Climate change-associated trends in biomass dynamics are consistent across soil drainage classes in western boreal forests of Canada

Eric B Searle,Han Y H Chen

doi:10.1186/s40663-017-0106-y

Abstract

BackgroundConsistent long-term declines in net aboveground biomass change have been reported in some boreal and tropical forests. Global change-type drought (i.e., demands of increased evapotranspiration exceeding soil water reserves) has been identified as the main driver for these declines. Despite the focus on reduced water availability, most studies relegate local site soil drainage to a plot random effect. However, if the major cause of some region’s recent loss in net aboveground biomass change is global change-type drought, those soils with less drainage capacity should help buffer against increased evapotranspiration, resulting in less negative effects of global change-type drought on growth, mortality and net biomass change.MethodsHere we used a network of 1279 permanent sampling plots, measured from 1958 to 2009, from western Canada, where long-term decline of climate moisture availability has been observed, to examine how soil drainage could affect the response of forest net biomass change and its components (growth and mortality) to global change-type drought.ResultsAfter accounting for the effects of endogenous forest age-related processes, temporal changes in absolute rates of biomass gain from growth did not differ among drainage classes, and temporal increases in biomass loss from tree mortality were also similar across drainage classes, resulting in similar decreases in net biomass change. Relative growth was significantly higher on moderately drained sites than well drained or poorly drained sites likely due to larger temporal decreases in standing biomass relative to declines in temporal growth on moderately drained soils. Moreover, growth, mortality, and net biomass change responded to atmospheric CO2, annual temperature anomaly, and standardized precipitation evapotranspiration index similarly across all drainage classes.ConclusionsOur results suggest that climate change serves as a top-down control on forest growth, mortality and net biomass change.

Highlights

Consistent long-term declines in net aboveground biomass change have been reported in some boreal and tropical forests
We selected plots according to four criteria (i) Permanent Sample Plots (PSPs) had a known origin date of stand replacing wildfire, and were unmanaged; (ii) PSPs had all trees marked and tagged with diameter at breast height (DBH) and species identification accurately tracked over multiple censuses; and, (iii) PSPs had to have a minimum of three censuses; and (iv) plot size, soil drainage class, and spatial location were available
Absolute growth declined at −0.001 ± 0.002 Mg ha−1 year−1 with calendar year, while absolute mortality increased at 0.015 ± 0.004 Mg ha−1 year−2, resulting in a decline of absolute net biomass change of 0.016 ± 0.004 Mg ha−1 year−2 (Table 1; Fig. 2a)

Summary

Methods

Study area To examine the simultaneous effects of endogenous processes and climate change, we used data from a network of Permanent Sample Plots (PSPs), established throughout Alberta and Saskatchewan. Explanatory variables We used calendar year to represent temporal changes in climatic conditions as a whole, following previous studies (Brienen et al 2015; Chen et al 2016; Searle and Chen 2017a), corresponding to each observation of ΔAGB, relative ΔAGB and their related components. This encompasses the systematic increases in atmospheric CO2 concentration and temperature and a decrease in climate moisture index, and the changes in other climatic and non-climatic drivers. Analysis was conducted in R 3.4.0 (R Core Development Team 2017), using the lme package (Bates et al 2015)

Results

Conclusions

Background

Discussion

Conclusion