Abstract
Scots pine (Pinus sylvestris L.) is one of the most important conifers in Northern Europe. In boreal forests, over one-third of net primary production is allocated to roots. Pioneer roots expand the horizontal and vertical root systems and transport nutrients and water from belowground to aboveground. Fibrous roots, often colonized by mycorrhiza, emerge from the pioneer roots and absorb water and nutrients from the soil. In this study, we installed three flatbed scanners to detect the daily growth of both pioneer and fibrous roots of Scots pine during the growing season of 2018, a year with an unexpected summer drought in Southern Finland. The growth rate of both types of roots had a positive relationship with temperature. However, the relations between root elongation rate and soil moisture differed significantly between scanners and between root types indicating spatial heterogeneity in soil moisture. The pioneer roots were more tolerant to severe environmental conditions than the fibrous roots. The pioneer roots initiated elongation earlier and ceased it later than the fibrous roots. Elongation ended when the temperature dropped below the threshold temperature of 4 °C for pioneer roots and 6 °C for fibrous roots. During the summer drought, the fibrous roots halted root surface area growth at the beginning of the drought, but there was no drought effect on the pioneer roots over the same period. To compare the timing of root production and the aboveground organs’ production, we used the CASSIA model, which estimates the aboveground tree carbon dynamics. In this study, root growth started and ceased later than growth of aboveground organs. Pioneer roots accounted for 87% of total root productivity. We suggest that future carbon allocation models should separate the roots by root types (pioneer and fibrous), as their growth patterns are different and they have different reactions to changes in the soil environment.
Highlights
Annual production of fine roots in boreal forests varies from 31 to 66% of total forest annual production (Kleja et al 2008, Hansson et al 2013a, Hansson et al 2013b, LeppälammiKujansuu et al 2014, Ding et al 2019), which is higher than the global average of 22–30% (Jackson et al 1997, McCormack et al 2015a)
Scots pine root growth phenology was mainly driven by temperature in boreal forests
The soil moisture had a variable effect on the root length growth indicating spatial variation in the soil
Summary
Annual production of fine roots in boreal forests varies from 31 to 66% of total forest annual production (Kleja et al 2008, Hansson et al 2013a, Hansson et al 2013b, LeppälammiKujansuu et al 2014, Ding et al 2019), which is higher than the global average of 22–30% (Jackson et al 1997, McCormack et al 2015a). In contrast to aboveground components, very little is known about fine root growth phenology (Steinaker and Wilson 2008) despite a large portion of belowground production (mainly on fine roots). Recent evidence shows that root growth phenology is in asynchrony with the shoot phenology across a variety of biomes (Steinaker et al 2010, Du and Fang 2014, Abramoff and Finzi 2015, McCormack et al 2015b, Blume-Werry et al 2016). 50 days later than shoot growth (Abramoff and Finzi 2015). Root growth initiated, peaked and ceased later than leaf growth in both broad-leaved and coniferous boreal forests (Du and Fang 2014)
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