plants is generally negligible during the snowmelt period,
many of the nutrients in the soil are leached out in the
meltwater (e.g. Park et al. 2010). Further research is needed
to clarify how material cycling in forests responds to variations in the winter climate, as recent changes in global
climate may trigger significant changes in the snowfall and
snowpack in Hokkaido (Park et al. 2010).
(2) Soil freeze-thaw cycles and nitrogen dynamics
As a porous medium, snowpack plays a role in insulating the
soil. If the snowpack on the soil is sufficiently thick (e.g. <50 cm), the soil does not freeze and the soil temperature
remains at around 0 °C. However, if the snowpack falls
below certain thresholds (e.g. 50 cm in thickness), the soil
freezes and thaws at variable frequencies and intensities as
the ambient air temperature changes (e.g. Shibata 2016).
Repeated soil freeze-thaw cycles physically crush litter, fine
roots, and microbes, providing a substrate and an energy
source for the microbes that survive and altering the nitrogen
and carbon cycling during the thaw period. Recent studies
have indicated that, as soil freeze-thaw cycles increase in
frequency, the ammonium production from organic nitrogen
via soil microbial reactions increases, and nitrification tends
to decrease in forest soils (Shibata 2016). Hishi et al. (2014)
indicated that slope topography (i.e. position and aspect)
promotes the development of different microclimates,
including the winter climate, and affects the soil nitrogen
dynamics. Additionally, emissions of greenhouse gases from
soil, such as nitrous oxide (N 2 O), are enhanced by soil
freeze-thaw cycles. Nitrogen produced in soil during winter
is an important source of nutrients to streams and the
atmosphere during the snowmelt period and also for plants
and microbes during the following growing season (Shibata
2016).
(3) Role of riparian wetlands
Unlike forests in the southern islands in Japan, the topography is relatively gentle in the Hokkaido forests, and riparian wetlands in forested watersheds are generally located
in areas adjacent to the stream channels. Organic matter
accumulates in riparian wetlands because of the anaerobic
conditions in the soil, resulting in emissions of the greenhouse gases, methane (CH 4 ), and N 2 O. Emissions of N 2 O
from riparian soil indicate nitrate (NO 3
− ) reduction via
denitrification in soil and groundwater just before NO 3
−
enters the stream water. The hyporheic zone, at the interface
between the stream channel and the surrounding sediments
where stream water and groundwater mix, plays an important role in buffering stream chemistry (Shibata et al. 2004).
In natural forest watersheds in the northern part of
Hokkaido, NO 3
− concentrations in stream water tend to be
high in relatively steep watersheds; in contrast, low concentrations of NO 3
− and high concentrations of dissolved
organic carbon (DOC) were observed in stream water
because of microbial denitrification in DOC-rich soil and
nutrient uptake by riparian plants in watersheds with relatively flat riparian wetlands in their lower reaches (Ogawa
et al. 2006).
Further research should address how changes in global
climate, including changes in the winter season, will impact
forest ecosystems such as those in Hokkaido. Furthermore,
there is little clarity about how increases in the magnitude
and frequency of extreme climate events (e.g. typhoons and
bomb cyclones) might influence material cycling in forests.
There is thus a need for integrated research programs that
assess the natural capital and ecosystem services of forest
ecosystems in the uppermost reaches of watersheds and that
pay special attention to both the local unique characteristics
of Hokkaido and the coupling between natural and social
systems in watersheds.
5.6.3 Carbon Cycle
(1) Carbon cycle of forest soils in Hokkaido
Andosols, brown forest soils, and Volcanogenous Regosols,
which are distributed widely in forests in Hokkaido, contain
large amounts of organic carbon in the top 1 m (Morisada
et al. 2004). A major carbon input to the soil comes from
dead forest vegetation, with leaves, branches, and fine roots
being a stable and main source. The annual rate of carbon
input from leaf and branch litter from trees ranges between
0.5 and 2 Mg C ha
−1 in several forest types in Northern
Hokkaido. Dwarf bamboo (Sasa spp.), a widely distributed
undergrowth plant, supplies up to 1 Mg C ha
−1 yr
−1 of these
litters (Watanabe et al. 2013). Additionally, fine roots are
considered to supply a comparable amount of litter to the
soil to that contributed by leaves. Coarse litter originated
from trunks and coarse roots and root exudates need to be
added to evaluate the total carbon input to the soil, however
because of the large temporal and spatial heterogeneity of
the coarse litter and the little quantitative evaluation for root
exudates, large uncertainty exists in the estimation. For
example, the long-term, large-scale averaged annual rate of
carbon input from coarse litter (corresponding to plant
mortality) ranges between 0.4 and 1 Mg C ha
−1 yr
−1 in
several forest types in the Northern USA.
A major carbon output from the soil is the emission of
CO 2 from the soil originating from the decomposition of soil
carbon by microorganisms, which is known as microbial
respiration. Root respiration also emits CO 2 from the soil
5 Hokkaido Region
155
many of the nutrients in the soil are leached out in the
meltwater (e.g. Park et al. 2010). Further research is needed
to clarify how material cycling in forests responds to variations in the winter climate, as recent changes in global
climate may trigger significant changes in the snowfall and
snowpack in Hokkaido (Park et al. 2010).
(2) Soil freeze-thaw cycles and nitrogen dynamics
As a porous medium, snowpack plays a role in insulating the
soil. If the snowpack on the soil is sufficiently thick (e.g. <50 cm), the soil does not freeze and the soil temperature
remains at around 0 °C. However, if the snowpack falls
below certain thresholds (e.g. 50 cm in thickness), the soil
freezes and thaws at variable frequencies and intensities as
the ambient air temperature changes (e.g. Shibata 2016).
Repeated soil freeze-thaw cycles physically crush litter, fine
roots, and microbes, providing a substrate and an energy
source for the microbes that survive and altering the nitrogen
and carbon cycling during the thaw period. Recent studies
have indicated that, as soil freeze-thaw cycles increase in
frequency, the ammonium production from organic nitrogen
via soil microbial reactions increases, and nitrification tends
to decrease in forest soils (Shibata 2016). Hishi et al. (2014)
indicated that slope topography (i.e. position and aspect)
promotes the development of different microclimates,
including the winter climate, and affects the soil nitrogen
dynamics. Additionally, emissions of greenhouse gases from
soil, such as nitrous oxide (N 2 O), are enhanced by soil
freeze-thaw cycles. Nitrogen produced in soil during winter
is an important source of nutrients to streams and the
atmosphere during the snowmelt period and also for plants
and microbes during the following growing season (Shibata
2016).
(3) Role of riparian wetlands
Unlike forests in the southern islands in Japan, the topography is relatively gentle in the Hokkaido forests, and riparian wetlands in forested watersheds are generally located
in areas adjacent to the stream channels. Organic matter
accumulates in riparian wetlands because of the anaerobic
conditions in the soil, resulting in emissions of the greenhouse gases, methane (CH 4 ), and N 2 O. Emissions of N 2 O
from riparian soil indicate nitrate (NO 3
− ) reduction via
denitrification in soil and groundwater just before NO 3
−
enters the stream water. The hyporheic zone, at the interface
between the stream channel and the surrounding sediments
where stream water and groundwater mix, plays an important role in buffering stream chemistry (Shibata et al. 2004).
In natural forest watersheds in the northern part of
Hokkaido, NO 3
− concentrations in stream water tend to be
high in relatively steep watersheds; in contrast, low concentrations of NO 3
− and high concentrations of dissolved
organic carbon (DOC) were observed in stream water
because of microbial denitrification in DOC-rich soil and
nutrient uptake by riparian plants in watersheds with relatively flat riparian wetlands in their lower reaches (Ogawa
et al. 2006).
Further research should address how changes in global
climate, including changes in the winter season, will impact
forest ecosystems such as those in Hokkaido. Furthermore,
there is little clarity about how increases in the magnitude
and frequency of extreme climate events (e.g. typhoons and
bomb cyclones) might influence material cycling in forests.
There is thus a need for integrated research programs that
assess the natural capital and ecosystem services of forest
ecosystems in the uppermost reaches of watersheds and that
pay special attention to both the local unique characteristics
of Hokkaido and the coupling between natural and social
systems in watersheds.
5.6.3 Carbon Cycle
(1) Carbon cycle of forest soils in Hokkaido
Andosols, brown forest soils, and Volcanogenous Regosols,
which are distributed widely in forests in Hokkaido, contain
large amounts of organic carbon in the top 1 m (Morisada
et al. 2004). A major carbon input to the soil comes from
dead forest vegetation, with leaves, branches, and fine roots
being a stable and main source. The annual rate of carbon
input from leaf and branch litter from trees ranges between
0.5 and 2 Mg C ha
−1 in several forest types in Northern
Hokkaido. Dwarf bamboo (Sasa spp.), a widely distributed
undergrowth plant, supplies up to 1 Mg C ha
−1 yr
−1 of these
litters (Watanabe et al. 2013). Additionally, fine roots are
considered to supply a comparable amount of litter to the
soil to that contributed by leaves. Coarse litter originated
from trunks and coarse roots and root exudates need to be
added to evaluate the total carbon input to the soil, however
because of the large temporal and spatial heterogeneity of
the coarse litter and the little quantitative evaluation for root
exudates, large uncertainty exists in the estimation. For
example, the long-term, large-scale averaged annual rate of
carbon input from coarse litter (corresponding to plant
mortality) ranges between 0.4 and 1 Mg C ha
−1 yr
−1 in
several forest types in the Northern USA.
A major carbon output from the soil is the emission of
CO 2 from the soil originating from the decomposition of soil
carbon by microorganisms, which is known as microbial
respiration. Root respiration also emits CO 2 from the soil
5 Hokkaido Region
155