TY - JOUR
T1 - Increasing tree density accelerates stand-level nitrogen cycling at the taiga–tundra ecotone in northeastern Siberia
AU - Hewitt, Rebecca E.
AU - Alexander, Heather D.
AU - Izbicki, Brian
AU - Loranty, Michael M.
AU - Natali, Susan M.
AU - Walker, Xanthe J.
AU - Mack, Michelle C.
N1 - Funding Information:
This research was supported by the Office of Polar Programs Arctic System Science grants to Michelle C. Mack (1545558), Michelle C. Mack and Rebecca E. Hewitt (1708344), and Heather D. Alexander (1304040 and 1708307), Susan M. Natali (1304007), and Michael M. Loranty (1304464 and 1623764). We thank Melissa Boyd and Samantha Miller for help in the field and laboratory.
Publisher Copyright:
© 2022 The Authors. Ecosphere published by Wiley Periodicals LLC on behalf of The Ecological Society of America.
PY - 2022/7
Y1 - 2022/7
N2 - As climate warms, tree density at the taiga–tundra ecotone (TTE) is expected to increase, which may intensify competition for belowground resources in this nitrogen (N)-limited environment. To determine the impacts of increased tree density on N cycling and productivity, we examined edaphic properties indicative of soil N availability along with aboveground and belowground tree-level traits and stand characteristics related to carbon (C) and N cycling across a tree density gradient of monodominant larch (Larix cajanderi) at the TTE in far northeastern Siberia. We found no consistent evidence from soil, tree, or stand-level N cycling characteristics of lower N availability or greater intraspecific competition for N with increased density. Active layer thickness declined, but resin-sorbed N and soil organic layer thickness did not covary with increased tree density. There was, however, greater allocation belowground to stand-level coarse and fine roots with increased tree density, an allocation pattern suggestive of limited soil resources. Foliar traits related to C (%C, δ13C, and resorption) were responsive to density indicating the importance of non-nutrient resources, like light, to foliar stoichiometry. As tree density increased and individual trees had lower productivity, tree-level N and biomass pools aboveground and belowground declined tracking decreases in N uptake, N resorption, N use efficiency, and allocation to slow cycling tissues like wood. At the stand level, our findings show high N turnover with increased N acquisition, allocation to short-lived tissues with relatively high N content and reduced N residence time, and greater stand productivity as tree density increased. Yet, these positive relationships were curtailed at the highest tree densities. Our observations of shifts in biomass, C and N allocation, and loss aboveground, along with greater root density with increased tree density, could have strong impacts on C and N cycling and should be represented in models of TTE dynamics and feedbacks to climate.
AB - As climate warms, tree density at the taiga–tundra ecotone (TTE) is expected to increase, which may intensify competition for belowground resources in this nitrogen (N)-limited environment. To determine the impacts of increased tree density on N cycling and productivity, we examined edaphic properties indicative of soil N availability along with aboveground and belowground tree-level traits and stand characteristics related to carbon (C) and N cycling across a tree density gradient of monodominant larch (Larix cajanderi) at the TTE in far northeastern Siberia. We found no consistent evidence from soil, tree, or stand-level N cycling characteristics of lower N availability or greater intraspecific competition for N with increased density. Active layer thickness declined, but resin-sorbed N and soil organic layer thickness did not covary with increased tree density. There was, however, greater allocation belowground to stand-level coarse and fine roots with increased tree density, an allocation pattern suggestive of limited soil resources. Foliar traits related to C (%C, δ13C, and resorption) were responsive to density indicating the importance of non-nutrient resources, like light, to foliar stoichiometry. As tree density increased and individual trees had lower productivity, tree-level N and biomass pools aboveground and belowground declined tracking decreases in N uptake, N resorption, N use efficiency, and allocation to slow cycling tissues like wood. At the stand level, our findings show high N turnover with increased N acquisition, allocation to short-lived tissues with relatively high N content and reduced N residence time, and greater stand productivity as tree density increased. Yet, these positive relationships were curtailed at the highest tree densities. Our observations of shifts in biomass, C and N allocation, and loss aboveground, along with greater root density with increased tree density, could have strong impacts on C and N cycling and should be represented in models of TTE dynamics and feedbacks to climate.
KW - Arctic
KW - Larix cajanderi
KW - boreal
KW - carbon cycling
KW - natural abundance isotopes
KW - roots
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U2 - 10.1002/ecs2.4175
DO - 10.1002/ecs2.4175
M3 - Article
AN - SCOPUS:85135009605
SN - 2150-8925
VL - 13
JO - Ecosphere
JF - Ecosphere
IS - 7
M1 - e4175
ER -