Search

Abstract: Wetland restoration is a common practice in the southeastern United States, where changing land use patterns have resulted in the widespread degradation of coastal ecosystems. Re-establishment of indigenous vegetation [...]

Determining which water sources a plant accesses throughout a year is an important step in understanding how changes in source characteristics affect utilization by plants. Water sources of Acer negundo L. and Betula nigra L. of the foothills of the southern Appalachians Mountains were examined during one full year, including the phenological stages of leaf bolt, flowering, and leaf senescence and abscission. Source utilization was monitored, comparing the isotopic composition of water samples from woody tissue with those of possible water sources at the site. Species used deep ground and shallow soil water, with a greater reliance on deeper sources during the late growing season. Betula nigra was typically more depleted in 82H than all water sources measured, while values from A. negundo were more variable throughout the study. Intraspecifically, isotopic values did not vary monthly or seasonally for either species (P > 0.56), while interspecific values were different for December, January, and July samplings (P < 0.02). Positive relationships occurred between air temperature and isotopic values of both species (P Keywords: deuterium, stream water, phenology, stable isotopes, water uptake. La determination des sources d'eau auxquelles une plante a acces au cours d'une annee constitue une etape importante pour comprendre comment les changements relatifs aux caracteristiques des sources affectent leur utilisation par les vegetaux. Les sources d'eau de Acer negundo L. et Betula nigra L. au piemont des Appalaches du sud ont ete examinees durant une annee complete, y compris les stades phenologiques de la feuillaison, de la floraison et de la senescence et abscission foliaire. L'utilisation de la source a ete repertoriee en comparant la composition isotopique des echantillons d'eau du tissu ligneux a celle des sources d'eau possibles du site. Les especes utilisaient l'eau souterraine profonde et l'eau superficielle du sol, avec une plus grande dependance aux sources profondes tard durant la saison de croissance. Betula nigra etait typiquement davantage depourvu de 82H que toutes les sources d'eau mesurees, alors que les valeurs de A. negundo etaient plus variables tout au long de l'etude. Les valeurs isotopiques intraspecifiques ne variaient pas en fonction du mois ou de la saison pour aucune des especes (P > 0,56), alors que les valeurs interspecifiques etaient differentes dans les echantillons de decembre, janvier et juillet (P < 0,02). Des relations positives etaient observees entre la temperature de l'air et les valeurs isotopiques des deux especes (P Mots-cles : deuterium, eau de ruisseau, phenologie, isotopes stables, absorption d'eau., Introduction Decreased water availability due to lower groundwater levels, increased evaporation from soils, or meteorological drought can lead to increased plant water stress and shifts in water sources (Burgess et [...]

Who steals my purse steals trash; 'tis something, nothing; 'Twas mine, 'tis his, and has been slave to thousands; But he that filches from me my good name Robs me [...]

Light quantity and quality strongly influence plant ecophysiology, especially in cloud forests. Only a few studies have investigated the quantitative effects of clouds on incident sunlight, and almost no studies have explored the specific effects of cloud immersion on light quality. In the present study, the spectral quantity and quality of sunlight during sunny (S), low cloud (LC), and cloud immersed (CI) days in open canopy and understory microsites are compared in high elevation cloud forests in the southern Appalachian Mountains, USA. Cumulative daily radiation measured on CI days was 85% less than on S days at open sites, but about the same as S days in understory sites. LC and CI conditions enhanced blue light by 5%-15% in open sites, but decreased it by 25%-60% in understory sites. In cloudy conditions, red light decreased by 6%-11% in open areas and increased by 25%-30% in understory sites. Cloudiness also resulted in important changes in spectral band ratios, and was associated with light diffusivity. Thus, cloud regimes had a strong influence on the photosynthetic light quality of incident sunlight. Key words: diffuse light. Southern Appalachians, spectroradiometry. La qualite el la quantite de lumiere influencent fortemenl l'ecophysiologie ties plantes, surtout dans les forets ennuagees. II n'existe que quelques etudes traitant des effets quantitatifs des nuages sur la lumiere solaire incidente, et presque aucune etude ayant explore les effets specifiques de l'immersion nuageuse sur la qualite de la lumiere. Les auteurs comparent la quantite et la qualite spectrales de la lumiere solaire au cours de journees ensoleillees (S), de faible nebulasite (LC) et d'immersion nuageuse (CI), dans la canopee ouverte et sur des microsites, et les comparent dans des forets en haute elevation dans le sud des montagnes Appalaches. aux Etats-Unis. La radiation quotidienne simulee mesuree en jours CI est 85% moindre par rapport aux jours S sur les sites ouverts, mais presque la meme que celle des jours S sur les sites en sous-etages. Les conditions des jours LC et CI augmentent la lumiere bleue de 5 a 15% dans les sites ouverts, mais la diminuent de 25 a 60%, sur les sites en sous-etages. Sous des conditions uuageuses, la lumiere rouge ditninue de 6 a 11% dans les aires ouvertes el augmente de 25 a 30% sur les sites en sous-etages. La nebulosite entralne egalement d'importantes modifications dans les ratios des bandes spectrales et s'associe a la diffusivite de la lumiere. Ainsi, les regimes nuageux influencent fortement la qualite pholosyniheiique de la lumiere solaire incidente. Mots-cies : lumiere diffuse, sud des Appalaches, spectroradiometrie. [Traduit par la Redaction], Introduction Cloud cover can have both direct and indirect impacts on plant biology. These include changes in microclimatological conditions and alterations of the spectral quality of light, all of which [...]

Because of the characteristically low temperatures and ambient C[O.sub.2] concentrations associated with greater altitudes, mountain forests may be particularly sensitive to global warming and increased atmospheric C[O.sub.2]. Moreover, the upper treeline is probably the most stressful location within these forests, possibly providing an early bellwether of forest response. Most treeline studies of the past century, as well as recently, have correlated temperatures with the altitudinal limits observed for treelines. In contrast, investigations on pre-establishment seedlings, the most vulnerable life stage of most tree species, are rare. There appears to be specific microclimatic factors dictated by wind and sky exposure that limit seedling survival, and also generate the distorted tree forms commonly observed at treeline. Seedling survival appears critical for creating the biological facilitation of microclimate at the community level which is necessary for the growth of seedlings to normal tree stature, forming new subalpine forest at a higher altitude. Es posible que--a causa de caracteristicas que estan asociadas con altitudes mas altas: las bajas temperaturas y las concentraciones ambientales de dioxido de carbono--los bosques en las montanas estan extra sensibles al calentamiento global y el aumento de dioxido de carbono en la atmosfera. El borde superior del bosque es probablemente el lugar con la mas estres y proviene uno de los primeros avisos de como reaccionara el bosque entero. En el pasado y hoy en dia, la mayoria de los estudios del borde del bosque ha conectado la temperatura con los limites de la altitud. En contraste, investigaciones de arboles infantiles son raras, y la infancia de los arboles es el periodo de vida mas vulnerable. Aparece que hay factores micro-climaticos dictados por la exposicion del viento y cielo que limitan la sobrevivencia de los arboles infantiles, y que generan arboles deformados observados al borde del bosque. Es mas, la sobrevivencia de arboles infantiles es critica para crear la facilitacion biologica del micro-clima en una comunidad arbolada. Esta facilitacion es necesaria para el crecimiento de arboles infantiles a arboles maduros, los que forman un nuevo bosque subalpino en una altitud mas alta. Keywords Treeline * Timberline * Ecotone * Seedling. Facilitation * Climate, Introduction Future impacts of global climate change such as warming temperatures could bring significant alterations in the altitudinal patterns of high-altitude forests and their contiguous alpine zones, potentially altering important [...]

Fraser fir (Abies fraseri (Pursh) Poir.) is an endemic tree species found only in refugial mountain-top forests in the southern Appalachian Mountains, USA. Very few studies have investigated the ecophysiology of this species in its natural environment. We measured and compared photosynthetic gas exchange and water relations of understory germinant seedlings (3 years old and ~0.15-1 m tall), and saplings (>10 years old and ~1-2 m tall) on a leaf-level basis. No significant differences in gas exchange were found among the age classes, although midday xylem water potentials were significantly different among the age classes. Differences in maximum photosynthesis ([A.sub.max]) were Le sapin de Fraser (Abies fraseri (Pursh) Poir.) est une espece d'arbre endemique dans les forets refuges sur le sommet des montagnes dans la partie sud des Appalaches, aux Etats-Unis d'Amerique. Tres peu d'etudes se sont interessees a l'ecophysiologie de cette espece dans son environnement naturel. Nous avons mesure et compare les echanges gazeux lies a la photosynthese et les relations hydriques sur la base des aiguilles des semis en voie de germination (GS, hauteur 3 ans et --0,15-1 m de haut) et des jeunes tiges (SA, >10 ans et ~1-2 m de haut) dans le sous-bois. Aucune difference significative dans les echanges gazeux n'a ete observee entre les differentes classes d'age malgre les differences significatives entre les differentes classes d'age dans le cas du potentiel hydrique du xyleme a midi. Les differences dans la photosynthese maximum ([A.sub.max]) etaient inferieures a 13% et variaient de 2,4 a 2,7 µmol*[m.sup.-2]*[s.sup.-1] (P = 0,26). La conductance foliaire maximum ([g.sub.max]) variait de 0,16 a 0,18 mol*[m.sup.-2][s.sup.-1] soit une variation de moins de 13% (P = 0,15). Le potentiel hydrique du xyleme a midi des SE et des SA etait semblable (-1,2 a -1,4 MPa) mais les valeurs des GS etaient significativement moins negatives (-0,2 a -0,5 MPa). Par consequent, les echanges gazeux et les relations hydriques dans ces classes d'age en sous-bois semblent similaires indiquant qu'il n'y a aucune contrainte particuliere chez la regeneration preetablie quelle que soit la classe d'age. [Traduit par la Redaction], Introduction Fraser fir (Abies fraseri (Pursh) Poir.) is an endemic tree species found only in seven mountain-top populations occurring from southwestern Virginia to southern North Carolina, USA (Ramseur 1960). Over [...]

Although anthocyanin coloration in lower (abaxial) leaf cells has been documented for numerous species, the functional significance of this character has not been comprehensively investigated according to habitat or leaf orientation. Here, we demonstrate that abaxial anthocyanin may function as a photoprotectant, similarly to its purported role in upper (adaxial) cells, in leaves vulnerable to high irradiance incident on abaxial surfaces. Spectral scans were derived for Galax urceolata leaves with the following phenotypes: abaxial or adaxial anthocyanin only, abaxial and adaxial anthocyanin, and no anthocyanin. To determine whether anthocyanins conferred protection from photoinhibition, maximum photosystem II efficiencies of red (anthocyanic) and green (acyanic) surfaces were compared during and after exposure to photoinhibitory conditions. Leaves were either positioned with their adaxial surfaces facing the light source or inverted to expose abaxial surfaces. Spectral scans showed increased absorptance of 500-600 nm wavelengths by red surfaces (consistent with the absorbance spectrum of anthocyanin), regardless of whether that surface was abaxial or adaxial. Leaves with anthocyanin in either illuminated surface were also photoinhibited less than leaves lacking anthocyanin in that surface. These results suggest that anthocyanic layers reduce absorbed sunlight in the mesophyll not only for adaxial surfaces, but also for the abaxial. Adaxial/abaxial anthocyanin plasticity may therefore be adaptive in high-light environments or during light-sensitive developmental stages where leaf orientation and/or substrate albedo are variable. Key words: abaxial; anthocyanin; Diapensiaceae; evergreen; Galax; leaf orientation; photoprotection, plasticity.

Abstract: Photosynthesis, water status, and associated physiological parameters were measured in chronically drought-stressed seedlings (5 years of below-average precipitation, 107 cm net deficit) of Abies lasiocarpa (Hook.) Nutt. above (treeline [...]

High-altitude forests of the southern Appalachian Mountains (USA) are frequently immersed in clouds, as are many mountain forests. They may be particularly sensitive to predicted increases in cloud base altitude with global warming. However, few studies have addressed the impacts of immersion on incident sunlight and photosynthesis. Understory sunlight (photosynthetically active radiation, PAR) was measured during clear, low cloud, and cloud-immersed conditions at Mount Mitchell and Roan Mountain, NC (USA) along with accompanying photosynthesis in tour representative understory species. Understory PAR was substantially less variable on immersed vs. clear days. Photosynthesis became light-saturated between ~100 and 400 [micro]mol x [m.sup.-2] x [s.sup.-1] PAR for all species measured, corresponding closely to the sunlight environment measured during immersion. Estimated daily carbon gain was 26% greater on clear days at a more open canopy site but was 22% greater on immersed/cloudy days at a more closed canopy site. [F.sub.v/[F.sub.m] (maximum photosystem II efficiency) in Abies fraseri seedlings exposed to 2.5 min full sunlight was significantly reduced (10%), indicating potential reductions in photosynthesis on clear days. In addition, photosynthesis in microsites with canopy cover was nearly 3-fold greater under immersed (2.6 mmol x [m.sup.-2] x [h.sup.-1]) vs. clear conditions (0.9 mmol x [m.sup.-2] x [h.sup.-1]). Thus, cloud immersion provided more constant PAR regimes that enhanced photosynthesis, especially in shaded microsites. Future studies are needed to predict the survival of these refugial forests under potential changes in cloud regimes. Key words: Abies fraseri; Aster divaricatus; diffuse light; Eupatorium rugosum; fog; photoinhibition; Sambucus pubens; seedling.

Abstract: Survival of seedlings is a demographic bottleneck for tree populations in alpine tree-line ecotones (ATE) and may play a pivotal role in determining the elevation of timberlines and tree [...]

Light propagation and distribution inside leaves have been recognized as important processes influencing photosynthesis. Monochromatic light absorption across the mesophyll was measured using chlorophyll fluorescence generated from illumination of the cut edge (cpi-illumination), as well as the adaxial or abaxial surfaces of the leaf. Species were selected that had basic leaf types: laminar leaf with adaxial palisade layer (Rhododendron catawbiense), needle with palisade (Abies fruseri), and needle without palisade (Picea rubens). Fluorescence was more evenly distributed across the mesophyll for adaxially illuminated leaves with a palisade cell layer, as well as for the needles (cylindrical) without palisade, when compared to fluorescence generated by abaxial illumination. Moreover, fluorescence from green light illumination remained high across the mesophyll of adaxially illuminated R. catawbiense, indicating a possible influence of mesophyll structure on internal light distribution beyond that of chlorophyll levels. These data support the idea that light propagation within the mesophyll is associated with asymmetric mesophyll structure, in particular the presence of palisade cell layers. In addition, we propose that the evolution of a more cylindrical leaf form, such as found in conifer species, may be a structural solution to excessive sunlight that replaces the highly differentiated mesophyll found in most laminar-leaved species. Key words: Abies fraseri; chlorophyll; Ericaecae; high-altitude species; leaf orientation: photosynthesis; Picea rubens: Pinaceae;

Co-occurring species may utilize vastly different strategies to cope with limited water resources, particularly in areas subjected to predictable and recurring drought. While these physiological responses have commonly been measured at the leaf level, in small seedlings, and integrated in fluxes of whole stands or watersheds, sap flux measurements in large trees have become a useful tool for monitoring transpiration of individual canopies over long time periods. In this study, sap flux (J(sub s)) was measured with constant heat sap flow gauges for co-occurring species which have been previously evaluated at the leaf level. Measurements were taken in a subalpine stand containing most of the dominant species of the central Rocky Mountains (Pinus contorta, Abies lasiocarpa, Populus trernuloides, and Pinus flexilis). Daily J(sub s) values were parabolically related to daytime average atmospheric vapor pressure deficit (D) in all species, with a broad range of maximum J(sub s) values occurring between 1.2 and 1.8 kPa. Populus tremuloides had the greatest increases in Js with increasing D, while Pinus contorta showed the lowest J(sub s)- A decrease in maximum J(sub s) was observed for all species later in the season when soil moisture declined from 0.35 to 0.24 m(super 3)/m(super 3) at 0-45 cm. Late-season J(sub s) in A. lasiocarpa decreased 50% due to stomatal closure in response to the soil moisture deficit, regardless of daily D. In contrast, the Pinus species were sensitive to D, showing larger late-season reductions in J(sub s) on high than on low D days. Populus tremuloides showed less sensitivity to soil moisture than the other species, with relatively high J(sub s) continuing late into the season and intermediate change in the response of J(sub s) to D with decreasing soil moisture. Stand-level estimates of transpiration by plots dominated by Pinus contorta and A. lasiocarpa (2.6 +/- 0.6 mm/d) were found to be similar to plots dominated by Populus trernuloides (2.7 +/- 0.6 mm/d) despite the nearly fourfold higher leaf area indices for the conifers. Key words: Abies lasiocarpa; hydraulic conductance, whole plant; leaf water potential; Pinus contorta; Pinus flexilis; Populus tremuloides; sap flow; seasonal drought; subalpine forest; transpiration; water relations.

Five plant communities in Western Australia, as well as selected desert and Rocky Mountain species of the western USA, were surveyed to evaluate associations among leaf structure, orientational properties, and the sunlight exposure and precipitation characteristic of each community. Selected leaf structural features have been associated previously with photosynthetic function and included shape, thickness, the ratio of thickness to width, stomatal distribution, leaf surface coloration, and the number and distribution of palisade cell layers. Decreases in annual precipitation (0.4 mm) leaves. Also, the percentage of species with a leaf thickness to width ratio >0.1, which were amphistomatous, or which had palisade cell layers beneath both leaf surfaces, increased from >20% in the highest rainfall and lowest sunlight community to >80% in the community with least rainfall but greatest sunlight exposure. Over 70% of the species in the most mesic, shaded community had lighter abaxial than adaxial leaf surfaces (leaf bicoloration). All of the above structural features were positively associated with a more inclined leaf orientation ([r.sup.2] = 0.79), except for leaf bicoloration, which was negatively associated ([r.sup.2] = 0.75). The ratio of adaxial to abaxial light was more strongly associated with leaf bicoloration ([r.sup.2] = 0.83) and the presence of multiple adaxial and isobilateral palisade cell layers([r.sup.2] = 0.80) than with total incident sunlight on just the adaxial leaf surface ([r.sup.2] = 0.69 and 0.73, respectively). These results provide field evidence that leaf orientation and structure may have evolved in concert to produce a photosynthetic symmetry in leaf structure in response to the amount of sunlight and other limiting factors of the community. This structural symmetry may serve fundamentally to regulate the distribution of both light and C[O.sub.2] levels inside the leaf and, thus, increase photosynthetic C[O.sub.2] uptake per unit leaf biomass. Key words: leaf structure; leaf orientation; photosynthesis; sunlight; rainfall; precipitation; Western Australia.

Terrestrial plants responded to the amount of sunlight and stress in a given habitat by evolving leaf structural properties in concert with leaf orientational capabilities Morphological and anatomical features of [...]

The typical conifer structure, including closely packed needle-like leaves on individual shoots, and conical crown shape may offer enhanced chances of survival during winter. This arrangement may also increase summer carbon accumulation, resulting from enhanced levels of sunlight interception, warming of the leaf and photosynthesis per unit of stem biomass.

Responses in net photosynthesis (A) and stomatal conductance to water vapor (g) in two Sonoran desert herbaceous perennials exposed to atypical conditions of variable sunlight were compared to previously documented responses in subalpine herbs that grow in an environment of extreme sunlight variability. In the desert herb Penstemon parryi, little variation in g (< 80 mmol [m.sup.-2] [s.sup.-1]) or xylem pressure potential ([psi]) was observed during periods of sunlight variability, although A varied substantially. This response was consistent with those subalpine plants that also maintained relatively constant [psi] during sun/shade periods. [psi] differed significantly between sun and shade periods in the desert herb Ambrosia ambrosioides (- 1.65 vs. - 1.36 MPa, respectively), and reductions in g during shade were as rapid (24.5 mmol [m.sup.-2] [s.sup.-1] [min.sup.-1]) as observed in subalpine species with similarly variable [psi]. However, recovery in g during sun periods occurred at a rate well below that measured in subalpine herbs. Thus, alterations in g did not closely correspond to variations in A and sunlight in this desert herb. This response pattern was unlike any measured in 22 subalpine species previously studied. Rapid recovery in g during sun periods that follow shade has likely been selected for in subalpine herbs that are exposed daily to sunlight variability. Such a response can substantially increase carbon gain. We suggest that the benefits of a strong tracking response in g to variations in A and sunlight are not realized in desert environments with relatively constant solar regimes. However, rapid stomatal closure during shade periods does insure higher water use efficiency and the potential for recovery in [psi] in this desert herb.

The relationship between near-infrared reflectance at 800 nm (NIRR) from leaves and characteristics of leaf structure known to affect photosynthesis was investigated in 48 species of alpine angiosperms. This wavelength was selected to discriminate the effects of leaf structure vs. chemical or water content on leaf reflectance. A quantitative model was first constructed correlating NIRR with leaf structural characteristics for six species, and then validated using all 48 species. Among the structural characteristics tested in the reflectance model were leaf trichome density, the presence or absence of both leaf bicoloration and a thick leaf cuticle ([is greater than] 1 [micro] m), leaf thickness, the ratio of palisade mesophyll to spongy mesophyll thickness (PM/SM), the proportion of the mesophyll occupied by intercellular air spaces (%IAS), and the ratio of mesophyll cell surface area exposed to IAS ([A.sub.mes]) per unit leaf surface area (A), or [A.sub.mes]/A. Multiple regression analysis showed that measured NIRR was highly correlated with [A.sub.mes]/A, leaf bicoloration, and the presence of a thick leaf cuticle ([r.sup.2] = 0.93). In contrast, correlations between NIRR and leaf trichome density, leaf thickness, the PM/SM ratio, or %IAS were relatively weak ([r.sup.2] [is less than] 0.25). A model incorporating [A.sub.mes]/A, leaf bicoloration, and cuticle thickness predicted NIRR accurately for 48 species ([r.sup.2] = 0.43; P [is less than] 0.01) and may be useful for linking remotely sensed data to plant structure and function. Key words: [A.sub.mes]/A; bicoloration; leaf structure; mesophyll; near-infrared; reflectance.

Periods of variable sunlight due to intermittent cloudcover are common in the subalpine zone of the central Rocky Mountains. To evaluate the ecophysiological significance of daily variations in sunlight, responses in net photosynthesis, stomatal conductance to water vapor, and water use efficiency ([CO.sub.2] uptake per unit [H.sub.2.O] lost) were measured in 12 subalpine species including trees, shrubs, and herbs. Measurements were made during intervals of alternating full sun and shade throughout a growth season that included daily and seasonal periods of water stress. In general, photosynthesis responded rapidly in all growth forms to changes in sunlight due to clouds, while stomatal conductance responded more slowly or not at all. Large reductions in photosynthesis were measured during cloud periods for all species, while stomatal conductance was considerably less responsive to changes in sunlight in woody species. The greatest and most variable reductions in stomatal conductance during shade ([is less than]10 to a 68% reduction of full sun levels) occurred in herbaceous species. Carbon gain in the woody species was enhanced because stomatal conductance remained relatively high during both sun and shade periods. Water use efficiency in most species was generally lower during shade periods compared to full sun periods. However, woody plants maintained higher xylem pressure potentials that were much nearer shade values compared to herbs. The greater decrease in stomatal conductance and photosynthesis in herbs during shade resulted in improved water use efficiency, although carbon gain was reduced during both shade and the subsequent full-sun period. These apparent growth strategies during non-steady-state response periods appear opposite to generalizations made previously for woody vs. herbaceous species under steady-state conditions.

Studies of floral movements in response to environmental change are rare in the literature, and information about possible adaptive benefits appears nonexistent. The closure of the upright, tubular flowers of alpine gentian (Gentiana algida) were observed during the frequent afternoon thunderstorms characteristic of the central and southern Rocky Mountains (USA). Flowers closed within minutes of an approaching thunderstorm and reopened after direct sunlight returned. Corolla opening widths decreased ~10%/min prior to rainfall, in close correspondence to declines in air and corolla temperatures. Identical floral behavior was also induced experimentally in the field and laboratory by artificial changes in corolla temperature. Corolla closure did not occur during experiments that simulated natural changes in solar irradiance, wind, or absolute humidity during a thunderstorm. Furthermore, individual G. algida plants forced experimentally to remain open during rain had substantial losses of pollen after single rain events (up to 34%) and if forced to remain open for the entire flowering period (59%). Subsequent seasonal reductions in female fitness (up to 73%) also occurred, including seed size and mass, number of ovules produced, number of viable seeds produced per ovule, and seed germination. Thus, corolla closing and opening in G. algida associated with frequent summer thunderstorms may be a behavioral adaptation that improves both paternal and maternal reproductive effort. Key words: alpine; floral movements; Gentiana; reproduction; temperature.

Mesophyll structure has been associated with the photosynthetic performance of leaves via the regulation of internal light and C[O.sub.2] profiles. Differences in mesophyll structure and chlorophyll distribution within three ontogenetically different leaf types of Eucalyptus globulus ssp. globulus were investigated. Juvenile leaves are blue-grey in color, dorsiventral (adaxial palisade layer only), hypostomatous, and approximately horizontal in orientation. In contrast, adult leaves are dark green in color, isobilateral (adaxial and abaxial palisade), amphistomatous, and nearly vertical in orientation. The transitional leaf type has structural features that appear intermediate between the juvenile and adult leaves. The ratio of mesophyll cell surface area per unit leaf surface area ([A.sub.mes]/A) of juvenile leaves was maximum at the base of a single, adaxial palisade layer and declined through the spongy mesophyll. Chlorophyll a + b content showed a coincident pattern, while the chlorophyll a:b ratio declined linearly from the adaxial to abaxial epidermis. In comparison, the mesophyll of adult leaves had a bimodal distribution of [A.sub.mes]/A, with maxima occurring beneath both the adaxial and abaxial surfaces within the first layer of multiple palisade layers. The distribution of chlorophyll a + b content had a similar pattern, although the maximum ratio of chlorophyll a:b occurred immediately beneath the adaxial and abaxial epidermis. The matching distributions of [A.sub.mes]/A and chlorophyll provide further evidence that mesophyll structure may act to influence photosynthetic performance. These changes in internal leaf structure at different life stages of E. globulus may be an adaptation for increased xeromorphy under increasing light exposure experienced from the seedling to adult tree, similar to the characteristics reported for different species according to sunlight exposure and water availability within their native habitats. Key words: [A.sub.mes]/A; chlorophyll; Eucalyptus; leaf structure; light; Myrtaceae; palisade mesophyll; [V.sub.mes]/V.