Tree Water Use Efficiency Research Articles

Water Use Efficiency of Trees Planted on Abandoned Farmland and Influencing Factors in a Loess Hilly Area

Water Use Efficiency of Trees Planted on Abandoned Farmland and Influencing Factors in a Loess Hilly Area

Typical Plantation Water Use Strategies Are Determined by Environmental Conditions and Plant Eco-Physiology in Beijing, China

The forest ecosystem of Beijing is an important barrier that preserves the ecological environment in the capital city of China. Therefore, the study of plant water utilization techniques in Beijing holds considerable importance in establishing a theoretical framework for the rehabilitation, administration, and preservation of forest ecosystem structures and functions. Here, the samples of precipitation, xylem water, and soil water were collected during the months of August and December 2021 from both mountainous and plains areas of Beijing. We measured the hydrogen and oxygen stable isotope values (δ2H and δ18O, respectively) and demonstrated the water use strategies of two typical tree species (Pinus tabuliformis Carr. and Acer truncatum Bunge) using the MixSIAR model. Divergent water use strategies were found in the mountainous and plains areas of Beijing. In the mountainous area, the two tree species exhibited seasonal differences in water use strategies. The xylem water of P. tabuliformis was mainly derived from the surface soil water (0~20 cm). In contrast, the xylem water of A. truncatum mainly originated from the surface soil water during the growing season, and it mainly originated from the deep soil water (60~100 cm) during the nongrowing season. However, in the plains area, the water sources of P. tabuliformis and A. truncatum did not show seasonal differences and originated mainly from the deep soil water. The findings of our study emphasize the notable disparity in water utilization strategies among tree species in the mountainous and plains areas. Consequently, it is imperative to formulate sustainable forestry management approaches that align with the water use efficiency of trees in various locations of Beijing.

High rainfall afforded resilience to tropical rainforests during Early Eocene Climatic Optimum

The rainforests near the early Eocene palaeo-equator were more resilient to greenhouse warming than mid-latitude vegetation. The mechanism underlying that resilience remains poorly known due to the lack of reliable terrestrial climate data from the palaeo-equator. In this paper, we quantify terrestrial temperature data using a plant proxy approach and infer that early Eocene climate near the palaeo-equator (∼2.6° N) was warmer than in mid- to high palaeolatitudes. The data also suggest that high levels of rainfall near the palaeo-equator might have afforded greater resilience to tropical rainforests by increasing the water use efficiency of trees during the warm greenhouse world of the Early Eocene Climatic Optimum.

Functional Groups Mask Inter- and Intraspecific Variation in Water Use Strategies in a Seasonally Dry Tropical Forest

Across the karst landscape of Quintana Roo, Mexico, plant access to nutrients and water appears limited by generally shallow soil. However, underlying this surface are heterogenous pockets in bedrock and deeper, stable groundwater, suggesting the potential for specialization by species in accessing soil resources. If species differentially access rock resources, divisions by functional groups may also be expected. In this study, shallow caves provided an opportunity to assess resource use strategies by direct, species-specific root observations coupled with traditional above ground measurements. Utilizing stable isotopes from stems and leaves (δ18O and δ13C), we investigated water access and water use efficiency of trees during the dry season to uncover relationships between rooting habit, tree size, and pre-determined functional groups based on leaf habit and wood density. Functional group membership did not predict measured stable isotope ratios, indicating that functional groups were poor predictors of resource use. We did find evidence for deep water use by select species and larger individuals. Interestingly, as trees became larger, δ13C increased to a threshold but then declined, suggesting increasing vulnerability to water limitation as trees increase in size, consistent with other seasonally dry tropical forests. Our work demonstrates that, although shallow soils likely drive strong resource limitations, co-occurring trees in karst ecosystems employ diverse resource acquisition strategies, suggesting important consequences for community composition and ecosystem function in the face of environmental change.

Water use efficiency of poplars grown for biomass production in the Midwestern United States

AbstractWater availability and other site conditions influence poplar biomass productivity and affect clonal performance due to genotype × environment interactions. It is important to select genotypes with high water use efficiency (WUE) that maximizes yield with available amounts of water at sites, while avoiding drought stress and growth impacts to the trees. During drought, stomatal closure induces increased accumulation of δ13C carbon isotope in tree tissues, which is strongly correlated with WUE of trees and usually expressed through carbon isotope discrimination (Δ). Our primary objectives were to evaluate differences in WUE among poplar genotypes grown in the Midwestern United States, and to identify genotypes with high WUE for future deployment on water‐limited sites in the region. Sites included 10‐year‐old biomass plantations in Escanaba, Michigan; Waseca, Minnesota; and Ames, Iowa established from 2000 to 2001 with seven poplar genotypes’ three genomic groups. Following harvest, height, diameter, and biomass were determined. Wood samples were collected from individual growth rings to assess annual ring width and WUE through δ13C and Δ. Aboveground dry biomass varied among sites (p = .0007), clones (p < .0001), and their interactions (p = .0134), ranging from 3.1 to 14.0 Mg ha−1 year−1. Δ varied among sites (Δ = 18.9–19.7‰; p < .0001) and clones (Δ = 18.6–19.9‰; p < .0001), indicating effects of site conditions on WUE of tested genotypes. Clones varied in their water‐conserving strategies. Some clones were characterized as water consumers with high growth and high WUE (“C916000”; “C916400”), while other genotypes were water conservers using lower amounts of water with moderate biomass production (“NC13624”; “NC13649”; “NM2” to a certain extent). Although δ13C carbon isotope accumulation correlates with WUE, Δ should only be used for selection when it is integrated with other parameters such as productivity, soils information, and climate data.

Combining carbon and oxygen isotopic signatures to identify ozone-induced declines in tree water-use efficiency.

Ground-level ozone (O3) pollution affects the plant carbon and water balance, but the relative contributions of impaired photosynthesis and the loss of stomatal functioning to the O3-induced reductions in water-use efficiency (WUE) remain unclear. We combined the leaf stable dual isotopic signatures of carbon (δ13C) and oxygen (δ18O) with related instantaneous gas exchange performance to determine the effects of O3 dose on the net photosynthetic rate (An), stomatal conductance (gs) and intrinsic WUE (iWUE=An/gs) in four tree species (one being a hybrid) exposed to five O3 levels. The iWUE declined for each step increase in O3 level, reflecting progressive loss of the coupling between leaf carbon gain and water loss. In ambient compared with charcoal-filtered air, the decreased iWUE was associated with reductions in both An and gs (i.e., decreased δ13C and increased δ18O). In elevated O3 treatments, however, the iWUE declines were caused by reduced An at constant or increased gs. The results show that the dual isotope approach provides a robust way to gather time-integrated information on how O3 pollution affects leaf gas exchange. Our study highlights that O3-induced decoupling between photosynthesis and stomatal regulation causes large and progressive declines in the WUE of forest trees, demonstrating the need for incorporating this hitherto unaccounted for effect into vegetation models.

Genetic and environmental influence on foliar carbon isotope composition, nitrogen availability and fruit yield of 5-year-old mango plantation in tropical Australia

The aim of this study was to quantify the effect of different varieties, planting densities, tree training systems and canopy aspect (north and south) on tree water use efficiency and nitrogen (N) availability in relation to mango fruit yield and fruit size as well as soil fertility (particularly total carbon (C) and total N as well as C and N isotope compositions) in a 5-year-old mango plantation of tropical Australia. Mango foliar samples were collected from a 5-year-old, factorial field experiment, where we tested the effects of two mango varieties (Calypso vs Keitt); two planting densities (medium vs high); two training systems (single leader vs conventional); and two sampling canopy aspects (north vs south) on foliar total C (%), total N (%), and stable C and N isotope compositions (δ13C and δ15N). The 0–10 cm of surface soils were also assessed for total C, total N, and δ13C and δ15N of different varieties, planting densities, and training systems. In addition, mango fruit yield, and fruit size were measured. There were significant genetic effects on foliar total N, tree water use efficiency (WUE) as reflected by foliar δ13C, N availability as indicated by foliar total N and δ15N, mango fruit yield, and fruit size in 5-year-old mango trees in tropical Australia. Overall, Keitt had higher foliar δ13C and N availability (higher total N and lower δ15N) as well as higher mango yield and greater fruit size, compared with those of Calypso. There were also significant environmental influences on foliar δ13C and N availability. In particular, high planting density had higher N availability (higher total N), and lower foliar δ13C as well as higher N loss (higher δ15N), compared with those of medium planting density. High planting density treatment also had higher soil total N, compared with that of medium planting density treatment. The conventional training system had higher N availability and foliar δ13C, compared with those of the single leader training system. The northern side of tree canopy (sunny side) had lower fruit number, compared with that of the southern side (shady side) of tree canopy. There were significant genetic and environmental influences on tree WUE and N availability as well as mango fruit yield and fruit size in the 5-year-old mango trees. Among them, Keitt’s fruit was significantly larger than Calypso and had higher foliar δ13C (higher WUE). Although high planting density increased soil total N, it also resulted in lower foliar δ13C and more N loss. Our results highlighted the significant potential for improvement of foliar δ13C and N availability as well as fruit yield and soil fertility with both genetic selection and site management regimes.

Effects of climate change and local environmental factors on long-term tree water-use efficiency and growth of Pseudolarix amabilis and Cryptomeria japonica in subtropical China

It has been well reported in many studies globally that the increased atmospheric carbon dioxide (CO2) concentration has positively influenced plant growth in the past century. However, it is crucial to understand how long the tree growth trend will remain alongside the rising CO2 concentration before it is tipped over. In this study, we focused on studying the long-term tree growth responses to the rising CO2 concentration, in combination with other environmental factors, and its feedback to the changing climate. This study has reported two tree species, Pseudolarix amabilis (P. amabilis) and Cryptomeria japonica (C. japonica), sampled from a subtropical monsoon forest located in eastern China, and established the long-term tree ring chronologies by applying tree ring width measurement and cross dating, and then, basal area increment (BAI) was calculated. Meanwhile, intrinsic water-use efficiency (iWUE) was calculated through the measurement of carbon isotope composition (δ13C) in tree ring samples, and its relationships with BAI and atmospheric CO2 concentration were also quantified. BAI values continuously increased in both P. amabilis and C. japonica with the rising of CO2 concentration until the atmospheric CO2 concentration tipping points were reached, after which tree growth started to decline with the rising CO2, while iWUE exerted a continuous increase trend with the increasing CO2 concentration. Tree growth of both species was more photosynthesis driven before reaching the maximum BAI points, once the tipping points of atmospheric CO2 were passed, leading to the positive feed of forest carbon cycling to climate change and global warming. Therefore, tree growth was more water limitation driven in the last 20 years.

Urban channel incision and stream flow subsidies have contrasting effects on the water status of riparian trees

The water relations of riparian trees are often closely tied to stream hydrology and channel morphology, which can be altered by urban development. In regions with limited precipitation, urban water use can generate or augment dry-season stream flows, potentially providing a water subsidy to riparian trees. However, urbanization is also associated with high storm flows that cause stream channel incision, or downcutting, which could limit the effect of flow subsidies by lowering riparian water tables. We investigated the effects of urban dry-season flow subsidies and stream channel incision on the water status of streamside trees in Sacramento, California, which has a Mediterranean climate with a distinct and lengthy dry season. For two common facultative riparian species, Quercus lobata (valley oak) and Fraxinus latifolia (Oregon ash), we analyzed both midday stem water potential (Ψstem) measurements and leaf carbon isotope signatures (∂13C) to determine whether channel incision and dry-season flow had interacting effects on seasonal midday water stress and longer-term water use efficiency. We found that stream flow could substantially reduce dry-season water stress in both tree species, but only at low levels of channel incision. Leaf ∂13C signatures for Q. lobata suggested increased water use efficiency in trees growing along incised streams and those lacking dry-season flow. Urban management decisions that affect both dry-season base flows and channel incision can thus influence the growth and health of riparian trees, potentially leading to broader changes in riparian ecosystems.

Inverse effects of recent warming on trees growing at the low and high altitudes of the Dabie Mountains, subtropical China

Understanding how tree rings of different species at different elevations respond to climate, and whether their relationship is stable over time is crucial not only for credible palaeoclimatic reconstructions, but also for better awareness of forest growth dynamics and therefore making scientific management decisions against a background of global warming. In this work, six tree-ring chronologies of Pinus taiwanensis at three sites from 800 to 1550 m above sea level (asl), and Pinus massoniana at three sites from 500 to 650 m asl were developed in the Dabie Mountains (DBS), subtropical China. In recent decades, only P. taiwanensis at 1550 m asl showed a positive growth-trend. Both P. taiwanensis at 800 m asl and three low-altitude P. massoniana at 500–650 m asl showed negative growth trends. Climate-growth relationship analyses revealed that 1) temperature was the dominant climatic factor that controlled tree-ring growth in DBS. It exerted most influence on trees growing at the lower and upper limit, than for trees growing at altitudes from 650 to 1450 m asl. Only precipitation in October positively influenced tree growth of P. massoniana at all three sites and P. taiwanensis at 800 m asl; 2) There was a shift in temperature’s impact on trees, that was from significantly negative relationship at the four lower altitudes, mainly for the current growing-season, to significantly positive relationship in previous February–July period at the highest altitude. The higher the altitude, the more significant the lag effect of temperature on trees; 3) The influence of temperature on tree growth at most altitudes were variable over time. The positive influence of temperature on trees at 1550 and 1450 m asl was comparatively stable during the early period of instrumental records. It strengthened evidently since the early 1990s, which is coincidental with the timing of the evident temperature increase in DBS. On the contrary, the negative impacts of temperature on tree growth at 800, 650 and 500 m asl had weakened since the early 1990s. Besides the increase of water use efficiency of trees, we speculated that the increasing influence of precipitation in May and July weakened the relationship between temperature and tree rings at low altitudes. This work points out that the upper and lower limits of forest in DBS offer the preferred locations for future sampling in climate reconstruction, but the stability of tree growth and climate over time should be considered. Moreover, forest management should give priority to altitude factors, in addition to tree species representation.

Tree water-use efficiency and growth dynamics in response to climatic and environmental changes in a temperate forest in Beijing, China

Understanding the responses of local forests to the gradually rising atmospheric CO2 concentrations (ca) and changing environment is critical for appropriate management activities. This work used tree ring width measures (i.e. basal areal increment, BAI) and carbon (C) and nitrogen (N) signals to explore the intrinsic water use efficiency (iWUE) and tree growth dynamics of three major tree species (Pinus massoniana, P.tabuliformis and Larix gmelinii) in the Miyun Reservae Basin (MRB) of Beijing. The results indicate that ca was a primary contributor to tree growths, especially at the remote site where rising ca accounted for 92% and 74% of BAI changes for P. tabuliformis and L. gmelinii, respectively. N deposition was found to have a positive effect on BAI at this site. The controlling effect of ca on tree growth at the close-to-city site was smaller (52% and 44% of the contributions for P. tabuliformis and P. massoniana, respectively), while the negative influences of N deposition on BAI tends to be intensified. iWUE showed consistent increase during the entire growth period at all sites. Quadratic relationships between iWUE and BAI were observed, which indicated that the rising ca stimulated photosynthesis, contributing to the initial BAI and iWUE increase. However, the intensified water stress resulting from reduced precipitation and increased temperature led to a reduction in tree stomatal conductance causing the subsequent increase in iWUE but decrease in BAI. Of the site- and species-related responses of tree growth to ca, climatic and environmental changes in the MRB, the site-related variation dominated. The non-linear relationship between BAI and ca combined with the quadratic relationship between BAI and iWUE indicate a decreased ability of forests to capture atmospheric CO2 once the CO2 tipping point has passed.

Effect of Mulching with Compost on Growth and Physiology of Ulmus ‘FL634’ Planted in an Urban Park

The effects of mixed compost as mulching material on growth and physiology of newly planted elm trees were evaluated over a three-year period after planting in an urban park. Trees mulched with compost generally had greater height (+10% and +19% for 5 cm layer and 10 cm layer treatments, respectively, if compared to control), trunk diameter (+13% and +29%) and current-year shoot growth (+46% and +56%). Limited effects were found with regard to carbon assimilation when considered on a per unit-leaf-area basis (-0.1% and +0.3%), but whole tree carbon assimilation increased in mulched trees (+7% and +59% for 5 cm and 10 cm treatments, respectively, if compared to control) because of the larger total tree leaf area of mulched plants. Mulching also increased chlorophyll content (+4% and +7% for 5 cm and 10 cm treatments, respectively). The results obtained in this study show how mulching with compost increased growth, carbon storage, and improved water use efficiency of trees planted in an urban environment characterized by hot dry summers.

Potassium fertilization increases water-use efficiency for stem biomass production without affecting intrinsic water-use efficiency in Eucalyptus grandis plantations

Adaptive strategies to improve tree water-use efficiency (WUE) are required to meet the global demand for wood in a future drier climate. A large-scale throughfall exclusion experiment was set up in Brazil to study the interaction between water status and potassium (K) or sodium (Na) availability on the ecophysiology of Eucalyptus grandis trees. This experiment focused primarily on the changes in aboveground net primary production, stand water use, phloem sap and leaf δ13C, net CO2 assimilation and stomatal conductance. The correlations between these response variables were determined to gain insight into the factors controlling water-use efficiency in tropical eucalypt plantations. The intrinsic WUE in individual leaves (the ratio of net CO2 assimilation to stomatal conductance) was estimated at a very short time scale from the leaf gas exchange. Sap flow measurements were carried out to assess the WUE for stemwood production (the ratio of wood biomass increment to stand water use).Averaged over the two water supply regimes, the stemwood biomass 3years after planting was 173% higher in trees fertilized with K and 79% higher in trees fertilized with Na than in trees with no K and Na addition. Excluding 37% of the throughfall reduced stemwood production only for trees fertilized with K. Total canopy transpiration between 1 and 3years after planting increased from about 750 to 1300mmy−1 in response to K fertilization with a low influence of the water supply regime. K fertilization increased WUE for stemwood production by approx. 60% with or without throughfall exclusion. There was a strong positive correlation between phloem sap δ13C and short-term leaf-level intrinsic WUE. Whatever the water and nutrient supply regime, the gas exchange WUE estimates were not correlated with WUE for stemwood production. Phloem sap δ13C and leaf δ13C were therefore not valuable proxies of WUE for stemwood production. The allocation pattern in response to nutrient and water supply appeared to be a major driver of WUE for stemwood production. In areas with very deep tropical soils and annual rainfall <1500mm, our results suggest that breeding programs selecting the eucalypt clones with the highest growth rates tend to select the genotypes with the highest water-use efficiency for wood production.

Effects of soil water and nitrogen availability on photosynthesis and water use efficiency of Robinia pseudoacacia seedlings

The efficient use of water and nitrogen (N) to promote growth and increase yield of fruit trees and crops is well studied. However, little is known about their effects on woody plants growing in arid and semiarid areas with limited water and N availability. To examine the effects of water and N supply on early growth and water use efficiency (WUE) of trees on dry soils, one-year-old seedlings of Robinia pseudoacacia were exposed to three soil water contents (non-limiting, medium drought, and severe drought) as well as to low and high N levels, for four months. Photosynthetic parameters, leaf instantaneous WUE (WUEi) and whole tree WUE (WUEb) were determined. Results showed that, independent of N levels, increasing soil water content enhanced the tree transpiration rate (Tr), stomatal conductance (Gs), intercellular CO2 concentration (Ci), maximum net assimilation rate (Amax), apparent quantum yield (AQY), the range of photosynthetically active radiation (PAR) due to both reduced light compensation point and enhanced light saturation point, and dark respiration rate (Rd), resulting in a higher net photosynthetic rate (Pn) and a significantly increased whole tree biomass. Consequently, WUEi and WUEb were reduced at low N, whereas WUEi was enhanced at high N levels. Irrespective of soil water availability, N supply enhanced Pn in association with an increase of Gs and Ci and a decrease of the stomatal limitation value (Ls), while Tr remained unchanged. Biomass and WUEi increased under non-limiting water conditions and medium drought, as well as WUEb under all water conditions; but under severe drought, WUEi and biomass were not affected by N application. In conclusion, increasing soil water availability improves photosynthetic capacity and biomass accumulation under low and high N levels, but its effects on WUE vary with soil N levels. N supply increased Pn and WUE, but under severe drought, N supply did not enhance WUEi and biomass.

Water savings, nutrient leaching, and fruit yield in a young avocado orchard as affected by irrigation and nutrient management

This project was designed to determine the effect of fertilizer rate and irrigation scheduling on water use, nutrient leaching, and fruit yield of young avocado trees (Persea americana Mill. cv. Simmonds). Seven nutrient and irrigation management practices were evaluated: (1) irrigation based on crop evapotranspiration (ET) with 50% fertilizer at a standard rate (FSR); (2) ET irrigation with FSR (typical for avocado production in the area); (3) ET irrigation with 200% FSR; (4) irrigation based on exceedance of 15-kPa (SW) soil water suction with 50% FSR; (5) SW with FSR; (6) SW with 200% FSR; and (7) irrigation at a set schedule (based on timing and frequency typically used in local avocado production) with FSR. The SW with FSR treatment saved 87% of the water volume applied and reduced total phosphorus leached by 74% compared to the set schedule irrigation with FSR. The SW with FSR treatment had higher avocado fruit production, tree water-use efficiency, and fertilizer-use efficiency than the other six treatments. Thus, the use of soil water monitoring for irrigation management can substantially increase sustainability of young avocado orchards in southern Florida.

Effects of Water Supply on the Growth Rate and Water Use Efficiency of Trees in Arid Land Afforestation in Western Australia

An afforestation field experiment studying Eucalyptus camaldulensis, E. torquata, and Casuarina obesa with five different irrigation intervals was set up in an arid region in Western Australia. The irrigation intervals used were 2 weeks, 1 month, 3 months, 6 months, and no irrigation, which resulted in water supplies of 1243, 895, 511, 423, and 313 mm/y averaged over the experimental period, respectively. The average growth rates of C. obesa in sections with 2-week and 1-month irrigation intervals were 1.9 and 1.3 kg/(m2·y), respectively, about double those of E. camaldulensis. However, the average growth rate of C. obesa was lower than E. camaldulensis in sections with less irrigation. For C. obesa, water use efficiency was highest with a 2-week irrigation interval and quickly decreased with decreasing water supply. For E. camaldulensis, it was highest with a 1-month irrigation interval and did not decrease as much as that of C. obesa with decreasing water supply. We concluded that C. obesa has a high water use efficiency with a water supply more than 890 mm/y. E. camaldulensis, however, can grow more efficiently than C. obesa when the water supply is less than 510 mm/y. E. torquata is unsuitable for arid land afforestation.

Modeling whole‐tree carbon assimilation rate using observed transpiration rates and needle sugar carbon isotope ratios

*Understanding controls over plant-atmosphere CO(2) exchange is important for quantifying carbon budgets across a range of spatial and temporal scales. In this study, we used a simple approach to estimate whole-tree CO(2) assimilation rate (A(Tree)) in a subalpine forest ecosystem. *We analysed the carbon isotope ratio (delta(13)C) of extracted needle sugars and combined it with the daytime leaf-to-air vapor pressure deficit to estimate tree water-use efficiency (WUE). The estimated WUE was then combined with observations of tree transpiration rate (E) using sap flow techniques to estimate A(Tree). Estimates of A(Tree) for the three dominant tree species in the forest were combined with species distribution and tree size to estimate and gross primary productivity (GPP) using an ecosystem process model. *A sensitivity analysis showed that estimates of A(Tree) were more sensitive to dynamics in E than delta(13)C. At the ecosystem scale, the abundance of lodgepole pine trees influenced seasonal dynamics in GPP considerably more than Engelmann spruce and subalpine fir because of its greater sensitivity of E to seasonal climate variation. *The results provide the framework for a nondestructive method for estimating whole-tree carbon assimilation rate and ecosystem GPP over daily-to weekly time scales.

Gas exchange and water use efficiency of trees and maize in agroforestry systems in semi-arid Kenya

Improved understanding of the impact of leafing phenology and soil water supplies on the gas exchange and water use efficiency ( WUE) of trees in agroforestry systems in the semi-arid tropics is essential to optimise tree/crop interactions and the use of scarce natural resources, particularly in view of recent initiatives to use carbon credits to reward subsistence farmers who plant and manage trees effectively. Understanding of the relationships between photosynthetic and transpiration rates and chlorophyll content may help to explain variation in growth rate, productivity and WUE between species and growth environments. Such information is lacking for the species examined here, the evergreen Grevillea robusta (A. Cunn), semi-deciduous Alnus acuminata (HBK) and deciduous Paulownia fortunei (Hemsl.). Although differences in leafing phenology influence the extent of competition or complementarity between trees and crops in agroforestry systems, and hence crop yield, farmers may deliberately plant fast-growing, competitive tree species if they provide attractive economic returns or carbon credit subsidies. The objectives were to characterise: (1) the extent of variation in foliar gas exchange and instantaneous water use efficiency ( WUE i ) between trees with contrasting leafing phenologies; (2) the influence of leaf age and available soil moisture on gas exchange and WUE i of trees; and (3) chlorophyll and foliar nitrogen (N) concentrations and gas exchange parameters for maize in agroforestry and sole crop systems. Foliar chlorophyll and N concentrations, assimilation ( A) and transpiration rate ( E) were much greater in P. fortunei than in A. acuminata or G. robusta. A, E and WUE i were greatest in young mature leaves of all three species but declined as they senesced. WUE i increased as soil moisture content declined and was similar in all species, suggesting their differing leafing phenologies were responsible for observed differences in long-term water use and growth. A and WUE i were greater in maize than in tree species, consistent with its C4 photosynthetic pathway. The implications for tree and crop growth, yield and acceptability to subsistence farmers in the semi-arid tropics are discussed.

Contrasting water-use strategies in two sympatric cool-temperate rainforest species, Nothofagus cunninghamii (Nothofagaceae) and Atherosperma moschatum (Atherospermataceae)

Nothofagus cunninghamii (Hook.) Oerst. and Atherosperma moschatum Labill. co-occur in cool-temperate rainforest across the wetter parts of Tasmania and Victoria, Australia, but A. moschatum extends to drier areas than N. cunninghamii. Possible reasons include differential tolerance of drought and fire or dispersal capacity. Here, we compare these species in their responses to water deficits. Differences in seedling survival, leaf tissue damage, shoot water relations, stomatal sensitivity, allocation of biomass and the long-term water-use efficiency of each species in response to water stress were investigated. N. cunninghamii showed traits typical of a high-water-use species, such as high stomatal conductance, a strategy that is not surprising in a rainforest species. However, it also displayed an exceptional ability to draw water from the soil and longer seedling roots, allowing replacement of water lost, at least in the short term. A. moschatum showed a more conservative water-use strategy, surviving greater internal dehydration with less damage, and displaying greater stomatal sensitivity to drought and long-term water-use efficiency in trees. The apparently superior long-term drought resistance of A. moschatum may in part explain its more common occurrence in drier regions than N. cunninghamii, at least in Tasmania, while the capacity of N. cunninghamii to survive short but severe periods of water stress correlates well with its higher position in the canopy and greater exposure to sunlight and desiccating winds. However, there is little evidence to suggest that the absence of N. cunninghamii from the rainforests of eastern Victoria is due to drought. We also suggest that the water-use strategy of N. cunninghamii may relate not just to surviving water deficits, but to maximising annual carbon gain in a temperate climate that is, on average, driest during the warmest time of the year.

Genetic variation in branchlet carbon and nitrogen isotope composition and nutrient concentration of 11-year-old hoop pine families in relation to tree growth in subtropical Australia

Genetic variation in tree growth and branchlet carbon isotope composition (δ 13C), nitrogen (N) isotope composition (δ 15N) and nutrient (N, P, K, Ca, Mg, Mn, Fe, Cu and Zn) concentration of 11-year-old hoop pine ( Araucaria cunninghamii Ait. ex D. Don) half-sib families were examined at two contrasting sites (31 families with 8 blocks sampled at a wet site and 27 families with 9 blocks at a dry site) in south-east Queensland, Australia. Genetic correlations were determined between the tree growth traits, between the physiological traits, and between the physiological and growth traits. The potential of using these tree physiological traits was evaluated for assisting in selection of elite hoop pine families with improved water-use efficiency (WUE) as reflected in branchlet δ 13C, nutrient (particularly N)-use efficiency (NUE) and tree growth. At the wet site there was significant genetic variation in tree growth, branchlet δ 13C, nutrient concentration and mineral concentration among the 11-year-old hoop pine families, with heritability estimates ranging from 0.26 to 0.72. At the dry site, there was also significant genetic variation in some of the corresponding variables with heritability estimates ranging from 0.26 to 0.54. There seemed to be significant family variation in branchlet δ 15N at the dry site ( P=0.068), with a moderate heritability estimate of 0.21, but not at the wet site. Some strong genetic correlations were also found between the tree growth traits, between the physiological traits, and between the tree growth and the physiological traits. Tree WUE and branchlet nutrient concentrations were significantly influenced by rainfall and soil fertility. Branchlet δ 15N at the wet site was significantly lower than that at the dry site, suggesting that soil microbial–plant interactions might play a role in discriminating soil available 15N in favor of 14N for tree uptake (lower δ 15N in plant tissues) at the wet site. Branchlet δ 13C δ 15N and nutrient (particularly N) concentration show promise as physiological and nutritional traits for assisting in the selection of elite hoop pine families with improved tree WUE, NUE and growth for the more water- and nutrient-limited environments in subtropical Australia.