Examining the Stand Level CO2 Fluxes of Spring Forest Geophytes

Agricultural and Forest Meteorology

We analysed 6 years (1997–2002) of continuous eddy covariance CO2 fluxes over a mixed temperate forest in the Belgian Campine region and present the long-term mean seasonal changes in net ecosystem exchange (NEE), gross primary production (GPP) and total ecosystem respiration (RE). Seasonal changes in NEE, GPP and RE were tightly correlated, with extremes in July and December/January. Nonetheless, maximum rates of RE slightly lagged behind the maximum rates of GPP. We observed a substantial increase in NEE at optimum light conditions (reduced uptake) above 24 °C. Because this increase is unlikely to be related only to the increase in RE, we hypothesize that it also indicates a significant decrease in GPP under dry and warm meteorological conditions. Because this increase in NEE was better correlated with atmospheric vapour pressure deficit than with temperature, we further hypothesize that the observed change in NEE was predominantly related to the effect of high atmospheric evapora...

Agricultural and Forest Meteorology, 2007

This paper reports 3-year measurements (2001-2003) of the net ecosystem CO 2 exchange (NEE) over a northern Japan larch plantation forest, using an eddy covariance technique with both open-and closed-path systems. The study evaluates interannual variations in the seasonal patterns of NEE, the annual NEE, clarifying the factors controlling the carbon balance. NEE for both open-and closed-path systems look equivalent from the half-hourly mean values; however, there is a small systematic bias in the annual cumulative totals. In this study, we used closed-path data because open-path data during winter are doubtful and the annual NEE from closed-path data was closer to the biometric NEP than that from open-path data. The 3-year averages of annual NEE, gross primary production (GPP), and ecosystem respiration (RE) were À212 AE 43 (standard deviation), À1673 AE 59, and 1462 AE 42 gC m À2 y À1 , respectively. The larch plantation forest absorbed large amounts of CO 2 during just 1 month, June. During that period, photosynthetic photon flux density (PPFD) was large while the temperature was not high. These meteorological conditions were suitable for CO 2 absorption before the leaf area index (LAI) reached its peak. Maximum GPP at light saturation (P max) was also large. During the 3-year measurement period, PPFD in summer, and temperatures in spring and summer engendered remarkable interannual differences of GPP, RE, and NEE. In spring, snowmelt and larch foliation occurred about 2 weeks earlier in 2002 than in either 2001 or 2003 because of higher temperatures; consequently, the ecosystem began to photosynthesize earlier. In July, NEE was more negative in 2003 than in other years. The GPP became larger because of higher PPFD, larger P max ; while the RE became smaller because of lower temperatures. Compared with similar forests, the amounts of GPP and RE measured here were large, but NEE was similar. The reason is that the photosynthetic capacity of larch is naturally large: the ample PPFD, and the lack of environmental stresses from dry air or soil, consequently allows high photosynthesis rates to be maintained. Moreover, it can be inferred that RE values are enhanced by the high photosynthetic activity of larch forest during the growing season.

Tree Physiology, 1996

Fluxes of carbon dioxide, water vapor and energy were measured above and below a temperate broad-leaved forest and a boreal jack pine (Pinus banksiania Lamb.) forest by the eddy covariance method. The aim of the work was to examine differences between the biological and physical processes that control the fluxes of mass and energy over these disparate forest stand types. Carbon and latent heat flux (LE) densities over the temperate broad-leaved forest were about three times larger than those observed over the boreal forest. Available energy was the key variable modulating LE over the temperate broad-leaved forest, whereas LE over the boreal jack pine stand was sensitive to variations in water vapor pressure deficits (VPDs) and available energy. It was also noted that VPDs had different impacts on transpiration rates of the two forest stands. Increasing VPDs forced a negative feedback on jack pine transpiration, whereas transpiration rates of the well-watered broadleaved forest responded favorably to increasing VPDs. Carbon dioxide flux densities over the broad-leaved forest stand were more sensitive to changes in absorbed photosynthetic photon flux density than those over the boreal forest. The efficiency of CO 2 uptake over the jack pine stand was reduced, in part, because the low leaf area of the stand caused a sizable fraction of available quanta to be absorbed by nonphotosynthetic organs, such as limbs and trunks. Over both forest stands, variations in photosynthetic photon flux density of photosynthetically active radiation (Q P) explained only 50 to 60% of the variance of CO 2 exchange rates. Consequently, caution should be exercised when scaling carbon fluxes to regional scales based on unmodified, satellite-derived indices. The more open nature of the boreal jack pine forest caused water vapor, CO 2 and heat fluxes at the forest floor to be a significant component of whole canopy mass and energy exchange rates. About 20 to 30% of net canopy mass and energy exchange occurred at the forest floor. Much smaller rates of mass and energy exchange occurred under the temperate broad-leaved forest.

Canadian Journal of Forest Research, 2002

The effects of rising atmospheric carbon dioxide concentration, [CO 2 ], and fertilization on gas exchange of four field-grown tree species were examined using the branch bag technique (Picea abies (L.) Karst., Pinus sylvestris L., Fagus sylvatica L.) or whole tree chambers (Populus trichocarpa Torr. & Gray). Results are presented on changes in light-saturated rates of net photosynthesis (A sat ), carboxylation efficiency (α), stomatal conductance (g s ), and stomatal limitation of photosynthesis (L s ) after 2-4 years of CO 2 exposure. Fertilization alone did not significantly change A sat , α, g s , or L s for any of the species, but α and A sat were linearly related to foliage nitrogen content when compared across all treatments. No significant CO 2 effects were detected for α, g s , or A sat when compared at the same intercellular [CO 2 ], i.e., no downregulation of A sat was apparent. "Long-term" CO 2 enrichment increased A sat significantly by 49, 53, 86, and 114% in Populus trichocarpa, Picea abies, F. sylvatica, and Pinus sylvestris, respectively. In all the species the relative CO 2 effect on A sat increased linearly with temperature. Thus, application of a simple linear relationship could improve predictions of future tree growth responses to increasing CO 2 and temperature in cool climates.

Canadian Journal of Botany, 1976

TAYLOR, R. J., and R. W. PEARCY. 1976. Seasonal patterns of the CO, exchange characteristics of understory plants from a deciduous forest. Can. J. Bot. 54: 10941 103. The seasonal patterns of CO, exchange characteristics were determined for six deciduous forest understory species that differed substantially in time of photosynthetic activity with regard to the time of overstory canopy development. Species that were photosynthetically active only before overstory canopy development were characterized by high maximum light-saturated net photosynthetic rates and high leaf and mesophyll conductances. These early species were, however, active for only a relatively brief period of time. Species that were present during overstory canopy closure initially had moderate light-saturated net CO, uptake rates that dropped markedly in response to the reduced light intensities after overstory canopy closure. All species present during the summer under the strongly reduced light intensities had low rates of light-saturated net CO, uptake and low leaf and mesophyll conductances. These species, however. possessed low dark respiration rates which resulted in higher rates of net CO, uptake at low light intensities than those of the early species. Light-saturated photosynthetic rates were correlated with ribulose-1.5-diphosphate (RuDP) carboxylase activities in mature but not in young expanding leaves. TAYLOR, R. J., et R. W. PEARCY. 1976. SeasonaLpatterns of the CO, exchange characteristics of understory plants from a deciduous forest. Can. J. Bot. 54: 10941 103. Les auteursont determine le patron saisonnier de l'echange de CO, chez six especes forestieres ddcidues de la strate infkrieure; ces especes different de f a~o n importante quant au moment de I'activitC photosynthetique compte tenu de la pCriode a laquelle le couvert feuillu se developpe dans la strate supkrieure. Les especes dont I'activitC photosynthetique est confinbe avant la fermeture du couvert se caracterisent par des taux ClevCs de photosynthese nette sous une illumination saturante, ainsi que par une conductance elevee dans la feuille et le mesophylle. Ces especes ne sont cependant actives que pendant une periode de temps relativement courte. Les especes prksentes au moment de la fermeture du couvert montrent au depart des taux modCrCs d'absorption nette du CO, sous une illumination saturante; ces taux diminuent fortement aux intensitds lumineuses reduites apres la fermeture du couvert dans les strates supkrieures. Toutes les especes prksentes pendant I'ete, sous une intensite lumineuse fortement rCduite, montrent de faibles taux d'absorption nette du CO, sous une illumination saturante et ont de faibles conductances dans la feuille et dans le mesophylle. Ces especes, cependant, possedent de faibles tauxde respiration I'obscuritC, lesquels entrainent des taux plus ClevCs d'absorption nette du CO, aux intensites lumineuses faibles que chez les especes hitives. Les taux photosynthetiques sous une illumination saturante sont en correlation avec I'activitC de la RuDP-carboxylase chez les feuilles adultes mais non chez lesjeunes feuilles en expansion.

Forests, 2020

We investigated how reduced summer precipitation modifies photosynthetic responses of two model tree species—coniferous Norway spruce and broadleaved sessile oak—to changes in atmospheric CO2 concentration. Saplings were grown under mountainous conditions for two growing seasons at ambient (400 μmol CO2 mol–1) and elevated (700 μmol CO2 mol–1) CO2 concentration. Half were not exposed to precipitation during the summer (June–August). After two seasons of cultivation under modified conditions, basic photosynthetic characteristics including light-saturated rate of CO2 assimilation (Amax), stomatal conductance (GSmax), and water use efficiency (WUE) were measured under their growth CO2 concentrations together with in vivo carboxylation rate (VC) and electron transport rate (J) derived from CO2-response curves at saturating light. An increase in Amax under elevated CO2 was observed in oak saplings, whereas it remained unchanged or slightly declined in Norway spruce, indicating a down-reg...

Photosynthetica, 2007

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F P >10 μmol m -2 s -1 ) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 ºC) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 ºC) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO 2 fluxes were ca. -1.75 and -0.60 μmol m -2 s -1 , respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m -2 s -1 , respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 ºC) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO 2 release of ca. 0.60 and 0.20 μmol m -2 s -1 , respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing-rather than amount-of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than F P , the model applied here failed to describe daytime carbon exchange only as a function of F P and could not be used over most of the year to fill gaps in the data.

Tree Physiology

Net CO 2 exchange in a 35-year-old boreal Nor-way spruce (Picea abies (L.) Karst.) forest in northern Sweden was measured at the shoot (NSE), tree (NTE) and ecosystem levels (NEE) by means of shoot cuvettes, whole-tree chambers and the eddy covariance technique, respectively. We com-pared the dynamics of gross primary production (GPP) at the three levels during the course of a single week. The diurnal dy-namics of GPP at each level were estimated by subtracting half-hourly or hourly model-estimated values of total respira-tion (excluding light-dependent respiration) from net CO 2 ex-change. The relationship between temperature and total respi-ration at each level was derived from nighttime measurements of NSE, NTE and NEE over the course of 1 month. There was a strong linear relationship (r 2 = 0.93) between the hourly esti-mates of GPP at the shoot and tree levels, but the correlation between shoot-and ecosystem-level GPP was weaker (r 2 = 0.69). However, the correlation between sh...

Global Change Biology, 1996

We measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO2 concentrations and low- and high-nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO2] generally had smaller canopies than their current [CO2]-grown counterparts throughout most of the senescent period. This was a result of smaller pre-senescent canopies or accelerated rates of leaf drop. Leaf-level photosynthetic rates were higher under elevated [CO2] for grey birch canopies and for low-nutrient red maple and high-nutrient ash canopies, but declined rapidly to values below those of their current [CO2] counterparts by midway through the senescent period. CO2 enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO2]-grown seedlings had lower indices of whole canopy end-of-season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO2] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end-of-season estimates of water use efficiency, however, were consistently lower under CO2 enrichment, with few exceptions. These results suggest that whole canopy end-of-season gas exchange may be altered significantly in an elevated [CO2] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO2-enriched forests of the future.

Agricultural and Forest Meteorology

We studied the inter-and intra-annual dynamics of the photosynthesis of forest floor vegetation and tree canopy in a subarctic Scots pine stand at the northern timberline in Finland. We tackled the issue using three different approaches: 1) measuring carbon dioxide exchange above and below canopy with the eddy covariance technique, 2) modelling the photosynthesis of the tree canopy based on shoot chamber measurements, and 3) upscaling the forest floor photosynthesis using biomass estimates and available information on the annual cycle of photosynthetic capacity of those species. The studied ecosystem was generally a weak sink of carbon but the sink strength showed notable year-to-year variation. Total ecosystem respiration and photosynthesis indicated a clear temperature limitation for the carbon exchange. However, the increase in photosynthetic production was steeper than the increase in respiration with temperature, indicating that warm temperatures increase the sink strength and do not stimulate the total ecosystem respiration as much in the 4-year window studied. The interannual variation in the photosynthetic production of the forest stand mainly resulted from the forest floor vegetation, whereas the photosynthesis of the tree canopy seemed to be more stable from year to year. Tree canopy photosynthesis increased earlier in the spring, whereas that of the forest floor increased after snowmelt, highlighting that models for photosynthesis in the northern area should also include snow cover in order to accurately estimate the seasonal dynamics of photosynthesis in these forests.