probable. A series of oxic fronts were developed at the footslope, with the most persistent one along the saprolite-saprock interface, due to seasonal fluctuations of the groundwater table. Cerium was immobilized there, predominantly through adsorption on Fe-Mn oxyhydroxides, causing enormous accumulation. Therefore, hillslope processes and groundwater flow could redistribute the REEs across the entire catchment, preferentially enriching the LREEs at the ridgetop and the HREEs at the footslope. Also, intense erosion facilitates chemical weathering and the accumulation of REEs, but the development of a thick weathering crust is favored by weak erosion. Repeated periods of high and low erosion rates in South China have enabled the gradual development of thick weathering crusts at the ridgetops that are sufficiently enriched in REEs to now constitute a major resource of these economically important elements.

Spatial optimization of watershed best management practice scenarios based on boundary-adaptive configuration units

Spatial optimization of watershed best management practice (BMP) scenarios based on watershed modeling is an effective decision support tool for watershed management. During such optimization, existing types of BMP configuration units for configuring BMPs (or BMP configuration units, e.g. subbasins, hydrologic response units, farms) remain fixed boundaries once they have been created through spatial discretization prior to BMP scenario optimization. This sort of “boundary-fixed” method does not allow for adjustments to the spatial characteristics of BMP configuration units. Hence, it runs the risk of missing superior BMP scenarios that could have been obtained by adjusting unit boundaries and may produce less effective spatial optimization. In this article, we propose a new approach to the spatial optimization of BMP scenarios based on boundary-adaptive configuration units. The proposed optimization approach adopts slope positions (basic landform units along hillslopes inherently related to physical hillslope processes) as BMP configuration units and dynamically adjusts their boundaries by using quantitative information about their spatial gradation (i.e. fuzzy slope positions) during the optimization. A case study conducted in the Youwuzhen watershed in Fujian, China, showed that the proposed optimization approach can significantly enlarge the search space and obtain optimal BMP scenarios with better cost-effectiveness and higher optimization efficiency than with boundary-fixed units. The proposed optimization approach provides a new alternative framework for spatial optimization of BMP scenarios, in which other watershed models, intelligent optimization algorithms, and BMP configuration units available for boundary adjustment can be applied to BMP scenario optimization in a boundary-adaptive manner. This study also exemplifies the potential for transforming qualitative, vague, and empirical geographical knowledge about slope position units related to physical hillslope processes and BMPs into quantitative, explicit, and automated geospatial algorithms for effectively resolving environmental management problems in a more geographically meaningful way.

Rivers as linear elements in landform evolution models

Models of detachment-limited fluvial erosion have a long history in landform evolution modeling in mountain ranges. However, they suffer from a scaling problem when coupled to models of hillslope processes due to the flux of material from the hillslopes into the rivers. This scaling problem causes a strong dependence of the resulting topographies on the spatial resolution of the grid. A few attempts based on the river width have been made in order to avoid the scaling problem, but none of them appear to be completely satisfying. Here a new scaling approach is introduced that is based on the size of the hillslope areas in relation to the river network. An analysis of several simulated drainage networks yields a power-law scaling relation for the fluvial incision term involving the threshold catchment size where fluvial erosion starts and the mesh width. The obtained scaling relation is consistent with the concept of the steepness index and does not rely on any specific properties of the model for the hillslope processes.

Characterizing Hydrological Fluxes of Lesser Himalayan hillslopes

&lt;p&gt;Hillslope-scale studies play a vital role in understanding the spatial and temporal dynamics of hydrological fluxes of an ungauged watershed. The linkage between static (i.e. topography, soil properties and landuse) and dynamic (i.e. runoff, soil moisture and temperature) characteristics of a hillslope provides a new insight towards hillslope processes. Thus, two Lesser Himalayan hillslopes of Aglar watershed have been selected in two different landuses (grass-covered and agro-forested) and aspects (south and north). In this study, we analyzed the different hydrological fluxes i.e. rainfall, runoff, soil moisture and soil temperature along with the soil properties to get a holistic understanding of hillslope processes. We used the soil moisture dynamics and soil hydraulic conductivity as the major components to derive the hillslope hydrological connectivity. It was observed that the grassed (GA) hillslope generates less runoff than the agro-forested (AgF) hillslope as the upslope runoff of GA hillslope re-infiltrated in the middle portion due to higher soil hydraulic conductivity and surface resistance. Further, this explains that the runoff contributing areas are located at the lower and upper portions of hillslopes due to the presence of low soil hydraulic conductivity zones. &amp;#160;As both the hillslopes are dominated with Hortonian overland flow, the negative correlation was found between topographic indices (TWI) and soil moisture and positive correlation was noticed between soil hydraulic conductivity. Higher runoff (less infiltration) from AgF hillslope results in a higher negative correlation between TWI and soil moisture in comparison to GA hillslope. This results in a higher rate of change in soil temperature of GA hillslope than the AgF hillslope. After analyzing 40 rainfall events, it was concluded that a temperature drop of more than 2&lt;sup&gt;o&lt;/sup&gt;C was recorded when the average rainfall intensity and event duration exceeds 7.5mm/hr and 7.5hr, respectively. The understanding of covariance of these hydrological fluxes will be used in the future to develop a hillslope-scale conceptual model.&lt;/p&gt;

Migrating divides induce drainage reversal toward cliffs and escarpments

&lt;p&gt;Observations from around the globe show that drainage reversal toward cliffs (and at a larger scale, toward escarpments) is a common phenomenon.&amp;#160; Drainage reversal occurs when a channel that used to grade in one direction reverses its gradient while exploiting its antecedent valley, forming barbed tributaries with junction angle &gt;90&amp;#176;. Drainage reversal is an important end-member of fluvial reorganization that drastically shifts the hydrologic and geomorphic functionality of the landscape.&amp;#160; The processes that induce drainage reversals, however, remain largely enigmatic. In many cases, tectonic or structural tilt of the surface is invoked to explain reversal toward the tilt direction, but independent evidence for tilting is rare. Moreover, in great escarpments, geodynamic models predict tilting away from the escarpment, opposite to the sense of reversal discussed here.&lt;/p&gt;&lt;p&gt;We study drainage reversals toward the southern Arava Valley escarpment in Israel, along the Sinai-Arabia transtentional plate boundary. In this area, we establish reversals by observations of barbed tributaries, valley-confined windgaps, and terraces and interfluves that grade opposite to the grading direction of the active channel. Detailed morphological and geological analysis of the field area gives rise to a new, tilting independent mechanism for drainage reversal toward cliffs. The initial condition for this mechanism is a cliff that truncates fluvial channels that flow over the highland and away from the cliff, and a water divide that coincides with the cliff. The truncated channels appear as saddles along the cliff and are commonly filled with alluvial and colluvial sediments. Such initial conditions characterize shoulder-type great escarpments and cliffs that form following river capture events. Importantly, in these settings, the sediments that fill the truncated channels are more erodible than the bedrock that builds the interfluves.&lt;/p&gt;&lt;p&gt;According to the mechanism we propose, the erodible valley fill near the steep cliff is initially transported down the cliff via hillslope processes, which results in a gradual migration of the divide along the antecedent valley and away from the cliff. A reversed channel segment forms between the receding divide and the cliff, such that along the channel, the divide and the cliff are not coincident anymore. The faster fluvial incision in the reversed segment with respect to the antecedent channel further pushes the divide away from the cliff. When the receding divide traverses a tributary confluence, a barbed tributary forms. The increased discharge of the reversed segment facilitates cliff embayment that eventually affects cliff retreat and morphology.&lt;/p&gt;&lt;p&gt;This new mechanism indicates that a relatively thin layer of erodible valley fill could be a tipping point that completely changes the trajectory of landscape evolution via drainage reversal. Importantly, however, flow reversal towards cliffs does not necessitate such a layer but instead could be triggered by other hydrological and geological conditions that promote faster erosion toward the cliff within the antecedent channel with respect to the interfluves.&amp;#160;&lt;/p&gt;