Jian-Guo Huang

Response of Forest Trees to Increased Atmospheric CO 2

The CO 2 fertilization hypothesis stipulates that rising atmospheric CO 2 has a positive effect on tree growth due to increasing availability of carbon. The objective of this paper is to compare the recent literature related to both field CO 2 -enriched experiments with trees and empirical dendrochronological studies detecting CO 2 fertilization effects in tree-rings. This will allow evaluation of tree growth responses to atmospheric CO 2 enrichment by combining evidence from both ecophysiology and tree-ring research. Based on considerable experimental evidence of direct CO 2 fertilization effect (increased photosynthesis, water use efficiency, and above- and belowground biomass), and predications from the interactions of enriched CO 2 with temperature, nitrogen and drought, we propose that warm, moderately drought-stressed ecosystems with an ample nitrogen supply might be the most CO 2 responsive ecosystems. Empirical tree-ring studies took the following three viewpoints on detecting CO 2 fertilization effect in tree-rings: 1) finding evidence of CO 2 fertilization effect in tree-rings, 2) attributing growth enhancement to favorable climate rather than atmospheric CO 2 enrichment, and 3) considering that tree growth enhancement might be caused by synergistic effects of several factors such as favorable climate change, CO 2 fertilization, and anthropogenic atmospheric deposition (e.g., nitrogen). At temperature-limiting sites such as high elevations, nonfindings of CO 2 fertilization evidence could be ascribed to the following possibilities: 1) cold temperatures, a short season of cambial division, and nitrogen deficiency that preclude a direct CO 2 response, 2) old trees past half of their maximum life expectancy and consequently only a small increase in biomass increment due to CO 2 fertilization effect might be diminished, 3) the elimination of age/size-related trends by statistical detrending of tree-ring series that might remove some long-term CO 2 -related trends in tree-rings, and 4) carbon partitioning and growth within a plant that is species-specific. Our review supports the atmospheric CO 2 fertilization effect hypothesis, at least in trees growing in semi-arid or arid conditions because the drought-stressed trees could benefit from increased water use efficiency to enhance growth.

Woody biomass production lags stem-girth increase by over one month in coniferous forests

Wood is the main terrestrial biotic reservoir for long-term carbon sequestration(1), and its formation in trees consumes around 15% of anthropogenic carbon dioxide emissions each year(2). However, the seasonal dynamics of woody biomass production cannot be quantified from eddy covariance or satellite observations. As such, our understanding of this key carbon cycle component, and its sensitivity to climate, remains limited. Here, we present high-resolution cellular based measurements of wood formation dynamics in three coniferous forest sites in northeastern France, performed over a period of 3 years. We show that stem woody biomass production lags behind stem-girth increase by over 1 month. We also analyse more general phenological observations of xylem tissue formation in Northern Hemisphere forests and find similar time lags in boreal, temperate, subalpine and Mediterranean forests. These time lags question the extension of the equivalence between stem size increase and woody biomass production to intra-annual time scales(3, 4, 5, 6). They also suggest that these two growth processes exhibit differential sensitivities to local environmental conditions. Indeed, in the well-watered French sites the seasonal dynamics of stem-girth increase matched the photoperiod cycle, whereas those of woody biomass production closely followed the seasonal course of temperature. We suggest that forecasted changes in the annual cycle of climatic factors(7) may shift the phase timing of stem size increase and woody biomass production in the future.

Impact of Future Climate on Radial Growth of Four Major Boreal Tree Species in the Eastern Canadian Boreal Forest

Immediate phenotypic variation and the lagged effect of evolutionary adaptation to climate change appear to be two key processes in tree responses to climate warming. This study examines these components in two types of growth models for predicting the 2010–2099 diameter growth change of four major boreal species Betula papyrifera, Pinus banksiana, Picea mariana, and Populus tremuloides along a broad latitudinal gradient in eastern Canada under future climate projections. Climate-growth response models for 34 stands over nine latitudes were calibrated and cross-validated. An adaptive response model (A-model), in which the climate-growth relationship varies over time, and a fixed response model (F-model), in which the relationship is constant over time, were constructed to predict future growth. For the former, we examined how future growth of stands in northern latitudes could be forecasted using growth-climate equations derived from stands currently growing in southern latitudes assuming that current climate in southern locations provide an analogue for future conditions in the north. For the latter, we tested if future growth of stands would be maximally predicted using the growth-climate equation obtained from the given local stand assuming a lagged response to climate due to genetic constraints. Both models predicted a large growth increase in northern stands due to more benign temperatures, whereas there was a minimal growth change in southern stands due to potentially warm-temperature induced drought-stress. The A-model demonstrates a changing environment whereas the F-model highlights a constant growth response to future warming. As time elapses we can predict a gradual transition between a response to climate associated with the current conditions (F-model) to a more adapted response to future climate (A-model). Our modeling approach provides a template to predict tree growth response to climate warming at mid-high latitudes of the Northern Hemisphere.

The contribution of carbon and water in modulating wood formation in black spruce saplings

During wood formation, water availability is the most important factor for cell production, while carbon is more important to sustain the differentiation of living cells. Nonstructural carbohydrates (NSCs) play a crucial role in xylem formation and represent, with water, the main constraint to plant growth. We assessed the relationships between xylogenesis and NSCs in order to (1) verify the variance explained by NSCs and (2) determine the influence of intrinsic (tissue supplying carbon) and extrinsic (water availability and temperature) factors. During 2 years, wood formation was monitored in saplings of black spruce (Picea mariana) subjected to a dry period of about 1 month in June and exposed to different temperature treatments in a greenhouse. In parallel, NSC concentrations were determined by extracting the sugar compounds from two tissues (cambium and inner xylem), both potentially supplying carbon for wood formation. A mixed-effect model was used to assess and quantify the potential relationships. Total xylem cells, illustrating meristematic activity, were modeled as a function of water, sucrose, and d-pinitol (conditional r2 of 0.79). Water availability was ranked as the most important factor explaining total xylem cell production, while the contribution of carbon was lower. Cambium stopped dividing under water deficit, probably to limit the number of cells remaining in differentiation without an adequate amount of water. By contrast, carbon factors were ranked as most important in explaining the variation in living cells (conditional r2 of 0.49), highlighting the functional needs during xylem development, followed by the tissue supplying the NSCs (cambium) and water availability. This study precisely demonstrates the role of carbon and water in structural growth expressed as meristematic activity and tissue formation.

CAN Canopy Addition of Nitrogen Better Illustrate the Effect of Atmospheric Nitrogen Deposition on Forest Ecosystem

Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Here we, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. UAN and reviewed the merits and pitfalls of the two approaches. The following hypotheses will be tested: i) UAN overestimates the N addition effects on understory and soil processes but underestimates those on canopy-associated biota and processes, ii) with low-level N addition, CAN favors canopy tree species and canopy-dwelling biota and promotes the detritus food web, and iii) with high-level N addition, CAN suppresses canopy tree species and other biota and favors rhizosphere food web. As a long-term comprehensive program, this experiment will provide opportunities for multidisciplinary collaborations, including biogeochemistry, microbiology, zoology, and plant science to examine forest ecosystem responses to atmospheric N deposition.

Variation in intra-annual wood formation, and foliage and shoot development of three major Canadian boreal tree species

PREMISE OF THE STUDY In a warming climate, boreal trees may have adjusted their growth strategy (e.g., onset and coordination of growth among different organs such as stem, shoot, and foliage, within and among species) to cope with the extended growing seasons. A detailed investigation into growth of different organs during a growing season may help us assess the potential effects of climate change on tree growth in the boreal forest. METHODS The intra-annual growth of stem xylem, shoot tips, and foliage area of Pinus banksiana, Populus tremuloides, and Betula papyrifera was monitored in a boreal forest in Quebec, Canada during the growing season of 2007. Xylem formation was measured at weekly intervals, and shoot elongation and foliage expansion were measured three times per week from May to September. Growth indices for stem, shoot, and foliage were calculated and used to identify any climate-growth dependence. KEY RESULTS The time periods required for stem growth, branch extension, and foliage expansion differed among species. Of the three species, P. banksiana had the earliest budburst (20 May) yet the latest completion date of the foliage growth (2 August); P. tremuloides had the latest budburst (27 May) yet the earliest completion date of the foliage growth (10 July). Air temperature positively affected shoot extension growth of all three species. Precipitation positively influenced stem growth of the two broadleaf species, whereas growing season temperature positively impacted stem growth of P. banksiana. CONCLUSION The results show that both the timing of growth processes and environmental dependences differ among co-occurring species, thereby leading to different adaptive capability of these boreal tree species to climate change.

Modelling growth-competition relationships in trembling aspen and white spruce mixed boreal forests of Western Canada.

We examined the effect of competition on stem growth of Picea glauca and Populus tremuloides in boreal mixedwood stands during the stem exclusion stage. We combined traditional approaches of collecting competition data with dendrochronology to provide retrospective measurements of stem diameter growth. Several competition indices including stand basal area (BA), the sum of stem diameter at breast height (SDBH), and density (N) for the broadleaf and coniferous species, as well as similar indices considering only trees with diameters greater than each subject (BAGR, SDBHGR, and NGR), were evaluated. We used a nonlinear mixed model to characterize the basal area increment over the past 5, 10, 15, 20, 25, 30, and 35 years as a function of growth of nearby dominant trees, the size of the subject trees, deciduous and coniferous competition indices, and ecoregions. SDBHGR and BAGR were better predictors for spruce, and SDBHGR and NGR were better for aspen, respectively, than other indices. Results showed strongest correlations with long-term stem growth, as the best models integrated growth for 10–25 years for aspen and ≥25 for spruce. Our model demonstrated a remarkable capability (adjusted R2>0.67) to represent this complex variation in growth as a function of site, size and competition.

Capturing spiral radial growth of conifers using the superellipse to model tree-ring geometric shape.

Tree-rings are often assumed to approximate a circular shape when estimating forest productivity and carbon dynamics. However, tree rings are rarely, if ever, circular, thereby possibly resulting in under- or over-estimation in forest productivity and carbon sequestration. Given the crucial role played by tree ring data in assessing forest productivity and carbon storage within a context of global change, it is particularly important that mathematical models adequately render cross-sectional area increment derived from tree rings. We modeled the geometric shape of tree rings using the superellipse equation and checked its validation based on the theoretical simulation and six actual cross sections collected from three conifers. We found that the superellipse better describes the geometric shape of tree rings than the circle commonly used. We showed that a spiral growth trend exists on the radial section over time, which might be closely related to spiral grain along the longitudinal axis. The superellipse generally had higher accuracy than the circle in predicting the basal area increment, resulting in an improved estimate for the basal area. The superellipse may allow better assessing forest productivity and carbon storage in terrestrial forest ecosystems.

Variation in intra-annual radial growth (xylem formation) of Picea mariana (Pinaceae) along a latitudinal gradient in western Quebec, Canada

PREMISE OF THE STUDY Climate warming might have resulted in altered initiation and termination dates of stem xylem growth in boreal stands. A systematic understanding of intra-annual xylem formation is thus needed for precise simulation of future growth in the context of sustainable forest management. METHODS A recently developed novel microsampling approach was employed over two growing seasons (2005-2006) to investigate the intra-annual stem xylem formation of Picea mariana at three sites along a latitudinal gradient (approximately 47.5-50°N) in western Quebec, Canada. The critical timings of xylem cell formation were determined and compared among sites/years. The relationships between xylem cell formation and meteorological variables were examined. KEY RESULTS From south to north, the onset of xylem cell production was detected on 20 May (SD ±3) at Angliers, 24 May (SD ±3) at Chicobi and 24 May (SD±4) at Muskuchii in 2005, and on 12 May (SD ±4) at Angliers, 14 May (SD ±3) at Chicobi and 20 May (SD ±3) at Muskuchii in 2006, respectively. Xylem cell production at each respective site terminated on 11 August (SD ±4), 7 August (SD ±3), and 7 August (SD ±4) in 2005, and on 8 August (SD ±4), 4 August (SD ±4), and 4 August (SD ±4) in 2006, respectively. CONCLUSION Our study implies that despite the expected occurrence of earlier phenological development due to early spring climate warming, boreal trees like P. mariana might not be producing wider rings if cold temperatures occur later in the growing season in June to August. These results may challenge the view that boreal trees could be benefiting from spring warming to enhance growth.

Effects of competition, drought stress and photosynthetic productivity on the radial growth of white spruce in western Canada

Understanding the complex interactions of competition, climate warming-induced drought stress, and photosynthetic productivity on the radial growth of trees is central to linking climate change impacts on tree growth, stand structure and in general, forest productivity. Using a mixed modeling approach, a stand-level photosynthetic production model, climate, stand competition and tree-ring data from mixedwood stands in western Canada, we investigated the radial growth response of white spruce [Picea glauca (Moench.) Voss] to simulated annual photosynthetic production, simulated drought stress, and tree and stand level competition. The long-term (~80-year) radial growth of white spruce was constrained mostly by competition, as measured by total basal area, with minor effects from drought. There was no relation of competition and drought on tree growth but dominant trees increased their growth more strongly to increases in modeled photosynthetic productivity, indicating asymmetric competition. Our results indicate a co-limitation of drought and climatic factors inhibiting photosynthetic productivity for radial growth of white spruce in western Canada. These results illustrate how a modeling approach can separate the complex factors regulating both multi-decadal average radial growth and interannual radial growth variations of white spruce, and contribute to advance our understanding on sustainable management of mixedwood boreal forests in western Canada.