J. Irvine

Spatial and temporal variation in respiration in a young ponderosa pine forest during a summer drought

Respiration rates of heterogeneous forest canopies arise from needles, stems, roots and soil microbes. To assess the temporal and spatial variation in respiration rates of these components in a heterogeneous ponderosa pine forest canopy, and the processes that control these fluxes, we conducted an intensive field study during the summer of 2000. We employed a combination of biological and micrometeorological measurements to assess carbon respiratory fluxes at the soil surface, within and above a 4-m-tall ponderosa pine forest. We also conducted manipulation studies to examine the carbon fluxes from the roots and heteorotrophs. Spatial variation in soil CO2 efflux was large, averaging 40% of the mean, which varied by nearly a factor of two between minima for bare soil to maxima beneath dense patches of understorey vegetation. The estimated vertical profile of respiration from chamber data, and the profile of nocturnal fluxes measured by the three eddy flux systems suggested that >70% of the ecosystem respiration was coming from below the 1.75-m measurement height of one of the flux systems, and 71% of photosynthetic carbon uptake in July was released by soil processes, thus there was a strong vertical gradient in respiration relatively close to the soil surface in this young forest. These results stress the importance of understanding spatial and temporal variation in soil processes when interpreting nocturnal eddy covariance data.

Seasonal differences in carbon and water vapor exchange in young and old-growth ponderosa pine ecosystems

Eddy covariance measurements of carbon dioxide and water vapor exchange were made above a young and an old-growth ponderosa pine (Pinus ponderosaDougl. ex P. & C. Laws) ecosystem located in a semiarid environment in central Oregon. The old-growth stand (O site) is a mixture of 250- and 50-year-old ponderosa pine trees with no significant understory (summer maximum leaf area index (LAI) (m 2 half-surface area foliage per m 2 ground) is 2.1). The young stand (Y site; 15 years old in 2000), about 10 km southeast of the old stand, is naturally regenerating following the clear-cut of an old stand in 1978 and has at present about 40% of its LAI in understory shrubs (summer maximum LAI of 1.0). Even though climatic conditions at both sites were very similar, ecosystem carbon exchange differed substantially between the two ecosystems. The old-growth forest with about two times the LAI of the young site, had higher carbon assimilation rates per unit ground area than the young forest, with trends similar between the two forests in spring and fall. Deviations from the trend occurred during summer when water stress in trees at the young site led to a significant reduction in transpiration, and consequently carbon assimilation due to stomatal limitations. Throughout the year, ecosystem respiration ( Re) and gross ecosystem production (GEP) were generally greater at the O site than Y site, and the net of these two processes resulted in a lower net carbon uptake at the Y site.

Climatic versus biotic constraints on carbon and water fluxes in seasonally drought‐affected ponderosa pine ecosystems

We investigated the relative importance of climatic versus biotic controls on gross primary production (GPP) and water vapor fluxes in seasonally drought-affected ponderosa pine forests. The study was conducted in young (YS), mature (MS), and old stands (OS) over 4 years at the AmeriFlux Metolius sites. Model simulations showed that interannual variation of GPP did not follow the same trends as precipitation, and effects of climatic variation were smallest at the OS ( 50%), and intermediate at the YS (<20%). In the young, developing stand, interannual variation in leaf area has larger effects on fluxes than climate, although leaf area is a function of climate in that climate can interact with age-related shifts in carbon allocation and affect whole-tree hydraulic conductance. Older forests, with well-established root systems, appear to be better buffered from effects of seasonal drought and interannual climatic variation. Interannual variation of net ecosystem exchange (NEE) was also lowest at the OS, where NEE is controlled more by interannual variation of ecosystem respiration, 70% of which is from soil, than by the variation of GPP, whereas variation in GPP is the primary reason for interannual changes in NEE at the YS and MS. Across spatially heterogeneous landscapes with high frequency of younger stands resulting from natural and anthropogenic disturbances, interannual climatic variation and change in leaf area are likely to result in large interannual variation in GPP and NEE.

Response of the carbon isotopic content of ecosystem, leaf, and soil respiration to meteorological and physiological driving factors in a Pinus ponderosa ecosystem

applications of isotope-based models of the global carbon budget as well as for understanding ecosystem-level variation in isotopic discrimination (D). Discrimination may be strongly dependent on synoptic-scale variation in environmental drivers that control canopy-scale stomatal conductance (Gc) and photosynthesis, such as atmospheric vapor pressure deficit (vpd) photosynthetically active radiation (PAR) and air temperature (Tair). These potential relationships are complicated, however, due to time lags between the period of carbon assimilation and ecosystem respiration, which may extend up to several days, and may vary with tissue (i.e., leaves versus belowground tissues). Our objective was to determine if relationships exist over a short-term period (2 weeks) between meteorological and physiological driving factors and d 13 CR and its components, soil-respired d 13 C( d 13 CR-soil) and foliage-respired d 13 C( d 13 CR-foliage). We tested for these hypothesized relationships in a 250-year-old ponderosa pine forest in central Oregon, United States. A cold front passed through the region 3 days prior to our first sample night, resulting in precipitation (total rainfall 14.6 mm), low vpd (minimum daylight average of 0.36 kPa) and near-freeze temperature (minimum air temperature of 0.18� C± 0.3� C), followed by a warming trend with relatively high vpd (maximum daylight average of 3.19 kPa). Over this 2-week period Gc was negatively correlated with vpd (P < 0.01) while net ecosystem CO2 exchange (NEE) was positively correlated with vpd (P < 0.01), consistent with a vpd limitation to conductance and net CO2 uptake. Consistent with a stomatal influence over D, a negative correlation was observed between d 13 CR and Gc measured 2 days prior (i.e., a 2-day time lag, P = 0.04); however, d 13 CR was not correlated with other measured variables. Also consistent with a stomatal influence over discrimination, d 13 CR-soil was negatively correlated with Gc (P < 0.01) and positively correlated with vpd and PAR measured one to 3 days prior (P = 0.01 and 0.04, respectively). In contrast, d 13 CR-foliage was not correlated with vpd or Gc, but was negatively correlated with minimum air temperature measured 5 days previously (P < 0.01) supporting the idea that cold air temperatures cause isotopic enrichment of respired CO2. The significant driving parameters differed for d 13 CR-foliage and d 13 CR-soil potentially due to different controls over the isotopic content of tissue-specific respiratory fluxes, such as differing carbon transport times from the site of assimilation to the respiring tissue or different reliance on recent versus old photosynthate. Consistent with Gc control over photosynthesis and D, both d 13 CR-soil and d 13 CR-foliage became enriched as net CO2 uptake decreased (more positive NEE, P < 0.01 for both). The d 13 C value of Pinus ponderosa foliage (� 27.1%, whole-tissue) was 0.5 to 3.0% more negative than any observed respiratory signature, supporting the contention that foliage d 13 C can be a poor proxy for the isotopic content of respiratory fluxes. The strong meteorological controls

High-frequency analysis of the complex linkage between soil CO 2 fluxes, photosynthesis and environmental variables

High-frequency soil CO(2) flux data are valuable for providing new insights into the processes of soil CO(2) production. A record of hourly soil CO(2) fluxes from a semi-arid ponderosa pine stand was spatially and temporally deconstructed in attempts to determine if variation could be explained by logical drivers using (i) CO(2) production depths, (ii) relationships and lags between fluxes and soil temperatures, or (iii) the role of canopy assimilation in soil CO(2) flux variation. Relationships between temperature and soil fluxes were difficult to establish at the hourly scale because diel cycles of soil fluxes varied seasonally, with the peak of flux rates occurring later in the day as soil water content decreased. Using a simple heat transport/gas diffusion model to estimate the time and depth of CO(2) flux production, we determined that the variation in diel soil CO(2) flux patterns could not be explained by changes in diffusion rates or production from deeper soil profiles. We tested for the effect of gross ecosystem productivity (GEP) by minimizing soil flux covariance with temperature and moisture using only data from discrete bins of environmental conditions (±1 °C soil temperature at multiple depths, precipitation-free periods and stable soil moisture). Gross ecosystem productivity was identified as a possible driver of variability at the hourly scale during the growing season, with multiple lags between ~5, 15 and 23 days. Additionally, the chamber-specific lags between GEP and soil CO(2) fluxes appeared to relate to combined path length for carbon flow (top of tree to chamber center). In this sparse and heterogeneous forested system, the potential link between CO(2) assimilation and soil CO(2) flux may be quite variable both temporally and spatially. For model applications, it is important to note that soil CO(2) fluxes are influenced by many biophysical factors, which may confound or obscure relationships with logical environmental drivers and act at multiple temporal and spatial scales; therefore, caution is needed when attributing soil CO(2) fluxes to covariates like temperature, moisture and GEP.