Leaf Longevity Research Articles

The phenology of leaf quality and its within-canopy variation is essential for accurate modeling of photosynthesis in tropical evergreen forests.

Leaf quantity (i.e., canopy leaf area index, LAI), quality (i.e., per-area photosynthetic capacity), and longevity all influence the photosynthetic seasonality of tropical evergreen forests. However, these components of tropical leaf phenology are poorly represented in most terrestrial biosphere models (TBMs). Here, we explored alternative options for the representation of leaf phenology effects in TBMs that employ the Farquahar, von Caemmerer & Berry (FvCB) representation of CO2 assimilation. We developed a two-fraction leaf (sun and shade), two-layer canopy (upper and lower) photosynthesis model to evaluate different modeling approaches andassessed three components of phenological variations (i.e., leaf quantity, quality, and within-canopy variation in leaf longevity). Our model was driven by the prescribed seasonality of leaf quantity and quality derived from ground-based measurements within an Amazonian evergreen forest. Modeled photosynthetic seasonality was not sensitive to leaf quantity, but was highly sensitive to leaf quality and its vertical distribution within the canopy, with markedly more sensitivity to upper canopy leaf quality. This is because light absorption in tropical canopies is near maximal for the entire year, implying that seasonal changes in LAI have little impact on total canopy light absorption; and because leaf quality has a greater effect on photosynthesis of sunlit leaves than light limited, shade leaves and sunlit foliage are more abundant in the upper canopy. Our two-fraction leaf, two-layer canopy model, which accounted for all three phenological components, was able to simulate photosynthetic seasonality, explaining ~90% of the average seasonal variation in eddy covariance-derived CO2 assimilation. This work identifies a parsimonious approach for representing tropical evergreen forest photosynthetic seasonality in TBMs that utilize the FvCB model of CO2 assimilation and highlights the importance of incorporating more realistic phenological mechanisms in models that seek to improve the projection of future carbon dynamics in tropical evergreen forests.

Ontogenetic changes in crown architecture and leaf arrangement: effects on light capture efficiency in three tree species differing in leaf longevity

AbstractPronounced strategy shifts along ontogeny have been observed in several tree species, mainly because of the trend to maximize growth during the seedling stage, which constitutes the most vulnerable part of the tree's life cycle. Our aim here was to analyze the ontogenetic changes in crown characteristics and light capture patterns in three Quercus species: the evergreens Quercus ilex and Quercus suber and the deciduous Quercus faginea co‐occurring in a Mediterranean open woodland. The seedlings were distributed in the large clearings among the adults and received full sunlight. We constructed three‐dimensional models of the aerial parts of seedlings and mature trees of the three species, using the YplantQMC program. Large differences between growth stages were observed for all variables. The seedlings exhibited smaller branch sizes and crown densities than those observed in the adult trees. Leaf angles to horizontal also tended to increase during ontogeny, whereas leaf dispersion and the observed distances between leaves tended to decrease. The amount of photosynthetic radiation absorbed per unit leaf area throughout the growing season was lower in adult specimens than in young specimens. Changes in absorption efficiency during ontogeny were more intense for the species with longer leaf life span at maturity. We conclude that more intense ontogenetic shifts in species with longer leaf life span reflect the priority change from the maximization of short‐term productivity at the seedling stage to maximizing leaf longevity during the adult stage.

Cost-benefit modelを用いた最適葉寿命モデル : 最適戦略基準の検討( なぜいま葉寿命なのか?)

Cost-benefit modelを用いた最適葉寿命モデル : 最適戦略基準の検討( なぜいま葉寿命なのか?)

Xylem traits, leaf longevity and growth phenology predict growth and mortality response to defoliation in northern temperate forests.

Defoliation outbreaks are biological disturbances that alter tree growth and mortality in temperate forests. Trees respond to defoliation in many ways; some recover rapidly, while others decline gradually or die. Functional traits such as xylem anatomy, growth phenology or non-structural carbohydrate (NSC) storage could explain these responses, but idiosyncratic measures used by defoliation studies have frustrated efforts to generalize among species. Here, I test for functional differences with published growth and mortality data from 37 studies, including 24 tree species and 11 defoliators from North America and Eurasia. I synthesized data into standardized variables suitable for numerical models and used linear mixed-effects models to test the hypotheses that responses to defoliation vary among species and functional groups. Standardized data show that defoliation responses vary in shape and degree. Growth decreased linearly or curvilinearly, least in ring-porous Quercus and deciduous conifers (by 10-40% per 100% defoliation), whereas growth of diffuse-porous hardwoods and evergreen conifers declined by 40-100%. Mortality increased exponentially with defoliation, most rapidly for evergreen conifers, then diffuse-porous, then ring-porous species and deciduous conifers (Larix). Goodness-of-fit for functional-group models was strong (R2c = 0.61-0.88), if lower than species-specific mixed-models (R2c = 0.77-0.93), providing useful alternatives when species data are lacking. These responses are consistent with functional differences in leaf longevity, wood growth phenology and NSC storage. When defoliator activity lags behind wood-growth, either because xylem-growth precedes budburst (Quercus) or defoliator activity peaks later (sawflies on Larix), impacts on annual wood-growth will always be lower. Wood-growth phenology of diffuse-porous species and evergreen conifers coincides with defoliation and responds more drastically, and lower axial NSC storage makes them more vulnerable to mortality as stress accumulates. These functional differences in response apply in general to disturbances that cause spring defoliation and provide a framework that should be incorporated into forest growth and vegetation models.

Root Branching Is a Leading Root Trait of the Plant Economics Spectrum in Temperate Trees.

Global vegetation models use conceived relationships between functional traits to simulate ecosystem responses to environmental change. In this context, the concept of the leaf economics spectrum (LES) suggests coordinated leaf trait variation, and separates species which invest resources into short-lived leaves with a high expected energy return rate from species with longer-lived leaves and slower energy return. While it has been assumed that being fast (acquisitive) or slow (conservative) is a general feature for all organ systems, the translation of the LES into a root economics spectrum (RES) for tree species has been hitherto inconclusive. This may be partly due to the assumption that the bulk of tree fine roots have similar uptake functions as leaves, despite the heterogeneity of their environments and resources. In this study we investigated well-established functional leaf and stature traits as well as a high number of fine root traits (14 traits split by different root orders) of 13 dominant or subdominant temperate tree species of Central Europe, representing two phylogenetic groups (gymnosperms and angiosperms) and two mycorrhizal associations (arbuscular and ectomycorrhizal). We found reflected variation in leaf and lower-order root traits in some (surface areas and C:N) but not all (N content and longevity) traits central to the LES. Accordingly, the LES was not mirrored belowground. We identified significant phylogenetic signal in morphological lower-order root traits, i.e., in root tissue density, root diameter, and specific root length. By contrast, root architecture (root branching) was influenced by the mycorrhizal association type which developed independent from phylogeny of the host tree. In structural equation models we show that root branching significantly influences both belowground (direct influence on root C:N) and aboveground (indirect influences on specific leaf area and leaf longevity) traits which relate to resource investment and lifespan. We conclude that branching of lower order roots can be considered a leading root trait of the plant economics spectrum of temperate trees, since it relates to the mycorrhizal association type and belowground resource exploitation; while the dominance of the phylogenetic signal over environmental filtering makes morphological root traits less central for tree economics spectra across different environments.

Isolation, Characterization and Transcriptome Analysis of a Cytokinin Receptor Mutant Osckt1 in Rice.

Cytokinins play important roles in regulating plant development, including shoot and root meristems, leaf longevity, and grain yield. However, the in planta functions of rice cytokinin receptors have not been genetically characterized yet. Here we isolated a rice mutant, Osckt1, with enhanced tolerance to cytokinin treatment. Further analysis showed that Osckt1 was insensitive to aromatic cytokinins but responded normally to isoprenoid and phenylurea-type cytokinins. Map-based cloning revealed that the mutation occurred in a putative cytokinin receptor gene, histidine kinase 6 (OsHK6). OsCKT1 was found to be expressed in various tissues throughout the plant and the protein was located in the endoplasmic reticulum. In addition, whole-genome gene expression profiling analysis showed that OsCKT1 was involved in cytokinin regulation of a number of biological processes, including secondary metabolism, sucrose and starch metabolism, chlorophyll synthesis, and photosynthesis. Our results demonstrate that OsCKT1 plays important roles in cytokinin perception and control of root development in rice.

Leaf phenology and morphofunctional variation in Myrcia amazonica DC. (Myrtaceae) in gallery forest and “campo rupestre” vegetation in the Chapada Diamantina, Brazil

Myrcia amazonica DC. occurs in gallery forest and “campo rupestre” vegetation in the Chapada Diamantina, Bahia State, Brazil—habitats with contrasting irradiance and humidity conditions. We evaluated variations in aspects of the cost–benefit relationships of leaf maintenance and photosynthetic activity (phenology, leaf age, gas exchange, and photosynthetic pigments) and the maintenance of a positive water balance (leaf water potential, wood density, and leaf attributes) in two populations of M. amazonica occurring in those habitats. Our hypothesis was that this species would show more efficient water use associated with maintaining its leaves for longer periods in “campo rupestre” vegetation rather than in gallery forests—linked to the environmental constraints on “campo rupestre” population (lower water availability and elevated irradiance). We found that individuals growing in “campo rupestre” showed greater stomatal conductance, CO2 assimilation, transpiration rates, leaf thicknesses, leaf succulence, daily water potential amplitudes, and leaf longevity but lower wood densities as compared to the gallery forest population. Even with wide contrasts in terms of all of these parameters, both populations maintained perennial leaf patterns, despite with variations in the intensities and durations of leaf production (being more intense and of shorter duration in the “campo rupestre” population). Myrcia amazonica demonstrated high functional and morphological plasticity of attributes related to its survival and growth in these habitats. Our hypothesis was confirmed, as “campo rupestre” plants showed morphofunctional strategies associated with resource conservation, including more efficient water use.

How <i>Betula ermanii</i> Maintains a Positive Carbon Balance at the Individual Leaf Level at High Elevations

Generally, plant species with shorter leaf longevity maintain a positive carbon balance by decreasing leaf mass per area (LMA) and increasing photosynthesis. However, plants at high elevations need to increase LMA against environmental stresses. Therefore, plants need to increase both LMA and photosynthesis at high elevations. To examine how deciduous plants maintain a positive carbon balance at high elevations, photosynthesis and related leaf traits for deciduous broad-leaved tree Betula ermanii were measured at three elevations. LMA was greater at middle and high elevations than at low elevation. Leaf δ13C was greater at higher elevations, and positively correlated with LMA, indicating greater long-term deficiency of CO2 in leaves at higher elevations. However, the Ci/Ca ratio at photosynthetic measurement was not low at high elevations. Nitrogen content per leaf mass and stomatal conductance were greater at higher elevations. Photosynthetic rates and photosynthetic nitrogen use efficiency (PNUE) did not differ among the three elevations. Photosynthetic rate showed a strong positive correlation with stomatal conductance on a leaf area basis (R2 = 0.83, P B. ermanii compensates the deficiency of CO2 in leaves at high elevation by increasing stomatal conductance, and maintains photosynthesis and PNUE at high elevation as much as at low elevation.

Turpanopitys taoshuyuanense gen. et sp. nov., a novel woody branch discovered in Early Triassic deposits of the Turpan Basin, Northwest China, and its palaeoecological and palaeoclimate implications

A novel well-preserved woody branch, Turpanopitys taoshuyuanense gen. et sp. nov. was collected in the early Triassic terrestrial deposits in the Turpan Basin, Northwest China. The permineralized wood is characterized by a heterogeneous pith, endarch primary xylem and Protophyllocladoxylon-type secondary xylem. The pith consists of parenchyma cells and supporting diaphragms formed by brick-like sclerenchyma cells. The quantitative growth-ring analyses of T. taoshuyuanense indicate that the species was evergreen, the leaf longevity being comprised between 3 and 15years. T. taoshuyuanense might indicate a warm humid climate with short dry periods in the Turpan basin in the Early Triassic. The Protophyllocladoxylon-type woods were widely distributed in different climate zone in both southern and northern hemispheres during the Palaeozoic and Early Triassic. The growth rings of the woods with Protophyllocladoxylon-type secondary xylem are good indicator for the palaeoclimate. The new specimen indicates that a warm humid climate with irregularly distributed short dry periods in the Early Triassic Turpan basin.

Global Transcriptional Analysis Reveals the Complex Relationship between Tea Quality, Leaf Senescence and the Responses to Cold-Drought Combined Stress in Camellia sinensis.

In field conditions, especially in arid and semi-arid areas, tea plants are often simultaneously exposed to various abiotic stresses such as cold and drought, which have profound effects on leaf senescence process and tea quality. However, most studies of gene expression in stress responses focus on a single inciting agent, and the confounding effect of multiple stresses on crop quality and leaf senescence remain unearthed. Here, global transcriptome profiles of tea leaves under separately cold and drought stress were compared with their combination using RNA-Seq technology. This revealed that tea plants shared a large overlap in unigenes displayed “similar” (26%) expression pattern and avoid antagonistic responses (lowest level of “prioritized” mode: 0%) to exhibit very congruent responses to co-occurring cold and drought stress; 31.5% differential expressed genes and 38% of the transcriptome changes in response to combined stresses were unpredictable from cold or drought single-case studies. We also identified 319 candidate genes for enhancing plant resistance to combined stress. We then investigated the combined effect of cold and drought on tea quality and leaf senescence. Our results showed that drought-induced leaf senescence were severely delayed by (i) modulation of a number of senescence-associated genes and cold responsive genes, (ii) enhancement of antioxidant capacity, (iii) attenuation of lipid degradation, (iv) maintenance of cell wall and photosynthetic system, (v) alteration of senescence-induced sugar effect/sensitivity, as well as (vi) regulation of secondary metabolism pathways that significantly influence the quality of tea during combined stress. Therefore, care should be taken when utilizing a set of stresses to try and maximize leaf longevity and tea quality.

Cover Caption: RLS3 Prevents Leaf Senescence

Senescence at the right time is important for crop yield. In this issue Lin et al. (pp.971–982) showed in rice that RLS3 plays a critical role in sustaining leaf longevity. RLS3 is expressed in various tissues, with the highest level in mesophyll cells. Overexpression of RLS3 prolonged leaf longevity and sustained normal growth, while loss of RLS3 function caused premature leaf senescence and retarded growth.

Morphological and physicochemical traits of leaves of different life-forms of various broadleaf woody plants in interior Alaska

The morphological and physicochemical traits of leaves are important in terms of plant adaptation to various growth environments, because such traits play central roles in various functions including photosynthesis. We measured the toughness, mass per unit area (LMA), nitrogen content, and δ15N levels of the leaves of different life-forms of 39 broadleaf woody plants in interior Alaska. The plants were divided into three life-forms based on the maximum height of adult plants: understory (<1 m), small (≥1 m to <5 m), and canopy (≥5 m). Evergreen species accounted for a large proportion of understory woody plants (44%), whereas most small (87%) and canopy (100%) trees were deciduous. Higher LMA, increased toughness, and higher C-to-N ratios prolonged leaf life-span in evergreen species. We also evaluated the presence or absence of bundle-sheath extensions (BSEs), which contribute to the mechanical support of leaves and enhance photosynthetic ability by improving hydraulic properties. In total, 11 species (28%) had homobaric leaves (BSEs were absent) and 28 species (72%) had heterobaric leaves; these proportions are similar to those of woody plants of other temperate deciduous forests. The δ15N values reflected the presence of root symbionts such as ericoid mycorrhiza or Frankia sp. Our results suggest that leaf traits may affect both the life-form of the host plants and leaf longevity in both evergreen and deciduous species.

Anatomical and Morphological Leaf Traits of Three Evergreen Ferns (Polystichum setiferum, Polypodium interjectum and Asplenium scolopendrium)

Leaf mass per area (LMA) is one of the key features that correlates with the ecological performance both of seed plants and ferns. For ferns LMA is at the lower end of the range for seed plants and increases with leaf longevity. Data concerning morphological and anatomical traits of ferns and their relationship with LMA are quite limited. The objectives of this study were to (1) break down LMA into anatomical components; and (2) analyze the trade-offs between anatomical and morphological leaf traits, which determine LMA variations for three evergreen ferns. Seven morphological and 16 anatomical leaf traits of Polystichum setiferum, Polypodium interjectum and Asplenium scolopendrium growing outdoors in the Botanical Garden of Rome were analyzed using light microscopy. LMA was not significantly different between the considered species (4.55±0.55, 4.34±0.47, 4.28±0.41 mg cm−2 respectively) and it was in the range of other evergreen fern species. The morphological and anatomical structure of all species was significantly different and reflected environmental adaptation of species to their natural habitats. In particular, total lamina thickness was163±16.1 μm (P. setiferum), 244±33.7 μm (P. interjectum) and 336±32.2 μm (A. scolopendrium); leaf tissues density ranged from 127±16.9 mg cm−3 (A. scolopendrium) to 277±37.1 mg cm−3 (P. setiferum); intercellular air space varied between 23.2±2.07% (P. interjectum) and 41.8±1.61% (A. scolopendrium). The overall results highlight that LMA is an integral feature that can be obtained by different anatomical structure.

RLS3, a protein with AAA+ domain localized in chloroplast, sustains leaf longevity in rice.

Leaf senescence plays an important role in crop developmental processes that dramatically affect crop yield and grain quality. The genetic regulation of leaf senescence is complex, involving many metabolic and signaling pathways. Here, we identified a rapid leaf senescence 3 (rls3) mutant that displayed accelerated leaf senescence, shorter plant height and panicle length, and lower seed set rate than the wild type. Map-based cloning revealed that RLS3 encodes a protein with AAA+ domain, localizing it to chloroplasts. Sequence analysis found that the rls3 gene had a single-nucleotide substitution (G→A) at the splice site of the 10th intron/11th exon, resulting in the cleavage of the first nucleotide in 11th exon and premature termination of RLS3 protein translation. Using transmission electron microscope, the chloroplasts of the rls3 mutant were observed to degrade much faster than those of the wild type. The investigation of the leaf senescence process under dark incubation conditions further revealed that the rls3 mutant displayed rapid leaf senescence. Thus, the RLS3 gene plays key roles in sustaining the normal growth of rice, while loss of function in RLS3 leads to rapid leaf senescence. The identification of RLS3 will be helpful to elucidate the mechanisms involved in leaf senescence in rice.

Phenodynamics of five orchids species growing on rock outcrops in the Chapada Diamantina Mountains in northeastern Brazil

We evaluated the vegetative and reproductive phenological patterns of the orchids Cattleya elongata, Cyrtopodium aliciae, Epidendrum orchidiflorum, Epistephium lucidum, and Sobralia liliastrum (Orchidaceae) growing on sandstone outcrops in the Chapada Diamantina Mountains in northeastern Brazil. Phenological events were associated with abiotic factors, leaf longevity, life form, plant growth pattern, and pollination syndrome. Phenological observations were made for 18 months and followed the phenophases of: stem/pseudobulb emission, leaf flush, leaf fall, flowering, immature fruit and mature fruit. Seasonality, synchrony, and correlations between phenophases and environmental variables were tested. The orchid species demonstrated aseasonal vegetative phenologies, except for pseudobulb emission. Reproductive events were seasonal, except for flowering in E. orchidiflorum. There was high flowering overlap between the species pairs S. liliastrum and C. aliciae (dry season) and C. elongata and E. lucidum (rainy season). Dispersal occurred during both the rainy and dry seasons. The vegetative phenophases exhibited low synchrony, and were related to certain morphofunctional characters (stems/pseudobulbs, CAM metabolism). The reproductive phenophases showed high synchrony consistent with the deceit pollination strategy usually associated with the group.

Photosynthesis in developing leaf of juveniles and adults of three Mediterranean species with different growth forms.

Leaf development is influenced by almost all the prevailing environmental conditions as well as from the conditions at the time of bud formation. Furthermore, the growth form of a plant determines the leaf longevity and subsequently the investment in biomass and the internal structure of the mesophyll. Therefore, photosynthetic traits of a growing leaf, though, partly predetermined, should also acclimate to temporal changes during developmental period. In addition, the age of the plant can affect photosynthesis of the growing leaf, yet, in the majority of studies, the age is associated to the size of the plant. To test if the reproductive status of the plant affects the time kinetics of the photosynthetic capacity of a growing leaf and the relative contribution of the plants' growth form to the whole procedure, field measurements were conducted in juveniles (prereproductive individuals) and adults (fully reproductive individuals) of an evergreen sclerophyllous shrub (Nerium oleander), a semi-deciduous dimorphic shrub (Phlomis fruticosa), and a winter deciduous tree with pre-leafing flowering (Cercis siliquastrum). PSII structural and functional integrity was progressively developed in all species, but already completed, only some days after leaf expansion in P. fruticosa. Developing leaf as well as fully developed leaf in adults of C. siliquastrum showed enhanced relative size of the pool of final PSI electron acceptors. Photosynthetic traits between juveniles and adults of P. fruticosa were similar, though the matured leaf of adults exhibited lower transpiration rates and improved water-use efficiency than that of juveniles. Adults of the evergreen shrub attained higher CO2 assimilation rate than juveniles in matured leaf which can be attributed to higher electron flow devoted to carboxylation, and lower photorespiration rate. The reproductive phase of the plant seemed to be involved in modifications of the PSII and PSI functions of the deciduous tree, in carboxylation and photorespiration traits of the evergreen shrub, and in water conductance efficiency of the semi-deciduous shrub. However, it is interesting, that regardless of the growth form of the plant and the prospective leaf longevity of the developing leaf, adults need to support flowering outmatch juveniles, in terms of photosynthesis.

Comparative assessment of ecosystem C exchange in Miscanthus and reed canary grass during early establishment

AbstractLand‐use change to bioenergy crop production can contribute towards addressing the dual challenges of greenhouse gas mitigation and energy security. Realisation of the mitigation potential of bioenergy crops is, however, dependent on suitable crop selection and full assessment of the carbon (C) emissions associated with land conversion. Using eddy covariance‐based estimates, ecosystem C exchange was studied during the early‐establishment phase of two perennial crops, C3 reed canary grass (RCG) and C4 Miscanthus, planted on former grassland in Ireland. Crop development was the main determinant of net carbon exchange in the Miscanthus crop, restricting significant net C uptake during the first 2 years of establishment. The Miscanthus ecosystem switched from being a net C source in the conversion year to a strong net C sink (−411 ± 63 g C m−2) in the third year, driven by significant above‐ground growth and leaf expansion. For RCG, early establishment and rapid canopy development facilitated a net C sink in the first 2 years of growth (−319 ± 57 (post‐planting) and −397 ± 114 g C m−2, respectively). Peak seasonal C uptake occurred three months earlier in RCG (May) than Miscanthus (August), however Miscanthus sustained net C uptake longer into the autumn and was close to C‐neutral in winter. Leaf longevity is therefore a key advantage of C4 Miscanthus in temperate climates. Further increases in productivity are projected as Miscanthus reaches maturity and are likely to further enhance the C sink potential of Miscanthus relative to RCG.

Patterns of climber distribution in temperate forests of the Americas

We propose a standard protocol at the landscape to continental scale for examining to what extent the range of ecological conditions found in temperate latitudes explains the variations in climber species richness and traits. The protocol was tested in forests of the two Americas. The data set included 151 climber species. We selected four categorical traits and grouped these species into six clusters with regard to these traits. Floristic records of American forests were first gathered into alliances, second combined with bioclimatic indices (rainfall, temperature, continentality). We obtained a total of 59 vegetational units in which we calculated values of climber species richness and proportion of clusters. Vegetational units were ultimately gathered into five forest formations (characterized by leaf longevity). Wetlands and uplands were considered separately. Our results emphasize clear trends in large-scale patterns of climber distribution, independently of taxonomy. Climber species richness (in particular woody climbers) peaks in moist and warm upland forests with oceanic climates, and where conifers are rare. In flooded areas, climber richness is also very high and peaks in seasonally flooded large floodplains. In ecological conditions of frost, dryness or lack of nutrients, climber species richness, abundance and trait diversity decline, resulting in the dominance of small, twining and deciduous life traits.

Cacao Cultivation under Diverse Shade Tree Cover Allows High Carbon Storage and Sequestration without Yield Losses.

One of the main drivers of tropical forest loss is their conversion to oil palm, soy or cacao plantations with low biodiversity and greatly reduced carbon storage. Southeast Asian cacao plantations are often established under shade tree cover, but are later converted to non-shaded monocultures to avoid resource competition. We compared three co-occurring cacao cultivation systems (3 replicate stands each) with different shade intensity (non-shaded monoculture, cacao with the legume Gliricidia sepium shade trees, and cacao with several shade tree species) in Sulawesi (Indonesia) with respect to above- and belowground biomass and productivity, and cacao bean yield. Total biomass C stocks (above- and belowground) increased fivefold from the monoculture to the multi-shade tree system (from 11 to 57 Mg ha-1), total net primary production rose twofold (from 9 to 18 Mg C ha-1 yr-1). This increase was associated with a 6fold increase in aboveground biomass, but only a 3.5fold increase in root biomass, indicating a clear shift in C allocation to aboveground tree organs with increasing shade for both cacao and shade trees. Despite a canopy cover increase from 50 to 93%, cacao bean yield remained invariant across the systems (variation: 1.1–1.2 Mg C ha-1 yr-1). The monocultures had a twice as rapid leaf turnover suggesting that shading reduces the exposure of cacao to atmospheric drought, probably resulting in greater leaf longevity. Thus, contrary to general belief, cacao bean yield does not necessarily decrease under shading which seems to reduce physical stress. If planned properly, cacao plantations under a shade tree cover allow combining high yield with benefits for carbon sequestration and storage, production system stability under stress, and higher levels of animal and plant diversity.

Determinants of some leaf characteristics of Australian mangroves

Leaf characteristics reflecting the size, lifespan (longevity), moisture content (degree of succulence) and complexity of structure of 20 mangrove species were studied over several years at 13 locations along the tropical and subtropical Australian coast. These characteristics were found to fall generally within the ranges of those for woody species from other ecosystems. With the exception of one species, it was found that leaf longevity was related inversely to leaf moisture content, increasing from nearly 6 months in more succulent species to over 2 years in less succulent species. This suggested that more succulent leaves are less complex in their structure because they have less well-developed ability to compartmentalize salt. There was a tendency also for leaf longevity to increase in species with larger leaves. These findings were consistent with the general view for land plants that leaf longevity is greater in species that have developed tolerance to environmental stress, salt particularly in the case of mangroves. Leaf tissue in such species is more robust or complex and requires greater metabolic resources in its construction; the plant is then advantaged by retaining the tissue for longer periods. Classification of the species considered here, based on their leaf longevity, moisture content and complexity, identified phylogenetically related species groupings that reflected these leaf longevity effects.