Root dynamics influence tree–grass coexistence in an Australian savanna

Both resource and disturbance controls have been invoked to explain tree persistence among grasses in savannas. Here we determine the extent to which competition for available resources restricts the rooting depth of both grasses and trees, and how this may influence nutrient cycling under an infrequently burned savanna near Darwin, Australia. We sampled fine roots <2 mm in diameter from 24 soil pits under perennial as well as annual grasses and three levels of canopy cover. The relative proportion of C₃ (trees) and C₄ (grasses) derived carbon in a sample was determined using mass balance calculations. Our results show that regardless of the type of grass both tree and grass roots are concentrated in the top 20 cm of the soil. While trees have greater root production and contribute more fine root biomass grass roots contribute a disproportional amount of nitrogen and carbon to the soil relative to total root biomass. We postulate that grasses maintain soil nutrient pools and provide biomass for regular fires that prevent forest trees from establishing while savanna trees, are important for increasing soil N content, cycling and mineralization rates. We put forward our ideas as a hypothesis of resource‐regulated tree–grass coexistence in tropical savannas.     

Physical and chemical response of juvenile Acacia tortilis trees to browsing. Experimental evidence

1. Changes in nutritional value and accessibility of leaves following browsing are important in the dynamics of plant–herbivore interactions because they influence the fitness of the plant attacked and the future utilization of it by the herbivore.
2. Hand pruning of Acacia tortilis , a spinescent tree common in savanna ecosystems of eastern Africa, resulted in higher biomass of spines and new shoots in pruned trees than in unpruned controls.
3. Pruned trees allocated a higher proportion of shoot biomass to spines than unpruned ones, whereas the proportion of leaf biomass in new shoots was slightly reduced. Because increases in spine biomass and density following pruning are coupled with an increase in shoot production, it is concluded that higher production of spines is an inducible response of Acacia tortilis to pruning.
4. No significant changes in the concentration of total phenolics, condensed tannins or leaf nitrogen were induced by pruning.
5. Irrespective of treatment, high foliar concentrations of nitrogen were correlated with an increase in twig production for a given leaf biomass and a reduction in the concentration of secondary substances in leaves. This relation may lead to a conflict between foraging efficiency and nutrition for browsers of A. tortilis.     

Influence of UV-B Radiation on Early Seedling Growth and Translocation of 65Zn from Cotyledons in Cotton

The influence of UV-B radiation from filtered or unfiltered fluorescent sunlamps on early seedling growth and translocation of ⁶⁵Zn from cotyledons to the shoot was examined in two cultivars of cotton, Acala and Gregg. Ten-day-old seedlings which had been irradiated in the greenhouse for 6 h continuously each day for 14 days with 0.81 or 1.61 W × m^-2 UV-B radiation under two unfiltered FS-40 sunlamps, showed pronounced phytotoxic damage. This was characterized at first by bronzing and glazing of the cotyledons and later by upward curling of the leaves and abscission. Leaf expansion, dry matter accumulation, and mobilization of ⁶⁵Zn from the cotyledons was severely impaired in the young developing shoot under unfiltered UV-B radiation. A significant stress response also was observed in seedlings exposed to 0.61 W × m^-2 UV-B radiation through a polystyrene filter and 0.73 W × m^-2 UV-B radiation through a cellulose-acetate filter. This stress response was characterized by the formation of a red pigment in the petioles of the cotyledons, reduced leaf expansion, and reduced transport of ⁶⁵Zn. Control seedlings exposed to 0.03 W × m^-2 UV-B radiation through a mylar filter were green, had maximum leaf size and dry-matter accumulation, and had the greatest percentage of ⁶⁵Zn translocated from the cotyledons.     

Influence of ultraviolet-B (UV-B) radiation on photosynthetic and growth characteristics in field-grown cassava (Manihot esculentum Crantz)

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5kJ m^?2 d^?1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (F_v/F_m) were determined for fully expanded leaves after 64–76 d of UV-B exposure. AH plants were harvested after 95 d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of F_v to F_m ratios indicated a significant decline in F_v/F_m ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800μmol m^?2 s^?1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO₂ and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation. The increase in shoot/root ratio was due primarily to a significant decrease in root weight (–32%) with UV-B exposure. Because root production determines the harvest-able portion of cassava, UV-B radiation may still influence the yield of an important tropical agronomic species, even though photosynthesis and total dry biomass may not be directly affected.     

Legume and Non-legume Trees Increase Soil Carbon Sequestration in Savanna

Savanna ecosystems are increasingly pressured by climate and land-use changes, especially around populous areas such as the Mt. Kilimanjaro region. Savanna vegetation consists of grassland with isolated trees or tree groups and is therefore characterized by high spatial variation and patchiness of canopy cover and aboveground biomass. Both are major regulators for soil ecological properties and soil-atmospheric trace gas exchange (CO₂, N₂O, CH₄), especially in water-limited environments. Our objectives were to determine spatial trends in soil properties and trace gas fluxes during the dry season and to relate above- and belowground processes and attributes. We selected a Savanna plain with vertic soil properties, south east of Mt. Kilimanjaro. Three trees were chosen from each of the two most dominant species: the legume Acacia nilotica and the non-legume Balanites aegyptiaca. For each tree, we selected one transect with nine sampling points, up to a distance of 4 times the crown radius from the stem. At each sampling point, we measured carbon (C) and nitrogen (N) content, δ¹³C of soil (0–10, 10–30 cm depth) and in plant biomass, soil C and N pools, water content, available nutrients, cation exchange capacity (CEC), temperature, pH, as well as root biomass and greenhouse-gas exchange. Tree species had no effect on soil parameters and gas fluxes under the crown. CEC, C, and N pools decreased up to 50% outside the crown-covered area. Tree leaf litter had a far lower C:N ratio than litter of the C₄ grasses. δ¹³C in soil under the crown shifted about 15% in the direction of tree leaf litter δ¹³C compared to soil in open area reflecting the tree litter contribution to soil organic matter. The microbial C:N ratio and CO₂ efflux were about 30% higher in the open area and strongly dependent on mineral N availability. This indicates N limitation and low microbial C use efficiency in the soil of open grassland areas. We conclude that the spatial structure of aboveground biomass in savanna ecosystems leads to a spatial redistribution of nutrients and thus C mineralization and sequestration. Therefore, the capability of savanna ecosystems to act as C sinks is both directly and indirectly dependent on the abundance of trees, regardless of their N-fixing status.     

Physiological and growth responses of two African species, Acacia karroo and Themeda triandra, to combined increases in CO2 and UV-B radiation

The interactive effects of increased carbon dioxide (CO₂) concentration and ultraviolet-B (UV-B, 280–320 nm) radiation on Acacia karroo Hayne, a C₃ tree, and Themeda triandra Forsk., a C₄ grass, were investigated. We tested the hypothesis that A. karroo would show greater CO₂-induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C₄ grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol^-1) and elevated (650 μmol mol^-1) CO₂, combined with ambient (1.56 to 8.66 kJ m^-2 day^-1) or increased (2.22 to 11.93 kJ m^-2 day^-1) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO₂ had no effect on biomass of A. karroo, despite increased net CO₂ assimilation rates. Interaction between UV-B and CO₂ on stomatal conductance was found, with conductances decreasing only where elevated CO₂ and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO₂. Elevated UV-B caused only an increase in foliar carbon concentrations. In T. triandra, net CO₂ assimilation rates were unaffected in elevated CO₂, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO₂. and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO₂. Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO₂-induced growth responses of the species therefore do not support the hypothesis of CO₂-driven woody encroachment of C₄ grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO₂ response of these species.     

Stable isotope characteristics across narrow savanna/woodland ecotones in Wolfe Creek Meteorite Crater,Western Australia

The stable isotopic composition (δ¹³C) of sediments from lakes are frequently analyzed to reconstruct the proportion of the regional vegetation that used either the C₃ or C₄ photosynthetic pathways, often without conducting a detailed survey of the current local vegetation. We performed a study on the modern vegetation composition within the Wolfe Creek Meteorite Crater to complement our future paleoecological investigation of the crater. A bull’s-eye pattern exists where C₄ grasses dominate an outer ring and salt tolerant species, including shrubs, herbs, chenopods, and halophytic algae, dominate the inner pan of the crater. The ecotone between the inner and outer zones is narrow and occupied by tall (>7 m) Acacia ampliceps, with some C₄ grasses in the understory. Along with the highest water table and most saline soils the center of the crater has C₃ plants present with the highest δ¹³C and δ¹⁵N values. The range of δ¹³C and δ¹⁵N values from the analysis of surface soil organic matter (OM) was much smaller compared with the range of values from plant materials implying that either: (1) the current plant OM has not yet been integrated into the soils, or (2) processes within the soil have acted to homogenize isotopic variability within the crater. The application of a two end member mixing model to calculate %C₄ and %C₃ biomass from the δ¹³C of surface soil OM was complicated by: (1) the crater containing both a dry habitat with C₄ grasses and a central pan with C₄ halophytic plants and, (2) the large variation in the δ¹³C of the plants and soil OM.     

Changes of epicuticular wax induced by enhanced UV-B radiation impact on gas exchange in Brassica napus

The epicuticular wax covering on plant surface plays important roles in protecting plants against UV radiation. However, the role of epicuticular wax in affecting leaf gas exchange under enhanced ultraviolet-B (UV-B) radiation remains obscure. In the present study, different aged leaves of Brassica napus were used to analyze the responses of crystal structure and chemical constituents of epicuticular wax to UV-B radiation and the effects of such responses on gas exchange indices. Enhanced UV-B radiation significantly decreased the amount of esters in all leaves except the first leaf, amount of secondary alcohols in the second, third and fourth leaves, and amount of primary alcohols in the second and third leaves, while increased the amounts of ketones and aldehydes in the first leaf. Enhanced UV-B level had no significant effect on the amounts of alkanes and total wax in all leaves. Exposure to UV-B radiation resulted in wax fusion on adaxial leaf and stomata opening on abaxial leaf. Fusions of plates and rods on adaxial leaf surface covered most of the stomata, thereby influencing the photosynthesis in the upper mesophyll of leaves. Enhanced UV-B level significantly reduced the net photosynthesis rate (P _N) but increased the stomata conductance (g _s), concentrations of intercellular CO₂ (C _i ), and transpiration rate (E) in all leaves. Both UV-B radiation and the wax fusion induced by enhanced UV-B radiation resulted in different stomata status on abaxial and adaxial leaf surface, causing decrease of P _N, and increase of g _s, C _i and E in leaves.     

Relationship between the structure of root systems and resource use for 11 North American grassland plants

Eleven Midwest North American grassland plant species differed in theirconstruction, production, and placement of fine and coarse belowground biomassin the soil profile after having been grown in containers in the field for twoand a half growing seasons. Based on the patterns of root system structure andresource utilization, the species we examined could be classified as 1)legumes,2) high-nitrogen rhizomatous C₃ species, and 3) a separategradient of differentiation from tall- to short-statured species(i.e. tallgrass to shortgrass species). Legumes depleted water evenlythroughoutthe soil profile, with little capacity for acquisition of inorganic nitrogenthroughout the 1m soil profile. The three rhizomatous species had shallow fineroot distributions, a large relative investment in shallow rhizomes, andmoisture and NO₃ ^– levels were low in shallow soils,but high at depth. Tallgrass species maintained a large standing root biomassofhigh-density, low-nitrogen fine roots, and acquire nitrogen andwater from a large, deep volume of soil, in which inorganic nitrogen is presentin low concentrations. Root systems ofshortgrass species lacked coarse belowground biomass, had fine roots that werefiner than those of the tallgrass species, and had a shallow root distribution.There was little support for functional dichotomies between the C₃and C₄ species or between the grasses and forbs. For example,Solidago rigida (C₃ forb) andAndropogon gerardii (C₄ grass) were moresimilarto each other than to other C₃ forbs or C₄ grasses,respectively.Across all species and depths examined, there were strong relationships betweenthe amount of fine root biomass present in a unit of volume of soil and thedepletion of soil water and nitrogen, but there were no relationships withcoarse belowground biomass. This reaffirms that differentiation of coarse andfine root biomass is as important as differentiating stems and leaves inevaluating plant allocation and ecosystem functioning.     

Interactive response of herbivores,soils and vegetation to annual burning in a South African savanna

Abstract Despite decades of research, the primary factors determining savanna structure remain elusive – a conundrum termed ‘the savanna problem’. After 47 years of annual burning in Terminalia woodland and Acacia/Combretum savanna on sandy, granite‐derived soils in the southern Kruger National Park, South Africa, a dense cover of trees and shrubs persists on some burnt plots and is largely absent from others. We postulated that intense browsing pressure by antelope and other herbivores prevents recruitment of trees in burnt plots and that herbivores concentrate on plots that are richest in nutrients. Herbivore abundance did not show a relationship with soil macronutrients and we consequently investigated micronutrient status. The reduction in tree cover as a result of annual burning was positively correlated with mass of herbivores (15–1500 kg) (r ² = 0.61, n = 8). This index of herbivore abundance was in turn positively correlated with total Zn (r ² = 0.64, n = 8). Other indices of herbivore abundance showed significant relationships with total clay content and total Mn. We suggest that herbivores concentrate on sites with greater clay content (possibly due to a greater availability of micronutrients), and that tree cover can remain relatively dense under a regime of annual burning if browsing pressure is not intense. The long‐term burn experiments in the Kruger National Park savanna provide a platform for unravelling the savanna problem. Determining possible interactions between soil properties, herbivory and fire is a step in this direction.     

Growth enhancement of soybean (Glycine max) upon exclusion of UV-B and UV-B/A components of solar radiation: characterization of photosynthetic parameters in leaves

Exclusion of UV (280–380 nm) radiation from the solar spectrum can be an important tool to assess the impact of ambient UV radiation on plant growth and performance of crop plants. The effect of exclusion of UV-B and UV-A from solar radiation on the growth and photosynthetic components in soybean (Glycine max) leaves were investigated. Exclusion of solar UV-B and UV-B/A radiation, enhanced the fresh weight, dry weight, leaf area as well as induced a dramatic increase in plant height, which reflected a net increase in biomass. Dry weight increase per unit leaf area was quite significant upon both UV-B and UV-B/A exclusion from the solar spectrum. However, no changes in chlorophyll a and b contents were observed by exclusion of solar UV radiation but the content of carotenoids was significantly (34–46%) lowered. Analysis of chlorophyll (Chl) fluorescence transient parameters of leaf segments suggested no change in the F _v/F _m value due to UV-B or UV-B/A exclusion. Only a small reduction in photo-oxidized signal I (P700⁺)/unit Chl was noted. Interestingly the total soluble protein content per unit leaf area increased by 18% in UV-B/A and 40% in UV-B excluded samples, suggesting a unique upregulation of biosynthesis and accumulation of biomass. Solar UV radiation thus seems to primarily affect the photomorphogenic regulatory system that leads to an enhanced growth of leaves and an enhanced rate of net photosynthesis in soybean, a crop plant of economic importance. The presence of ultra-violet components in sunlight seems to arrest carbon sequestration in plants. An erratum to this article can be found at     

Potential of dendrochronology to assess annual rates of biomass productivity in savanna trees of West Africa

We examined the potential of dendrochronology to assess biomass productivity of individual savanna species from a semi-arid ecosystem in southern Senegal. The 9 tree species examined in this dendrochronologial study included: Acacia macrostachya, Acacia seyal, Balanites aegyptiaca, Combretum glutinosum, Cordyla pinnata, Pterocarpus erinaceus, Terminalia macroptera, Daniellia oliveri, and Combretum nigricans. Dendrochronologial analyses were applied on cross-sectional disks obtained from the tree stem to reconstruct past tree growth (diameter and biomass) histories. Despite challenges with discerning annual tree rings in these savanna species (associated with ring suppression, wedging, indistinct ring boundaries, and fires), tree species (A. macrostachya, A. seyal, and T. macroptera) with the highest dendrochronology potential produced a clear thin band of marginal parenchyma. A. macrostachya had rapid annual diameter and biomass growth increments in the juvenile years (ages 1–10), compared to T. macroptera which showed greater growth past this early juvenile period. Given the same species, generally wetter forests had lower annual and cumulative growth rates that were likely due to increased inter-tree and tree-grass competition for soil moisture in the wetter forests. We concluded that dendrochronology is well suited for retrospective annual biomass assessment in savanna trees of Senegal, West Africa.     

Contrasting impacts of grass species on nitrogen cycling in a grazed Sudanian savanna

We investigated the impact of perennial and annuals grass species on nitrogen cycling in a Sudanian savanna of Burkina Faso. We also analysed how the local context in terms of grazing and soil properties modifies these impacts. We selected four plots differing both by the intensity of grazing by cattle and soil depth, and used soil and grass biomass ¹⁵N as integrative indicators of N cycle. If perennials are able to foster a more efficient nitrogen cycling there should be lower ¹⁵N abundances in their biomass and soil. If soil depth and cattle pressure significantly modify nitrogen fluxes, soil depth and cattle pressure should influence ¹⁵N signatures. Our results suggest that perennial grasses are more conservative for nitrogen (inhibition of nitrification, less leaching via a perennial root system, slower cycling). The increase in leaf δ¹⁵N with N concentration is steeper in Loudetia togoensis than in the three other grasses. No significant difference was found between the ¹⁵N signatures of the four plots. Our results on ¹⁵N signatures and the fact that perennial grasses are much more abundant in the plots that are less grazed and have deeper soils, confirm that the switch from perennial to annual grasses is linked to a degradation in soil fertility and pasture quality. This suggests that ¹⁵N signatures can be used as indicators of fertility.     

Responses of tropical native and invader C<Subscript>4</Subscript> grasses to water stress,clipping and increased atmospheric CO<Subscript>2</Subscript> concentration

The invasion of African grasses into Neotropical savannas has altered savanna composition, structure and function. The projected increase in atmospheric CO₂ concentration has the potential to further alter the competitive relationship between native and invader grasses. The objective of this study was to quantify the responses of two populations of a widespread native C₄ grass (Trachypogon plumosus) and two African C₄ grass invaders (Hyparrhenia rufa and Melinis minutiflora) to high CO₂ concentration interacting with two primary savanna stressors: drought and herbivory. Elevated CO₂ increased the competitive potential of invader grasses in several ways. Germination and seedling size was promoted in introduced grasses. Under high CO₂, the relative growth rate of young introduced grasses was twice that of native grass (0.58 g g⁻¹ week⁻¹ vs 0.25 g g⁻¹ week⁻¹). This initial growth advantage was maintained throughout the course of the study. Well-watered and unstressed African grasses also responded more to high CO₂ than did the native grass (biomass increases of 21–47% compared with decreases of 13–51%). Observed higher water and nitrogen use efficiency of invader grasses may aid their establishment and competitive strength in unfertile sites, specially if the climate becomes drier. In addition, high CO₂ promoted lower leaf N content more in the invader grasses. The more intensive land use, predicted to occur in this region, may interact with high CO₂ to fincreasesavor the African grasses, as they generally recovered faster after simulated herbivory. The superiority of invader grasses under high CO₂ suggests further in their competitive strength and a potential increased rate of displacement of the native savannas in the future by grasslands dominated by introduced African species.     

Occurrence of C3 and C4 photosynthesis in cotyledons and leaves of Salsola species (Chenopodiaceae)

Most species of the genus Salsola (Chenopodiaceae) that have been examined exhibit C₄ photosynthesis in leaves. Four Salsola species from Central Asia were investigated in this study to determine the structural and functional relationships in photosynthesis of cotyledons compared to leaves, using anatomical (Kranz versus non-Kranz anatomy, chloroplast ultrastructure) and biochemical (activities of photosynthetic enzymes of the C₃ and C₄ pathways, ¹⁴C labeling of primary photosynthesis products and ¹³C/¹²C carbon isotope fractionation) criteria. The species included S. paulsenii from section Salsola, S. richteri from section Coccosalsola, S. laricina from section Caroxylon, and S. gemmascens from section Malpigipila. The results show that all four species have a C₄ type of photosynthesis in leaves with a Salsoloid type Kranz anatomy, whereas both C₃ and C₄ types of photosynthesis were found in cotyledons. S. paulsenii and S. richteri have NADP- (NADP-ME) C₄ type biochemistry with Salsoloid Kranz anatomy in both leaves and cotyledons. In S. laricina, both cotyledons and leaves have NAD-malic enzyme (NAD-ME) C₄ type photosynthesis; however, while the leaves have Salsoloid type Kranz anatomy, cotyledons have Atriplicoid type Kranz anatomy. In S. gemmascens, cotyledons exhibit C₃ type photosynthesis, while leaves perform NAD-ME type photosynthesis. Since the four species studied belong to different Salsola sections, this suggests that differences in photosynthetic types of leaves and cotyledons may be used as a basis or studies of the origin and evolution of C₄ photosynthesis in the family Chenopodiaceae.This revised version was published online in October 2005 with corrections to the Cover Date.     

Plant responses to atmospheric carbon dioxide enrichment: interactions with some soil and atmospheric conditions

In general, C₃ plant species are more responsive to atmospheric carbon dioxide (CO₂) enrichment than C₄-plants. Increased relative growth rate at elevated CO₂ primarily relates to increased Net Assimilation Rate (NAR), and enhancement of net photosynthesis and reduced photorespiration. Transpiration and stomatal conductance decrease with elevated CO₂, water use efficiency and shoot water potential increase, particularly in plants grown at high soil salinity. Leaf area per plant and leaf area per leaf may increase in an early growth stage with increased CO₂, after a period of time Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) generally decrease. Starch may accumulate with time in leaves grown at elevated CO₂. Plants grown under salt stress with increased (dark) respiration as a sink for photosynthates, may not show such acclimation to increased atmospheric CO₂ levels. Plant growth may be stimulated by atmospheric carbon dioxide enrichment and reduced by enhanced UV-B radiation but the limited data available on the effect of combined elevated CO₂ and ultraviolet B (280–320 nm) (UV-B) radiation allow no general conclusion. CO₂-induced increase of growth rate can be markedly modified at elevated UV-B radiation. Plant responses to elevated atmospheric CO₂ and other environmental factors such as soil salinity and UV-B tend to be species-specific, because plant species differ in sensitivity to salinity and UV-B radiation, as well as to other environmental stress factors (drought, nutrient deficiency). Therefore, the effects of joint elevated atmospheric CO₂ and increased soil salinity or elevated CO₂ and enhanced UV-B to plants are physiologically complex.     

Interactive effects of α-NAA and UV-B radiation on the endogenous hormone contents and growth of Trichosanthes kirilowii Maxim seedlings

The impact of UV-B radiation on endogenous hormones in plants has recently drawn attention from researchers. The mechanism for reduced stem elongation by UV-B might be due to changes in the phytohormone levels, especially IAA, which plays a role in stem elongation. In this study, effects of UV-B radiation on Trichosanthes kirilowii Maxim (T. kirilowii) seedlings in greenhouse-grown plants were investigated. The results indicated that: (1) In comparison to controls, exposure to 0.029 Jm^?2 s^?1. UV-B radiation led to accumulation of endogenous abscisic acid (ABA) and zeatinriboside (ZR) in the plant contents, and decreased contents of endogenous indole-3-acetic acid (IAA) and gibberellic acid (GA_1/3). Exposure to UV-B radiation reduced the height and leaf area of plants. As a result, total biomass (plant dry weight) was lower. (2) In comparison to controls, addition of 2 mg l^?1 α-naphthaleneacetic acid (α-NAA) slightly increased the contents of IAA, GA_1/3 and ZR, and decreased the content of ABA in leaves. This addition of α-NAA significantly increased plant height and leaf area, but only slightly increased total biomass. (3) Addition of α-NAA to UV-B-exposed plants: increased the content of endogenous IAA, GA_1/3 and ZR; decreased accumulation of endogenous ABA; and increased plant height and leaf area in comparison to plants that only were exposed to UV-B. Moreover, total biomass increased slightly. This suggests that addition of α-NAA may compensate to a certain extent for the lack of IAA resulting from UV-B radiation; it also increases the content of GA_1/3 and ZR, decreases the accumulation of ABA, and promotes the growth of plants.     

Influence of rainfall and grazing on herbage dynamics in a seasonally dry tropical savanna

Species composition and herbage dynamics in relation to rainfall variability and cattle grazing were studied in permanently protected, grazed, and temporarily fenced treatments on three sites in a seasonally dry tropical savanna. Permanently protected sites, established between 1979 and 1984, were 55–79% similar with each other in species composition, and 14–25% similar with grazed sites during the period 1986–1988. Similarity among grazed sites was only 36–43%. Number of species was greater in the grazed treatment than in the permanently protected treatment. The percentages of annual grasses and non-leguminous forbs were greater in grazed savanna than in permanently protected savanna. Species diversity was higher in grazed savanna than in the corresponding permanently protected savanna. Species the two annual cycles studied, peak live shoot biomass was 614 g m^-2 in permanently protected savanna, 109 g m^-2 in grazed savanna, and 724 g m^-2 in temporarily fenced savanna. Live shoot biomass in temporarily fenced savanna was 18 to 44% greater than in permanently protected savanna. Peak canopy biomass ranged from 342 to 700 g m^-2 in permanently protected savanna. It was related with total rainy season rainfall, and was particularly sensitive to late rainy season rainfall. On the other hand, peak canopy biomass in grazed savanna ranged from 59 to 169 g m^-2 and was related to grazing intensity rather than either total rainy season rainfall or late rainy season rainfall. Coefficient of variation of green biomass in permanently protected savanna was related with rainfall variability indicating it to be a pulsed system which responds quickly to rainfall events. Biomass of woody species ranged from 2466 to 5298 g m^-2 in permanently protected savanna and from 744 to 1433 g m^–2 in the grazed savanna. Green foliage biomass was 3.7 to 6.4% of the woody biomass in permanently protected and 5.6 to 5.9% in grazed savanna, and supplements substantially the fodder resource during the dry periods of the year.     

Hydraulic lift in<Emphasis Type="Italic"> Acacia tortilis</Emphasis> trees on an East African savanna

Recent studies suggest that savanna trees in semi-arid areas can increase understorey plant production. We hypothesized that one of the mechanisms that explains the facilitation between trees and grasses in East African savannas is hydraulic lift (HL). HL in large Acacia tortilis trees was studied during the first 3 months of the dry season during a relatively wet year (1998) and a very dry year (2000). In 1998, we found distinct diel fluctuation in soil water potential (psi(s)), with increasing values during the night and decreasing again the following day. These fluctuations in psi(s )are consistent with other observations of HL and in A. tortilis were found up to 10 m from the tree. In 2000, during a severe drought, psi(s) measurements indicated that HL was largely absent. The finding that HL occurred in wetter years and not in drier years was supported by data obtained on the delta(18)O values in soil, rain and groundwater. The delta(18)O of water extracted from the xylem water of grasses indicated that when they grew near trees they had values similar to those of groundwater. This could be because they either (1) use water from deeper soil layers or (2) use hydraulically lifted water provided by the tree; this was not seen in the same grass species growing outside tree canopies. While our data indicate that HL indeed occurs under Acacia trees, it is also true that psi(s) was consistently lower under trees when compared to outside tree canopies. We believe that this is because tree-grass mixtures take up more water from the upper soil layers than is exuded by the tree each night. This limits the beneficial effect of HL for understorey grasses and suggests that in savannas both facilitation via HL and competition are active processes. The importance of each process may depend upon how wet or dry that particular site or year is.     

Effects of N application on N accumulation of two Erigeron breviscapus populations from different altitudes under enhanced ultraviolet-B radiation

To study intraspecific differences in N utilization in response to enhanced UV-B radiation, field experiments were conducted on two Erigeron breviscapus populations (Huguo and Cangshan), which were respectively obtained from low altitude (UV-B sensitive) and high altitude (UBV-B resistant).The effects of soil nitrogen (N) application (0, 15, 30, 45 g m²) on free amino acid content, the activities of nitrate reductase (NR) and glutamine synthetase (GS), total nitrogen content and N mass in leaves were determined under enhanced UV-B radiation (5 kJ m²) for both populations. The results showed that under enhanced UV-B radiation: (1) increases in total N contents in leaves of the Huguo and Cangshan populations correlated with the amount of N applied. Additionally, leaf biomass of Huguo treated with 15 g m^?2 N application and Cangshan with 30 g m^?2 N application were higher than that of other treatments. Leaf N masses were highest in both E. breviscapus populations treated with 30 g m^?2 N; (2) increases in contents of free amino acids in leaves of both E. breviscapus populations correlated with the amount of applied nitrogen; (3) increases of NR activity in leaves correlated with the amount of applied nitrogen; (4) GS activity in leaves of the Huguo and Cangshan E. breviscapus populations were highest with respective N applications of 15 g m^?2 N and 30 g m^?2 N. In general, under enhanced UV-B radiation, N application might affect NR and GS and change free amino acid content, resulting in changes in total nitrogen content, biomass and N mass. The optimal amount of supplemental N for N accumulation in E. breviscapus was 30 g m^?2 N under enhanced UV-B radiation.