Nutrient limitation of plant growth on the floodplain of the Narran River, Australia: growth experiments and a pilot soil survey

Water resource development has altered the hydrological regime on the Lower Balonne River in Queensland, Australia. Concerns have been raised about possible impacts to floodplain plant communities, which support a pastoral industry and a range of native fauna. Water and nutrients commonly limit plant growth in south central Queensland, where the climate is semi-arid and the soils are infertile. Floodplain plant productivity is boosted by inundation with water, but the role of flooding in nutrient provision is not known. Growth experiments and a pilot soil survey were conducted to help determine if soil nutrient deficiencies exist and if regular flooding is required to maintain floodplain soil fertility. Soils were sampled from areas representing three flood frequency classes: high, moderate, and low. Chemical extractions were performed as a surrogate for `bioavailable' nutrients. Soil nitrogen (N) but not phosphorus (P) limited the growth of seedlings of wheat (Triticum aestivum L. Gardia) based on responses to nutrient additions: plants supplied with N had greater shoot length and total biomass than plants without N. Clear evidence of an effect of flood frequency on fertility was not revealed. Neither soil P, soil N, nor plant growth varied significantly with flood frequency. However, this analysis had low statistical power. There were trends for greater biomass of seedlings grown on moderately flooded soils and thinner roots for seedlings grown on frequently flooded soils, but neither of these growth responses was clearly linked to nutrient limitation. Nevertheless, the possibility that flooding provides a nutrient subsidy to plants cannot be ruled out because of a number of factors, including the statistical power of this analysis and the possibility that short-term nutrient subsidies occur with floods.     

Effects of flood pre-conditioning on responses of three bottomland tree species to soil waterlogging

The potential for improving the performance of seedlings of woody species under flood stress was evaluated. Seedlings of baldcypress (Taxodium distichum), nuttall oak (Quercus nuttallii), and swamp chestnut oak (Quercus michauxii) were subjected to a two-phase study in which soil moisture regime was controlled. During Phase I, plants were randomly assigned to either: Control, well-watered and drained conditions; or intermittent flooding, alternating flooding and drained conditions. Following completion of Phase I, seedlings in each treatment were assigned randomly to one of two treatments: well-watered but drained or continuously flooded. Thus during Phase II, plants were divided into four groups: Control (A), intermittent flooding during Phase I and draining during Phase II (B), control during Phase I and continuously flooding during Phase II (C), and intermittently flooded during Phase I and continuously flooded during Phase II (D). Stomatal conductance (gw) in baldcypress increased in flooded treatments ranging from 112 to 128 percnt; of controls. Net photosynthesis (Pn) in baldcypress was not significantly affected by flooding while in oaks, Pn and gw decreased significantly in response to flooding. In oaks, flood pre-conditioning did not affect growth or physiological responses during phase II significantly. However, stomatal conductance of flood-hardened baldcypress was increased compared to seedlings that were not hardened. The study species developed significantly higher root porosity under flooded conditions compared to controls. Flood pre-conditioning did not appear to increase root porosity although it may have accelerated the process.     

Physiological,morphological, and growth responses ofPlatanus occidentalis seedlings to flooding

Summary Flooding ofPlatanus occidentalis L. seedlings for up to 40 days induced several changes including early stomatal closure, greatly accelerated ethylene production by stems, formation of hypertrophied lenticels and adventitious roots on submerged portions of stems, and marked growth inhibition. Poor adaptation ofPlatanus occidentalis seedlings to soil inundation was shown in stomatal closure during the entire flooding period, inhibition of root elongation and branching, and death of roots. Some adaptation to flooding was indicated by (1) production of hypertrophied lenticels which may assist in exchange of dissolved gases in flood water and in release of toxic compounds, and (2) production of adventitious roots on stems which may increase absorption of water. These adaptations appeared to be associated with greatly stimulated ethylene production in stems of flooded plants. The greater reduction of root growth over shoot growth in flooded seedlings will result in decreased drought tolerance after the flood waters recede. The generally low tolerance to flooding of seedlings of species that are widely rated as highly flood tolerant is emphasized.     

Soybean dry matter and N accumulation responses to flooding stress, N sources and hypoxia

Soybean (Glycine max [L.] Merr.) is generally considered sensitive to flooding stress. Data on relative sensitivities of biomass accumulation and N2 fixation to flooding stress, however, are limited. Additionally, it is not clear why plants dependent on N2 fixation appear to be more flood-sensitive than plants supplemented with inorganic N. This study evaluated the response to flooding and N source of biomass and N accumulation in various soybean genotypes. Soybean plants were grown in a potting mixture in a greenhouse and flooded for 21 d in degassed nutrient solution. An additional experiment evaluated root hypoxia by exposing roots of plants to a gas mixture supplying 1.5 kPa pO2. Dry matter and N were determined at various times following the initiation of flood or low O2 treatment. In all experiments, N2 fixation was more sensitive to flooding than was biomass accumulation. The decrease in N2 fixation occurred faster (within 7 d of flooding) than the decrease in biomass (within 14-21 d), and the decrease in N2 fixation was more pronounced than the decrease in biomass. Addition of nitrate decreased flood sensitivity relative to plants dependent on N2 fixation. Plant response to hypoxia was similar to flooding. Biomass of plants with roots exposed to 1.5 kPa pO2 was decreased by 34% when dependent on N2 fixation and 12% when supplemented with nitrate. Collectively, the data indicate that decreased soybean growth under flooding is a result of decreased N2 fixation and that supplementation of soybean plants with nitrate may improve their tolerance to flooding relative to those relying on N2 fixation.Keywords: Soybean, Glycine max, flooding stress, hypoxia, N source, nitrogen fixation.      

Effects of soil moisture regimes on photosynthesis and growth in cattail (Typha latifolia)

Both waterlogging and water deficiency are major environmental factors affecting plant growth and functioning in many wetland and floodplain ecosystems across North America. Wetland plants possess various characteristics that enable them to survive and function in the intermittently flooded wetland environments, while their sensitivity to drought has received less attention. The present study quantified the photosynthetic and growth responses of cattail (Typha latifolia), an important species of freshwater wetlands, to a wide range of soil moisture regimes. In addition, changes in the efficiency of photosynthetic apparatus following initiation of the treatments were investigated. Under greenhouse conditions, seedlings were subjected to four soil moisture regimes: (1) drained (control), (2) continuous flooding, (3) periodic flooding, and (4) periodic drought. Results indicated that dark fluorescence yield was increased in response to periodic drought, while it showed decreases under continuous flooding. Net photosynthesis and stomatal conductance were enhanced by continuous flooding and periodic flooding. In contrast, these parameters exhibited reduction under periodic drought. In addition, leaf chlorophyll content was adversely affected by periodic drought. Recovery of net photosynthesis was noted, along with enhanced height growth, in both continuously and periodically flooded plants. Meanwhile, continuous flooding enhanced biomass production while periodic drought led to biomass reduction. Periodic drought also contributed to substantial reduction in root growth compared with shoot growth. Therefore, the combined photosynthetic performance and growth responses of cattail are likely to contribute to the ability of this species to thrive in flooded condition but be susceptive to periodic drought.     

Effects of flooding on leaf litter decomposition in microcosms

Summary The effects of hydroperiod on decomposition rates of senescent Acer rubrum leaves were tested in microcosms in a controlled laboratory environment. Microcosm treatments included continuously flooded, continuously unflooded, and fluctuating hydroperiods. All flooding treatments promoted decomposition but variations in hydroperiod had no significant effects. A leaching experiment indicated the higher decay rates under flooded conditions were primarily due to high leaching losses from soaking. Unlike nutrient dynamics in the field, where net accumulation occurs, nitrogen and phosphorus in the litter in the microcosms exhibited net losses. The major external inputs which provide a source of nitrogen and phosphorus for immobilization in the field were lacking in the microcosms. Calcium, magnesium, and potassium exhibited net losses except for calcium in the unflooded microcosms. The microcosm results demonstrated the importance of external inputs to litter nutrient relations.     

Effects of Flooding on Susceptibility of Taxodium distichum L. Seedlings to Drought

Responses of baldcypress (Taxodium distichum) seedlings to soil moisture were studied to test the hypothesis that flooding may lead to seedling's higher susceptibility to drought. Treatments included a well-watered but drained control (C), continuously flooded (CF), control followed by drought (CD), and flooded followed by drought (FD). Gas exchange values revealed no significant effects on net photosynthetic rate (P _N) in response to flooding. In contrast, after the onset of drought, P _N was significantly reduced in CD and FD plants. Significant growth reductions under mild drought conditions indicated that baldcypress seedlings were drought sensitive. However, comparison of gas exchange rates and growth responses between CD and FD plants indicated that prior flooding had no detectable effect on subsequent sensitivity of baldcypress to drought. These findings explain baldcypress persistence in wetland habitats characterized by periodic flooding and mild drought.     

Nutrients stimulate leaf breakdown rates and detritivore biomass: bottom-up effects via heterotrophic pathways

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to long-term nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that differed in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the effects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment affected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3× faster during the first and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3× higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2–3× with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6× for red maple and up to 44× for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by different leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may affect detrital resource quality and subsequently lead to altered energy and nutrient flow in detrital food webs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Responses of three bottomland species with different flood tolerance capabilities to various flooding regimes

Seedlings of baldcypress (Taxodium distichum), nuttall oak (Quercus nuttalli), and cherrybark oak (Quercus falcata var.pagodaefolia) were subjected to four flooding treatments: control, continuously flooded, intermittently flooded, and partially flooded for 70 days in a greenhouse. The treatments imposed various durations and intensities of soil redox potential (Eh) conditions representing a range encountered by plants in their habitats. Morphological changes and gas exchange responses to the treatments differed among the study species. Rapid development of adventitious root and hypertrophied lenticels were observed in baldcypress and nuttall oak under all flooded treatments. Cherrybark oak had the highest percentage reduction in net photosynthesis ranging from 65–87%, whereas reductions in nuttall oak ranged between 35–68% and in baldcypress between 6–21% in response to various treatments. Recovery of gas exchange was noted in baldcypress but no significant recovery was found in oaks. The recovery in baldcypress contributed to the continued growth and biomass accumulation under various treatments. Little evidence of consistent changes in biomass allocation patterns in response to the treatments was found in baldcypress but total biomass decreased significantly under the continuously flooded treatment. In oaks, total biomass decreased significantly in all flooded treatments. The present findings demonstrated that physiological functions are adversely affected by low soil Eh conditions and the extent of such effects are dependent on the intensity and duration of soil reduction as well as the species' capability to respond to such conditions rapidly. Management plans concerned with regeneration of bottomland forested ecosystems should consider the species flood response capabilities at seedling stages as well as the timing, durations, and intensities of soil reduction at the specific site.     

VARIATION IN ALCOHOL DEHYDROGENASE ACTIVITY AND FLOOD TOLERANCE IN WHITE CLOVER,TRIFOLIUM REPENS

Flooding results in induction of anaerobic metabolism in many higher plants. As an important component of anaerobic energy production, alcohol dehydrogenase (ADH) activity increases markedly in response to flooding in white clover, Trifolium repens. Significant inter-individual variation in flood-induced ADH activity exists in natural populations of T. repens. The genetic basis of this variation was analyzed by offspring-midparent regression of data from 75 greenhouse reared families; the estimated heritability of flood-induced ADH activity was 0.55 (±0.13). Genetic variation in flood-induced ADH activity has pronounced effects on physiological response and flood tolerance in this species. ADH activity is positively correlated with the rate of ethanol production, indicating that observed in vitro activity differences are manifested in in vivo physiological function. T. repens plants with higher ADH activities during flooding have greater flood tolerance (measured as growth rate when flooded/unflooded growth rate). Variation in ADH activity during flooding accounts for more than 79% of the variance in flood tolerance. On the basis of a limited field survey of populations occupying three sites differing in exposure to flooding conditions, individuals from site C, the most frequently flooded site, expressed significantly higher average ADH activity when flooded than individuals from site A, a site with no history of flooding. Since ADH activity levels are not correlated with electrophoretic mobility variation in T. repens, this work supports previous suggestions that regulatory variation in enzyme activity may play a central role in biochemical adaptations to environmental stress.     

Morphological and photosynthetic responses of riparian plant Distylium chinense seedlings to simulated Autumn and Winter flooding in Three Gorges Reservoir Region of the Yangtze River, China

To evaluate the tolerance of riparian plant Distylium chinense in Three Gorges Reservoir Region to anti-season flooding, a simulation flooding experiment was conducted during Autumn and Winter, and morphology and photosynthesis of D. chinense seedlings and their recovery growth after soil drainage were analyzed in different duration of flooding and flooding depth. The seedlings were submitted to four treatments: (1) 40 seedlings unflooded and watered daily as control (Unflooded, CK); (2) 120 seedlings flooded at 1 cm above the ground level (F-1 cm); (3) 120 seedlings flooded at 12 cm above the ground level (F-12 cm) and (4) 120 seedlings completely submerged with 2 m water depth (F-2 m, top of plants at 2 m below water surface). The flooding survival, plant height, stem diameter, adventitious roots, stem lenticels, epicormic shoots, chlorophyll content and photosynthesis parameters were determined at 0, 15, 30, 90 days in flooding stress and 15, 60 days after soil drainage. The results showed that the survival of the seedlings subjected to flooding was 100% for all repeated measurements in all treatments. Adventitious roots, hypertrophied lenticels and stem hypertrophy were observed in the seedlings flooded for more than 15 d, and increased with the prolonged flooding duration, while disappeared after the soil was drained. Flooding duration and flooding depth showed significant individual and interactive effects on leaf chlorophyll a (Chl a), chlorophyll b (Chl b), and their ratio, chlorophyll (a + b), the net photosynthesis rate (P_n), transpiration rate (T_r), stomatal conductance (C_s), and inter-cellular CO₂ concentration (C_i) of D. chinense seedlings (P < 0.01). After 15 days of flooding, there was no significant decrease in P_n of the flooded seedlings as compared with that of the control seedlings. P_n of the flooded seedlings was significantly lower than that of the control seedlings after 30 days of flooding (P < 0.05), whereas P_n showed no significant difference among seedlings from three flooding depths. After 90 days of flooding, P_n of the F-2 m flooded seedlings was significantly lower than that of the controls, F-1 cm and F-12 cm flooded seedlings (P < 0.05), but still maintained high photosynthetic capacity. P_n of the F-1 cm and F-12 cm flooded seedlings rose gradually after soil drainage, while, it was significantly lower than that of the control seedlings after 15 days of recovery (P < 0.05). After 60 days of recovery, P_n of all seedlings flooded with different depths showed no significant difference as compared with that of the control seedlings and new leaves grew out in the F-2 m flooded seedlings. The effect of all flooding treatments on Gs, T_r, Chl a, Chl b, Chl a/Chl b and chl (a + b) was basically the same as their effect on P_n, while the effect of all flooding treatments on C_i was quite the contrary. Correlation analysis showed that P_n was positively relative with Gs, T_r, Chl a, Chl b and chl (a + b) (P < 0.05) and significantly negative with C_i (P < 0.05). Therefore, the present study demonstrates that D. chinense has high survival and good recovery growth after long-term flooding in anti-season flooding and could be taken as an excellent candidate species in the re-vegetation of water-level-fluctuation areas in Three Gorges Reservoir Region.     

Morphological and photosynthetic responses of riparian plant Distylium chinense seedlings to simulated Autumn and Winter flooding in Three Gorges Reservoir Region of the Yangtze River, China

To evaluate the tolerance of riparian plant Distylium chinense in Three Gorges Reservoir Region to anti-season flooding, a simulation flooding experiment was conducted during Autumn and Winter, and morphology and photosynthesis of D. chinense seedlings and their recovery growth after soil drainage were analyzed in different duration of flooding and flooding depth. The seedlings were submitted to four treatments: (1) 40 seedlings unflooded and watered daily as control (Unflooded, CK); (2) 120 seedlings flooded at 1 cm above the ground level (F-1 cm); (3) 120 seedlings flooded at 12 cm above the ground level (F-12 cm) and (4) 120 seedlings completely submerged with 2 m water depth (F-2 m, top of plants at 2 m below water surface). The flooding survival, plant height, stem diameter, adventitious roots, stem lenticels, epicormic shoots, chlorophyll content and photosynthesis parameters were determined at 0, 15, 30, 90 days in flooding stress and 15, 60 days after soil drainage. The results showed that the survival of the seedlings subjected to flooding was 100% for all repeated measurements in all treatments. Adventitious roots, hypertrophied lenticels and stem hypertrophy were observed in the seedlings flooded for more than 15 d, and increased with the prolonged flooding duration, while disappeared after the soil was drained. Flooding duration and flooding depth showed significant individual and interactive effects on leaf chlorophyll a (Chl a), chlorophyll b (Chl b), and their ratio, chlorophyll (a + b), the net photosynthesis rate (P_n), transpiration rate (T_r), stomatal conductance (C_s), and inter-cellular CO₂ concentration (C_i) of D. chinense seedlings (P < 0.01). After 15 days of flooding, there was no significant decrease in P_n of the flooded seedlings as compared with that of the control seedlings. P_n of the flooded seedlings was significantly lower than that of the control seedlings after 30 days of flooding (P < 0.05), whereas P_n showed no significant difference among seedlings from three flooding depths. After 90 days of flooding, P_n of the F-2 m flooded seedlings was significantly lower than that of the controls, F-1 cm and F-12 cm flooded seedlings (P < 0.05), but still maintained high photosynthetic capacity. P_n of the F-1 cm and F-12 cm flooded seedlings rose gradually after soil drainage, while, it was significantly lower than that of the control seedlings after 15 days of recovery (P < 0.05). After 60 days of recovery, P_n of all seedlings flooded with different depths showed no significant difference as compared with that of the control seedlings and new leaves grew out in the F-2 m flooded seedlings. The effect of all flooding treatments on Gs, T_r, Chl a, Chl b, Chl a/Chl b and chl (a + b) was basically the same as their effect on P_n, while the effect of all flooding treatments on C_i was quite the contrary. Correlation analysis showed that P_n was positively relative with Gs, T_r, Chl a, Chl b and chl (a + b) (P < 0.05) and significantly negative with C_i (P < 0.05). Therefore, the present study demonstrates that D. chinense has high survival and good recovery growth after long-term flooding in anti-season flooding and could be taken as an excellent candidate species in the re-vegetation of water-level-fluctuation areas in Three Gorges Reservoir Region.     

Responses of Wetland Tree Species to Hydrology and Soils

We conducted a study of the flood tolerance of nine wetland tree species on seven soil types. Seedlings were subjected to 11 months of continuous shallow inundation or moist soil conditions on three mineral soils, two organic soils, a manufactured soil designed to mimic the practice of layering muck over mineral soil, and a stockpiled topsoil. Taxodium ascendens, T. distichum, Acer rubrum, and Pinus serotina suffered no mortality; Fraxinus carolininna (1%), Liquidambar styraciflua (8%), P. elliottii (8%), and Gordonia lasianthus (24%) suffered low to moderate mortality; and Persea palustris (46%) suffered significant mortality. In general, greatest net height and total biomass were achieved on moist organic soils, and least net height and total biomass were achieved on stockpiled topsoil and inundated soils. Responses to hydrological conditions were less pronounced for Taxodium spp. If the results of this experiment are transferable to the field, then Acer rubrum, Fraxinus caroliniana, Pinus serotina, Taxodium ascendens, and Taxodium distichum seedlings can reasonably be expected to survive at least one year under a broad range of hydrological and edaphic conditions. With the exception of Taxodium spp., first-year growth for the species of this study can be facilitated by maintaining moist but not inundated conditions. These findings suggest that transfer of organic soils will benefit restoration and creation efforts, and that layering organic soil over mineral soil is more effective than using mineral soils or stockpiled topsoil.     

Seasonal flooding regimes influence survival,nitrogen fixation,and the partitioning of nitrogen and biomass in Alnus incana ssp. rugosa

Alteration of natural flooding regimes can expose lowlands to waterlogged soil conditions during any month of the year. The seasonality of flooding may have profound effects on the carbon and nitrogen budgets of N-fixing alders (Alnus spp.), and in turn, may impact the C and N economy of extensive alder-dominated, wetland ecosystems, including those dominated by speckled alder (Alnus incana ssp. rugosa). To better understand this process, two-year-old, nodulated seedlings of speckled alder were subjected to late spring (May 10 – July 10), summer (July 10 – September 8), and fall (September 8 – November 8) flooding treatments. Alders were root-flooded outdoors in tanks containing an N-free nutrient solution and compared with unflooded alders at the experimental site. Flooding arrested N fixation, photosynthesis, and growth of alders without recovery in all flooding treatments for the remainder of the growing season. Late spring and summer flooding resulted in complete mortality of alders while all seedlings survived flooding in the fall. Fall flooding increased foliar N resorption by 140% over unflooded seedlings. Eighty-seven percent of the total N fixed and 89% of biomass accumulation for the entire growing season occurred in unflooded alders after July 10. In unflooded alders, nitrogen fixation rates per unit mass declined by 63% for nodules, 28% for leaves, and 48% for whole seedlings during the fall, while total N fixed per plant in the fall was similar to that fixed in the summer. The majority of newly fixed N in unflooded alders was allocated to leaves before September 8 and to roots/nodules combined after September 8. In unflooded plants, the greatest proportion of new biomass was partitioned to leaves before July 10, to stems between July 10 and September 8, and equally to stems and roots/nodules after September 8. Fall-flooded alders did not increase root or nodule biomass. Proportional allocation of plant resources were such that the ratio of N fixed to seedling growth of unflooded alders decreased by 19% during summer before rebounding by 6% in fall. Seasonality of flooding alters seedling survival, growth, and resource allocation, and may be a critical determinant of speckled alder recruitment and occurrence in wetlands.     

Arbuscular mycorrhizal colonization and nodulation improve flooding tolerance in <Emphasis Type="Italic">Pterocarpus officinalis</Emphasis> Jacq. seedlings

Pterocarpus officinalis (Jacq.) seedlings inoculated with the arbuscular mycorrhizal fungus, Glomus intraradices, and the strain of Bradyrhizobium sp. (UAG 11A) were grown under stem-flooded or nonflooded conditions for 13 weeks after 4 weeks of nonflooded pretreatment under greenhouse conditions. Flooding of P. officinalis seedlings induced several morphological and physiological adaptive mechanisms, including formation of hypertrophied lenticels and aerenchyma tissue and production of adventitious roots on submerged portions of the stem. Flooding also resulted in an increase in collar diameter and leaf, stem, root, and total dry weights, regardless of inoculation. Under flooding, arbuscular mycorrhizas were well developed on root systems and adventitious roots compared with inoculated root systems under nonflooding condition. Arbuscular mycorrhizas made noteworthy contributions to the flood tolerance of P. officinalis seedlings by improving plant growth and P acquisition in leaves. We report in this study the novel occurrence of nodules connected vascularly to the stem and nodule and arbuscular mycorrhizas on adventitious roots of P. officinalis seedlings. Root nodules appeared more efficient fixing N₂ than stem nodules were. Beneficial effect of nodulation in terms of total dry weight and N acquisition in leaves was particularly noted in seedlings growing under flooding conditions. There was no additive effect of arbuscular mycorrhizas and nodulation on plant growth and nutrition in either flooding treatment. The results suggest that the development of adventitious roots, aerenchyma tissue, and hypertrophied lenticels may play a major role in flooded tolerance of P. officinalis symbiosis by increasing oxygen diffusion to the submerged part of the stem and root zone, and therefore contribute to plant growth and nutrition.     

Adaptive strategies to tolerate prolonged flooding in seedlings of floodplain and upland populations of Himatanthus sucuuba, a Central Amazon tree

Long-term flooding imposes a strong selection pressure on plants for the development of protective mechanisms to alleviate the harmful effects of hypoxic and anoxic conditions. This is particularly critical in the Amazonian floodplains where plants withstand annual periods of flooding lasting 7 months and mean flooding amplitude reaching 10 m or more. Himatanthus sucuuba (Apocynaceae) is a tree that is found in the varzea (VZ) floodplains and non-flooded terra firme (TF) forests. It was examined whether individuals from these two contrasting habitats respond differently when subjected to extreme flooding conditions. TF and VZ seedlings were experimentally well-watered, waterlogged (roots and parts of the stems flooded), or submerged (whole plant flooded) during a 4-month period. Anaerobic respiration, evaluated by measuring alcohol dehydrogenase (ADH) activity, and root carbohydrate reserves were quantified, given that the availability of readily fermentable carbohydrates is essential to sustain an active fermentative metabolism. We also assessed changes in morphoanatomy, seedling survival, biomass accumulation and distribution. VZ seedlings had greater root concentrations of soluble sugars and starch, larger seedling mass and accumulated more biomass in roots and stems while TF seedlings allocated more towards stem and leaves. ADH activity was low in seedlings of both populations before exposure to flooding. Waterlogging induced an increase in ADH activity that reached a maximum value in 15 days. Thereafter activity decreased slowly, meanwhile a rapid formation of lenticels, adventitious roots and aerenchyma was observed. Submergence induced leaf shedding and the development of aerenchyma in the root cortex. While VZ seedlings maintained high levels of ADH activity throughout the whole 4-month period, ADH activity in TF seedlings peaked about 15 days after submersion followed by a continuous decrease and death of all the plants. Thus, VZ and TF seedlings differed considerably in terms of tolerating long-term exposure to flooding, especially under total submersion. These results suggest that the predictability and long-term duration of flooding in Central Amazon rivers can impose a selective pressure that is strong enough to result in large phenotypic differences between the two populations of H. sucuuba in the two habitat types.     

Stem hypoxia and root respiration of flooded maple and birch seedlings

Some researchers have attributed flood tolerance of woody plants to air entering the shoot through stems, leaves, or lenticels and diffusing to the roots to sustain aerobie respiration. The purpose of this study was to determine if internal aeration of roots by lower stems, changes in gross morphology of lower stems, or both, contribute to flood tolerance of certain tree species. Greenhouse-grown seedlings of red maple ( Acer rubrum L.) and river birch ( Betula nigra L.) tolerated at least 30 days of flooding, where as sugar maple ( Acer saccharum Marsh) and European white birch (also called silver birch, Betula pendula Roth) were intolerant. Flood treatment induced lentieel intumescences and adventitious root formation on red maple stems, but only adventitious roots formed on river birch stems. Stem morphology of sugar maple and European birch was unchanged by flooding. Flood stress decreased oxygen consumption capacity of excised roots from both tolerant and intolerant species. Exclusion of oxygen from the lower stems of flooded red maple and river birch prevented lenticel intumescence and adventitious root formation, but flood tolerance and root respiration capacity were unchanged. Neither internal aeration nor changes in stem morphology appear to account for flood tolerance of red maple and river birch.     

Facilitation of Nothofagus antarctica (Fagaceae) seedlings by the prostrate shrub Empetrum rubrum (Empetraceae) on glacial moraines in Patagonia

Abstract Although positive interactions among plants are believed to be common in primary succession, they have rarely been demonstrated on glacial moraines. In Patagonia, the prostrate shrub Empetrum rubrum is dominant in early succession in recently deglaciated valleys, eventually being replaced by the small tree Nothofagus antarctica. This study experimentally evaluates the effects of E. rubrum shrubs on density, biomass, growth and survival of N. antarctica seedlings. The initial density of the seedlings was significantly higher under the canopy of Empetrum than in open areas. A removal experiment was performed to evaluate the influence of Empetrum on survival and growth of seedlings that germinated naturally beneath the canopy of this species. The shrub cover was removed from half of a sample of randomly selected seedlings, and left intact above the remainder. One year later, seedlings with an intact cover of E. rubrum showed significantly higher survival, growth and leaf number than seedlings which had been exposed. The results suggest that the presence of E. rubrum facilitates the establishment of N. antarctica in post‐glacial succession, mainly as a result of more favourable microclimatic conditions under the shrub canopy.     

Ecophysiological adaptations of black spruce (<Emphasis Type="Italic"> Picea mariana</Emphasis>) and tamarack (<Emphasis Type="Italic"> Larix laricina</Emphasis>) seedlings to flooding

Black spruce [ Picea mariana (Mill.) B.S.P.] and tamarack [ Larix laricina (Du Roi) K. Koch] are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiological responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electrolyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained significantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings developed adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.     

Interactive effects of substrate,hydroperiod, and nutrients on seedling growth of Salix nigra and Taxodium distichum

The large river swamps of Louisiana have complex topography and hydrology, characterized by black willow (Salix nigra) dominance on accreting alluvial sediments and vast areas of baldcypress (Taxodium distichum) deepwater swamps with highly organic substrates. Seedling survival of these two wetland tree species is influenced by their growth rate in relation to the height and duration of annual flooding in riverine environments. This study examines the interactive effects of substrate, hydroperiod, and nutrients on growth rates of black willow and baldcypress seedlings. In a greenhouse experiment with a split-split-plot design, 1-year seedlings of black willow and baldcypress were subjected to two nutrient treatments (unfertilized versus fertilized), two hydroperiods (continuously flooded versus twice daily flooding/draining), and two substrates (sand versus commercial peat mix). Response variables included height, diameter, lateral branch count, biomass, and root:stem ratio. Black willow growth in height and diameter, as well as all biomass components, were significantly greater in peat substrate than in sand. Black willow showed a significant hydroperiod–nutrient interaction wherein fertilizer increased stem and root biomass under drained conditions, but flooded plants did not respond to fertilization. Baldcypress diameter and root biomass were higher in peat than in sand, and the same two variables increased with fertilization in flooded as well as drained treatments. These results can be used in Louisiana wetland forest models as inputs of seedling growth and survival, regeneration potential, and biomass accumulation rates of black willow and baldcypress.