Species selection in hardwoods research: variations in baseline physiological responses of select temperate hardwood tree species

Drought periods are becoming more extreme worldwide and the ability of plants to contribute towards atmospheric flux is being compromised. Properly functioning stomata provide an exit for water that has been absorbed by the roots, funneled into various cell parts, and eventually released into the atmosphere via transpiration. By observing the effects that weather conditions such as climate change may have on stomatal density, distribution, and functioning, it may be possible to elucidate a portion of the mechanisms trees use to survive longer periods of water stress. This study analyzed stomatal density (SD), stomatal conductance (gs ), CO2 assimilation (A), instantaneous water-use efficiency (WUEi ), and transpiration (E) rates in six native tree species in the Midwestern USA and showed that trees within the same ecotype followed similar trends, but that trees within the same family did not when exposed to identical greenhouse conditions. Naturally drought tolerant tree species demonstrated lower g s and higher WUEi , while intolerant species had higher SD. This study showed negative or no correlation between SD and g s , A, E, and WUEi and positive correlations between E and A and gs and E.     

Sap flow characteristics and whole-tree water use of Pinus cembra across the treeline ecotone of the central Tyrolean Alps

Whole-tree water use of nine Pinus cembra trees was estimated in the treeline ecotone of the Central Austrian Alps. Sap flow density using Granier-type thermal dissipation probes and environmental parameters was monitored along an elevational gradient from the forest limit up to treeline and finally mediating the krummholz limit throughout two growing seasons. Normalized sap flow density (Q _s) was significantly correlated with solar radiation (R _s) and vapor pressure deficit (D) throughout the treeline ecotone. Multiple regression analysis indicated that at the forest limit and at treeline, D had a similar effect on Q _s than R _s. At the krummholz limit by contrast, D had a greater effect on Q _s than R _s due to partially stomatal closure and wind-induced clustering of the needles, which impaired their response to available irradiance. Whole-tree water use scaled to crown surface area estimated for an entire growing (172 days) declined from 449 mm at the forest limit to 274 mm at treeline and was 251 mm at the krummholz limit, which is within the values estimated for other European forest ecosystems. Nevertheless, the observation above the forest limit in the central Tyrolean Alps tree transpiration scaled to crown surface area is comparable to the water use of adjacent low-stature vegetation, which should also be taken into account when forecasting potential effects of global change on the water balance of the treeline ecotone.     

Glycosylsphingolipids from Euonymus japonicus Thunb

The stem bark of Euonymus japonicus Thunb. led to the isolation of three new glycosylsphingolipids (1–3), 1-O-[-L-rhamnopyranosyl-(1 → 2)-β-D-glucopyranosyl]-(2S,3R,9E)-2-N-[(2R)-hydroxystearoyl]-octadecasphinga-9-ene (euojaposphingoside A, 1), 1-O-[β-D-glucopyranosyl-(1 → 2)-β-D-glucopyranosyl]-(2S,3R,4R,11E)-2-N-[(2R)-hydroxydocasanoyl]-octadecasphinga-11-ene (euojaposphingoside B, 2), 1-O-[β-D-glucopyranosyl]-2′-O-[β-D-glucopyranosyl]-(2S,3R,4R,11E)-2-N-[(2R)-hydroxytetracosanoyl]-octadecasphinga-11-ene (euojaposphingoside C, 3) along with three known glycosylsphingolipids (4–6), 1-O-[β-D-glucopyranosyl]-(2S,3R,9E)-3-hydroxymethyl-2-N-[(2R)-hydroxynonacosanoyl)-tridecasphinga-9-ene (4), 1-O-[β-D-glucopyranosyl]-(2S,3R,9E,12E)-2-N-[(2R)-hydroxytetracosanoyl] octadecasphinga-9,12–diene (5), 1-O-[β-D-glucopyranosyl]-(2S,3R,5R,9E)-2-N-[tridecanoyl] nonacosasphinga-9-ene (6), lupeol (7), stigmasterol (8), sitosterol (β and α) (9,10) and β-carotene (11). The structure of all the compounds was achieved by spectroscopic and chemical data analysis. The antiplasmodial, antileismanial and cytotoxic activity of all compounds was tested.     

Effects of forest cover types and environmental factors on soil respiration dynamics in a coastal sand dune of subtropical China

Trees on sand dunes are more sensitive to environmental changes because sandy soils have extremely low water holding capacity and nutrient availability. We investigated the dynamics of soil respiration(Rs) for secondary natural Litsea forest and plantations of casuarina,pine, acacia and eucalyptus. Results show that significant diurnal variations of Rsoccurred in autumn for the eucalyptus species and in summer for the pine species, with higher mean soil respiration at night. However, significant seasonal variations of Rswere found in all five forest stands. Rschanged exponentially with soil temperatures at the 10-cm depth; the models explain 43.3–77.0% of Rs variations. Positive relationships between seasonal Rsand soil moisture varied with stands. The correlations were significant only in the secondary forest, and the eucalyptus and pine plantations. The temperature sensitivity parameter(Q10 value) of Rsranged from 1.64 in casuarina plantation to 2.32 the in secondary forest; annual Rswas highest in the secondary forest and lowest in the pine plantation. The results indicate that soil temperatures and moisture are the primary environmental controls of soil respiration and mainly act through a direct influence on roots and microbial activity. Differences in root biomass, quality of litter,and soil properties(pH, total N, available P, and exchangeable Mg) were also significant factors.     

Effects of soil drought stress on photosynthetic gas exchange traits and chlorophyll fluorescence in <Emphasis Type="Italic">Forsythia suspensa</Emphasis>

To clarify the changes in plant photosynthesis and mechanisms underlying those responses to gradually increasing soil drought stress and reveal quantitative relationships between photosynthesis and soil moisture, soil water conditions were controlled in greenhouse pot experiments using 2-year-old seedlings of Forsythia suspensa (Thunb.) Vahl. Photosynthetic gas exchange and chlorophyll fluorescence variables were measured and analyzed under 13 gradients of soil water content. Net photosynthetic rate (P _N), stomatal conductance (g _s), and water-use efficiency (W _UE) in the seedlings exhibited a clear threshold response to the relative soil water content (R _SWC). The highest P _N and W _UE occurred at R _SWC of 51.84 and 64.10%, respectively. Both P _N and W _UE were higher than the average levels at 39.79% ≤ R _SWC ≤ 73.04%. When R _SWC decreased from 51.84 to 37.52%, P _N, g _s, and the intercellular CO₂ concentration (C _i) markedly decreased with increasing drought stress; the corresponding stomatal limitation (L _s) substantially increased, and nonphotochemical quenching (N _PQ) also tended to increase, indicating that within this range of soil water content, excessive excitation energy was dispersed from photosystem II (PSII) in the form of heat, and the reduction in P _N was primarily due to stomatal limitation. While R _SWC decreased below 37.52%, there were significant decreases in the maximal quantum yield of PSII photochemistry (F _v/F _m) and the effective quantum yield of PSII photochemistry (ΦPSII), photochemical quenching (q _P), and N _PQ; in contrast, minimal fluorescence yield of the dark-adapted state (F ₀) increased markedly. Thus, the major limiting factor for the P _N reduction changed to a nonstomatal limitation due to PSII damage. Therefore, an R _SWC of 37.52% is the maximum allowable water deficit for the normal growth of seedlings of F. suspensa, and a water content lower than this level should be avoided in field soil water management. Water contents should be maintained in the range of 39.79% ≤ R _SWC ≤ 73.04% to ensure normal function of the photosynthetic apparatus and high levels of photosynthesis and efficiency in F. suspensa.     

Impacts of understory species removal and/or addition on soil respiration in a mixed forest plantation with native species in southern China

Although the removal or addition of understory vegetation has been an important forest management practice in forest plantations, the effects of this management practice on soil respiration are unclear. The overall objective of this study was to measure and model soil respiration and its components in a mixed forest plantation with native species in south China and to assess the effects of understory species management on soil respiration and on the contribution of root respiration (R_r) to total soil respiration (R_s). An experiment was conducted in a plantation containing a mixture of 30 native tree species and in which understory plants had been removed or replaced by Cassia alata Linn. The four treatments were the control (Control), C. alata addition (CA), understory removal (UR) and understory removal with C. alata addition (UR + CA). Trenched subplots were used to quantify R_r by comparing R_s outside the 1-m² trenched subplots (plants and roots present) and inside the trenched subplots (plants and roots absent) in each treatment. Annual soil respiration were modeled using the values measured for R_s, soil temperature and soil moisture. Our results indicate that understory removal reduced R_s rate and soil moisture but increased soil temperature. Regression models revealed that soil temperature was the main factor and soil moisture was secondary. Understory manipulations and trenching increased the temperature sensitivity of R_s. Annual R_s for the Control, CA, UR and UR + CA treatments averaged 594, 718, 557 and 608 g C m⁻² yr⁻¹, respectively. UR decreased annual R_s by 6%, but CA increased R_s by about 21%. Our results also indicate that management of understory species increased the contribution of R_r to R_s.     

Incident rainfall partitioning and canopy interception modeling for an abandoned Japanese cypress stand

Interception loss (E _i) in forests has been studied widely. However, E _i parameters and modeling as well as spatial patterns of throughfall (TF) in abandoned Japanese cypress (Chamaecyparis obtusa) plantations remain poorly documented. In this study, gross precipitation (P _G), stemflow (SF), and TF were monitored in an unmanaged 32-year-old Japanese cypress stand throughout the 2011 rainy season. Results indicate that P _G partitioning into TF, SF, and E _i were, respectively, 64.2 ± 3.6, 10.6 ± 0.6, and 25.2 ± 1.1 % of the 880.8 mm cumulative P _G from 29 rainfall events. Direct throughfall proportion (p) and drainage from the canopy contributed about 14 ± 7 and 50 ± 21 % of the total TF for the events, respectively. The mean canopy storage capacity (S) was 2.4 ± 0.7 mm. The coefficient of variability (CV) of TF rate decreased asymptotically with increasing P _G amount, ranging from 16 to 56 % with median 26 %. The CV of TF rate was not significantly correlated with canopy cover (r = 0.152, P = 0.521, n = 20) and distance from the nearest trunk (r = 0.196, P = 0.408, n = 20). Based on the revised Gash analytical model, the total simulated E _i was close to the observed, with a general underestimation magnitude of 5.7 %. The E _i components were quantified, and most of the interception loss (62.9 %) evaporated during rainfall, while 26.8 % evaporated after rainfall ceased. Climatic and forest structural parameters required by the model were identified and analyzed by sensitivity analysis, implying that the revised Gash analytical model is robust and reliable enough for abandoned Japanese cypress plantations in a maritime climate.     

Heterotrophic respiration causes seasonal hysteresis in soil respiration in a warm-temperate forest

To assess the effect of changes in organic litter stock on seasonal changes in heterotrophic respiration (R _H), soil respiration (R _S), and total ecosystem respiration (R _E), we measured seasonal changes in leaf litter respiration (R _LL) by the chamber method and estimated the seasonal change in total R _H using the RothC model in a warm-temperate mixed deciduous?Cevergreen forest in Japan. Both R _E and R _S had seasonal hysteresis and were higher in spring than at the same temperature during autumn. Under warm and humid conditions, the rate of decomposition of newly supplied leaf litter in one?year was high (60% loss). Consequently, R _LL and R _H were higher in spring after leaf drop, when more fresh material was available, than in autumn. In this study, 42 and 88% of the difference in R _E and R _S between spring and autumn (soil temperature 16?C18°C) could be accounted for by the difference in R _H, respectively, and 71% of the difference in R _H could be accounted for by the difference in R _LL. This study showed that seasonal changes in heterotrophic respiration (R _LL and R _H) could be a major factor in the seasonal hysteresis of R _E and R _S.     

Robust relationships for simple plantation growth models based on sparse data

Three equations predicting height H = β₁(t − 0.5)^0.5, diameter D = β₂(H − 1.3)/ln N, and mortality dN/N = −2(G/G_max)³dD/D from plantation age (t), stocking (N) and basal area (G) can be calibrated with few data (even a single observation) for plantations in which re-measured data and growth models are unavailable. Despite having only three parameters to be estimated, these equations extrapolate reliably and allow objective forecasts of future plantation growth performance that may serve as useful first approximations until more precise growth models can be developed.     

盐胁迫下3个楸树无性系光合特征研究

以3个楸树无性系为材料进行盐胁迫试验,分析了叶片叶绿素含量、净光合速率(P_n)、气孔导度(G_s)、胞间CO₂浓度(C_i)等指标的变化规律。结果表明:随盐浓度增加,所有无性系的叶绿素a、叶绿素b及总叶绿素含量呈下降趋势;方差分析表明:处理间及无性系间差异显著(P＜0.05)。叶绿素a/叶绿素b值呈上升趋势,YQ₁无性系各盐处理与CK差异不显著,02-2-5和07-1无性系各盐处理与CK存在一定差异。从光合日变化看,3个无性系的P_n随盐浓度的升高明显降低,降幅为07-1〉YQ₁〉02-2-5,净光合速率最大值(P_max)都出现在8:00时,无性系间差异显著,且02-2-5〉YQ₁〉07-1。回归分析和通径分析表明:02-2-5和YQ₁无性系主要受胞间CO₂浓度(C_i)、叶温(T_l)、光合有效辐射(PAR)的影响,07-1无性系主要受G_s和C_i的影响。初步认为3个无性系的耐盐能力为02-2-5〉YQ₁〉07-1。     

Diversity of water use efficiency among Quercus robur genotypes: contribution of related leaf traits

? Previously, a large intra-specific diversity and a tight genetic control have been shown for Δ¹³C (carbon isotope discrimination) in a pedunculate oak (Quercus robur L.) family, which is an estimator for intrinsic water use efficiency (W _i), a complex trait defined as the ratio of net CO₂ assimilation rate (A) to stomatal conductance for water vapour (g _s ).

? In the present study, twelve genotypes with extreme phenotypic values of Δ¹³C were selected within this family to (i) asses the stability of genotype differences across contrasting environments and for different measures W _i; (ii) quantify the relationship between Δ¹³C and W_i within this family; (iii) identify which leaf traits drive the diversity in W _i observed in this family.

? Genetic variability of Δ¹³C and W _i was largely independent from different temporal integration scales and their correlation was found to be strong (R ² = 88% for leaf sugars) within this family.

? Weak correlations between measures of W _i with estimators of photosynthetic capacity, suggest a minor role of the latter in the diversity of W _i.

? However, the tight correlation between g _s and Δ¹³C as well as W _i, and the related genotypic variation in stomatal density, suggest that the genotypic diversity in W _i within this pedunculate oak family might be due to differences in g _s .

     

Soil water affects transpiration response to rainfall and vapor pressure deficit in poplar plantation

Influences of environmental factors on transpiration are interactive. Sensitivities of transpiration responses to both evaporative demand and rainfall under contrasting soil water conditions constitute the physiological basis of the drought tolerance of trees. Such knowledge is practically significant for plantation management, especially for irrigation management. We therefore conducted a 6-year study on the transpiration of a poplar plantation in temperate China to elucidate the existence and pattern of the influence of the soil water over stand transpiration responses to (1) vapor pressure deficit (VPD), the major indicator of air dryness and (2) the rainfall, in terms of total amount and event size. The results showed that the response of plantation transpiration (E _c ) to VPD was conditioned by soil moisture. There was a significant difference in the frequency distribution of maximum sap flux under contrasting soil relative extractable water. E _c after rainfall of different sizes varied under similar VPD. The increasing occurrences of only large rainfall events led to enhanced total E _c during the growing season, but prolonged rainless intervals did not lead to a continuous decrease of E _c , suggesting appreciable supplements from the soil water were present to sustain transpiration. In addition, the balance of soil water between replenishment and extraction also conditioned the influence of rainfall over subsequent E _c during the respective rainless intervals. Based on the E _c responses to VPD and rainfall under different soil moisture levels, irrigation that directly replenishes the deep soil layers in order to alleviate water stress on transpiration during the small-rain event-dominated growing season is an effective and water-saving approach to guarantee trees survival during drought period.     

Transpiration along an age series of Eucalyptus globulus plantations in southeastern Australia

Many of the world's Eucalyptus plantations are grown on short rotations of 15 years or less, which often covers the most rapid phase of stand development and peaks in growth rates and leaf areas. Since transpiration is related to stand leaf area these short rotations that make use of rapid early growth rates, may also maximise plantation water use, which has implications for predicting their water requirements and impacts on catchment hydrology. This study examined the transpiration, leaf area and growth rates of Eucalyptus globulus Labill. plantations aged 2–8 years. Transpiration (E), estimated using the heat pulse technique, increased from 0.4 mm day⁻¹ at age 2 years to a peak of about 1.6–1.9 mm day⁻¹ in stands aged 5–7 years. This was associated with similar trends for stand leaf area index (LAI) and periodic annual increments of aboveground biomass, which both peaked at about age 4–6 years resulting in a linear relationship between E and LAI. While stand sapwood areas were continuing to increase at age 8 years, E was already declining due to reductions in sap velocity, from 13.5 cm h⁻¹ at age 2 years to 6.3 cm h⁻¹ at age 8 years and reduced sapwood area growth rates. Trees compensated for this reduction in sap velocity with declines in the leaf area (A_L) to sapwood area (A_S) relationship (A_L:A_S) with age. There was also a reduction in growth efficiency (aboveground biomass increment per LAI) with age. However, reductions in WUE were small after age 4 years, which explained the linear relationship between E and LAI. If E continues to decline successive short rotation lengths may not only make use of rapid early growth rates but could also increase plantation water use compared to longer rotations over the same period of time.     

Comparison of gas exchanges between in situ and abscised leaves of high arbor trees: a case study of Cylobalanopsis glauca under three habitats

It has been difficult to monitor the gas exchanges of tall arbor trees using portable equipment, e.g. Li-6400, as it is hard to reach to the leaf samples in situ. Due to the tree heights, we aimed to compare the gas exchange between in situ and abscised leaves of Cylobalanopsis glauca under three habitats, with the purpose of identifying a possible methodology with which we might be able to use abscised rather than in situ leaves. The results showed that after the samples were abscised, the leaf stomatal conductance (g _s) immediately increased before dropping gradually later. The extent of this change was found to be temperature-dependent. The linear relationships either between gs and transpiration (E), or between g _s and net photosynthesis (P _n) were significant but the former had a higher correlation, indicating that the opening of the stomata has a more intensive effect on E than on Pn. Temperature is a key factor affecting the variation of gas exchanges of the abscised leaves, with higher temperature in the karst area resulting in a rapid E loss and leading to the fact that water stress occurs earlier to the part stomatal closure, which in turn decreases P _n. A reliable duration for measuring the gas exchanges from the abscised leaves is determined by leaf temperature. In the karst area where the leaf temperature is frequently over 32°C in the summer, the reliable duration can last only 3–6 min after abscission, while in the Guilin Botanical Garden (non-karst area) with leaf temperatures of about 32°C, the reliable duration can last 10 min; in the green house, it is at around 20°C, and the reliable duration will possibly last 20 min.     

Na2SO4胁迫对沙枣幼苗生长和光合生理的影响

采用盆栽控制试验,研究了不同浓度(0、60、120和180 mmol·L-1)Na2SO4胁迫对沙枣幼苗生长和光合特性的影响。结果表明:(1)盐胁迫对沙枣幼苗生长具有显著的抑制效应。不同浓度Na2SO4胁迫沙枣的株高、侧枝数、总叶面积、单株叶片数、比叶面积以及各组织(除根)生物量均显著低于对照,且均随盐胁迫浓度的升高呈下降趋势,而根冠比值则由对照的0.153 1显著增加到180 mmol·L-1Na2SO4胁迫幼苗的0.348 7。(2)盐胁迫显著降低了沙枣幼苗的光合能力。随着Na2SO4胁迫的加剧,净光合速率(Pn)、气孔导度(Gs)、胞间CO2浓度(Ci)和蒸腾速率(Tr)均呈下降的趋势,而气孔限制值(Ls)和水分利用效率(WUE)则依次增加,且Pn下降主要受气孔限制;180 mmol·L-1Na2SO4胁迫沙枣幼苗的Pn、Gs、Ci和Tr分别为对照的71.57%、30.85%、67.15%和51.65%,而Ls和WUE则分别为对照的1.91、1.38倍。(3)盐胁迫强度与幼苗株高、总叶面积、单株叶片数、比叶面积、茎生物量、叶生物量、总生物量等生长指标以及Pn、Gs、Ci、Tr等光合参数呈极显著负相关,叶片的光合参数与总叶面积、单株叶片数呈显著或极显著正相关,而叶片的生长指标、光合参数与幼苗的株高生长和生物量累积也呈显著或极显著正相关。     

Stand-scale transpiration estimates in a Moso bamboo forest: II. Comparison with coniferous forests

In western Japan, Moso bamboo (Phyllostachys pubescens) forests have been expanding by replacing surrounding vegetation such as coniferous plantation forests and natural broadleaved forests. It has been speculated that the replacement of surrounding vegetation by bamboo forests could alter the vegetation water cycle and available water resources. We quantified stand-scale transpiration (E) in a bamboo forest on the basis of sap-flux measurements and compared the E value with values for coniferous forests. The annual E was estimated to be 567 mm. Seasonal trends in E primarily corresponded to seasonal trends in the vapor pressure deficit. Annual E for the bamboo forest was higher than that for the coniferous forests by 12% of annual precipitation (P). This difference in annual E is comparable with the difference in annual interception evaporation (I) relative to P between bamboo and coniferous forests; previous studies reported lower I for bamboo forests by ∼10% of P. Thus, the sum of E and I was comparable for bamboo and coniferous forests. As this study is the first measuring E of bamboo forests, further studies are required to examine the generality of our results.     

Dynamic modulus of elasticity and bending properties of large beams of Taiwan-grown Japanese cedar from different plantation spacing sites

The effect of plantation spacings (types A-E) on the bending strength and dynamic modulus of elasticity of 41-year-old Taiwan-grown cedar (Cryptomeria japonica D. Don) was investigated. The results indicate that the highest values for the static bending modulus of elasticity (MOE), modulus of rupture (MOR), and dynamic modulus of elasticity (E_D, E _Dt) occurred in trees obtained from those most densely planted (type A); there was a significant difference between type A and the other four spacing types (B, C, D, and E), but there were no significant differences among those four types. Interrelations among MOE, MOR, E _D, and E _Dt could be represented by positive linear regression formulas, which revealed highly significant differences. The relations among the square value of stress-wave transmission velocity (V_t ² and V_t ²⁾ and MOE, MOR, E _Dl, and E _Dt, respectively, could be represented by positive linear regression formulas. The differences were highly significant.Part of this report was presented at the International Wood Engineering Conference '96, New Orleans, LA, USA, October 1996     

黄河小浪底库区山地栓皮栎人工林土壤呼吸的季节动态

[目的]分离并量化土壤自养呼吸和异养呼吸,探讨各自贡献率及其随季节变化的动态特征。[方法]采用壕沟法和气体红外分析法,研究黄河小浪底库区山地栓皮栎人工林土壤总呼吸、自养呼吸和异养呼吸速率的季节动态变化、贡献率和环境影响因子。[结果]表明:栓皮栎人工林总土壤呼吸、自养呼吸和异养呼吸均呈夏季速率高、冬季速率低。栓皮栎土壤总呼吸、自养呼吸及异养呼吸速率与5 cm土壤温度均呈极显著指数相关,温度敏感性系数Q_(10)值大小为自养呼吸(3.40)异养呼吸(2.90)土壤总呼吸(2.45);栓皮栎土壤总呼吸、自养呼吸、异养呼吸速率与0 10 cm土壤体积含水量均显著线性相关;土壤总呼吸、自养呼吸速率与0 10 cm土壤电导率显著相关。土壤总呼吸和异养呼吸的温度敏感系数Q_(10)值均在冬季最大,夏秋季最小;而自养呼吸的Q_(10)值则呈相反的变化趋势。栓皮栎人工林自养呼吸和异养呼吸对土壤总呼吸的月贡献率为13.23%37.33%和62.67%86.76%,且自养呼吸的贡献率与土壤温度的季节变化规律相似。土壤总呼吸、异养呼吸与自养呼吸的CO2年通量分别为1 616.41、1 199.39、417.02 g·m~(-2)·a~(-1)。[结论]经过区分与定量化土壤总呼吸及其组分,确定异养呼吸为本研究区栓皮栎人工林土壤总呼吸的主要组分,作用于异养呼吸的生物与非生物因子均能显著影响整个森林生态系统表层CO_2总排放通量的大小,进一步为该研究区森林生态系统碳循环与能量流动的进一步量化研究提供参考。     

Calibrating the self-thinning frontier

Calibration of the self-thinning frontier in even-aged monocultures is hampered by scarce data and by subjective decisions about the proximity of data to the frontier. We present a simple model that applies to observations of the full trajectory of stand mean diameter across a range of densities not necessarily close to the frontier. Development of the model is based on a consideration of the slope s = ln(N_t/N_t−1)/ln(D_t/D_t−1) of a log-transformed plot of stocking N_t and mean stem diameter D_t at time t. This avoids the need for subjective decisions about limiting density and allows the use of abundant data further from the self-thinning frontier. The model can be solved analytically and yields equations for the stocking and the stand basal area as an explicit function of stem diameter. It predicts that self-thinning may be regulated by the maximum basal area with a slope of −2. The significance of other predictor variables offers an effective test of competing self-thinning theories such Yoda's −3/2 power rule and Reineke's stand density index.     

Physiological response of irrigated and non-irrigated Norway spruce trees as a consequence of drought in field conditions

Physiological reactions of 25-year-old Norway spruce (Picea abies (L.) Karst.) trees to drought were examined during 2009 vegetation period. During the second half of summer, the decrease in soil water content was observed and irrigation was applied to a group of spruce trees, while the second group was treated under natural soil drought. The response to water deficit was recorded at the level of leaf water potential (Ψ_L). However, it appears that Ψ_L plays minor role in early stomata regulation of Norway spruce as CO₂ assimilation rate (P _N) and stomatal conductance (g _S) were reduced already before water potential decrease. Leaf water potential decreased significantly only in case when soil water content was low in the long run and when transpiration losses were simultaneously relatively high. Almost complete stomatal closure even of the irrigated trees was caused by the increase in the vapour pressure deficit of the air (D) above the value of approximately 1.5?kPa. Low values of D were accompanied by partial stomata opening of drought-treated trees. In non-irrigated spruce trees, the values of P _N decreased by 35–55% in comparison with irrigated trees. No drought-induced significant changes were found either in chlorophyll and carotenoid concentration (chl a?+?b, car) or in maximal photochemical efficiency of photosystem II (F _v/F _m). High rates of sap flow (F) did not always lead to stomatal closure during midday. It?appears that high transpiration rates do not control stomatal response to D.