ABSTRACT
Bamboos stand out among other tall plants in being able to generate positive pressure in the xylem at night, pushing water up to the leaves and causing drops to fall from leaf tips as guttation that can amount to a steady nocturnal 'bamboo rain'. The location and mechanism of nocturnal pressure generation in bamboos are unknown, as are the benefits for the plants. We conducted a study on the tall tropical bamboo species Bambusa oldhamii (giant timber bamboo) growing outdoors in southern California under full irrigation to determine where in the plant the nocturnal pressure is generated, when it rises in the evening, and when it dissipates in the morning. We hypothesized that the build-up of positive pressure would be triggered by the cessation of transpiration-driven sap flow and that resumption of sap flow in the morning would cause the pressure to dissipate. Nocturnal pressure was observed in mature stems and rhizomes, but never in roots. The pressure was episodic and associated with stem swelling and was usually, but not always, higher in rhizomes and basal stems than in stems at greater height. Time series analyses revealed that dry atmospheric conditions were followed by lower nocturnal pressure and rainfall events by higher stem pressure. Nocturnal pressure was unrelated to sap flow and even was generated for a short time in isolated stem pieces placed in water. We conclude that nocturnal pressure in bamboo is not 'root pressure' but is generated in the pseudo-woody rhizomes and stems. It is unrelated to the presence or absence of sap flow and therefore must be created outside of vessels, such as in phloem, parenchyma, or fibres. It is unlikely to be a drought adaptation and may benefit the plants by maximizing stem water storage for daytime transpiration or by transporting nutrients to the leaves.
ABSTRACT
We tested an approach to estimate daily canopy net photosynthesis, A, based on estimates of transpiration, E, using measurements of sap flow and water-use efficiency, ω, by measuring δ13C in CO2 respired from shoots in the canopies of two conifers (Podocarpaceae) native to New Zealand. The trees were planted in adjacent 20-year-old stands with the same soil and environmental conditions. Leaf area index was lower for Dacrycarpus dacrydioides (1.34 m2 m-2) than for Podocarpus totara (2.01 m2 m-2) but mean (± standard error) stem diameters were the same at 152 ± 21 mm for D. dacrydioides and 154 ± 25 mm for P. totara. Over a 28-day period, daily A (per unit ground area) ranged almost five-fold but there were no significant differences between species (mean 2.73 ± 1.02 gC m-2 d-1). This was attributable to higher daily values of E (2.63 ± 0.83 mm d-1) and lower ω (1.35 ± 0.53 gC kg H2O-1) for D. dacrydioides compared with lower E (1.82 ± 0.72 mm d-1) and higher ω (1.90 ± 0.77 gC kg H2O-1) for P. totara. We attributed this to higher nitrogen availability and nitrogen concentration per unit foliage area, Na, and greater exposure to irradiance in the D. dacrydioides canopy compared with P. totara. Our findings support earlier observations that D. dacrydioides is more adapted to sites with poor drainage. In contrast, the high retention of leaf area and maintaining low rates of transpiration by P. totara, resulting in higher water-use efficiency, is an adaptive response to survival in dry conditions. Our findings show that physiological adjustments for two species adapted to different environments led to similar canopy photosynthesis rates when the trees were grown in the same conditions. We demonstrated consistency between whole-tree and more intensive shoot-scale measurements, confirming that integrated approaches are appropriate for comparative estimates of carbon uptake in stands with different species.
ABSTRACT
Carbon dioxide sequestration from the atmosphere is commonly assessed using the eddy covariance method. Its net flux signal can be decomposed into gross primary production and ecosystem respiration components, but these have seldom been tested against independent methods. In addition, eddy covariance lacks the ability to partition carbon sequestration among individual trees or species within mixed forests. Therefore, we compared gross primary production from eddy covariance versus an independent method based on sap flow and water-use efficiency, as measured by the tissue heat balance method and δ13C of phloem contents, respectively. The latter measurements were conducted on individual trees throughout a growing season in a mixed broadleaf forest dominated by three tree species, namely English oak, narrow-leaved ash and common hornbeam (Quercus robur L., Fraxinus angustifolia Vahl, and Carpinus betulus L., respectively). In this context, we applied an alternative ecophysiological method aimed at verifying the accuracy of a state-of-the-art eddy covariance system while also offering a solution to the partitioning problem. We observed strong agreement in the ecosystem gross primary production estimates (R2 = 0.56; P < 0.0001), with correlation being especially high and nearly on the 1:1 line in the period before the end of July (R2 = 0.85; P < 0.0001). After this period, the estimates of gross primary production began to diverge. Possible reasons for the divergence are discussed, focusing especially on phenology and the limitation of the isotopic data. English oak showed the highest per-tree daily photosynthetic rates among tree species, but the smaller, more abundant common hornbeam contributed most to the stand-level summation, especially early in the spring. These findings provide a rigorous test of the methods and the species-level photosynthesis offers avenues for enhancing forest management aimed at carbon sequestration.
Subject(s)
Forests , Photosynthesis , Trees , Photosynthesis/physiology , Trees/physiology , Quercus/physiology , Quercus/metabolism , Carbon Sequestration , Fraxinus/physiology , Fraxinus/metabolismABSTRACT
Transpiration is a significant part of water cycle in forest ecosystems, influenced by meteorological factors and potentially constrained by soil moisture. We used Granier-type thermal dissipation probes to monitor xylem sap flow dynamics of three tree species (Quercus liaotungensis, Platycladus orientalis, and Robinia pseudoacacia) in a semi-arid loess hilly region, and to continuously monitor the key meteorological factors and soil water content (SWC). We established the SWC thresholds delineating soil moisture-limited and -unlimited sap flow responses to transpiration drivers. The results showed that mean sap flux density (Js) of Q. liaotungensis and R. pseudoacacia was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in Js for P. orientalis between the two periods was not significant. We used an exponential saturation function to fit the relationship between the Js of each tree species and the integrated transpiration variable (VT) which reflected solar radiation and vapor pressure deficit. The difference in the fitting curve parameters indicated that there were distinct response patterns between Js and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m3·m-3 for Q. liaotungensis, 0.116 m3·m-3 for P. orientalis, and 0.108 m3·m-3 for R. pseudoacacia. Below the thresholds, Js was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for Q. liaotungensis and P. orientalis reached saturation, while that of R. pseudoacacia did not reach saturation but exhibited a significant reduction in moisture limitation. Among the three species, P. orientalis was the most capable of overcoming soil moisture constraints.
Subject(s)
Plant Transpiration , Soil , Trees , Water , Soil/chemistry , Water/metabolism , Water/analysis , Trees/growth & development , Trees/physiology , Trees/metabolism , China , Quercus/physiology , Quercus/growth & development , Quercus/metabolism , Ecosystem , Robinia/physiology , Robinia/growth & development , Robinia/metabolism , Forests , Xylem/physiology , Xylem/metabolism , Plant Exudates/metabolismABSTRACT
Stem CO2 efflux is an important component of the carbon balance in forests. The efflux is considered to principally reflect the net result of two dominating and opposing processes: stem respiration and stem photosynthesis. In addition, transport of CO2 in xylem sap is thought to play an appreciable role in affecting the net flux. This work presents an approach to partition stem CO2 efflux among these processes using sap-flux data and CO2-exchange measurements from dark and transparent chambers placed on mature Scots pine (Pinus sylvestris) trees. Seasonal changes and monthly parameters describing the studied processes were determined. Respiration contributed most to stem net CO2 flux, reaching up to 79% (considering the sum of the absolute values of stem respiration, stem photosynthesis and flux from CO2 transported in xylem sap to be 100%) in June, when stem growth was greatest. Photosynthesis contribution accounted for up to 13 % of the stem net CO2 flux, increasing over the monitoring period. CO2 transported axially with sap flow, decreased towards the end of the growing season. At a reference temperature, respiration decreased starting around midsummer, while its temperature sensitivity increased during the summer. A decline was observed for photosynthetic quantum yield around midsummer together with decreasing light-saturation point. The proposed approach facilitates modeling net stem CO2 flux at a range of time scales.
ABSTRACT
Aerosols could significantly influence ecosystem carbon and water fluxes, potentially altering their interconnected dynamics, typically characterized by water-use efficiency (WUE). However, our understanding of the underlying ecophysiological mechanisms remains limited due to insufficient field observations. We conducted 4-yr measurements of leaf photosynthesis and transpiration, as well as 3-yr measurements of stem growth (SG) and sap flow of poplar trees exposed to natural aerosol fluctuation, to elucidate aerosol's impact on plant WUE. We found that aerosol improved sun leaf WUE mainly because a sharp decline in photosynthetically active radiation (PAR) inhibited its transpiration, while photosynthesis was less affected, as the negative effect induced by declined PAR was offset by the positive effect induced by low leaf vapor pressure deficit (VPDleaf). Conversely, diffuse radiation fertilization (DRF) effect stimulated shade leaf photosynthesis with minimal impact on transpiration, leading to an improved WUE. The responses were further verified by a strong DRF on SG and a decrease in sap flow due to the suppresses in total radiation and VPD. Our field observations indicate that, contrary to the commonly assumed coupling response, carbon uptake and water use exhibited dissimilar reactions to aerosol pollution, ultimately enhancing WUE at the leaf and canopy level.
Subject(s)
Aerosols , Carbon , Photosynthesis , Plant Leaves , Plant Transpiration , Populus , Water , Water/metabolism , Photosynthesis/radiation effects , Photosynthesis/drug effects , Carbon/metabolism , Plant Leaves/physiology , Plant Leaves/radiation effects , Plant Leaves/drug effects , Plant Transpiration/physiology , Plant Transpiration/radiation effects , Populus/physiology , Populus/radiation effects , Populus/drug effects , Plant Stems/radiation effects , Plant Stems/drug effects , Plant Stems/physiologyABSTRACT
Greenhouse gas (GHG) fluxes from peatland soils are relatively well studied, whereas tree stem fluxes have received far less attention. Simultaneous year-long measurements of soil and tree stem GHG fluxes in northern peatland forests are scarce, as previous studies have primarily focused on the growing season. We determined the seasonal dynamics of tree stem and soil CH4, N2O and CO2 fluxes in a hemiboreal drained peatland forest. Gas samples for flux calculations were manually collected from chambers at different heights on Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees (November 2020-December 2021) and analysed using gas chromatography. Environmental parameters were measured simultaneously with fluxes and xylem sap flow was recorded during the growing season. Birch stems played a greater role in the annual GHG dynamics than spruce stems. Birch stems were net annual CH4, N2O and CO2 sources, while spruce stems constituted a CH4 and CO2 source but a N2O sink. Soil was a net CO2 and N2O source, but a sink of CH4. Temporal dynamics of stem CH4 and N2O fluxes were driven by isolated emissions' peaks that contributed significantly to net annual fluxes. Stem CO2 efflux followed a seasonal trend coinciding with tree growth phenology. Stem CH4 dynamics were significantly affected by the changes between wetter and drier periods, while N2O was more influenced by short-term changes in soil hydrologic conditions. We showed that CH4 emitted from tree stems during the wetter period can offset nearly half of the soil sink capacity. We presented for the first time the relationship between tree stem GHG fluxes and sap flow in a peatland forest. The net CH4 flux was likely an aggregate of soil-derived and stem-produced CH4. A dominating soil source was more evident for stem N2O fluxes.
Subject(s)
Betula , Environmental Monitoring , Forests , Greenhouse Gases , Methane , Soil , Greenhouse Gases/analysis , Soil/chemistry , Methane/analysis , Seasons , Carbon Dioxide/analysis , Nitrous Oxide/analysis , Picea , Plant Stems , Air Pollutants/analysisABSTRACT
What represents a water source for the ecological restoration of a plant in an arid region is still up to debate. To address this issue, we conducted an in situ experiment in the Ulan Buh Desert of China, to study desert plants absorbing atmospheric water vapor. We selected Tamarisk, a common drought-salt-tolerant species in the desert, for ecological restoration as our research subject, used a newly designed lysimeter to monitor precipitation infiltration, and a sap flow system to track reverse sap flow that occurred in the shoot, branch, and stem during the precipitation event, and observed the precipitation redistribution process of the Tamarisk plot. The results showed that Tamarisk indeed directly absorbs precipitation water: when precipitation occurs, the main stem, lateral branch, and shoot all show the signs of reversed sap flow, and the reversed sap flow accounted for 21.5% of the annual sap flow in the shoot and branch, and 13.6% in the stem. The precipitation event in the desert was dominated by light precipitation events, which accounted for 81% of the annual precipitation events. It was found that light precipitation can be directly absorbed by the Tamarisk leaves, especially during nighttime or cloudy days. Even when the precipitation is absent, it was found that desert plants can still absorb water from the unsaturated atmospheric vapor; even the absorbed atmospheric water vapor was transported from the leaves to the stem, forming a reversed sap flow, as a reversed sap flow was observed when the atmospheric relative humidity reached 75%. This study indicated that the effect of light precipitation on desert plants was significant and should not be overlooked in terms of managing the ecological and hydrological systems in arid regions.
ABSTRACT
BACKGROUND AND AIMS: Plant water status is important for fruit development, because many fleshy fruits contain large amounts of water. However, there is no information on vascular flows of Persea americana 'Hass' avocado. The aims of this research were to explore the impact of drought stress on the water relationships of the 'Hass' avocado plant and its fruit growth. METHODS: Well-watered and water-stressed 'Hass' avocado plants were compared. Over 4 weeks, water flows through the shoot and fruit pedicel were monitored using external sap flow gauges. Fruit diameter was monitored using linear transducers, and stomatal conductance (gs), photosynthesis (A) and leaf and stem water potentials (Ñ°leaf and Ñ°stem) were measured to assess the response of the plants to water supply. KEY RESULTS: In well-watered conditions, the average water inflow to the shoot was 72 g day-1. Fruit water inflow was 2.72 g day-1, but there was water loss of 0.37 g day-1 caused by the outflow (loss back into the tree) through the vascular tissues and 1.06 g day-1 from the fruit skin. Overall, fruit volume increased by 1.4 cm3 day-1. In contrast, water flow into fruit of water-stressed plants decreased to 1.88 g day-1, with the outflow increasing to 0.61 g day-1. As a result, increases in fruit volume were reduced to 0.4 cm3 day-1. The values of A, gs and sap flow to shoots were also reduced during drought conditions. Changes in the hourly time-courses of pedicel sap flow, fruit volume and stem water potential during drought suggest that the stomatal response prevented larger increases in outflow from the fruit. Following re-watering, a substantial recovery in growth rate was observed. CONCLUSIONS: In summary, a reduction in growth of avocado fruit was observed with induced water deficit, but the isohydric stomatal behaviour of the leaves helped to minimize negative changes in water balance. Also, there was substantial recovery after re-watering, hence the short-term water stress did not decrease avocado fruit size. Negative impacts might appear if the drought treatment were prolonged.
Subject(s)
Droughts , Fruit , Persea , Photosynthesis , Plant Stomata , Water , Persea/physiology , Persea/growth & development , Plant Stomata/physiology , Fruit/physiology , Fruit/growth & development , Water/physiology , Water/metabolism , Photosynthesis/physiology , Plant Leaves/physiology , Plant Leaves/growth & development , Plant Transpiration/physiology , Plant Stems/physiology , Plant Stems/growth & development , Plant Stems/anatomy & histology , Plant Shoots/physiology , Plant Shoots/growth & development , Stress, Physiological/physiology , DehydrationABSTRACT
Accurate estimation of desert vegetation transpiration is key to regulating desert water resources of desert ecosystems. Sap flow density (SFD) can indirectly reflect a tree's transpiration consumption, and it has been affected by climate warming and groundwater depths in desert ecosystems. Sap flow responses to meteorological conditions and groundwater depths are further affected by tree of different sizes. However, how meteorological factors and groundwater depths affects tree sap flow among tree sizes remains poorly understand. In this study, a 50 × 50 m P. euphratica stand was selected as a sample plot in the hinterland of the Taklamakan Desert, and the SFD of P. euphratica of different sizes was measured continuously using the thermal diffusion technique from May to October of 2021 and 2022. The results showed that SFD of large P. euphratica was consistently higher than that of small P. euphratica in 2021 and 2022. and the SFD of P. euphratica was significantly and positively correlated with solar radiation (Rad) and vapor pressure deficit (VPD), and the correlation was higher than that of the air temperature (Ta) and relative humidity (RH), and also showed a strong non-linear relationship. Analysis of the hour-by-hour relationship between P. euphratica SFD and VPD and Rad showed a strong hysteresis. Throughout the growing season, there was no significant relationship between SFD of P. euphratica and groundwater depth, VPD and Rad were still the main controlling factors of SFD in different groundwater depths. However, during the period of relative groundwater deficit, the effect of groundwater depth on the SFD of P. euphratica increased, and the small P. euphratica was more sensitive, indicating that the small P. euphratica was more susceptible to groundwater changes. This study emphasized that Rad and VPD were the main drivers of SFD during the growing season, as well as differences in the response of different sizes of P. euphratica to groundwater changes. The results of the study provide a scientific basis for future modeling of transpiration consumption in P. euphratica forests in desert oases, as well as the regulation and allocation of water resources.
ABSTRACT
Climate change significantly impacts global forests, leading to tree decline and dieback. To cope with climate change, trees develop several functional traits, such as intra-annual density fluctuations (IADFs) in tree rings. The formation of these traits facilitates trees to optimize resource allocation, allowing them to withstand periods of stress and eventually recover when the conditions become favourable again. This study focuses on a Pinus pinaster Aiton forest in a warm, drought-prone Mediterranean area, comparing two growing seasons with different weather patterns. The innovative continuous monitoring approach used in this study combines high-resolution monitoring of sap flow (SF), analysis of xylogenesis and quantitative wood anatomy. Our results revealed the high plasticity of P. pinaster in water use and wood formation, shedding light on the link between IADFs and tree conductance. Indeed, the capacity to form large cells in autumn (as IADFs) improves the total xylem hydraulic conductivity of this species. For the first time, a continuous SF measurement system captured the dynamics of bimodal SF during the 2022 growing season in conjunction with the bimodal growth pattern observed through xylogenesis monitoring. These results highlight the intricate interplay between environmental conditions, water use, wood formation and tree physiology, providing valuable insights into the acclimation mechanisms employed by P. pinaster to cope with weather fluctuations.
Subject(s)
Pinus , Wood , Wood/anatomy & histology , Xylem/physiology , Pinus/physiology , Seasons , Droughts , WaterABSTRACT
Background: Southeast Asia has experienced widespread deforestation and change in land use. Consequently, many reforestation projects have been initiated in this region. However, it is imperative to carefully choose the tree species for planting, especially in light of the increasing climate variability and the potential alteration of plantation on the watershed water balance. Thus, the information regarding water-use characteristics of various tree species and sizes is critical in the tree species selection for reforestation. Methods: We estimated tree water use (T) of dominant species including Syzygium antisepticum and Adinandra integerrima, hereafter Sa and Ai, respectively, in a secondary tropical forest in Khao Yai National Park, Thailand, using sap flow data, and compared T between species and size classes. Additionally, we evaluated the responses of T of both species in each size class to environmental factors including soil moisture and vapor pressure deficit (VPD). Results: Results showed consistently higher T in Sa compared to Ai across ranges of VPD and soil moisture. Under low soil moisture, T of Sa responded to VPD, following a saturating exponential pattern while Ai maintained T across different VPD levels, irrespective of tree size. No responses of T to VPD were observed in either species when soil water was moderate. When soil moisture was high, T of both species significantly increased and saturated at high VPD, albeit the responses were less sensitive in large trees. Our results imply that Ai may be suitable for reforestation in water-limited areas where droughts frequently occur to minimize reforestation impact on water availability to downstream ecosystems. In contrast, Sa should be planted in regions with abundant and reliable water resources. However, a mixed species plantation should be generally considered to increase forest resilience to increasing climate variation.
Subject(s)
Ecosystem , Syzygium , Conservation of Natural Resources , Thailand , Parks, Recreational , Forests , Trees/physiology , Soil , WaterABSTRACT
Determining the differences of water use characteristics of a tree species with different origins (natural forests and introduced plantations) is significantly important for forest sustainable management. Pinus sylvestris var. mongolica is an important tree species of afforestation in the 'Three North' project in China. In this study, with Pinus sylvestris var. mongolica from two origins, we monitored the sap flow velocity of sapwood (Js) of trees by thermal dissipation sap flow probes, and analyzed the relationship between water transportation and the environmental factors during the growing season. The results showed that under the typical sunny day, daily sap flow velocity (Js-daily) of trees from plantations was significantly higher than that from natural forests. The mean value of Js-daily was 132.98 and 114.86 cm·d-1 for the two origins, respectively. Trees from plantations showed higher water transportation potential than natural forests. Vapor pressure deficit (VPD) mainly showed the driving effect on the water use process of trees from natural forests. In the plantations, there was an obvious threshold effect, and the inflection point of VPD was about 1.91 kPa, with the boundary function of Js-hour increased to the maximum of 17.88 cm·h-1. Atmospheric driven transpiration potential (Js-hour/VPD) of P. sylvestris var. mongolica trees with two origins decreased with the aggravation of soil drought, but sensitivity to drought was higher in the plantations than in the natural forests, suggesting the strong ability of Pinus sylvestris var. mongolica to regulate water use process.
Subject(s)
Pinus sylvestris , Pinus , Pinus sylvestris/physiology , Water/analysis , Forests , Trees , Soil , ChinaABSTRACT
The sap flow of trees is complex and difficult to express with multivariate linear or empirical models. A simple and feasible method on the basis of understanding sap flow variation to simulate its variation with environmental factors is of special importance for quantitatively analyzing forest ecohydrological processes and regional water demand. In this study, with one of the shelter forest species Euonymus bungeanus in the east sandy land of Yellow River in Ningxia as the research object, we continuously measured the trunk sap flow velocity by thermal diffusion sap flow meter, and analyzed the effects of environmental factors on stem sap flow. We used the particle swarm optimization (PSO) and sparrow search algorithm (SSA) optimized neural network model to predict sap flow velocity of E. bungeanus. Results showed that the main environmental factors influencing sap flow were solar radiation, vapor pressure deficit, air temperature, and relative humidity, with the influencing importance of 32.5%, 25.3%, 22.0% and 16.1%, respectively. The response process between sap flow and environmental factors presented a hysteresis loop relationship. The optimized BP, Elman and ELM neural network models improved the comprehensive evaluation index (GPI) by 1.5%, 30.0% and 5.3%, respectively. Compared with the PSO-Elman and SSA-ELM optimization models, the SSA-BP optimization model had the best prediction results with an improvement of 1.0% and 23.2% in GPI, respectively. Therefore, the prediction results of the BP neural network model based on the sparrow search algorithm could be used as an optimal model for predicting instantaneous sap flow velocity of E. bungeanus.
Subject(s)
Euonymus , Neural Networks, Computer , Computer Simulation , Algorithms , ForestsABSTRACT
Polylepis trees occur throughout the Andean mountain region, and it is the tree genus that grows at the highest elevation worldwide. In the humid Andes where moisture is rarely limiting, Polylepis trees must adapt to extreme environmental conditions, especially rapid fluctuations in temperature, ultraviolet radiation and vapor pressure deficit (VPD). However, Polylepis' water-use patterns remain largely unknown despite the importance of understanding their response to microclimate variation to determine their capacity to maintain resilience under future environmental change. We conducted a study in a Polylepis reticulata Kunth forest in the Ecuadorian Andes to evaluate its tree water-use dynamics and to identify the main environmental drivers of transpiration. Tree sap flow was monitored simultaneously with soil volumetric water content (VWC) and microclimate during 2 years for trees growing in forest edge and interior locations. We found that sap flow was primarily controlled by VPD and that VWC exerted a secondary role in driving sap flow dynamics. The highest values for sap flow rates were found when VPD > 0.15 kPa and VCW < 0.73 cm3 cm-3, but these threshold conditions only occurred during brief periods of time and were only found in 11% of our measurements. Moreover, these brief windows of more favorable conditions occurred more frequently in forest edge compared with forest interior locations, resulting in edge trees maintaining 46% higher sap flow compared with interior trees. Our results also suggest that P. reticulata has a low stomatal control of transpiration, as the sap flow did not decline with increasing VPD. This research provides valuable information about the potential impacts of projected future increases in VPD due to climate change on P. reticulata water-use dynamics, which include higher sap flow rates leading to greater transpirational water loss due to this species' poor stomatal control.
Subject(s)
Trees , Water , Trees/physiology , Water/physiology , Altitude , Ultraviolet Rays , Plant Transpiration/physiology , Forests , SoilABSTRACT
Introduction: Automated plant-based measurements of water stress have the potential to advance precision irrigation in orchard crops. Previous studies have shown correlations between sap flow, line variable differential transform (LVDT) dendrometers and fruit tree drought response. Here we report season-long automated measurement of maximum daily change in trunk diameter using band dendrometers and heated needles to measure a simplified sap flow index (SFI). Methods: Measurements were made on two apple cultivars that were stressed at 7 to 12 day intervals by withholding irrigation until the average stem water potential (ΨStem) dropped below -1.5 MPa, after which irrigation was restored and the drought cycle repeated. Results: Dendrometer measurements of maximum daily trunk shrinkage (MDS) were highly correlated (r² = 0.85) with pressure chamber measurements of stem water potential. The SFI measurements were less correlated with stem water potential but were highly correlated with evaporative demand (r² = 0.82) as determined by the Penman-Monteith equation (ETr). Discussion: The high correlation of SFI to ETr suggests that high-density orchards resemble a continuous surface, unlike orchards with widely spaced trees. The correlations of MDS and SFI to ΨStem were higher during the early season than the late season growth. Band dendrometers are less labor intensive to install than LVDT dendrometers and are non-invasive so are well suited to commercialization.
ABSTRACT
Introduction: Rainfall events can determine a cascade of plant physiological and ecological processes, and there is considerable interest in the way that rainfall modifies plant water flux dynamics. Methods: The sap flow density (SF) of the planted species of Vitex negundo and Hippophae rhamnoides, on the Loess Plateau of China was monitored using the heat balance method from 2015 to 2017. Results and discussion: The results showed that SF responded differently to rainfall classes because of the changing meteorological and soil water content (SWC) conditions. For class 1: 0.2-2 mm, SF increased by 14.36-42.93% for the two species, which were mainly attributable to the effect of solar radiation and vapor pressure deficit after rainfall. For class 2: 2-10 mm, SF remained nearly stable for V. negundo and decreased for H. rhamnoides because of the relative humidity's effect. For class 3: > 10 mm, SF increased significantly because of increased SWC and the increasing response to solar radiation. The increased percentage of SF was relatively higher for V. negundo when rainfall was less than 20 mm, while the value was higher for H. rhamnoides when rainfall was greater than 10 mm. Further, V. negundo's water potential increased at the soil-root interface (ψ0) and ψL, indicating that the plant, which has shallower roots and a coarser of leaf and bark texture, considered as anisohydric species and used precipitation-derived upper soil water to survive. The relatively consistent ψL and ψ0 for H. rhamnoides, which has deep roots and leathery leaves, indicated that this species was considered as isohydric species and insensitive to the slight change in the soil water status. The differed response patter and water use strategies between the two species showed that species as V. negundo are more susceptible to frequent, but small rainfall events, while larger, but less frequent rainfall events benefit such species as H. rhamnoides. This study quantified the effect of environmental factors for SF variation. The results could help formulate a selection process to determine which species are more suitable for sustainable management in the afforestation activities under the context of more frequent and intense rainfall events.
ABSTRACT
Tropical forests are experiencing increases in vapour pressure deficit (D), with possible negative impacts on tree growth. Tree-growth reduction due to rising D is commonly attributed to carbon limitation, thus overlooking the potentially important mechanism of D-induced impairment of wood formation due to an increase in turgor limitation. Here we calibrate a mechanistic tree-growth model to simulate turgor limitation of radial stem growth in mature Toona cilitata trees in an Asian tropical forest. Hourly sap flow and dendrometer measurements were collected to simulate turgor-driven growth during the growing season. Simulated seasonal patterns of radial stem growth matched well with growth observations. Growth mainly occurred at night and its pre-dawn build-up appeared to be limited under higher D. Across seasons, the night-time turgor pressure required for growth was negatively related to previous midday D, possibly due to a relatively high canopy conductance at high D, relative to stem rehydration. These findings provide the first evidence that tropical trees grow at night and that turgor pressure limits tree growth. We suggest including turgor limitation of tree stem growth in models also for tropical forest carbon dynamics, in particular, if these models simulate effects of warming and increased frequency of droughts.
Subject(s)
Rainforest , Trees , Vapor Pressure , Water , Forests , Carbon , Tropical ClimateABSTRACT
The interactive role of life-history traits and environmental factors on plant water relations is crucial for understanding the responses of species to climate change, but it remains poorly understood in secondary tropical montane forests (TMFs). In this study, we examined differences in sap flow between the pioneer species Symplocos racemosa and Eurya acuminata, and the late-successional species Castanopsis hystrix that co-occur in a biodiverse Eastern Himalayan secondary broadleaved TMF. The fast-growing pioneers had sap flux densities that were 1.6-2.1 times higher than the late-successional species, and exhibited characteristics of long-lived pioneer species. Significant radial and azimuthal variability in sap flow (V) between species was observed and could be attributed to the life-history trait and the access of the canopy to sunlight. Nocturnal V was 13.8% of the daily total and was attributable to stem recharge during the evening period (18.00-23.00 h) and to endogenous stomatal controls during the pre-dawn period (00.00-05.00 h). The shallow-rooted pioneer species both exhibited midday depression in V that was attributable to photosensitivity and diel moisture stress responses. In contrast, the deep-rooted late-successional species showed unaffected transpiration across the dry season, indicating their access to groundwater. Thus, our results suggest that secondary broadleaved TMFs, with a dominance of shallow-rooted pioneers, are more prone to the negative impacts of drier and warmer winters than primary forests, which are dominated by deep-rooted species. Our study provides an empirical understanding of how life-history traits coupled with microclimate can modulate plant water use in the widely distributed secondary TMFs in Eastern Himalaya, and highlights their vulnerability to warmer winters and reduced winter precipitation due to climate change.
Subject(s)
Forests , Trees , Trees/physiology , Plants , Biodiversity , Water/physiology , Plant Transpiration/physiology , Tropical ClimateABSTRACT
Phloem sap transport, velocity and allocation have been proposed to play a role in physiological limitations of crop yield, along with photosynthetic activity or water use efficiency. Although there is clear evidence that carbon allocation to grains effectively drives yield in cereals like wheat (as reflected by the harvest index), the influence of phloem transport rate and velocity is less clear. Here, we took advantage of previously published data on yield, respiration, carbon isotope composition, nitrogen content and water consumption in winter wheat cultivars grown across several sites with or without irrigation, to express grain production in terms of phloem sucrose transport and compare with xylem water transport. Our results suggest that phloem sucrose transport rate follows the same relationship with phloem N transport regardless of irrigation conditions and cultivars, and seems to depend mostly on grain weight (i.e., mg per grain). Depending on the assumption made for phloem sap sucrose concentration, either phloem sap velocity or its proportionality coefficient to xylem velocity change little with environmental conditions. Taken as a whole, phloem transport from leaves to grains seems to be homeostatic within a narrow range of values and following relationships with other plant physiological parameters across cultivars and conditions. This suggests that phloem transport per se is not a limitation for yield in wheat but rather, is controlled to sustain grain filling.