Northern Terrestrial Ecosystems Research Articles

Impact of Preseason Climate Factors on Vegetation Photosynthetic Phenology in Mid-High Latitudes of the Northern Hemisphere.

During the period preceding the vegetation growing season (GS), temperature emerges as the pivotal factor determining phenology in northern terrestrial ecosystems. Despite extensive research on the impact of daily mean temperature (Tmean) during the preseason period, the influence of diurnal temperature range (DTR) on vegetation photosynthetic phenology (i.e., the impact of the plant photosynthetic cycle on seasonal time scale) has largely been neglected. Using a long-term vegetation photosynthetic phenology dataset and historical climate data, we examine vegetation photosynthetic phenology dynamics and responses to climate change across the mid-high latitudes of the Northern Hemisphere from 2001 to 2020. Our data reveal an advancing trend in the start of the GS (SOS) by -0.15 days per year (days yr-1), affecting 72.1% of the studied area. This is particularly pronounced in western Canada, Alaska, eastern Asia, and latitudes north of 60°N. Conversely, the end of the GS (EOS) displays a delaying trend of 0.17 days yr-1, impacting 62.4% of the studied area, especially northern North America and northern Eurasia. The collective influence of an earlier SOS and a delayed EOS has resulted in the notably prolonged length of the GS (LOS) by 0.32 days yr-1 in the last two decades, affecting 70.9% of the studied area, with Eurasia and western North America being particularly noteworthy. Partial correlation coefficients of the SOS with preseason Tmean, DTR, and accumulated precipitation exhibited negative values in 98.4%, 93.0%, and 39.2% of the study area, respectively. However, there were distinct regional variations in the influence of climate factors on the EOS. The partial correlation coefficients of the EOS with preseason Tmean, DTR, and precipitation were positive in 58.6%, 50.1%, and 36.3% of the region, respectively. Our findings unveil the intricate mechanisms influencing vegetation photosynthetic phenology, holding crucial significance in understanding the dynamics of carbon sequestration within terrestrial ecosystems amidst climate change.

Improved Vegetation Photosynthetic Phenology Monitoring in the Northern Ecosystems Using Total Canopy Solar‐Induced Chlorophyll Fluorescence Derived From TROPOMI

AbstractSolar‐Induced chlorophyll Fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI) with substantially improved spatiotemporal resolutions provides a new potential to improve satellite‐based phenology monitoring. The performance of TROPOMI SIF for tracking vegetation photosynthetic phenology, and how it compares to conventional vegetation indices (VIs)‐based approaches, however, have not been adequately assessed. Total canopy SIF, as a better proxy of Gross Primary Productivity (GPP) than original directional SIF, is a new SIF to estimate phenology while its performance has not been investigated. This study assesses the capability of TROPOMI SIF before and after canopy correction for phenology monitoring and improves our understanding of these questions. Benchmarked by tower‐based GPP, TROPOMI SIF generally performed better than VIs, especially for capturing the End Of Season (EOS) of vegetation photosynthetic activity at deciduous broadleaf forest (DBF), evergreen forest (ENF), and croplands (CRO) sites, but not for Start Of Season (SOS). This suggested that the advantage of SIF over VIs depended on phenological metrics. The total canopy SIF emission obtained through canopy correction generally performed better than the original SIF retrievals, especially in estimating the EOS of forest sites (DBF, MF, ENF), but soil correction did not further improve the accuracy of phenological monitoring. When comparing SIF‐ and VI‐based phenological metrics over northern terrestrial ecosystems, SIF showed earlier senescence date widely, while the differences in onset date were region dependent. These results indicate the necessity of canopy correction to convert directional SIF to canopy total SIF when using satellite SIF products to estimate phenological metrics.

Vegetation photosynthetic phenology dataset in northern terrestrial ecosystems

Vegetation phenology can profoundly modulate the climate-biosphere interactions and thus plays a crucial role in regulating the terrestrial carbon cycle and the climate. However, most previous phenology studies rely on traditional vegetation indices, which are inadequate to characterize the seasonal activity of photosynthesis. Here, we generated an annual vegetation photosynthetic phenology dataset with a spatial resolution of 0.05 degrees from 2001 to 2020, using the latest gross primary productivity product based on solar-induced chlorophyll fluorescence (GOSIF-GPP). We combined smoothing splines with multiple change-point detection to retrieve the phenology metrics: start of the growing season (SOS), end of the growing season (EOS), and length of growing season (LOS) for terrestrial ecosystems above 30° N latitude (Northern Biomes). Our phenology product can be used to validate and develop phenology or carbon cycle models and monitor the climate change impacts on terrestrial ecosystems.

Quantifying the role of moss in terrestrial ecosystem carbon dynamics in northern high latitudes

Abstract. Mosses are ubiquitous in northern terrestrial ecosystems, and play an important role in regional carbon, water and energy cycling. Current global land surface models that do not consider mosses may bias the quantification of regional carbon dynamics. Here we incorporate mosses as a new plant functional type into the process-based Terrestrial Ecosystem Model (TEM 5.0), to develop a new model (TEM_Moss). The new model explicitly quantifies the interactions between vascular plants and mosses and their competition for energy, water, and nutrients. Compared to the estimates using TEM 5.0, the new model estimates that the regional terrestrial soils currently store 132.7 Pg more C and will store 157.5 and 179.1 Pg more C under the RCP8.5 and RCP2.6 scenarios, respectively, by the end of the 21st century. Ensemble regional simulations forced with different parameters for the 21st century with TEM_Moss predict that the region will accumulate 161.1±142.1 Pg C under the RCP2.6 scenario and 186.7±166.1 Pg C under the RCP8.5 scenario over the century. Our study highlights the necessity of coupling moss into Earth system models to adequately quantify terrestrial carbon–climate feedbacks in the Arctic.

Exposures to temperature beyond threshold disproportionately reduce vegetation growth in the northern hemisphere.

In recent decades, terrestrial vegetation in the northern hemisphere (NH) has been exposed to warming and more extremely high temperatures. However, the consequences of these changes for terrestrial vegetation growth remain poorly quantified and understood. By examining a satellite-based vegetation index, tree-ring measurements and land-surface model simulations, we discovered a consistent convex pattern in the responses of vegetation growth to temperature exposure (TE) for forest, shrub and grass in both the temperate (30°−50° N) and boreal (50°−70° N) NH during the period of 1982−2012. The response of vegetation growth to TE for the three vegetation types in both the temperate and boreal NH increased convergently with increasing temperature, until vegetation type-dependent temperature thresholds were reached. A TE beyond these temperature thresholds resulted in disproportionately weak positive or even strong negative responses. Vegetation growth in the boreal NH was more vulnerable to extremely high-temperature events than vegetation growth in the temporal NH. The non-linear responses discovered here provide new insights into the dynamics of northern terrestrial ecosystems in a warmer world.

Global change-driven effects on dissolved organic matter composition: Implications for food webs of northern lakes.

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial-aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)-the messenger between terrestrial and lake ecosystems-with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.

Icelandic grasslands as long-term C sinks under elevated organic N inputs

About 10% of the anthropogenic CO2 emissions have been absorbed by northern terrestrial ecosystems during the past decades. It has been hypothesized that part of this increasing carbon (C) sink is caused by the alleviation of nitrogen (N) limitation by increasing anthropogenic N inputs. However, little is known about this N-dependent C sink. Here, we studied the effect of chronic seabird-derived N inputs (47–67 kg N ha−1 year−1) on the net soil organic C (SOC) storage rate of unmanaged Icelandic grasslands on the volcanic Vestmannaeyjar archipelago by using a stock change approach in combination with soil dating. We studied both early developmental (young) soils that had been receiving increased N inputs over a decadal timescale since an eruption in 1963, and well-developed soils, that had been receiving N inputs over a millennial timescale. For the latter, however, the effects on both decadal (topsoil; 40 years) and millennial (total soil profile; 1600 years) SOC storage could be studied, as the age of topsoil and the total soil profile could be determined from volcanic ash layers deposited in 1973 and 395 AD. We found that enhanced N availability—either from accumulation over time, or seabird derived—increased the net SOC storage rate. Under low N inputs, early developmental soils were weak decadal C sinks (0.018 ton SOC ha−1 year−1), but this increased quickly under ca. 30 years of elevated N inputs to 0.29 ton SOC ha−1 year−1, thereby equalling the decadal SOC storage rate of the unfertilized well-developed soils. Furthermore, for the well-developed soils, chronically elevated N inputs not only stimulated the decadal SOC storage rate in the topsoil, but also the total millennial SOC storage was consistently higher. Hence, our study suggests that Icelandic grasslands, if not disturbed, can remain C sinks for many centuries under current climatic conditions and that chronically elevated N inputs can induce a permanent strengthening of this sink.

Changes in vegetation phenology are not reflected inatmospheric CO2 and 13 C/12 C seasonality.

Northern terrestrial ecosystems have shown global warming-induced advances in start, delays in end, and thus increased lengths of growing season and gross photosynthesis in recent decades. The tradeoffs between seasonal dynamics of two opposing fluxes, CO2 uptake through photosynthesis and release through respiration, determine the influence of the terrestrial ecosystem on the atmospheric CO2 and 13 C/12 C seasonality. Here, we use four CO2 observation stations in the Northern Hemisphere, namely Alert, La Jolla, Point Barrow, and Mauna Loa Observatory, to determine how changes in vegetation productivity and phenology, respiration, and air temperature affect both the atmospheric CO2 and 13 C/12 C seasonality. Since the 1960s, the only significant long-term trend of CO2 and 13 C/12 C seasonality was observed at the northern most station, Alert, where the spring CO2 drawdown dates advanced by0.65±0.55daysyr-1 , contributing to a nonsignificant increase in length of the CO2 uptake period (0.74±0.67daysyr-1 ). For Point Barrow station, vegetation phenology changes in well-watered ecosystems such as the Canadian and western Siberian wetlands contributed the most to 13 C/12 C seasonality while the CO2 seasonality was primarily linked to nontree vegetation. Our results indicate significant increase in the Northern Hemisphere soil respiration. This means, increased respiration of 13 C depleted plant materials cancels out the 12 C gain from enhanced vegetation activities during the start and end of growing season. These findings suggest therefore that parallel warming-induced increases both in photosynthesis and respiration contribute to the long-term stability of CO2 and 13 C/12 C seasonality under changing climate and vegetation activity. The summer photosynthesis and the soil respiration in the dormant seasons have become more vigorous which lead to increased peak-to-through CO2 amplitude. As the relative magnitude of the increased photosynthesis in summer months is more than the increased respiration in dormant months, we have the increased overall carbon uptake rates in the northern ecosystems.

Increased light‐use efficiency in northern terrestrial ecosystems indicated by CO2 and greening observations

AbstractObservations show an increasing amplitude in the seasonal cycle of CO2 (ASC) north of 45°N of 56 ± 9.8% over the last 50 years and an increase in vegetation greenness of 7.5–15% in high northern latitudes since the 1980s. However, the causes of these changes remain uncertain. Historical simulations from terrestrial biosphere models in the Multiscale Synthesis and Terrestrial Model Intercomparison Project are compared to the ASC and greenness observations, using the TM3 atmospheric transport model to translate surface fluxes into CO2 concentrations. We find that the modeled change in ASC is too small but the mean greening trend is generally captured. Modeled increases in greenness are primarily driven by warming, whereas ASC changes are primarily driven by increasing CO2. We suggest that increases in ecosystem‐scale light use efficiency (LUE) have contributed to the observed ASC increase but are underestimated by current models. We highlight potential mechanisms that could increase modeled LUE.

Relative importance between biogeochemical and biogeophysical effects in regulating terrestrial ecosystem‐climate feedback in northern high latitudes

AbstractWarming‐induced changes in structures and functions of northern terrestrial ecosystems (NTEs), including their regulation on terrestrial biogeochemistry and surface energy balance, may exert positive or negative feedback to the climate system. However, the relative importance among these biogeochemical and biogeophysical feedback is not well understood. Here we use a terrestrial ecosystem model to quantify spatially explicit ecosystem‐climate feedback over NTEs (north of 50°N) under four climate change scenarios from 2010 to 2100, including biogeochemical feedback from climate‐induced changes in net CH4 exchanges (NME) and net CO2 exchanges (NCE) and biogeophysical feedback from changes in surface energy partitioning associated with snow cover and vegetation biomass dynamics. Our results indicate that (1) biogeochemical and biogeophysical feedback are attributed more to the changes in NME and snow cover dynamics, respectively; (2) net biogeophysical feedback is much larger than net biogeochemical feedback; (3) NTEs will cause a net positive radiative forcing of 0.04–0.26 W m−2 between 2010 and 2100. Our findings support the notion that NTEs will exert positive net climate feedback; however, our estimation of positive net biogeochemical feedback including NME‐ and NCE‐induced effects is contrary to previous studies showing negative net biogeochemical feedback including NCE‐induced effect only. This study highlights the importance of NME‐induced biogeochemical effect in regulating ecosystem‐climate feedback in NTEs and implies that previous studies without considering NME‐induced effect might have underestimated the intensity of total terrestrial feedback to the climate system.

Impacts of future climate change on the carbon budget of northern high-latitude terrestrial ecosystems: An analysis using ISI-MIP data

This paper assesses future changes in the carbon budgets of northern terrestrial ecosystems (above 60°N) using data from the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP). By analyzing simulations from seven biome models driven by five climate scenarios under two representative concentration pathways (RCP2.6 and RCP8.5), the range of responses and their uncertainty in the 21st century was evaluated. The biome models consistently simulated a gradual increase in vegetation productivity driven by an elevated atmospheric CO2 concentration and a longer growing period. By the 2090s, most simulations showed average net carbon uptake into the northern terrestrial ecosystems of +27 Pg C for RCP2.6 and +48 Pg C for RCP8.5. These estimates showed a wide range of variability among simulations, especially for soil carbon stocks. Even under low greenhouse gas concentrations (RCP2.6), most simulations indicated that vegetation productivity and biomass would change by more than 10%, implying that it will be difficult to completely prevent climatic impacts in northern regions. Simulated spatial patterns and seasonality in the carbon budget can be used to identify sensitive areas and seasons, allowing for improved monitoring. Further research combining observations and modeling will be required to reduce estimation uncertainty and devise ecosystem management options.

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

Recent warming has stimulated the productivity of boreal and Arctic vegetation by reducing temperature limitations. However, several studies have hypothesized that warming may have also increased moisture limitations because of intensified summer drought severity. Establishing the connections between warming and drought stress has been difficult because soil moisture observations are scarce. Here we use recently developed gridded datasets of moisture variability to investigate the links between warming and changes in available soil moisture and summer vegetation photosynthetic activity at northern latitudes (>45°N) based on the Normalized Difference Vegetation Index (NDVI) since 1982. Moisture and temperature exert a significant influence on the interannual variability of summer NDVI over about 29% (mean r2 = 0.29 ± 0.16) and 43% (mean r2 = 0.25 ± 0.12) of the northern vegetated land, respectively. Rapid summer warming since the late 1980s (~0.7 °C) has increased evapotranspiration demand and consequently summer drought severity, but contrary to earlier suggestions it has not changed the dominant climate controls of NDVI over time. Furthermore, changes in snow dynamics (accumulation and melting) appear to be more important than increased evaporative demand in controlling changes in summer soil moisture availability and NDVI in moisture-sensitive regions of the boreal forest. In boreal North America, forest NDVI declines are more consistent with reduced snowpack rather than with temperature-induced increases in evaporative demand as suggested in earlier studies. Moreover, summer NDVI variability over about 28% of the northern vegetated land is not significantly associated with moisture or temperature variability, yet most of this land shows increasing NDVI trends. These results suggest that changes in snow accumulation and melt, together with other possibly non-climatic factors are likely to play a significant role in modulating regional ecosystem responses to the projected warming and increase in evapotranspiration demand during the coming decades.

As different as night and day

An analysis of northern ecosystems shows that the effects on plant growth of rising night-time temperatures are opposite to those of increasing daytime temperatures — a finding that has implications for carbon-cycle models. See Letter p.88 Modelling and observational studies suggest that climate warming is likely to enhance vegetation growth in northern terrestrial ecosystems, increasing carbon storage. However, for the past 50 years the global land surface temperature has been warming faster during the night than during the day, and little is known about the potential effects of this asymmetry on the productivity of plants. Multiple lines of evidence presented in this study, including 28 years of satellite-derived data on vegetation greenness and extensive atmospheric CO2 measurements, suggest that day and night-time warming have divergent effects on vegetation productivity in the Northern Hemisphere. Daytime warming is associated with increasing productivity in wet and cool areas over boreal regions, but with decreasing productivity in dry temperate regions. In contrast, night-time warming decreases vegetation growth in boreal regions, but has mixed effects in dry temperate regions.

Large‐scale variations in the vegetation growing season and annual cycle of atmospheric CO2 at high northern latitudes from 1950 to 2011

We combine satellite and ground observations during 1950-2011 to study the long-term links between multiple climate (air temperature and cryospheric dynamics) and vegetation (greenness and atmospheric CO(2) concentrations) indicators of the growing season of northern ecosystems (>45°N) and their connection with the carbon cycle. During the last three decades, the thermal potential growing season has lengthened by about 10.5 days (P < 0.01, 1982-2011), which is unprecedented in the context of the past 60 years. The overall lengthening has been stronger and more significant in Eurasia (12.6 days, P < 0.01) than North America (6.2 days, P > 0.05). The photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn when factors such as light and moisture limitation may constrain photosynthesis. The autumnal extension of the photosynthetic growing season since 1982 appears to be about half that of the thermal potential growing season, yielding a smaller lengthening of the photosynthetic growing season (6.7 days at the circumpolar scale, P < 0.01). Nevertheless, when integrated over the growing season, photosynthetic activity has closely followed the interannual variations and warming trend in cumulative growing season temperatures. This lengthening and intensification of the photosynthetic growing season, manifested principally over Eurasia rather than North America, is associated with a long-term increase (22.2% since 1972, P < 0.01) in the amplitude of the CO(2) annual cycle at northern latitudes. The springtime extension of the photosynthetic and potential growing seasons has apparently stimulated earlier and stronger net CO(2) uptake by northern ecosystems, while the autumnal extension is associated with an earlier net release of CO(2) to the atmosphere. These contrasting responses may be critical in determining the impact of continued warming on northern terrestrial ecosystems and the carbon cycle.

Recent dynamics of arctic and sub-arctic vegetation

We present a focus issue of Environmental Research Letters on the ‘Recent dynamics of arctic and sub-arctic vegetation’. The focus issue includes three perspective articles (Verbyla 2011 Environ. Res. Lett. 6 041003, Williams et al 2011 Environ. Res. Lett. 6 041004, Loranty and Goetz 2012 Environ. Res. Lett. 7 011005) and 22 research articles. The focus issue arose as a result of heightened interest in the response of high-latitude vegetation to natural and anthropogenic changes in climate and disturbance regimes, and the consequences that these vegetation changes might have for northern ecosystems. A special session at the December 2010 American Geophysical Union Meeting on the ‘Greening of the Arctic’ spurred the call for papers. Many of the resulting articles stem from intensive research efforts stimulated by International Polar Year projects and the growing acknowledgment of ongoing climate change impacts in northern terrestrial ecosystems.

A comparative analysis of changes in the phasing of temperature and satellite-derived greenness at northern latitudes

Plant growth at northern latitudes is highly responsive to the climatic changes that have occurred over recent decades. However, the sensitivity of the phasing of the seasonal cycle of terrestrial ecosystems to a changing environment remains less widely understood. We present an investigation and comparative study of large-scale changes in seasonal cy- cling of both land surface temperature and plant growth. Our results have shown trends in- dicating a marked increased towards overall plant productivity by ~3% from 1982 to 2005, reduced trends in seasonal variation at low-mid latitudes by ~2%, increased trends in sea- sonal variations at mid-high latitudes by ~7%, and an earlier phase in northern terrestrial ecosystems (~1.1 days) in parallel with changes in the phasing of surface temperatures at northern latitudes over the 24 years in this study. These shifts in annual cycles of terrestrial vegetation appear to have a distinct geographical zonality and are dependent upon latitudinal changes in climatic variables. More conspicuous changes in overall vegetation productivity and the seasonal phase of ecosystems have been observed in Eurasia compared to North America, largely because of a more rapid rise in temperature. Our results state that changing climate boosts plant growth at northern latitudes, but also alters the phase and seasonal variations of the annual cycle of terrestrial ecosystems.

Effects of observed and experimental climate change on terrestrial ecosystems in northern Canada: results from the Canadian IPY program

Tundra and taiga ecosystems comprise nearly 40 % of the terrestrial landscapes of Canada. These permafrost ecosystems have supported humans for more than 4500 years, and are currently home to ca. 115,000 people, the majority of whom are First Nations, Inuit and Métis. The responses of these ecosystems to the regional warming over the past 30–50 years were the focus of four Canadian IPY projects. Northern residents and researchers reported changes in climate and weather patterns and noted shifts in vegetation and other environmental variables. In forest-tundra areas tree growth and reproductive effort correlated with temperature, but seedling establishment was often hindered by other factors resulting in site-specific responses. Increased shrub cover has occurred in sites across the Arctic at the plot and landscape scale, and this was supported by results from experimental warming. Experimental warming increased vegetation cover and nutrient availability in most tundra soils; however, resistance to warming was also found. Soil microbial diversity in tundra was no different than in other biomes, although there were shifts in mycorrhizal diversity in warming experiments. All sites measured were sinks for carbon during the growing season, with expected seasonal and latitudinal patterns. Modeled responses of a mesic tundra system to climate change showed that the sink status will likely continue for the next 50–100 years, after which these tundra systems will likely become a net source of carbon dioxide to the atmosphere. These IPY studies were the first comprehensive assessment of the state and change in Canadian northern terrestrial ecosystems and showed that the inherent variability in these systems is reflected in their site-specific responses to changes in climate. They also showed the importance of using local traditional knowledge and science, and provided extensive data sets, sites and researchers needed to study and manage the inevitable changes in the Canadian North.

Regional drought-induced reduction in the biomass carbon sink of Canada's boreal forests

The boreal forests, identified as a critical "tipping element" of the Earth's climate system, play a critical role in the global carbon budget. Recent findings have suggested that terrestrial carbon sinks in northern high-latitude regions are weakening, but there has been little observational evidence to support the idea of a reduction of carbon sinks in northern terrestrial ecosystems. Here, we estimated changes in the biomass carbon sink of natural stands throughout Canada's boreal forests using data from long-term forest permanent sampling plots. We found that in recent decades, the rate of biomass change decreased significantly in western Canada (Alberta, Saskatchewan, and Manitoba), but there was no significant trend for eastern Canada (Ontario and Quebec). Our results revealed that recent climate change, and especially drought-induced water stress, is the dominant cause of the observed reduction in the biomass carbon sink, suggesting that western Canada's boreal forests may become net carbon sources if the climate change-induced droughts continue to intensify.

Interannual variation of evapotranspiration from forest and grassland ecosystems in western canada in relation to drought

Climate models predict rising temperatures and more frequent and prolonged droughts, particularly in the northern hemisphere and in the Canadian Prairies. However, few studies have examined the interannual variation in evapotranspiration (E) of northern terrestrial ecosystems in relation to drought. This study analyses multi-year (1998-2006), eddy-covariance measurements to quantify the seasonal and interannual variability in E from grassland and mature aspen, black spruce, and jack pine forest ecosystems in the dry interior plains of western Canada. It also investigates the response of E to the historic 2001-2003 drought in this region. Leaf area index (LAI) was a primary factor controlling the difference in E among different ecosystems in the same ecozone. Annual E was higher and more variable for the aspen forest (405 ± 84 mm, mean ± s.d.) and grassland (395 ± 90 mm) than the black spruce (374 ± 34 mm) and jack pine (300 ± 20 mm) forests. Interannual variation of E was controlled by early spring soil temperature at all four sites, with warm springs enhancing annual E. Only the aspen forest and grassland showed a significant suppression of E by summer drought, related to reduced surface conductance at the aspen site, and to reduced surface conductance and early leaf senescence at the grassland. We conclude that the potential for drought impacts on annual E of northern ecosystems is greatest for grasslands, moderate for deciduous broadleaf aspen forests, and smallest for coniferous black spruce and jack pine forests. The forests of the Boreal Plains, adjacent to the prairie region, may ameliorate the onset of drought through the recycling of moisture to the atmosphere, whereas the prairie grasslands have only limited capacity to counteract drought through moisture recycling.

Water availability controls microbial temperature responses in frozen soil CO2 production

AbstractSoil processes in high‐latitude regions during winter are important contributors to global carbon circulation, but our understanding of the mechanisms controlling these processes is poor and observed temperature response coefficients of CO2 production in frozen soils deviate markedly from thermodynamically predicted responses (sometimes by several orders of magnitude). We investigated the temperature response of CO2 production in 23 unfrozen and frozen surface soil samples from various types of boreal forests and peatland ecosystems and also measured changes in water content in them after freezing. We demonstrate that deviations in temperature responses at subzero temperatures primarily emanates from water deficiency caused by freezing of the soil water, and that the amount of unfrozen water is mainly determined by the quality of the soil organic matter, which is linked to the vegetation cover. Factoring out the contribution of water limitation to the CO2 temperature responses yields response coefficients that agree well with expectations based on thermodynamic theory concerning biochemical temperature responses. This partitioning between a pure temperature response and the effect of water availability on the response of soil CO2 production at low temperatures is crucial for a thorough understanding of low‐temperature soil processes and for accurate predictions of C‐balances in northern terrestrial ecosystems.