Intermittent low temperatures constrain spring recovery of photosynthesis in boreal Scots pine forests

During winter and early spring, evergreen boreal conifers are severely stressed because light energy cannot be used when photosynthesis is pre‐empted by low ambient temperatures. To study photosynthetic performance dynamics in a severe boreal climate, seasonal changes in photosynthetic pigments, chloroplast proteins and photochemical efficiency were studied in a Scots pine forest near Zotino, Central Siberia. In winter, downregulation of photosynthesis involved loss of chlorophylls, a twofold increase in xanthophyll cycle pigments and sustained high levels of the light stress‐induced zeaxanthin pigment. The highest levels of xanthophylls and zeaxanthin did not occur during the coldest winter period, but rather in April when light was increasing, indicating an increased capacity for thermal dissipation of excitation energy at that time. Concomitantly, in early spring the D1 protein of the photosystem II (PSII) reaction centre and the light‐harvesting complex of PSII dropped to their lowest annual levels. In April and May, recovery of PSII activity, chloroplast protein synthesis and rearrangements of pigments were observed as air temperatures increased above 0°C. Nevertheless, severe intermittent low‐temperature episodes during this period not only halted but actually reversed the physiological recovery. During these spring low‐temperature episodes, protective processes involved a complementary function of the PsbS and early light‐induced protein thylakoid proteins. Full recovery of photosynthesis did not occur until the end of May. Our results show that even after winter cold hardening, photosynthetic activity in evergreens responds opportunistically to environmental change throughout the cold season. Therefore, climate change effects potentially improve the sink capacity of boreal forests for atmospheric carbon. However, earlier photosynthesis in spring in response to warmer temperatures is strongly constrained by environmental variation, counteracting the positive effects of an early recovery process.     

Photoinhibition of photosynthesis in intact bean leaves: role of light and temperature,and requirement for chloroplast-protein synthesis during recovery

Photoinhibition of photosynthesis was induced in intact leaves of Phaseolus vulgaris L. grown at a photon flux density (PFD; photon fluence rate) of 300 mol·m^-2·s^-1, by exposure to a PFD of 1400 mol·m^-2·s^-1. Subsequent recovery from photoinhibition was followed at temperatures ranging from 5 to 35°C and at a PFD of either 20 or 140 mol·m^-2·s^-1 or in complete darkness. Photoinhibition and recovery were monitored mainly by chlorophyll fluorescence emission at 77K but also by photosynthetic O₂ evolution. The effects of the protein-synthesis inhibitors, cycloheximide and chloramphenicol, on photoinhibition and recovery were also determined. The results demonstrate that recovery was temperature-dependent with rates slow below 15°C and optimal at 30°C. Light was required for maximum recovery but the process was light-saturated at a PFD of 20 mol·m^-2·s^-1. Chloramphenicol, but not cycloheximide, inactivated the repair process, indicating that recovery involved the synthesis of one or more chloroplast-encoded proteins. With chloramphenicol, it was shown that photoinhibition and recovery occurred concomitantly. The temperature-dependency of the photoinhibition process was, therefore, in part determined by the effect of temperature on the recovery process. Consequently, photoinhibition is the net difference between the rate of damage and the rate of repair. The susceptibility of chilling-sensitive plant species to photoinhibition at low temperatures is proposed to result from the low rates of recovery in this temperature range.Abbreviations and symbols Da Dalton - Fo, Fm, Fv instantaneous, maximum, variable fluorescence emission - PFD photon flux density - PSII photosystem II - photon yield C.I.W.-D.P.B. Publication No. 871     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: effect of growth temperature on photoinhibition and recovery

Intact leaves of kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) from plants grown in a range of controlled temperatures from 15/10 to 30/25°C were exposed to a photon flux density (PFD) of 1500 μmol·m⁻²·s⁻¹ at leaf temperatures between 10 and 25°C. Photoinhibition and recovery were followed at the same temperatures and at a PFD of 20 μmol·m⁻²·s⁻¹, by measuring chlorophyll fluorescence at 77 K and 692 nm, by measuring the photon yield of photosynthetic O₂ evolution and light-saturated net photosynthetic CO₂ uptake. The growth of plants at low temperatures resulted in chronic photoinhibition as evident from reduced fluorescence and photon yields. However, low-temperature-grown plants apparently had a higher capacity to dissipate excess excitation energy than leaves from plants grown at high temperatures. Induced photoinhibition, from exposure to a PFD above that during growth, was less severe in low-temperature-grown plants, particularly at high exposure temperatures. Net changes in the instantaneous fluorescence,F ₀, indicated that little or no photoinhibition occurred when low-temperature-grown plants were exposed to high-light at high temperatures. In contrast, high-temperature-grown plants were highly susceptible to photoinhibitory damage at all exposure temperatures. These data indicate acclimation in photosynthesis and changes in the capacity to dissipate excess excitation energy occurred in kiwifruit leaves with changes in growth temperature. Both processes contributed to changes in susceptibility to photoinhibition at the different growth temperatures. However, growth temperature also affected the capacity for recovery, with leaves from plants grown at low temperatures having moderate rates of recovery at low temperatures compared with leaves from plants grown at high temperatures which had negligible recovery. This also contributed to the reduced susceptibility to photoinhibition in low-temperature-grown plants. However, extreme photoinhibition resulted in severe reductions in the efficiency and capacity for photosynthesis.     

Requirements for obtaining maximum indices of photosynthesis and transpiration in attached leaves of willow plants grown in short-rotation forest

The results of multiyear studies of gas exchange in intact attached leaves of several willow species (Salix sp.) were analyzed. Measurements were performed with a portable Li-6400 infrared gas analyzer both on plants in their natural environment and on rooted cuttings grown in a greenhouse. Individual attached leaves were placed into the leaf chamber where climatic conditions were either similar to or different from those outside the chamber. The maximal rates of net photosynthesis (P _n) and transpiration (E) were only observed with the provision that the environmental variables inside and outside the chamber were identical. On rainy or cloudy days, the P _n and E values observed under optimum conditions inside the leaf chamber were lower than their potential maxima by 12–18% and 35–45%, respectively. Deviation of temperature in the chamber by 5–7°C from the external level and fluctuations of ambient temperature affected P _n but not E rates of tested leaves. Variations in relative air humidity in the chamber directly influenced E but had no effect on P _n of attached leaves. It was shown that the maximum rates of gas exchange in the attached willow leaf could be only attained by providing optimum conditions for the whole plant.     

Ecosystem CO2 exchange and plant biomass in the littoral zone of a boreal eutrophic lake

• 1 In order to study the dynamics of primary production and decomposition in the lake littoral, an interface zone between the pelagial, the catchment and the atmosphere, we measured ecosystem/atmosphere carbon dioxide (CO₂) exchange in the littoral zone of an eutrophic boreal lake in Finland during two open water periods (1998–1999). We reconstructed the seasonal net CO₂ exchange and identified the key factors controlling CO₂ dynamics. The seasonal net ecosystem exchange (NEE) was related to the amount of carbon accumulated in plant biomass.
• 2 In the continuously inundated zones, spatial and temporal variation in the density of aerial shoots controlled CO₂ fluxes, but seasonal net exchange was in most cases close to zero. The lower flooded zone had a net CO₂ uptake of 1.8–6.2 mol m^?2 per open water period, but the upper flooded zone with the highest photosynthetic capacity and above‐ground plant biomass, had a net CO₂ loss of 1.1–7.1 mol m^?2 per open water period as a result of the high respiration rate. The excess of respiration can be explained by decomposition of organic matter produced on site in previous years or leached from the catchment.
• 3 Our results from the two study years suggest that changes in phenology and water level were the prime cause of the large interannual difference in NEE in the littoral zone. Thus, the littoral is a dynamic buffer and source for the load of allochthonous and autochthonous carbon to small lakes.

     

Effects of soil warming during spring on photosynthetic recovery in boreal Norway spruce stands

The effect of soil thawing and soil temperature on postwinter recovery of photosynthetic capacity was studied, during late spring and early summer, in Norway spruce stands in northern Sweden. Soil temperature was manipulated by means of buried heating cables. The warming treatment was applied to stands with low (natural) and high (fertilized) availability of nutrients. Soil thawing, expressed as water availability, was followed by means of sapflow in stems, and shoot water potentials. The recovery of photosynthetic capacity was assessed by measuring the rate of light-saturated photosynthesis (A_max), and maximum photochemical efficiency of photosystem II in detached shoots, and chlorophyll a fluorescence. Accumulation of starch reserves in the needles was followed as an independent indicator of photosynthetic performance in situ. Snowmelt and soil thawing occurred more than one month earlier in heated than in unheated plots. This was expressed both as sapflow and as differences in shoot water potential between treatments. During May, the rates of A_max were significantly higher on heated than on control plots. The effect of soil warming on A_max was, however, not reflected in chlorophyll fluorescence or needle starch content. The time course of the recovery of photosynthetic capacity was mainly controlled by mean air temperature and by the frequency of severe night frosts, and to a lesser extent by earlier soil thawing and higher soil temperatures.     

Temperature-dependent photoinhibition and recovery of photosynthesis in the green alga Chlamydomonas reinhardtii acclimated to 12 and 27°C

Photoinhibition of photosynthesis and subsequent recovery were studied in cultures of the unicellular green alga Chlamydomonas reinhardtii L. (wt strain 137 c mating type +) acclimated at high (27°C) and low (12°C) temperature, Photoinhibition was assayed by fluorescence kinetics (77K) and oxygen evolution measurements under growth temperature conditions Inhibition of 50% was obtained by exposing cultures acclimated at high temperature to a photosynthetic photon flux density (PPFD) of 1 600 μmol m⁻² S⁻¹ at. 27°C. and cultures acclimated at low temperature to a PPFD of 900 μmol m⁻² s⁻¹ at 12°C When the photoinhibitory conditions were shifted it was revealed that algae acclimated at low temperature had acquired an increased resistance to photoinhibition at both 12 and 27°C. Furthermore, acclimation at low temperature increased the capacity to recover from 50% photoinhibition at both 12 and 27°C Studies of photoinhibition in the presence of the protein synthesis inhibitor, chloramphenicol, revealed that in response to acclimation at low temperature during growth the algae became more dependent on protein synthesis to avoid photoinhibition. It is suggested that acclimation at low temperature rendered C. reinhardtii an increased resistance to photoinhibition by. increasing the rate of turnover of photodamaged proteins in photosystem II (PS II). However, we cannot exclude the possibility that the increased resistance to photoinhibition of C. reinhardtii acclimated at low temperature also involves modifications of the mechanism of photoinhibition.     

Photoinhibition of photosynthesis in intact willow leaves in response to moderate changes in light and temperature

When willow leaves were transferred from 270 to 650 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), partial photoinhibition developed over the next hours. This was manifested as roughly parallel inhibitions of the ratio of variable over maximal chlorophyll fluorescence (F_v/F_M), and of the maximal quantum yield and the capacity of photosynthesis. This occurred even though photosynthesis was operating well below its capacity and only about one fourth of the reaction centres of photosystem (PS) II were in the closed state. When the air temperature was lowered from 25 to 15°C (18°C leaf temperature) photoinhibition was markedly accelerated. This temperature effect is suggested to be mediated largely by a decrease in the rate of energy dissipation through photosynthesis and indicated by a 50% increase in the number of closed PSII reaction centres. The pool size of the carotcnoid zeaxanthin and the extent of inhibition of the F_v/F_M ratio were positively correlated during the treatment. However, the relaxation following imposition of darkness was much faster for zeaxanthin than for the F_v/F_M ratio, ruling out the possibility of a direct causal relationship. The energy distribution between PSII and PSI was unaltered upon photoinhibition. However, the functioning of the PSII reaction centres was altered, as indicated by a rise in the minimal fluorescence, Fa.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Changes in susceptibility to photoinhibition and recovery during the growth season

Kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) plants grown in an outdoor enclosure were exposed to the natural conditions of temperature and photon flux density (PFD) over the growing season (October to May). Temperatures ranged from 14 to 21° C while the mean monthly maximum PFD varied from 1000 to 1700 mol · m^–2 · s^–1, although the peak PFDs exceeded 2100 mol · m^–2 · s^–1. At intervals, the daily variation in chlorophyll fluorescence at 692 nm and 77K and the photon yield of O₂ evolution in attached leaves was monitored. Similarly, the susceptibility of intact leaves to a standard photoinhibitory treatment of 20° C and a PFD of 2000 mol · m^–2 · s^–1 and the ability to recover at 25° C and 20 mol · m^–2 · s^–2 was followed through the season. On a few occasions, plants were transferred either to or from a shade enclosure to assess the suceptibility to natural photoinhibition and the capacity for recovery. There were minor though significant changes in early-morning fluorescence emission and photon yield throughout the growing season. The initial fluorescence, F_o, and the maximum fluorescence, F_m, were, however, significantly and persistently different from that in shade-grown kiwifruit leaves, indicative of chronic photoinhibition occurring in the sun leaves. In spring and autumn, kiwifruit leaves were photoinhibited through the day whereas in summer, when the PFDs were highest, no photoinhibition occurred. However, there was apparently no non-radiative energy dissipation occurring then also, indicating that the kiwifruit leaves appeared to fully utilize the available excitation energy. Nevertheless, the propensity for kiwifruit leaves to be susceptible to photoinhibition remained high throughout the season. The cause of a discrepancy between the severe photoinhibition under controlled conditions and the lack of photoinhibition under comparable, natural conditions remains uncertain. Recovery from photoinhibition, by contrast, varied over the season and was maximal in summer and declined markedly in autumn. Transfer of shade-grown plants to full sun had a catastrophic effect on the fluorescence characteristics of the leaf and photon yield. Within 3 d the variable fluorescence, F_v, and the photon yield were reduced by 80 and 40%, respectively, and this effect persisted for at least 20 d. The restoration of fluorescence characteristics on transfer of sun leaves to shade, however, was very slow and not complete within 15 d.Abbreviations and Symbols F_o, F_m, F_v initial, maximum, variable fluorescence - F_i F_v at t = 0 - F F_v at t = - PFD photon flux density - PSII photosystem II - leaf absorptance ratio - (_a photon yield of O₂ evolution (absorbed basis) - _{i a} at t = 0 - _a at t = We thank Miss Linda Muir and Amanda Yeates for their technical assistance in this study.     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Comparison of carbon dioxide fluxes over three boreal black spruce forests in Canada

Although mature black spruce forests are a dominant cover type in the boreal forest of North America, it is not clear how their carbon (C) budgets vary across the continent. The installation of an eddy covariance flux tower on an Old Black Spruce (OBS) site in eastern Canada (EOBS, Québec) provided a first opportunity to compare and contrast its annual (2004) and seasonal C exchange with two other pre-existing OBS flux sites from different climatic regions located in Saskatchewan [Southern OBS (SOBS)] and Manitoba [Northern OBS (NOBS)]. Although there was a relatively uniform seasonal pattern of net ecosystem productivity (NEP) among sites, EOBS had a lower total annual NEP than the other two sites. This was primarily because warmer soil under a thicker snowpack at EOBS appeared to increase winter C losses and low light suppressed both NEP and gross ecosystem productivity (GEP) in June. Across sites, greater total annual GEP and ecosystem respiration ( R ) were associated with greater mean annual air temperatures and an earlier beginning of the growing season. Also, GEP at all three sites showed a stronger relationship with air temperature in spring and early summer compared with later in the growing season, highlighting the importance of springtime conditions to the C budget of these boreal ecosystems. The three sites had different parameter estimates describing the responses of R and GEP at the half hour time scale to near surface temperature and light, respectively. On the other hand, the responses of both R and GEP to temperature at the monthly scale did not differ among sites. These results suggest that a general parameterization could be sufficient at coarse time resolutions to model the response of C exchange to environmental factors of mature black spruce forests from different climatic regions.     

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO₂ assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO₂ uptake (A_max). Decrease in A_max correlated with a halving of the leaf internal CO₂ concentration (c_i), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.     

Carbon dioxide exchange of a Russian boreal forest after disturbance by wind throw

The exchange of carbon dioxide (CO₂) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO₂ uptake flux was ?3 µmol m^?2s^?1, whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m^?2s^?1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* ≤ 0.08 ms^?1). On average, 164 mmol CO₂ m^?2d^?1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO₂ m^?2 (180 g CO₂ m^?2) released during the 3‐month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of ?0.04 yr^?1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.     

Carbon sequestration in arid-land forest

Rising atmospheric CO₂ concentrations may lead to increased water availability because the water use efficiency of photosynthesis (WUE) increases with CO₂ in most plant species. This should allow the extension of afforestation activities into drier regions. Using eddy flux, physiological and inventory measurements we provide the first quantitative information on such potential from a 35‐year old afforestation system of Aleppo pine (Pinus halepensis Mill.) at the edge of the Negev desert. This 2800 ha arid‐land forest contains 6.5 ± 1.2 kg C m^?2, and continues to accumulate 0.13–0.24 kg C m^?2 yr^?1. The CO₂ uptake is highest during the winter, out of phase with most northern hemispheric forest activity. This seasonal offset offers low latitude forests ～10 ppm higher CO₂ concentrations than that available to higher latitude forests during the productive season, in addition to the 30% increase in mean atmospheric CO₂ concentrations since the 1850s. Expanding afforestation efforts into drier regions may be significant for C sequestration and associated benefits (restoration of degraded land, reducing runoff, erosion and soil compaction, improving wildlife) because of the large spatial scale of the regions potentially involved (ca. 2 × 10⁹ ha of global shrub‐land and C4 grassland). Quantitative information on forest activities under dry conditions may also become relevant to regions predicted to undergo increasing aridity.     

Photosynthetic strategies of summergreen and evergreen understory herbs of the boreal mixedwood forest

Seasonal differences in photosynthesis and stomatal conductance of four herbaceous perennials from beneath a deciduous canopy was assessed at two light levels (60 and 400 μmol m⁻² s⁻¹ photosynthetic photon flux density, PPFD) and two leaf temperatures (7 and 20°C). Leaves of an evergreen, Pyrola asarifolia Michx., a wintergreen, Cornus canadensis L., and two summergreen species, Rubus pubescens Raf. and Aralia nudicaulis L., were collected at four times during the growing season. In addition, midsummer light response curves were obtained for one summergreen (A. nudicaulis) and one evergreen species (P. asarifolia) at both 7 and 20°C. Gas exchange measurements were made in the laboratory under controlled environmental conditions. For leaves collected in April, when insolation was high due to the leafless overstory, only P. asarifolia had green leaves, and there was no effect of temperature or light on this species' photosynthesis. P. asarifolia's net assimilation rate (NA) in April was about 30% of it's maximum in late summer. In early summer (June), A. nudicaulis and R. pubescens had higher NA at the higher temperature; at this time, these summergreen species also reached their maximum NA. Midsummer photosynthetic light response curves showed that the light-saturation point was higher and more responsive to leaf temperature in the summergreen A. nudicaulis than in the evergreen P. asarifolia. The summergreen species appear to have a photosystem which performs at high rates during early- and mid-summer, as well as a taller stature which allows them to intercept more light. The photosynthetic system of the ever/wintergreen species is adapted to the low ground-level light conditions in the summer and there does not appear to be an adjustment to take further advantage of the higher light in the spring and fall period. The adaptation of the evergreen and wintergreen understory species is tolerance to low temperatures, enabling them to photosynthesize into the fall till the first continuous frosts occur in the understory and also permitting the evergreen species to begin photosynthesis early in the spring. Received: 17 October 1996 / Accepted: 2 May 1997     

Climatic controls on the carbon and water balances of a boreal aspen forest, 1994–2003

The carbon and water budgets of boreal and temperate broadleaf forests are sensitive to interannual climatic variability and are likely to respond to climate change. This study analyses 9 years of eddy‐covariance data from the Boreal Ecosystem Research and Monitoring Sites (BERMS) Southern Old Aspen site in central Saskatchewan, Canada and characterizes the primary climatic controls on evapotranspiration, net ecosystem production (F_NEP), gross ecosystem photosynthesis (P) and ecosystem respiration (R). The study period was dominated by two climatic extremes: extreme warm and cool springs, which produced marked contrasts in the canopy duration, and a severe, 3‐year drought. Annual F_NEP varied among years from 55 to 367 g C m⁻² (mean 172, SD 94). Interannual variability in F_NEP was controlled primarily by factors that affected the R/P ratio, which varied between 0.74 and 0.96 (mean 0.87, SD 0.06). Canopy duration enhanced P and F_NEP with no apparent effect on R. The fraction of annual photosynthetically active radiation (PAR) that was absorbed by the canopy foliage varied from 38% in late leaf‐emergence years to 51% in early leaf‐emergence years. Photosynthetic light‐use efficiency (mean 0.0275, SD 0.026 mol C mol⁻¹ photons) was relatively constant during nondrought years but declined with drought intensity to a minimum of 0.0228 mol C mol⁻¹ photons during the most severe drought year. The impact of drought on F_NEP varied with drought intensity. Years of mild‐to‐moderate drought suppressed R while having little effect on P, so that F_NEP was enhanced. Years of severe drought suppressed both R and P, causing either little change or a subtle reduction in F_NEP. The analysis produced new insights into the dominance of canopy duration as the most important biophysical control on F_NEP. The results suggested a simple conceptual model for annual F_NEP in boreal deciduous forests. When water is not limiting, annual P is controlled by canopy duration via its influence on absorbed PAR at constant light‐use efficiency. Water stress suppresses P, by reducing light‐use efficiency, and R, by limiting growth and/or suppressing microbial respiration. The high photosynthetic light‐use efficiency showed this site to be a highly productive boreal deciduous forest, with properties similar to many temperate deciduous forests.     

Winter photosynthesis by saplings of evergreen broad-leaved trees in a deciduous temperate forest

* Here we investigated photosynthetic traits of evergreen species under a deciduous canopy in a temperate forest and revealed the importance of CO2 assimilation during winter for annual CO2 assimilation. * Saplings were shaded by the canopy trees from spring through to autumn, but were less shaded during the winter months. Photosynthetic rates at light saturation (Aarea) were lower during winter than during the growing season. Aarea was higher in Camellia, Ilex and Photinia than in Castanopsis, Cleyera and Quercus during the winter, but differed little during summer and autumn. * Estimated daily CO2 assimilation (Aday) was higher during the winter than during the growing season in Camellia, Ilex and Photinia but was higher than that during the growing season only at the beginning and end of winter in Castanopsis, Cleyera and Quercus. Aday was higher in Camellia, Ilex and Photinia than in Castanopsis, Cleyera and Quercus but differed little among them during the growing season. * These results reveal the importance of winter CO2 assimilation for the growth of Camellia, Ilex and Photinia. Furthermore, differences in annual CO2 assimilation among species are strongly modified by species-specific photosynthetic traits during the winter under deciduous canopy trees.     

A long-term record of carbon exchange in a boreal black spruce forest: means, responses to interannual variability, and decadal trends

We present a decadal (1994–2004) record of carbon dioxide flux in a 160‐year‐old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m⁻², 1995) to a sink (−21 g C m⁻², 2004) of CO₂ over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate.     

Photoinhibition and recovery of photosynthesis in intact barley leaves at 5 and 20°C

Photoinhibition of photosynthesis and its recovery were studied in intact barley ( Hordeum vuigare L. cv. Gunilla) leaves grown in a controlled environment by exposing them to two temperatures, 5 and 20°C, and a range of photon flux densities in excess of that during growth. Additionally, photoinhibtion was examined in the presence of chloramphenicol (CAP, an inhibitor of chloroplast protein synthesis) and of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Susceptibility to photoinhibition was much higher at 5 than at 20°C. Furthermore, at 20°C. CAP exacerbated photoinhibition strongly, whereas CAP had little additional effect (10%) at 5°C. These results support the model that net photoinhibition is the difference between the inactivation and repair of photosystem II (PSII); i.e. the degradation and synthesis of the reaction centre protein, Dl. Furthermore, the steady-state extent of photoinhibition was strongly dependent on temperature and the results indicated this was manifested through the effects of temperature on the repair process of PSII. We propose that the continuous repair of PS II at 20°C conferred at least some protection from photoinhibition. At 5°C the repair process was largely inhibited, with increased photoinhibition as a consequence. However, we suggest where repair is inhibited by low temperature, some protection is alternatively conferred by the photoinhibited reaction centres. Providing they are not degraded, such centres could still dissipate excitation energy non-radiatively, thereby conferring protection of remaining photochemically active centres under steady-state conditions.
A fraction of PS II centres were capable of resisting photoinhibition when the repair process was inhibited by CAP. This is discussed in relation to PS II heterogeneity. Furthermore, the repair process was not apparently activated within 3 h when barley leaves were transferred to photoinhibitory light conditions at 20°C.     

Warmer and drier conditions stimulate respiration more than photosynthesis in a boreal peatland ecosystem: Analysis of automatic chambers and eddy covariance measurements

Continuous half‐hourly net CO₂ exchange measurements were made using nine automatic chambers in a treed fen in northern Alberta, Canada from June–October in 2005 and from May–October in 2006. The 2006 growing season was warmer and drier than in 2005. The average chamber respiration rates normalized to 10 °C were much higher in 2006 than in 2005, while calculations of the temperature sensitivity (Q₁₀) values were similar in the two years. Daytime average respiration values were lower than the corresponding, temperature‐corrected respiration rates calculated from night‐time chamber measurements. From June to September, the season‐integrated estimates of chamber photosynthesis and respiration were 384 and 590 g C m^?2, respectively in 2006, an increase of 100 and 203 g C m^?2 over the corresponding values in 2005. The season‐integrated photosynthesis and respiration rates obtained using the eddy covariance technique, which included trees and a tall shrub not present in the chambers, were 720 and 513 g C m^?2, respectively, in 2006, an increase of 50 and 125 g C m^?2 over the corresponding values in 2005. While both photosynthesis and respiration rates were higher in the warmer and drier conditions of 2006, the increase in respiration was more than twice the increase in photosynthesis.