Should I stay or should I go? Stagnant and advancing
treelines in subarctic and alpine localities
Steven D. Mamet & G. Peter Kershaw
1
2
Biology Department, University of Saskatchewan, Saskatoon, SK, Canada; 2Department of Earth and Atmospheric Sciences, Edmonton, AB, Canada
1
Email: steven.mamet@usask.ca
1
SUBARCTIC AND ALPINE TREELINE DYNAMICS
4. Seedling establishment, recruitment & climate (HBL)
Attempts have been made to find a functional explanation for global-scale treeline position. Körner
[1] suggested separating regional-scale modulative drivers from global-scale fundamental drivers
(temperature), & identified five hypotheses of partly interrelated mechanisms:
1.
2.
3.
4.
5.
Stress: repeated damage which impairs tree growth;
Disturbance: mechanical damage or herbivory & fungal pathogens;
Reproduction: limitation of fecundity, natality, & seedling establishment;
Carbon balance: insufficient uptake or balance between uptake & loss;
Growth limitation: growth reduced independently of supply of raw materials (i.e., photosimilates).
Four treeline forms [2]: diffuse, abrupt, tree island, & krummholz.
•
•
Mean JJAS temps. & precip.
departures (w.r.t. 1971-00),
growth indices with 5-year
moving average &
establishment residuals
(5-year classes). Note the
change in y-axis scale for larch
residuals. The most recent age
class was removed as it was
likely incomplete and not
indicative of seedling
frequency during that period.
Examples of treeline forms observed in this study. (a)
Diffuse white spruce treeline near Churchill, MB (30 June
2009). (b) Subalpine fir tree island east of Macmillan Pass,
NWT (28 July 2008).
Treeline forms not arbitrary, but represent a clear signal inferring underlying process.
This poster presents results of research within the forest-tundra ecotone (FTE) of the western Mackenzie
Mountains (WMM), NWT (tree islands), & the western Hudson Bay Lowlands (HBL), northern MB (diffuse).
5. Contrasting responses of subarctic & alpine treelines to warming
Observations:
• Treeline position advanced & stand density increased in WMM & HBL during the early 20th century.
• Rapid change in recruitment ~1920–50 coincided with a period of above average
temperatures. Highest correlations obtained using a 30-50 yr window of forward
averaging—comparable to other studies [8,9].
Interpretations:
• Abundant seedlings & low mortality suggest stand infilling & treeline advance in HBL will
continue. WMM treeline stagnant, due to lack of sexual reproduction.
• Increased HBL larch establishment & recruitment mirrored by dramatic growth increase in old
& young trees—reiterated throughout the circumboreal treeline [10-16].
• Many larch are seedlings/saplings—will population dynamics will continue?
Fundamental research questions:
1. What are the historical & current characteristics of trees at altitudinal & latitudinal treelines?
2. What factors contributed most to the current configuration of treeline at its altitudinal & latitudinal limits?
MATERIALS & METHODS
1.
2.
3.
4.
5.
Assessment of growth of treeline trees using dendroclimatology
Monitoring the extent of mid-winter desiccation of coniferous foliage
Determination of environmental controls on seedling establishment
Contrasting seedling distribution in wetland & upland environments
Treeline stand reconstruction using dendroecology
Location of the (a) WMM and (b) HBL study areas (inset)
and the sampling sites in each region. The contour
interval in (a) is 150 m.
Relationship between
regional tree growth &
climatic change during
the 20th century around
the WMM and HBL. Grey
lines represent
chronologies from trees
established prior to 1920
& the black lines show
chronologies derived
from all sampled trees.
RESULTS & DISCUSSION
1. Growth limitation & climate at treeline (WMM & HBL)
Observations:
• Growth correlated with summer & autumn temperatures—similar for trees at & distal to treeline.
• Regionally: growth of all species responded positively to 20th century warming except for black spruce.
• Growth since the 1990s was greater than any time during the last 250 years, particularly for larch.
Interpretations:
• Site factors & tree age can supersede regional climate forcing at temperature-sensitive subarctic treelines.
• Future research should include seasonal monitoring of soil moisture at treeline to help decipher actual
mechanisms that control growth at treeline.
2. Needle health & biomass dieback (HBL)
Observations:
• Minimum epidermal conductance (gmin) varied little with height & orientation.
• Highest gmin in 2010 -->June (t-1) temperatures 2.9°C cooler than normal; lowest gmin in 2009-->June
(t-1) 1.2°C warmer.
• Viability & water content correlated in NW-facing treeline needles.
Interpretations:
• Some evidence of winter desiccation at HBL treelines, though debatable influence on treeline dynamics.
• Little winterkill at diffuse treelines, though abundant winterkill at tree islands—scale of study could
impact the magnitude of desiccation.
• Further research should occur throughout the year; focus on tree & krummholz islands in the HBL [3].
Observations:
• N=270: 205 (76%) larch & all but one were in the wetland. Low establishment in upland lichens.
• Establishment correlated with JJAS temperatures—particularly larch.
• Absence of upland seedlings suggests superseding influence—probably lichen seedbed.
• Establishment of wetland black spruce related more to edaphic influences.
Interpretations:
• Temperature after establishment is important driver of population dynamics for larch & white
spruce, but not black spruce.
• Establishment can be limited by extensive lichen mats [5,6].
• If germination occurs within desiccation cracks or at mat margins, seedlings could be shielded
from excessive sun & receive adequate moisture supply for growth & survival [6,7].
• Further study: moisture beneath lichens & nature/longevity of establishment within lichens.
CONCLUSIONS
Age distributions of living individuals sampled in the HBL (a–l),
and WMM (m–o), divided into decadal age classes. The mean
& SE of the site elevation within the WMM is shown. Note the
change in scale of the y-axis among sites & species.
Relationship
between treeline
dynamics & 20th
century climatic
change. Top: (a)
Mean JJAS temps. &
(b) mean MJJAS
temps [17]. Bottom:
Proportion of stems
established in the
noted decade up to
the 1990s. Horizontal
solid & dashed lines
represent the mean
proportion of each
species established
during the 20th
century.
Minimum epidermal conductance
and June temperatures (Env. Can.,
2012). Solid circle=mean. Outliers (±
2σ) are indicated by ‘x’. Different
lowercase letters at the top of each
box show significant differences
between years.
First-year conifer needles
sampled 2008-10.
This is the first multi-site, in-depth study of treeline pattern & process in the WMM & HBL.
• Basic methods here have been used elsewhere, but by integrating them within a single
study-->able to elucidate influences on pattern & process across FTE.
– Rapid increase in growth, regeneration, densification, treeline shift, & vegetative facilitation.
• In absence of increased disturbances or moisture stress, HBL treeline shall continue to
advance, potentially with a change from spruce to larch-dominated communities.
• Despite significantly increased growth rates during the last several decades, no seedlings
were found within WMM since at least 2006, & reproduction exclusively asexual.
• Disparity in response between the two locales highlights the need for consideration not
just of climate, but also of treeline form, plant interactions & autecology in treeline
dynamics.
Tree rings: assess growth
& population dynamics
3. Seedling establishment, environment, & plant interactions (WMM & HBL)
Observations:
• Many HBL spruce seeds produced—high germination (> 88%) of filled seeds. WMM: No viable seed or seedlings.
• HBL: more larch (n = 187) than spruce seedlings (white: n = 172; black: n = 9). % dead seedlings: 0–20%.
• Preferential establishment in moss & duff. Change from competition in forest to facilitation at treeline.
Interpretations:
• No reproduction limitation to HBL treeline advance as in WMM. Future advance likely contingent on
seedbed facilitation.
• Future work: seasonal population censuses [4] & annual assessment of mortality of younger life stages.
Seedling preference (P2)
for vegetation cover
stratified by plant height.
P2<0.3 suggests
avoidance, P2=0.3
suggests random use,
P2>0.3 suggests
preference for that
vegetation category.
REFERENCES
Seedling, sapling &
plant surveys
Lab work: needle
transpiration & seed
germination
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Körner, 1998. Oecologia 115, 445–459.
[11]
Harsch & Bader, 2011. GEB 20, 582–596.
[12]
Cairns, 2001. GA 83A, 157–168.
[13]
Kullman, 2007. J. Ecology 95, 41–52.
[14]
Allen, 1929. Ecology 10, 354–355.
Houle & Filion, 2003. Écoscience 10, 80–84. [15]
[16]
Fraser, 1956. McGill University.
Danby & Hik, 2007. J. Ecology 95, 352–363. [17]
Szeicz & MacDonald, 1995. J. Eco. 83, 873–885.
Baker & Moseley, 2007. AAAR 39, 200–209.
Heiri et al., 2006. J. Ecology 94, 206–216.
Kullman, 1998. GEB Letters 7, 181–188.
MacDonald et al., 2008. PTRS 363, 2283–2299.
Peñalba & Payette, 1997. QR 48, 114–121.
Roush, 2009. M.Sc. thesis, U. Vic.
Treter, 2000. Mar. Geo. Sch. 135, 156–170.
New et al., 2000. J. Clim. 13, 2217–2238.
ACKNOWLEDGEMENTS
This research was supported by grants from the
Government of Canada, IPY, Canadian Circumpolar
Institute at the University of Alberta, the Northern Research
Fund administered by the Churchill Northern Studies
Centre (CNSC) and Earthwatch Institute. This paper is a
product under the International Polar Year (IPY) core
project PPS Arctic as part of IPY 2007-08, sponsored by the
International Council for Science and the World
Meteorological Organization.