The water relations of two evergreen tree species in a karst savanna

The ecohydrology of karst has not received much attention, despite the disproportionally large contribution of karst aquifers to freshwater supplies. Karst savannas, like many savannas elsewhere, are encroached by woody plants, with possibly negative consequences on aquifer recharge. However, the role of savanna tree species in hydrological processes remains unclear, not least because the location and water absorption zones of tree roots in the spatially complex subsurface strata are unknown. This study examined the water sources and water relations of two savanna trees, Quercus fusiformis (Small) and Juniperus ashei (Buchholz) in the karst region of the eastern Edwards Plateau, Texas (USA). Stable isotope analysis of stem water revealed that both species took up evaporatively enriched water during the warm season, suggesting a relatively shallow water source in the epikarst, the transition zone between soil and bedrock. Q. fusiformis had consistently higher predawn water potentials than J. ashei during drought, and thus was probably deeper-rooted and less capable of maintaining gas exchange at low water potentials. Although the water potential of both species recovered after drought-breaking spring and summer rain events, associated shifts in stem water isotope ratios did not indicate significant uptake of rainwater from the shallow soil. A hypothesis is developed to explain this phenomenon invoking a piston-flow mechanism that pushes water stored in macropores into the active root zones of the trees. Epikarst structure varied greatly with parent material and topography, and had strong effects on seasonal fluctuations in plant water status. The study suggests that tree species of the Edwards Plateau do not commonly reduce aquifer recharge by tapping directly into perched water tables, but more likely by reducing water storage in the epikarst. A more general conclusion is that models of savanna water relations based on Walter's two-layer model may not apply unequivocally to karst savannas.     

Seasonal patterns of gas exchange,water relations and growth of it Roupala montana,an evergreen savanna species

Roupala montana is an evergreen species widespread in the seasonal savannas of the central plains of Brazil. I examined the degree of coupling of photosynthetic gas-exchange characteristics, water relations and growth responses of R. montana with regard to seasonal changes in soil water availability. Despite a rainless period of over three months soil water potential at 60 cm depth reached values of only about -1.0 MPa, while pre-dawn leaf water potential (^l) reached about -0.4 MPa by the end of the three-month drought. Thus, R. montana had access to deep soil water in the dry period, but pre-dawn ^l did not reach the high wet season values of -0.2 MPa. Most of the shoot growth was concluded in the onset of the rainy season. Although some individual branches might have shown some extension thereafter, most of them remained inactive during the rest of the rainy season and the subsequent dry season. New leaf production was also restricted to the first part of the wet period. R. montana remained evergreen in the dry season, but there was a 27% decrease in the number of leaves and herbivory removed about 16% of the leaf area still present in the plant. CO₂-exchange rates of these leaves reached only ca. 55% of the maximum rainy season values of 14 µmol m^-2 s^-1. Thus, the estimated potential daily carbon gain was about 34% of the maximum by the end of the dry period. These values will be even lower, if we considered the decrease in photosynthetic rates that occurred around midday. These reductions in photosynthetic rates as a result of partial stomatal closure were measured both in the wet and dry season and they were related to increases in the evaporative demand of the atmosphere. In conclusion, the combined effect of herbivory, leaf loss and reductions in photosynthetic rates limited plant productivity in the dry season.     

Effects of short-term NaCl stress on water relations and gas exchange of two jute species

Thirty-day-old seedlings of two jute species (Corchorus capsularis L. cv. JRC 212 and C. olitorius L. cv. JRO 632) were subjected to short-term salinity stress (160 and 200 mM NaCl for 1 and 2 d). Relative water content, leaf water potential, water uptake, transpiration rate, water retention, stomatal conductance, net photosynthetic rate and water use efficiency of both jute species decreased due to salinity stress. The decrease was greater in C. olitorius than in C. capsularis and with higher magnitude of stress. Greater accumulation of Na+ and Cl- and a lower ratio of K+/Na+ in the root and shoot of C. olitorius compared with C. capsularis were also recorded. Pretreatment of seedlings with kinetin (0.09 mM), glutamic acid (4 mM) and calcium nitrate (5 mM) for 24 h significantly improved net photosynthesis, transpiration and water use efficiency of salinity stressed plants, the effect being more marked in C. olitorius. Among the pre-treatment chemicals, calcium nitrate was most effective. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diurnal changes in leaf gas exchange and chlorophyll fluorescence in tropical tree species with contrasting light requirements

Interspecific ecophysiological differences in response to different light environments are important to consider in regeneration behavior and forest dynamics. The diurnal changes in leaf gas exchange and chlorophyll fluorescence of two dipterocarps, Shorea leprosula (a high light-requiring) and Neobalanocarpus heimii (a low light-requiring), and a pioneer tree species (Macaranga gigantea) growing in open and gap sites were examined. In the open site, the maximum net photosynthetic rate (P_n), photosystem II (PSII) quantum yield (; F/Fm), and relative electron transport rate (r-ETR) through PSII at a given photosynthetic photon flux density (PPFD) was higher in S. leprosula and M. gigantea than in N. heimii, while non-photochemical quenching (NPQ) at a given PPFD was higher in N. heimii. The maximum values of net photosynthetic rate (P_n) in M. gigantea and S. leprosula was higher in the open site (8–11 mol m^–2 s^–1) than in the gap site (5 mol m^–2 s^–1), whereas that in N. heimii was lower in the open site (2 mol m^–2 s^–1) than in the gap site (4 mol m^–2 s^–1), indicating that N. heimii was less favorable to the open site. These data provide evidence to support the hypothesis that ecophysiological characteristics link with plants regeneration behavior and successional status. Although P_n and stomatal conductance decreased at midday in M. gigantea and S. leprosula in the open site, both r-ETR and leaf temperature remained unchanged. This indicates that stomatal closure rather than reduced photochemical capacity limited P_n in the daytime. Conversely, there was reduced r-ETR under high PPFD conditions in N. heimii in the open site, indicating reduced photochemical capacity. In the gap site, P_n increased in all leaves in the morning before exposure to direct sunlight, suggesting a relatively high use of diffuse light in the morning.     

Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species

Rapidly induced responses can alter host plant suitability for insect growth and survival. The effects of defoliation on the suitability of potted 5-year-old red pine, Pinus resinosa Ait., for the sawfly Neodiprion sertifer (Hymenoptera: Diprionidae), were measured in two experiments. In the first, overall larval growth rate increased on seedlings within 8 days of low (<15%) defoliation. Suitability varied among larval age groups: defoliation increased the performance of older larvae, but not that of young larvae. In the second experiment, larval survival and weight varied non-linearly with defoliation intensity 8 weeks after treatment. Similar responses were observed following artificial and natural defoliation, and on early- and late-season seedlings. These results suggest that some evergreen conifers can respond rapidly to defoliation injury, and that herbivores may simultaneously acclimate to the induced response as they develop. The relative importance of induced response rates to plant-insect interactions is discussed.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Responses of leaf gas exchange, water relations, and water consumption in seedlings of four semiarid tree species to soil drying

To understand the response patterns to soil drying and the water use properties of commonly reforested trees in the semiarid Loess Plateau region of China, a glasshouse experiment was carried out with the seedlings of four species, i.e., Robinia pseudoacacia, Armeniaca sibirica, Syringa oblata, and Quercus liaotungensis. Severe water stress induced by withholding water resulted in permanent wilting of most of the seedlings pot-cultured with sandy soil in 8–12 days. Predawn and midday leaf water potentials and gas exchange characteristics (e.g., stomatal conductance) in the seedlings did not show marked changes until the volumetric soil water content decreased to about 0.05. As the soil water content decreased further, these physiological parameters rapidly declined, approaching their minimal levels at the stage of permanent wilting. The response of each parameter to soil water content changes was fitted with a non-linear saturation curve. Though the results suggested that the general pattern of responses to soil drying was identical among the species, quantitative differences in drought tolerance and water use properties were detected. Leaf stomatal conductance in R. pseudoacacia and A. sibirica showed earlier responses to reduced predawn leaf water potentials. However, water use characteristics and specific leaf area indicated that these two species consumed more water and may not be as drought tolerant as S. oblata and Q. liaotungensis. These results may provide important information to compare the reforestation species with respect to soil drying.     

Effects of Quaternary climatic fluctuations on the distribution of Neotropical savanna tree species

In order to develop niche models for tree species characteristic of the cerrado vegetation (woody savannas) of central South America, and to hindcast their distributions during the Last Glacial Maximum and Last Inter‐Glacial, we compiled a dataset of tree species checklists for typical cerrado vegetation (n = 282) and other geographically co‐occurring vegetation types, e.g. seasonally dry tropical forest (n = 355). We then performed an indicator species analysis to select ten species that best characterize typical cerrado vegetation and developed niche models for them using the Maxent algorithm. We used these models to assess the probability of occurrence of each species across South America at the following time slices: Current (0 ka pre‐industrial), Holocene (6 ka BP), Last Glacial Maximum (LGM – 21 ka BP), and Last Interglacial (LIG – 130 ka BP). The niche models were robust for all species and showed the highest probability of occurrence in the core area of the Cerrado Domain. The palaeomodels suggested changes in the distributions of cerrado tree species throughout the Quaternary, with expansion during the LIG into the adjacent Amazonian and Atlantic moist forests, as well as connections with other South American savannas. The LGM models suggested a retraction of cerrado vegetation to inter‐tableland depressions and slopes of the Central Brazilian Highlands. Contrary to previous hypotheses, such as the Pleistocene refuge theory, we found that the widest expansion of cerrado tree species seems to have occurred during the LIG, most probably due to its warmer climate. On the other hand, the postulated retractions during the LGM were likely related to both decreased precipitation and temperature. These results are congruent with palynological and phylogeographic studies in the Cerrado Domain.     

Distinct roles of savanna and forest tree species in regeneration under fire suppression in a Brazilian savanna

Questions: Has fire suppression relaxed barriers to the exchange of species between savanna and forest? Do all species or a subset of species participate in this exchange? Would current vegetation structure persist if fire suppression were to cease? Location: A gallery forest edge in the Cerrado region of central Brazil that burned only once in the past 35 years. Methods: Density of tree seedlings, saplings and adults, leaf area index (LAI), tree basal area and diameter were surveyed in 12, 10 m × 70 m transects centred on and perpendicular to the forest–savanna boundary. Community composition was assessed using non‐metric multi‐dimensional scaling (NMDS). Results: Basal area and LAI declined substantially from forest to savanna, with an associated shift in species composition. Savanna tree species were nearly absent in the forest, but accounted for the majority of stems in the savanna. In contrast, forest species comprised 14% of adults and more than one‐third of juveniles in the savanna. Despite the high diversity of trees (85 species) in the forest, five species play a particularly large role in this initial phase of forest expansion. Reintroduction of fire, however, would result in widespread topkill of juveniles and the majority of adult forest trees, thereby interrupting the succession towards forest. Conclusions: After 35 years during which the site burned only once, the savanna still remains dominated by savanna species. Nevertheless, the dominance of forest juveniles in border and savanna tree communities suggests that with a continued policy of fire suppression, the forest will continue to expand.     

Environmental control of gas exchange in some savanna woody species

Summary The effects of temperature, radiation, water stress and humidity on net photosynthesis, transpiration and leaf resistances of the two savanna woody species, Grewia flavescens and Terminalia sericea were studied under controlled environmental conditions. Both species have developed morphological and physiological features resulting in low rates of transpiration even under non-stress conditions and are able to tolerate quite severe water stress. Grewia seemed particularly well adapted in this regard and prestressed plants of Grewia were able to photosynthesise down to leaf water potentials of-42 bars. The photosynthetic responses of both species to all the environmental parameters studied were predominantly under stomatal control. The mechanisms employed for maintaining production and conserving water are examined.This work was undertaken as part of the South African Savanna Ecosystem Project (SASEP) and was supported by a CSIR grant to Prof. C.F. Cresswell     

Differential gender selection on flower size in two Neotropical savanna congeneric species

Larger flowers greatly increase among-individual pollen exchange within populations. However, water costs associated to transpirational cooling also increase with increasing flower size. Overall, the interplay between pollen and resource limitation determines the intensity of selection on flower size and this process is mostly dependent on gender and ecological context. To examine how pollinators and water use affect flower size, we determined corolla transpiration, pollen limitation, and selection through male and female fitness in two Kielmeyera species from the Brazilian cerrado flowering at different seasons. Hand-pollination experiments suggested pollen limitation through female fitness in both species, but K. coriacea showed lower limitation levels than K. regalis. For male fitness, the percentage of pollen removal was 1.5-times higher in K. coriacea. Higher air temperature and water deficit during flowering season of K. coriacea resulted in 4-fold higher corolla transpiration rates compared to K. regalis. Selection on flower size through male function was positive and significantly higher than selection through female components in both species. We also detected stabilizing selection in K. coriacea and positive selection in K. regalis on flower size through seed number. Our results suggest that selection on flower size in K. coriacea was mainly limited by water, whereas in K. regalis it was more limited by pollen. We demonstrate that differences in pollen and abiotic resource limitation determine gender-specific selection on flower size.     

Climatic changes can drive the loss of genetic diversity in a Neotropical savanna tree species

The high rates of future climatic changes, compared with the rates reported for past changes, may hamper species adaptation to new climates or the tracking of suitable conditions, resulting in significant loss of genetic diversity. Trees are dominant species in many biomes and because they are long‐lived, they may not be able to cope with ongoing climatic changes. Here, we coupled ecological niche modelling (ENM) and genetic simulations to forecast the effects of climatic changes on the genetic diversity and the structure of genetic clusters. Genetic simulations were conditioned to climatic variables and restricted to plant dispersal and establishment. We used a Neotropical savanna tree as species model that shows a preference for hot and drier climates, but with low temperature seasonality. The ENM predicts a decreasing range size along the more severe future climatic scenario. Additionally, genetic diversity and allelic richness also decrease with range retraction and climatic genetic clusters are lost for both future scenarios, which will lead genetic variability to homogenize throughout the landscape. Besides, climatic genetic clusters will spatially reconfigure on the landscape following displacements of climatic conditions. Our findings indicate that climate change effects will challenge population adaptation to new environmental conditions because of the displacement of genetic ancestry clusters from their optimal conditions.     

Seasonal progressions of tissue and cell water relations parameters in evergreen and deciduous perennials

Abstract. Tissue and cell water relations parameters were followed for Heteromeles arbutifolia, Cercis occidentalis and Aesculus californica , in an environment exhibiting seasonally increasing drought. The extensive seasonal osmotic adjustment of evergreen H. arbutifolia and the moderate adjustment in C. occidentalis closely matched their respective seasonal decreases in minimum daily water potential. Summer deciduous A. californica exhibited only small drops in osmotic potential and water potential. Experiments with irrigated plants indicated that drought was not required for the osmotic adjustment of H. arbutifolia and C. occidentalis. However, in H. arbutifolia drought treatment enhanced osmotic adjustment. In irrigated H. arbutifolia , osmotic adjustment was mainly the result of an accumulation of osmotica. In drought-stressed plants, the same change in osmotic potential resulted from a combination of accumulation of osmotica and a decrease in symplast volume.     

Leaf gas exchange characteristics and water- and nitrogen-use efficiencies of dominant grass and tree species in a West African savanna

Whereas leaf gas exchange properties are important to assess carbon and water fluxes in ecosystems worldwide, information of this type is scarce for savanna species. In this study, gas exchange characteristics of 2 C₄ grass species (Andropogon canaliculatus and Hyparrhenia diplandra) and 2 C₃ tree species (Crossopteryx febrifuga and Cussonia arborea) from the West-African savanna of Lamto (Ivory Coast) were investigated in the field. Measurements were done in order to provide data to allow the parameterization of biochemically-based models of photosynthesis (for C₄ and C₃ plant metabolic types) and stomatal conductance ; and to compare gas exchange characteristics of coexisting species. No systematic difference was found between grass and tree species for reference stomatal conductance, under standard environmental conditions, or stomatal response to incident light or vapour pressure deficit at leaf surface. Conversely, grass species displayed higher water (1.5-2 fold) and nitrogen (2-5 fold) photosynthetic use efficiencies (WUE and NUE, ratio of net photosynthesis to transpiration and leaf nitrogen, respectively). These contrasts were attributed to the CO₂ concentrating mechanism of C₄ plants. When looking within plant life forms, no important difference was found between grass species. However, significant contrasts were found between tree species, Cussonia showing higher NUE and reference stomatal conductance than Crossopteryx. These results stress the need to account for functional diversity when estimating ecosystem carbon and water fluxes. In particular, our results suggest that the tree/grass ratio, and also the composition of the tree layer, could strongly affect WUE and NUE at the ecosystem scale in West African savannas.     

Relative growth rate variation of evergreen and deciduous savanna tree species is driven by different traits

Background and Aims

Plant relative growth rate (RGR) depends on biomass allocation to leaves (leaf mass fraction, LMF), efficient construction of leaf surface area (specific leaf area, SLA) and biomass growth per unit leaf area (net assimilation rate, NAR). Functional groups of species may differ in any of these traits, potentially resulting in (1) differences in mean RGR of groups, and (2) differences in the traits driving RGR variation within each group. We tested these predictions by comparing deciduous and evergreen savanna trees.

Methods

RGR, changes to biomass allocation and leaf morphology, and root non-structural carbohydrate reserves were evaluated for juveniles of 51 savanna species (34 deciduous, 17 evergreen) grown in a common garden experiment. It was anticipated that drivers of RGR would differ between leaf habit groups because deciduous species have to allocate carbohydrates to storage in roots to be able to flush leaves again, which directly compromises their LMF, whereas evergreen species are not subject to this constraint.

Key Results

Evergreen species had greater LMF and RGR than deciduous species. Among deciduous species LMF explained 27 % of RGR variation (SLA 34 % and NAR 29 %), whereas among evergreen species LMF explained between 2 and 17 % of RGR variation (SLA 32–35 % and NAR 38–62 %). RGR and LMF were (negatively) related to carbohydrate storage only among deciduous species.

Conclusions

Trade-offs between investment in carbohydrate reserves and growth occurred only among deciduous species, leading to differences in relative contribution made by the underlying components of RGR between the leaf habit groups. The results suggest that differences in drivers of RGR occur among savanna species because these have different selected strategies for coping with fire disturbance in savannas. It is expected that variation in the drivers of RGR will be found in other functional types that respond differently to particular disturbances.     

Water-use efficiency and whole-plant performance of nine tropical tree species at two sites with contrasting water availability in Panama

Across their natural distributions, tropical tree species are regularly exposed to seasonal droughts of varying intensities. Their ability to tolerate drought stress plays a vital role in determining growth and mortality rates, as well as shaping the functional composition of tropical forests. In order to assess the ability of species to acclimate to contrasting levels of drought stress, physiological and structural traits involved in drought adaptation—wood C isotope discrimination (δ¹³C), wood specific gravity, and wood C content—of 2-year-old saplings of nine tropical tree species were evaluated in common garden experiments at two study sites in Panama with contrasting seasonality. We assessed co-variation in wood traits with relative growth rates (RGR_BD), aboveground biomass, and basal diameter and the plasticity of wood traits across study sites. Overall, species responded to lower water availability by increasing intrinsic water-use efficiency, i.e., less negative wood δ¹³C, but did not exhibit a uniform, directional response for wood specific gravity or wood C content. Trait plasticity for all wood traits was independent of RGR_BD and tree size. We found that the adaptive value of intrinsic water-use efficiency varied with water availability. Intrinsic water-use efficiency increased with decreasing RGR_BD at the more seasonal site, facilitating higher survival of slower growing species. Conversely, intrinsic water-use efficiency increased with tree size at the less seasonal site, which conferred a competitive advantage to larger individuals at the cost of greater susceptibility to drought-induced mortality. Our results illustrate that acclimation to water availability has negligible impacts on tree growth over short periods, but eventually could favor slow-growing species with conservative water-use strategies in tropical regions experiencing increasingly frequent and severe droughts.     

Seasonal patterns of leaf water relations in four co-occurring forest tree species: Parameters from pressure-volume curves

Summary Leaf water relationships were studied in four widespread forest tree species (Ilex opaca Ait., Cornus florida L., Acer rubrum L., and Liriodendron tulipifera L.). The individuals studied all occurred on the same site and were selected to represent a range of growth forms and water relationships in some of the principal tree species of the region. The water relations of the species were analyzed using the concept of the water potential-water content relationship. The pressure-volume method was used to measure this relationship using leaf material sampled from naturally occurring plants in the field. Water potential components (turgor, osmotic, and matric) were obtained by analysis of the pressure-volume curves.Initial osmotic potentials (the value of the osmotic component at full turgidity) were highest (least negative) at the start of the growing season. They decreased (becoming progressively more negative) as the season progressed through a drought period. Following a period of precipitation at the end of the drought period, initial osmotic potentials increased toward the values measured earlier in the season.Seasonal osmotic adjustments were sufficient in all species to allow maintenance of leaf turgor through the season, with one exception: Acer appeared to undergo some midday turgor loss during the height of the July drought period.In addition to environmental influences, tissue stage of development played a role; young Ilex leaves had higher early season initial osmotic potentials than overwintering leaves from the same tree.The seasonal pattern of initial osmotic potential in Liriodendron and the observed pattern of leaf mortality suggested a possible role of osmotic potentials in the resistance of those leaves to drought conditions. The fraction of total leaf water which is available to affect osmotic potentials, called the osmotic water fraction in this study, was greatest in young tissue early in the season and declined as the season progressed.The results of this study showed that the water potential-water content relationship represents a dynamic mechanism by which plant internal water relations may vary in response to a changing external water-availability regime. The measured water relationships confirmed the relative positions of the species along a water-availability gradient, with Cornus at the wettest end and Ilex at the driest end of the gradient. Acer and Liriodendron were intermediate in their water relations. The spread of these species along a water-availability gradient on the same site suggested that coexistence is partially based on differential water use patterns.     

Photosynthetic acclimation to light in juveniles of two cloud forest tree species

The photosynthetic response of juveniles of Decussocarpus rospigliosii, an emergent primary forest species and shade tolerant in its juvenile stages and Alchornea triplinervia, a gap-colonizing species of tropical cloud forest in Venezuela was studied. Daily courses of microenvironmental variables and gas exchange under contrasting light conditions (gap and understory) were carried out in their natural environment and transplanted to different light regimes (shade and sun) in the field. The photosynthetic response and some anatomical characteristics of plants from different treatments were analyzed in the laboratory. Photosynthetic rates were low for both species, and were negative during some diurnal periods, related to the low photosynthetically active radiation levels obtained at both gap (6% of total radiation) and understory (2%). A. triplinervia shows higher rates (1.5–3.0 molm^{-2 -1}) than D. rospigliosii (0.7–1.1 molm^-2s^-1). Both species showed increased photosynthetic rates when grown in gaps. A. triplinervia did not adjust its maximum photosynthetic rates to the prevailing light conditions. In contrast, D. rospigliosii responded to increased light levels. Both species showed low light compensation points when grown under total shade. There was a partial stomatal closure generally during midday in D. rospigliosii. A. triplinervia presented lower leaf conductances, transpiration rates and lesser stomatal control. Some leaf anatomical characteristics, in both species, were affected by variations in the light regime (i.e. increased leaf thickness, leaf specific weight and stomatal density). These results suggest that both species have the ability to respond to variations in their natural light environments, therefore maintaining a favorable carbon balance during the day.