The effect of growing season and summer greenness on northern forests (original) (raw)
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Identifying Climatic Controls on Ring Width: The Timing of Correlations between Tree Rings and NDVI
Earth Interactions, 2008
The authors examine the effects of latitude and life history on the timing of relationships between satellite measures of normalized difference vegetation index (NDVI) and ground-based measures of tree-ring width in forests at mid- and high latitudes in the Northern Hemisphere. Results indicate a correlation between NDVI and tree rings over the entire growing season for all areas analyzed. For sites south of 40°N, a correlation appears in early spring and late fall while a correlation appears during summer months north of 40°N. For conifers, the correlation appears in summer while deciduous trees show the relationship during early spring and late fall. Of these two correlations, the effect of life history seems to dominate the effect of latitude. The timing of these correlations may help clarify the relationship between climate and tree rings and the effect of canopy duration on carbon uptake by trees.
As the climate changes, warmer spring temperatures are causing earlier leaf-out1–6 and commencement of net carbon dioxide (CO2) sequestration2,4 in temperate deciduous forests, resulting in a tendency towards increased growing season length1,4,5,7–9 and annual CO2 uptake2,4,10–14. However, less is known about how spring temperatures affect tree stem growth, which sequesters carbon (C) in wood that has a long residence time in the ecosystem15,16. Using dendrometer band measurements from 463 trees across two forests, we show that warmer spring temperatures shifted the woody growth of deciduous trees earlier but had no consistent effect on peak growing season length, maximum daily growth rates, or annual growth. The latter finding was confirmed on the centennial scale by 207 tree-ring chronologies from 108 forests across eastern North America, where annual growth was far more sensitive to temperatures during the peak growing season than in the spring. These findings imply that extra CO...
Influence of spring and autumn phenological transitions on forest ecosystem productivity
Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 2010
We use eddy covariance measurements of net ecosystem productivity (NEP) from 21 FLUXNET sites (153 site-years of data) to investigate relationships between phenology and productivity (in terms of both NEP and gross ecosystem photosynthesis, GEP) in temperate and boreal forests. Results are used to evaluate the plausibility of four different conceptual models. Phenological indicators were derived from the eddy covariance time series, and from remote sensing and models. We examine spatial patterns (across sites) and temporal patterns (across years); an important conclusion is that it is likely that neither of these accurately represents how productivity will respond to future phenological shifts resulting from ongoing climate change. In spring and autumn, increased GEP resulting from an 'extra' day tends to be offset by concurrent, but smaller, increases in ecosystem respiration, and thus the effect on NEP is still positive. Spring productivity anomalies Downloaded from appear to have carry-over effects that translate to productivity anomalies in the following autumn, but it is not clear that these result directly from phenological anomalies. Finally, the productivity of evergreen needleleaf forests is less sensitive to phenology than is productivity of deciduous broadleaf forests. This has implications for how climate change may drive shifts in competition within mixedspecies stands.
Oecologia, 2009
While changes in spring phenological events due to global warming have been widely documented, changes in autumn phenology, and therefore in growing season length, are less studied and poorly understood. However, it may be helpful to assess the potential lengthening of the growing season under climate warming in order to determine its further impact on forest productivity and C balance. The present study aimed to: (1) characterise the sensitivity of leaf phenological events to temperature, and (2) quantify the relative contributions of leaf unfolding and senescence to the extension of canopy duration with increasing temperature, in four deciduous tree species (Acer pseudoplatanus, Fagus sylvatica, Fraxinus excelsior and Quercus petraea). For 3 consecutive years, we monitored the spring and autumn phenology of 41 populations at elevations ranging from 100 to 1,600 m. Overall, we found significant altitudinal trends in leaf phenology and species-specific differences in temperature sensitivity. With increasing temperature, we recorded an advance in flushing from 1.9 ± 0.3 to 6.6 ± 0.4 days°C -1 (mean ± SD) and a 0 to 5.6 ± 0.6 days°C -1 delay in leaf senescence. Together both changes resulted in a 6.9 ± 1.0 to 13.0 ± 0.7 days°C -1 lengthening of canopy duration depending on species. For three of the four studied species, advances in flushing were the main factor responsible for lengthening canopy duration with increasing temperature, leading to a potentially larger gain in solar radiation than delays in leaf senescence. In contrast, for beech, we found a higher sensitivity to temperature in leaf senescence than in flushing, resulting in an equivalent contribution in solar radiation gain. These results suggest that climate warming will alter the C uptake period and forest productivity by lengthening canopy duration. Moreover, the between-species differences in phenological responses to temperature evidenced here could affect biotic interactions under climate warming.
Earlier springs decrease peak summer productivity in North American boreal forests
Environmental Research Letters, 2013
Recent change of vegetation growth trend in China Shushi Peng, Anping Chen, Liang Xu et al.-Recent citations Carbon fluxes and interannual drivers in a temperate forest ecosystem assessed through comparison of top-down and bottom-up approaches Andrew P. Ouimette et al-Detecting early warning signals of tree mortality in boreal North America using multiscale satellite data
International Journal of Biometeorology
The inter- and intra-annual variability in radial growth reflects responses to climatic variability and water shortage, especially in areas subjected to seasonal drought. However, it is unknown how this variability is related to forest productivity, which can be assessed by measuring changes in canopy greenness and cover through remote sensing products as the Normalized Difference Vegetation Index (NDVI). We combine xylogenesis with measurements of inter-annual changes in seasonal wood production (earlywood width, adjusted latewood width) and NDVI to improve the understanding of climate and drought impacts on growth and forest productivity in a Pinus teocote stand located in northern Mexico. Cambial dynamics accelerated in March and a high production of radially enlarging and thickening tracheids were observed from April to October and from June to October, respectively. Tracheid maturation was very active in October when latewood production peaked. Wet conditions in winter-spring a...
On linking interannual tree ring variability with observations of whole-forest CO2 flux
Global Change Biology, 2006
We used a 10-year record of the CO2 flux by an old growth boreal forest in central Manitoba (the Northern Old Black Spruce Site (NOBS)), a ∼150-year-old Picea mariana [Mill.] stand) to determine whether and how whole-forest CO2 flux is related to tree ring width. We compared a 37-year ring width chronology collected at NOBS to a second chronology that was collected at a nearby Black Spruce stand with a different disturbance history, and also to three measures of annual whole-forest photosynthesis [gross ecosystem production (GEP)], two measures of annual respiration (R), and one measure of annual carbon balance [net ecosystem production (NEP)]. The year-to-year ring width fluctuations were well correlated between the two sites; increasing our confidence in the NOBS chronology and implying that ring width variation is driven and synchronized by the physical environment. Both chronologies exhibited serial correlation, with a fluctuation in ring width that had an apparent periodicity of ∼7 years. Neither chronology was correlated with variation in annual precipitation or temperature. Ring width and NEP increased, while R decreased from 1995 to 2004. GEP either remained constant or decreased from 1995 to 2004, depending on which measure was considered. The lack of relationship between ring width and GEP may indicate that ring growth is controlled almost entirely by something other than carbon uptake. Alternative explanations for the ring width chronologies include the possibility that wood production varies as a result of shifts in respiration, or that an unidentified aspect of the environment, rather than the balance between GEP and respiration, controls wood production. The serial correlation in ring width may be related to increases and decreases in carbohydrate pools, or to gradual changes in nutrient availability, pathogens, herbivores, soil frost or soil water table. The cause or causes of serial correlation, and the controls on the allocation of photosynthate to wood production, emerge as critical uncertainties for efforts in predicting the carbon balance of boreal ecosystems and inferring past climate from tree rings.
Forests
Forests play a vital role in sequestering carbon dioxide from the atmosphere. Vegetation phenology is sensitive to climate changes and natural environments. Exploring the patterns in phenological events of the forests can provide useful insights for understanding the dynamics of vegetation growth and their responses to climate variations. Deciduous forest in North America is an important part of global forests. Here we apply time-series remote sensing imagery to map the critical dates of vegetation phenological events, including the start of season (SOS), end of season (EOS), and growth length (GL) of the deciduous forests in North America during the past two decades. The findings show that the SOS and EOS present considerable spatial and temporal variations. Earlier SOS, delayed EOS, and therefore extended GL are detected in a large part of the study area from temporal trend analysis over the years, though the magnitude of the trend varies at different locations. The phenological e...