Seasonal water uptake and movement in root systems of Australian phraeatophytic plants of dimorphic root morphology: a stable isotope investigation

A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native treeBanksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4–6 month) dry season. Parallel studies of other native species and two plantation-grown species ofEucalyptus all demonstrated behavior similar to that ofB. prionotes. ForB. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study supported the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heathlands where the acquisition of seasonally limited water is at a premium.     

Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangroves,Rhizophora mangle L.

Summary The red mangrove (Rhizophora mangle L.) occurs frequently in both scrub and fringe mangrove forests. Our previous study demonstrated that individuals of this mangrove species growing in scrub and fringe forests differ significantly in both morphological and physiological characteristics. To further characterize physiological differences between scrub and fringe mangroves, we compared their differences in water uptake and photosynthetic gas exchange during different seasons. In the wet season (June–October, 1990), scrub mangroves showed lower D and ¹⁸O values of stem water than fringe mangroves, indicating more usage of rain-derived freshwater. In the dry season (Jan–April, 1991), however, scrub mangroves utilized the same water source as fringe mangroves, reflected by their similar D and ¹⁸O values of stem water. Consistently, there were significant differences in predawn water potentials between scrub and fringe mangroves in the wet season (October 1990) with higher values for scrub mangroves, but no significant differences in the dry season (January 1991). Higher elevation in the scrub forest seems to be the major factor responsible for the shift of water sources in scrub mangroves. On Apr. 27 and Aug. 8, 1990, scrub mangroves showed lower CO₂ assimilation rate, stomatal conductance, and intercellular CO₂ concentration than fringe mangroves. There were no differences in these gas exchange characteristics on the other two measuring dates: Oct. 17, 1990 and Jan. 11, 1991. Instantaneous water use efficiency was significantly higher for scrub mangroves than for fringe mangroves on three of the four sampling dates. Similarly, leaf carbon isotope discrimination of scrub mangroves was always significantly lower than that of fringe mangroves, indicating higher long-term water use efficiency. Higher water use efficiency in scrub mangroves is a result of stomatal limitation on photosynthesis, which may entail considerable carbon cost to the plants.     

Sources of water used by riparian Eucalyptus camaldulensis overlying highly saline groundwater

Water sources of Eucalyptus camaldulensis Dehn. trees were investigated on a semiarid floodplain in south-eastern Australia. The trees investigated ranged in distance from 0.5 to 40 m from a stream, with electrical conductivity 0.8 dSm^–1, and grew over groundwater with electrical conductivity ranging from 30 to 50 dSm^–1. The sources of water being used by the trees were investigated using the naturally occurring stable isotopes of water and measurements of soil water potential. Xylem water potential and leaf conductance were also examined to identify the trees' response to using these sources of water. Trees at distances greater than about 15 m from the stream used no stream water. The trees used groundwater in summer and a combination of groundwater and rain-derived surface-soil water (0.05–0.15 m depth) in winter. In doing so they suffered water stress at electrical conductivities higher than approximately 40 dSm^–1 (equivalent to approximately –1.4 MPa). Trees adjacent to the stream used stream water directly in summer, but may have used stream water from the soil profile in winter, after the stream had risen and recharged the soil water. E. camaldulensis appeared to be partially opportunistic in the sources of water they used.     

Leaf functional traits of Neotropical savanna trees in relation to seasonal water deficit

The seasonal savannas (cerrados) of Central Brazil are characterized by a large diversity of evergreen and deciduous trees, which do not show a clear differentiation in terms of active rooting depth. Irrespective of the depth of the root system, expansion of new foliage in deciduous species occurs at the end of the dry season. In this study, we examined a suite of leaf traits related to C assimilation, water and nutrients (N, P) in five deciduous and six evergreen trees that were among the dominant families of cerrado vegetation. Maximum CO₂ assimilation on a mass basis (A_mass) was significantly correlated with leaf N and P, and specific leaf area (SLA; leaf area per unit of leaf mass). The highest leaf concentrations of both nutrients were measured in the newly mature leaves of deciduous species at the end of the dry period. The differences in terms of leaf N and P between evergreen and deciduous species decreased during the wet season. Deciduous species also invested less in the production of non-photosynthetic leaf tissues and produced leaves with higher SLA and maintained higher water use efficiency. Thus, deciduous species compensated for their shorter leaf payback period by maintaining higher potential payback capacity (higher values of A_mass) and lower leaf construction costs (higher SLA). Their short leafless period and the capacity to flush by the end of the dry season may also contribute to offset the longer payback period of evergreen species, although it may involve the higher cost of maintaining a deep-root system or a tight control of plant water balance in the shallow-rooted ones.     

Comparisons in water relations of plants between newly formed riparian and non-riparian habitats along the bank of Three Gorges Reservoir,China

The water table in the Three Gorges Reservoir (TGR) rose significantly since the construction of the Three Gorges Dam across the Yangtze River. Little is known about how such a change in local hydrological condition will affect the ecophysiology of plants along the bank of the world’s largest reservoir. In this study, water relations of the dominant plants were investigated over an entire year period by comparing stable isotope compositions of xylem water, leaf water potentials and foliar carbon isotope ratios at a newly-formed riparian site near the river bank and two non-riparian sites at higher elevations. The isotopic compositions of xylem water indicated that the plants in the newly-formed riparian zone acquired water mainly from the soil previously infiltrated by local rain rather than the Yangtze River water. Predawn and midday water potentials of the riparian trees were also similar to those at the non-riparian sites. Leaf δ¹³C values of plants across the three sites also showed no difference, even though there was a significant difference among species. Our results indicate that plants at the newly formed transient riparian area along the bank of TGR do not depend on the Yangtze River water for their water sources and that rising water levels in this reservoir may not lead to immediate changes in water relations of the plants along the bank. Communicated by K. Winter.     

Variations in life-form composition and foliar carbon isotope discrimination among eight plant communities under different soil moisture conditions in the Xilin River Basin,Inner Mongolia,China

Water is one of the key limiting factors for the survival and growth of plant species in arid and semi-arid steppe regions. Different plant functional groups (PFGs) based on life-forms differ in their strategies to cope with limited water availability. The foliar carbon isotope discrimination () value provides an integrated measurement of internal plant physiological and external environmental properties affecting photosynthetic gas exchange over the time interval when the carbon was fixed. In this study, we surveyed the composition and values of various life-forms (shrubs, sub-shrubs, perennial grasses, perennial forbs and annuals) in eight different plant communities along a soil moisture gradient in the Xilin River Basin, Inner Mongolia, China. Our results showed that: (1) life-forms occurred variously in eight steppe communities with different soil moisture status; (2) in wetter habitats, forbs were more abundant and accounted for the majority of aboveground biomass, whereas grasses became more important in drier habitats. Shrubs and sub-shrubs increased with decreasing soil water availability and their relative biomass rapidly increased in degraded steppe and sand dune communities. (3) The numerical order of the mean values of life-forms is as follows: perennial grasses (15.86) < shrubs (16.10) < perennial forbs (16.45)=annuals (16.41) < sub-shrubs (17.55), reflecting their differences in water use efficiencies. The significant differences in the values among these life-forms suggested that life-form-based PFGs not only represent a morphological classification of these plants, but could also represent a functional group integrating different physiological processes such as water use strategies, which may partially explain the differences in PFG composition and competitive ability of co-existing species along environmental gradients in the Xilin River Basin.     

A preliminary study of the responsiveness to seasonal atmospheric and rainfall patterns of wash woodland species in the arid Richtersveld

Seasonal carbon and water relations were compared among seven tree or shrub wash woodland species in the winter rainfall desert of the Richtersveld National Park, South Africa. Plants were generally aseasonal with respect to gas exchange, but responsive to rainfall events with respect to water relations and phenology. Relatively narrow annual ranges in potential evapotranspiration due to the maritime influence could explain why these plants respond more to fluctuations in water acquisition potential than to evaporative demand. Two species were summer-deciduous, but one of them (Ozoroa concolor) responded to aseasonal summer rainfall by leafing out and flowering. These two species had high shoot xylem water potentials when in leaf. All other species were sclerophyllous evergreens with low water potentials, particularly the shallow-rooted shrub Zygophyllum prismatocarpum, and Boscia albitrunca which may have a different rooting pattern to the other phreatophytes. The latter species was also unique due to its high leaf nitrogen contents, photosynthetic rates and stomatal conductances, despite very low leaf water potentials. Leaf stable carbon isotope composition C¹³C) varied between species (–22 to –27), but was lower than the mean for arid regions worldwide. The values indicated moderately high levels of water use efficiency, but a less conservative strategy in two species, including Boscia albitrunca. The affinities of these species to summer rainfall biomes, their apparent decline in the western arid regions in recent geological history following aridification, and their absence southwards in the winter rainfall regions, suggest that these wash species rely on sporadic summer rainfall events to some extent. They may be at risk if predicted increases in temperature and changes in rainfall patterns alter their effective moisture availability.     

Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quereus gambelii Nutt)

Winter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (D) and oxygen (¹⁸O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem D and ¹⁸O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. D values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.     

Modeling soil water movement with water uptake by roots

Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil-plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tracing water uptake by jarrah (Eucalyptus marginata) trees using natural abundances of deuterium

 Seasonal change in the δ²H content of water from twig sap, soil, rainfall and groundwater were measured to determine the water sources accessed by jarrah (Eucalyptus marginata) trees at three sites in Western Australia with differing soils and depths to water table. During winter and spring the main contributor to the water uptake of the trees was stored water in the surface layers of the soil replenished by predominantly winter rainfall. With the onset of summer drought jarrah became more reliant on water from deeper down the profile. There was no clear evidence that jarrah could tap water from groundwater more than 14 m deep in deep sands. Defining the source of water for trees in deep lateritic soils using stable isotopes is hampered by the uniform deuterium profiles down most of the unsaturated zone and into the groundwater. There was a limited response in the δ²H values of sapwater in twigs to changes in the δ²H of the upper layers of the deep sand following input of rainfall in autumn. The damped response was related to the small variation in the δ²H composition of rainfall in most events during the year and the mixing in the tree of water extracted from different locations in the soil profile. Received: 21 August 1995 / Accepted: 3 December 1995     

Oxygen isotope ratio stratification in a tropical moist forest

Summary Oxygen isotope ratios were determined in leaf cellulose from two plant species at Barro Colorado (Republic of Panama) in 4 different plots, two of which were undergoing an irrigation treatment during the dry season. There is a gradient in ¹⁸O values of leaf cellulose from the understory to canopy leaves, reflecting the differences in relative humidity between these two levels of the forest. This gradient is most pronounced in irrigated plots. For irrigated plots there was a highly significant correlation between ¹⁸O and ¹³C values, which was not observed in control plots. This relationship can be explained by humidity controlling stomatal conductance. Low humidity affects ¹⁸O values of leaf water during photosynthesis, which isotopically labels cellulose during its synthesis. Low humidity also decreases stomatal conductance, which affects discrimination against carbon-13 by photosynthetic reactions, thus affecting the ¹³C values of photosynthates. WUE values calculated by using plant carbon and oxygen isotope ratios were similar to those observed with gas exchange measurements in other tropical and temperate area. Thus the concurrent analysis of carbon and oxygen isotope ratios of leaf material can potentially be useful for long term estimation of assimilation and evapotranspiration regimes of plants.     

Effects of varying levels and frequencies of irrigation on growth,yield, nutrient uptake and water use efficiency of maize and cowpeas on a sandy loam ultisol

Summary A greenhouse study was carried out on an Nsukka sandy loam Ultisol having low soil moisture retention capacity to investigate the soil moisture regime and irrigation frequency required for optimum growth, yield, nutrient uptake and water use efficiency of maize (Zea mays L.) and cowpeas (Vigna unguiculata L. Walp). Four irrigation amounts (400 cm³, 300 cm³, 200 cm³ and 100 cm³ equivalent to 100, 75, 50 and 25% of field capacity, respectively) and four irrigation frequencies (daily, 2-day, 3-day and 4-day intervals) were tested in a factorial randomized design with three replications.Growth of maize was best when irrigation with water equivalent to 75% field capacity at daily interval but the optimum yields and nutrient uptake of both crops as well as cowpea nodulation were obtained when irrigating with water equivalent to 100% field capacity at daily or 2-day interval. The optimum water use efficiency was, however, achieved when irragating with amount equivalent to 100% field capacity at a 2-day interval. Irrigation with water equivalent to 50 or 25% field capacity at any interval resulted in various degrees of moisture stress which manifested in poor crop performance.     

Linking water uptake with rooting patterns in grassland species

Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004–2005) in which we measured the isotopic signature of plant xylem water from seven species (including C₃ forbs and shrubs and C₄ grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (δ¹⁸O) values were compared to soil water δ¹⁸O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C₃ species used proportionally more water from deeper portions of the soil profile relative to the C₄ grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth.     

Nitrogen uptake and use of two contrasting maize hybrids differing in leaf senescence

In eastern Canada, the use of fertilizer N has been identified as the most energy-consuming component of maize (Zea mays L.) grain production. As the economic and environmental costs of excessive N fertilization rise, there is an increased emphasis on selection of hybrids with greater N use efficiency (NUE; defined as the ratio of the amount of ¹⁵N recovered in grain or stover dry matter to the amount of fertilizer ¹⁵N applied to the soil in this study). Using an ¹⁵N-labelling approach, a field study was conducted on a tile-drained Brandon loam soil (Typic Endoaquoll) on the Central Experimental Farm at Ottawa, Canada (45°22 N, 75°43 W) in 1993 and 1994. Fertilizer N uptake and partitioning within the plant in relation to dry matter changes were monitored during development of a current stay-green maize hybrid and an older early-senescing hybrid grown with three fertilizer N levels (0, 100, 200 kg N ha^-1). Dry matter, N concentration and¹⁵ N atom% enrichment of plant components were determined at five growth stages. The current stay-green hybrid, Pioneer 3902 had greater NUE than the old early-senescing hybrid, Pride 5, which was associated with 24% more dry matter production and 20% more N uptake during grain fill for Pioneer 3902. There was no indication of greater allocation of N to the grain in Pioneer 3902. Our data suggest that prolonged maintenance of green leaf area for photosynthate production during grain fill and the ability to take up available soil N later in grain filling are characteristics of maize hybrids with greater NUE.     

13C isotopic discrimination: a starting point for new insights in competition for nitrogen and water under contour hedgerow systems in tropical mountainous regions

Competition for nutrients and water between crops and associated hedgerows reduces overall performance of contour hedgerow systems and hampers its acceptance by rural communities in tropical mountainous regions. Therefore, it is imperative to better understand competition leading to a decline in crop response close to hedges. In the highlands of North East Thailand spatial variability in grain yield of maize (Zea mays L., cv. Suwan 1) was assessed for two contour hedgerow systems based on Brachiaria ruziziensis Germain et Evrard (Ruzi grass) barriers or Leucaena leucocephala (Lam) de Wit hedges without or with fertilizer (60 kg N ha⁻¹ and 14 kg P ha⁻¹). Available was analyzed across the slope. In addition, shoot N concentration and δ¹³C values in leaves were measured for maize plants in the center of the alley and in the row next to and at the upper side of barriers or hedges. Despite variable field conditions, δ¹³C values were significantly (p < 0.05) less depleted close to the barriers or hedges, except for 2 out of 16 plots, suggesting that water deficiency was not the main driver for spatial variability along the alleys. The negative correlation between ¹³C isotopic discrimination and available in the soil, with R ² ranging from 0.5 (p < 0.10) to 0.9 (p < 0.01), assigned a major role to N availability in the reduced crop response towards the barriers. The proposed framework of ¹³C isotopic discrimination, together with plant and soil N data, is a new approach and was shown to be suitable to determine N and water competition between hedgerows and crops grown in alleys under field conditions.     

碳稳定同位素技术在植物水分胁迫研究中的应用

植物体的碳稳定同位素组成主要由植物本身的生物学特性决定 ,但环境胁迫对其影响也十分明显。综述了碳稳定同位素技术在研究植物水分利用效率、生物量高低及判断历史气候依据等研究领域的进展 ,阐明了植物体的 δ1 3C值对干旱、盐分及其它环境因素的变化所引起的水分胁迫的响应 ,并对碳稳定同位素对水分胁迫的响应机理进行了归纳和推断     

Photosynthetic gas exchange under emersed conditions in eulittoral and normally submersed members of the Fucales and the Laminariales: interpretation in relation to C isotope ratio and N and water use efficiency

Summary Ten species of brown macroalgae (five eulittoral and one submersed species of the Fucales; four submersed species of the Laminariales) from a rocky shore at Arbroath, Scotland, were examined for characteristics of emersed photosynthesis in relation to the partial pressure of CO₂ and O₂. The five eulittoral species of the Fucaceae were approaching CO₂ saturation for light-saturated photosynthesis at normal air levels of CO₂ (35 Pa) in 21 kPa O₂. The normally submersed algae are further from CO₂ saturation under these conditions, especially in the case of the four members of the Laminariales. The rate of net photosynthesis in the Fucaceae is O₂-independent in the range 2–21 kPa O₂ over the entire range of CO₂ partial pressure tested (compensation up to 95 Pa). For the other five algae tested, net photosynthesis is slightly inhibited by O₂ at 21 kPa relative to 2 kPa over the entire range of CO₂ partial pressures tested (compensation up to 95 Pa). CO₂ compensation partial pressures are low (<0.5 Pa) for the Fucaceae and independent of O₂ in the range 2–42 kPa. For the other five algae, the CO₂ compensation partial pressure are higher, and increased with O₂ partial pressure in the range 2–42 kPa. These gas exchange data show that the Fucaceae exhibit more C₄-like characteristics of their photosynthetic physiology than do the other five species tested, although even the Laminariales and Halidrys siliquosa are not classic C₃ plants in their photosynthetic physiology. These data suggest that, in emersed conditions as well as in the previously reported work on submersed photosynthesis, a CO₂ concentrating mechanism is operating which, by energized transmembrane transport of inorganic C, accumulates CO₂ at the site of RUBISCO and, at least in part, suppresses the oxygenase activity. Work with added extracellular carbonic anhydrase (CA), and with a relatively membrane-impermeant inhibitor of the native extracellular CA activity (acetazolamide), suggests that, in emersed conditions as well as in the previously reported work on algae submersed in seawater at pH 8, HCO _inf3 ^sup– is the major inorganic C species entering the cell. At optimal hydration, the rate of emersed photosynthesis in air is not less than the rate of photosynthesis when submersed in seawater, at least for the Fucaceae. ¹³C ratios of organic C for the Fucaceae are slightly more negative than is the case for the other five algae; these data are consitent with substantial (half or more of the entering inorganic C) leakage of CO₂ from the accumulated pool, and with some contribution of atmospheric CO₂ to the organic C gain by the eulittoral algae. The predicted increase in N use efficiency of photosynthesis in the Fucaceae, with their more strongly developed CO₂ concentrating mechanism, is consistent with data on emersed, but not submersed, photosynthesis for the algae collected from the wild and thus at a poorly defined N status. The more C₄-like gas exchange charateristics of photosynthesis in the eulittoral Fucaceae may be important in increasing the water use efficiency of emersed photosynthesis from the limited capital of water available for transpiration by a haptophyte.     

Internal efficiency, nutrient uptake, and the relation to field water resources in rainfed lowland rice of northeast Thailand

Rice-based (Oryza sativa L.) rainfed lowlands are the major cropping system in northeast Thailand. Average yields are low, which is generally explained by frequent drought events, low soil fertility, and poor fertilizer response. However, neither the relative importance of these factors nor their interaction is well understood. Therefore, we analyzed an existing database on fertilizer trials conducted between 1995 and 1997 at eight different sites in northeast Thailand with the objective to determine indigenous nutrient supplies, internal efficiencies, and recovery efficiencies of applied nutrients in rainfed lowland rice. Of particular interest was the effect of variety type (traditional) and water supply on these components. Comparison of N, P, and K concentrations in grain and straw (average N–P–K grain concentration of 11.0–2.7–3.4 g kg⁻¹; average N–P–K straw concentration of 5.2–0.9–16.4 g kg⁻¹) in the traditional-type varieties used at all trial sites with literature values showed no differences for these parameters between traditional and modern-type varieties or between irrigated and rainfed environments. In contrast, internal efficiencies of N, P, and K (average IEN: 46 kg grain per kg N uptake; IEP: 218 kg grain per kg P uptake; IEK: 25 kg grain per kg K uptake) were much lower than reported for irrigated systems, and the difference was greatest for K, which is mainly accumulated in the straw. Indigenous nutrient supply (average INS: 38 kg ha⁻¹; IPS: 10 kg ha⁻¹; IKS: 89 kg ha⁻¹) and recovery efficiency (average REN: 0.28 kg kg⁻¹; REP: 0.13 kg kg⁻¹; REK: 0.49 kg kg⁻¹) were low but comparable to the lower values reported from irrigated systems. Average seasonal field water resources seemed to reduce the indigenous nutrient supply but had no or little effect on internal efficiency and recovery efficiency. We concluded that the main reason for the low system productivity without and with fertilizer in northeast Thailand is the dominant use of traditional-type varieties with low harvest indices, which was the dominant cause for the observed low internal nutrient efficiency. Therefore, intensification of rainfed systems through substantially increased nutrient inputs can be recommended only where varieties with an average harvest index of close to 0.4 or higher are available.     

Morphological and physiological variation in western redcedar (<Emphasis Type="Italic">Thuja plicata</Emphasis>) populations under contrasting soil water conditions

Adaptation to precipitation conditions may induce genetic diversity that changes morphological and physiological traits. This hypothesis was investigated in the seedlings of seven western redcedar (Thuja plicata Donn ex D. Don) populations, which were collected along a precipitation transect from the Pacific coast to the southern interior of British Columbia, Canada. The experimental seedlings were either well-watered or soil-droughted and measured for growth, gas exchange rates, transpiration efficiency, and carbon isotope discrimination during or at the end of the third growing season. Significant variation was found in most of these morphological and physiological traits among the populations. Much of this variation occurred under well-watered, but not so much under droughted conditions. Mean height increments and transpiration efficiency showed a significant linear relationship, but biomass increments exhibited a quadratic relationship with precipitation on the origin site of these populations. Measurements of water use efficiency obtained from instantaneous gas exchange measurements, carbon isotope discrimination, and transpiration efficiency were intercorrelated in the seedlings. However, neither did any of these measurements consistently rank the populations, nor were they indicative of adaptation to climatic precipitation conditions in these western redcedar populations.