In a cool-temperate forest, deciduous (summer-green) broad-leaved trees are apparently dominant, although in other temperate forests evergreen tree species are dominant. However, the structure of a cool-temperate forest is not fully understood; dominant deciduous trees cannot seem to regenerate successfully; evergreen coniferous species are also distributed in a cool-temperate forest, tough, their growth and regeneration have been never compared to deciduous species'. In this study, we detected which is later-successional, evergreen coniferous species or deciduous broad-leaved tree species in a cool-temperate forest, to understand the structure of a cool-temperate forest.

First, we investigated the light availability for photosynthesis in the understory of a cool-temperate forest. Second, we contrasted the regeneration success of an evergreen conifer, Abies firma (AF), and typical late-successional deciduous broad-leaved tree species, Fagus crenata (FC) and F. japonica (FJ), at various light-availability sites. Last, we tried to clarify what physiological- and morphological traits explain the differences in growth and regeneration success in the forest understory.

Long-term, short-interval measurements of incident photosynthetically active photon flux density (PPFD; μmol m(-2) s(-1)) on the forest floor are essential for estimating the leaf carbon gain of understory plants. Such PPFD data, however, are scarce. We measured PPFD at 1-min intervals for more than 12 months in cool-temperate forest sites and reported the data as a PPFD frequency distribution. We chose five sites: an open site (OPN), the understory of a deciduous broad-leaved tree stand with no visible gaps (DCD), that of an evergreen conifer stand (EVG), that of a deciduous broad-leaved tree stand with a gap of approximately 80m2 (GAPDCD), and that of an evergreen conifer stand with a gap of approximately 100m2 (GAPEVG). DCD were divided into three sub sites (DCD1-3) to investigate variation within a small area. GAP-sites were consisted of two sub sites (GAPDCD1-2 and GAPEVG1-2) differing in the distance from the gap center. Using the PPFD data, we estimated the summer seasonal (May-October) net assimilation rate of leaves (NARL) at each site for various photosynthetic capacities (Amax: μmol m(-2) s(-1)) and other parameters of a light response curve of CO2 assimilation rates. At OPN, the average daily accumulated PPFD (mol m(-2) day(-1)) was highest in May (28.2) and lowest in December (8.2). Even at OPN, the class of instantaneous PPFD that contributed most to NARL was 250-300 μmol m(-2) s(-1). Such a large contribution of lower PPFD is suggested to be an important feature of a field light-availability. At DCD, the relative PPFD (RPPFD, %) to OPN was 7.2 during canopy closure and 49.4 after leaf shedding (averaged for 3 sites). EVG had the lowest light environment throughout the year. Its average RPPFD was 3%. For GAP sites, summer seasonal RPPFD (%) was 15.6, 18.8, 6.4 and 15.6 for GAPDCD1, GAPDCD2, GAPEVG1 and GAPEVG2, respectively. At OPN, the NARL increased with Amax (which ranged from 1 to 40), suggesting that plants at OPN do not maximize NARL. In contrast, at DCD and EVG, Amax values were attained that did maximize NARL, suggesting that plants at these sites could maximize the NARL. Amax-NARL relationships for GAPDCD and GAPEVG showed similar trend to closed canopy sites, DCD and EVG, while NARL and Amax* of GAP sites were larger than at these sites. Among DCD1-3, the daily accumulated PPFD (mol m(-2) day(-1)) averaged in summer ranged 1.3-1.8 and the maximum NARL value differed up to 1.5 times. It indicates that Amax and NARL can be various among plants under a similar canopy conditions.

At field sites under the three different canopy conditions like OPN, DCD and EVG, we evaluated regeneration success for AF, FC and FJ mainly by sapling height-growth rate and seedling relative growth rate (RGR) and survival rate. In parallel, we measured physiological- and morphological traits of leaf for seedlings, such as net assimilation rate (NAR), leaf mass per area (LMA), leaf-lifespan (LL), allocation rate of assimilates to leaf (Lp), etc. Using these traits, we offered "leaf reproductive index (LRI)". LRI is expressed as (NAR*Lp*LL / LMA), and indicates long-term whole-plant carbon balance, or potential RGR. LRI also shows multiple effects of the traits on potential RGR. By LRI, we tried to explain how these traits affect plant growth and regeneration in the forest understory. Our result shows that an evergreen conifer, AF grow well and can regenerate without gap opening at DCD, whereas, deciduous broad-leaved trees, FC and FJ cannot. FC and FJ had smaller growth- and survival rate than AF. In addition, at EVG, AF continues to grow for longer time than FC and FJ, yet, for the regeneration success, improvement in light-availability will be needed. LRI showed that at DCD evergreen species can maintain positive carbon balance because of sufficiently large NAR by assimilating during leafless period of deciduous trees and long LL which can make potential RGR > 0 even in the case of NAR < LMA. On the other hand, late-successional deciduous broad-leaved species like Fagus spp. was indicated to hardly keep the positive carbon balance under the closed canopy. This is because their LMA is too large relative to the NAR. For maintaining positive carbon balance, for example, FC must have Lp of over 0.9 at DCD, it is not realistic. LRI can be applied to any plant and light-availability if NAR and leaf traits are collected.

Our result clearly indicated that an evergreen conifer, AF, is later-successional than deciduous broad-leaved trees, FC and FJ. We concluded that evergreen tree species can be the latest-successional also in a cool-temperate forest. Deciduous broad-leaved trees will be replaced by evergreen conifers with the succession. However, now in Japanese cool-temperate forest, deciduous forests are seen. It is clear that another mechanism rather than "most shade-tolerant" is needed to explain the current dominance of FC and FJ. Past time human disturbance like logging exerted strongly on conifers can be one of the plausible factors.

本論文は、研究の背景を概観した前文、実際の研究に関する2つの章、および今後の研究の展望も含む総合討論からなっており、冷温帯林の構造解明を目的として、常緑針葉樹と落葉広葉樹とではどちらがより遷移後期的であるかを、光環境と樹木の成長速度に焦点をあてて解明した研究が著されている。

第1章では、冷温帯林の林床における光利用可能性と葉の純生産速度を明らかにした研究が述べられている。まず、これまで不足していた野外での長期的かつ詳細な光量子束密度データを、複数の環境について測定したデータが示されている。1年間をこえる期間を通じて1分刻みで測定した種々の環境における光合成有効光量子密度束のデータは貴重であり、実際の野外の光環境が、これまでに種々の試算に用いられたsinの自乗関数などによって表現されるものとは、大きく異なることが明らかにされている。続いて、この詳細な光環境データにもとづいて、葉の純生産量と最適光合成能力の推定が行われている。正確な光環境データが、このような推定にとって必須であることが明確に示されている。本章の内容は、冷温帯林に限らず、あらゆる植物の生産速度推定に応用できる利用価値の高いものであり、特に植物の成長を確実に測定することが困難な、林床のような暗い環境における葉の生産量が明確に把握できる点で、高く評価できるものである。

第2章では、冷温帯林において常緑針葉樹(モミ)の方が落葉広葉樹(ブナ、イヌブナ)よりも遷移後期的であるということが、多様な光環境における成長速度の比較から明らかにされ、同時に各生活型の生理的形態的特性からその説明が試みられた研究が述べられている。本章では、葉の生理的形態的特性に基づいた新規の炭素収支の判別式が提示され、この判別式を通常の成長解析の手法と組み合わせて、様々な光環境下における、ブナ、イヌブナ、およびモミの樹高成長が予測されている。予測値は、実測値ときわめてよい一致を示しており、落葉樹林床では常緑広葉樹の成長がよいことが明確に示されている。したがって、本章の内容は、まず、落葉広葉樹が最も遷移後期的であるとされてきた、従来の冷温帯林の構造理解を大きく覆した点で評価することができる。また、本章の手法や結果は、冷温帯林同様に常緑樹と落葉樹が混交している他の温帯林にも応用できることも重要である。特に、葉の生理的形態的特性に基づく簡便な炭素収支の判別式は、あらゆる植物の成長予測に応用できるものであり、その提案は極めて高く評価できる。

第1章と第2章を通して、森林の光環境と植物の成長に関する知見が理路整然と述べられ、対象とした種の数は限られるものの、冷温帯においては、常緑針葉樹が落葉広葉樹よりも遷移後期的であることが説得力をもって示されている。また、研究手法や新たに提出された判別式も汎用性の高いものである。このように、論文提出者の研究は、結果が斬新であるばかりでなく、冷温帯に限らず多様な森林の構造理解に貢献しうる基礎的なものである。

なお、本論文第1章は杉浦大輔氏、澤上航一郎氏、市橋隆自氏、谷友和氏、舘野正樹氏との共同研究であるが、論文提出者が主体となって分析および検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。