Sapwood Area Research Articles

鼎湖山针阔叶混交林4种优势树种树干液流特征

PDF HTML阅读 XML下载 导出引用 引用提醒 鼎湖山针阔叶混交林4种优势树种树干液流特征 DOI: 10.5846/stxb201310202533 作者: 作者单位: 中国科学院华南植物园;中国科学院大学,中国科学院华南植物园;中国科学院大学,中国科学院华南植物园;中国林业科学研究院热带林业实验中心,中国科学院华南植物园,中国科学院华南植物园,中国科学院华南植物园 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目(31170375); 中国科学院知识创新工程重要方向项目(KZCX2-YW-GJ01); 广东省自然科学基金项目(9151008901000130) Sap flow characteristics of four dominant tree species in a mixed conifer-broadleaf forest in Dinghushan Author: Affiliation: South China Botanical Garden,Chinese Academy of Sciences,South China Botanical Garden,Chinese Academy of Sciences,South China Botanical Garden,Chinese Academy of Sciences,South China Botanical Garden,Chinese Academy of Sciences,South China Botanical Garden,Chinese Academy of Sciences,South China Botanical Garden,Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:运用Granier热消散式探针法,对鼎湖山自然保护区针阔叶混交林4种优势树种(马尾松、木荷、锥栗和广东润楠)的树干液流密度进行了长期连续观测,并同步监测林分的环境因子(光合有效辐射(PAR)、空气温度(T)、空气湿度(RH)、土壤含水量)。结果表明:(1)4种优势树种的边材面积(As)与胸径(DBH)均存在显著相关关系(P < 0.05);(2)各优势树种树干液流均呈现"昼高夜低"单峰曲线,且液流速率存在明显的季节性差异;(3)无论湿季还是干季,光合有效辐射(PAR)和水汽压亏缺(VPD) 均为控制蒸腾的主要驱动因子;(4) 4种优势树种湿季平均日蒸腾量高于干季,马尾松、木荷、锥栗、广东润楠湿季平均日蒸腾量分别为29.52、39.29、30.40、9.41 kgH2O/d,干季分别为20.91、24.84、24.26、8.43 kgH2O/d,干季和湿季的平均日蒸腾量(kgH2O/d)大小均为木荷>锥栗>马尾松>广东润楠,这种种间差异是由边材面积大小和树种本身的生物学特性共同决定的。 Abstract:The plant water use efficiency is not only crucial for plant to maintain a relatively high growth rate but also an important metric of water balance in forest ecosystem. Recent years, sap flow measurement has been widely used as a basis for assessing the water exchange between tree crown and ambient atmosphere for it can keep trees in natural condition without damaging the growth of trees. Many studies on trees' sap flow in single species plantations have been carried out across China, but the studies in natural forests are still few. We measured sap flow of four dominant tree species (Pinus manssoniana, Schima suerba, Castanopsis chinensis and Machilus kwangtungensis) in a mixed conifer-broadleaf natural forest in the Dinghushan Reserve of South China using the Granier's thermal dissipation probe method. The sap flow was measured daily in July and November, 2010, which represented the wet and dry seasons in the region, respectively. The environmental factors such as photosynthetically active radiation (PAR), air temperature (T), relative air humidity (RH) and soil moisture were also recorded. The aims of this study were (1) to investigate the characteristics of sap flow of four dominant tree species, (2) to estimate their relationships with the driving factors of transpiration, (3) and to compare the transpiration rates of different tree species calculated from sap flux density combing with trees' morphological characteristics. The results showed that sapwood area of the four dominant tree species had a significantly (P < 0.05) positive relationship with diameter at breast height (DBH). The coefficient of determination R2 of four trees species were 0.99、0.96、0.93、0.81 respectively. Sap flux density displayed an obvious pattern with higher values during daytime and also showed a significant seasonal variation with higher value in wet season. The average sap flow velocity of P. manssoniana, S. superba, C. chinensis and M. kwangtungensis were (10.07±1.47), (10.07±1.51), (12.20±1.73), (8.35±1.28) gH2O m-2 s-1 in wet season, and were (7.22±1.33), (6.41±1.15), (9.78±1.64), (7.37±1.34) gH2O m-2 s-1 in dry season. Contrast to previous study in Dinghushan Reserve of South China at an altitude of 50m, our research found that sap flow velocity displayed a significant variation along the gradient of altitude, which was induced by the differentiation of microenvironment and plant morphological characteristics. Partial correlation analysis indicated that photosynthetically active radiation (PAR) and vapor pressure deficiency (VPD) were the two main environmental factors determining the tree transpiration in both seasons. Transpiration of the four dominant tree species was much higher during the wet season than in the dry season. The average daily transpiration rates of P. manssoniana, S. superba, C. chinensis and M. kwangtungensis during the wet season were 29.52, 39.29, 30.40, 9.41 kgH2O/d, respectively, while those during the dry season the rates were 20.91, 24.84, 24.26 and 8.43 kgH2O/d, respectively. The order of the average daily transpiration was: S. superba > C. chinensis > P. manssoniana > M. kwangtungensis both in wet and dry seasons. Such difference in daily transpiration among tree species was mainly determined by the sapwood area and biological characteristics. 参考文献 相似文献 引证文献

The hydraulic architecture of Eucalyptus trees growing across a gradient of depth-to-groundwater.

Heterogeneity in water availability acts as an important driver of variation in plant structure and function. Changes in hydraulic architecture represent a key mechanism by which adaptation to changes in water availability can be expressed in plants. The aim of this study was to investigate whether differences in depth-to-groundwater influence the hydraulic architecture of Eucalyptus trees in remnant woodlands within mesic environments. Hydraulic architecture of trees was examined in winter and summer by measuring the following traits: Huber value (HV: the ratio between sapwood area and leaf area), branch hydraulic conductivity (leaf and sapwood area specific), sapwood density, xylem vulnerability (P50 and Pe) and hydraulic safety margins across four sites where depth-to-groundwater ranged from 2.4 to 37.5m. Huber value increased significantly as depth-to-groundwater increased. Neither sapwood density nor branch hydraulic conductivity (sapwood and leaf area specific) varied significantly across sites. Xylem vulnerability to embolism (represented by P50 and Pe) in both seasons was significantly and negatively correlated with depth-to-groundwater. Hydraulic safety margins increased with increasing depth-to-groundwater and therefore trees growing at sites with deeper water tables were less sensitive to drought induced embolism. These results showed plasticity in some, but not all, hydraulic traits (as reflected in HV, P50, Pe and hydraulic safety margin) in response to increase in depth-to-groundwater in a mesic environment.

Sapwood and intermediate wood thickness variation in Japanese cedar: impacts on sapwood area estimates

Sapwood and intermediate wood thickness variation in Japanese cedar: impacts on sapwood area estimates

Stomatal and hydraulic conductance and water use in a eucalypt plantation in Guangxi, southern China

Previous work has demonstrated that constant water use per unit leaf area results from the combined control of stomatal and hydraulic conductance holds for plant water use; however, few studies have ever explored the water use of a popularly planted fast-grown tree species, eucalyptus, in southern China. In this study, seasonal variations in hydraulic traits and ecophysiological parameters were monitored via the sap flux, leaf water potential (ψleaf) and associated environmental variables to investigate the water use of a five-year-old Eucalyptus grandis×Eucalyptus urophylla plantation in Guangxi province, China, in July and October 2012 and January and April 2013. The results show that predawn ψleaf was similar among all months, suggesting an abundance of soil water in the study site. There was a significant seasonal variation in midday ψleaf. Moreover, canopy stomatal conductance (Gs) was higher in July than in October and linearly decreased with the natural logarithm of the vapor pressure deficit (ln(VPD)). Furthermore, the hydraulic conductance from soil to leaves (K) per unit sapwood area in the dry season (October) was relatively high compared to that in the wet season (July). Whole-tree water use per day was estimated to be 7.7kgd−1 and 6.7kgd−1 in July and October, respectively, and linearly increased with leaf area. However, the slope of the regression line between whole-tree water use per day and leaf area was similar in July and October, clearly indicating that this eucalyptus stand had a constant water use per unit leaf area, which confirms the generally accepted concept. The findings of this study should help address the increasing ecohydrological and water resource concerns related to the rapid expansion of Eucalyptus spp. plantations in southern China, which recently underwent a severe drought.

Influence of temporospatial variation in sap flux density on estimates of whole-tree water use in Avicennia marina

Our study shows that sap flow in Avicennia marina varies significantly throughout the sapwood and that spatial patterns in sap flux density are dependent on meteorological conditions. Sap flux density measurements are used worldwide as a relatively inexpensive means to provide estimates of whole-tree and whole-stand water use in forest ecosystems. However, erroneous upscaling from point measurements to the entire sapwood area remains an issue, since sap flow is hardly ever constant throughout the tree. In this study, two widely used sap flow methodologies (the Heat Ratio or HR method and the Heat Field Deformation or HFD method) are used to assess radial and azimuthal variations in sap flux density in three mature trees of the mangrove species Avicennia marina in Brisbane, Australia. The genus Avicennia is characterised by secondary growth via successive cambia, resulting in an atypical sapwood pattern of xylem patches braided with phloem strings. Water use estimates were calculated in different ways. At first, spatial variation was ignored when upscaling from point measurements. Then, radial and azimuthal variations were incorporated subsequently by measuring at different depths and aspects around the tree. Ignoring azimuthal variation led to over- or underestimations of up to 102 %, while radial variation accounted for discrepancies of up to 25 %. Furthermore, the influence of changing meteorological conditions was assessed, which showed that radial profiles changed in shape during rain events, such that maximum sap flow rates occurred at different depths compared to dry periods. Our study thus indicates that spatial variation in sap flux density is highly unpredictable in A. marina due to its hydraulic architecture, and that changing meteorological conditions alter the pattern of this variation. These two factors should be accounted for when assessing whole-tree water use.

A model relating transpiration for Japanese cedar and cypress plantations with stand structure

Previous studies have revealed that changes in forest structure due to management (e.g., thinning, aging, and clearcutting) could affect the forest water balance. However, there are unexplained variability in changes in the annual water balance with changing structure among different sites. This is the case even when analyzing data for specific species/regions. For a more advanced and process-based understanding of changes in the water balance with changing forest structure, we examined transpiration (E) observed using the sap-flux method for 14 Japanese cedar and cypress plantations with various structure (e.g., stem density and diameter) in Japan and surrounding areas and developed a model relating E with structural parameters. We expressed E using the simplified Penman–Monteith equation and modeled canopy conductance (Gc) as a product of reference Gc (Gcref) when vapor pressure deficit is 1.0kPa and functions expressing the responses of Gc to meteorological factors. We determined Gcref and parameters of the functions for the sites separately. E observed for the 14 sites was not reproduced well by the model when using mean values of Gcref and the parameters among the sites. However, E observed for the sites was reproduced well when using Gcref determined for each site and mean values of the parameters of the functions among the sites, similar to the case when using Gcref and the parameters of the functions determined for each site. These results suggest that considering variations in Gcref among the sites was important to reproduce variations in E, but considering variations in the parameters of the functions was not. Our analysis revealed that Gcref linearly related with the sapwood area on a stand scale (A) and that A linearly related with stem density (N) and powers of the mean stem diameter (dm). Thus, we proposed a model relating E with A (or N and dm), where Gcref was calculated from A (or N and dm) and the parameters of the functions were assumed to be the mean values among the sites. This model estimates changes in E with changing structure from commonly available data (N and dm), and therefore helps improve our understanding of the underlying processes of the changes in the water balance for Japanese cedar and cypress plantations.

Sapling performance along resource gradients drives tree species distributions within and across tropical forests

Niche differentiation is a major hypothesized determinant of species distributions, but its practical importance is heavily debated and its underlying mechanisms are poorly understood. Trait‐based approaches have been used to infer niche differentiation and predict species distributions. For understanding underlying mechanisms, individual traits should be scaled up to whole‐plant performance, which has rarely been done.We measured seven key traits that are important for carbon and water balance for 37 tropical tree species. We used a process‐based plant physiological model to simulate the carbon budget of saplings along gradients of light and water availability, and quantified the performance of the species in terms of their light compensation points (a proxy for shade tolerance), water compensation points (proxy for drought tolerance), and maximum carbon gain rates (proxy for potential growth rate). We linked species performances to their observed distributions (the realized niches) at two spatial scales in Bolivian lowland forests: along a canopy openness gradient at local scale (∼1 km2) and along a rainfall gradient (1100–2200 mm/yr) at regional (∼1000 km) scale.We show that the water compensation point was the best predictor of species distributions along water and light resource gradients within and across tropical forests. A sensitivity analysis suggests that the stomatal regulation of minimum leaf water potentials, rather than stem hydraulic traits (sapwood area and specific conductivity), contributed to the species differences in the water compensation point of saplings. The light compensation point and maximum carbon gain, both driven by leaf area index and leaf nitrogen concentration, also contributed to differential species distributions at the local scale, but not or only marginally at the regional scale. Trait‐and‐physiology‐based simulations of whole‐plant performance thus help to evaluate the possible roles of individual traits in physiological processes underlying species performance along environmental gradients. The development of such whole‐plant concepts will improve our ability to understand responses of plant communities to shifts in resource availability and stress under global change.

Do alien and native tree species from Central Argentina differ in their water transport strategy?

In this study we assessed the water transport strategies and the abundance of alien and native tree species at a regional scale in Cordoba Mountains, Central Argentina. The aims of this study were: (i) to analyse whether alien and native tree species show divergent water transport strategies; and (ii) to explore whether species abundances of alien and natives are associated with specific trait attributes. Eight alien species and 12 native species were recorded in 50 complete vascular plant vegetation releves. Water transport strategies were assessed through the following functional traits: minimum leaf water potential, potential water content of wood, effective leaf area, leaf area per sapwood area and wood density. Also, resource use strategies were assessed throughout the measurement of specific leaf area. We found that alien species had a higher efficiency in water transport (i.e. higher minimum leaf water potential and lower wood density values) and faster resource acquisition and use (higher specific leaf area values) than native species. We did not find evidence suggesting that the relative abundance of species was associated to water transport strategies and faster resource acquisition and use. Alien species seem to differ from natives in specific functional attributes that are absent in the resident community and might allow aliens to use more resources and at a higher rate than native species. Finally, our results show the potential of a trait-based predictive framework for alien species, and the possible effects on ecosystem functions.

A new method for measuring stand sapwood area in forests

AbstractWe introduce a novel methodology for measuring stand sapwood area (SA), which provides a useful indicator of evapotranspiration from forest stands. The method is demonstrated in a 73‐year‐oldEucalyptus regnansforest comprising 784 stems over a 5 ha area. We used photos of stump cross‐sections to differentiate sapwood from heartwood and found 90% of stump segments to have a visible transition boundary. The digital images were corrected for lens distortion and scaled to an actual cross‐sectional area, with resulting stump perimeters corresponding well with field‐measured perimeters traced using string (root‐mean‐square error = 4 cm,R2 = 0.99). Calculated SA and basal area (BA) at stump height were coupled with tree and sapwood taper data to predict the SA:BA ratio at 1.3 m height (). Tree taper data were coupled with stump dimensions data in a mixed‐effects model to predict each stump's BA at 1.3 m, and sapwood taper data from buttress logs were used to improve each stems. Using this procedure, we found our study site to have anof 0.21 and total stand SA at 1.3 m height of 9.3 m2 ha−1.We quantified the bias in traditionalestimates that use cores to measure sapwood thickness and diameter tape to calculate SA. We show traditional methods underestimatethat increases with tree diameter and decreases with stem circularity, whereas our methodology gave more accurate measures of SA in large buttressing trees. Our methodology also provides a more efficient way of generating maps of SA variation across large forested catchments. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Stand‐scale transpiration of two Moso bamboo stands with different culm densities

AbstractThe large‐scale expansion of Moso bamboo forests in Japan raises concerns about how vegetation will affect the hydrological cycle and local water resources. However, information on transpiration (E) is limited to study on a single bamboo stand with low culm density (4 000 ha−1). The present study conducted a year‐round measurement of E in a high‐density (11 000 ha−1) stand using sap‐flow measurements, and additional measurements were made during summer in the previously studied low‐density stand and a neighbouring Japanese cedar stand for comparison. Annual E of the high‐density stand was estimated at 455 mm. Although the value was greater than those reported for neighbouring coniferous forests (250–370 mm), it was lower than previous measurements made in the low‐density stand (567 mm). The daytime sap flux density of the high‐density stand was generally less than half that in the low‐density stand, resulting in similar daily maximum E values in those stands. In addition, E typically peaked before midday in the high‐density stand followed by a rapid decline; the midday suppression of E led to decreased total E in the stand. Based on present and previous measurements of E, combined with the lower rainfall interception (I) reported for bamboo stands, water loss through evapotranspiration would be comparable between bamboo and coniferous forests. Present results also suggest that the E of bamboo stands does not have strong positive relationships with stand structural parameters (e.g., culm density and sapwood area) but possibly depends more on the sites' microenvironments (e.g., solar radiation). Copyright © 2014 John Wiley &amp; Sons, Ltd.

The Effect of Raffaelea quercus-mongolicae Inoculations on the Formation of Non-conductive Sapwood of Quercus mongolica

In Korea, mass mortality of Quercus mongolica trees has become obvious since 2004. Raffaelea quercus-mongolicae is believed to be a causal fungus contributing the mortality. To evaluate the pathogenicity of the fungus to the trees, the fungus was multiple- and single-inoculated to the seedlings and twigs of the mature trees, respectively. In both the inoculations, the fungus was reisolated from more than 50% of inoculated twigs and seedlings. In the single inoculations, proportions of the transverse area of non-conductive sapwood at inoculation points and vertical lengths of discoloration expanded from the points were significantly different between the inoculation treatment and the control. In the multiple inoculations, no mortality was confirmed among the seedlings examined. These results showed that R. quercus-mongolicae can colonize sapwood, contribute to sapwood discoloration and disrupt sap flows around inoculation sites of Q. mongolica, although the pathogenicity of the fungus was not proven.

Seeing the trees for the forest: drivers of individual growth responses to climate in Pinus uncinata mountain forests

Summary Individual trees, not forests, respond to climate. Such an individual‐scale approach has seldom been used to retrospectively track the radial growth responses of trees to climate in dendrochronology. The aim of this study was to adopt this individual view to retrospectively assess tree sensitivity to climate warming, and to evaluate and compare the potential drivers of tree growth responses to climate acting at species, site and individual scales. Following a dendroecological framework, we sampled a network of 29 Pinus uncinata forests in NE Spain and obtained tree‐ring widths series from 642 trees. Individual features as northness, elevation, slope, basal area, sapwood area, tree height and tree age were used to evaluate the potential drivers of tree growth responses to climate. The analysed data set includes diverse ecological and biogeographical conditions. The tree growth responses to climate were assessed by relating growth indices to climatic variables using linear‐mixed effects models. Maximum November temperatures during the year prior to tree‐ring formation enhanced P. uncinata growth mainly in mid‐elevation sites, whereas at higher elevations growth was more dependent on the positive effect of warmer minimum May temperatures during the year of tree‐ring formation. Current June precipitation was the positive main climatic driver of growth in sites prone to water deficit such as the southernmost limit of the species distribution area or very steep sites. Elevation was the main factor controlling how much growth variability is explained by climate at the site and tree scales. Climate warming was more intense during the early 20th century, when the importance of elevation as an indirect modulator of growth declined as compared with the late 20th century. Synthesis. The individual‐scale approach taken in this study allowed detecting that trees growing at southern and low‐elevation sites were the most negatively affected by warm and dry summer conditions. Our results emphasize that both (i) an individual‐scale approach to quantify tree growth responses to climate and (ii) a detailed evaluation of the potential biotic and abiotic drivers of those individual responses are necessary to understand climate sensitivity of trees.

Comparing ecophysiological traits and evapotranspiration of an invasive exotic, Pinus halepensis in native woodland overlying a karst aquifer

AbstractInvasion by exotic plant species into water‐limited environments has the potential to change the ecosystem water balance and may further exacerbate water scarcity issues. Here, we compared ecophysiological traits related to tree water use (sap flux, sapwood density, leaf and soil water potentials) and actual evapotranspiration (ETa) of the invasive Pinus halepensis to native tree and shrub species. We hypothesized that the invasive pine species would possess traits that are consistent with the potential to use more water than native species, which would be supported by higher ETa in areas invaded by pine. We found higher rates of sap flux for the invasive P. halepensis (5.5 cm h−1) per unit sapwood area compared with the native species (&lt;3.5 cm h−1). In addition, P. halepensis sapwood was significantly less dense than the sapwood of the native species, suggesting higher sapwood conductivity facilitated the faster sap flux. Comparison of remotely sensed ETa before and after P. halepensis removal within a Eucalyptus camaldulensis stand demonstrated a reduction in ETa by an average of 50 (±11 SE) mm year−1 in the 2 years after removal, relative to the ETa from an undisturbed, intact E. camaldulensis stand. This study suggests that active management of this invasive species should reduce overall ETa losses and thereby exert a positive influence on the availability of soil moisture for groundwater recharge. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Stand characteristics and water use at two elevations in a sub-tropical evergreen forest in southern China

The humid evergreen monsoon forests in China and other parts of Asia contribute significantly to the global and regional atmospheric circulation and hydrological cycle. The forests are, however, changing in structure due to increasing droughts, with potential impacts on forest hydrology. Long-term monitoring of these forests has also shown strong shifts in tree size distribution and tree species composition, changes that are likely to impose shifting functional demands on wood traits, leading to modification of the wood anatomy, stem sapflow and forest stand water use, as a consequence of changing hydraulic structures. We investigated the roles of tree size, species composition, elevation and microclimate on tree water use (TWU) and stand transpiration (Ec) in two forest stands located at 20m (Low-site) and 300m (High-site) elevations in the Dinghushan Biosphere forest reserve in South China (23̊09′21″N–23̊11′30″N, 112̊30′39″E–112̊33′41″E). The reserve was established in 1956 to protect the natural monsoon evergreen evergreen-broad-leaved forests. Sap flux density (SFD) was measured for a period of 1 year on 34 trees belonging to five different species common to the two stands. Species abundance, social position of the crown and crown size, tree diameter at breast height (DBH) and the area of the conductive sapwood (SA) were measured in plots of 400 and 600m2, respectively. TWU was calculated from SFD and SA, while Ec was estimated from TWU and stand SA derived from DBH/SA relationship. SA was positively and exponentially (R2=0.91; P<0.001) correlated with DBH. The mean maximum SFD from the 34 trees, belonging to the five species ranged between 3.9±0.6 and 16.4±1.8gcm2h−1, while the mean TWUmax ranged between 5 and 50kgd−1. Differences among individuals were significant (P<0.05) and were attributed to social position of the crown and tree size. The average daily canopy transpiration (Ec) from trees with DBH >10cm was 1.1±0.6 and 0.9±0.4mmd−1, accounting for 36–43% of the total soil water uptake at the ET and GH sites, respectively. Daily fluctuations in SFD and Ec were determined by VPD and PAR. Despite the low TWU and Ec from trees with DBH >10cm, the total potential evapotranspiration (ET) from the forest was estimated at 3.6mmd−1, which is within the range for most tropical forests. It is likely that the understory, with DBH <10cm plays an important role in forest water use, an attribute of a relatively young forest stand.

Interannual variation in stand transpiration estimated by sap flow measurement in a semi‐arid black locust plantation, Loess Plateau, China

AbstractThe semi‐arid region of the Loess Plateau in China is characterized by its fragile ecosystems. Soil erosion and water shortage are major factors influencing the ecology and hydrology of vegetation in this area. For proper management of watershed ecosystems, quantification of water use by major vegetation types is needed. We used Granier‐type thermal dissipation probes in a black locust (Robinia pseudoacacia) plantation and upscaled stand transpiration from individual measurements of stem sap flow. Stand transpiration water use during the growing seasons of 2008–2010 was estimated on the basis of measured sap flux densities and sapwood areas. Seasonal and interannual variations were observed in stand‐scale transpiration. Within a monthly timescale, water use by daily transpiration was correlated with daily solar radiation and daytime average vapour pressure deficit. The monthly changes in stand transpiration were mainly correlated with changes in leaf area index and with mean daytime vapour pressure deficit, solar radiation and potential evapotranspiration at lower significance. A positive relationship between stand transpiration and soil moisture conditions was detected at an annual scale, but not at a monthly scale. Precipitation did not show a clear correlation with stand transpiration at either seasonal or annual timescale. The results suggest that although the stand transpiration is controlled by multiple factors, those contributing to a significant difference vary with timescales. Some factors (e.g. soil moisture) may only be detected upon long‐term observations. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Difference in shade tolerance affects foliage–sapwood response to thinning by two eucalypts

In eucalypt plantations in subtropical Australia, Eucalyptus dunnii exhibits greater diameter increment after thinning than more shade-tolerant Corymbia citriodora. To elucidate the mechanism underlying this difference, we investigated relationships between tree leaf area and sapwood area following thinning in 11-year-old E. dunnii and C. citriodora plantations. There was no significant difference in specific leaf area (SLA) between thinned and unthinned stands in E. dunnii and C. citriodora, whereas crown zone significantly affected SLA in the two eucalypts. At the whole-tree level non-linear leaf area (Al)–sapwood area (As) relationships were measured in both eucalypts by thinning treatment. A significant increase in Al/As ratio was only observed at the upper crown in thinned E. dunnii. The present results suggested the plastic nature of response of leaf characteristics in both eucalypts grown in different light environments and the species-specific pattern of crown-zone leaf responses to thinning in the two species, i.e. the top of the canopy appears to be driving greater growth response to thinning in the less shade tolerant E. dunnii compared with the more shade-tolerant C. citriodora. It is concluded that different thinning regimes should be applied in shade-tolerant and shade-intolerant eucalypt forests.

Forest transpiration—targeted through xylem sap flux assessment versus hydrological modeling

The assessment of forest transpiration rates is crucial for determining plant-available soil water consumption and drought risk of trees. Xylem sap flux measurements have been used increasingly to quantify stand transpiration in forest ecosystems. Here, we compare this empirical approach with hydrological modeling on the basis of a stand transpiration dataset of adult beech (Fagus sylvatica), which was acquired across Bavaria, Germany, at eight forest sites. Xylem sap flux sensors were installed in five dominant trees each. Two tree to stand upscaling approaches, related to site-specific (1) sapwood area or (2) to leaf area index, were compared. The outcome was examined each in relation to process-based stand hydrological modeling, using LWF-BROOK90. Distinct relationships between tree diameter at breast height (1.30 m) and sapwood area-weighted sap flux along the radial profile became apparent across the study sites, confirming a generic allometric basis for stand-level upscaling of transpiration. The two upscaling approaches did not differ in outcome, representatively covering stand structure for comparison with modeling. Differential analysis yielded high agreement between the empirical and modeling approaches throughout most of the study period, although LWF-BROOK90 tended to overestimate sap flux measurements under low soil moisture. The two empirical approaches proved reliable for even-aged beech stands, as performance under high stand-structural heterogeneity awaits clarification. Findings advance stand-level hydrological modeling regarding coverage of stomatal behavior during temporary limitation in water availability.

Ceratocystis smalleyi colonization of bitternut hickory and host responses in the xylem

SummaryColonization of Carya cordiformis sapwood by Ceratocystis smalleyi and subsequent host defence responses following artificial inoculation were investigated using anatomical and histological techniques. Hyphae of C. smalleyi were observed in all sapwood xylem features confirming the ability of the pathogen to invade and colonize the xylem tissues of the host species. The fungus was isolated from within and at the margins of discoloured sapwood areas at 2 and 12 months after inoculation. General host defence responses that included vessel occlusion with gels or tyloses, lipid accumulation, and production of phenolic compounds were observed in xylem tissues of inoculated C. cordiformis stems. Pectic substances, lipids, and to a rare extent, phenolic compounds were detected in vascular gels. The lipid‐rich barriers observed likely prevent lateral expansion of the fungus in the sapwood. Furthermore, lack of fungus sporulation within vessels may restrict axial spread of the fungus. C. smalleyi appears to be a limited vascular wilt pathogen of bitternut hickory based on these observations and previously reported sap flow reduction correlated with multiple infections in artificially inoculated trees.

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.

Seasonal dynamics of night-time stem water recharge of Schima superba and its relation to tree architecture and leaf biomass

Aims Our objectives are to study the effects of tree architecture and leaf biomass of Schima superba on seasonal variations of nocturnal stem water recharge and to elucidate the compensation mechanism for hydraulic limitation and response to the hydraulic limitation hypothesis proposed by Roan and Yoder. Methods Night-time sap flow of 15 trees in a S. superba plantation was measured with Granier-type sensors installed in the sapwood at a height of 1.3 m for calculating night-time stem water recharge. Single and multiple linear regressions were performed in data analysis. Important findings The ending time of nocturnal sap flow was not followed by photosynthetically active radiation (PAR) but was the active time, which was unrelated to diameter at breast height (DBH). Night-time stem water recharge (W) was strongly linearly related with DBH, canopy size, sapwood area and leaf biomass (r = 0.596 to 0.895), except in winter. The linear relationship between W and tree height was found only in spring. W was positively affected by canopy in spring and summer, but by DBH in autumn and by tree height in winter. The results show that tree architecture and leaf biomass had significant effects on W and the degree changed with seasons.