Soil moisture effects on the carbon isotope composition of soil respiration.

The carbon isotopic composition (delta(13)C) of recently assimilated plant carbon is known to depend on water-stress, caused either by low soil moisture or by low atmospheric humidity. Air humidity has also been shown to correlate with the delta(13)C of soil respiration, which suggests indirectly that recently fixed photosynthates comprise a substantial component of substrates consumed by soil respiration. However, there are other reasons why the delta(13)CO(2) of soil efflux may change with moisture conditions, which have not received as much attention. Using a combination of greenhouse experiments and modeling, we examined whether moisture can cause changes in fractionation associated with (1) non-steady-state soil CO(2) transport, and (2) heterotrophic soil-respired delta(13)CO(2). In a first experiment, we examined the effects of soil moisture on total respired delta(13)CO(2) by growing Douglas fir seedlings under high and low soil moisture conditions. The measured delta(13)C of soil respiration was 4.7 per thousand more enriched in the low-moisture treatment; however, subsequent investigation with an isotopologue-based gas diffusion model suggested that this result was probably influenced by gas transport effects. A second experiment examined the heterotrophic component of soil respiration by incubating plant-free soils, and showed no change in microbial-respired delta(13)CO(2) across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired delta(13)CO(2), but they indicate that the expected impacts of photosynthetic discrimination may be similar in direction and magnitude to those from gas transport-related fractionation. Gas transport-related fractionation may operate as an alternative or an additional factor to photosynthetic discrimination to explain moisture-related variation in soil-respired delta(13)CO(2).

Bias and uncertainty of delta13CO 2 isotopic mixing models.

Patterns in the isotopic signal (stable C isotope composition; delta(13)C) of respiration (delta(13)C(R)) have led to important gains in understanding the C metabolism of many systems. Contained within delta(13)C(R) is a record of the C source mineralized, the metabolic pathway of C and the environmental conditions during which respiration occurred. Because gas samples used for analysis of delta(13)C(R) contain a mixture of CO(2) from respiration and from the atmosphere, two-component mixing models are used to identify delta(13)C(R). Measurement of ecosystem delta(13)C(R), using canopy airspace gas samples, was one of the first applications of mixing models in ecosystem ecology, and thus recommendations and guidelines are based primarily on findings from these studies. However, as mixing models are applied to other experimental conditions these approaches may not be appropriate. For example, the range in [CO(2)] obtained in gas samples from canopy air is generally less than 100 micromol mol(-1), whereas in studies of respiration from soil, foliage or tree stems, the range can span as much as 10,000 micromol mol(-1) and greater. Does this larger range in [CO(2)] influence the precision and accuracy of delta(13)C(R) estimates derived from mixing models? Does the outcome from using different regression approaches and mixing models vary depending on the range of [CO(2)]? Our research addressed these questions using a simulation approach. We found that it is important to distinguish between large (>1,000 micromol mol(-1)) and small (<100 micromol mol(-1)) ranges of CO(2) when applying a mixing model (Keeling plot or Miller-Tans) and regression approach (ordinary least squares or geometric mean regression) combination to isotopic data. The combination of geometric mean regression and the Miller-Tans mixing model provided the most accurate and precise estimate of delta(13)C(R) when the range of CO(2) is >or=1,000 micromol mol(-1).

A laboratory comparison of two methods used to estimate the isotopic composition of soil delta13CO2 efflux at steady state.

The stable isotopic composition of soil (13)CO(2) flux is important for monitoring soil biological and physical processes. While several methods exist to measure the isotopic composition of soil flux, we do not know how effective each method is at achieving this goal. To provide clear evidence of the accuracy of current measurement techniques we created a column filled with quartz sand through which a gas of known isotopic composition (-34.2 per thousand) and concentration (3,000 ppm) diffused for 7 h. We used a static chamber at equilibrium and a soil probe technique to test whether they could identify the isotopic signature of the known gas source. The static chamber is designed to identify the source gas isotopic composition when in equilibrium with the soil gas, and the soil probe method relies on a mixing model of samples withdrawn from three gas wells at different depths to identify the gas source. We sampled from ports installed along the side of the sand column to describe the isotopic and concentration gradient as well as to serve as a control for the soil probe. The soil probe produced similar isotopic and concentration values as the control ports, as well as Keeling intercepts. The static chamber at equilibrium did not identify the source gas but, when applied in a two end-member mixing model, did produce a similar Keeling intercept produced from the control ports. Neither of the methods was able to identify the source gas via the Keeling plot method probably because CO(2) profiles did not reach isotopic steady state. Our results showed that the static chamber at equilibrium should be used only with a Keeling plot approach and that the soil probe is able to provide estimates of uncertainty for the isotopic composition of soil gas as well as information pertinent to the soil profile.

Biophysical constraints on leaf expansion in a tall conifer.

The physiological mechanisms responsible for reduced extension growth as trees increase in height remain elusive. We evaluated biophysical constraints on leaf expansion in old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Needle elongation rates, plastic and elastic extensibility, bulk leaf water (Psi(L)) and osmotic (Psi(pi)) potential, bulk tissue yield threshold and final needle length were characterized along a height gradient in crowns of > 50-m-tall trees during the period between bud break and full expansion (May to June). Although needle length decreased with increasing height, there was no height-related trend in leaf plastic extensibility, which was highest immediately after bud break (2.9%) and declined rapidly to a stable minimum value (0.3%) over a 3-week period during which leaf expansion was completed. There was a significant positive linear relationship between needle elongation rates and plastic extensibility. Yield thresholds were consistently lower at the upper and middle crown sampling heights. The mean yield threshold across all sampling heights was 0.12 +/- 0.03 MPa on June 8, rising to 0.34 +/- 0.03 MPa on June 15 and 0.45 +/- 0.05 MPa on June 24. Bulk leaf Psi(pi) decreased linearly with increasing height at a rate of 0.004 MPa m(-1) during the period of most rapid needle elongation, but the vertical osmotic gradient was not sufficient to fully compensate for the 0.015 MPa m(-1) vertical gradient in Psi(L), implying that bulk leaf turgor declined at a rate of about 0.011 MPa m(-1) increase in height. Although height-dependent reductions in turgor appeared to constrain leaf expansion, it is possible that the impact of reduced turgor was mitigated by delayed phenological development with increasing height, which resulted in an increase with height in the temperature during leaf expansion.

Age-related trends in red spruce foliar plasticity in relation to declining productivity.

Phenotypic plasticity in needle morphology with increasing tree size and age was investigated by comparing four age classes of red spruce (Picea rubens Sarg.) ranging from juvenile (3-12 years old) to mature (over 100 years old). With increase in tree age there were significant increases in leaf mass per unit area (LMA), mesophyll and vascular bundle area as a percentage of total needle cross-sectional area, and stomatal density. Within the vascular bundle, both xylem cross-sectional area and tracheid lumen area increased significantly, whereas air space as a percentage of total cross-sectional area decreased. These morphological changes were associated with a significant decrease in photosynthetic capacity and stomatal conductance, and an increase in (13)C enrichment. Although both photosynthetic capacity and whole-tree conductance decreased significantly between age classes 3 and 12 years, they did not differ between age classes 53 and 127 years, even though needle (13)C enrichment was significantly greater in the 127-year age class. Thus there appear to be compensatory mechanisms that maintain photosynthetic capacity as trees increase in size and vascular complexity, which in red spruce and other species, may affect leaf hydraulic conductance. Although increased LMA may contribute to reduced photosynthetic capacity in red spruce, similar relationships are not seen in other conifers.

Using nocturnal cold air drainage flow to monitor ecosystem processes in complex terrain.

This paper presents initial investigations of a new approach to monitor ecosystem processes in complex terrain on large scales. Metabolic processes in mountainous ecosystems are poorly represented in current ecosystem monitoring campaigns because the methods used for monitoring metabolism at the ecosystem scale (e.g., eddy covariance) require flat study sites. Our goal was to investigate the potential for using nocturnal down-valley winds (cold air drainage) for monitoring ecosystem processes in mountainous terrain from two perspectives: measurements of the isotopic composition of ecosystem-respired CO2 (delta13C(ER)) and estimates of fluxes of CO2 transported in the drainage flow. To test if this approach is plausible, we monitored the wind patterns, CO2 concentrations, and the carbon isotopic composition of the air as it exited the base of a young (approximately 40 yr-old) and an old (>450 yr-old) steeply sided Douglas-fir watershed. Nocturnal cold air drainage within these watersheds was strong, deep, and occurred on more than 80% of summer nights. The depth of cold air drainage rapidly increased to tower height or greater when the net radiation at the top of the tower approached zero. The carbon isotope composition of CO2 in the drainage system holds promise as an indicator of variation in basin-scale physiological processes. Although there was little vertical variation in CO2 concentration at any point in time, we found that the range of CO2 concentration over a single evening was sufficient to estimate delta 13C(ER) from Keeling plot analyses. The seasonal variation in delta 13C(ER) followed expected trends: during the summer dry season delta 13C(ER) became less negative (more enriched in 13C), but once rain returned in the fall, delta 13C(ER) decreased. However, we found no correlation between recent weather (e.g., vapor pressure deficit) and delta 13C(ER) either concurrently or with up to a one-week lag. Preliminary estimates suggest that the nocturnal CO2 flux advecting past the 28-m tower is a rather small fraction (<20%) of the watershed-scale respiration. This study demonstrates that monitoring the isotopic composition and CO2 concentration of cold air drainage at the base of a watershed provides a new tool for quantifying ecosystem metabolism in mountainous ecosystems on the basin scale.

Developmental decline in height growth in Douglas-fir.

The characteristic decline in height growth that occurs over a tree's lifespan is often called "age-related decline." But is the reduction in height growth in aging trees a function of age or of size? We grafted shoot tips across different ages and sizes of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees to determine whether the decline in height growth is mediated by tree size or by the age of the apical meristem. We also evaluated whether reduced carbon assimilation plays an important role in height growth decline. In one experiment we cut shoot tips from old-growth, young-mature and seedling trees and grafted them onto 2-year-old graft-compatible rootstock in a seed orchard in Lebanon, Oregon. In another experiment we performed reciprocal grafts between lateral branches of old-growth trees accessible from the canopy crane at Wind River, Washington and young-mature trees in a nearby plantation. We measured growth (diameter and elongation of the dominant new stem) and mortality annually for three years in the Seed Orchard experiment and for two years in the Reciprocal Graft experiment. In the Seed Orchard experiment we also measured photosynthetic capacity (determined from the response of net carbon assimilation to the intercellular CO(2) concentration of the leaf, or A/C(i) curves), leaf mass per area (LMA) and carbon isotope composition (delta(13)C) of cellulose in 1-year-old foliage. Grafting caused changes in both growth and physiology of the grafted stems. Within two years after grafting, growth and physiology of all combinations of scions and rootstock exhibited characteristics of the rootstock. In some cases, the change in growth was dramatic-cuttings from old-growth trees showed a 10-fold increase in stem elongation rate within 2 years of grafting onto seedling rootstock. Similarly, carbon isotope discrimination of new foliage on shoots from old-growth trees increased by nearly 3 per thousand and 2 per thousand after grafting onto young-mature and seedling rootstock, respectively, whereas discrimination decreased by a similar magnitude in scions from young-mature trees after grafting on old- growth trees. Furthermore, differences in carbon assimilation estimated from carbon isotope discrimination and A/C(i )relationships were small relative to growth differences. Our results confirm that size, not age, drives developmental changes in height growth in Douglas-fir. Reduced carbon assimilation does not play an important role in height growth decline.

Contributions of evaporation, isotopic non-steady state transpiration and atmospheric mixing on the delta18O of water vapour in Pacific Northwest coniferous forests.

Changes in the 2H and 18O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio (delta 18O) as high as 4% per hundred were observed for water vapour (delta (18)Ovp) above and within an old-growth coniferous forest in the Pacific Northwest region of the United States. Values of delta 18Ovp decreased in the morning, reached a minimum at midday, and recovered to early-morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the 18O of canopy water vapour over a 2-d period by considering the 18O-isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do delta 18O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of delta 18Ovp? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted 18O-isoflux in the morning of day 1, causing values of delta 18Ovp, to decrease. An isotopically enriched 18O-isoflux resulting from transpiration then offset this decreased delta 18Ovp later during the day. Contributions of 18O-isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H2(16)O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas-fir trees as approximately 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non-steady state model for predicting delta 18O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled delta 18Ovp was clearly detectable, suggesting the importance of considering isotopic non-steady state of transpiration in studies of forest 18O water balance.

The hydraulic limitation hypothesis revisited.

We proposed the hydraulic limitation hypothesis (HLH) as a mechanism to explain universal patterns in tree height, and tree and stand biomass growth: height growth slows down as trees grow taller, maximum height is lower for trees of the same species on resource-poor sites and annual wood production declines after canopy closure for even-aged forests. Our review of 51 studies that measured one or more of the components necessary for testing the hypothesis showed that taller trees differ physiologically from shorter, younger trees. Stomatal conductance to water vapour (g(s)), photosynthesis (A) and leaf-specific hydraulic conductance (K L) are often, but not always, lower in taller trees. Additionally, leaf mass per area is often greater in taller trees, and leaf area:sapwood area ratio changes with tree height. We conclude that hydraulic limitation of gas exchange with increasing tree size is common, but not universal. Where hydraulic limitations to A do occur, no evidence supports the original expectation that hydraulic limitation of carbon assimilation is sufficient to explain observed declines in wood production. Any limit to height or height growth does not appear to be related to the so-called age-related decline in wood production of forests after canopy closure. Future work on this problem should explicitly link leaf or canopy gas exchange with tree and stand growth, and consider a more fundamental assumption: whether tree biomass growth is limited by carbon availability.

Use of the Conners-wells adolescent self-report scale (short form) with children.

The short form of the Conners-Wells Adolescent Self-Report Scale (CASS:Short) is administered to 319 children and 844 adolescents to investigate whether the instrument can be used with respondents under the age of 12. Mothers of all respondents are asked to rate each child using a measure designed to assess a parallel set of problem behaviors. The factor structure of the CASS:Short is tested in both samples and is found to have good fit to the data. Mean levels of ADHD symptomatology are also examined for possible sex and age effects, as was the relationship between parent and self-report ratings. The overall results provide empirical support for the use of the CASS:Short with children ages 7 to 11.

Environmental sensitivity of gas exchange in different-sized trees.

The carbon isotope signature (delta13C) of foliar cellulose from sunlit tops of trees typically becomes enriched as trees of the same species in similar environments grow taller, indicative of size-related changes in leaf gas exchange. However, direct measurements of gas exchange in common environmental conditions do not always reveal size-related differences, even when there is a distinct size-related trend in delta13C of the very foliage used for the gas exchange measurements. Since delta13C of foliage predominately reflects gas exchange during spring when carbon is incorporated into leaf cellulose, this implies that gas exchange differences in different-sized trees are most likely to occur in favorable environmental conditions during spring. If gas exchange differs with tree size during wet but not dry conditions, then this further implies that environmental sensitivity of leaf gas exchange varies as a function of tree size. These implications are consistent with theoretical relationships among height, hydraulic conductance and gas exchange. We investigated the environmental sensitivity of gas exchange in different-sized Douglas-fir (Pseudotsuga menziesii) via a detailed process model that specifically incorporates size-related hydraulic conductance [soil-plant-atmosphere (SPA)], and empirical measurements from both wet and dry periods. SPA predicted, and the empirical measurements verified, that differences in gas exchange associated with tree size are greatest in wet and mild environmental conditions and minimal during drought. The results support the hypothesis that annual net carbon assimilation and transpiration of trees are limited by hydraulic capacity as tree size increases, even though at particular points in time there may be no difference in gas exchange between different-sized trees. Maximum net ecosystem exchange occurs in spring in Pacific Northwest forests; therefore, the presence of hydraulic limitations during this period may play a large role in carbon uptake differences with stand-age. The results also imply that the impacts of climate change on the growth and physiology of forest trees will vary depending on the age and size of the forest.

Alexithymia in young adulthood: a risk factor for pathological gambling.

BACKGROUND: Pathological gambling is more prevalent among postsecondary students than among the general adult population. While the prevalence of pathological gambling in this group has risen over the past decade, factors underlying the development of problem gambling among university students remain largely unexplored. One early study found alexithymia to be associated with pathological gambling. The aim of the present study was to further examine the relationship between alexithymia and gambling among postsecondary students. METHODS: The relationship between alexithymia and pathological gambling was examined in 562 postsecondary students who completed the South Oaks Gambling Screen (SOGS) and the 20-item Toronto Alexithymia Scale (TAS-20). RESULTS: Approximately 12% of the sample was classified as alexithymic according to the TAS-20. These individuals were found to have significantly more gambling problems, as measured by the SOGS, than nonalexithymic individuals. Approximately 9% of the sample was classified as pathological gamblers according to the SOGS. These individuals were found to have significantly higher levels of alexithymia, as measured by the TAS-20, than nonproblem gamblers. CONCLUSIONS: Alexithymia is associated with pathological gambling and may be a risk factor among postsecondary students for developing severe gambling problems.

Structural and compositional controls on transpiration in 40- and 450-year-old riparian forests in western Oregon, USA.

Large areas of forests in the Pacific Northwest are being transformed to younger forests, yet little is known about the impact this may have on hydrological cycles. Previous work suggests that old trees use less water per unit leaf area or sapwood area than young mature trees of the same species in similar environments. Do old forests, therefore, use less water than young mature forests in similar environments, or are there other structural or compositional components in the forests that compensate for tree-level differences? We investigated the impacts of tree age, species composition and sapwood basal area on stand-level transpiration in adjacent watersheds at the H.J. Andrews Forest in the western Cascades of Oregon, one containing a young, mature (about 40 years since disturbance) conifer forest and the other an old growth (about 450 years since disturbance) forest. Sap flow measurements were used to evaluate the degree to which differences in age and species composition affect water use. Stand sapwood basal area was evaluated based on a vegetation survey for species, basal area and sapwood basal area in the riparian area of two watersheds. A simple scaling exercise derived from estimated differences in water use as a result of differences in age, species composition and stand sapwood area was used to estimate transpiration from late June through October within the entire riparian area of these watersheds. Transpiration was higher in the young stand because of greater sap flux density (sap flow per unit sapwood area) by age class and species, and greater total stand sapwood area. During the measurement period, mean daily sap flux density was 2.30 times higher in young compared with old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Sap flux density was 1.41 times higher in young red alder (Alnus rubra Bong.) compared with young P. menziesii trees, and was 1.45 times higher in old P. menziesii compared with old western hemlock (Tsuga heterophylla (Raf.) Sarg.) trees. Overall, sapwood basal area was 21% higher in the young stand than in the old stand. In the old forest, T. heterophylla is an important co-dominant, accounting for 58% of total sapwood basal area, whereas P. menziesii is the only dominant conifer in the young stand. Angiosperms accounted for 36% of total sapwood basal area in the young stand, but only 7% in the old stand. For all factors combined, we estimated 3.27 times more water use by vegetation in the riparian area of the young stand over the measurement period. Tree age had the greatest effect on stand differences in water use, followed by differences in sapwood basal area, and finally species composition. The large differences in transpiration provide further evidence that forest management alters site water balance via elevated transpiration in vigorous young stands.

An investigation of hydraulic limitation and compensation in large, old Douglas-fir trees.

The hydraulic limitation hypothesis (Ryan and Yoder 1997) proposes that leaf-specific hydraulic conductance (kl) and stomatal conductance (gs) decline as trees grow taller, resulting in decreased carbon assimilation. We tested the hydraulic limitation hypothesis by comparison of canopy-dominant Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) trees in stands that were approximately 15 m (20 years old), 32 m (40 years old) and 60 m (> 450 years old) tall in Wind River, Washington, USA. Carbon isotope discrimination (Delta) declined with tree height (18.6, 17.6 and 15.9 per thousand for stands 15, 32 and 60 m tall, respectively) indicating that gs may have declined proportionally with tree height in the spring months, when carbon used in the construction of new foliage is assimilated. Hydraulic conductance decreased by 44% as tree height increased from 15 to > 32 m, and showed a further decline of 6% with increasing height. The general nonlinear pattern of kl versus height was predicted by a model based on Darcy's Law. Stemwood growth efficiency also declined nonlinearly with height (60, 35 and 28 g C m-2 leaf area for the 15-, 32- and 60-m stands, respectively). Unlike kl and growth efficiency, gs and photosynthesis (A) during summer drought did not decrease with height. The lack of decline in cuvette-based A indicates that reduced A, at least during summer months, is not responsible for the decline in growth efficiency. The difference between the trend in gs and A and that in kl and D may indicate temporal changes (spring versus summer) in the response of gas exchange to height-related changes in kl or it may be a result of measurement inadequacies. The formal hydraulic limitation hypothesis was not supported by our mid-summer gs and A data. Future tests of the hydraulic limitation hypothesis in this forest should be conducted in the spring months, when carbon uptake is greatest. We used a model based on Darcy's Law to quantify the extent to which compensating mechanisms buffer hydraulic limitations to gas exchange. Sensitivity analyses indicated that without the observed increases in the soil-to-leaf water potential differential (DeltaPsi) and decreases in the leaf area/sapwood area ratio, kl would have been reduced by more than 70% in the 60-m trees compared with the 15-m trees, instead of the observed decrease of 44%. However, compensation may have a cost; for example, the greater DeltaPsi of the largest trees was associated with smaller tracheid diameters and increased sapwood cavitation, which may have a negative feedback on kl and gs.

Effects of branch height on leaf gas exchange, branch hydraulic conductance and branch sap flux in open-grown ponderosa pine.

Recent studies have shown that stomata respond to changes in hydraulic conductance of the flow path from soil to leaf. In open-grown tall trees, branches of different heights may have different hydraulic conductances because of differences in path length and growth. We determined if leaf gas exchange, branch sap flux, leaf specific hydraulic conductance, foliar carbon isotope composition (delta13C) and ratios of leaf area to sapwood area within branches were dependent on branch height (10 and 25 m) within the crowns of four open-grown ponderosa pine (Pinus ponderosa Laws.) trees. We found no difference in leaf gas exchange or leaf specific hydraulic conductance from soil to leaf between the upper and lower canopy of our study trees. Branch sap flux per unit leaf area and per unit sapwood area did not differ between the 10- and 25-m canopy positions; however, branch sap flux per unit sapwood area at the 25-m position had consistently lower values. Branches at the 25-m canopy position had lower leaf to sapwood area ratios (0.17 m2 cm-2) compared with branches at the 10-m position (0.27 m2 cm-2) (P = 0.03). Leaf specific conductance of branches in the upper crown did not differ from that in the lower crown. Other studies at our site indicate lower hydraulic conductance, sap flux, whole-tree canopy conductance and photosynthesis in old trees compared with young trees. This study suggests that height alone may not explain these differences.

Aging in Pacific Northwest forests: a selection of recent research.

This paper provides an overview of the work reported at a symposium on age-related changes in the structure and function of forests in the United States Pacific Northwest. Some of the work presented at this meeting is reported in the peer-reviewed papers comprising this journal issue. Age-related changes in leaf structure, CO2 assimilation rate, stable carbon isotope ratio, nitrogen concentration and stomatal limitation were demonstrated at many organizational scales. At larger scales, age-related changes were reported in canopy structure and light profile, stand productivity, tree mortality and respiration. These data raise new questions about the potential interaction among the structural and functional changes in aging forests, and indicate many avenues for future research concerning tree growth and ecosystem functioning.

Needle anatomy changes with increasing tree age in Douglas-fir.

Morphological differences between old-growth trees and saplings of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) may extend to differences in needle anatomy. We used microscopy with image analysis to compare and quantify anatomical parameters in cross sections of previous-year needles of old-growth Douglas-fir trees and saplings at the Wind River Canopy Crane site in Washington and at three sites in the Cascade Mountains of Oregon. We also compared needle anatomy across a chronosequence of 10-, 20-, 40- and 450-year-old Douglas-fir trees from the Wind River site. Anatomy differed significantly between needles of old-growth trees and saplings at all sites, suggesting a developmental change in needle anatomy with increasing tree age. Compared with needles of old-growth trees, needles of saplings were longer and had proportionately smaller vascular cylinders, larger resin canals and few hypodermal cells. Astrosclereids, which sequester lignin in their secondary cell walls and occupy space otherwise filled by photosynthetic cells, were scarce in needles of saplings but abundant in needles of old-growth trees. Needles of old-growth trees had an average of 11% less photosynthetic mesophyll area than needles of saplings. The percentage of non-photosynthetic area in needles increased significantly with increasing tree age from the chronosequence of 10-, 20-, 40- and 450-year-old trees at the Wind River site. This reduction in photosynthetic area may contribute to decreased growth rates in old trees.

Biophysical controls of carbon flows in three successional Douglas-fir stands based on eddy-covariance measurements.

We measured net carbon flux (F(CO2)) and net H2O flux (F(H2O)) by the eddy-covariance method at three Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)-western hemlock (Tsuga heterophylla (Raf.) Sarg.) sites located in the Wind River Valley of southern Washington State, USA. Stands were approximately 20, 40 and 450 years old and measurements were made between June 15 and October 15 of 1998 in the 40- and 450-year-old stands, and of 1999 in the 20- and 450-year-old stands. Our objectives were to determine if there were differences among the stands in: (1) patterns of daytime F(CO2) during summer and early autumn; (2) empirically modeled relationships between local climatic factors (e.g., light, vapor pressure deficit (VPD), soil water content, temperature and net radiation) and daytime F(CO2); and (3) water-use efficiency (WUE). We used the Landsberg equation, a logarithmic power function and linear regression to model relationships between F(CO2) and physical variables. Overall, given the same irradiance, F(CO2) was 1.0-3.9 mol m-2 s-1 higher (P < 0.0001 for both seasons) at the two young stands than at the old-growth stand. During summer and early autumn, F(CO2) averaged 4.2 and 6.1 mol m-2 s-1 at the 20- and 40-year-old stand, respectively. In contrast, the 450-year-old forest averaged 2.2 and 3.2 mol m-2 s-1 in 1998 and 1999, respectively. Increases in VPD were associated with reduced F(CO2) at all three stands, with the greatest apparent constraints occurring at the old-growth stand. Correlations between F(CO2) and all other environmental variables differed among ecosystems, with soil temperature showing a negative correlation and net radiation showing a positive correlation. In the old-growth stand, WUE was significantly greater (P < 0.0001) in the drier summer of 1998 (2.7 mg g-1) than in 1999 (1.0 mg g-1). Although we did not use replicates in our study, the results indicate that there are large differences in F(CO2) among Douglas-fir stands of different ages growing in the same general area, and that variations in age structure and site conditions need to be considered when scaling flux measurements from individual points to the landscape level.

Canopy and hydraulic conductance in young, mature and old Douglas-fir trees.

We tested for reductions in water transport with increasing tree size, a key component in determining whether gas exchange and growth are hydraulically limited in tall trees. During the summers of 1998 and 1999, we measured water transport with Granier-type, constant-heat sap flow probes, vapor pressure deficit, and leaf and soil water potentials in overstory Pseudotsuga menziesii (Mirb.) Franco trees in three stands differing in size and age (15, 32 and 60 m in height and about 20, 40 and 450 years in age, respectively) in a P. menziesii-dominated forest in the Pacific Northwest, USA. A total of 24 trees were equipped with sap flow sensors--six 60-m trees, nine 32-m trees and nine 15-m trees. Based on the sap flow measurements and leaf area information estimated from leaf area-sapwood area relationships, we estimated crown-averaged stomatal conductance (GS) and leaf-specific hydraulic conductance (KL). We tested the hypothesis that GS and KL vary inversely with tree height (15 > 32 > 60 m). Analysis of variance of GS ranked as 15 = 60 > 32 m during the early summer and 15 > 60 > 32 m during late season drought. Over the growing season, mean daily GS (+/- SE) was 29.2 +/- 4.4, 24.0 +/- 6.8 and 17.7 +/- 7.2 mmol m-2 s-1 for the 15-, 60- and 32-m trees, respectively. The value of K(L) differed among tree heights only during late season drought and ranked 15 > 32 = 60 m. A hydraulic mass balance suggests that greater sapwood conductivity in 60-m trees compared with 32- and 15-m trees is a likely cause for the departure of the above rankings from those predicted by height and leaf-to-sapwood area ratio.

13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit.

Variation in the carbon isotopic composition of ecosystem respiration (δ13CR) was studied for 3  years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall δ13CR varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual δ13CR differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between δ13CR and the vapor saturation deficit of air (vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the δ13CR versus vpd relationship, resulting in higher than expected δ13CR values.