Phenological Types Research Articles

Changes in leaf functional traits with leaf age: when do leaves decrease their photosynthetic capacity in Amazonian trees?

Most leaf functional trait studies in the Amazon basin do not consider ontogenetic variations (leaf age), which may influence ecosystem productivity throughout the year. When leaf age is taken into account, it is generally considered discontinuous, and leaves are classified into age categories based on qualitative observations. Here, we quantified age-dependent changes in leaf functional traits such as the maximum carboxylation rate of ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) (Vcmax), stomatal control (Cgs%), leaf dry mass per area and leaf macronutrient concentrations for nine naturally growing Amazon tropical trees with variable phenological strategies. Leaf ages were assessed by monthly censuses of branch-level leaf demography; we also performed leaf trait measurements accounting for leaf chronological age based on days elapsed since the first inclusion in the leaf demography, not predetermined age classes. At the tree community scale, a nonlinear relationship between Vcmax and leaf age existed: young, developing leaves showed the lowest mean photosynthetic capacity, increasing to a maximum at 45days and then decreasing gradually with age in both continuous and categorical age group analyses. Maturation times among species and phenological habits differed substantially, from 8±30 to 238±30days, and the rate of decline of Vcmax varied from -0.003 to -0.065μmol CO2 m-2s-1day-1. Stomatal control increased significantly in young leaves but remained constant after peaking. Mass-based phosphorus and potassium concentrations displayed negative relationships with leaf age, whereas nitrogen did not vary temporally. Differences in life strategies, leaf nutrient concentrations and phenological types, not the leaf age effect alone, may thus be important factors for understanding observed photosynthesis seasonality in Amazonian forests. Furthermore, assigning leaf age categories in diverse tree communities may not be the recommended method for studying carbon uptake seasonality in the Amazon, since the relationship between Vcmax and leaf age could not be confirmed for all trees.

Impacts of soil water stress on the acclimated stomatal limitation of photosynthesis: Insights from stable carbon isotope data.

Atmospheric aridity and drought both influence physiological function in plant leaves, but their relative contributions to changes in the ratio of leaf internal to ambient partial pressure of CO2 (χ) - an index of adjustments in both stomatal conductance and photosynthetic rate to environmental conditions - are difficult to disentangle. Many stomatal models predicting χ include the influence of only one of these drivers. In particular, the least-cost optimality hypothesis considers the effect of atmospheric demand for water on χ but does not predict how soils with reduced water further influence χ, potentially leading to an overestimation of χ under dry conditions. Here, we use a large network of stable carbon isotope measurements in C3 woody plants to examine the acclimated response of χ to soil water stress. We estimate the ratio of cost factors for carboxylation and transpiration (β) expected from the theory to explain the variance in the data, and investigate the responses of β (and thus χ) to soil water content and suction across seed plant groups, leaf phenological types and regions. Overall, β decreases linearly with soil drying, implying that the cost of water transport along the soil-plant-atmosphere continuum increases as water available in the soil decreases. However, despite contrasting hydraulic strategies, the stomatal responses of angiosperms and gymnosperms to soil water tend to converge, consistent with the optimality theory. The prediction of β as a simple, empirical function of soil water significantly improves χ predictions by up to 6.3±2.3% (mean±SD of adjusted-R2 ) over 1980-2018 and results in a reduction of around 2% of mean χ values across the globe. Our results highlight the importance of soil water status on stomatal functions and plant water-use efficiency, and suggest the implementation of trait-based hydraulic functions into the model to account for soil water stress.

Monitoring Broadscale Vegetational Diversity and Change across North American Landscapes Using Land Surface Phenology

We describe a polar coordinate transformation of vegetation index profiles which permits a broad-scale comparison of location-specific phenological variability influenced by climate, topography, land use, and other factors. We apply statistical data reduction techniques to identify fundamental dimensions of phenological variability and to classify phenological types with intuitive ecological interpretation. Remote sensing-based land surface phenology can reveal ecologically meaningful vegetational diversity and dynamics across broad landscapes. Land surface phenology is inherently complex at regional to continental scales, varying with latitude, elevation, and multiple biophysical factors. Quantifying phenological change across ecological gradients at these scales is a potentially powerful way to monitor ecological development, disturbance, and diversity. Polar coordinate transformation was applied to Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index (NDVI) time series spanning 2000-2018 across North America. In a first step, 46 NDVI values per year were reduced to 11 intuitive annual metrics, such as the midpoint of the growing season and degree of seasonality, measured relative to location-specific annual phenological cycles. Second, factor analysis further reduced these metrics to fundamental phenology dimensions corresponding to annual timing, productivity, and seasonality. The factor analysis explained over 95% of the variability in the metrics and represented a more than ten-fold reduction in data volume from the original time series. In a final step, phenological classes (‘phenoclasses’) based on the statistical clustering of the factor data, were computed to describe the phenological state of each pixel during each year, which facilitated the tracking of year-to-year dynamics. Collectively the phenology metrics, factors, and phenoclasses provide a system for characterizing land surface phenology and for monitoring phenological change that is indicative of ecological gradients, development, disturbance, and other aspects of landscape-scale diversity and dynamics.

Seasonal patterns of gas exchange and water relations in juveniles of two Neotropical savanna tree species differing in leaf phenology

Ecophysiological studies on Neotropical savanna trees have shown that there is a resistance gradient of seasonal water stress in deciduous and evergreen species. Some species perform like drought avoiders, characterized by high transpiration rates during the wet season that can be sustained by evergreen deep rooted species during the dry season. Nevertheless, water deficit in superficial soil layers, fire damages and nutritional requirements could differentially affect juveniles of both phenological types determining their spatial patterns of distribution (evergreen species sparse in open grasslands and deciduous species gathered in groves). This study examined seasonal variations in water relations and photosynthesis in juveniles of two tree species differing in leaf phenology: Curatella americana (evergreen) and Cochlospermum vitifolium (deciduous). We wanted to test the hypothesis that the improved ability of evergreen deep rooted species like C. americana to colonize open savannas is related to drought tolerance and a conservative gas exchange pattern in juveniles that photosynthesize even during the dry season, while one might expect a water spender and avoider character in propagules of deciduous species like C. vitifolium that are active mostly during the rainy season. The studied species showed high transpiration rates during the rainy period, indicating little water availability limitations on surface soil layers. Even so, both species exhibited a distinctive decrease in stomatal conductance towards the end of the rainy season, an indication of water availability restrictions and/or a response to higher air evaporative demands in the shallow rooted juveniles. At the peak of the dry season, C. americana presented comparable transpiration rates with wet season measurements, indicating that the studied juveniles had reached more permanent (or stable) water sources at soil depths of 30 cm (approximate maximum root lengths measured). Yet, C. americana showed a significant decrease in water potentials from wet to dry seasons, and osmotic adjustment as a drought tolerance mechanism. Overall, the evergreen functional type (juvenile plants of C. americana) showed tolerance to water deficit, maintaining functional leaves in the dry season. On the other hand, in absence of a water deficit, juveniles of C. vitifolium showed higher photosynthetic rates that could be translated into high growth rates in wet habitats. However, the low water deficit tolerance of the juveniles of C. vitifolium seems to be the main handicap for its colonization potential of the open savanna environment. In conclusion, as predicted, the greater tolerance to water deficit of the evergreen studied species suggests that resistance to water stress determines the observed distribution pattern of tree species in the study area, where only the evergreen ones colonize the open savanna, acting in its adult stages as a focus for the recruitment of woody species.

Street tree stormwater control measures can reduce runoff but may not benefit established trees

Directing stormwater runoff to irrigate urban trees has the potential to simultaneously: (i) reduce the volume of runoff generated by impervious surfaces and (ii) reduce tree drought stress and increase growth. Many papers promote this concept, but few have quantified both potential benefits. In this study, we quantified both the runoff retention performance of infiltration trenches retrofitted alongside established street trees and tree drought stress and growth. We compared retention for different soil types, tree phenological types and inlet designs over a period of 18 months. Retention was low on average (18.3%) but was highly variable among infiltration trenches (5.2–43.7%), driven by variation in inlet capacity due to blockages, as well as the flow velocity of runoff leading to the inlet. With appropriately designed inlets, street tree stormwater control measures have significant potential to retain runoff. We observed no clear reduction in tree drought stress or increase in tree growth. This suggests that the established trees in our study already had access to adequate soil water within their rooting volumes, supplemented by nearby irrigated private gardens, or leaky water infrastructure. We suggest that i) mature trees surrounded by greater impervious surface area, ii) newly planted trees, or iii) trees growing in hotter, drier climates, may respond more positively to similar passive irrigation systems. The evolving practice of passive irrigation of street trees with stormwater requires innovative design to maximise runoff capture efficiency under typical sediment and debris loads.

Identifying phenological functional types in savanna trees

Savannas are notable for the varied patterns of leaf display in the tree layer. Characterising this variation, and assessing the range of phenological strategies apparent in savanna ecosystems will be helpful for understanding savanna tree life-histories and their response to global change. We recorded leaf phenological metrics on a monthly basis for 28 woody species for one year in a semi-arid savanna at Nylsvley Nature Reserve, South Africa. Using these metrics four plant functional types were described: (1) evergreen species, which retained full canopy throughout the year; (2) semi-evergreen species, which lost a significant amount of canopy during the dry season, but not less than 60%; (3) obligate deciduous species in which the canopy was leafless for about 3 months during the dry season; and (4) rain-stimulated species, in which canopy flushing was driven by rainfall patterns. Of the 28 species, 18% were evergreen, 18% obligate deciduous, 32% semi-evergreen and 32% were rain-stimulated species. These results confirm that phenological strategies are variable in savannas, and highlight that there are numerous cues controlling the timing of leaf out and leaf drop. By defining distinct phenological strategies we can predict which plant functional types will have phenological cues that will be vulnerable to shifting climates.

Studies of Phenological Forms of Pedunculate Oak

We review the specificity of early and late phenological types of pedunculate oak differing in the spring flushing of leaves, namely, its occurrence in topographical elements, soil conditions and hydrology, responses to weather, and vulnerability to unfavorable factors. We show that phenological forms differ in a set of morphological and physiological indicators, growth energy, and peculiarities of the wood. We focus on the importance of the phenological specificity of planting materials when organizing oak plantations with regard to ongoing climatic changes and the ubiquitous decline of oak forests. The late form of oak is more preferable in most cases, because it is more resistant to spring frosts and insect damage, as well as the fact that it has higher quality wood in solid-volume trunks.

Influence of vegetation spatial structure on growth and water fluxes of a mixed forest: Results from the NOTG 3D model

Many natural terrestrial ecosystems, agroforestry systems, and, increasingly, forest plantations, are made of a mixture of species and display significant structural heterogeneity. Yet most carbon-based biophysical models are limited to homogeneous, monospecific canopies.We explored whether the spatial structure at the Font-Blanche experimental site, located in a mixed Mediterranean forest of Aleppo pine and holm oak in south-east France, could affect the components of the carbon and water cycles. We used NOTG, a new 3D, process-based, and individual-based model specifically developed to study the importance of the spatial structure on the carbon and water cycles of complex ecosystems. It incorporates processes involved in the energy, carbon, and water cycles at the tree (radiation interception, photosynthesis, phenology, allocation, senescence, transpiration), and soil (soil moisture, soil organic matter decomposition) levels. It is designed to handle various plant phenological types (deciduous, non opportunistic evergreen, and opportunistic evergreen), and a comprehensive description of plant C pools. The model is designed to study water and carbon fluxes at the stand scale over one to a few years.In this study, NOTG was parameterized for the Font-Blanche forest and two five-year simulations were made using either the full 3D representation, or a simplified 1D representation. Comparison of model outputs against observations at the tree, soil, and ecosystem scales, showed a much better accuracy when using the 3D representation. This illustrates the dependence of plant and ecosystem function to the above- and below-ground spatial structures, and the usefulness of such detailed models when dealing with structurally complex ecosystems.

Victims of agricultural intensification: Mowing date affects Rhinanthus spp. regeneration and fruit ripening

The recent population declines of annual hemiparasitic Rhinanthus species may be a result of changes in mowing dates associated with the intensification of grassland management, but the causal mechanisms are not well understood. We aimed to determine the dynamics of Rhinanthus regeneration after cutting and of fruit ripening under silage or hay making. Mowing was simulated on several dates from mid-May to mid-July in populations of a vernal ecotype of Rhinanthus minor and an aestival ecotype of Rh. alectorolophus. Survival and regeneration of clipped plants, as well as fruit ripening were monitored in the experiments. We showed that Rhinanthus species were capable of resprouting, albeit with high mortality, but only in early spring before the lower leaves were shed. The time of fruit ripening differed among phenological types by over a month and a considerable number of fruits ripened during hay making. If meadows are mown in the period when plants are not able to regenerate and not enough fruits have ripened, Rhinanthus populations could undergo a massive decline. Postponing the first cut until fruits start to ripen is necessary for the protection of these species. Current Czech agri-environmental measures (AEMs) subsidize postponing the first cut only in protected areas, which limits the distribution of Rhinanthus. Moreover, the earliest date for a postponed cut is mid-July, which is too late considering forage quality. We suggest implementing a late-June mowing, which would diversify the mosaic of various mowing dates within protected areas, and could be widely acceptable for farmers in non-protected landscapes.

Identifying phenological functional types of semi-arid savanna trees

Identifying phenological functional types of semi-arid savanna trees

Diversity assessment of vertebrate fauna in a wetland of hot hyperarid lands

A great paradox arises when we talk about biodiversity in wetlands located at hot-hyperarid lands. Ayata Lake (155 ha) belongs to the complex of wetlands of Oued Righ Valley in Algerian Lower-Sahara. Surveys conducted between October 2009 and June 2010 has allowed assessing vertebrate diversity living in the lake and its surrounding areas. Several methods and techniques were used for sampling and censing each group of vertebrate (fish, amphibians, reptiles, birds and mammals). Inventories revealed the existence of 03 fish, 02 amphibians, 06 reptiles, 55 birds, and 07 mammals. Phenological types (winter or summer migrant, resident-breeder occasional visitor) were attributed to birds then population dynamics were discussed. Birds used generally the waterbody in winter where migrants, mainly waterbirds, were abundant. We investigated for most inventoried species the specific habitats where they are occurring, and which respond to their behavioural and diet ecology. Biogeography status of vertebrate species revealed a desert affinity, which is represented mainly by Saharan and Saharo-sindian bio-models; except for birds which revealed dominance of Palaearctic biogeographical categories. This approach helped to highlight the biological resources of Ayata Lake and determine its actual ecological value. Furthermore, this study proposes some suggestions for management and conservation purposes.

Seasonal variation in photosynthesis in six woody species with different leaf phenology in a valley savanna in southwestern China

During 2003–2005, we examined the effect of seasonal drought on water status, gas exchange, δ13C, chlorophyll fluorescence and spectral reflectance in six woody species in a valley savanna near the Yuanjiang River (the upper Red River) in southwestern China. Three different phenological types of these woody species were compared, i.e., an evergreen species, Cyclobalanopsis helferiana, two winter-deciduous (WD) species, Buchanania latifolia and Symplocos racemosa, and three drought-deciduous (DD) species, Terminthia paniculata, Wendlandia tinctoria and Woodfordia fruticosa. We aimed to test the following three hypotheses: (1) the evergreen and WD species employ a drought avoidance strategy, whereas DD species employ a drought tolerance strategy; (2) the evergreen and WD species have a more economical water use strategy than the DD species and (3) the evergreen and WD species have a stronger photoprotection capacity through thermal dissipation than the DD species. At the end of a prolonged drought, the predawn leaf water potential (Ψpd) in C. helferiana and S. racemosa dropped to ca. −0.8 MPa, whereas the Ψpd in B. latifolia remained close to zero and DD species were leafless. In the rainy seasons, maximal photosynthetic rates of the evergreen (18.4 μmol m−2 s−1) and W. fruticosa (18.0 μmol m−2 s−1) were higher than those of the other four species (12.2−13.8 μmol m−2 s−1). The evergreen and WD species responded to drought by closing stomata and thus maintained a constant relative water content (RWC), which is a typical drought avoidance strategy; however, it is at the expense of carbon gain. DD species maintained a high photosynthetic capacity with a decrease in both stomatal conductance and RWC until the driest period, and then shifted from the drought tolerance strategy to the avoidance mechanism by shoot dieback. There was no significant difference in the means of δ13C across the phenological groups. The evergreen and WD species had stronger heat dissipation than the DD species in dry seasons. All species increased leaf spectral reflectance, probably because of degradation of chlorophyll as indicated by the leaf reflectance index, which should reduce light harvesting. All species showed a strong increase in the ratio of red to green spectral reflectance of leaves during dry seasons, indicating the accumulation of anthocyanin, which may contribute to screening sunlight and scavenging reactive oxygen species. Different responses to drought of savanna woody species with different leaf phenologies may facilitate the partitioning of resource use and hence their co-existence.

Seed mass, seed production, germination and seedling traits in two phenological types of Bidens pilosa (Asteraceae)

Bidens pilosa L. is a summer annual that shows a particular phenological pattern in the Córdoba mountains, Argentina. Some individuals start flowering 1 month after germination (early type), but most of the population starts flowering 4 months after germination (normal type). The aims of this study were to (1) analyse whether differences in flowering phenology affect seed mass and seed production, and (2) assess whether possible differences in seed traits of the two parental phenological types would affect germinability, germination rate, seedling growth and flowering phenology of offspring under laboratory conditions. The study showed that the numbers of seeds per capitulum and per plant were greater in the normal type than in the early type plants. This can be related to plant height, since in the field, normal-type plants are larger than early type plants. However, early type plants produced heavier seeds than normal-type plants. Germination rate was faster in the early type seeds, but total germination was higher in the normal ones. Seedling growth, in terms of height and the date of first flowering, did not differ between the phenological types of parental seed sources. Our results showed that differences in flowering phenology were associated with seed mass and seed production differences. Seed mass appears to be related to germination characteristics but not to seedling growth nor to flowering phenology of offspring.

LEAF PHENOLOGY OF WOODY SPECIES IN A NORTH AUSTRALIAN TROPICAL SAVANNA

Leaf phenology was monitored for 49 woody species (trees and tall shrubs) each month over a 2.5-year period in a humid, wet–dry tropical eucalypt savanna at Solar Village, near Darwin, Australia. In the 10 most common species, which spanned the range of phenological types, phenology was monitored every two weeks. To investigate the relationships between leaf phenology and plant water status, pre-dawn leaf water potential was monitored in eight common species every 4–6 weeks. Four main phenological types were described: (1) evergreen species, which retained full canopy throughout the year; (2) brevi- or partly deciduous species, in which the amount of canopy fell significantly, but briefly, during at least one dry season during the study period, but to levels not below 50% of full canopy; (3) semideciduous species in which canopy fell to below 50% of full canopy in each of the dry seasons; and (4) fully deciduous species, which lost all leaves during the early-mid dry season, and remained leafless for at least one month. Of these 49 species, 24% were evergreen, 20% were brevideciduous, 29% were semideciduous, and 27% were fully deciduous. Leaf fall occurred 1–2 months earlier in the dry season for the fully deciduous species than for the semideciduous species. Leaf fall in all species was coincident with the attainment of seasonal minima in leaf water potential, which were, on average, about −1.5 to −2.0 MPa in the evergreen and semideciduous species, compared with about −0.5 to −1.0 MPa in the fully deciduous species. Leaf flushing occurred throughout the dry season in the two evergreen species, with a major peak in the late dry season. In five semideciduous species and one of the fully deciduous species, leaf flushing commenced in the late dry season prior to the occurrence of any rain. In two fully deciduous species, leaf flushing occurred only after the first storms of the early wet season. There was variation in the timing of flushing, both between species within years and between years for some species. However, all species commenced leaf flushing after water potentials rose, following the attainment of seasonal minima in pre-dawn leaf water potential. Soil moisture at 1 m did not fall below permanent wilting point during the dry season; hence, reserves of soil water at the end of the dry season were sufficient to support the whole-plant rehydration that preceded leaf flushing in the absence of rain. These results are consistent with hypotheses, developed in the neotropics, that leaf phenology in trees from the wet–dry tropics is largely controlled by endogenous mechanisms.

Leaf Phenology of Woody Species in a North Australian Tropical Savanna

Leaf phenology was monitored for 49 woody species (trees and tall shrubs) each month over a 2.5-year period in a humid, wet–dry tropical eucalypt savanna at Solar Village, near Darwin, Australia. In the 10 most common species, which spanned the range of phenological types, phenology was monitored every two weeks. To investigate the relationships between leaf phenology and plant water status, pre-dawn leaf water potential was monitored in eight common species every 4–6 weeks. Four main phenological types were described: (1) evergreen species, which retained full canopy throughout the year; (2) brevi- or partly deciduous species, in which the amount of canopy fell significantly, but briefly, during at least one dry season during the study period, but to levels not below 50% of full canopy; (3) semideciduous species in which canopy fell to below 50% of full canopy in each of the dry seasons; and (4) fully deciduous species, which lost all leaves during the early-mid dry season, and remained leafless for at least one month. Of these 49 species, 24% were evergreen, 20% were brevideciduous, 29% were semideciduous, and 27% were fully deciduous. Leaf fall occurred 1–2 months earlier in the dry season for the fully deciduous species than for the semideciduous species. Leaf fall in all species was coincident with the attainment of seasonal minima in leaf water potential, which were, on average, about −1.5 to −2.0 MPa in the evergreen and semideciduous species, compared with about −0.5 to −1.0 MPa in the fully deciduous species. Leaf flushing occurred throughout the dry season in the two evergreen species, with a major peak in the late dry season. In five semideciduous species and one of the fully deciduous species, leaf flushing commenced in the late dry season prior to the occurrence of any rain. In two fully deciduous species, leaf flushing occurred only after the first storms of the early wet season. There was variation in the timing of flushing, both between species within years and between years for some species. However, all species commenced leaf flushing after water potentials rose, following the attainment of seasonal minima in pre-dawn leaf water potential. Soil moisture at 1 m did not fall below permanent wilting point during the dry season; hence, reserves of soil water at the end of the dry season were sufficient to support the whole-plant rehydration that preceded leaf flushing in the absence of rain. These results are consistent with hypotheses, developed in the neotropics, that leaf phenology in trees from the wet–dry tropics is largely controlled by endogenous mechanisms.

Leaf dynamics and shoot phenology of eleven warm-temperate evergreen broad-leaved trees near their northern limit in central Japan

Dynamic features of shoot phenology including leaf emergence and leaf fall, and leaf life span for eleven evergreen broad-leaved tree species were investigated in a warm-temperate rain forest in Mount Kiyosumi, central Japan. All species had periodic leaf emergence or flushing pattern, and were classified into two types; single and multiple flush and only one species, Eurya japonica, represented the latter type and the rest had single flush in spring. The single flush type can further be subdivided into two groups according to their duration of shoot growth; short and long flush. Seasonal patterns of leaf fall were categorized into four; unimodal, bimodal, broad unimodal, and multimodal type though they were not fixed pattern. The leaf emergence and leaf fall patterns were correlated for the eleven species, and five phenological types were categorized. Four of them were the single flush types, i.e., short flush of leaf emergence with unimodal leaffall (SSU) type of Castanopsis sieboldii and Quercus salicina, short flush with bimodal leaf fall (SSB) type of Quercus acuta, Machilus thunbergii, Neolitsea sericea, and Cinnamomum japonicum, long flush with bimodal leaffall (SLB) type of Myrsine seguinii, and long flush with broad unimodal leaffall (SLR) type of Symplocos prunifolia, Cleyera japonica, and Illicium anisatum. The multiple flush type is only one species, Eurya japonica, and it had multimodal leaffall pattern (MM type). The phenological pattern varied in relation to leaf life span, leaf size, and tree habit. Leaf life span ranged from 1.1 to 5.8 yr. The short flush species or SSU and SSB types were all canopy or subcanopy trees, and the former had short and the latter had long leaf life spans. The long flush species were all microphyllous small trees, and SLB type had a relatively long leaf life span in understory, SLR type had a long leaf life span in understory or in open habitat and/or forest gap as a pioneer tree. MM type had a long leaf life span and colonizing species in open habitat but they can survive in understory as well. The phenological attributes of evergreen trees were well corresponded to the ecological guild of the tree in both forest structure and successional stage, and were also constrained by phylogenetic groups.

Site fertility and leaf nutrients of sympatric evergreen and deciduous species of Quercus in central coastal California

Leaf and soil nutrient levels interact with and may each influence the other. We hypothesize that to the extent soil fertility influences the nutritional state of trees, soil fertility should correlate with summer leaf nutrient levels, whereas to the extent that trees influence soil nutrient levels, the quality of leaf litterfall should correlate with soil fertility. We examined these correlations for five sympatric oak species (genus Quercus) in central coastal California. Soil fertility, including both nitrogen and especially phosphorus, correlated significantly with summer leaf nutrient levels. In contrast, phosphorus, but not nitrogen, in the leaf litterfall correlated positively with soil nutrients. These results suggest that soil nitrogen and phosphorus influence tree nutrient levels and that leaf phosphorus, but not leaf nitrogen, influence soil fertility under the trees. Feedback between the soil and the tree for phosphorus, but not nitrogen, is apparently significant and caused by species-specific differences in leaf quality and not by litterfall quality differences within a species. We also compared functional differences between the evergreen and deciduous oak species at our study site. There were no differences in soil nitrogen and only small differences for soil phosphorus between the phenological types. Differences in leaf nutrient concentration were much more pronounced, with the evergreen species having substantially lower levels of both nitrogen and phosphorus. Evergreen species conserved more phosphorus, but not more nitrogen, than the deciduous species, but there was no consistent relationship between retranslocation and either soil nitrogen or phosphorus. These results do not support the hypothesis that evergreenness is an adaptation to low soil fertility in this system.

Seasonal Variation in Water Relations of Trees of Differing Leaf Phenology in a Wet - Dry Tropical Savanna near Darwin, Northern Australia

The seasonal variation in leaf xylem pressure potential at dawn (ψdawn), leaf tissue water characteristics and daily maximum leaf conductance was measured in eight woody species in a wet–dry tropical savanna near Darwin, northern Australia, between October 1992 and October 1993. The species were Eucalyptus miniata, E. tetrodonta, E. clavigera, Xanthostemon paradoxus, Erythrophleum chlorostachys, Planchonia careya, Terminalia ferdinandiana and Cochlospermum fraseri. The species represented the major leaf phenological types, evergreen, semi-deciduous and fully deciduous. The climate of the region is characterised by annual drought during the winter months, when virtually no rain falls and vapour pressure deficit (VPD) in the afternoon reaches 3 kPa for 5 consecutive months each year. Despite this drought, ψdawn remained high (–1.3 to –1.5 MPa in evergreen species and –0.5 to –1.5 MPa in deciduous species) relative to those trees that experience summer drought in temperate and arid Australia. There was a tendency for evergreen and semi-deciduous species to maintain positive turgor to lower xylem pressure potentials (mean osmotic potential at incipient plasmolysis, π0 = –2.15 MPa) than the fully deciduous species (π0 = –2.03 MPa). For all species, the daily maximum leaf conductance (gmax) was maximal in the wet and decreased during the dry season. Diurnally, (gmax occurred near midday in the wet season, but at about 0800–1000 hours during the dry season and the ‘buildup’, the transitional period between the dry and wet seasons. There was substantial decrease in (gmax (from 650–1000 mmol m-2 s-1 in March to 200 mmol m-2 s-1 in May) early in the dry season in two of the three fully deciduous species (Planchonia careya and Cochlospermum fraseri). The dominant evergreen species Eucalyptus miniata, by contrast, had high gmax (> 400 mmol m-2 s-1) throughout the dry season, suggesting it had access to groundwater. For each species, gmax declined with decreasing dawn water potential in a log-linear manner; the slope of this relationship tended to increase with increasing degree of deciduousness.

A coupled carbon and water flux model to predict vegetation structure

Abstract. A coupled carbon and water flux model (BIOME2) captures the broad‐scale environmental controls on the natural distribution of vegetation structural and phenological types in Australia. Model input consists of latitude, soil type, and mean monthly climate (temperature, precipitation, and sunshine hours) data on a 1/10° grid. Model output consists of foliage projective cover (FPC) for the quantitative combination of plant types that maximizes net primary production (NPP). The model realistically simulates changes in FPC along moisture gradients as a consequence of the trade‐off between light capture and water stress. A two‐layer soil hydrology model also allows simulation of the competitive balance between grass and woody vegetation including the strong effects of soil texture.

Variation in a Mutualism: Phenology and the Maintenance of Gynodioecy in Two Indian Fig Species

11 studied phenological variation in the coevolved mutualism between gynodioecious figs (genus Ficus) and their species-specific pollinator wasps. The phenology of tropical gynodioecious figs is important to vertebrates relying on fig fruit as a resource, and to understanding the evolution of gynodioecy in Ficus. 2 Gynodioecious fig species have hermaphrodite trees with closed, urn-shaped inflorescences bearing male and female flowers, and female trees with inflorescences bearing female flowers. Pollinators entering receptive inflorescences on hermaphrodite trees pollinate, lay eggs in female flowers, and die; weeks later, offspring emerge, mate and carry pollen from hermaphrodite trees to other trees. Wasps entering inflorescences on female trees pollinate female flowers but cannot lay eggs, and die without leaving offspring. Hermaphrodite ('male') trees produce pollen and wasps, and no seeds; female trees produce only seeds. 3 During 1991 and 1992, I observed the phenology of two gynodioecious fig tree species, Ficus exasperata and F. hispida, in a dry and a wet site in Coorg District, south India. 4 Ficus exasperata trees displayed a synchronous phenology very similar to that predicted by one hypothesis relating the evolution of gynodioecy to phenology. Female trees of F. exasperata produced seeds synchronously during the monsoon season and 'male' trees produced pollinator wasps and pollen synchronously during the peak period of female receptivity. Crop production occurred more frequently in the dry than in the wet site. Female trees bore mature fruit for longer periods than 'male' trees. 5 In contrast, several phenological traits of F. hispida departed strongly from prediction. This species displayed an asynchronous phenology at the population level. Receptivity and wasp or seed production in F. hispida occurred year-round, female trees produced more crops than 'male' trees, and there was considerable overlap in timing of 'male' and female inflorescence production. Durations of phenological phases were longer on female than on 'male' trees at the wet site. 6 An asynchronous phenology appears to be more advantageous in aseasonal than in seasonal environments. If the two phenological types observed in this study are found to be widespread, then the possibility that functional dioecy in Ficus evolved via more than one pathway must be considered.