Understanding the relationship between plant water status and productivity and between plant water status and plant mortality is required to effectively quantify and predict the effects of drought on ...plants. Plant water status is closely linked to leaf water content that may be estimated using remote sensing technologies. Here, we used an inexpensive miniature hyperspectral spectrometer in the 1550–1950 nm wavelength domain to measure changes in silver birch (Betula pendula Roth) leaf water content combined with leaf gas exchange measurements at a sub-minute time resolution, under increasing vapor pressure deficit, CO2 concentrations, and light intensity within the measurement cuvette; we also developed a novel methodology for calibrating reflectance measurements to predict leaf water content for individual leaves. Based on reflectance at 1550 nm, linear regression modeling explained 98–99% of the variation in leaf water content, with a root mean square error of 0.31–0.43 g cm−2. The prediction accuracy of the model represents a c. ten-fold improvement compared to previous studies that have used destructive sampling measurements of several leaves. This novel methodology allows the study of interlinkages between leaf water content, transpiration, and assimilation at a high time resolution that will increase understanding of the movement of water within plants and between plants and the atmosphere.
Key message
Abies fargesii
var.
faxoniana
(Rehder et E.H. Wilson) Tang S Liu seedlings at high elevations compensate for the low efficiency of their water conducting system and high phloem hydraulic ...resistance by the enhancement in xylem:leaf area, phloem:leaf area, and phloem:xylem area.
Context
Maintenance of xylem and phloem transport is particularly important for the survival and growth of trees at the treeline. How plants modify the allocation to leaf, xylem, and phloem structures to adapt to the treeline environment is an important issue.
Aims
The purpose of this study was to estimate how xylem and phloem anatomy and volume as well as leaf functional traits of
A. fargesii
seedlings vary with elevation.
Methods
We examined elevation-related differences in a variety of phloem and xylem functional areas and hydraulic conduit diameters of
A. fargesii
seedlings growing at elevations between 2600 and 3200 m in the subalpine conifer forest of southwest China.
Results
Xylem area, last xylem ring area, and leaf:sapwood area significantly decreased, while xylem:leaf area, phloem:leaf area, and non-collapsed phloem:xylem area significantly increased with elevation. Principal components analysis showed that xylem area, non-collapsed phloem area, and xylem:phloem area were positively correlated with growth rates.
Conclusion
Our results showed that
A. fargesii
tree seedlings at the treeline tend to facilitate growth and maintain functional water and sugar balance between stem and leaves by the enhancement in xylem:leaf area, phloem:leaf area, and phloem:xylem area, but not through differences in vessel lumen diameter.
•Raw dendrometer time-series data are usually analyzed focusing just on specific periods.•New procedure to automatically filter dendrometer data.•Daily temporal features including stem shrinkage and ...swelling.•Rigorous accuracy assessment and method validation.
Automatic analysis of point dendrometer time series (DTS) registering radial stem variations of trees is of relevant interest to study tree water use and growth. Unfortunately, data from such sensors are often characterized by a large amount of noise that needs to be distinguished by sensors responses induced by biological processes (signal) and irregular fluctuations due to electrical disturbances, malfunctions, or external inputs (noise) such as stem flow, perturbation induced by animals, stem temperature and humidity variations. Although some algorithms have been developed to adjust jumps and correct peaks in DTS, it is nowadays challenging to extract biological signals after a large number of corrections and artifacts introduced during denoising processes. In this study, we present an alternative methodology consisting of the first attempt to automatically identify days in which the dendrometers are registering information related to the activity of the tree and relevant for a specific study (days-of-signal). Through (i) per-day temporal segmentation of different stem behaviors, (ii) daily temporal features extraction, and (iii) automatic days-of-signal and days-of-noise discrimination, we automatically analyzed 19 million DTS records acquired during three years by 12 dendrometers installed on xylem and bark at different stem heights from the collar, at the bottom and top-level, of Pinus sylvestris trees. To train and assess the performance of the model, we constructed a reference dataset by labelling 600 daily DTS into days-of-signal or days-of-noise. As a result of our model application, we detected 3,534 days-of-signal among the altogether 13,152 measurement days with a per sensor overall accuracy, calculated using the reference dataset, ranging between 100% and 82%. Finally, we showed the trend of stem shrinkage and swelling over the three years study period. The large accuracies obtained over the different sensors suggest that our method is versatile and generalizable.
Trees experience low apoplastic water potential frequently in most environments. Low apoplastic water potential increases the risk of embolism formation in xylem conduits and creates dehydration ...stress for the living cells. We studied the magnitude and rate of xylem diameter change in response to decreasing apoplastic water potential and the role of living parenchyma cells in it to better understand xylem diameter changes in different environmental conditions. We compared responses of control and heat-injured xylem of Pinus sylvestris (L.) and Populus tremula (L.) branches to decreasing apoplastic water potential created by osmotic stress, desiccation and freezing. It was shown that xylem in control branches shrank more in response to decreasing apoplastic water potential in comparison with the samples that were preheated to damage living xylem parenchyma. By manipulating the osmotic pressure of the xylem sap, we observed xylem shrinkage due to decreasing apoplastic water potential even in the absence of water tension within the conduits. These results indicate that decreasing apoplastic water potential led to withdrawal of intracellular water from the xylem parenchyma, causing tissue shrinkage. The amount of xylem shrinkage per decrease in apoplastic water potential was higher during osmotic stress or desiccation compared with freezing. During desiccation, xylem diameter shrinkage involved both dehydration-related shrinkage of xylem parenchyma and water tension-induced shrinkage of conduits, whereas dehydration-related shrinkage of xylem parenchyma was accompanied by swelling of apoplastic ice during freezing. It was also shown that the exchange of water between symplast and apoplast within xylem is clearly faster than previously reported between the phloem and the xylem. Time constant of xylem shrinkage was 40 and 2 times higher during osmotic stress than during freezing stress in P. sylvestris and P. tremula, respectively. Finally, it was concluded that the amount of water stored in the xylem parenchyma is an important reservoir for trees to buffer daily fluctuations in water relations.
ABSTRACT
Efficient water transport from the soil to the leaves is essential for plant function, while building and maintaining the water transport structure in the xylem require a major proportion of ...the assimilated carbon of the tree. Xylem transport also faces additional challenges as water in the xylem is under tension and therefore cavitation cannot be completely avoided. We constructed a model that calculates the xylem structure that maximizes carbon‐use efficiency while simultaneously taking into account pit structure in increasing the resistance to water transport and constricting the spreading of embolisms. The optimal xylem structure predicted by the model was found to correspond well to the generally observed trends: xylem conduits grew in size from the apex towards the base while simultaneously decreasing in number, and vulnerability to cavitation increased with conduit size. These trends were caused primarily by the axial water potential gradient in the xylem. The pits have to be less porous near the apex where water potential is lower to restrict the spreading of embolisms, while whole‐plant carbon‐use efficiency demands that conduit size decreases and conduit number increases simultaneously. The model predictions remained qualitatively the same regardless of the exact optimality criterion used for defining carbon‐use efficiency.
We constructed a model which calculates the xylem structure that maximizes carbon use efficiency while simultaneously taking into account pit structure in increasing the resistance to water transport and constricting the spreading of embolisms. The optimal xylem structure predicted by the model was found to correspond well to the generally observed trends: xylem conduits grew in size from the apex towards the base while simultaneously decreasing in number, and vulnerability to cavitation increased with conduit size. These trends were caused primarily by the axial water potential gradient in the xylem.
Tree stems have been identified as sources of volatile organic compounds (VOCs) that play important roles in tree defence and atmospheric chemistry. Yet, we lack understanding on the magnitude and ...environmental drivers of stem VOC emissions in various forest ecosystems. Due to the increasing importance of extreme drought, we studied drought effects on the VOC emissions from mature Scots pine (Pinus sylvestris L.) stems. We measured monoterpenes, acetone, acetaldehyde and methanol emissions with custom‐made stem chambers, online PTR‐MS and adsorbent sampling in a drought‐prone forest over the hot‐dry summer of 2018 and compared the emission rates and dynamics between trees in naturally dry conditions and under long‐term irrigation (drought release). The pine stems were significant monoterpene sources. The stem monoterpene emissions potentially originated from resin, based on their similar monoterpene spectra. The emission dynamics of all VOCs followed temperature at a daily scale, but monoterpene and acetaldehyde emission rates decreased nonlinearly with drought over the summer. Despite the dry conditions, large peaks of monoterpene, acetaldehyde and acetone emissions occurred in late summer potentially due to abiotic or biotic stressors. Our results highlight the potential importance of stem emissions in the ecosystem VOC budget, encouraging further studies in diverse environments.
Scots pine stems in dry environment were considerable monoterpene sources. The emission rates of monoterpenes, acetone, acetaldehyde from mature Scots pine stems decreased with reduced water availability in both dry and irrigated treatments, but methanol emission rates seemed unaffected.
Abstract
Waterlogging causes hypoxic or anoxic conditions in soils, which lead to decreases in root and stomatal hydraulic conductance. Although these effects have been observed in a variety of plant ...species, they have not been quantified continuously over a range of water table depths (WTD) or soil water contents (SWC). To provide a quantitative theoretical framework for tackling this issue, we hypothesized similar mathematical descriptions of waterlogging and drought effects on whole-tree hydraulics and constructed a hierarchical model by connecting optimal stomata and soil-to-leaf hydraulic conductance models. In the model, the soil-to-root conductance is non-monotonic with WTD to reflect both the limitations by water under low SWC and by hypoxic effects associated with inhibited oxygen diffusion under high SWC. The model was parameterized using priors from literature and data collected over four growing seasons from Scots pine (Pinus sylvestris L.) trees grown in a drained peatland in Finland. Two reference models (RMs) were compared with the new model, RM1 with no belowground hydraulics and RM2 with no waterlogging effects. The new model was more accurate than the RMs in predicting transpiration rate (fitted slope of measured against modeled transpiration rate = 0.991 vs 0.979 (RM1) and 0.984 (RM2), R2 = 0.801 vs 0.665 (RM1) and 0.776 (RM2)). Particularly, RM2’s overestimation of transpiration rate under shallow water table conditions (fitted slope = 0.908, R2 = 0.697) was considerably reduced by the new model (fitted slope = 0.956, R2 = 0.711). The limits and potential improvements of the model are discussed.
Cavitation resistance is a key parameter to understand tree drought tolerance but little is known about the mechanisms of air entry into xylem conduits. For conifers three mechanisms have been ...proposed: (1) a rupture of pit margo microfibrils, (2) a displacement of the pit torus from its normal sealing position over the pit aperture, and (3) a rupture of an air-water menisci in a pore of the pit margo. In this article, we report experimental results on three coniferous species suggesting additional mechanisms. First, when xylem segments were injected with a fluid at a pressure sufficient to aspirate pit tori and well above the pressure for cavitation induction we failed to detect the increase in sample conductance that should have been caused by torus displacement from blocking the pit aperture or by membrane rupture. Second, by injecting xylem samples with different surfactant solutions, we found a linear relation between sample vulnerability to cavitation and fluid surface tension. This suggests that cavitation in conifers could also be provoked by the capillary failure of an air-water meniscus in coherence with the prediction of Young-Laplace's equation. Within the bordered pit membrane, the exact position of this capillary seeding is unknown. The possible Achilles' heel could be the seal between tori and pit walls or holes in the torus. The mechanism of water-stress-induced cavitation in conifers could then be relatively similar to the one currently proposed for angiosperms.
Sugar transport in the phloem is driven by turgor pressure gradients which are created by osmotic gradients resulting from sugars loaded to the phloem at the source tissue and unloaded at the sink ...tissue. Therefore, osmolality is a key parameter that can be used to evaluate sugar status and get an indication of the driving force for phloem transport. Here we describe how osmotic concentration measurements from inner bark (practically, the phloem) and needles of trees can be measured. This protocol presents the procedure used by Lintunen et al. (Front Plant Sci 7:726, 2016) and Paljakka et al. (Plant Cell Environ 40:2160-2173, 2017), extended by practical advice and discussion of potential errors and caveats. We describe how to implement this procedure for gymnosperm as well as angiosperm trees. This method uses mechanical sap extraction with a centrifuge from inner bark and leaf samples, which have gone through a deep freeze treatment and thawing. The osmotic potential of these samples is then analyzed with a freezing point or vapor pressure osmometer. The aim of these measurements is to study the spatial and temporal dynamics of phloem function.
•Low-severity fire can result in lower transpiration and stem growth in the surviving trees.•Low-severity fire affects soil temperature and moisture conditions.•Our findings highlight the longevity ...of fire impacts on the forest ecosystem.
Boreal forests are exposed to larger and more frequent fires due to climate change, with significant consequences for their carbon and water balances. Low-severity fires (trees charred but surviving) are the most common fire regime in the Eurasian boreal forest, but we still lack understanding on how they impact tree functioning. This study focused on the dynamics of tree transpiration and stem growth of Pinus sylvestris in central Sweden after a large wildfire in 2018. We compared a stand impacted by low-severity fire (LM) with an unburnt stand (UM), over three years following the fire (2020–2022). We found that transpiration was on average lower and more variable within the stand at LM compared to UM. LM also had consistently lower stem growth compared to UM, resulting in larger accumulated growth for the unburnt site in the second to fourth year after the fire. Our results highlight the complex effects of low-severity fire on tree water cycling, with both direct (damage to tree xylem and roots) and indirect fire impacts (due to loss of understory vegetation and changes in soil properties). Trees affected by low-severity fire also exhibited reduced resilience to water shortages. Considering the expected increase in frequency of droughts and forest fires at higher northern latitudes, such indirect fire impacts may put additional pressure on the boreal forest.
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