Báo cáo khoa học: "Variation in leaf morphology and branching pattern of some tropical rain forest species from Guadeloupe (French West Indies) under semi-controlled light conditions"

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Original article Variation in leaf morphology and branching pattern of some tropical rain forest species from Guadeloupe (French West Indies) under semi-controlled light conditions M Ducrey INRA, Laboratoire de Recherches Forestières Méditerranéennes, Avenue A Vivaldi, F-84000 Avignon, France accepted 7 July 1992) 18 March 1992; (Received of 7 canopy from the Guadeloupe tropical rain forest (Dacryodes Summary — Seedlings species excelsa, Amanoa caribaea, Richeria grandis, Simaruba amara, Symphonia globulifera, Byrsonima coriacea and Podocarpus coriaceus) were raised in full sunlight and under artifical neutral shade transmitting 6, 11, 19 and 54% light for 2 to 3 years. At the end of this period, the number of leaves and branches, leaf size, specific leaf area and stomatal density were observed for each plant. For all species, the maximum number of leaves was obtained in partial shade (11 or 19% sunlight). Branch- ing occurrence depended more on species type than on light conditions. Both individual leaf size and specific leaf area increased regularly with shade, but in a proportion which varied according to the species. Stomatal density was highly variable from one species to another and increased with greater light. The morphological plasticity of species response to light conditions was then analysed and related to shade tolerance. In order of decreasing plasticity, the first species found were R gran- dis, S amara and B coriacea, which were the most plastic and the most shade intolerant, followed by A caribaea and P coriaceus, less plastic but shade-tolerant species. Finally, D excelsa and S globu- lifera were found to be the least plastic species and highly or moderately shade-tolerant. tropical rain forest / leaf morphology / specific leaf area / branching pattern / shade tolerance Résumé — Variations de la morphologie foliaire et branchaison de quelques espèces de la forêt tropicale humide de Guadeloupe en conditions semi-contrôlées d’éclairement. De jeunes semis de 7 espèces de la strate arborescente de la forêt tropicale humide de Guadeloupe (Da- cryodes excelsa, Amanoa caribaea, Richeria grandis, Simaruba amara, Symphonia globulifera, Byr- sonima coriacea et Podocarpus coriaceus) ont été élevés pendant 2-3 ans en pleine lumière et sous ombrages artificiels neutres laissant passer 6%, 11%, 19% et 54% de la pleine lumière. À la fin de cette période on a observé sur chaque plant, le nombre de feuilles et de ramifications, la taille et la surface spécifique des feuilles ainsi que la densité stomatique. Pour toutes les espèces étudiées, le nombre de feuilles est maximal pour des ombrages moyens (11 ou 19% de la pleine lumière). La présence de ramifications dépend davantage des espèces que des conditions d’éclairement. La sur- face individuelle des feuilles ainsi que leur surface spécifique augmentent régulièrement avec l’om-
mais dans des proportions variables selon les espèces. La densité stomatique, très variable brage d’une espèce à l’autre, augmente avec l’éclairement. La plasticité morphologique des espèces en ré- ponse aux conditions d’éclairement est ensuite analysée et interprétée en termes de tolérance à l’om- brage. Par ordre de plasticité décroissante, on trouve R grandis, S amara et B coriacea qui sont les espèces les plus plastiques et les plus intolérantes à l’ombrage. On trouve ensuite A caribaea et P coriaceus, moins plastiques mais tolérantes à l’ombrage. D excelsa, et S globulifera sont les moins plastiques et sont modérément ou fortement tolérantes à l’ombrage. tropicale humide / morphologie foliaire / surface foliaire spécifique / blanchaison / tolé- forêt à l’ombrage rance studied under 2 different thinning intensi- INTRODUCTION ties. The variations in environmental condi- tions due to the different silvicultural treat- The reaction of trees to varying light envi- ments were then used as a means of be ronments, particularly to shade, can determining the range of light requirements compared at different levels. First of all, at in the species studied, from the most the species level, we find species which shade-intolerant to the most shade-tolerant. require full sunlight and others which are A uniquely silvicultural approach is not more or less shade-tolerant. On the indi- sufficient to understand the forest behavi- vidual level, within the same species or our of a given species and its relative genotype, we find trees which have grown place in a forest succession. It therefore in different light environments and have seemed of interest to further the know- different phenotypes (shade phenotypes ledge on these species by studying mor- or sun phenotypes). Finally, within the phological variations in leaves and branch- same individual, particularly within a stand, ing pattern in response to light conditions sun and shade leaves are found, depend- during growth. This approach is of value ing on their position in the tree crown. for 2 reasons. First of all, the use of mor- These facts known for generally are phological criteria to account for physiolog- most tree species growing in temperate cli- ical potentials under varying light condi- mates, but have been less studied for trop- tions appears to be possible using existing ical species. In particular, the shade re- relationships between physiological and sponse of the main commercial species in morphogenetic processes (Tsel’Niker, the tropical rainforest of Guadeloupe is 1977). Secondly, the range of morphologi- practically unknown. cal variations in the leaf system under ex- treme light conditions is a good means of The conducted experiments (Ducrey, determining the forest behaviour of a given 1982; Ducrey and Labbé, 1985) stimu- on species (Smith, 1982; Fetcher et al, 1983; lated and controlled natural regeneration Goulet and Bellefleur, 1986). in the Guadeloupe rainforest provided the first results (Ducrey and Labbé, 1986) on This article examines the morphological the forest behaviour of the main tree spe- variations in leaves and branching pattern cies favoured for natural regeneration. for 7 evergreen species subjected to 5 dif- Methods similar to the progressive felling ferent light conditions. The experiment also regeneration and the tropical shelterwood took into account photosynthetic response, system were adopted. Survival and growth growth and biomass production, which will of seedlings from different species were be discussed in further papers.
full MATERIALS AND METHODS sunlight. The 4 tunnel shelters were 15 m long and 6 m wide and covered with reinforced transparent PVC as a protection against rainfall. Three of them were shaded with different black Description of seedlings neutral shade screens in order to obtain various of species studied shade conditions. Finally, global radiation meas- urements with Li-Cor pyranometers indicated 6.4% light under tunnel I, 11.4% under tunnel II, The seedlings used for the experiment were 18.8% under tunnel III and 54.3% under tunnel sampled from the tropical rainforest of Guade- IV. loupe, French West Indies. They came from the Table I summarizes climatic data under tun- "Débauchée" area (Ducrey, 1986) at an eleva- nel shelters. These were opened and oriented in tion of 250 m. Mean temperatures were 23 °C in the direction of prevailing winds. The microcli- January and 26 °C in July. Mean annual rainfall matic conditions under the tunnels were the was > 3 000 mm. There was a short dry season same as those in the open air treatment (meteo- from January to April, but the monthly rainfall rological data measured by a weather station), always > 100 was mm. except for tunnel IV whose maximum tempera- The 7 species studied were evergreen domi- tures were slightly higher than the others. This nant and co-dominant trees from the middle and could be explained, as the shade under this tun- late successional gradient of the Guadeloupe nel was only created by the reinforced transpar- rainforest: Dacryodes excelsa Vahl, Amanoa ca- ent plastic cover which caused a more signifi- ribaea Kr et Urb and Podocarpus coriaceus LC cant warming effect. Rich are late successional shade-tolerant spe- The protocol was applied to all the species cies; Simaruba amara Aubl and Richeria grandis except P coriaceus and A caribaea. The P coria- Vahl are middle successional shade-intolerant ceus seedlings were placed under the same species; Byrsonima coriacea is present in mid- moderately shaded tunnel (tunnel III) in March dle and late succession, whereas Symphonia 1981 and then subjected to the different experi- globulifera L, a wet soil specialist, is a late suc- mental conditions in January 1982. The experi- cessional species. However, their shade reac- ment with A caribaea started in March 1982. tion is not well known. In each tunnel, plants were grouped by spe- D excelsa and S amara have compound cies with a container density of 16 plants per m . 2 leaves, while the other species have simple All the plant groups were moved once a week in- leaves. All could be easily identified in the forest side each tunnel so that they occupied the same with the exception of B coriacea, understorey place every 8 weeks. This was undertaken to which was difficult to differentiate when young uniformize growth light conditions. At the begin- from 2 neighbouring forms, the "Patagonian" ning of the shading experiment, there were be- Byrsonima and the "Coal wood" Byrsonima. tween 30 and 40 plants per species and per treatment. The number of plants remaining at the end of the experiment is given in tables II Experimental treatments and III. Containers were watered twice a week. No fertilizer was used during the experiment. The 1-yr-old seedlings were sampled from the for- margin in January 1981, transplanted in 9-I est Plant observations and measurements containers filled with surface forest soil, and placed under the forest canopy to ensure better recovery. After 3 months, the containers were At the end of the experiment (between March transferred to tunnel shelters covered with shade 1983 and January 1984 depending on the spe- cloths to obtain the required amount of shade. cies) when the plants were approximately 1.00- Seedlings were then between 10 and 20 cm 1.50 m in height, several observations were height. made: counting leaves on the main stem and on The seedlings were separated into 5 different branches, dry weight and surface area of 2 ran- treatment groups: 4 treatments under plastic domly selected leaves from the stem and 2 tunnels and one treatment in the open air and leaves from the branches on each plant. The
RESULTS data used to calculate the specific leaf were 2 -1 (cm g ) of each species for each light area condition. Leaf counting The leaf stomatal density (number of stomata per leaf area unit) was determined during the last quarter of 1982 via leaf prints. A thin collod- Table II summarizes data concerning the ion film was spread on the leaf surface to pre- mean number of leaves per seedling for pare a print of epidermic and stomatal cells that simple-leaved species. The mean number could be observed by optical microscopy. These of leaves varied from one species to an- leaf prints were taken for 2-6 leaves per species other: 22 on average for R grandis, 54 for and per tunnel and were made systematically on the lower and upper side of the leaves. B coriacea, 95 for A caribaea, 140 for
S globulifera and 317 for P coriaceus. For Study of branching pattern each species, the maximum number of leaves was observed either in tunnel II or All the seedlings studied were very young. III and some statistical differences might It was thus interesting to note the appear- have occurred among tunnels. The distri- ance of branches and their variations un- bution of leaves on the main axis or on the der different light conditions (tables II, III). branches was related to the percentages The compound-leaved species D excel- of branched seedlings and to the number and S amara had no branches. These sa of branches per branched seedling. only appeared under natural forest condi- R grandis leaves were almost entirely situ- tions in larger and older trees. ated on the main axis while those of A ca- ribaea, B coriacea and P coriaceus were simple-leaved species had different The mainly located on the branches. degrees of branching. R grandis had only Table III provides the same information just begun to ramify and had very few for compound-leaved species. D excelsa branches. All the S globulifera seedlings had an average of 11 leaves per plant, but were highly branched and had between 15 the number of leaflets per leaf increased and 17 branches per seedling. The other with increasing shade from 3 in the open species also had a high percentage of air to 5 in the darkest tunnel. S amara had branched often close to 100%. This plants, between 5-10 leaves. It would appear that maximum under low light percentage was the number of leaflets per leaf increased conditions for A caribaea and P coriaceus with exposure to shade, but the repeated and under sunlight conditions for B coria- attacks of phyllophagous caterpillars typi- cea. However, it appeared that branching cal of this species made the results difficult species-dependent occurrence was more than to interpret. light regime-dependent.
leaved species) for each species in rela- Leaf characteristics tion to relative light intensity which they re- ceived during growth. First of all, there was Figure 1 indicates the variations in area of a high variability in leaf size from one spe- individual leaves or leaflets (for compound-
close to 50 cm g S amara was the 2 -1 . cies to another. Taking all the tunnels to- was gether, the average leaf areas increased most affected by increasing shade: 149% from 10 cm for P coriaceus to nearly 200 2 increase in specific leaf area when going 2 cm for R grandis. from full sunlight to shadiest tunnel. It was followed by R grandis, B coriacea and P co- There was also a regular decrease in riaceus with 100% increase, then by D ex- = leaf area for all species when relative light celsa and A caribaea with = 75% increase, increased. Some species reacted strongly and finally by S globulifera which had < to shade and the area of individual leaves 50% increase. As already mentioned for in- more than doubled when going from full dividual leaf area, an exponential decrease sunlight to 6% sunlight. This was the case in specific leaf area in relation to relative for R grandis (150% increase), B coriacea light intensity was found except for A cari- (120% increase) and S amara (100% in- baea, R grandis and S amara which were crease), followed by A caribaea (65% in- less affected by deep shading. crease), D excelsa, S globulifera and P co- riaceus (50% increase for each species) which reacted less strongly to variations in Stomatal density light conditions. The right side of figure 1 shows that for most species there was a The leaf prints showed that for all the stud- quasi-linear decrease in individual leaf area in relation to the logarithm of relative ied species, stomata were present only on the lower side of the leaves. The stomata light intensity. This demonstrated an expo- as well as the epidermic cells had a large nential variation in relation to relative light variety of forms and sizes, as shown in fig- intensity, a relationship which has fre- quently been found for similar phenomena. ure 3. This variability was demonstrated by means comparisons of stomatal density Specific leaf area (leaf area recorded by (number of stomata per mm for each ) 2 unit of dry leaf biomass) is shown in figure species in each light treatment (table IV). 2. Leaves of all species in full sunlight had a specific area close to 100 cm g except 2 -1 Stomatal density for full sunlight condi- for P coriaceus, whose leaves were thicker tions showed the highest values for D ex- celsa (661 stomata per mm and A cari- ) 2 and tougher and whose specific leaf area
Stomatal density was highest under full (325 stomata per mm The 5 other ). 2 baea sunlight conditions and decreased as light species had a stomatal density close to intensity diminished. All the species did not 150 stomata per mm . 2
react in the same manner. R grandis was B coriacea (58%), A caribaea (43%), P co- the most affected species with 67% de- riaceus (38%), and D excelsa (35%). In creased from full sunlight to the shadiest contrast, S globulifera, with only 3% de- crease, did not appear to be affected by environment. The decrease in stomatal density was smaller for S amara (59%), shading.
Some authors (Logan and Krotkov, Morphological plasticity 1969; Loach, 1970) found that with temper- species the number of leaves reached ate Species plasticity for a given trait - leaf maximum in full sunlight. In many decid- a size, specific leaf area or stomatal density uous leaved species, the number of leaves may be calculated as the range of this - is set from budbreak, while in evergreen trait from full sunlight to the shadiest condi- tropical species growth is more or less tion, divided by corresponding data under continuous and the number of leaves full sunlight conditions. For each trait, spe- present at a given moment is more highly cies plasticity was calculated, and species related to environmental conditions. ranked from the most plastic to the least According to Smith (1982), the branch- plastic species (table V). Then a mean ing ability could be considered as a criteri- ranking was calculated which gave an on for adaptation to shade. This hypothe- overall appreciation of the morphological sis agrees with that of Bazzaz and Pickett plasticity for each species. (1980), who found that trees in the first Ranked by decreasing order of plastici- successional stages ramify little and have it was found that R grandis, S amara ty, weak branches. Such a lack of branches and B coriacea were the most plastic spe- was observed from young-aged pioneer cies, A caribaea and P coriaceus the medi- species present in Guadeloupe: Cecropia um-plastic species, and finally D excelsa peltata and to a lesser degree Miconia mi- and S globulifera the least plastic species. rabilis. For the species studied, a lack of branches was also the case for R grandis DISCUSSION AND CONCLUSION and S amara which appear in the middle successional stages of species during col- Large differences were observed regarding onization of open areas by forest. On the leaf morphology and branching pattern be- other hand, D excelsa, a final species in tween the species depending on light con- the succession, was not branched either. ditions. The interpretation of these differ- There are thus species-specific differences ences in terms of light behaviour could independent of adaptation to shade. The improve knowledge on the ability of differ- other species were all more or less ent species to grow under determined light branched. Except for B coriacea, more conditions. seedlings were branched in the shadiest tunnels than in the open air. These results show a tendency towards a greater occu- Counting of leaves and ramifications pation of available space for better energy capture by plants grown in the shade. general for all the studied species there In leaves in tunnels II and III than were more Individual leaf area in the others. The decrease under strong light conditions could be due to a more For all the species studied, shade in- rapid aging which brought about premature creased leaf size. Some species reacted leaf fall. The decrease under lower light very strongly: R grandis, B coriacea and conditions could be due to a decrease in S amara; other species reacted less: P co- nutrition- morphogenetic activity following a riaceus, S globulifera and D excelsa; al andenergetic deficiency.
A caribaea fell between the 2 groups. The of specific leaf in response to light en- area results from various reports in the litera- vironment. ture, in particular those of Logan and Krot- S amara was the most plastic species. It kov (1969), Logan (1970), Loach (1970), followed in decreasing order of leaf was McClendon and McMillan (1982) showed plasticity by R grandis, B coriacea and P that shade does not always have the same coriaceus, then by D excelsa and A cari- effect. finally by S globulifera. baea and From these authors, it appears that This ranking is basically the same as some species, such as Populus deltoides, the typical forest ranking for increasing Populus tremuloides or Prunus american- shade adaptation as found previously (Du- us react negatively to shade. Others such crey and Labbé, 1986). Similar results as Quercus rubra or Acer saccharum bare- have been reported by Fetcher et al (1983) ly show any reaction. Still others such as who found that in very shady conditions, Morus alba, Fraxinus pennsylvanica or Li- Heliocarpus appendiculatus, a pioneer or riodendron tulipifera react very positively large gap species, was twice as plastic as to shade (leaves twice the size). In the lat- Dipteryx panamensis, a small gap species ter species, however, too much shade can (see table VI). Among the temperate spe- have a depressive effect. This is what was cies, Loach (1967) found results along the also observed in R grandis. lines: Liriodendron tulipifera, a same shade-intolerant species, was more plastic than Fagus grandifolia, a shade-tolerant Specific leaf area species. However, Populus tremuloides, a highly intolerant shade species, does not conform to this rule (table VI). Shade has the most noticeable and con- sistent effect on the specific leaf area. Results obtained for other species Leaves of equal dry weight always had a (table VI) show the regular increase in spe- larger surface area in shade than in sun- cific leaf area as light decreases. It is al- light. The effect of shade, however, dif- ways hazardous to compare results ob- fered depending on the species, illus- tained under different experimental as trated in table V which shows the conditions. Nevertheless, looking at results plasticity
tion to light can be found in Platanus occi- obtained for conditions ranging from 13- dentalis (Duba and Carpenter, 1980), and 20% light, it can be seen that shade- Quercus robur (Tronchet and Grandgirard, tolerant species have a specific leaf area 1956), as well as in Quercus sessiliflora close to 1.4-fold greater than those in full and Fagus silvatica (Aussenac and Du- sunlight, while shade-intolerant species have values from 1.8-2.0-fold greater. crey, 1977). Studies on non-woody plants (Schoch specific leaf area in shade is Increase in et al, 1980) showed that the stomatal in- generally accompanied by a decrease in dex, ie the number of stomata related to leaf thickness. Leaves exposed to full sun- the total number of epidermic cells, de- light could be twice as thick as leaves in pends on light conditions. The stomatal in- the shade, as shown by Tronchet and dex increases when light increases during Grangirard (1956), Aussenac and Ducrey the ontogenic phase of the leaf. Regarding (1977), Duba and Carpenter (1980), Fet- our results this could indicate that shade cher et al (1983) and Nygren and Kelloma- has a doubly negative effect on stomatal ki (1983). These modifications are accom- density: a), by increasing cell size; and b), panied by variations in the relative impor- by decreasing the percentage of stomata tance of the lacunose parenchyma and the in relation to epidermic cells. This is obvi- palisade parenchyma of the leaf (Star- ously important to the physiological func- zecki, 1974) which may cause changes in tions of the leaf, particularly to their stoma- the diffusion of carbon dioxide within the tal conductance. leaf and thus in photosynthetic processes. Differences in stomatal plasticity among species occurred, as shown in table V. Stomatal density Variations in stomatal density from sunlight to shade environments were greater for R grandis, S amara and B coriacea (more Different stomatal densities were observed shade-intolerant species) than for A cari- from one species to another: in general, baea, P coriaceus and D excelsa (more many small stomata or few large-sized shade-tolerant species). S globulifera, an- stomata were found. Our results agree other shade-tolerant species, had no stom- with those of Carpenter and Smith (1975) atal plasticity at all. who found a stomatal density for some 50 shade-grown shrub and arborescent spe- cies ranging from 65-900 stomata per Species plasticity and shade adaptation mm and also with those species reviewed 2 by Willmer (1983). studied all reacted to shade increase was observed The species In particular an by increasing individual leaf area and spe- in stomatal density with increase in light cific leaf area and by decreasing stomatal conditions. Similar results were obtained by Fetcher et al (1983) for H appendicula- density. tus whose stomatal density more than specific leaf area, which is Variations in doubled when exposed to between 2 and generally accompanied by variations in 100% light. This species also has the par- leaf thickness, demonstrate an adaptation ticular trait of possessing stomata on the by decreasing the distance trav- to shade upper side of leaves when in full sunlight, by photons to carboxylation sites and elled which are absent in strong shade. The by decreasing resistance to the diffusion of same increase in stomatal density in rela- carbon dioxide in the mesophile. More
generally, reducing leaf biomass per unit least plastic species are D excelsa which area in shade leaves is a plant strategy found to be more shade-tolerant than was used to reduce leaf cost under limiting light A caribaea, and S globulifera, another spe- environment. cies with a shade-tolerant reputation. In the same manner, the increase in the The agreement between these aspects amount of stomata in full sunlight shows is thus not perfect and morphological crite- that the leaf must have a better control of ria alone are insufficient. In fact, many oth- increase er characters should be examined to inves- through temperature as seen an in stomatal conductance and thus tigate tree plasticity in response to light transpi- ration. environment. In particular, plasticity should be analysed at a leaf level for photosyn- The morphological plasticity of leaves thetic light response, biochemistry, anato- differs from one species to another. Many my, ultrastructure and morphology, at a authors have attempted to link this mor- plant level and at a canopy level (Board- phological plasticity to species shade be- man, 1977; Bjorkman, 1981; Givnish, haviour. For temperate species whose for- 1988). est behaviour is fairly well known, it is possible to rank species from most to least From a forester’s point of view forest be- shade tolerant (Baker, 1949). There is a haviour is not a well-defined concept, as shown by the following examples. Shade- good agreement between degree of leaf plasticity and shade tolerance where the tolerant versus shade-intolerant behaviour support the assumption that a full sunlight most plastic species are the most shade- intolerant (see Specific leaf area). environment is the standard reference. The light-demander species notion implies In tropical species, empirical and silvi- some species need more light than that cultural knowledge is basically non- others, although most species may grow existent and forest behaviour can only be sunlight environments. Other under full deduced from morphological variations. means of explaining differences in tree In the species we have studied in response are to consider their place in light Guadeloupe, initial insight into their forest forest successional cycle (Bazzaz and a behaviour was obtained through studies Pickett, 1980) from pioneer species to late on natural regeneration. The results re- successional species, or to emphasize garding the morphological plasticity of growth response to gap size in the forest these species are in approximate agree- canopy (Whitmore, 1978; Denslow, 1980, ment with the preceding results. 1987). Other authors (Grime, 1979; Kolb et In order of decreasing plasticity, the first al, 1990) consider that competition or plant species found is R grandis, followed imme- tolerance strategy in response to stresses diately by S amara and then by B coria- should include all stress factors and not cea. From growth studies in experimental only light stress. conditions of natural regeneration (Ducrey and Labbé, 1986), S amara was found to be slightly less shade-tolerant than R gran- REFERENCES dis. No information was obtained for B co- riacea. The following species, in decreas- Aussenac G, Ducrey M (1977) Étude bioclima- ing order of plasticity, are A caribaea tique d’une futaie feuillue (Fagus silvatica L which was found to be shade-tolerant and et Quercus sessiliflora Salisb) de l’Est de la P coriaceus which usually had a reputation France. I. Analyse des profils microclima- of being very shade-tolerant. Finally, the tiques et des caractéristiques anatomiques et
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