Aug. 2002, International Congress of Ecology, Outstanding Student Poster, 2nd place award
International society

Changes in leaf lifespan, nitrogen resorption, and mean residence time of leaf nitrogen along a soil fertility gradient in an evergreen oak tree
Shimpei Oikawa, Lead, Abstract

The ability of plants to retain nitrogen (N) for a long period of time is critical to their N use efficiency, growth, and fitness, particularly in infertile environments. The mean residence time of leaf N (MRT_L) and its two determinants, leaf lifespan and N resorption efficiency (r_N, the fraction of the total leaf N pool that is resorbed during leaf senescence), have been hypothesized to increase plastically with decreasing soil N fertility but this remains to be fully tested. To avoid confusion by random changes in these characteristics in a relatively narrow N fertility range, MRT_L, leaf lifespan, and N resorption efficiency were measured in Quercus glauca over a broad N fertility range. In the high to moderate N fertility range, leaf lifespan and r_N increased with decreasing N addition rate, and thus the MRT_L increased. However, in the moderate to low N fertility range, leaf lifespan increased but r_N decreased significantly, so MRT_L decreased. The decrease in r_N occurred because the senesced leaf N concentration was almost constant at the lower limit while the green leaf N concentration decreased in this range. The hump‐shaped quadratic responses of MRT_L and r_N along the N fertility gradient suggest that incorrect conclusions about the response of these traits to N fertility variation may be drawn from experiments that include only a few fertility levels, and N recycling within leaf canopy alone cannot achieve efficient N use in infertile environments., Wiley
Physiologia Plantarum, 12 Sep. 2024, [Reviewed]

Herbicide management of roadside kudzu (Pueraria lobata) vegetation doubles soil nitrification and nitrate leaching rates　　　　　　　　　　　　　　　
Miki U. Ueda; Nobuo Sakagami; Tadao Enomoto; Genki Katata; Yugo Suzuki; Yui Takase; Shimpei Oikawa, Last
Landscape and Ecological Engineering, Jul. 2024, [Reviewed]

Competition between the invasive alien species Solidago altissima and the native Pueraria lobata in Japan
Naoki Masada; Tadao Enomoto; Genki Katata; Nobuo Sakagami; Yugo Suzuki; Shimpei Oikawa, Last, Taylor & Francis
Botany Letters, Jun. 2023, [Reviewed]

Evidence of NOx and O3 concentration reduction by kudzu Pueraria lobata invasion at a Japanese highway
Genki Katata; Mirai Watanabe; Shimpei Oikawa; Akiko Takahashi; Tomohiro Kubota; Yui Takase; Tadao Enomoto; Keitaro Fukushima; Nobuo Sakagami; Miki U. Ueda; Yugo Suzuki, Elsevier
Atmospheric Pollution Research, Jan. 2023, [Reviewed]

〔Major achievements〕Effect of symbiotic N2-fixation on leaf protein contents, protein degradation and nitrogen resorption during leaf senescence in temperate deciduous woody species
Ryo Tanabe; Shin-Ichi Miyazawa; Osamu Kitade; Shimpei Oikawa, Last, Springer
Oecologia, 17 Sep. 2022, [Reviewed]

〔Major achievements〕Supra-optimal leaf area index of a temperate liana Pueraria lobata for competition with Solidago altissima at the expense of canopy photosynthesis.
Shimpei Oikawa, Lead, Oxford
Tree Physiology, Jul. 2022, [Reviewed]

Leaf productivity and persistence have been improved during soybean (Glycine max) domestication and evolution
Ayaka Togashi; Shimpei Oikawa, Last, Springer
Journal of Plant Research, Jan. 2021, [Reviewed]

People's attitude towards strategies to control kudzu (Pueraria lobar (Wield.) Ohwi) during road slope revegetation　　　　　　　　　　　　　　　
高瀬唯; 及川真平; 榎本忠夫; 堅田元喜; 坂上伸生, 一般社団法人 環境情報科学センター
環境情報科学研究論文集, Dec. 2020, [Reviewed]

Species-specific nitrogen resorption proficiency in legumes and nonlegumes
Shimpei Oikawa · Yusuke Matsui · Michio Oguro · Masanori Okanishi · Ryo Tanabe · Tomoki Tanaka · Ayaka Togashi · Tomoyuki Itagaki, Lead, Nitrogen (N) resorption from senescing leaves enables plants to reuse N, thereby making them less dependent on current N uptake from the environment. Therefore, N resorption is important for survival and fitness, particularly for plants growing under low N supply. We studied N resorption from senescing leaves of 25 legumes and 25 nonlegumes in a temperate region of Japan to test the hypothesis that high N resorption has not evolved in legumes that fix atmospheric N-2. The extent of N resorption was quantified by N resorption proficiency that is measured as the level to which leaf N concentration was reduced during senescence, i.e., the lower the senesced leaf N concentration, the lower the N loss through leaf fall and higher the N resorption proficiency. In support of the hypothesis, senesced leaf N concentration was higher in legumes than in nonlegumes, but there was considerable overlap between the groups. The higher senesced leaf N concentration of legumes was associated with a lower proportion of leaf N resorbed during senescence, particularly in species with higher leaf N concentrations. According to a hierarchical partitioning analysis, there was a large contribution of species to the total variance in the senesced leaf N concentration as opposed to a minor contribution of functional group (legume/nonlegume). This study reveals that legumes are not proficient at resorbing N from senescing leaves but that N-2-fixation might not be the single most important determinant of N resorption., Springer
Journal of Plant Research, Aug. 2020, [Reviewed]

Leaf shedding increases the photosynthetic rate of the canopy in N2-fixing and non-N2-fixing woody species.
Tomoki Tanaka; Chiharu Kurokawa; Shimpei Oikawa, Last, Oxford University Press
Tree Physiology, Sep. 2018, [Reviewed]

Nitrogen resorption in senescing leaf blades of rice exposed to Free-Air CO2 Enrichment (FACE) under different N fertilization levels
Shimpei Oikawa; Hitomi Ehara; Mika Koyama; Tadaki Hirose; Kouki Hikosaka; Charles P. Chen; Hirofumi Nakamura; Hidemitsu Sakai; Takeshi Tokida; Yasuhiro Usui; Toshihiro Hasegawa, Lead, Springer
Plant and Soil, Sep. 2017, [Reviewed]

Inconsistent intraspecific pattern in leaf life span along nitrogen-supply gradient
Shimpei Oikawa; Koya Suno; Noriyuki Osada, Lead, PREMISE OF THE STUDY: Leaf life span (LLS) has long been hypothesized to plastically increase with decreasing nitrogen (N) supply from soil to maximize N retention, carbon assimilation, and fitness; however, accumulating evidence shows no consistent trend. The apparent inconsistencies are explained by a recent model that assumes LLS has a hump-shaped quadratic response to the N-supply gradient. The available evidence mostly originates from comparisons of LLS at only two levels of N availability, and the hypothesis remains unanswered.
METHODS: We investigated LLS of two asteraceous forbs (Adenocaulon himalaicum and Xanthium canadense) experimentally grown at eight levels of N supply, which covered a range of N supply in their natural habitats. We additionally conducted a literature search to retrieve studies reporting LLS response along an N-supply gradient.
KEY RESULTS: The LLS of neither species showed a hump-shaped response along the N-supply gradient. Past studies examining the LLS of an aquatic forb and terrestrial shrubs and trees along the N-supply gradient (more than four levels of N supply) also refuted the hypothesis.
CONCLUSIONS: The LLS of a single species exhibited neither an increase nor a hump-shaped response to decreased N supply in a variety of life forms. Comparisons at only a few N levels are misleading with regard to LLS response to N supply., BOTANICAL SOC AMER INC
AMERICAN JOURNAL OF BOTANY, Feb. 2017, [Reviewed]

Changes in leaf area, nitrogen content and canopy photosynthesis in soybean exposed to an ozone concentration gradient
Shimpei Oikawa; Elizabeth A. Ainsworth, Lead, Influences of ozone (O-3) on light-saturated rates of photosynthesis in crop leaves have been well documented. To increase our understanding of O-3 effects on individual- or stand level productivity, a mechanistic understanding of factors determining canopy photosynthesis is necessary. We used a canopy model to scale photosynthesis from leaf to canopy, and analyzed the importance of canopy structural and leaf ecophysiological characteristics in determining canopy photosynthesis in soybean stands exposed to 9 concentrations of [O-3] (37-116 ppb; 9-h mean). Light intensity and N content peaked in upper canopy layers, and sharply decreased through the lower canopy. Plant leaf area decreased with increasing [O-3] allowing for greater light intensity to reach lower canopy levels. At the leaf level, light-saturated photosynthesis decreased and dark respiration increased with increasing [O-3]. These data were used to calculate daily net canopy photosynthesis (P-c). P-c decreased with increasing [O-3] with an average decrease of 10% for an increase in [O-3] of 10 ppb, and which was similar to changes in above-ground dry mass production of the stands. Absolute daily net photosynthesis of lower layers was very low and thus the decrease in photosynthesis in the lower canopy caused by elevated [O-3] had only minor significance for total canopy photosynthesis. Sensitivity analyses revealed that the decrease in Pc was associated with changes in leaf ecophysiology but not with decrease in leaf area. The soybean stands were very crowded, the leaves were highly mutually shaded, and sufficient light for positive carbon balance did not penetrate to lower canopy leaves, even under elevated [O-3]. (C) 2016 Elsevier Ltd. All rights reserved., ELSEVIER SCI LTD
ENVIRONMENTAL POLLUTION, Aug. 2016, [Reviewed]

A meta-analysis of leaf nitrogen distribution within plant canopies
Kouki Hikosaka; Niels P. R. Anten; Almaz Borjigidai; Chiho Kamiyama; Hidemitsu Sakai; Toshihiro Hasegawa; Shimpei Oikawa; Atsuhiro Iio; Makoto Watanabe; Takayoshi Koike; Kazuya Nishina; Akihiko Ito, Background and aims Leaf nitrogen distribution in the plant canopy is an important determinant for canopy photosynthesis. Although the gradient of leaf nitrogen is formed along light gradients in the canopy, its quantitative variations among species and environmental responses remain unknown. Here, we conducted a global meta-analysis of leaf nitrogen distribution in plant canopies.
Methods We collected data on the nitrogen distribution and environmental variables from 393 plant canopies (100, 241 and 52 canopies for wheat, other herbaceous and woody species, respectively).
Key Results The trends were clearly different between wheat and other species; the photosynthetic nitrogen distribution coefficient (K-b) was mainly determined by leaf area index (LAI) in wheat, whereas it was correlated with the light extinction coefficient (K-L) and LAI in other species. Some other variables were also found to influence K-b. We present the best equations for K-b as a function of environmental variables and canopy characteristics. As a more simple function, K-b = 0 center dot 5K(L) can be used for canopies of species other than wheat. Sensitivity analyses using a terrestrial carbon flux model showed that gross primary production tended to be more sensitive to the K-b value especially when nitrogen content of the uppermost leaf was fixed.
Conclusion Our results reveal that nitrogen distribution is mainly driven by the vertical light gradient but other factors such as LAI also have significant effects. Our equations contribute to an improvement in the projection of plant productivity and cycling of carbon and nitrogen in terrestrial ecosystems., OXFORD UNIV PRESS
ANNALS OF BOTANY, Aug. 2016, [Reviewed]

Trait-Based Approaches for Understanding Species Niche, Coexistence, and Functional Diversity in Subalpine Moorlands　　　　　　　　　　　　　　　
Kouki Hikosaka; Takehiro Sasaki; Chiho Kamiyama; Masatoshi Katabuchi; Shimpei Oikawa; Masaya Shimazaki; Hiroshi Kimura; Tohru Nakashizuka
Structure and Function of Mountain Ecosystems in Japan, 05 Apr. 2016, [Reviewed]

Nitrogen-utilization efficiency in rice: an analysis at leaf, shoot, and whole-plant level
Takahiro Ogawa; Shimpei Oikawa & Tadaki Hirose, Aims If rice has a higher nitrogen use efficiency for dry matter production (NUE) and grain yield (NUEGY) than wild annuals, we may question whether the higher NUE is due to a higher productivity per plant N (NP) or a longer retention time of plant N (MRT) or both, and whether the higher NUEGY results also from a higher harvest index (HI).
Methods Stands of rice were established at three N levels. Censuses were done for birth and death of every shoot and leaf from germination to full maturity. Nitrogen uptake, dry matter production, grain yield, HI, NUE, and NUEGY were determined at shoot and whole-plant levels.
Results Rice had a higher NUE, NUEGY and NP, but hardly higher plant-N MRT and HI than wild annuals. Leaf-N MRT was higher than leaf longevity in fertile shoots, while the opposite was true in sterile tillers. Shoot NUEGY was higher in late tillers due to higher HI.
Conclusions High NUE of rice results from its high NP, not from high MRT at both whole-plant and leaf levels. Revenues gained from enhancing MRT would have been lower than the opportunity costs of reducing NP. N recycles between shoots. Sterile tillers function as an N storage for grain yield in fertile shoots., Springer
Plant and Soil, Mar. 2016, [Reviewed]

Leaf dynamics in growth and reproduction of Xanthium canadense as influenced by stand density
Takahiro Ogawa; Shimpei Oikawa; Tadaki Hirose, Background and Aims Leaf longevity is controlled by the light gradient in the canopy and also by the nitrogen (N) sink strength in the plant. Stand density may influence leaf dynamics through its effects on light gradient and on plant growth and reproduction. This study tests the hypothesis that the control by the light gradient is manifested more in the vegetative period, whereas the opposite is true when the plant becomes reproductive and develops a strong N sink.
Methods Stands of Xanthium canadense were established at two densities. Emergence, growth and death of every leaf on the main stem and branches, and plant growth and N uptake were determined from germination to full senescence. Mean residence time and dry mass productivity were calculated per leaf number, leaf area, leaf mass and leaf N (collectively termed 'leaf variables') in order to analyse leaf dynamics and its effect on plant growth.
Key Results Branching and reproductive activities were higher at low than at high density. Overall there was no significant difference in mean residence time of leaf variables between the two stands. However, early leaf cohorts on the main stem had a longer retention time at low density, whereas later cohorts had a longer retention time at high density. Branch leaves emerged earlier and tended to live longer at low than at high density. Leaf efficiencies, defined as carbon export per unit investment of leaf variables, were higher at low density in all leaf variables except for leaf number.
Conclusions In the vegetative phase of plant growth, the light gradient strongly controls leaf longevity, whereas later the effects of branching and reproductive activities become stronger and over-rule the effect of light environment. As leaf N supports photosynthesis and also works as an N source for plant development, N use is pivotal in linking leaf dynamics with plant growth and reproduction., OXFORD UNIV PRESS
ANNALS OF BOTANY, Oct. 2015, [Reviewed]

Implications of lifespan variation within a leaf cohort for evaluation of the optimal timing of leaf shedding
Noriyuki Osada; Shimpei Oikawa; Kaoru Kitajima, Wiley
Functional Ecology, Sep. 2014, [Reviewed]

Seasonal change in light partitioning among coexisting species of different functional groups along elevation gradient in subalpine moorlands
Chiho Kamiyama; Shimpei Oikawa; Kouki Hikosaka, <list list-type="bulleted" id="nph12960-list-0001"> Species niches are expected to differ between different functional groups and between species with different functional traits. However, it is still unclear how functional traits contribute to niche separation between species coexisting in a community and between sites along environmental gradients. We studied seasonal changes in light partitioning among coexisting species belonging to different functional groups in moorland plant communities at different altitudes. We estimated the lifetime light absorption per unit invested leaf biomass ((Lleafmass)) as a measure of the benefit/cost ratio of light acquisition. Evergreen species absorbed more light in spring, whereas deciduous species absorbed more light in summer. A similar tradeoff was also found between short and tall species within each functional group. As a result, evergreen and shorter species had comparable phi(Lleafmass) values to those of deciduous and taller species. Evergreen species had higher phi(Lleafmass) at higher altitudes relative to deciduous species, suggesting that evergreen habit is more advantageous for the lifetime light interception at higher altitudes. Our results demonstrate that phenological tradeoffs for light partitioning can contribute to the coexistence of species with different functional traits. Our results also reveal that the most advantageous traits differ depending on environment. <doi origin="wiley" registered="yes">10.1111/(ISSN)1469-8137</doi, Wiley
New Phytologist, Aug. 2014, [Reviewed]

機能形質から種のニッチ，共存，そして消失を理解する：亜高山帯湿原群集を対象とした群集生態学と生理生態学からのアプローチ
彦坂 幸毅; 佐々木 雄大; 神山 千穂; 片渕 正紀; 及川 真平; 嶋崎 仁哉; 木村 啓; 中静 透, 国際環境研究協会
地球環境, 2014, [Reviewed]

Effects of elevated CO2 on leaf area dynamics in nodulating and non-nodulating soybean stands
Shimpei Oikawa; Masumi Okada; Kouki Hikosaka, Lead, The effects of elevated CO2 on leaf area index (LAI) vary among studies. We hypothesized that the interactive effects of CO2 and nitrogen on leaf area loss have important roles in LAI regulation.
We studied the leaf area production and loss using nodulating soybean and its non-nodulating isogenic line in CO2-controlled greenhouse systems.
Leaf area production increased with elevated CO2 levels in the nodulating soybean stand and to a lesser extent in the non-nodulating line. Elevated CO2 levels accelerated leaf area loss only in nodulating plants. Consequently, both plants exhibited a similar stimulation of peak LAI with CO2 elevation. The accelerated leaf loss in nodulating plants may have been caused by newly produced leaves shading the lower leaves. The nodulating plants acquired N throughout the growth phase, whereas non-nodulating plants did not acquire N after flowering due to the depletion of soil N. N retranslocation to new organs and subsequent leaf loss were faster in non-nodulating plants compared with nodulating plants, irrespective of the CO2 levels.
LAI regulation in soybean involved various factors, such as light availability within the canopy, N acquisition and N demands in new organs. These effects varied among the growth stages and CO2 levels., Springer
Plant and Soil, Jul. 2013, [Reviewed]

Mean residence time: leaf number, area, dry mass, and nitrogen in canopy photosynthesis
Tadaki Hirose; Shimpei Oikawa, Last, Springer
Oecologia, Feb. 2013, [Reviewed]

Effects of elevated [CO2] on stem and root lodging among rice cultivars
Zhu Chunwu; Weigo Chen; Hidemitsu Sakai; Shimpei Oikawa; Rebecca C Laza; Yasuhiro Usui; Toshihiro Hasegawa, Studies showed that elevated [CO2] would improve photosynthetic rates and enhance yields of rice; however, few studies have focused on the response of rice lodging, which is a major cause of cereal yield loss and quality reduction, under elevated [CO2]. In this study, we examined the effects of elevated [CO2] on stem and root lodging using 4 rice cultivars (86Y8, japonica hybrid; LYP9, 2-line indica hybrid; variety 9311, type of indica inbred rice, and SY63, 3-line indica hybrid) grown under two [CO2] levels: 400 and 680 mu mol mol(-1). Our results indicated that under elevated [CO2], the stem-lodging risk (SLR) of 9311 decreased, while in SY63 the SLR increased, 86Y8 and LYP9 were not significantly affected; the risk of root lodging was reduced for all cultivars, because root biomass (instead of root number) and bending strength were increased significantly, and then the increase of anti-lodging ability is far higher than that of self-weight mass moment for all cultivars. These findings suggested that higher [CO2] can enhance the risk of stem-lodging for cultivars with strong-[CO2]-responses, but may not aggravate the root lodging for all rice cultivars., SCIENCE PRESS
Chinese Science Bulletin, Jan. 2013, [Reviewed]

Demand and supply of N in seed production of soybean (Glycine max) at different N fertilization levels after flowering
Toshihiko Kinugasa; Takashi Sato; Shimpei Oikawa; Tadaki Hirose, Springer
Journal of Plant Research, Jun. 2011, [Reviewed]

Effects of elevated CO2 concentration on seed production in C-3 annual plants
Kouki Hikosaka; Toshihiko Kinugasa; Shimpei Oikawa; Yusuke Onoda; Tadaki Hirose, The response of seed production to CO2 concentration ([CO2]) is known to vary considerably among C-3 annual species. Here we analyse the interspecific variation in CO2 responses of seed production per plant with particular attention to nitrogen use. Provided that seed production is limited by nitrogen availability, an increase in seed mass per plant results from increase in seed nitrogen per plant and/or from decrease in seed nitrogen concentration ([N]). Meta-analysis reveals that the increase in seed mass per plant under elevated [CO2] is mainly due to increase in seed nitrogen per plant rather than seed [N] dilution. Nitrogen-fixing legumes enhanced nitrogen acquisition more than non-nitrogen-fixers, resulting in a large increase in seed mass per plant. In Poaceae, an increase in seed mass per plant was also caused by a decrease in seed [N]. Greater carbon allocation to albumen (endosperm and/or perisperm) than the embryo may account for [N] reduction in grass seeds. These differences in CO2 response of seed production among functional groups may affect their fitness, leading to changes in species composition in the future high-[CO2] ecosystem., OXFORD UNIV PRESS
JOURNAL OF EXPERIMENTAL BOTANY, Feb. 2011, [Reviewed]

Light interception in species with different functional groups coexisting in moorland plant communities
Chiho Kamiyama; Shimpei Oikawa; Takuya Kubo; Kouki Hikosaka, Competition for light is one of the most essential mechanisms affecting species composition. It has been suggested that similar light acquisition efficiency (I broken vertical bar(mass), absorbed photon flux per unit aboveground mass) may contribute to species coexistence in multi-species communities. On the other hand, it is known that traits related with light acquisition vary among functional groups. We studied whether I broken vertical bar(mass) was similar among species with different functional groups coexisting in moorland communities. We conducted stratified clipping in midsummer when the stand biomass reached a maximum. Light partitioning among species was estimated using a model accounting for both direct and diffuse light. Evergreen species were found to have a significantly lower I broken vertical bar(mass) than deciduous species, which resulted from their lower absorbed photon flux per unit leaf area and lower specific leaf area. Shrubs had a smaller leaf mass fraction, but their I broken vertical bar(mass) was not lower than that of herbs because they had a higher leaf position due to the presence of wintering stems. Species with vertical leaves had a higher I broken vertical bar(mass) than those with horizontal leaves despite vertical leaves being a decided disadvantage in terms of light absorption. This higher I broken vertical bar(mass) was achieved by a greater leaf height in species with vertical leaves. Our results clearly demonstrate that light acquisition efficiency was different among the functional groups. However, the trend observed is not necessarily the same as that expected based on prior knowledge, suggesting that disadvantages in some traits for light acquisition efficiency are partly compensated for by other traits., Springer
Oecologia, Jun. 2010, [Reviewed]

Interactions between elevated CO2 and N2-fixation determine soybean yield—a test using a non-nodulated mutant
Shimpei Oikawa; Kay-May Miyagi; Kouki Hikosaka; Masumi Okada; Toshinori Matsunami; Makie Kokubun; Toshihiko Kinugasa & Tadaki Hirose, Lead, Springer
Plant and Soil, Oct. 2009, [Reviewed]

Reproductive yield of individuals competing for light in a dense stand of an annual, Xanthium canadense
Yosuke Matsumoto; Shimpei Oikawa; Yuko Yasumura; Tadaki Hirose; Kouki Hikosaka, In a dense stand, individuals compete with each other for resources, especially for light. Light availability decreases with increasing depth in the canopy, thus light competition becoming stronger with time in the vegetative phase. In the reproductive phase, on the other hand, leaves start senescing, and the light environment, particularly of smaller individuals, will be improved. To study the effect of change in light climate on reproduction of individuals, we established an experimental stand of an annual, Xanthium canadense, and assessed temporal changes in whole plant photosynthesis through the reproductive phase with particular reference to light availability of individuals. At flowering, 83% of individuals were still alive, but only 27% survived to set seeds. Most of the individuals that died in the reproductive phase were smaller than those that produced seeds. Individuals that died at the early stage of the reproductive phase had a lower leaf to stem mass ratio, suggesting that the fate of individuals was determined partly by the pattern of biomass allocation in this period. At the early stage of the reproductive phase, leaf area index (LAI) of the stand was high and larger individuals had higher whole plant photosynthesis than smaller individuals. Although light availability at later stages was improved with reduction in LAI, whole plant photosynthesis was very low in all individuals due to a lower light use efficiency, which was caused by a decrease in photosynthetic N use efficiency. We conclude that light competition was still strong at the early stage of the reproductive phase and that later improvement of light availability did not ameliorate the photosynthesis of smaller individuals., SPRINGER
OECOLOGIA, Aug. 2008, [Reviewed]

Does leaf shedding increase the whole-plant carbon gain despite some nitrogen being lost with shedding?
Shimpei Oikawa; Kouki Hikosaka; Tadaki Hirose, Lead, When old leaves are shed, part of the nitrogen in the leaf is retranslocated to new leaves. This retranslocation will increase the whole-plant carbon gain when daily C gain : leaf N ratio (daily photosynthetic N-use efficiency, NUE) in the old leaf, expressed as a fraction of NUE in the new leaf, becomes lower than the fraction of leaf N that is resorbed before shedding (R-N).
We examined whether plants shed their leaves to increase the whole-plant C gain in accord with this criterion in a dense stand of an annual herb, Xanthium canadense, grown under high (HN) and low (LN) nitrogen availability.
The NUE of a leaf at shedding expressed as a fraction of NUE in a new leaf was nearly equal to the R-N in the LN stand, but significantly lower than the R-N in the HN stand. Thus shedding of old leaves occurred as expected in the LN stand, whereas in the HN stand, shedding occurred later than expected.
Sensitivity analyses showed that the decline in NUE of a leaf resulted primarily from a reduction in irradiance in the HN stand. On the other hand, it resulted from a reduction in irradiance and also in light-saturated photosynthesis : leaf N content ratio (potential photosynthetic NUE) in the LN stand., Wiley
New Phytologist, May 2008, [Reviewed]

Leaf lifespan and lifetime carbon gain of individual leaves in a stand of an annual herb, Xanthium canadense
Shimpei Oikawa; Kouki Hikosaka; Tadaki Hirose, Lead, Wiley
New Phytologist, Jul. 2006, [Reviewed]

Leaf lifespan and lifetime carbon balance of individual leaves in a stand of an annual herb, Xanthium canadense
Oikawa, Shimpei; Hikosaka, Kouki; Hirose, Tadaki
New Phytologist, 2006, [Reviewed]

Dynamics of leaf area and nitrogen in a canopy of an annual herb, Xanthium canadense
Shimpei Oikawa; Kouki Hikosaka; Tadaki Hirose, Lead, We Studied leaf area and nitrogen dynamics in the canopy of stands of an annual herb Xanthium canadense, grown at a high (HN)- and a low-nitorgen (LN) availability. Standing leaf area increased continuously through the vegetative growth period in the LN stand, or leveled off in the later stage in the HN stand. When scaled against standing leaf area, both production and loss rates of leaf area increased but with different patterns: the production rate was retarded, while the loss rate was accelerated, implying an upper limit of standing leaf area of the canopy. The rate of leaf-area production was higher in the HN than in the LN stand, which was caused by the higher rate of leaf production per standing leaf area as well as the greater standing leaf area in the HN stand. Although the rate of leaf-area loss was higher in the HN than in the LN stand, it was not significantly different between the two stands when compared at a common standing leaf area, suggesting involvement of light climate in determination of the leaf-loss rate. On the other hand, the rate of leaf-area loss was positively correlated with nitrogen demand for leaf area development across the two stands, suggesting that leaf loss was caused by retranslocation of nitrogen for construction of new leaves. A simple simulation model of leaf and nitrogen dynamics in the canopy showed that, at steady state, where the rate of leaf-area loss becomes equal to the production rate, the standing leaf area was still greater in the HN than in the LN stand. Similarly, when the uptake and loss of nitrogen are equilibrated., the standing nitrogen was greater in the HN than in the LN stand. These results suggest that leaf-area production is strongly controlled by nitrogen availability, while both nitrogen and light climate determine leaf-loss rates in the canopy., Springer
Oecologia, May 2005, [Reviewed]

Cost-benefit relationships in fronds emerging at different times in a deciduous fern, Pteridium aquilinum
Shimpei Oikawa; Kouki Hikosaka; Tadaki Hirose; Masae Shiyomi; Shigeo Takahashi; Yoshimichi Hori, Lead, We studied the life-span and ecophysiological traits of "leaves" (fronds) that emerged at different times during the growing season in a deciduous herbaceous fern, Pteridium aquilinum (L.) Kuhn (in this paper we use "leaves" for fronds to be consistent with other studies on leaf phenology). Leaf life-span was shorter in later than in early cohorts. Leaf construction cost per unit mass was nearly constant among cohorts. Later cohorts had a lower leaf construction cost per unit leaf area with a lower leaf mass per area. Leaf life-span was positively correlated with construction cost per unit area. Late cohorts showed a slightly higher leaf nitrogen content per unit mass, but a lower nitrogen content per unit leaf area with a lower light-saturated photosynthesis per unit area. Light-saturated photosynthesis per unit area was positively correlated with leaf life-span. The ratio of leaf construction cost to light-saturated photosynthesis was lower in late cohorts, and the ratio was nearly proportional to leaf life-span, suggesting that late cohorts may pay back the construction cost despite their shorter life-span., Canadian Science Publishing
Canadian Journal of Botany, Apr. 2004, [Reviewed]

グリーンムーブメント "確かに「望ましくない」のだけれど"　　　　　　　　　　　　　　　
及川真平
高速道路と自動車, 01 Jun. 2023, [Invited]

グリーンムーブメント "国内外で雑草扱いされるクズ（葛）"　　　　　　　　　　　　　　　
及川真平
高速道路と自動車, 01 May 2023, [Invited]

グリーンムーブメント "雑草とは何か"　　　　　　　　　　　　　　　
及川真平
高速道路と自動車, 01 Apr. 2023, [Invited]

葉寿命研究の歴史と近況
及川真平; 長田典之; 宮沢良行; 宮田理恵; Onno Muller
日本生態学会誌, 2013, [Reviewed]
Lead

環境条件に応じた葉寿命の種内変異：一般的傾向と機能型間の差異
長田典之; 及川真平; 宮田理恵; 神山千穂
日本生態学会誌, 2013, [Reviewed]

葉の枯死と個体の炭素収支
及川真平; 長田典之
日本生態学会誌, 2013, [Reviewed]
Lead

葉寿命研究のこれから　　　　　　　　　　　　　　　
長田典之; 及川真平; Onno Muller; 宮田理恵
日本生態学会誌, 2013, [Reviewed]

Lifetime carbon gain of individual leaves in a stand of an annual herb, Xanthium canadense
Oikawa Shimpei; Hikosaka Kouki; Hirose Tadaki
Abstracts of the Annual Meeting of the Ecological Society of Japan, 2005

葉群内における個葉、葉面積、窒素のターンオーバー
及川 真平; 彦坂 幸毅; 広瀬 忠樹
日本生態学会大会講演要旨集, 2003

Plants and microclimate (3rd edition)　　　　　　　　　　　　　　　
Hamlin G. Jones, Joint translation
森北出版, 2017

植物学の百科事典　　　　　　　　　　　　　　　
Joint work
丸善出版, 2016

Structure and Function of Mountain Ecosystems in Japan　　　　　　　　　　　　　　　
Kouki Hikosaka; Takehiro Sasaki; Chiho Kamiyama; Masatoshi Katabuchi; Shimpei Oikawa; Masaya Shimazaki; Hiroshi Kimura and Tohru Nakashizuka
Springer, 2015

Structure of herbaceous plant stands and canopy photosynthesis models　　　　　　　　　　　　　　　
Single work
2008

〔Major achievements〕Does plant strategy on resource investment in seeds and appendages vary depending on the cost of seed production and the function of the appendages?
衣笠利彦; 永松大; 板垣智之; 向井駿広; 及川真平
第７２回 日本生態学会大会, 17 Mar. 2025
20250315, 20250318

〔Major achievements〕土壌肥沃度勾配に沿った葉の寿命, 窒素吸収, 葉窒素の平均滞留時間の変化　　　　　　　　　　　　　　　
及川真平
第７２回 日本生態学会大会, 17 Mar. 2025
20250315, 20250318

富士スバルラインの路傍植生における種構成の標高に伴う変化　　　　　　　　　　　　　　　
紺野由佳; 及川真平; 塩見正衛; 中野隆志; 安田泰輔; 山村靖夫
2024年度 システム農学会, 01 Nov. 2024
20241101, 20241102

高速道路盛土法面におけるクズ群落のオゾン・窒素酸化物の低減効果は樹林に匹敵するか？　　　　　　　　　　　　　　　
堅田元喜; 和田龍一; 及川真平; 榎本忠夫; 坂上伸生; 高瀬唯; 鈴木雄吾
第65回大気環境学会年会, Sep. 2024
20240911, 20240913

道路緑地における植生管理が法面土壌の化学性および物理性に及ぼす影響ついて
宮﨑永遠・バティン一希・鈴木雄吾・西澤智康・堅田元喜・榎本忠夫・高瀬 唯・及川真平・坂上伸生
日本土壌肥料学会 関東支部大会, 20 Nov. 2022, 日本土壌肥料学会
20221120, 20221120

〔Major achievements〕群落光合成を最大化しない葉面積指数の適応的意義: クズ群落のケース
及川真平
The 69 th Annual Meeting of the Ecological Society of Japan, Mar. 2022, Ecological Society of Japan
20220314, 20220319

〔Major achievements〕ダイズの栽培化に伴う葉の生産性と被食防御の変化
戸田風香・及川真平
第69回 日本生態学会大会, Mar. 2022, 日本生態学会
20220314, 20220319

〔Major achievements〕強害雑草クズの繁茂がソバの送粉と結実に与える影響
斎藤歩乃花・板垣智之・及川真平
第69回 日本生態学会大会, Mar. 2022, 日本生態学会
20220314, 20220319

高速道路法面における除草剤の散布が土壌の化学性および土壌微生物群集に与える影響　　　　　　　　　　　　　　　
坂上伸生; 迫田翠; 竹内琴音; 西澤智康; 堅田元喜; 福島慶太郎; 榎本忠夫; 高瀬唯; 及川真平
日本土壌肥料学会 2020年度 岡山大会, Sep. 2020
202009

高速道路法面における除草剤の散布が土壌微生物群集に与える影響　　　　　　　　　　　　　　　
迫田翠; 坂上伸生; 郭永; 西澤智康; 堅田元喜; 榎本忠夫; 高瀬唯; 及川真平
日本土壌微生物学会 2020年度大会, Jun. 2020
202006

クズの繁茂した高速道路法面における除草剤散布が窒素流出に及ぼす影響　　　　　　　　　　　　　　　
福島慶太郎; 吉村佳一郎; 堅田元喜; 坂上伸生; 榎本忠夫; 高瀬 唯; 及川真平
日本生態学会 第67回大会, Mar. 2020
202003

側道から見た高速道路の盛土法面に繁茂するクズに対する市民の景観評価　　　　　　　　　　　　　　　
高瀬唯; 及川真平; 榎本忠夫; 堅田元喜; 坂上伸生
日本造園学会関東支部大会, 23 Nov. 2019, 日本造園学会
20191123

高速道路盛土法面の植生が及ぼす大気質への局地的影響の予備的観測　　　　　　　　　　　　　　　
堅田元喜; 渡邉未来; 高橋晃子; 福島慶太郎; 久保田智大; 及川真平; 高瀬唯; 榎本忠夫; 坂上伸生
第60回 大気環境学会年会, 18 Sep. 2019, 大気環境学会
201909

〔Major achievements〕Leaf shedding optimizes the photosynthetic rate of the canopy in deciduous woody species
Tomoki Tanaka; Chiharu Kurokawa; *Shimpei OIKAWA
The 66th Annual Meeting of the Ecological Society of Japan, 18 Mar. 2019, Ecological Society of Japan

〔Major achievements〕Comparison of leaf traits between wild progenitor of soybean and soybean cultivars
Ayaka TOGASHI; Shimpei Oikawa
The 66th Annual Meeting of the Ecological Society of Japan, 17 Mar. 2019, Ecological Society of Japan

〔Major achievements〕Is leaf protein composition determinant of nitrogen resorption?　　　　　　　　　　　　　　　
Ryo Tanabe; Shinpei Oikawa
The 66th Annual Meeting of the Ecological Society of Japan, 17 Mar. 2019, Ecological Society of Japan

〔Major achievements〕植物の落葉タイミングは最適か: 野外での検証　　　　　　　　　　　　　　　
日本生態学会第63回全国大会, 24 Mar. 2016

〔Major achievements〕ダイズの栽培化に伴う形質の変化：何を得て何を失ったのか？　　　　　　　　　　　　　　　
日本生態学会第63回全国大会, 22 Mar. 2016

A meta-analysis of leaf nitrogen distribution within plant canopies　　　　　　　　　　　　　　　
第63回日本生態学会仙台大会, Mar. 2016

植物葉群内における葉間窒素分配のメタ解析　　　　　　　　　　　　　　　
東北植物学会第5回大会, 19 Dec. 2015

窒素固定植物の老化葉からの窒素回収　　　　　　　　　　　　　　　
日本生態学会第62回全国大会, Mar. 2015

〔Major achievements〕栽培化によって損なわれた植物機能の解明：増大する環境ストレスへの応答（研究代表者）　　　　　　　　　　　　　　　
基盤研究 (B)
Apr. 2023 - Mar. 2027

〔Major achievements〕種子の防御と散布のためのエネルギーコスト：繁殖資源分配研究の新視点（研究分担者）
挑戦的研究(萌芽)
Jun. 2022 - Mar. 2024

高速道路緑地における新たな管理手法の開発・管理　　　　　　　　　　　　　　　
Apr. 2019 - Mar. 2024

マメ科植物は窒素を贅沢に使っているのか：老化する葉からの窒素回収とその要因（研究代表者）　　　　　　　　　　　　　　　
基盤研究（C）
Apr. 2017 - Mar. 2021

マメ科植物は窒素を贅沢に使っているのか：老化する葉からの窒素回収とその要因（研究代表者）　　　　　　　　　　　　　　　
2017 - 2020

寄生性植物ネナシカズラの嗜好性：ジェネラリストか, スペシャリストか?（研究代表者）　　　　　　　　　　　　　　　
Apr. 2018 - Mar. 2019

安全で快適な高速走行環境の構築に向けた新たな緑地管理手法の開発ー大学・企業・住民で考える未来志向型の高速道路緑地管理手法（研究代表者）　　　　　　　　　　　　　　　
Apr. 2018 - Mar. 2019

茨城大学イノベーション研究推進プログラム　　　　　　　　　　　　　　　
Apr. 2015 - Mar. 2016

マメ科植物の窒素再利用とリターフォールを介した土壌への影響に関する研究（研究代表者）　　　　　　　　　　　　　　　
Apr. 2014 - Mar. 2015

育種過程で植物が獲得・喪失した形質の探索（研究代表者）　　　　　　　　　　　　　　　
2014 - 2015

植物の栄養・繁殖成長が個葉の枯死に及ぼす影響（研究代表者）　　　　　　　　　　　　　　　
若手研究
Apr. 2011 - Mar. 2014

植物の栄養・繁殖成長が個葉の枯死に及ぼす影響（研究代表者）　　　　　　　　　　　　　　　
若手B
2011 - 2013

生物季節モニタリング（生物季節観測）
informant
国立環境研究所気候変動適応センター（CCCA）、気象庁、環境省, Aug. 2021 - Present

生物季節モニタリング調査　　　　　　　　　　　　　　
investigator
Aug. 2021 - Present

環境省 重要生態系監視地域モニタリング推進事業（モニタリングサイト1000）里地里山調査（第４期）
informant
環境省、日本自然保護協会, Apr. 2018 - Present

環境省 重要生態系監視地域モニタリング推進事業（モニタリングサイト1000）里地里山調査
informant
環境省、日本自然保護協会, Apr. 2018 - Present

環境省 重要生態系監視地域モニタリング推進事業（モニタリングサイト1000）里地里山調査（第5期: 2023年度~2027年度）　　　　　　　　　　　　　　
informant
環境省、日本自然保護協会, Apr. 2023 - Mar. 2028

気候変動対策と生物多様性の保全〜持続可能な地球環境のために〜　　　　　　　　　　　　　　
lecturer
茨城県水戸生涯学習センター「三の丸カレッジ」, 19 Oct. 2024

環境省 重要生態系監視地域モニタリング推進事業（モニタリングサイト1000）里地里山調査（第４期: 2018年度~2022年度）
informant
環境省、日本自然保護協会, Apr. 2018 - Mar. 2022

茨城県建設コンサルタンツ協会 技術力向上セミナー　　　　　　　　　　　　　　
appearance
一般社団法人 茨城県建設コンサルタンツ協会, 06 Oct. 2021 - 06 Oct. 2021

水戸芸術館企画展「道草展」・道草資料室
informant
水戸芸術館, 29 Aug. 2020 - 08 Nov. 2020

茨城県技術懇談会　　　　　　　　　　　　　　
lecturer
Nov. 2020