Page 149 - PC2019 Program & Proceedings

PLANT CANADA 2019

S83. How is the understory vegetation influenced by changes in tree canopy dominance in black
spruce and trembling aspen in a Canadian boreal forest?
2
1
1
*1
Rodríguez, J. ; É. Mestre ; N. Fenton ; S. Kembel ; Y. Bergeron
2
1 Université du Québec en Abitibi-Témiscamingue (UQAT)
2 Université du Québec à Montréal (UQAM))

The Canadian boreal forest has globally significant ecosystem services related with nutrient and water
cycles, as well as wood production. Canopy composition influences habitat conditions for plants in the
understory of these forests and consequently affects ecosystem processes. In the clay belt of Québec, we
manipulated understory conditions in adjacent black spruce (Picea mariana) and trembling aspen
(Populus tremuloides) stands to understand the mechanism of impact on the understory community. We
found that litter deposition from trembling aspen trees is the principal mechanism that significantly affects
moss establishment though a physical rather than a chemical effect. In addition, understory plants from
trembling aspen stands are more resilient to individual and global changes in canopy composition after 5
years of treatment, in contrast to the less resilient layer of mosses of black spruce stands. Black spruce
stands are stable ecosystems that resist colonization by other herbaceous plants. However, once the
system is disturbed and herbs and shrubs are established, the system is not resilient. In contrast, trembling
aspen stands with an understory dominated by herbs and shrubs is much more resilient to disturbances as
the native vegetation invaded the transplanted mosses. In these boreal forests, a feedback loop between
overstory and understory creates microconditions affecting decomposition and nutrient cycling, in turn
influencing tree regeneration and growth.

Juanita Rodríguez (rodj06@uqat.ca)

S84. Compensatory growth release in surviving lodgepole pine in Northern BC after Mountain Pine
Beetle attack
McEwen, J.
University of Northern BC

The most recent Mountain Pine Beetle (MPB) epidemic in forests across western North America,
beginning in the late 1990s, caused widespread mortality in BC pine forests with major consequences for
present and future forest carbon (C) stocks. Early modeling had predicted these stands would transition
from C-sinks to C-sources and remain in that state for decades. However, Eddy-Covariance (EC)
measurements showed that these stands, when left unharvested, could recover to C-sinks in 3-5years
following MPB attack. This was attributed to a growth release in the residual live trees and advanced
regeneration in stands with 80-95% overstory mortality. At our northern BC site, attacked in 2006-2007,
we measured growing season height and diameter changes in 160 residual trees in 2017 and 2018. Height
growth of residual trees averaged ~10cm annually in these both years, while diameter growth was much
greater in 2018 than 2017. The heights and diameters over the 2017 and 2018 growing seasons were used
to determine annual C-storage in residual tree stems, contributing ~8 and ~17 g m-2 yr-1, respectively.
Using dendrochronology approaches, we determined that our sample trees increased their annual wood
volume by 295% in the decade following the outbreak when compared to the decade prior. There were
strong positive linear relationships between stem C-storage and EC NEP (R2=0.657) as well as stem C-
2
storage and % of downed trees (R =0.712).

Jesse McEwen (mcewen@unbc.ca)

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