Interspecific variation of photosynthesis and leaf characteristics in canopy trees of five species of Dipterocarpaceae in a tropical rain forest

Photosynthetic rate, nitrogen concentration and morphological properties of canopy leaves were studied in 18 trees, comprising five dipterocarp species, in a tropical rain forest in Sarawak, Malaysia. Photosynthetic rate at light saturation (Pmax) differed significantly across species, varying from 7 to 18 micro mol m(-2) s(-1). Leaf nitrogen concentration and morphological properties, such as leaf blade and palisade layer thickness, leaf mass per area (LMA) and surface area of mesophyll cells per unit leaf area (Ames/A), also varied significantly across species. Among the relationships with leaf characteristics, Pmax had the strongest correlation with leaf mesophyll parameters, such as palisade cell layer thickness (r2 = 0.76, P < 0.001) and Ames/A (r2 = 0.73, P < 0.001). Leaf nitrogen concentration and Pmax per unit area also had a significant but weaker correlation (r2 = 0.46, P < 0.01), whereas Pmax had no correlation, or only weakly significant correlations, with leaf blade thickness and LMA. Shorea beccariana Burck, which had the highest P(max) of the species studied, also had the thickest palisade layer, with up to five or more layers. We conclude that interspecific variation in photosynthetic capacity in tropical rain forest canopies is influenced more by leaf mesophyll structure than by leaf thickness, LMA or leaf nitrogen concentration.     

Diurnal changes in leaf gas exchange characteristics in the uppermost canopy of a rain forest tree, Dryobalanops aromatica Gaertn. f

Dryobalanops aromatica Gaertn. f. is a major tropical canopy species in lowland tropical rain forests in Peninsular Malaysia. Diurnal changes in net photosynthetic rate (A) and stomatal conductance to water vapor (g(s)) were measured in fully expanded young and old leaves in the uppermost canopy (35 m above ground). Maximum A was 12 and 10 micro mol m(-2) s(-1) in young and old leaves, respectively; however, because of large variation in A among leaves, mean maximum A in young and old leaves was only 6.6 and 5.5 micro mol m(-2) s(-1), respectively. Both g(s) and A declined in young leaves when T(leaf) exceeded 34 degrees C and leaf-to-air vapor pressure deficit (DeltaW) exceeded 0.025, whereas in old leaves, g(s) and A did not start to decline until T(leaf) and DeltaW exceeded 36 degrees C and 0.035, respectively. Under saturating light conditions, A was linearly related to g(s). The coefficient of variation (CV) for the difference between the CO(2) concentrations of ambient air and the leaf intercellular air space (C(a) - C(i)) was smaller than the CV for A or g(s), suggesting that maximum g(s) was mainly controlled by mesophyll assimilation (A/C(i)). Minimum C(i)/C(a) ratios were relatively high (0.72-0.73), indicating a small drought-induced stomatal limitation to A and non-conservative water use in the uppermost canopy leaves.     

Sap-flux-scaled transpiration responses to light,vapor pressure deficit,and leaf area reduction in a flooded Taxodium distichum forest

We used 20-mm-long, Granier-type sensors to quantify the effects of tree size, azimuth and radial position in the xylem on the spatial variability in xylem sap flux in 64-year-old trees of Taxodium distichum L. Rich. growing in a flooded forest. This information was used to scale flux to the stand level to investigate variations in half-hourly and daily (24-hour) sums of sap flow, transpiration per unit of leaf area, and stand transpiration in relation to vapor pressure deficit (D) and photosynthetically active radiation (Q(o)). Measurements of xylem sap flux density (J(s)) indicated that: (1) J(s) in small diameter trees was 0.70 of that in medium and large diameter trees, but the relationship between stem diameter as a continuous variable and J(s) was not significant; (2) J(s) at 20-40 mm depth in the xylem was 0.40 of that at 0-20 mm depth; and (3) J(s) on the north side of trees was 0.64 of that in directions 120 degrees from the north. Daily transpiration was linearly related to daily daytime mean D, and reached a modest value of 1.3 mm day(-1), reflecting the low leaf area index (LAI = 2.2) of the stand. Because there was no soil water limitation, half-hourly water uptake was nearly linearly related to D at D < 0.6 kPa during both night and day, increasing to saturation during daytime at higher values of D. The positive effect of Q(o) on J(s) was significant, but relatively minor. Thus, a second-order polynomial with D explained 94% of the variation in J(s) and transpiration. An approximately 40% reduction in LAI by a hurricane resulted in decreases of about 18% in J(s) and stand transpiration, indicating partial stomatal compensation.     

Photosynthesis of a tropical canopy tree, Ceiba pentandra, in a lowland forest in Panama

Diel (24 h) courses of CO(2) and water-vapor exchange of Ceiba pentandra (L.) Gaertn. (Bombacaceae) were studied under natural tropical conditions in the semi-evergreen moist forest of Barro Colorado Island, Panama. Measurements were conducted from early February 1991 (dry season), shortly after new leaves emerged, until mid-October 1991 (wet season), when leaves were shed. Rates of net CO(2) uptake were significantly higher in the dry season than in the wet season, and showed a linear decrease with leaf age. Leaf nitrogen concentrations and contents also decreased with age. Our estimate of annual carbon gain (2640 g CO(2) m(-2) year(-1) or 21 g CO(2) g(DW) (-1) year(-1)) is considerably higher than estimates available for temperate forest trees.     

Leaf CO(2) exchange of Erythrina poeppigiana (Leguminosae: Phaseolae) in humid tropical field conditions

An idealized model was developed to describe leaf CO(2) exchange in the leguminous tree Erythrina poeppigiana (Walpers) O.F. Cook under well-watered field conditions. Photosynthetic rate in mature leaves (p) was modeled as a rectangular hyperbolic function of photon flux density (q) and ambient CO(2) concentration (c(a)), relative photosynthetic capacity (pi) was modeled as a logistic s-function of leaf age (l(a)), metabolic dark respiration rate (r(m)) was modeled as an exponential function of leaf temperature (T(l)), and photorespiration rate (r(p)) was modeled as a hyperbolic function of c(a). Assimilation rate (a(c)) was modeled as the difference between the product of p and pi and the sum of r(m) and r(p): a(c) = p(q,c(a))pi(l(a)) - [r(m)(T(l)) + r(p)(c(a))]. The model parameters were estimated separately for five sources of E. poeppigiana (Clones 2660, 2662, 2687 and 2693 and half-sib Family 2431) from field data measured with a portable closed-loop gas exchange system at a humid tropical site in Costa Rica. The between-source differences in leaf CO(2) exchange characteristics were small, but statistically significant. Aboveground biomass production was highest in sources that maintained high relative photosynthetic capacity throughout the leaf life span. Quantum yield varied between 0.046 and 0.067, and light-saturated assimilation rate (q = 2000 micro mol m(-2) s(-1) and T(l) = 28 degrees C) at natural atmospheric c(a) (350 micro mol mol(-1)) was 16.8-19.9 micro mol m(-2) s(-1). Increasing c(a) to 1000 micro mol mol(-1) resulted in an approximate doubling of the light-saturated assimilation rate. Foliole nitrogen concentration, which was 45.3-51.2 mg g(-1) in mature leaves, was positively correlated with relative photosynthetic capacity. Foliole nitrogen concentration, quantum yield and maximum assimilation rate of E. poeppigiana are among the highest values observed in tropical woody legumes.     

Diurnal patterns of water use in Eucalyptus victrix indicate pronounced desiccation-rehydration cycles despite unlimited water supply

Knowledge about nocturnal transpiration (E(night)) of trees is increasing and its impact on regional water and carbon balance has been recognized. Most of this knowledge has been generated in temperate or equatorial regions. Yet, little is known about E(night) and tree water use (Q) in semi-arid regions. We investigated the influence of atmospheric conditions on daytime (Q(day)) and nighttime water transport (Q(night)) of Eucalyptus victrix L.A.S. Johnson & K.D. Hill growing over shallow groundwater (not >1.5 m in depth) in semi-arid tropical Australia. We recorded Q(day) and Q(night) at different tree heights in conjunction with measurements of stomatal conductance (g(s)) and partitioned E(night) from refilling processes. Q of average-sized trees (200-400 mm diameter) was 1000-3000 l month(-1), but increased exponentially with diameter such that large trees (>500 mm diameter) used up to 8000 l month(-1). Q was remarkably stable across seasons. Water flux densities (J(s)) varied significantly at different tree heights during day and night. We show that g(s) remained significantly different from zero and E(night) was always greater than zero due to vapor pressure deficits (D) that remained >1.5 kPa at night throughout the year. Q(night) reached a maximum of 50% of Q(day) and was >0.03 mm h(-1) averaged across seasons. Refilling began during afternoon hours and continued well into the night. Q(night) eventually stabilized and closely tracked D(night). Coupling of Q(night) and D(night) was particularly strong during the wet season (R2?=?0.95). We suggest that these trees have developed the capacity to withstand a pronounced desiccation-rehydration cycle in a semi-arid environment. Such a cycle has important implications for local and regional hydrological budgets of semi-arid landscapes, as large nighttime water fluxes must be included in any accounting.     

The effect of aqueous transport of CO(2) in xylem sap on gas exchange in woody plants

The influence of CO(2) transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO(2) partial pressure in the gas phase within the woody stems of trees. High CO(2) partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO(2) concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO(2) in the xylem sap was calculated by multiplying sap CO(2) concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 &mgr;mol CO(2) m(-2) s(-1), representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO(2) on stem gas exchange, we made simultaneous measurements of stem CO(2) efflux and sap flow on the same stem. After removing the effect of temperature, stem CO(2) efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 &mgr;mol m(-2) s(-1), representing ~12% of peak stem respiration rates.     

Environmental controls over isoprene emission in deciduous oak canopies

In summer 1992, isoprene emission was measured on intact leaves and branches of Quercus alba (L.) at two heights in a forest canopy. Isoprene emission capacity (measured at 30 degrees C and a photosynthetic photon flux density of 1000 micro mol m(-2) s(-1)) was significantly higher in sun leaves than in shade leaves when expressed on a leaf area basis (51 versus 31 nmol m(-2) s(-1); P < 0.01). Because leaf mass per unit area (LMA, g m(-2)) was higher in sun leaves than in shade leaves, emissions of sun and shade leaves expressed on a dry mass basis did not differ significantly (99 versus 89 micro g C g(DW) (-1) h(-1); P = 0.05). Similar measurements in 1995 were consistent with the 1992 data, but data from leaves in more shaded locations demonstrated that isoprene emission capacity decreased with decreasing growth irradiance, irrespective of units of expression. Isoprene emission capacity in leaves of Q. coccinea Muenchh. and Q. velutina Lam. also declined steeply with canopy depth. Emission capacity, on a dry mass basis, showed no obvious pattern with canopy position in Q. prinus L. There was no difference in the temperature response of sun versus shade leaves of Q. alba, but shade leaves exhibited a greater quantum efficiency and saturated at lower irradiance than sun leaves. Rates of isoprene emission measured on branches of Q. alba were approximately 60% of those measured on individual leaves, as a result of self-shading within branch enclosures. It is recommended that within-canopy variation in isoprene emission capacity be incorporated into regional emission models.     

欧亚大陆地上森林凋落物的研究

根据一个包括471个林分的森林凋落物数据库,本文对欧亚大陆森林生态系统的凋落物产量进行了估计。结果表明:欧亚大陆森林生态系统凋落物产量为每年6.53 Pg (1 Pg = 1015g),其中二分之一发生在热带和亚热带森林,三分之一发生在寒带森林,剩余的发生在温带森林。因此,在欧亚大陆森林生态系统中,森林凋落物每年从森林植被转移大约2.94 Pg 碳到土壤亚系统。表3参18。 关键词:碳流动;欧亚大陆森林;森林凋落物     

Diurnal patterns of leaf photosynthesis, conductance and water potential at the top of a lowland rain forest canopy in Cameroon: measurements from the Radeau des Cimes

Diurnal patterns of leaf conductance, net photosynthesis and water potential of five tree species were measured at the top of the canopy in a tropical lowland rain forest in southwestern Cameroon. Access to the 40 m canopy was by a large canopy-supported raft, the Radeau des Cimes. The measurements were made under ambient conditions, but the raft altered the local energy balance at times, resulting in elevated leaf temperatures. Leaf water potential was equal to or greater than the gravitational potential at 40 m in the early morning, falling to values as low as -3.0 MPa near midday. Net photosynthesis and conductance were typically highest during midmorning, with values of about 10-12 micro mol CO(2) m(-2) s(-1) and 0.2-0.3 mol H(2)O m(-2) s(-1), respectively. Leaf conductance and net photosynthesis commonly declined through midday with occasional recovery late in the day. Photosynthesis was negatively related to leaf temperature above midday air temperature maxima. These patterns were similar to those observed in other seasonally droughted evergreen communities, such as Mediterranean-climate shrubs, and indicate that environmental factors may cause stomatal closure and limit photosynthesis in tropical rain forests during the midday period.     

Photosynthetic responses of cottonwood seedlings grown in glacial through future atmospheric [CO2] vary with phosphorus supply

Plants often exhibit proportionately larger photosynthetic responses to the transition from glacial to modern [CO(2)] than from modern to future [CO(2)]. Although this pattern may reflect increased nutrient demand with increasing [CO(2)], few studies have examined the role of nutrient supply in regulating responses to the range of [CO(2)] from glacial to future [CO(2)]. In this study, we examined the effects of P supply (0.004-0.5 mM) on photosynthetic responses of Populus deltoides (cottonwood) seedlings to glacial (200 micromol mol(-1)), modern (350 μmol mol(-1)) and future (700 micromol mol(-1)) [CO(2)]. The A(sat) (light-saturated net photosynthetic rates at the growth [CO(2)]) response to future [CO(2)] decreased with decreasing P supply such that there was no response at the lowest P supply. However, P supply did not affect A(sat) responses to an increase from glacial to modern [CO(2)]. Photosynthetic capacity [e.g., final rubisco activity, apparent, maximal Rubisco-limited rate of photosynthesis (V(cmax)), apparent, maximal electron transport-limited rate of photosynthesis (J(max))], stomatal conductance (g(s)) and leaf P generally increased with increasing P supply but decreased with increasing [CO(2)]. Measures of carbohydrate sink capacity (e.g., leaf mass per unit leaf area, leaf starch) increased with both increasing P supply and increasing [CO(2)]. Changes in V(cmax) and g(s) together accounted for 78% of the variation in A(sat) among [CO(2)] and P treatments, suggesting significant biochemical and stomatal controls on photosynthesis. However, A(sat) responses to increasing [CO(2)] did not reflect the changes in the carbohydrate sink capacity. These results have important implications because low P already constrains responses to increasing [CO(2)] in many ecosystems, and our results suggest that the P demand will increasingly affect A(sat) in cottonwood as [CO(2)] continues to increase.     

Effects of mutual shading of tree crowns on prediction of photosynthetic light-use efficiency in a coastal Douglas-fir forest

Gross primary production (GPP) is often expressed as the product of absorbed photosynthetically active radiation and the efficiency (epsilon) with which a plant community uses absorbed radiation in biomass production. Light-use efficiency is affected by environmental stresses, and varies diurnally and seasonally. Uncertainty about epsilon can be a serious limitation when modeling GPP. An important determinant of epsilon is the amount and type of solar radiation incident on a canopy, because an abundance of light can trigger a photo-protective reaction, diminishing GPP. The radiation regime in a forest canopy is determined by the predominant sky conditions and by mutual shading of tree crowns. Shading effects, producing shifts in the amount of incident direct and diffuse solar radiation, have been largely ignored, however, because they depend on forest structure and are difficult to measure. We describe a new approach for estimating changes in mutual canopy shading throughout the day and year based on a canopy structure model derived from light detection and ranging (LiDAR). Proportions of canopy shading were then combined with eddy covariance data to assess the explanatory power for variance in epsilon by regression tree analysis over half-hourly, daily and weekly time scales. The approach explained between 75 and 97% of variance in epsilon, representing an increase of between 5 and 16% compared with models driven solely by meteorological variables.     

Physiological adjustment of two full-sib families of ponderosa pine to elevated CO(2)

Seeds from two full-sib families of ponderosa pine (Pinus ponderosa) with known differences in growth rates were germinated and grown in an ambient (350 micro l l(-1)) or elevated (700 micro l l(-1)) CO(2) concentration. Gas exchange at both ambient and elevated CO(2) concentrations was measured 1, 6, 39, and 112 days after the seed coat was shed. Initial stimulation of CO(2) exchange rate (CER) by elevated CO(2) was large (> 100%). On Day 1, CER of seedlings grown in elevated CO(2) and measured at ambient CO(2) was significantly lower than the CER of seedlings grown and measured at ambient CO(2), indicating physiological adjustment of the seedlings exposed to elevated CO(2). Physiological acclimation to elevated CO(2) was complete by Day 39 when there was no significant difference in CER between seedlings grown and measured at ambient CO(2) and seedlings grown and measured at elevated CO(2). After 4 months, the light response of seedlings in the two treatments was determined at both ambient and elevated CO(2). Light compensation point, CER at light saturation, and apparent quantum efficiency of seedlings grown and measured at ambient CO(2) were not significantly different from those of seedlings grown and measured at elevated CO(2). With a short-term increase in CO(2), CER at light saturation (5.16 +/- 0.52 versus 3.13 +/- 0.30 micro mol CO(2) m(-2) s(-1)) and apparent quantum efficiency (0.082 +/- 0.011 versus 0.045 +/- 0.003 micro mol CO(2) micro mol(-1) quanta) were significantly increased. Leaf C/N ratio was significantly increased in the elevated CO(2) treatment. There were few significant differences between families for any response to elevated CO(2). Under the experimental conditions, high growth rate was not correlated with a greater response to elevated CO(2).     

Is elevation of carbon dioxide concentration beneficial to seedling photosynthesis in the understory of tropical rain forests?

Cuttings of the southeast Asian tropical rain forest tree species, Pongamia pinnata (L.) Pierre were raised in growth chambers providing a photosynthetic photon flux density (PPFD) of 60 micromol m(-2) s(-1) with either a low or a high red:far-red light ratio (LR and HR, repectively). The chambers were supplied with air containing CO(2) at a concentration of either 400 (LR4 and HR4, respectively) or 800 micromol mol(-1) (HR8 and LR8, respectively). After 4 months, leaf morphology and photosynthetic characteristics were determined. Relative to HR4, the LR4 treatment increased leaf area and total chlorophyll concentration (Chl) by 24 and 25%, respectively, but reduced leaf mass per unit area (LMA) by 19%. Elevated [CO(2)] significantly increased leaf area and LMA but did not affect Chl of LR or HR plants. Leaf nitrogen concentration was unaffected by the red:far-red light ratio but decreased significantly in seedlings in the elevated [CO(2)] treatment. Photosynthesis measured in situ under the growth conditions of ambient light and [CO(2)] (A(amb)) was 30% lower on an area basis and 14% lower on a mass basis in LR4 plants than in HR4 plants. Elevated [CO(2)] reduced the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and thus decreased light-saturated photosynthetic rate in both HR and LR plants. Elevated [CO(2)] increased mean leaf area and decreased respiration rates in both LR and HR plants. The LR8 plants had significantly higher A(amb) than LR4 plants, but similar A(amb) to HR8 plants.     

Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings

To study the effects of elevated CO2 concentration ([CO2]) on relationships between nitrogen (N) nutrition and foliar gas exchange parameters, white birch (Betula papyrifera Marsh.) seedlings were exposed to one of five N-supply regimes (10, 80, 150, 220, 290 mg N l(-1)) in either ambient [CO2] (360 micromol mol(-1)) or elevated [CO2] (720 micromol mol(-1)) in environment-controlled greenhouses. Foliar gas exchange and chlorophyll fluorescence were measured after 60 and 80 days of treatment. Photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated [CO2] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO2], elevated [CO2] increased the overall rate of photosynthesis (P(n)) and instantaneous water-use efficiency (IWUE) by up to 69 and 236%, respectively, but decreased transpiration (E) and stomatal conductance (g(s)) in all N treatments. However, the degree of stimulation of photosynthesis by elevated [CO2] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO2] significantly increased total photosynthetic electron transport in all N treatments at 60 days of treatment, but the effect was insignificant after 80 days of treatment. Both P(n) and IWUE generally increased with increasing leaf N concentration except at very high leaf N concentrations, where both P(n) and IWUE declined. The relationships of P(n) and IWUE with leaf N concentration were modeled with both a linear regression and a second-order polynomial function. Elevated [CO2] significantly and substantially increased the slope of the linear regression for IWUE, but had no significant effect on the slope for P(n). The optimal leaf N concentration for P(n) and IWUE derived from the polynomial function did not differ between the CO2 treatments when leaf N was expressed on a leaf area basis. However, the mass-based optimal leaf N concentration for P(n) was much lower in seedlings in elevated [CO2] than in ambient [CO2] (31.88 versus 37.00 mg g(-1)). Elevated [CO2] generally decreased mass-based leaf N concentration but had no significant effect on area-based leaf N concentration; however, maximum N concentration per unit leaf area was greater in elevated [CO2] than in ambient [CO2] (1.913 versus 1.547 g N m(-2)).     

Limitation of leaf carbon gain by stomatal and photochemical processes in the top canopy of Macaranga conifera,a tropical pioneer tree

Diurnal changes in gas exchange and chlorophyll fluorescence were measured in the top canopy leaves of the tropical rainforest tree species, Macaranga conifera (Zoll.) Muell. Arg. during a drought year. Maximum values of net photosynthetic rate (P(n), 10 &mgr;mol m(-2) s(-1)) and stomatal conductance (g(s), 0.2 mol m(-2) s(-1)) were found in east-facing leaves in early morning. After 1000 h, both P(n) and g(s) decreased. Minimum daytime values of P(n), g(s), and photosystem II (PSII) quantum yield (DeltaF/F(m)') were found in horizontally fixed leaves. At a given electron transport rate through PSII (ETR), P(n) was higher in early morning than at midday, suggesting a high rate of photorespiration at midday. We tested the hypothesis that the effect of low leaf temperature (T(leaf)) on P(n) is significant in the early morning, whereas the effect of low g(s) on P(n) predominates at midday. In the early morning, when T(leaf) was increased from 32 to 38 degrees C by artificial heating, P(n) at a given ETR decreased 29%, suggesting that the low T(leaf) was associated with a high P(n). When T(leaf) at midday was decreased from 37 to 32 degrees C by artificial cooling, P(n) increased 22%, but P(n) at a given ETR was higher in early morning than at midday, even at the same low T(leaf) (32 degrees C). This suggests that the rate of photorespiration was higher at midday than in early morning because low g(s) at midday caused a reduction in leaf intercellular CO(2) concentration. We conclude that low P(n) at midday was the result of both a reduction in the photochemical process and an increase in stomatal limitation.     

A simple model of light and water use evaluated for Pinus radiata

An existing model of light and water use by crops (RESCAP) was adapted and evaluated for trees. In the model, growth on any given day is determined either by the amount of intercepted radiation (by means of the light utilization coefficient, epsilon) or by the maximum rate of water extraction by roots (a function of root biomass and soil water content). In either case, transpiration and growth are related by the water-use efficiency (q), which is inversely proportional to the daily mean saturation vapor pressure deficit (D). The model was applied to two Pinus radiata (D. Don) stands (control (C) and fertilized (F)) growing near Canberra, Australia, using data collected during the Biology of Forest Growth experiment (1983-1988). For both stands, predicted and measured soil water contents were in close agreement (r(2) > 0.9) over a 4-year period involving several wet-dry cycles. The parameter combination epsilon/qD was estimated to be 0.28 and 0.26 kg H(2)O (MJ total)(-1) kPa(-1) for the C and F stands, respectively. Because of the close physiological link between water use and CO(2) uptake, the results suggest that tree growth may be realistically simulated by simple models based on conservative values for epsilon and qD.     

Photosynthesis and light-use efficiency by plants in a Canadian boreal forest ecosystem

Measurements of the photosynthetic response to midsummer irradiance were made for 11 species representing the dominant trees, understory shrubs, herbaceous plants and moss species in an old black spruce (Picea mariana (Mill.) B.S.P.) boreal forest ecosystem. Maximum rates of photosynthesis per unit foliage area at saturating irradiance, A(max), were highest for aspen (Populus tremuloides Michx.), reaching 16 micromol m(-2) s(-1). For tamarack (Larix laricina (Du Roi) K. Kock) and P. mariana, Amax was only 2.6 and 1.8 micromol m(-2) s(-1), respectively. Values of A(max) for understory shrubs and herbaceous plants were clustered between 9 and 11 micromol m(-2) s(-1), whereas A(max) of feather moss (Pleurozium schreberi (Brid.) Mitt.) reached only 1.9 micromol m(-2) s(-1). No corrections were made for differences in shoot structure, but values of photosynthetic light-use efficiency were similar for most species (70-80 mmol CO2 mol(-1)); however, they were much lower for L. laricina and P. mariana (15 mmol CO2 mol(-1)) and much higher for P. schreberi (102 m;mol CO2 mol(-1)). There was a linear relationship between Amax and foliage nitrogen concentration on an area basis for the broad-leaved species in the canopy and understory, but the data for P. mariana, L. laricina and P. schreberi fell well below this line. We conclude that it is not possible to scale photosynthesis from leaves to the canopy in this ecosystem based on a single relationship between photosynthetic rate and foliage nitrogen concentration.     

Component and whole-system respiration fluxes in northern deciduous forests

We measured component and whole-system respiration fluxes in northern hardwood (Acer saccharum Marsh., Tilia americana L., Fraxinus pennsylvanica Marsh.) and aspen (Populus tremuloides Michx.) forest stands in Price County, northern Wisconsin from 1999 through 2002. Measurements of soil, leaf and stem respiration, stem biomass, leaf area and biomass, and vertical profiles of leaf area were combined with biometric measurements to create site-specific respiration models and to estimate component and whole-system respiration fluxes. Hourly estimates of component respiration were based on site measurements of air, soil and stem temperature, leaf mass, sapwood volume and species composition. We also measured whole-system respiration from an above-canopy eddy flux tower. Measured soil respiration rates varied significantly among sites, but not consistently among dominant species (P < 0.05 and P > 0.1). Annual soil respiration ranged from 8.09 to 11.94 Mg C ha(-1) year(-1). Soil respiration varied linearly with temperature (P < 0.05), but not with soil water content (P > 0.1). Stem respiration rates per unit volume and per unit area differed significantly among species (P < 0.05). Stem respiration per unit volume of sapwood was highest in F. pennsylvanica (up to 300 micro mol m(3) s(-1)) and lowest in T. americana (22 micro mol m(3) s(-1)) when measured at peak summer temperatures (27 to 29 degrees C). In northern hardwood stands, south-side stem temperatures were higher and more variable than north-side temperatures during leaf-off periods, but were not different statistically during leaf-on periods. Cumulative annual stem respiration varied by year and species (P < 0.05) and averaged 1.59 Mg C ha(-1) year(-1). Leaf respiration rates varied significantly among species (P < 0.05). Respiration rates per unit leaf mass measured at 30 degrees C were highest for P. tremuloides (38.8 nmol g(-1) s(-1)), lowest for Ulmus rubra Muhlenb. (13.1 nmol g(-1) s(-1)) and intermediate and similar (30.2 nmol g(-1) s(-1)) for T. americana, F. pennsylvanica and Q. rubra. During the growing season, component respiration estimates were dominated by soil respiration, followed by leaf and then stem respiration. Summed component respiration averaged 11.86 Mg C ha(-1) year(-1). We found strong covariance between whole-ecosystem and summed component respiration measurements, but absolute rates and annual sums differed greatly.     

Sapwood temperature gradients between lower stems and the crown do not influence estimates of stand-level stem CO(2) efflux

Temperature plays a critical role in the regulation of respiration rates and is often used to scale measurements of respiration to the stand-level and calculate annual respiratory fluxes. Previous studies have indicated that failure to consider temperature gradients between sun-exposed stems and branches in the crown and shaded lower stems may result in errors when deriving stand-level estimates of stem CO(2) efflux. We measured vertical gradients in sapwood temperature in a mature lowland podocarp rain forest in New Zealand to: (1) estimate the effects of within-stem temperature variation on the vertical distribution of stem CO(2) efflux; and (2) use these findings to estimate stand-level stem CO(2) efflux for this forest. Large within-stem gradients in sapwood temperature (1.6 +/- 0.1 to 6.0 +/- 0.5 degrees C) were observed. However, these gradients did not significantly influence the stand-level estimate of stem CO(2) efflux in this forest (536 +/- 42 mol CO(2) ha(-1) day(-1)) or the vertical distribution of stem CO(2) efflux, because of the opposing effects of daytime warming and nighttime cooling on CO(2) efflux in the canopy, and the small fraction of the woody biomass in the crowns of forest trees. Our findings suggest that detailed measurements of within-stand temperature gradients are unlikely to greatly improve the accuracy of tree- or stand-level estimates of stem CO(2) efflux.