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.     

The Effects of Intermittent Flooding on Seedlings of Three Forest Species

Under greenhouse conditions, seedlings of three forest species, baldcypress (Taxodium distichum), nuttall oak (Quercus nuttallii), and swamp chestnut oak (Quercus michauxii) were subjected to an intermittent flooding and subsequent physiological and growth responses to such conditions were evaluated. Baldcypress showed no significant reductions in stomatal conductance (g _s) or net photosynthetic rate (P _N) in response to flood pulses. In nuttall oak seedlings g _s and P _N were significantly decreased during periods of inundation, but recovered rapidly following drainage. In contrast, in swamp chestnut oak g _s was reduced by 71.8 % while P _N was reduced by 57.2 % compared to controls. Baldcypress displayed no significant changes in total mass while oak species had significantly lower leaf and total mass compared to their respective controls. Thus baldcypress and nuttall oak showed superior performance under frequent intermittent flooding regimes due to several factors including the ability for rapid recovery of gas exchange soon after soil was drained. In contrast, swamp chestnut oak seedlings failed to resume gas exchange functions after the removal of flooding.     

Comparative studies on physiological and biochemical adaptation of Taxodium distichum and Taxodium ascendens seedlings to different soil water regimes

Responses of baldcypress (Taxodium distichum) and pondcypress (Taxodium ascendens) seedlings in leaf gas exchange and root metabolism to a wide range of hydrological regimes were studied. Four water treatments included control (C), mild drought (MD), wet soil (WS) and flooding (FL). Both species showed no significant change in net photosynthetic rate (Pn) in response to flooding in contrast to significant reduction in Pn under mild drought conditions. In baldcypress, Pn was also maintained at control level compared to significant increase of Pn in pondcypress when subjected to the wet soil treatment. In flooding, both species showed significant increase in contents of malate and shikimate in their lateral roots and further in total roots, as compared to control. However, different adaptation to flooding was demonstrated in their taproots in contents of malate and shikimate. Furthermore, baldcypress seedlings showed significant reductions in lateral and total root biomass, in contrast with no significant change in that of pondcypress seedlings in response to flooding. In wet soil, no significant effects were revealed in contents of malate and shikimate in roots (including different root portions) of baldcypress seedlings in contrast to significant increase of their contents in lateral roots of pondcypress seedlings. No significant effects on root biomass were detected in either baldcypress or pondcypress seedlings in response to wet soil. When subjected to mild drought, baldcypress seedlings displayed no significant change in contents of malate and shikimate in roots, whereas pondcypress seedlings exhibited significant reduction in content of shikimate in lateral roots and total roots. In addition, root biomass of baldcypress seedlings showed no significant change, while lateral and total root biomass of pondcypress seedlings demonstrated significant increase when mild drought was imposed. These findings indicate that baldcypress and pondcypress appear to be promising candidates for reforestation in the Three Gorges Reservoir region due to their characteristics of water-tolerance and mild drought endurance. However, pondcypress is more resilient than baldcypress to the continuum of water availability conditions expected in the Three Gorges Reservoir region.     

前期水淹对牛鞭草后期干旱胁迫光合生理响应的影响

水淹和干旱是限制植物生长的两种主要环境因子。三峡库区消落带由于其特殊的地形条件和人工水文节律,呈现以年度为周期的“水淹-落干”交替变化的水文变动特征,在消落带生长的植物因此受到水淹和干旱交替胁迫的双重影响。为了探究库区蓄水对消落带植被干旱耐受性的影响,以当年生牛鞭草扦插苗为试验对象,设置对照组(CK)、表土水淹组(SF)、全淹组(TF)、对照-干旱组(CD)、表土水淹-干旱组(SFD)、全淹-干旱组(TFD)6个处理组,研究不同水分处理对牛鞭草光合特性的影响。结果表明:(1)水淹和干旱胁迫均对牛鞭草光合特性造成显著影响;(2)水淹胁迫阶段,与CK组相比,牛鞭草SF和TF组净光合速率、气孔限制值和水分利用效率显著下降,胞间CO_2浓度显著上升;(3)干旱胁迫阶段,牛鞭草CD和SFD组净光合速率、气孔导度、胞间CO_2浓度和蒸腾速率等光合参数显著低于CK组,TFD组净光合速率、气孔导度、胞间CO_2浓度和蒸腾速率等指标与CK组无显著差异;(4)复水阶段,各处理组净光合速率、气孔导度、胞间CO_2浓度和蒸腾速率等指标均与CK组无显著差异。研究表明,前期水淹并未增加牛鞭草对后期干旱胁迫的敏感性,牛鞭草对水淹和干旱胁迫均具有较好的耐受性,有助于牛鞭草对库区消落带生境变化的适应性。     

Leaf gas exchange,chlorophyll fluorescence and growth responses of <Emphasis Type="Italic">Melaleuca alternifolia</Emphasis> seedlings to flooding and subsequent recovery

Periodic flooding of trees in tropical floodplains and reservoirs where water levels fluctuate is a common phenomenon. The effects of flooding and subsequent recovery on gas exchange, chlorophyll fluorescence and growth responses of Melaleuca alternifolia seedlings, a tall shrub species used in floodplain and reservoir forest restoration in southern China, were studied during a grow season (from March to December in 2007). M. alternifolia seedlings were flooded for 180 days, drained and left to recover for another 60 days. Survival rates of the seedlings were 100% during the 180-day flooding period. Chlorophyll (Chl) content, net photosynthetic rate (P _N), stomatal conductance (g _s), and transpiration rate (E) of the flooded seedlings were all significantly lower than those of the control. Significant reductions of photochemical quenching coefficient (q_p) and increases of nonphotochemical quenching (NPQ) in the flooded seedlings were observed. However, there were no significant differences in the maximal quantum yield of PSII photochemistry (F_v/F_m) between treatments. All seedlings survived during the two-month recovery period after the flooded treatment was drained, and the biomass and height of the recovered seedlings approached those of the control at the end of the experiment. During the first-month recovery period, Chl content, P _N, g _s and E in the recovered seedlings were all obviously low, then increased gradually and rose to the levels similar to the control by the end of the experiment. Quenching analysis revealed significant reductions of q_p and increments of NPQ in the recovered seedlings at the beginning of draining, and a nearly complete recovery for both parameters by the end of the experiment. However, F_v/F_m of the recovered seedlings did not differ significantly from the control during the recovery period. Our study demonstrated that M. alternifolia seedlings can survive and grow through 180 days of flooding with a subsequent 60-day recovery period in drained conditions, indicating that seedlings of this species would be suitable for afforestation in areas exposed to intermittent flooding.     

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.     

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.     

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.     

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.     

Effects of flooding and salinity on photosynthesis and water relations of four Southeastern Coastal Plain forest species

The influence of flooding and salinity on photosynthesis and water relations was examined for four common coastal tree species [green ash (Fraxinus pennsylvanica Marshall), water tupelo (Nyssa aquatica L.). Chinese tallow (Sapium sebiferum (L.) Roxb.), and baldcypress (Taxodium distichum (L.) Richard)]. Both chronic (as might be associated with sea level rise) and acute (similar to hurricane storm surges) exposures to these stresses were examined. Chronic freshwater flooding of green ash, water tupelo, and Chinese tallow seedlings reduced photosynthesis (A) relative to that of watered seedlings, while baldcypress was unaffected. Chinese tallow A declined with increasing length of flooding. A salinity increase of the floodwater to 2 ppt decreased A of baldcypress and water tupelo, but not A of green ash and Chinese tallow, which was already severely reduced by freshwater flooding. All seedlings of the four species died within 2 to 6 weeks when flooded with 10 ppt saltwater. Photosynthesis of all four species did not differ between 0 and 2 ppt watering. Watering with 10 ppt salinity initially reduced A of all four species, but the seedlings recovered over time. Photosynthesis was severely decreased for all species when flooded with 21 ppt salinity for 48 hours. Reduced A continued following the treatment. Photosynthesis of only green ash and water tupelo was reduced by watering with 21 ppt salinity for 48 hours. Flooding of low-lying areas with increased salinity would lead to shifts in species composition of coastal forests due to these differential tolerances.     

Flooding and salinity effects on growth and survival of four common forested wetland species

The survival, growth, and biomass of baldcypress (Taxodium distichum (L.) Rich.), water tupelo (Nyssa aquatica L.), Chinese tallow (Sapium sebiferum (L.) Roxb.), and green ash (Fraxinus pennsylvanica Marsh.) seedlings were examined in an experiment varying water levels (watered, flooded) and salinity levels (0, 2, and 10 ppt, plus a simulated storm surge with 32 ppt saltwater). All seedlings, except for those flooded with 10 ppt saltwater, survived to the end of the experiment. In 10 ppt saltwater, flooded baldcypress, water tupelo, and green ash survived two weeks whereas Chinese tallow survived for 6 weeks. However, a second set of slightly older baldcypress, water tupelo, and Chinese tallow seedlings survived eight weeks of flooding with 10 ppt saltwater. When carried through the winter to the beginning of the second growing season, flooded baldcypress and Chinese tallow seedlings from the 0 and 2 ppt treatments leafed out, but only Chinese tallow recovered from the saltwater surge treatment. The diameter and growth (height) of each species was not affected when watered with 2 ppt saltwater, except for the effects of the height growth of baldcypress. Growth was reduced for all species when watered with 10 ppt saltwater. The diameter growth of green ash was reduced by freshwater flooding. The diameter growth of baldcypress and water tupelo was greater when flooded with fresh water. Flooding with 2 ppt saltwater caused a significant reduction in diameter growth in water tupelo, green ash, and Chinese tallow, but not in baldcypress. Root and stem biomass values were not significantly different for any species between the 0 and 2 ppt salinity watering treatments. However, seedlings watered with 10 ppt saltwater had significantly lower root and stem biomass values, except for baldcypress roots and green ash stems. Baldcypress was least affected by flooding with 0 and 2 ppt saltwater, although there were slight reductions in root biomass with increasing salinity. Because of the susceptibility of the seedlings of these four species to increases in flooding and salinity, their regeneration may be limited in the future, thereby causing shifts in species composition.     

Developing a mathematical model for variation of physiological responses of Jatropha curcas leaves depending on leaf positions under soil flooding

The effect of soil flooding on photosynthesis, transpiration and stomatal conductance of Jatropha curcas seedlings were studied under natural environmental variables. Soil flooding reduced photosynthesis (P _N), transpiration (E) and stomatal conductance (gs) in response to leaf positions of Jatropha curcas plants. Based on the results, we conclude that decrease in stomatal opening and stomatal limitation of photosynthesis, followed by decrease in individual leaf area are the main causes of reductions in carbon uptake of flooded seedlings. A mathematical relationship was successfully developed to describe photosynthesis, transpiration and stomatal response of Jatropha under soil flooding stress.     

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.     

Disentangling the effects of drought,salinity, and sulfate on baldcypress growth in a coastal plain restored wetland

Because of their dominance in swamps of the southeastern United States, baldcypress (Taxodium distichum) trees are commonly used in wetland restoration. Though baldcypress are known to tolerate moderate flooding and salinity, their growth has been shown to decrease when they experience drought and high salinity. This study examined the effects of drought and elevated salinity on the growth of baldcypress seedlings and saplings. In a restored wetland in North Carolina, we examined the growth of 8‐year‐old baldcypress by measuring height and diameter at breast height (DBH) along salinity, nutrient, and flooding gradients. In a greenhouse, we placed 1‐year‐old baldcypress seedlings in either drought or saturated conditions and applied different water treatments: fresh water, sulfate, and artificial salt water (5 ppt [parts per thousand]). Over 26 weeks, we measured diameter at root collar, height, and biomass. In the field, chloride concentrations in soil solution had a negative effect on DBH and height (51 and 36% decrease, respectively); high water levels had a negative effect on height (47% decrease) and DBH (46% decrease). In the greenhouse, both drought and salinity decreased diameter growth (43 and 61% decline, respectively) and height (64 and 43% decline, respectively). Sulfate did not have a significant effect on diameter growth, but caused a 14% decrease in height. Our results suggest that both drought and salinity (even as low as 1 ppt) can lead to a 20–60% decline in baldcypress growth. Restoration practitioners should consider the negative consequences of both drought and increased salinity on baldcypress growth when planning for wetland restoration.     

Physiological behaviors of <Emphasis Type="Italic">Acer mono</Emphasis> under drought and low light

The seedlings of Acer mono Maxim. were exposed to two watering regimes (well watered (100% of field capacity) and drought (30% of field capacity)) and two light levels (high light (100% of full sunlight) and low light (15% of full sunlight)) in a greenhouse to assess growth, photosynthesis, and foliar nutrient traits of woody plants under drought and low light. Drought significantly reduced growth and gas exchange characteristics of A. mono, including net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C_i), and photosynthetic nitrogen use efficiency (PNUE). Also, drought reduced relative water content (RWC) and foliar C and N concentrations, but increased the C/N ratio. P _N and C concentration were lower under drought and low light than in any other treatment, indicating that deep shade might seriously decrease C assimilation. However, the negative effect induced by drought was alleviated by improving RWC and maintaining C balance, and therefore low-light seedlings accumulated more biomass than those under high light when they were exposed to drought. Our results showed that trade-off and facilitation effects of drought and low light might be complementary and alter in different species.     

Comparative ecophysiological responses to drought of two shrub and four tree species from karst habitats of southwestern China

Drought stress is one of the most important factors in limiting the survival and growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. The ecophysiological response to drought stress of native plants with different growth forms is useful for re-vegetation programs. Two shrub and four tree species were studied, including Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and other three deciduous trees, Broussonetia papyrifera, Platycarya longipes, and Pteroceltis tatarinowii. The seedlings were randomly assigned to four drought treatments, i.e., well-watered, mild drought stress, moderate drought stress, and severe drought stress. Leaf water relations, gas exchange, chlorophyll fluorescence, and growth of the seedlings were investigated. Under severe drought stress, the two shrubs with low leaf area ratio (LAR) maintained higher water status, higher photosynthetic capacity, and larger percent biomass increase than the most of the trees. The two shrubs also had lower specific leaf area, greater intrinsic water use efficiency, and thermal dissipation than the trees. This suggested that the two shrubs had high tolerance to severe drought and were suitable for re-vegetation in harsh habitats. The evergreen C. bodinieri exhibited higher leaf mass ratio (LMR) and LAR than the deciduous species under mild and moderate stress. However, the low maximum quantum efficiency of PSII photochemistry (F _v/F _m) and net assimilation rate, and the sharp decreases of water potential, LMR, LAR, and biomass under severe stress indicated C. bodinieri’s weak tolerance to severe drought. In response to drought stress, the three deciduous trees revealed sharp reductions of biomass due to the large drought-induced decreases of gas exchange, LAR, and LMR. Under drought conditions, the deciduous trees minimized water loss by stomatal closure and by reducing transpiration leaf area and light harvesting through shedding leaves. This suggested that the three deciduous trees were more sensitive to water availability than the shrubs and used avoidance strategies against drought stress. However, the better growth performance of the deciduous trees than that of the shrubs under favorable conditions suggested that deciduous trees could be suitable for habitats with mild and temporary drought stress.     

Arbuscular Mycorrhizal Fungi Alter Fractal Dimension Characteristics of Robinia pseudoacacia L. Seedlings Through Regulating Plant Growth,Leaf Water Status,Photosynthesis, and Nutrient Concentration Under Drought Stress

The influence of arbuscular mycorrhizal fungi (AMF), Funneliformis mosseae and Rhizophagus intraradices, on plant growth, leaf water status, chlorophyll concentration, photosynthesis, nutrient concentration, and fractal dimension (FD) characteristics of black locust (Robinia pseudoacacia L.) seedlings was studied in pot culture under well-watered, moderate drought stress, and severe drought stress treatments. Mycorrhizal seedlings had higher dry biomass, leaf relative water content (RWC), and water use efficiency (WUE) compared with non-mycorrhizal seedlings. Under all treatments, AMF colonization notably enhanced net photosynthetic rate, stomatal conductance, and transpiration rate, but decreased intercellular CO₂ concentration. Leaf chlorophyll a and total chlorophyll concentrations were higher in AM seedlings than those in non-AM seedlings although there was no significant difference between AMF species. AMF colonization improved leaf C, N, and P concentrations, but decreased C:N, C:P, and N:P ratios. Mycorrhizal seedlings had a larger FD value than non-mycorrhizal seedlings. The FD value was positively and significantly correlated to the plant growth parameters, photosynthesis, RWC, WUE, and nutrient concentration but negatively correlated to leaf/stem ratio, C:N and C:P ratios, and intercellular CO₂ concentration. We conclude that AMF lead to an improvement of growth performance of black locust seedlings under all growth conditions, including drought stress via improving leaf water status, chlorophyll concentration, photosynthesis, and nutrient uptake. Moreover, FD technology proved to be a powerful non-destructive method to characterize the effect of AMF on the physiology of host plants during drought stress.     

COMPARISON OF THE VEGETATION OF THREE LOUISIANA SWAMP SITES WITH DIFFERENT FLOODING REGIMES

Net primary productivity and community structure were studied at three adjacent swamp sites that were all once subject to seasonal flooding. Because of man-related activities one is now permanently flooded, and a second is managed on a summer-fall dry period and a winter-spring flooded period. In the permanently flooded area such flood-intolerant trees as green ash are dying while shrubs and aquatic plants are increasing. Competition among overstory trees is reduced, enhancing growth of existing baldcypress and water tupelo trees. Total areal production, however, is low (887 g/m²/yr) because of fewer trees. Productivity in the managed area is high (1,780 g/m²/yr). The baldcypress-water tupelo community is being replaced by a swamp maple-ash community typical of bottomlands. In the natural swamp forest, baldcypress and water tupelo have remained dominant. Canopy closure has limited understory and aquatic growth but has also increased competition among overstory trees, resulting in a slow increase in basal area. Productivity is about 1,166 g/m²/yr. It is suggested that the flooding regime is an important controlling factor in each of the areas.     

涝渍对3个树种生长、组织孔隙度和渗漏氧的影响

为了了解落羽杉(Taxodium distichum)、乌桕(Sapium sebiferum)和美国山核桃(Carya illinoensis)等树种的耐涝机制, 采用盆栽模拟涝渍环境的试验方法, 设置了淹水、渍水和对照3个处理, 测定了一年生落羽杉、乌桕和美国山核桃实生苗的生长、组织孔隙度、根氧消耗等指标。结果表明, 涝渍处理抑制了落羽杉、乌桕和美国山核桃的生物量和生物量增量(渍水处理下落羽杉的生长得到了促进), 增加了3树种的根冠比, 从生物量和生物量增量下降幅度来评价, 落羽杉的耐涝性最强, 其次为美国山核桃。淹水和渍水处理下, 落羽杉、乌桕和美国山核桃的根、茎和叶中的组织孔隙度显著增加, 且随着处理时间的延长, 各器官的组织孔隙度有增加的趋势, 3个树种中, 落羽杉的根、茎和叶中的组织孔隙度均较其他2个树种高。淹水和渍水处理下, 移除茎明显增加了落羽杉、美国山核桃和乌桕的根的氧消耗, 表明涝渍处理增强了O₂在3个树种体内的运输并通过根系扩散到涝渍土壤中的能力, 并且随着处理时间的延长, 3个树种体内运输O₂并扩散到土壤中的能力有逐渐增强的趋势。因此, 涝渍环境总体上抑制了落羽杉、乌桕和美国山核桃等树种的生长, 但各树种为了适应这种生长环境, 形成了大量的通气组织, 从而导致各器官组织孔隙度的增加, 增强了O₂通过植物体运输到根系并扩散到土壤中的能力, 解决了根系及根际缺氧的矛盾。