Publications by R.S. Jassal

Interannual variability of net ecosystem productivity in forests is explained by carbon flux phenology in autumn.

Thu, 07 Feb 2013

Aim To investigate the importance of autumn phenology in controlling interannual variability of forest net ecosystem productivity (NEP) and to derive new phenological metrics to explain the interannual variability of NEP.

Location North America and Europe.

Method Flux data from nine deciduous broadleaf forests (DBF) and 13 evergreen needleleaf forests (ENF) across North America and Europe (212 site-years) were used to explore the relationships between the yearly anomalies of annual NEP and several carbon flux based phenological indicators, including the onset/end of the growing season, onset/end of the carbon uptake period, the spring lag (time interval between the onset of growing season and carbon uptake period) and the autumn lag (time interval between the end of the carbon uptake period and the growing season). Meteorological variables, including global shortwave radiation, air temperature, soil temperature, soil water content and precipitation, were also used to explain the phenological variations.

Results We found that interannual variability of NEP can be largely explained by autumn phenology, i.e. the autumn lag. While variation in neither annual gross primary productivity (GPP) nor in annual ecosystem respiration (R_e) alone could explain this variability, the negative relationship between annual NEP and autumn lag was due to a larger R_e/GPP ratio in years with a prolonged autumn lag. For DBF sites, a longer autumn lag coincided with a significant decrease in annual GPP but showed no correlation with annual R_e. However, annual GPP was insensitive to a longer autumn lag in ENF sites but annual R_e increased significantly.

Main conclusions These results demonstrate that autumn phenology plays a more direct role than spring phenology in regulating interannual variability of annual NEP. In particular, the importance of respiration may be potentially underestimated in deriving phenological indicators.

Interannual and spatial impacts of phenological transitions, growing season length, and spring and autumn temperatures on carbon sequestration: a North America flux data synthesis.

Tue, 19 Jun 2012

Understanding feedbacks of ecosystem carbon sequestration to climate change is an urgent step in developing future ecosystem models. Using 187 site-years of flux data observed at 24 sites covering three plant functional types (i.e. evergreen forests (EF), deciduous forests (DF) and non-forest ecosystems (NF) (e.g., crop, grassland, wetland)) in North America,we present an analysis of both interannual and spatial relationships between annual net ecosystem production (NEP) and phenological indicators, including the flux-based carbon uptake period (CUP) and its transitions, degree-day-derived growing season length (GSL), and spring and autumn temperatures. Diverse responses were acquired between annul NEP and these indicators across PFTs. Forest ecosystems showed consistent patterns and sensitivities in the responses of annual NEP to CUP and its transitions both interannually and spatially. The NF ecosystems, on the contrary, exhibited different trends between interannual and spatial relationships. The impact of CUP onset on annual NEP in NF ecosystems was interannually negative but spatially positive. Generally, the GSL was observed to be a likely good indicator of annual NEP for all PFTs both interannually and spatially, although with relatively moderate correlations in NF sites. Both spring and autumn temperatures were positively correlated with annual NEP across sites while this potential was greatly reduced temporally with only negative impacts of autumn temperature on annual NEP in DF sites. Our analysis showed that DF ecosystems have the highest efficiency in accumulating NEP from warmer spring temperature and prolonged GSL, suggesting that future climate warming will favor deciduous species over evergreen species, and supporting the earlier observation that ecosystemswith the greatest net carbon uptake have the longest GSL.

Decarbonization of the atmosphere: role of the boreal forest under changing climate.

Thu, 07 Jun 2012

The boreal forest, with an area of about 11.4 million km², is the second largest terrestrial biome and plays a critical role in the global carbon (C) cycle. Its role in either accelerating or slowing climate change depends on whether the boreal forest is a net C source or a net C sink. The boreal forest stores 715.2 Pg C with 430.2 Pg present in peatlands and the remaining in forest ecosystems. In forest, about 60% of total C is present in the soil. The boreal forest sequesters C in surface vegetation, and has accumulated and conserved annual increments of C for millennia in soils, permafrost deposits, wetlands and peatlands. The net annual C sink of the boreal forest increased signifi cantly over the last 20 years, from 0.54 Pg C year −1 to 1.07 Pg C year −1 . The future C balance of the boreal forest largely depends on the frequency and intensity of different disturbances, changes in species composition, forest management regimes and alterations to the nutrient and moisture regimes under changing climate conditions. The role of the boreal forest in the decarbonization of the atmosphere can be strengthened through techniques that reduce the time for stand establishment (such as site preparation, planting, and weed control) or increase the available nutrients for growth, or through the selection of species that are more productive. Fire- and insect-protection activities have a strong impact on the C sink strength of the boreal landscape. Therefore, reducing the area prone to fi re and insect mortality, and extending the rotation age for holding C longer in older age classes will strongly increase the capacity of the boreal forest to decarbonize the atmosphere.

Assessing the impact of N-fertilization on biochemical composition and biomass of a Douglas-fir canopy—A remote sensing approach

Wed, 27 Apr 2011

Vegetation biochemistry is a critical driver of the forest carbon and water cycle and the fluxes between the land surface and the atmosphere. As result, monitoring biochemistry is a key to improving our estimates of the terrestrial carbon and energy budget. While destructive sampling techniques have been widely applied to determine nutrient content in foliage, scaling of these measurements to the stand and landscape is challenging. As an alternative to traditional field-based approaches, optical remote sensing is a powerful technique for sampling biochemical constituents in a spatially continuous fashion. Remote sensing of biochemical constituents is based on the understanding that leaf biochemistry is closely linked to absorption and reflectance properties in characteristic, often spectrally narrow, wavebands. Spectral absorption features can be identified to characterize and quantify biochemical properties at the leaf, stand and landscape level. At the same time, Light Detection and Ranging (LiDAR) remote sensing can allow inference about the impact of leaf biochemistry on tree growth and canopy structure. In this study, we report the effect of nitrogen-fertilization of a Douglas-fir dominated forest on Vancouver Island, British Columbia, Canada using active and passive remote sensing techniques. Leaf pigment concentrations were estimated from inversion of a canopy reflectance model (PROSAIL) and canopy nitrogen (N) was inferred from an airborne imaging spectrometer (AVIRIS). The impact of leaf biochemistry on canopy structure and tree growth was then investigated using a temporal sequence of LiDAR data acquired two years before, and after, the fertilization treatment. Results indicate that while fertilization had a significant impact on canopy pigment concentrations, it did not impact canopy nitrogen. A notable increase in tree growth was found for younger stands of less than 15 m of height, but not for more mature stand with taller trees. Fertilization had no immediate impact on canopy density measured from LiDAR derived leaf area and canopy volume. The use of advanced remote sensing tools and techniques such as those demonstrated in this study can be a powerful addition to ongoing efforts to model carbon and water fluxes throughout the landscape.

Soil respiration in four different land use systems in north central Alberta, Canada

Tue, 02 Feb 2010

This study compares soil respiration and its heterotrophic and autotrophic components in four land use types: agriculture, 2 and 9 year old hybrid poplar plantations, grassland, and a native aspen stand in north central Alberta, Canada, over a period of two growing seasons (2006 and 2007). The differences were examined with respect to substrate quality and quantity, fine root biomass, and nutrient availability, in addition to soil temperature and soil water content. Cumulative soil C loss via soil respiration averaged over the two growing seasons was (in decreasing order) 781, 551, 523, 502, and 428 g C m-2 for native aspen stand, 9 year old hybrid poplar plantation, grassland, agriculture and 2 year old hybrid poplar plantation, respectively. We found that ~75% of soil respiration in the native aspen stand originated from the top 7.5–10 cm litter-fibric-humus layer. Seasonal heterotrophic and autotrophic respiration among the land uses ranged from 97 to 272 and 333 to 560 g C m-2, respectively, contributing up to 35% and 83% of total soil respiration, respectively. The variability in soil respiration across different land uses was explained mainly by site differences in soil temperature (88–94%). Soil respiration followed a pronounced seasonal trend: increasing during the growing season and converging to a minimum in the fall. Soil respiration under different land uses was influenced by (1) ecosystem C stock, (2) temperature sensitivity (Q10) of organic matter present, and (3) organic matter decomposability as indicated by the natural abundance of d13C. Heterotrophic respiration was influenced by soil temperature, while autotrophic respiration was influenced by fine root biomass and nutrient (NO3- and P) availability. These results are useful in estimating potential responses of soil respiration and its components to future land management and climate change.

N2O emissions and carbon sequestration in a nitrogen-fertilized Douglas fir stand

Wed, 05 Nov 2008

This study investigated how nitrogen (N) fertilization with 200 kg N/ha of a 58-year-old West Coast Douglas fir stand influenced its net greenhouse gas (GHG) global warming potential (GWP) in the first year after fertilization. Effects of fertilization on GHG GWP were calculated considering changes in soil N2O emissions, measured using the static chamber technique and the soil N2O gradient technique; eddy covariance (EC) measured net ecosystem productivity (NEP); and energy requirements of fertilizer production, transport, and its aerial spreading. We found significant N2O losses in fertilized plots compared to a small uptake in nonfertilized plots. Chamber-measured N loss in the fertilized plots was about 16 kg N2O/ha in the first year, which is equivalent to 10 kg N/ha or 5% of the applied fertilizer N. Soil N2O emissions measured using the gradient technique, however, exceeded the chamber measurements by about 50%. We also compared a polymer-coated slow-release urea with regular urea and found that the former delayed N2O emissions but the year-end total loss was about the same as that from regular urea. Change in NEP due to fertilization was determined by relating annual NEP for the nonfertilized stand to environmental controls using an empirical and a process-based model. Annual NEP increased by 64%, from 326 g C m-2, calculated assuming that the stand was not fertilized, to the measured value of 535 g C m-2 with fertilization. At the end of the year, net change in GHG GWP was -2.28 t CO2 /ha compared to what it would have been without fertilization, thereby indicating favorable effect of fertilization even in the first year after fertilization with significant emissions of N2O.

Comparison of MODIS, eddy covariance determined and physiologically modelled gross primary production (GPP) in a Douglas-fir forest stand

Mon, 15 Jun 2009

Quantification of the magnitude of net terrestrial carbon (C) uptake, and how it varies inter-annually, is an important question with future potential sequestration influenced by both increased atmospheric CO2 and changing climate. However the assessment of differences in measured and modeled C accumulation is a challenging task due to the significant fine scale variation occurring in terrestrial productivity due to soil, climate and vegetation characteristics as well as difficulties in measuring carbon accumulation over large spatial areas. The Moderate Resolution Imaging Spectroradiometer (MODIS) offers a means of monitoring gross primary production (GPP), both spatially and temporally, routinely from space. However it is critical to compare and contrast the temporal dynamics of the C and water fluxes with those measured from ground-based networks, or estimated using physiological models. In this paper, using a number of approaches, our objective is to determine if any systematic biases exists in either the MODIS, or the modeled estimates of fluxes, relative to the measurements made over an evergreen, needleleaf temperate rainforest on Vancouver Island, Canada. Results indicate that 8-day GPP as predicted with a simple physiological model (3PGS), forced using local meteorology and canopy characteristics, matched measured fluxes very well (r2 = 0.86, p < 0.001) with no significant difference between eddy covariance (EC) and modeled GPP (p < 0.001). In addition, modeled water supply closely matched measured relative available soil water content at the site. Using canopy characteristics from the MODIS fraction of photosynthetically active radiation (fPAR) algorithm, slightly reduced the correspondence of the predictions due to a large number of unsuccessful retrievals (83%) due to sun angle, snow and cloud. Predictions of GPP based on the MODIS GPP algorithm, forced using local meteorology and canopy characteristics, were also highly correlated with EC measurements (r2 = 0.89, p < 0.001) however these estimates were biased under predicting GPP. Estimates of GPP based on the most recent MODIS reprocessing (collection 4.5) remained highly correlated (r2 = 0.88, p < 0.001) yet were also the most biased with the estimates being 30% less than the EC-measured GPP. Most of the variance in GPP at the site was explained by the absorbed photosynthetically active radiation. We also compared the nighttime respiration as measured over 2 years at the site with the minimum 8-day MODIS land surface temperature and found a significant relationship (r2 = 0.57), similar to other studies.

Impact of changing soil moisture distribution on net ecosystem productivity of a boreal aspen forest during and following drought

Wed, 04 Apr 2007

The interannual and seasonal variability of gross ecosystem photosynthesis (P), ecosystem respiration (R) and evapotranspiration (E), and their relationships to environmental factors were used to explain changes in net ecosystem productivity (FNEP) at the onset of, during, and following a 3-year-long (2001–2003) drought in a mature boreal aspen stand in central Saskatchewan, Canada. The forest was a moderate carbon (C) sink over its entire 11-year data record (FNEP = 153 ± 99 g C m-2 year-1), including the peak drought years of 2002 and 2003. In 2001, the depletion of water near the soil surface likely reduced heterotrophic soil respiration while water remaining deep in the root zone maintained P above the pre-drought mean, resulting in above-average FNEP. In 2002 and 2003, the forest remained a C sink even though P was below average because R was also below average—a likely consequence of the influence of low soil water content in deep and shallow soil layers on both autotrophic and heterotrophic respiration. In 2004, the recharge of soil water in shallow soil layers allowed R to recover to its pre-drought values, whereas low spring temperatures, the slow recharge of soil water in deep soil layers in spring, late leaf emergence and diminished leaf area index combined to suppress P and produce the lowest annual FNEP of the 11-year record (4 g C m-2 year-1). The low FNEP and P were mirrored in the lowest stem growth and LAI values of the 11-year record. In 2005, a warm wet year, both the annual values and seasonal variations of FNEP, P and R returned to those of pre-drought years; the partial recovery of LAI to pre-drought values suggests that aspen P was able to adjust to this restriction on C assimilation. Growing season average dry surface conductance (gsv), the Priestley–Taylor coefficient (a) and light use efficiency (LUE) reached their lowest values in 2003 and became similar to pre-drought years in 2004–2005. Water use efficiency (WUE) was highest in 2003 and remained above average in 2004 and 2005. At the ecosystem scale, the above-average gains made in C sequestration in the first year of the drought were significantly offset by below-average stand FNEP in the final 2 years of the drought, and in the year following the drought.