京都大学 / 新制・課程博士 / 博士(農学) / 甲第24656号 / 農博第2539号 / 新制||農||1097(附属図書館) / 学位論文||R5||N5437(農学部図書室) / 京都大学大学院農学研究科森林科学専攻 / (主査)教授 北山 兼弘, 教授 田中 千尋, 教授 松浦 健二 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DGAM

<p dir="ltr">Woody debris (WD) is an important element in forest ecosystems. It provides critical habitat for plants, animals, and insects; but it is also a source of fuel contributing to fire propagation and sometimes leads to catastrophic wildfire. Usually, WD inventory is conducted through field surveys using transects and sample plots. Light Detection and Ranging (LiDAR) point clouds are emerging as a valuable source for the development of comprehensive WD detection strategies. Although results from previous LiDAR-based WD detection approaches have been promising, there is still a lack of a general strategy for handling acquired point clouds by different platforms with varying characteristics (e.g., point density) in a complex environment, especially in natural forests. Here, we propose a general morphological WD detection strategy which requires a few intuitive thresholds, making it applicable to multi-platform LiDAR datasets in both plantation and natural forests. The conceptual basis of the strategy is that WD LiDAR points exhibit non-planar characteristics, distinct intensity, and comprise clusters that exceed a minimum size. The developed strategy is tested using leaf-off point clouds acquired by Geiger-mode, uncrewed aerial vehicle (UAV), and backpack LiDAR systems. The developed approach achieved an average recall of 0.83 indicating a low rate of omission errors. Datasets with higher point density (i.e., from UAV and backpack LiDAR) showed better performance. As for the precision evaluation metric, it ranges from 0.40 to 0.85, indicating a higher level of commission errors at the lower range. The commission errors depend on the presence of bushes and undergrowth, with a lower percentage in forest plantations.</p>

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The forestry is the main use of forests in Sweden which results in a number of positive aspects but also a number of negative effects on our nature. Aquatic ecosystems are specifically exposed to land use by the forest industry. The problem areas that arise are mainly discharge of nutrients, mercury, change in hydromorphology and local environment, sludge transport, absence of dead wood in streams and altered solar radiation. In order to reduce negative impact and protect natural populations of species but also to protect important ecosystems, there are a number of national prospective objectives and laws. From the year 2014, a number of prospective aims have been introduced to get a common view of the problem factors, but also to get a common representation of how the forestry measures should be carried out.  This work evaluated the width of the riparian buffer zone left along the 138 streams at final clearcutting in the Vindelälven catchment, comparing three periods: period 1 (year 2001-2007), period 2 (year 2008-2013) and period 3 (year 2014-2018).  The work also included a count of the number of dead wood objects that occur in 16 of the 138 streams in the study area. The method of the study is based on remote sensing using ArcGIS software, along with a field inventory to verify the remote sensing with reality. The results found from the study were that a significant increase in average minimum-width was shown between periods 1 and 3 (ANOVA, n = 138, df = 2, F = 5.083 and p = 0.007). The results from the average width were not significantly different but suggest a positive correlation between lower age of final clearcutting and the average width of the riparian zone. The density of dead wood in the streams of the study site did not give any significant differences or correlations depending on time period. What could be explained from the result was that the presence of beaver (Castor fiber) positively affected the amount of dead wood. To conclude, positive effects of the introduction of new objectives for riparian buffer zone management were indicated.  This is a sign that the forestry industry is moving towards implementing more sustainable methods. In addition, a method based on remote sensing for measuring riparian buffer zone widths was found to provide reliable estimates in the Vindelälven catchment, Västerbotten County. / Grip on Life IP

Coastal foredune evolution involves complex processes and controls. Although a great deal is known about the effects of vegetation cover, moisture, and fetch distance on sediment supply, and of topographic forcing on airflow dynamics, the role of large woody debris (LWD) as a modulator of sediment supply and a control on foredune growth is understudied. Large assemblages of LWD are common on beaches near forested watersheds and collectively have a degree of porosity that increases aerodynamic roughness and provides substantial sand trapping volume. To date, no research has attempted to understand the geomorphic role that LWD matrices, as a whole, have as roughness elements affecting airflow and sediment transport across a beach-dune system, or, what the long-term implications of these impacts are on beach and foredune erosion recovery and evolution. This four-year research initiative investigated the role of a LWD matrix on beach-dune morphodynamics on West Beach, Calvert Island on the central coast of British Columbia, Canada. This study integrated data from research that spanned three temporal scales, 1) event-scale (10 min) flow and sediment transport patterns, 2) daily frequency and relative magnitude of landscape changing events, 3) seasonal to interannual-scale volumetric and LWD changes. An event-scale experiment to characterise airflow dynamics and related sand transport patterns showed that LWD distinctly alters wind flow patterns and turbulence levels from that of incoming flow over a flat beach. Overall, mean wind speed and fluctuating flow properties declined as wind transitioned across the LWD. Streamwise mean energy was converted to turbulent energy, however, the reductions in mean flow properties were too great for the increased streamwise turbulence to have an effect on transport. In response to these flow alterations and more limited sand transport pathways to the foredune, sediment flux was reduced by 99% in the LWD compared to the open beach, thereby reducing sand supply to the foredune. Sand grains rebounding off of the LWD were carried higher into the flow field resulting in greater mass flux recorded at 20-50 cm in the LWD as opposed to the flat beach. This effect was only recorded 6 m into the LWD. As such, LWD has the potential to modulate rates of foredune recovery, growth, and evolution. Time-lapse photography collected at 15 min intervals during the study revealed that storm events lead to wave-induced erosion of the backshore and reworking of the LWD matrix. The exposed LWD matrix subsequently traps aeolian sediment that leads to rapid burial of the LWD and building of a raised platform for emergent vegetation. However, infilling of the accommodation space within the LWD matrix is so rapid, that sediment starvation of the foredune is short-lived. While the LWD at this site does trap sediment in the backshore, helping to protect the dune from scarping, LWD at this study site maintains an overall lower impact on transport to the foredune. Critical to this relationship is the frequency and magnitude of nearshore events that erode the beach periodically and re-organize the LWD matrix, which directly impacts the ability of LWD to store sediment and modulate transport to the foredune. A conceptual model exploring these relationships is presented. / Graduate

Few field studies have assessed the temporal and spatial dynamics of wood in small streams (bankfull widths < 5 m) flowing through forest ecosystems dominated by stand replacing wildfires. Comparisons of instream wood loads associated with clearcut harvesting, wildfire, and undisturbed, old forests are also scarce. The two main objectives of this research were: (1) to document the temporal and spatial variability of wood and its geomorphic role in relation to stand development stage; and (2) to compare wood loads and its geomorphic role in relation to streamside clearcut harvesting, wildfires and older, undisturbed forest stands. This research focused on 38 small streams with gradients less than 14% situated in the plateau regions of south-central British Columbia, Canada. A distinct temporal trend in wood loading was observed, with elevated volumes present 30-50 years subsequent to the wildfire disturbances following a “reverse J-shaped” trend in relation to time since the last major wildfire disturbance. The number of wood pieces was highly variable and few of the wood characteristics exhibited a significant trend in relation to time since the last major wildfire disturbance. Except at the smallest spatial scale (<3 m segments longitudinally along the stream) the spatial distribution of wood followed a random pattern with no trend, indicating that wood loads are related to local wood recruitment processes associated with episodic or chronic tree mortality and low wood transport. Instream wood volumes were three times higher in streams recently (30 – 50 years ago) disturbed by wildfire as compared to the older riparian forest stands, confirming that wildfire disturbance is an important mechanism to recruit wood into streams. No significant differences in wood loads were identified between the streamside clearcut streams and the wildfire-disturbed or older, undisturbed streams. The lack of reductions in wood loads are likely related to the low transport capacity of our study streams, retention of non-merchantable trees and recruitment of slash from harvesting. A lack of morphologic variability was observed in relation to the disturbances indicating that the streams included in this study are relatively robust and unresponsive to wildfire or streamside clearcut harvesting disturbances.

River-groundwater exchange (hyporheic exchange) influences temperature, water chemistry, and ecology within rivers and alluvial aquifers. Rates and patterns of hyporheic exchange depend on riverbed permeability, pressure gradients created by current-obstacle interactions, and river stage fluctuations. I demonstrate the response of hyporheic exchange to three examples of these driving forces: fine-scale permeability structure in cross-bedded sediment, current interactions with large woody debris (LWD), and anthropogenic river stage fluctuations downstream of dams. Using numerical simulations, I show that cross-bedded permeability structure increases hyporheic path lengths and modifies solute residence times in bedforms. The tails of residence time distributions conform to a power law in both cross-bedded and internally homogeneous riverbed sediment. Current-bedform interactions are responsible for the decade-scale tails, rather than permeability heterogeneity. Like bedforms, wood debris interacts with currents and drives hyporheic exchange. Laboratory flume experiments and numerical simulations demonstrate that the amplitude of the pressure wave (and thus hyporheic exchange) due to a channel-spanning log increases with channel Froude number and blockage ratio (log diameter : flow depth). Upstream from LWD, downwelling water transports the river’s diel thermal signal deep into the sediment. Downstream, upwelling water forms a wedge of buffered temperatures. Hyporheic exchange associated with LWD does not significantly impact diel surface water temperatures. I tested these fluid and heat flow relationships in a second-order stream in Valles Caldera National Preserve (NM). Log additions created alternating zones of upwelling and downwelling in a reach that was previously losing throughout. By clearing LWD from channels, humans have reduced hydrologic connectivity at the meter-scale and contributed to degradation of benthic and hyporheic habitats. Dams also significantly alter hydrologic connectivity in modern rivers. Continuous water table measurements show that 15 km downstream of the Longhorn dam (Austin, Texas), river stage fluctuations of almost 1 m induce a large, unsteady hyporheic exchange zone within the bank. Dam-induced hyporheic exchange may impact thermal and geochemical budgets for regulated rivers. Together, these three case studies broaden our understanding of complex drivers of hyporheic exchange in small, natural streams as well as large, regulated rivers. / text

Forest managers concerned with maintaining soil productivity must consider the impacts of forestry practices upon the features of a site. One critical feature is the amount and type of organic matter on a site, which may affect soil development. This study addresses the question of whether CWD accumulations increase the intensity of podzolization, thus reducing the long-term productivity of a site.

Few field studies have assessed the temporal and spatial dynamics of wood in small streams (bankfull widths < 5 m) flowing through forest ecosystems dominated by stand replacing wildfires. Comparisons of instream wood loads associated with clearcut harvesting, wildfire, and undisturbed, old forests are also scarce. The two main objectives of this research were: (1) to document the temporal and spatial variability of wood and its geomorphic role in relation to stand development stage; and (2) to compare wood loads and its geomorphic role in relation to streamside clearcut harvesting, wildfires and older, undisturbed forest stands. This research focused on 38 small streams with gradients less than 14% situated in the plateau regions of south-central British Columbia, Canada. A distinct temporal trend in wood loading was observed, with elevated volumes present 30-50 years subsequent to the wildfire disturbances following a “reverse J-shaped” trend in relation to time since the last major wildfire disturbance. The number of wood pieces was highly variable and few of the wood characteristics exhibited a significant trend in relation to time since the last major wildfire disturbance. Except at the smallest spatial scale (<3 m segments longitudinally along the stream) the spatial distribution of wood followed a random pattern with no trend, indicating that wood loads are related to local wood recruitment processes associated with episodic or chronic tree mortality and low wood transport. Instream wood volumes were three times higher in streams recently (30 – 50 years ago) disturbed by wildfire as compared to the older riparian forest stands, confirming that wildfire disturbance is an important mechanism to recruit wood into streams. No significant differences in wood loads were identified between the streamside clearcut streams and the wildfire-disturbed or older, undisturbed streams. The lack of reductions in wood loads are likely related to the low transport capacity of our study streams, retention of non-merchantable trees and recruitment of slash from harvesting. A lack of morphologic variability was observed in relation to the disturbances indicating that the streams included in this study are relatively robust and unresponsive to wildfire or streamside clearcut harvesting disturbances.

Downed woody debris (DWD) is a carbon-rich form of forest litter and plays a unique role in carbon and nutrient cycling. I present a novel modeling approach describing DWD decomposition and nutrient storage in a managed northern hardwood forest. The predicted half-life of DWD carbon was 7 years, less than previously observed in similar northern hardwood forests. A stage-based nutrient model indicated that harvest slash DWD was a net nitrogen and phosphorus sink for eight years following harvest and accumulated calcium during decay. Field observations of respiration and leaching supported model results with a respired C half-life of 8 years, while leached carbon export constituted 1.37% of the respired flux. DWD leachate carbon and nitrogen concentrations were 11× and 2× greater than from litter, respectively, and DWD leachate contributed disproportionately to soil C stocks. This work represents an expedient means of forecasting DWD abundance and partitioning carbon flux from DWD.