A dynamic model for water flow in a single tree: evidence that models must account for hydraulic architecture

A model is presented for the dynamics of water flow in a single eastern white cedar tree (Thuja occidentalis L.). The model takes into account the spatial and temporal dependence of the evaporative flux from leaves in the crown. It also accounts for the quantitative hydraulic architecture of the tree, i.e., the model characterizes the tree as a branched catena of > 4000 stem segments in which account is taken of the segment length, diameter, hydraulic resistance, and the total area of leaves attached to the segment. Input values needed to run the model are measurements of evaporative flux, hydraulic conductance of stems versus stem diameter, and leaf and stem water storage capacitances. Output parameters are the spatial and temporal characterization of stem and leaf water potentials, stem and leaf water deficits, sap flow rate, and relative sap velocity. The input and output values of the branched catena model are compared and contrasted to that of an unbranched catena model. It is shown that the branched catena model fits independently measured field parameters better than an unbranched catena model. Close correspondence is found between model predictions and field measurements of shoot water potential, pressure gradients in stems, hysteresis in sap velocity between the lower and upper parts of the tree, and diurnal changes in stem and leaf water deficits. This model is discussed in terms of both the hydraulic architecture of trees and the potential application of the model to questions of tree morphology, ecology, physiology and evolution.     

Estimating water use by sugar maple trees: considerations when using heat-pulse methods in trees with deep functional sapwood

Accurate estimates of sapwood properties (including radial depth of functional xylem and wood water content) are critical when using the heat pulse velocity (HPV) technique to estimate tree water use. Errors in estimating the volumetric water content (V(h)) of the sapwood, especially in tree species with a large proportion of sapwood, can cause significant errors in the calculations ofsap velocity and sap flow through tree boles. Scaling to the whole-stand level greatly inflates these errors. We determined the effects of season, tree size and radial wood depth on V(h) of wood cores removed from Acer saccharum Marsh. trees throughout 3 years in upstate New York. We also determined the effects of variation in V(h) on sap velocity and sap flow calculations based on HPV data collected from sap flow gauges inserted at four depths. In addition, we compared two modifications of Hatton's weighted average technique, the zero-step and zero-average methods, for determining sap velocity and sap flow at depths beyond those penetrated by the sap flow gauges. Parameter V(h) varied significantly with time of year (DOY), tree size (S), and radial wood depth (RD), and there were significant DOY x S and DOY x RD interactions. Use of a mean whole-tree V(h) value resulted in differences ranging from -6 to +47% for both sap velocity and sap flow for individual sapwood annuli compared with use of the V(h) value determined at the specific depth where a probe was placed. Whole-tree sap flow was 7% higher when calculated on the basis of the individual V(h) value compared with the mean whole-tree V(h) value. Calculated total sap flow for a tree with a DBH of 48.8 cm was 13 and 19% less using the zero-step and the zero-average velocity techniques, respectively, than the value obtained with Hatton's weighted average technique. Smaller differences among the three methods were observed for a tree with a DBH of 24.4 cm. We conclude that, for Acer saccharum: (1) mean V(h) changes significantly during the year and can range from nearly 50% during winter and early spring, to 20% during the growing season;(2) large trees have a significantly greater V(h) than small trees; (3) overall, V(h) decreases and then increases significantly with radial wood depth, suggesting that radial water movement and storage are highly dynamic; and (4) V(h) estimates can vary greatly and influence subsequent water use calculations depending on whether an average or an individual V(h) value for a wood core is used. For large diameter trees in which sapwood comprises a large fraction of total stem cross-sectional area (where sap flow gauges cannot be inserted across the entire cross-sectional area), the zero-average modification of Hatton's weighted average method reduces the potential for large errors in whole-tree and landscape water balance estimates based on the HPV method.     

Estimating stand water use of large mountain ash trees and validation of the sap flow measurement technique

Mountain ash (Eucalyptus regnans F.J. Muell.) forest catchments exhibit a strong relationship between stand age and runoff, attributed inter alia to differences in tree water use. However, the tree water use component of the mountain ash forest water balance is poorly quantified. We have used the sap flow technique to obtain estimates of daily water use in large mountain ash trees. First, the sap flow technique was validated by means of an in situ cut tree experiment. Close agreement was obtained between the sap flow estimate of water use and the actual uptake of water by the tree from a reservoir. Second, we compared the variability in sap velocity between a symmetric and an asymmetric tree by using multiple sap flow loggers. In the symmetric tree, velocity was fairly uniform throughout the xylem during the day, indicating that accurate sap flow estimates can be obtained with a minimal number of sampling points. However, large variations in sap velocity were observed in the asymmetric tree, indicating that much larger sampling sizes are required in asymmetric stems for an accurate determination of mean sap velocity. Finally, we compared two procedures for scaling individual tree sap flow estimates to the stand level based on stem diameter and leaf area index measurements. The first procedure was based on a regression between stem diameter and tree water use, developed on a small sample of trees and applied to a stand-level census of stem diameter values. Inputs to the second procedure were tree water use and leaf area of a single tree and the leaf area index of the stand. The two procedures yielded similar results; however, the first procedure was more robust but it required more sampling effort than the second procedure.     

天目山柳杉古树的液流特征研究

应用热扩散技术法,于2010年4—8月对浙江天目山自然保护区内的2株柳杉古树的树干液流进行连续观测,结合所测定的相关环境因子,分析了柳杉树干液流和耗水量的变化规律,以及液流与各环境因子的关系。结果表明:不同季节柳杉树干液流速率日变化规律基本一致,呈单峰波动曲线,但树干液流启动时间、达到峰值时间及迅速下降时间存在明显差异;树干液流密度与光合有效辐射、空气温度和水汽压差间存在极显著正相关,与空气相对湿度和CO₂浓度呈极显著负相关;液流密度随树干直径的增加,无明显变化差异,但液流速率和日均耗水量均随树干直径的增加而增大;胸径75.6 cm柳杉和胸径62.8 cm柳杉的日均耗水量季节变化过程相同,但2者变化差异较大,6月份日均耗水量最低,分别为(49.356±14.883) kg和(9.531±4.297)kg;4月份日均耗水量最高,分别为(110.022±21.890)kg和(49.352±2.629)kg。     

The relationship between sap flow rate and diurnal change of tangential strain on inner bark in Cryptomeria japonica saplings

The relationship between sap flow rates and diurnal fluctuation of stems was investigated in cloned 3-year-old saplings of Cryptomeria japonica D. Don grown in a phytotron with irrigation every 2 days. The improved stem heat balance method and a strain gauge were used to measure sap flow rate and diurnal fluctuation of the stem. The sap flow rate reacted to lighting conditions, increasing and decreasing immediately after lights-on and lights-off, respectively. The tangential strain on the surface of the inner bark exhibited a reaction that followed but opposed the reaction of the sap flow rate to lighting conditions. Based on the changes in sap flow rate, there seemed to be four phases in diurnal sap flow: phase A₁ began with lights-on, when the sap flow rate increased, and lasted about 2 hours. In the following phase, A₂, the sap flow rate remained almost constant at 1.3 g/min for about 10 h, and then declined for about 2 h as lights-off approached. In phase B, the early period of darkness, the sap flow declined quickly and then more slowly, for about 4 h, until the start of the second dark period, phase C, when the sap flow rate became almost constant at 0.05 g/min for about 6 h. The first derivative of each sap flow rate and the corresponding tangential strain were calculated, and the results indicated a negative correlation between the two variables in all periods. In particular, the relationship between the first derivative values exhibited a highly negative correlation in phases A₁ and B, expressed as a primary formula. Sap flow rate was found to continue for some time after lights-off, and this compensated for reduced evaporative effects, albeit at a slow rate, over 4 h. The total amount of sap flow in the dark was only about 9% of that in the light, disregarding transpiration in the dark for simplicity. Thus, the total amount of sap flow responsible for swelling of the stem was about 9% of that consumed in transpiration during the light period.     

Sap flow characteristics of three afforestation species during the wet and dry seasons in a dry-hot valley in Southwest China

Assessing and using tree species (exotic or native) with superior tolerance to environmental stresses (such as drought and high temperature) play an important role in afforestation practices. In the present study, stem sap flow characteristics and responses to ambient meteorological factors of three tree species, Albizzia kalkora (native), Azadirachta indica (exotic), and Acacia auriculaeformis (exotic), in a dry-hot valley (Yuanmou, Yunnan Province, China) were investigated using thermal dissipation probes. The diurnal dynamics of sap flow in three studied species displayed an obvious circadian rhythm during the wet and dry seasons, with the exception of A. indica during the dry season. The sap flow velocity (SFV) in A. kalkora and A. auriculaeformis was significantly positively correlated with photosynthetically active radiation (PAR), air temperature, vapour pressure deficit (VPD) and wind speed, but negatively correlated with atmospheric relative humidity over the two seasons. The cross-correlation analysis also revealed that the SFV of the three species was significantly correlated with PAR and VPD (P < 0.001). Additionally, stem sap flow lagged behind PAR but ahead of VPD, and the diurnal sap flow was more dependent on PAR than on VPD. However, we found that the dominant climatic factor influencing the stem sap flow differed between daytime and nighttime. PAR was more influential than other meteorological factors during the daytime, while VPD or other factors were more influential overnight. When the nighttime refilling ability of the three tree species was compared, our results suggest that A. indica has higher drought resistance and better for afforestation of the studied region.     

Radial profiles of sap flow with increasing tree size in maritime pine

We investigated the radial variation of sap flow within sapwood below the live crown in relation to tree size in 10-, 32-, 54- and 91-year-old maritime pine stands (Pinus pinaster Ait.). Radial variations were determined with two thermal dissipation sensors; one measured sap flux in the outer 20 mm of the xylem (Jref), whereas the other was moved radially across the sapwood in 20-mm increments to measure sap flux at multiple depths (Jref). For all tree sizes, sap flow ratios (Ri = JiJref (-1)) declined with increasing sapwood depth, but the decrease was steeper in trees with large diameters. Correction factors (C) were calculated to extrapolate Jref for an estimate of whole-tree sap flux. A negative linear relationship was established between stem diameter and C, the latter ranging from 0.6 to 1.0. We found that neglecting these radial corrections in 10-, 32-, 54- and 91-year-old trees would lead to overestimation of stand transpiration by 4, 14, 26 and 47%, respectively. Therefore, it is necessary to account for the differential radial profiles of sap flow in relation to tree size when comparing tree transpiration and hydraulic properties among trees differing in size.     

雷州半岛加勒比松人工林旱季液流特征与耗水量研究

利用热脉冲技术对雷州半岛25年生加勒比松人工林旱季 (2000年10月至2001年2月)树干液流特征、耗水及环境因子的关系进行研究.结果表明:加勒比松边材不同位点的液流密度由外到内呈低-高-低的态势;观测期间林分日液流密度变幅为411～4 120 L·m-2·d-1,日平均液流密度为2 084 L·m-2·d-1;旱季无雨时,加勒比松日液流密度与太阳辐射、饱和蒸汽压差、气温及湿度相关性显著;有雨时,加勒比松日液流密度仅与饱和蒸汽压及湿度相关性显著;单株日耗水量变幅为6.4～169.5 L,日平均耗水量为55.5 L,林分日耗水量变幅为0.35～3.52 mm·d-1,日均耗水量为1.71 mm·d-1,旱季总耗水量为257.7 mm,占同期降水量的72.3%.     

洞庭湖区不同立地杨树生长规律研究

于洞庭湖区的岳阳市君山区、沅江市、临澧县、常德市鼎城区,选择2008年春造林的杨树人工林,按平原、滩地、丘陵平地与丘陵坡地4种立地类型设立标准样地,对洞庭湖区不同立地杨树的生长规律进行研究。结果表明:在平原、滩地、丘陵平地、丘陵坡地等4种立地,6年生杨树林的年均树高生长量分别为2.8m、2.3 m、2.6 m、1.5 m,年均胸径生长量分别为2.6 cm、2.6 cm、2.3 cm、1.9 cm,年均单株材积生长量分别为0.024 0 m3、0.018 0 m3、0.018 0 m3、0.006 3 m3,年均单位面积蓄积生长量分别为20.0 m3/hm2、15.5m3/hm2、15.5 m3/hm2、7.8 m3/hm2;4种立地以平原地的杨树生长最好,并且立地条件越好,杨树进入快速生长期也越早;在平原,6年生杨树的胸径生长已经因林分过密而放缓,但树高、单株材积、单位面积蓄积生长量都还处于快速生长期。