Sap flow rates and sapwood density are critical factors in within- and between-tree variation in CO2 efflux from stems of mature Dacrydium cupressinum trees

Measurements of CO2 efflux from stems and branches, sap velocity, and respiratory activity of excised wood cores were conducted in Dacrydium cupressinum trees that differed in diameter, age, and canopy emergence. The objective of this study was to determine if consistent linkages exist among respiratory production of CO2 within stems, xylem transport of CO2, and the rate of CO2 diffusing from stem surfaces. Stem CO2 efflux was depressed during periods of sap flow compared with the efflux rate expected for a given stem temperature and was positively correlated with sapwood density. By contrast, no significant relationships were observed between CO2 efflux and the respiratory activity of wood tissues. Between 86 and 91% of woody tissue respiration diffused to the atmosphere over a 24-h period. However, at certain times of the day, xylem transport and internal storage of CO2 may account for up to 13-38% and 12-18%, respectively, of woody tissue respiration. These results demonstrate that differences in sap flow rates and xylem anatomy are critically important for explaining within- and between-tree variation in CO2 efflux from stems.     

Relations between water content, potential and permeability in stems of conifers

Abstract. The influence of sapwood water content on the conductivity of sapwood to water was measured on stem sections of Pinus contorta. A reduction in relative water content from 100 to 90% caused permeability to fall to about 10% of the saturated value.
Pressure–volume curves of branchwood and stem sapwood of Pinus contorta and Picea sitchensis have been analysed to definè the tissue capacitance and the time constant and resistance for water movement between stored water and the functional xylem as functions of tissue water potential. Three phases in water loss were discernible. In the initial phase at high water potentials (> –0.5 MPa), the capacitance was large, the time constant long and the resistance to flow large in comparison with intermediate water potentials (−0.5 to −1.5 MPa). At still lower water potentials (−1.5 to −3.0 MPa), the time constant and resistance declined still further but the capacitance had a tendency to increase again, especially in the stemwood of Sitka spruce. Typical values in the second phase were for the time constant 5 s, for the resistance 4 × 10⁻¹³ N s m⁻⁵ and for the capacitance (change in relative water content per unit change in potential) 1×10⁻¹¹ m³ Pa⁻¹. These parameters define the availability of stored water and are being used in a dynamic model of water transport in trees.     

Components of ecosystem evaporation in a temperate coniferous rainforest, with canopy transpiration scaled using sapwood density

Here we develop and test a method to scale sap velocity measurements from individual trees to canopy transpiration (E(c)) in a low-productivity, old-growth rainforest dominated by the conifer Dacrydium cupressinum. Further, E(c) as a component of the ecosystem water balance is quantified in relation to forest floor evaporation rates and measurements of ecosystem evaporation using eddy covariance (E(eco)) in conditions when the canopy was dry and partly wet. Thermal dissipation probes were used to measure sap velocity of individual trees, and scaled to transpiration at the canopy level by dividing trees into classes based on sapwood density and canopy position (sheltered or exposed). When compared with ecosystem eddy covariance measurements, E(c) accounted for 51% of E(eco) on dry days, and 22% of E(eco) on wet days. Low transpiration rates, and significant contributions to E(eco) from wet canopy evaporation and understorey transpiration (35%) and forest floor evaporation (25%), were attributable to the unique characteristics of the forest: in particular, high rainfall, low leaf area index, low stomatal conductance and low productivity associated with severe nutrient limitation.     

桉树人工林树液流动密度随边材径向深度的变化

树液流动密度(SFD)随边材径向深度的变化对于准确估测流经边材的树液通量是非常重要的,后者又制约着Heat Pulse的应用精度.但迄今为止,只有很少的研究估计了由于SFD随径向的梯度变化而带来的误差,SFD沿树干径向分布规律的获得往往依靠对少数几棵树的观测资料.基于在广东雷州半岛对两块3～4年生桉树(Eucalyptus urophylla S.T. Blake)人工林1年的Heat Pulse观测,探讨了对来自39株立木大量观测资料的综合处理方法,发现这两个样地(纪家和河头)的林分中SFD随边材径向深度的变化可以用如下回归方程来描述:纪家: y=3.667 5x3-7.295 5x2+3.682 6x+0.567 4 (R2=0.939 1, n=80, P=0.01)河头: y=5.006 2x3-9.116 1x2+4.454 4x+0.463 4 (R2=0.806 9, n=72, P=0.01)式中:y--某一树液感应器所测得的SFD与不同深度的4个感应器所测得的SFD的平均值之比;x-某一树液感应器在边材中的深度与边材厚度之比.从形成层到心材,SFD最初有所增加,随后持续减小,但由于树木年龄很小,最大的SFD只比最小的SFD大0.33～0.36倍.     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞,对这一现象的解释存在很多争议.成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢.营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低,细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓.呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少.水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降,分配到高生长的生物量减少.树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力,包括叶片结构和生理特征的变化,叶/边材面积比降低,边材渗透性和树干储水能力的增加等.水力限制假说得到了较多的关注,对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说.但对这一假说也存在很多的争议,主要表现在:水力阻力是否确实随高度的增加而增加,水力阻力的分布,补偿机制的作用和生物量分配转变等.本文综述了树木高生长限制的4个假说以及争论的焦点,并总结了目前研究的热点问题和今后的研究方向.     

桉树人工林树液流动密度随边材径向深度的变化

树液流动密度 (SFD)随边材径向深度的变化对于准确估测流经边材的树液通量是非常重要的 ,后者又制约着HeatPulse的应用精度。但迄今为止 ,只有很少的研究估计了由于SFD随径向的梯度变化而带来的误差 ,SFD沿树干径向分布规律的获得往往依靠对少数几棵树的观测资料。基于在广东雷州半岛对两块 3～ 4年生桉树 (Euca lyptusurophyllaS .T .Blake)人工林 1年的HeatPulse观测 ,探讨了对来自 39株立木大量观测资料的综合处理方法 ,发现这两个样地 (纪家和河头 )的林分中SFD随边材径向深度的变化可以用如下回归方程来描述 :纪家 :y =3.6 6 75x3 - 7.2 95 5x2 3.6 82 6x 0 .5 6 74 (R2 =0 .9391,n =80 ,P =0 .0 1)河头 :y =5 .0 0 6 2x3 - 9.116 1x2 4.4 5 4 4x 0 .4 6 34(R2 =0 .80 6 9,n =72 ,P =0 .0 1)式中 :y———某一树液感应器所测得的SFD与不同深度的 4个感应器所测得的SFD的平均值之比 ;x—某一树液感应器在边材中的深度与边材厚度之比。从形成层到心材 ,SFD最初有所增加 ,随后持续减小 ,但由于树木年龄很小 ,最大的SFD只比最小的SFD大 0 .33～ 0 .36倍。     

Climate-driven changes in biomass allocation in pines

Future increases in air temperature resulting from human activities may increase the water vapour pressure deficit (VPD) of the atmosphere. Understanding the responses of trees to spatial variation in VPD can strengthen our ability to predict how trees will respond to temporal changes in this important variable. Using published values, we tested the theoretical prediction that conifers decrease their investment in photosynthetic tissue (leaves) relative to water‐conducting tissue in the stem (sapwood) as VPD increases. The ratio of leaf/sapwood area (A_L/A_S) decreased significantly with increasing VPD in Pinus species but not in Abies, Pseudotsuga, Tsuga and Picea, and the average A_L/A_S was significantly lower for pines than other conifers (pines: 0.17 m² cm^?2; nonpines: 0.44 m² cm^?2). Thus, pines adjusted to increasing aridity by altering above‐ground morphology while nonpine conifers did not. The average water potential causing a 50% loss of hydraulic conductivity was ?3.28 MPa for pines and ?4.52 MPa for nonpine conifers, suggesting that pines are more vulnerable to xylem embolism than other conifers. For Pinus ponderosa the decrease in A_L/A_S with high VPD increases the capacity to provide water to foliage without escalating the risk of xylem embolism. Low A_L/A_S and plasticity in this variable may enhance drought tolerance in pines. However, lower A_L/A_S with increasing VPD and an associated shift in biomass allocation from foliage to stems suggests that pines may expend more photosynthate constructing and supporting structural mass and carry less leaf area as the climate warms.     

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 Córdoba 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 relevés. 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.