Air temperature triggers the recovery of evergreen boreal forest photosynthesis in spring

Abstract

The timing of the commencement of photosynthesis (P*) in spring is an important determinant of growing‐season length and thus of the productivity of boreal forests. Although controlled experiments have shed light on environmental mechanisms triggering release from photoinhibition after winter, quantitative research for trees growing naturally in the field is scarce. In this study, we investigated the environmental cues initiating the spring recovery of boreal coniferous forest ecosystems under field conditions. We used meteorological data and above‐canopy eddy covariance measurements of the net ecosystem CO2 exchange (NEE) from five field stations located in northern and southern Finland, northern and southern Sweden, and central Siberia. The within‐ and intersite variability for P* was large, 30–60 days. Of the different climate variables examined, air temperature emerged as the best predictor for P* in spring. We also found that ‘soil thaw’, defined as the time when near‐surface soil temperature rapidly increases above 0°C, is not a useful criterion for P*. In one case, photosynthesis commenced 1.5 months before soil temperatures increased significantly above 0°C. At most sites, we were able to determine a threshold for air‐temperature‐related variables, the exceeding of which was required for P*. A 5‐day running‐average temperature (T5) produced the best predictions, but a developmental‐stage model (S) utilizing a modified temperature sum concept also worked well. But for both T5 and S, the threshold values varied from site to site, perhaps reflecting genetic differences among the stands or climate‐induced differences in the physiological state of trees in late winter/early spring. Only at the warmest site, in southern Sweden, could we obtain no threshold values for T5 or S that could predict P* reliably. This suggests that although air temperature appears to be a good predictor for P* at high latitudes, there may be no unifying ecophysiological relationship applicable across the entire boreal zone.

Attribution of divergent northern vegetation growth responses to lengthening non-frozen seasons using satellite optical-NIR and microwave remote sensing

“…Abundant spring solar radiation coupled with earlier seasonal thawing promotes deeper soil active layer development and release of plant-available moisture and nutrients, leading to rapid vegetation carbon uptake and earlier C spr (Bergh and Linder 1999;Tanja et al 2003;Wang et al 2011). However, our results indicate that longer NF seasons also promote increased amplitude of the atmospheric CO 2 seasonal cycle; thus the relative increase in CO 2 uptake by ecosystem productivity is being offset by enhanced carbon emissions, likely due to commensurate increases in regional disturbance and respiration processes (Angert et al 2005;Buermann et al 2013;Kasischke et al 2005).…”

Detecting Inter-Annual Variations in the Phenology of Evergreen Conifers Using Long-Term MODIS Vegetation Index Time Series

“…Determining the metrics based on the raw flux data according to Tanja et al [60] and Thum et al [61]. SGS and EGS were defined as the first and the last date at which the daily mean NEE first fall below 20% of the maximum summer uptake of CO 2 defined as the lowest one percentile of NEE values.…”

“…Therefore, some time lag between the estimates is not surprising considering that the same methods have been applied to two types of data based on fundamentally different observation techniques. The 20% threshold was selected according to Tanja et al [60] and Thum et al [61], who found this to produce similar predictions to those of temperature data, however, relative thresholds used in the reviewed literature vary between 10% [62] and 50% [9]. Derivative methods to identify the points of maximum increase on the seasonal curve have also been applied in previous studies (e.g., Tan et al [54]), which is particularly beneficial when applied to regional and global scale studies and ecosystems characterized by multiple growth cycles [55].…”

Species-specific temporal variation in photosynthesis as a moderator of peatland carbon sequestration

“…1a and b). Temperature limitation of springtime photosynthesis is well known for boreal forests (Tanja et al, 2003;Ensminger et al, 2004), as well as for bog Sphagna (Moore et al, 2006). Mean annual temperature together with PPFD during the growing season are the most important factors explaining Sphagnum productivity on the global scale (Gunnarsson, 2005;Loisel et al, 2012), and the temperature optimum of Sphagnum photosynthesis is known to change over the growing season (Gaberščik and Martinčič, 1987).…”

“…This hypothesis has gained support from testing in several ecosystem types, especially in grasslands (Hector et al, 2010;Cardinale et al, 2011;Morin et al, 2014). In addition to species and genotype diversity (Hughes et al, 2008), the functional diversity, i.e., the presence of species and plant functional types with different physiology, morphology, resource requirements, seasonal growth patterns and life history may increase the productivity of an ecosystem (Tilman et al, 1997;Cadotte et al, 2008). Although this study did not directly test the insurance hypothesis, our results also indicate that functional diversity, especially in regard of differences in phenology and seasonal changes in photosynthetic parameters, decreased the temporal variation of ecosystemlevel P G and could therefore decrease the variation of the ecosystem C sink.…”