@misc{oai:miyazaki-u.repo.nii.ac.jp:00004047.1,
 author = {Richard, Gyimah},
 month = {2020-06-21},
 note = {木材資源-の過度な依存によるアフリカ熱帯林の減少に対して､近年
関心が高まっている｡しかし､ガーナを含む多くのアフリカの国々での
植林を成功させるための課題の一つは､環境条件の異なる立地で植林に
用いる樹種､特に郷土種の生理生態的な特性の情報が不足していること
である｡本研究では､ガーナの森林再生に有用と考えられる数種の熱帯樹につ
いて土壌水分､土壌養分及び光環境をコントロールした条件下での､生
長及び生理生態的反応を調べた｡
1.まず､ガーナの半落葉樹林帯の異なる立地に生育する2種のアフ
リカマホガニー(Khaya anthotheca and K. ivorensis)の実生苗を､
ガラス室内で3レベルの土壌水分条件(過湿､適潤及び乾燥)で育て､
苗高､乾物重､物質配分に対する土壌水分の影響を調べた｡K.
ivorensisでは土壌水分の不足は､乾物重を著しく減少させ､乾燥に対
する順応戦略によって葉と根の物質配分を変化させた｡K.anthotheca
の実生苗は､適潤及び乾燥処理においてK.ivorensisの実生苗よりも
乾物重が多かった｡これらの結果からK.anthothecaはアフリカ湿潤熱
帯の中で4-5ケ月の乾期のある立地での造林樹種に､K.ivorensisは
同地域の低地など土壌の乾燥が起こりにくい立地の造林樹種に適してい
ると考えられる｡
2.次に､乾燥半落葉樹林帯からサバンナまでのより乾燥ストレスの
大きい地域での多様な植林目的に適する樹種を検討した.Cadrela
odorata､Cassia siamea, Khaya senegalensisの実生苗の耐乾性を､
土壌の乾燥過程での蒸散速度､気孔コンダクタンス及びクロロフィル蛍
光の変化から比較した結果､C.siameaとK.senegalensisはより強い
耐乾性を示し､C.odoramaは乾燥回避性を示し土壌水分減少の早い段階
から消費水分の減少が見られた｡この結果から､この地域の中で強い乾
燥ストレスとなる立地ではC.siameaとK.senegalensisを植栽する
方が有利である｡
3.立地の光環境との関係を検討するために､3樹種の実生苗を相対
照度(RLI)10､22､45､100%の光条件下で育て､乾物重､物質分配､
最大光合成速度(Amax)､水分利用効率(WUE)､光飽和点(LSE)及び
最大光量子収率(Fv/Fm)から比較した｡先駆種であるTerminalia
superba は､RLI100%でAmax､WUE､LSE及びFv/Fnの平均値が最も高く､
RLI10%では､Amax、WUE､LSEの平均値は最も低くなった。遷移中期
種であるCedlela odorataはRLI22%でMansonia altissimaより高い
Amax 及びLSEを示した｡また､3樹種ともRLI45%で乾物重は最も大き
くなった｡全光下及び日陰の条件下における葉と根-の物質分配は､典
型的なパターンを示した。これらの結果からT.superbaは全光条件と
なる立地､C.odorataは光に関して広い適応域を持ち林内の広いギャッ
プから全光条件下まで､M altissimaは林内での小さなギャップ-の造
林樹種に適していると考えられる｡
4.初期生長-の施肥の効果をCedrela oforataとTerminalia
superba の実生苗を用いて光環境(RLI33%および100%)との関係で
調べた｡両種とも施肥､RLI33%で高い相対成長率を示し､乾物重は施
肥､RLI100%で最も高くなった｡施肥で両種の葉の窒素濃度と相対クロ
ロフィル含有量が増加し､無施肥ではRLI100%でクロロフィル含有量
が低下した｡また施肥は､RLI100%でのC.odorataのFv/Fmを増加さ
せ､光阻害の影響を低減させるなど､初期生長への施肥の大きな効果が
認められた｡
この研究により､ガーナの森林再生に有望な樹種の光､土壌水分､養
分などの立地条件に対する反応特性が明らかになり､研究の成果はこれ
からの西アフリカ地域での､森林再生､造林の際の立地に応じた樹種選
択に実際に適用できると考えられる｡
\nDeforestation of African tropical forests as a result of greater
dependence on wood resources is a growing concern. However, one of the
challenges to successful reforestation efforts in many African countries,
including Ghana, is the lack of knowledge on ecophysiological traits of
potential plantation species, especially native species, under different site
conditions.
A series of experiments were conducted under controlled conditions
to examine morphological and/ or physiological responses of some
selected tropical tree species to soil water, nutrient and/ or light treatments.
First, soil water availability on seedling height, dry matter production
and allocation were investigated for two African mahogany species
(Khaya anthotheca and K. ivorensis) from contrasting semi-deciduous
forest sub-types of Ghana. Seedlings of the two species were subjected to
three soil watering treatments, which were termed wet, moderate and dry
treatment, in a glass house facility. Soil water deficit greatly reduced total
dry matter production in K. ivorensis and altered proportional allocation to
leaves and roots as a possible strategy for acclimation. In the moderate and
dry treatments, K. anthotheca seedlings had greater total plant dry matter
than K. ivorensis seedlings, which suggests that K. anthotheca have
considerable potential for plantation forestry in the humid tropics ofAfrica
where distinct drought periods (4-5 months) occur.
Second, drought resistance in seedlings of Cedrela odorata, Cassia
siamea and Khaya senegalensis were estimated through measurements of
transpiration rate, stomatal conductance and chlorophyll fluorescence, and
growth responses under drought conditions of a greenhouse experiment.
This study was conducted in order to assess the suitability of these species
for plantation forestry in semi arid and drier sites of Ghana. Results of this
study indicated that C. siamea and K. senegalensis exhibited stronger
drought resistance, whereas C. odorata followed a drought-avoiding
strategy and was more conservative in its water use. At soil water potential
of -0.4 MPa (mild drought condition), C. odorata showed restricted
growth and lowered physiological traits. In contrast, C. siamea and K.
senegalensis showed reduced growth and physiological traits at -1.0 and
-0.8 MPa, respectively, conferring greater advantage to them than C.
odorata as plantation forestry species in semi-dry environments.
Third, containerized seedlings of three tropical species were grown
under four different light treatments [i.e., 100, 45, 22 and 10% sunlight].
Photosynthetic properties and growth analysis reflected the species
successional status with Terminalia superba, the pioneer species showing
largest mean light-saturated photosynthesis (Amax), light saturation
estimates (LSE), intrinsic water-use efficiency (WUE) and maximal
quantum yield of photosystem II (Fv/Fm) at 100% sunlight, whereas at
10% sunlight, it showed the lowest Amax, WUE and LSE. At 22% sunlight,
Cedrela odorata, an intermediate successional species had greater Amax
and LSE than Mansonia altissima, a non-pioneer light demander and T.
superba. Superior biomass production was found at 45% sunlight for all
the species. Allocation to leaf and root followed patterns typical of plants
growing in sun or shade conditions. Light requirement for species growth
and physiological functions may be specie-specific. This study therefore,
suggests that T. superba may have greater potential in plantation forestry
at exposed sites; C. odorata has broad ecological amplitude making it
suitable for planting in medium gaps and exposed sites; M altissima may
be suitable for enrichment planting in smaller gaps.
Furthermore, growth responses to fertilization in seedlings of Cedrela
odorata and Terminalia superba were studied under 100 and 33% sunlight.
Both species attained greatest relative height growth rate at 33% sunlight
with fertilization and showed superior biomass production at 100%
sunlight with fertilization. N fertilization enhanced foliar N concentration
and relative Chl content of both species and without fertilization, 100%
sunlight-grown foliage of C. odorata showed reduced Chl content. At
100% sunlight, fertilization enhanced Fv/Fm of C. odorata and thus,
reduced the specie's susceptibility to photoinhibition. Enhancing
acclimation of C. odorata and T. superba seedlings through longer
hardening periods plus nutrient additions in an exposed nursery conditions
may stimulate their initial growth when planted in the field.
Results from these experimental studies may have practical use in
screening tropical tree species for use in reforestation and plantation
forestry in degraded forest sites of West Africa where site resources such
as soil water, light and nutrient availability may vary considerably.},
 title = {Studies on growth  and ecophysiological characteristics of tropical tree species useful in Ghana},
 year = {}
}