Trade-off of leaf-scale resource-use efficiencies along the vertical canopy of the subtropical forest

Leaf resource-use efficiencies are key indicators of plant adaptability to climate change, as they depend on both photosynthetic carbon assimilation and available resources. However, accurately quantifying the response of the coupled carbon and water cycles is challenging due to the canopy vertical variability in resource-use efficiencies, which introduces greater uncertainty into the calculations. Here we experimented to ascertain the vertical variations of leaf resource-use efficiencies along three canopy gradients of coniferous (Pinus elliottii Engelmann.) and broad-leaved (Schima Superba Gardn & Champ.) forests over one year in the subtropical region of China. The efficiency of water (WUE), and nitrogen (NUE) showed higher values in the top canopy level for the two species. The maximum efficiency of light (LUE) occurred in the bottom canopy level for both species. The impact of photosynthetic photon flux density (PPFD), leaf temperature (T_leaf), and vapor pressure deficit (VPD) on leaf resource-use efficiencies varied with canopy gradients in slash pine and schima superba. We also observed a trade-off between NUE and LUE for slash pine and between NUE and WUE for schima superba. Moreover, the variation in the correlation between LUE and WUE indicated a change in resource-use strategies for slash pine. These results emphasize the significance of vertical variations in resource-use efficiencies to enhance the prediction of future carbon-water dynamics in the subtropical forest.

Recent global decline in rainfall interception loss due to altered rainfall regimes

Evaporative loss of interception (Ei) is the first process occurring during rainfall, yet its role in large-scale surface water balance has been largely underexplored. Here we show that Ei can be inferred from flux tower evapotranspiration measurements using physics-informed hybrid machine learning models built under wet versus dry conditions. Forced by satellite and reanalysis data, this framework provides an observationally constrained estimate of Ei, which is on average 84.1 ± 1.8 mm per year and accounts for 8.6 ± 0.2% of total rainfall globally during 2000-2020. Rainfall frequency regulates long-term average Ei changes, and rainfall intensity, rather than vegetation attributes, determines the fraction of Ei in gross precipitation (Ei/P). Rain events have become less frequent and more intense since 2000, driving a global decline in Ei (and Ei/P) by 4.9% (6.7%). This suggests that ongoing rainfall changes favor a partitioning towards more soil moisture and runoff, benefiting ecosystem functions but simultaneously increasing flood risks.

Embolism resistance in petioles and leaflets of palms

BACKGROUND AND AIMS

Hydraulic studies are currently biased towards conifers and dicotyledonous angiosperms; responses of arborescent monocots to increasing temperature and drought remain poorly known. This study aims to assess xylem resistance to drought-induced embolism in palms.

METHODS

We quantified embolism resistance via P50 (xylem pressure inducing 50 % embolism or loss of hydraulic conductivity) in petioles and leaflets of six palm species differing in habitat and phylogenetic relatedness using three techniques: in vivo X-ray-based microcomputed tomography, the in situ flow centrifuge technique and the optical vulnerability method.

KEY RESULTS

Our results show that P50 of petioles varies greatly in the palm family, from -2.2 ± 0.4 MPa in Dypsis baronii to -5.8 ± 0.3 MPa in Rhapis excelsa (mean ± s.e.). No difference or weak differences were found between petioles and leaf blades within species. Surprisingly, where differences occurred, leaflets were less vulnerable to embolism than petioles. Embolism resistance was not correlated with conduit size (r = 0.37, P = 0.11).

CONCLUSIONS

This study represents the first estimate of drought-induced xylem embolism in palms across biomes and provides the first step towards understanding hydraulic adaptations in long-lived arborescent monocots. It showed an almost 3-fold range of embolism resistance between palm species, as large as that reported in all angiosperms. We found little evidence for hydraulic segmentation between leaflets and petioles in palms, suggesting that when it happens, hydraulic segregation may lack a clear relationship with organ cost or replaceability.

Dynamics of photosynthetic responses in 10 rubber tree (Hevea brasiliensis) clones in Colombian Amazon: Implications for breeding strategies

The rubber tree [Hevea brasiliensis (Willd. Ex Adr. de Juss.) Muell.-Arg] is the main source of natural rubber in the world. However, in the Amazon region, its production is reduced by biotic and abiotic limitations, which have prompted breeding programs in order to identify desirable agronomic and physiological indicators. The objective of this study was to analyze the temporal dynamics of photosynthetic responses based on the parameters of leaf gas exchange and chlorophyll a fluorescence in 10 rubber tree clones during the immature phase (pre-tapping) in three large-scale clone trials, during daily cycles and under two climatic periods (dry and rainy) in the Caquetá region (Colombian Amazon). The variables A, LT, Φ_PSII, ETR and qP were significantly higher in the dry period, where the highest values of PAR, AT and VPD were seen. In San Vicente del Caguán and Florencia, the highest averages were estimated for A, E and g_s, as compared with Belén de los Andaquíes. In Florencia, the highest fluorescence parameters of chlorophyll a were recorded. At 9:00 h and 12:00 h, the highest means of A, E, Φ_PSII and ETR were observed. The majority of the clones displayed the highest F_v/F_m mean (0.82–0.84) in the dry period. The clones FX 4098, FDR 4575, MDF 180, GU198 and FDR 5788 represent genotypes with the best photosynthetic performance (greater photosynthetic rates and better ability of the photosynthetic apparatus to capture, use and dissipate light energy). These desirable genotypes constitute a promising gene pool for expanding the genetic resource of rubber trees in the Colombian Amazon.

A comparative structural and functional study of leaf traits and sap flow in Dracaena cinnabari and Dracaena draco seedlings

Water relations for two remote populations of Dracaena tree species from the dragon tree group, Dracaena cinnabari Balfour f. and Dracaena draco (L.) L., were studied to test our hypothesis that morphological and anatomical differences in leaf structure may lead to varied functional responses to changing environmental conditions. Sap flow measurements were performed using the heat field deformation method for four Dracaena seedlings grown in one glasshouse and two greenhouses, and leaf traits related to plant-water relationships were characterised. All traits studied confirmed that D. cinnabari leaves are more xeric in their morpho-anatomical structure compared with D. draco leaves. No radial sap flow variability was detected in D. draco plant stems, whereas sap flow was found to be higher in the inner part of D. cinnabari stems. The regular occurrence of reverse sap flow at night in both Dracaena species was consistent with a staining experiment. Vapour pressure deficit (VPD) was found to be the main driver for transpiration for both Dracaena species. However, the relationship between VPD and sap flow appeared to be different for each species, with a clockwise or no hysteresis loop for D. draco and a counter-clockwise hysteresis loop for D. cinnabari. This resulted in a shorter transpiration cycle in D. cinnabari. The observed superior water-saving strategy of D. cinnabari corresponds to its more xeric morpho-anatomical leaf structure compared with D. draco.

Oil palm water use: calibration of a sap flux method and a field measurement scheme

Oil palm (Elaeis guineensis Jacq.) water use was assessed by sap flux density measurements with the aim to establish the method and derive water-use characteristics. Thermal dissipation probes were inserted into leaf petioles of mature oil palms. In the laboratory, we tested our set-up against gravimetric measurements and derived new parameters for the original calibration equation that are specific to oil palm petioles. In the lowlands of Jambi, Indonesia, in a 12-year-old monoculture plantation, 56 leaves on 10 palms were equipped with one sensor per leaf. A 10-fold variation in individual leaf water use among leaves was observed, but we did not find significant correlations to the variables trunk height and diameter, leaf azimuthal orientation, leaf inclination or estimated horizontal leaf shading. We thus took an un-stratified approach to determine an appropriate sampling design to estimate stand transpiration (Es, mm day(-1)) rates of oil palm. We used the relative standard error of the mean (SEn, %) as a measure for the potential estimation error of Es associated with sample size. It was 14% for a sample size of 13 leaves to determine the average leaf water use and four palms to determine the average number of leaves per palm. Increasing these sample sizes only led to minor further decreases of the SEn of Es. The observed 90-day average of Es was 1.1 mm day(-1) (error margin ± 0.2 mm day(-1)), which seems relatively low, but does not contradict Penman-Monteith-derived estimates of evapotranspiration. Examining the environmental drivers of Es on an intra-daily scale indicates an early, pre-noon maximum of Es rates (11 am) due to a very sensitive reaction of Es to increasing vapor pressure deficit in the morning. This early peak is followed by a steady decline of Es rates for the rest of the day, despite further rising levels of vapor pressure deficit and radiation; this results in pronounced hysteresis, particularly between Es and vapor pressure deficit.

Direct in vivo evidence of immense stem water exploitation in irrigated date palms

During the summer, evaporative demand at midday often exceeds the transport capacity of most desert plants. However, date palms maintain their ecological dominance with sustained and uniquely high rates of transpiration. This high rate of flow cannot be attributed to soil water supply alone. In order to quantify intra-plant water allocation in irrigated date palms, three water-sensing techniques have been incorporated: heat dissipation, gravimetric sampling, and time domain reflectrometry. Each of these methods has known limitations but their integration resulted in a quantitative in vivo accounting of the date palm diurnal and seasonal water mass balance. By incorporating these methods it was possible to determine that date palms substantially rely on the exploitation and recharge of the stem reservoir in their water budget. The stem of mature date palms can hold up to 1 m(3) of water and supply 25% of daily transpiration (i.e. 5000 l of water in 100 d of summer). The internal stem water reservoir is consistently recharged by over 50 l per night which allows for successive daytime reuse throughout the entire growing season. More broadly, these findings suggest that internal water allocation and night-time soil-water availability could provide useful information for improving date palm irrigation practices.

Response of transpiration to rain pulses for two tree species in a semiarid plantation

Responses of transpiration (Ec) to rain pulses are presented for two semiarid tree species in a stand of Pinus tabulaeformis and Robinia pseudoacacia. Our objectives are to investigate (1) the environmental control over the stand transpiration after rainfall by analyzing the effect of vapor pressure deficit (VPD), soil water condition, and rainfall on the post-rainfall Ec development and recovery rate, and (2) the species responses to rain pulses and implications on vegetation coverage under a changing rainfall regime. Results showed that the sensitivity of canopy conductance (Gc) to VPD varied under different incident radiation and soil water conditions, and the two species exhibited the same hydraulic control (-dG c/dlnVPD to Gcref ratio) over transpiration. Strengthened physiological control and low sapwood area of the stand contributed to low Ec. VPD after rainfall significantly influenced the magnitude and time series of post-rainfall stand Ec. The fluctuation of post-rainfall VPD in comparison with the pre-rainfall influenced the Ec recovery. Further, the stand Ec was significantly related to monthly rainfall, but the recovery was independent of the rainfall event size. Ec enhanced with cumulative soil moisture change (ΔVWC) within each dry-wet cycle, yet still was limited in large rainfall months. The two species had different response patterns of post-rainfall Ec recovery. Ec recovery of P. tabulaeformis was influenced by the pre- and post-rainfall VPD differences and the duration of rainless interval. R. pseudoacacia showed a larger immediate post-rainfall Ec increase than P. tabulaeformis did. We, therefore, concluded that concentrated rainfall events do not trigger significant increase of transpiration unless large events penetrate the deep soil and the species differences of Ec in response to pulses of rain may shape the composition of semiarid woodlands under future rainfall regimes.

Estimating sap flux densities in date palm trees using the heat dissipation method and weighing lysimeters

In a world of diminishing water reservoirs and a rising demand for food, the practice and development of water stress indicators and sensors are in rapid progress. The heat dissipation method, originally established by Granier, is herein applied and modified to enable sap flow measurements in date palm trees in the southern Arava desert of Israel. A long and tough sensor was constructed to withstand insertion into the date palm's hard exterior stem. This stem is wide and fibrous, surrounded by an even tougher external non-conducting layer of dead leaf bases. Furthermore, being a monocot species, water flow does not necessarily occur through the outer part of the palm's stem, as in most trees. Therefore, it is highly important to investigate the variations of the sap flux densities and determine the preferable location for sap flow sensing within the stem. Once installed into fully grown date palm trees stationed on weighing lysimeters, sap flow as measured by the modified sensors was compared with the actual transpiration. Sap flow was found to be well correlated with transpiration, especially when using a recent calibration equation rather than the original Granier equation. Furthermore, inducing the axial variability of the sap flux densities was found to be highly important for accurate assessments of transpiration by sap flow measurements. The sensors indicated no transpiration at night, a high increase of transpiration from 06:00 to 09:00, maximum transpiration at 12:00, followed by a moderate reduction until 08:00; when transpiration ceased. These results were reinforced by the lysimeters' output. Reduced sap flux densities were detected at the stem's mantle when compared with its center. These results were reinforced by mechanistic measurements of the stem's specific hydraulic conductivity. Variance on the vertical axis was also observed, indicating an accelerated flow towards the upper parts of the tree and raising a hypothesis concerning dehydrating mechanisms of the date palm tree. Finally, the sensors indicated reduction in flow almost immediately after irrigation of field-grown trees was withheld, at a time when no climatic or phenological conditions could have led to reduction in transpiration.

Intrinsic and extrinsic hydraulic factors in varying sizes of two Amazonian palm species (Iriartea deltoidea and Mauritia flexuosa) differing in development and growing environment

PREMISE OF THE STUDY

This study seeks to determine how hydraulic factors vary with ontogeny and whether they begin to limit further height growth in palms. Palms are an attractive group for physiological research because their columnar trunks and simple leaf habit allow key intrinsic and extrinsic hydraulic variables to be estimated more easily than in complex arborescent dicotyledons. •

METHODS

We measured various biometric and physiological factors including sap flux, leaf areas, turnover rates, and internode lengths in two Amazonian rainforest species: terra firme Iriartea deltoidea and swamp-adapted Mauritia flexuosa. These two palm species differ markedly in edaphic conditions, leaf type (pinnately compound vs. palmate), and bole development, making physiological comparisons between them important as well. •

KEY RESULTS

The species exhibited differing patterns in height growth rate along boles, which appear to relate to their differences in bole development. Growth rates ultimately slowed at the tops of tall palms in both species. We also found a high degree of convergence in total leaf area with height in both species even though they exhibited contrasting patterns in both live frond number and leaf area per frond with height. Sap flux density from leaves was constant with height but four times greater in M. flexuosa than in I. deltoidea. •

CONCLUSIONS

Although height growth rates slow considerably in tall palms, neither species shows evidence that hydraulic factors become limiting because they are able to support much greater leaf areas with similar sap flux densities as shorter palms.