Nate McDowell

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

Abstract. Variation in the carbon isotopic composition of ecosystem respiration (δ13CR) was studied for 3xa0 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 2,760xa0mm. 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) 5–10xa0days 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.

The relationship between tree height and leaf area: sapwood area ratio

Abstract. The leaf area to sapwood area ratio (Al:As) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that Al:As must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hydraulic conductance. We tested the hypothesis that Al:As declines with tree height. Whole-tree Al:As was measured on 15 individuals of Douglas-fir (Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62xa0m (aged 20–450 years). Al:As declined substantially as height increased (P=0.02). Our test of the hypothesis that Al:As declines with tree height was extended using a combination of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and significant reduction in Al:As with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in Al:As with height, although the reason for this is not clear. The slope of the relationship between Al:As and tree height (ΔAl:As/Δh) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with ΔAl:As/Δh. The decrease in Al:As with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic conductance as trees grow in height.

Age-related development of crown structure in coastal Douglas-fir trees

We compared crown structure among 20-, 40-, and 450-year-old Douglas-fir (Pseudotsuga menziesii Mirb. (Franco) var. menziesii) trees, and present a conceptual model of crown development. The model is based on the idea that the tree crown can be considered a vertical chronosequence of cohorts of branches that increase in age from upper- to lower-crown. Mean branch volume increased from upper- to lower-crown following the exponential or general logistic growth curve. Maximum branch volume occurred in the lower-crown for 20- and 40-year-old trees, while it occurred in the middle-crown for 450-year-old trees. For the 20- and 40-year-old trees, branch death did not occur in the upper-most part of the crown, and branch density decreased exponentially for the lower one-half and two-thirds of the crown, respectively. For the 450-year-old trees, branch death occurred and branch density decreased exponentially for the full extent of the crown. Epicormic branches increased branch density in the lower-crown, and moderated the rate of decrease in branch density. For the 20- and 40-year-old trees, branch diameter distributions changed from an abundance of small-diameter branches in the upper-crown, to positively skewed bimodal distributions in the middle-crown, and unimodal distributions comprised of surviving large-diameter branches in the lower-crown. For the 450-year-old trees, branch diameter distributions in the upper-crown were unimodal consisting mostly of original branches. In the middle- to lower-crown, branch diameter distributions were bimodal comprised of small-diameter epicormic branches and large-diameter original branches. For the 20- and 40-year-old trees, the relationship between mean branch volume and branch density showed two distinct phases. In the upper-crown where branch death was not observed, and mean branch volume increased with decreasing height while branch density remained relatively constant. In the middle- to lower-crown where branch death occurred, mean branch volume increased while branch density decreased exponentially with decreasing height. For the 450-year-old trees, branch death occurred, and mean branch volume increased while branch density decreased exponentially with decreasing height for the full extent of the crown. The relationship between mean branch volume and branch density after the onset of branch death defined the branch self-pruning line/curve. This relationship reflected sequential changes in the population structure of cohorts of branches growing under increasingly shady conditions as the crown grows taller and new cohorts develop above old ones. As a result of the combined effects of branch growth and death, vertical distribution of branch volume shifted toward the upper-crown with increasing tree age.

Carbon and oxygen isotope ratios of tree ring cellulose along a precipitation transect in Oregon, United States

[1]xa0The carbon and oxygen isotopic compositions of tree ring cellulose were examined for trees along a precipitation gradient in western Oregon, United States. Two years of cellulose from four sites dominated by coniferous forests ranging in precipitation from 227 to 2129 mm were sampled in conjunction with studies that measured the δ18O and δ13C of ecosystem respiration. The mean tree ring cellulose δ13C varied from −22.1 to −26.3‰ among sites and showed enrichment with decreasing water availability across the transect. The δ13C in cellulose varied across the precipitation transect in a similar pattern to the δ13C of leaf and root tissues as well as ecosystem respiration, although tree ring cellulose was enriched in 13C by over 3‰ compared to other organic matter components. The mean tree ring cellulose δ18O varied from 28.1 to 30.3‰. However, trends of cellulose δ18O change with water availability were obscured by differences in stem water δ18O. When calculated as deviation from stem water (δ18Ocellulose − δ18Ostem water) the differences in evaporative enrichment between sites was more pronounced (range of 9.6‰). The limited observed variation in tree ring cellulose δ18O of field grown trees despite large site difference in stem and leaf water δ18O across the transect agreed with predictions from a mechanistic model. Tree ring records of cellulose δ18O may provide useful proxy information regarding humidity and site water balance especially if combined with δ13C records that also vary with plant water status.

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 2. Short-term dynamics of foliar and soil component fluxes in an old-growth ponderosa pine forest

[1]xa0The oxygen isotope contents (δ18O) of soil, xylem, and leaf water and ecosystem respiration were studied in a ponderosa pine forest during summer 2001. Our goal was to assess whether δ18O of CO2 could be used to quantify the relative contributions of soil and foliar respiration to total nocturnal ecosystem respiration. The δ18O in leaf and soil water showed enrichment over a 2-week sampling period as the weather became hot and dry (leaves 0.9 to 15.0‰, and soil −10.4 to −3.1‰), while δ18O of xylem water remained constant (−12.9‰). Water in the soil was enriched in 18O near the soil surface (−6.4‰ at 5 cm depth) relative to greater depths (−11.1‰ at 20 cm). The δ18O of ecosystem respiration became gradually enriched over the 2-week sampling period (from 24.2 initially to 32.9‰ at the end, VSMOW scale). Soil respiration contributed 80 ± 12 percent to the total respiratory flux, close to estimates from scaled-up chamber data (77% [Law et al., 2001a]). Quantitative application of the isotopic approach to determine respiratory proportions required direct measurement of δ18O of soil and xylem water, air and soil temperature, and humidity. Better estimates of the isotopic signatures of component fluxes could be achieved with additional measurements and more detailed modeling. Results demonstrate that (1) there is variability in δ18O of precipitation inputs to ecosystems, (2) immediately following a precipitation event, δ18O of ecosystem respiration may reflect δ18O of precipitation, (3) periods of hot dry weather can substantially enrich ecosystem water pools and subsequently alter the isotope content of CO2 in ecosystem respiration, and (4) stable oxygen isotopes in CO2 can be used to quantify the foliar and soil components of ecosystem respiration.

Canopy carbon gain and water use: Analysis of old-growth conifers in the Pacific Northwest

This report summarizes our current knowledge of leaf-level physiological processes that regulate carbon gain and water loss of the dominant tree species in an old-growth forest at the Wind River Canopy Crane Research Facility. Analysis includes measurements of photosynthesis, respiration, stomatal conductance, water potential, stable carbon isotope values, and biogenic hydrocarbon emissions from Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata). Leaf-level information is used to scale fluxes up to the canopy to estimate gross primary production using a physiology-based process model. Both light-saturated and in situ photosynthesis exhibit pronounced vertical gradients through the canopy, but are consistently highest in Douglas-fir, intermediate in western hemlock, and lowest in western red cedar. Net photosynthesis and stomatal conductance are strongly dependent on vapor-pressure deficit in Douglas-fir, and decline through the course of a seasonal drought. Foliar respiration is similar for Douglas-fir and western hemlock, and lowest for western red cedar. Water-use efficiency varied with species and tree height, as indexed using stable carbon isotopes values for foliage. Leaf water potential is most negative for Douglas-fir and similar for western hemlock and western red cedar. Terpene fluxes from foliage equal approximately 1% of the net carbon loss from the forest. Modeled estimates based on physiological measurements show gross primary productivity (GPP) to be about 22xa0Mg C m−2 y−1. Physiological studies will be necessary to further refine estimates of stand-level carbon balance and to make long-term predictions of changes in carbon balance due to changes in forest structure, species composition, and climate.

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 1. Observations in forests along a precipitation transect in Oregon, USA

precipitation from 227 to 2760 mm. There was a gradient in the isotopic content (d 18 O) of precipitation, with inland sites receiving isotopically depleted precipitation (more negative d 18 O) relative to coastal sites. The d 18 O of water in plant xylem generally followed the isotopic pattern of precipitation. Inland forests were drier than coastal forests, leading to a gradient in the vapor pressure deficit of air that caused isotopic enrichment of soil and leaf water. The enriched soil and leaf water pools influenced the isotopic composition of respired CO2, leading to variation in observed d 18 OR (Keeling-plot intercepts). Keeling plots with non-significant (p > 0.01) regression slopes and those sampled over a time period (t) greater than 5 hours yielded unacceptably high uncertainty in d 18 OR. The range of observed d 18 OR was 21.7 to 35.3% (VSMOW), with variation within a single site as large as 10.7% (range 24.2 to 34.9% at different sites). The results suggested a trend of more positive d 18 OR at inland sites relative to those nearer the coast, indicating that fractionation due to evaporative enrichment overshadowed the original isotopic composition of precipitation as a first order control on d 18 OR. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1615 Global Change: Biogeochemical processes (4805); 1818 Hydrology: Evapotranspiration; KEYWORDS: carbon cycle, coniferous forest, OTTER, rainfall gradient, d 18 O

Does foliage on the same branch compete for the same water? Experiments on Douglas-fir trees

Abstract. Do branchlets within a branch have autonomous water supplies, or do they share a common water supply system? We hypothesized that if branchlets shared a common water supply, then stomatal conductance (gs) on sunlit foliage would increase with reduced transpiration of competing foliage on the branch. We reduced transpiration of other foliage on the branch through bagging and shading, and we monitored the gas-exchange responses of the remaining sunlit foliage on the branch relative to control branches for several age classes of Douglas-fir trees (aged ~10xa0years, 20xa0years, and 450xa0years old). Contrary to our hypothesis, we found no increases in gs in either young or old trees following transient reductions in the amount of transpiring leaf area. The diurnal change in water potential, mid-day stomatal closure and associated photosynthetic decline occurred at the same time and were of the same magnitude on both treated and untreated branches, with the exception of photosynthesis in one 450-year-old tree. Hydraulic conductance measurements of branch junctions indicate that xylem within branches is only partially interconnected which would reduce the effectiveness of shading as a means of increasing water supply to the remaining sunlit foliage. The lack of a response implies that when a branch is in partial shade, the remaining sunlit foliage has no advantage with respect to water status over foliage on a branch completely in the sun.