Disturbance legacies and climate jointly drive tree growth and mortality in an intensively studied boreal forest

Most North American forests are at some stage of post‐disturbance regrowth, subject to a changing climate, and exhibit growth and mortality patterns that may not be closely coupled to annual environmental conditions. Distinguishing the possibly interacting effects of these processes is necessary to put short‐term studies in a longer term context, and particularly important for the carbon‐dense, fire‐prone boreal forest. The goals of this study were to combine dendrochronological sampling, inventory records, and machine‐learning algorithms to understand how tree growth and death have changed at one highly studied site (Northern Old Black Spruce, NOBS) in the central Canadian boreal forest. Over the 1999–2012 inventory period, mean tree diameter increased even as stand density and basal area declined significantly. Tree mortality averaged 1.4 ± 0.6% yr^?1, with most mortality occurring in medium‐sized trees; new recruitment was minimal. There have been at least two, and probably three, significant influxes of new trees since stand initiation, but none in recent decades. A combined tree ring chronology constructed from sampling in 2001, 2004, and 2012 showed several periods of extreme growth depression, with increased mortality lagging depressed growth by ~5 years. Higher minimum and maximum air temperatures exerted a negative influence on tree growth, while precipitation and climate moisture index had a positive effect; both current‐ and previous‐year data exerted significant effects. Models based on these variables explained 23–44% of the ring‐width variability. We suggest that past climate extremes led to significant mortality still visible in the current forest structure, with decadal dynamics superimposed on slower patterns of fire and succession. These results have significant implications for our understanding of previous work at NOBS, the carbon sequestration capability of old‐growth stands in a disturbance‐prone landscape, and the sustainable management of regional forests in a changing climate.     

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.     

Carbon sequestration in boreal jack pine stands following harvesting

A large area of boreal jack pine (Pinus banksiana Lamb.) forest in Canada is recovering from clear‐cut harvesting, and the carbon (C) balance of these regenerating forests remains uncertain. Net ecosystem CO₂ exchange was measured using the eddy‐covariance technique at four jack pine sites representing different stages of stand development: three postharvest sites (HJP02, HJP94, and HJP75) and one preharvest site (OJP). The four sites, located in the southern Canadian boreal forest, Saskatchewan, Canada, are typical of low productivity jack pine stands and were 2, 10, 29, and 90 years old in 2004, respectively. Mean annual net ecosystem production (NEP) for 2004 and 2005 was ?137±11, 19±16, 73±28, and 22±30 g C m^?2 yr^?1 at HJP02, HJP94, HJP75 and OJP, respectively, showing the postharvest jack pine stands to be moderate C sources immediately after harvesting, weak sinks at 10 years, moderate C sinks at 30 years, then weak C sinks at 90 years. Mean annual gross ecosystem photosynthesis (GEP) for the 2 years was 96±10, 347±20, 576±34, and 583±35 g C m^?2 yr^?1 at HJP02, HJP94, HJP75, and OJP, respectively. The ratio of annual ecosystem respiration (R) to annual GEP was 2.51±0.15, 0.95±0.04, 0.87±0.03, and 0.96±0.03. Seasonally, NEP peaked in May or June at all four sites but GEP and R were highest in July. R at a reference soil temperature of 10 °C, ecosystem quantum yield and photosynthetic capacity were lowest for the 2‐year‐old stand. R was most sensitive to soil temperature for the 90‐year‐old stand. The primary source of variability in NEP over the course of succession of the jack pine ecosystem following harvesting was stand age due to the changes in leaf area index. Intersite variability in GEP and R was an order of magnitude greater than interannual variability at OJP. For both young and old stands, GEP had greater interannual variability than R and played a more important role than R in interannual variation in NEP. Based on year‐round flux measurements from 2000 to 2005, the 10‐year stand had larger interannual variability in GEP and R than the 90‐year stand. Interannual variability in NEP was driven primarily by early‐growing‐season temperature and growing‐season length. Photosynthesis played a dominant role in the rapid rise in NEP early in stand development. Late in stand development, however, the subtle decrease in NEP resulted primarily from increasing respiration.     

Modeling analysis of primary controls on net ecosystem productivity of seven boreal and temperate coniferous forests across a continental transect

Process‐based models are effective tools to synthesize and/or extrapolate measured carbon (C) exchanges from individual sites to large scales. In this study, we used a C‐ and nitrogen (N)‐cycle coupled ecosystem model named CN‐CLASS (Carbon Nitrogen‐Canadian Land Surface Scheme) to study the role of primary climatic controls and site‐specific C stocks on the net ecosystem productivity (NEP) of seven intermediate‐aged to mature coniferous forest sites across an east–west continental transect in Canada. The model was parameterized using a common set of parameters, except for two used in empirical canopy conductance–assimilation, and leaf area–sapwood relationships, and then validated using observed eddy covariance flux data. Leaf Rubisco‐N dynamics that are associated with soil–plant N cycling, and depend on canopy temperature, enabled the model to simulate site‐specific gross ecosystem productivity (GEP) reasonably well for all seven sites. Overall GEP simulations had relatively smaller differences compared with observations vs. ecosystem respiration (RE), which was the sum of many plant and soil components with larger variability and/or uncertainty associated with them. Both observed and simulated data showed that, on an annual basis, boreal forest sites were either carbon‐neutral or a weak C sink, ranging from 30 to 180 g C m^?2 yr^?1; while temperate forests were either a medium or strong C sink, ranging from 150 to 500 g C m^?2 yr^?1, depending on forest age and climatic regime. Model sensitivity tests illustrated that air temperature, among climate variables, and aboveground biomass, among major C stocks, were dominant factors impacting annual NEP. Vegetation biomass effects on annual GEP, RE and NEP showed similar patterns of variability at four boreal and three temperate forests. Air temperature showed different impacts on GEP and RE, and the response varied considerably from site to site. Higher solar radiation enhanced GEP, while precipitation differences had a minor effect. Magnitude of forest litter content and soil organic matter (SOM) affected RE. SOM also affected GEP, but only at low levels of SOM, because of low N mineralization that limited soil nutrient (N) availability. The results of this study will help to evaluate the impact of future climatic changes and/or forest C stock variations on C uptake and loss in forest ecosystems growing in diverse environments.     

Landscape-variability of the carbon balance across managed boreal forests

Boreal forests are important global carbon (C) sinks and, therefore, considered as a key element in climate change mitigation policies. However, their actual C sink strength is uncertain and under debate, particularly for the actively managed forests in the boreal regions of Fennoscandia. In this study, we use an extensive set of biometric- and chamber-based C flux data collected in 50 forest stands (ranging from 5 to 211 years) over 3 years (2016–2018) with the aim to explore the variations of the annual net ecosystem production (NEP; i.e., the ecosystem C balance) across a 68 km² managed boreal forest landscape in northern Sweden. Our results demonstrate that net primary production rather than heterotrophic respiration regulated the spatio-temporal variations of NEP across the heterogeneous mosaic of the managed boreal forest landscape. We further find divergent successional patterns of NEP in our managed forests relative to naturally regenerating boreal forests, including (i) a fast recovery of the C sink function within the first decade after harvest due to the rapid establishment of a productive understory layer and (ii) a sustained C sink in old stands (131–211 years). We estimate that the rotation period for optimum C sequestration extends to 138 years, which over multiple rotations results in a long-term C sequestration rate of 86.5 t C ha⁻¹ per rotation. Our study highlights the potential of forest management to maximize C sequestration of boreal forest landscapes and associate climate change mitigation effects by developing strategies that optimize tree biomass production rather than heterotrophic soil C emissions.     

Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

In 2001–03, continuous eddy covariance measurements of carbon dioxide (CO₂) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3−year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100−km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions.     

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.     

Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

In 2000–03, continuous eddy covariance measurements of carbon dioxide (CO₂) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3-year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100-km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions. The online version of the original article can be found under doi:     

Patterns of NPP,GPP, respiration,and NEP during boreal forest succession

We combined year‐round eddy covariance with biometry and biomass harvests along a chronosequence of boreal forest stands that were 1, 6, 15, 23, 40, ～74, and ～154 years old to understand how ecosystem production and carbon stocks change during recovery from stand‐replacing crown fire. Live biomass (C_live) was low in the 1‐ and 6‐year‐old stands, and increased following a logistic pattern to high levels in the 74‐ and 154‐year‐old stands. Carbon stocks in the forest floor (C_{forest floor}) and coarse woody debris (C_CWD) were comparatively high in the 1‐year‐old stand, reduced in the 6‐ through 40‐year‐old stands, and highest in the 74‐ and 154‐year‐old stands. Total net primary production (TNPP) was reduced in the 1‐ and 6‐year‐old stands, highest in the 23‐ through 74‐year‐old stands and somewhat reduced in the 154‐year‐old stand. The NPP decline at the 154‐year‐old stand was related to increased autotrophic respiration rather than decreased gross primary production (GPP). Net ecosystem production (NEP), calculated by integrated eddy covariance, indicated the 1‐ and 6‐year‐old stands were losing carbon, the 15‐year‐old stand was gaining a small amount of carbon, the 23‐ and 74‐year‐old stands were gaining considerable carbon, and the 40‐ and 154‐year‐old stands were gaining modest amounts of carbon. The recovery from fire was rapid; a linear fit through the NEP observations at the 6‐ and 15‐year‐old stands indicated the transition from carbon source to sink occurred within 11–12 years. The NEP decline at the 154‐year‐old stand appears related to increased losses from C_live by tree mortality and possibly from C_{forest floor} by decomposition. Our findings support the idea that NPP, carbon production efficiency (NPP/GPP), NEP, and carbon storage efficiency (NEP/TNPP) all decrease in old boreal stands.     

Seasonal variability of forest sensitivity to heat and drought stresses: A synthesis based on carbon fluxes from North American forest ecosystems

Climate extremes such as heat waves and droughts are projected to occur more frequently with increasing temperature and an intensified hydrological cycle. It is important to understand and quantify how forest carbon fluxes respond to heat and drought stress. In this study, we developed a series of daily indices of sensitivity to heat and drought stress as indicated by air temperature (T_a) and evaporative fraction (EF). Using normalized daily carbon fluxes from the FLUXNET Network for 34 forest sites in North America, the seasonal pattern of sensitivities of net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (RE) in response to T_a and EF anomalies were compared for different forest types. The results showed that warm temperatures in spring had a positive effect on NEP in conifer forests but a negative impact in deciduous forests. GEP in conifer forests increased with higher temperature anomalies in spring but decreased in summer. The drought‐induced decrease in NEP, which mostly occurred in the deciduous forests, was mostly driven by the reduction in GEP. In conifer forests, drought had a similar dampening effect on both GEP and RE, therefore leading to a neutral NEP response. The NEP sensitivity to T_a anomalies increased with increasing mean annual temperature. Drier sites were less sensitive to drought stress in summer. Natural forests with older stand age tended to be more resilient to the climate stresses compared to managed younger forests. The results of the Classification and Regression Tree analysis showed that seasons and ecosystem productivity were the most powerful variables in explaining the variation of forest sensitivity to heat and drought stress. Our results implied that the magnitude and direction of carbon flux changes in response to climate extremes are highly dependent on the seasonal dynamics of forests and the timing of the climate extremes.     

Annual Growth Ring Patterns in Brachystegia spiciformis Reveal Influence of Precipitation on Tree Growth1

The availability of exactly dated tree‐ring chronologies is limited in tropical regions. However, these chronologies could contribute widely to studies of the influence of natural and human‐induced factors on tropical forests. We examine the potential for building a chronology based on three sites in the miombo woodland of western Zambia. Brachystegia spiciformis Benth., a dominant species from this vegetation type, is used. Response of the chronology to several climatic factors is examined. All specimens showed very clear growth rings, and cross‐dating between radii of a tree was successful for all trees. Site chronologies could be constructed after cross‐dating of growth ring series of individual trees. The mean growth ring curves of the three sites were significantly similar, allowing for the construction of a regional chronology. Correlation function analysis between the tree‐ring chronology and regional climatic variables revealed that climate at the core of the rainy season, in December and January, has an explicit influence on tree growth. Where precipitation and relative humidity in these months influence tree growth positively, temperature correlates in a negative way. Some 20 percent of the variance in the B. spiciformis tree‐ring chronology is accounted for by wet season rainfall. The successful cross‐dating and correlation between a tree‐ring chronology and climate demonstrated in this study indicate annual ring formation in B. spiciformis trees and sensitivity to climatic conditions.     

Measurements of gross and net ecosystem productivity and water vapour exchange of a Pinus ponderosa ecosystem,and an evaluation of two generalized models

Net ecosystem productivity (NEP), net primary productivity (NPP), and water vapour exchange of a mature Pinus ponderosa forest (44°30′ N, 121°37′ W) growing in a region subject to summer drought were investigated along with canopy assimilation and respiratory fluxes. This paper describes seasonal and annual variation in these factors, and the evaluation of two generalized models of carbon and water balance (PnET‐II and 3‐PG) with a combination of traditional measurements of NPP, respiration and water stress, and eddy covariance measurements of above‐and below‐canopy CO₂ and water vapour exchange. The objective was to evaluate the models using two years of traditional and eddy covariance measurements, and to use the models to help interpret the relative importance of processes controlling carbon and water vapour exchange in a water‐limited pine ecosystem throughout the year. PnET‐II is a monthly time‐step model that is driven by nitrogen availability through foliar N concentration, and 3‐PG is a monthly time‐step quantum‐efficiency model constrained by extreme temperatures, drought, and vapour pressure deficits. Both models require few parameters and have the potential to be applied at the watershed to regional scale. There was 2/3 less rainfall in 1997 than in 1996, providing a challenge to modelling the water balance, and consequently the carbon balance, when driving the models with the two years of climate data, sequentially. Soil fertility was not a key factor in modelling processes at this site because other environmental factors limited photosynthesis and restricted projected leaf area index to ～1.6. Seasonally, GEP and LE were overestimated in early summer and underestimated through the rest of the year. The model predictions of annual GEP, NEP and water vapour exchange were within 1–39% of flux measurements, with greater disparity in 1997 because soil water never fully recharged. The results suggest that generalized models can provide insights to constraints on productivity on an annual basis, using a minimum of site data.     

Spatial patterns and climate drivers of carbon fluxes in terrestrial ecosystems of China

Understanding the dynamics and underlying mechanism of carbon exchange between terrestrial ecosystems and the atmosphere is one of the key issues in global change research. In this study, we quantified the carbon fluxes in different terrestrial ecosystems in China, and analyzed their spatial variation and environmental drivers based on the long‐term observation data of ChinaFLUX sites and the published data from other flux sites in China. The results indicate that gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP) of terrestrial ecosystems in China showed a significantly latitudinal pattern, declining linearly with the increase of latitude. However, GEP, ER, and NEP did not present a clear longitudinal pattern. The carbon sink functional areas of terrestrial ecosystems in China were mainly located in the subtropical and temperate forests, coastal wetlands in eastern China, the temperate meadow steppe in the northeast China, and the alpine meadow in eastern edge of Qinghai‐Tibetan Plateau. The forest ecosystems had stronger carbon sink than grassland ecosystems. The spatial patterns of GEP and ER in China were mainly determined by mean annual precipitation (MAP) and mean annual temperature (MAT), whereas the spatial variation in NEP was largely explained by MAT. The combined effects of MAT and MAP explained 79%, 62%, and 66% of the spatial variations in GEP, ER, and NEP, respectively. The GEP, ER, and NEP in different ecosystems in China exhibited ‘positive coupling correlation’ in their spatial patterns. Both ER and NEP were significantly correlated with GEP, with 68% of the per‐unit GEP contributed to ER and 29% to NEP. MAT and MAP affected the spatial patterns of ER and NEP mainly by their direct effects on the spatial pattern of GEP.     

Characteristics of a multi-species conifer network of wood properties chronologies from Southern Australia

Progress in quantitative wood anatomy has resulted in a growing number of increasingly understood proxies from the tree-ring archive. Much of this work has been based on tree species in the Northern Hemisphere. Here, we present and examine a relatively dense network of wood property chronologies (wood density, tracheid radial diameter, cell wall thickness and ring width) from several species in Tasmania, southern Australia. We ask how the relationships amongst the different types of chronologies differ within and amongst species. We also consider how each chronology responds to monthly climate. In general terms, and similar to findings in the Northern Hemisphere, relationships between the various wood properties and climate are stronger than those between climate and ring width chronologies. An important exception to this is the highest elevation Lagarostrobos franklinii site. Additionally, strongest response to climate for the wood properties generally occurs for the concurrent growing season compared to the prior growing season for ring width. Relationships amongst the various chronology types differ for the various species, with L. franklinii also showing some variation in these relationships by site (possibly associated with elevation). Results suggest there is considerable value in further exploring the potential for developing anatomical wood chronologies for climate reconstruction from other species for which ring widths do not exhibit a strong climate signal.     

Dendroclimatic signals deduced from riparian versus upland forest interior pines in North Karelia, Finland

Radial growth of boreal tree species is only rarely studied in riparian habitats. Here we investigated chronologies of earlywood, latewood, and annual ring widths and blue intensity (BI; a surrogate to latewood density) from riparian lake shore and upland forest interior pines (Pinus sylvestris L.) growing in boreal forest in eastern Finland. Riparian and upland chronologies were compared to examine differences in the pine growth variability and growth response to climatic variation in the two habitats. It was found that the climatic variables showing statistically significant correlations with the tree-ring chronologies were related to snow conditions at the start of the growing season. Deeper snowpack led to reduced upland pine growth, possibly due to delayed snowmelt and thus postponed onset of the growing season. Warm late winters were followed by increased riparian pine growth because of earlier start of the snow-melt season and thus a lower maximum early summer lake level. Moreover, riparian pines reacted negatively to increased rainfall in June, whereas the upland pines showed a positive response. Latewood growth reacted significantly to summer temperatures. The BI chronology showed a strong correlation with warm-season temperatures, indicating an encouraging possibility of summer temperature reconstruction using middle/south boreal pine tree-ring archives.     

CO2 exchange and evapotranspiration across dryland ecosystems of southwestern North America

Global‐scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO₂ sink. However, such analyses are poorly constrained by measured CO₂ exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water‐limited Southwest region of North America with observed ranges in annual precipitation of 100–1000 mm, annual temperatures of 2–25°C, and records of 3–10 years (150 site‐years in total). Annual fluxes were integrated using site‐specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from ‐350 to +330 gCm^?2 across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest‐dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (R_eco) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and R_eco were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross‐site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS‐based models captured only 20–30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO₂ exchange may be up to 3–5 times larger than current estimates.     

季节性高温和干旱对亚热带毛竹林碳通量的影响

采用涡度相关技术对安吉亚热带毛竹林生态系统碳通量进行连续观测,选取2011和2013年月尺度净生态系统生产力（NEP）、生态系统呼吸（R_e）和总生态系统生产力（GEP）数据,探讨季节性高温、干旱对毛竹林生态系统碳通量的影响.结果表明： 安吉毛竹林年际间NEP有较大差异;2013年7和8月由于水热不同步而造成的高温干旱使其NEP明显下降,相比于2011年同期分别下降了59.9%和80.0%.对2011和2013年月尺度下NEP、R_e和GEP与环境因子进行相关分析发现,R_e和GEP与温度因子均呈显著相关（P<0.05）,但两者对空气和土壤水分的响应方式和程度有所不同,GEP相比于R_e更易受到土壤水分降低的影响,而饱和水汽压差的升高会在一定程度上促进R_e、同时抑制GEP,这是造成2013年7和8月安吉毛竹林NEP降低的根本原因.     

CO2 balance of boreal, temperate, and tropical forests derived from a global database

Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome‐specific carbon budgets; to re‐examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO₂ balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 °C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO₂ balance required the introduction of substantial biome‐specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non‐CO₂ carbon fluxes are not presently being adequately accounted for.     

The Suceava oak chronology: A new 804 years long tree-ring chronology bridging the gap between central and south Europe

Although the importance of centuries-long tree ring width oak chronologies in dating wood, eastern European regions are still lacking a reference chronology. To fill this geographical gap, we combined living and historical chronologies from the Moldova region, including part of Romania and the Republic of Moldova. Here we present the “Suceava oak chronology” as the first 804 years long chronology continuously covering the 1216–2019 period. Our analyses further revealed the strong teleconnection of the Suceava oak chronology with south European ones. The link between Romania and northern Turkey chronologies is of particular importance in the context of the high abundance of undated subfossil and archaeological/historical oak wood in eastern Europe and intense wood trades with central Europe in XV-XVII centuries. The availability of the Suceava oak chronology will provide a good base for precise dating and climatic reconstructions.     

DROUGHT EFFECTS ON CARBON EXCHANGE IN A SUBTROPICAL CONIFEROUS PLANTATION IN CHINA

 干旱对陆地生态系统的影响已成为全球变化研究的焦点问题之一。该研究基于生态系统过程模型——CEVSA2, 结合涡度相关通量观测, 分析了不同程度干旱对亚热带人工针叶林碳交换的影响及其关键控制因素。结果表明: 1)干旱使生态系统碳交换显著下降, 2003和2004年的干旱使得年净生态系统生产力(Net ecosystem production, NEP)相比无干旱影响情景的模拟结果分别减少了63%和47%; 2)光合和呼吸对干旱具有不同的响应, 干旱时光合的下降比呼吸更为显著, 这导致了NEP的显著下降; 3)当饱和水气压差(Vapor pressure deficit, VPD)达到1.5 kPa以上时, 生态系统的光合、呼吸和净碳吸收均开始下降, 当VPD大于2.5 kPa、土壤相对含水量(土壤含水量/土壤饱和含水量)(Relative soil water content, RSW)低于40%时, 生态系统的碳收支由碳汇转为碳源; 4)土壤干旱是造成碳交换下降的主要驱动因素, 对年NEP下降的平均贡献率为46%, 而大气干旱的贡献率仅为4%。