Whitebark pine facilitation at treeline: potential
interactions for disruption by an invasive
pathogen
Diana F. Tomback , Sarah C. Blakeslee , Aaron C. Wagner , Michael B.
Wunder , Lynn M. Resler , Jill C. Pyatt & Soledad Diaz
Published 2016 In Ecology and Evolution
Outline
Background
Introduction to Paper
Summary
Take Home Messages
Critique
Discussion
Whitebark pine
(Pinus albicaulis
Engelm) Growing
conditions
-
Nutrient poor soils
-
Aridity
-
Cold temperatures
-
Slow growing
-
Reaches sexual
maturity at 20-30
years
-
“Stress tolerant” spp.
& poor competitor on
productive sites
P. albicaulis
krummholz
communities
- Most common tree
island initiator in many
Rocky Mountain
communities
- Most often the
‘windward conifer’
-
Important for
microclimate/ protection
P. albicaulis
krummholz
communities
- Tree isl. initiation likely due to
climate effects not latitude
- Whitebark pine depends on
Clark’s nutcracker (Nucifraga
columbiana)
-
Nutcrackers cache
seeds near nurse
objects - treeline
Whitebark pine threatened by
Cronartium ribicola J. C. Fisch
-
subalpine/treeline communities of 5 needle
pines in Northern Rocky Mountains are
increasingly threatened by white pine blister
rust (Cronartium ribicola)
-
Often fatal, non-native fungal pathogen
-
Causes branch dieback, reproductive
failure, and tree mortality
Whitebark pine threatened by
C. ribicola
-
Small diameter trees may die within a few
years
-
May limit tree island initiation or reduce
survival of tree island components
-
Potentially impacts conifer community
structure at treeline
Tomback et al. compared two treeline communities in
Montana’s Rocky Mountains
1. Examined whether or not P. albicaulis may facilitate the early life-history stages of
tree island initiation and provide protection for leeward krummholz trees
1. Compared quality of protection provided by leeward microsites of whitebark pine,
engelmann spruce, rocks and unprotected microsites by looking at the germination of
sown seeds and cotyledon seedling survival and survival of planted seedlings
1. Examined whether the death of windward whitebark pine (simulating death from
white pine blister rust) potentially reduced the growth and vigor of leeward conifer
Study Areas
- Treeline at 2100m
- Steep, north facing slopes
- Infection incidence of P.
albicaulis by C. ribicola was
23.4%
- Treeline at 2980m
- More moderate, North
facing slopes
- Infection incidence of P.
albicaulis by C. ribicola
was 19.2%
Relative abundance and density of solitary trees
- Established 20 random point locations for each study area
- Used belt transect (50 m X 10 m) and recorded the occurrence of all solitary
krummholz conifers by species (P. albicaulis, A. lasiocarpa, P. engelmannii)
- Results indicated that in both study areas P. albicaulis densities were the
highest of the 3 conifer species
- Highest overall at Divide Mountain
Microsite quality: seed germination and seedling
survival
- compared protective quality of 4 common treeline leeward microsites
(krummholz P. albicaulis and P. engelmannii, rock and exposed)
- via examining distribution of seed germination, cotyledon seedling survival,
and planted (7 month-old) seedling survival among microsite types 1 year
after planting in each study area
Results: Microsite quality: seed germination and
seedling survival
- At Line Creek there was no significant difference between any of the
microsite types in terms of germination or seedling survival
- At Divide Mountain there was also no increase in odds of seed germination,
but there was an increase in the odds of summer survival for cotyledon
seedlings in whitebark pine protected microsites
Examining the effects of increased stress on shoot
lengths
- Measured total length of new branch
shoots of isolated solitary krummholz
trees (unprotected) across study site
- Repeated for trees (upright) in
subalpine forest
- Results indicated that krummholz trees of both P. albicaulis and P.
engelmannii had shorter shoots than subalpine (upright trees)
Simulating loss of facilitation
- Found tree islands that had windward krummholz P. albicaulis sheltering
either leeward P. engelmannii or A. lasiocarpa
- Assigned trees to 22 pairs of control and experimental units of the same
leeward species
- Collected baseline measurements of shoot lengths on the leeward conifer
nearest to, and sheltered by, windward P. albicaulis
- After obtaining baseline measurements, defoliated and girdled the
experimental P. albicaulis leaving only tree skeleton
- used tree boring methods and dendrochronology to determine tree age
Results: Simulating loss of facilitation
- After baseline measurements 3/22 (14%) of control P. albicaulis were
infected by blister rust and died over the course of the study & removed from
analysis
- Windward P. albicaulis was taller or similar in height to the shoots measured
on the leeward conifer
- 2 years after girdling and defoliation of experimental windward P. albicaulis,
shoots of experimental leeward conifers were much shorter than control
leeward conifers
- P. albicaulis krummholz age ranged from 20-124 years
Statistical Analysis
- Used R 2.11.1 for all analyses
- Multinomial tests to compute
probability all 3 spp. Occur with equal
probability in belt transects
- Bootstrap analysis to compensate for
nonrandom sampling and multiple
samples per tree
Discussion: P. albicaulis relative abundance and
stress tolerance
- P. albicaulis is the most common solitary conifer and most frequent tree
island initiator at both study sites
- Other studies found similar results (Resler and Tomback 2008,Tomback et al.
2016)
- Suggests that as P. albicaulis succumbs to C. ribicola, opportunities for tree
island initiation may decline
- Further supported that P. albicaulis is a highly stress tolerant species,
adapted for harsh conditions and persists as a climax species on exposed,
windy sites
Discussion: Facilitation and early stages in tree
island initiation
- On Divide Mountain sown seeds germinate best in microsites leeward of
rocks (soil temp)
- After 1 yr cotyledon seedlings that germinated from sown seeds had > chance
of survival in summer microsites leeward of Whitebark pine than other
microsites
- microsites leeward of P. albicaulis may foster survival of new seedlings better
than other microsites examined
Discussion: Simulating loss of facilitation
- Measured shoots on exposed leeward conifers were significantly shorter than
those for control dyads with healthy windward P. albicaulis
- The removal of protection increased environmental stress and impacted shoot
growth
- Windward shelter may be esp. Important in years with low snowpack and high
winds
- loss of P. albicaulis as a tree island initiator can have neg. Cascading
influences on leeward krummholz trees esp. Wrt windflow patterns
Take Home Messages: The impact of Cronartium
ribicola on Rocky Mountain treeline communities
- Widespread mortality of P. albicaulis from C. ribicola will influence treeline community
composition and structure
- As P. albicaulis declines from continued spread and intensification of C. ribicola the rate
of tree island initiation may also decline
- This may also cause structural disruptions of current tree islands from the cascading
effects of the loss of windward conifers
Take Home Messages: The impact of Cronartium
ribicola on Rocky Mountain treeline communities
- Climate change will impact future treeline dynamics
- Warmer temperatures predicted to shift treeline upwards in elevation
- Fewer whitebark pine means this may be delayed or precluded
- This could lead to the perception that treeline is not moving up or moving more slowly
than current models suggest
Critique
Critique
- Tomback cites herself often (approximately 25 times) and Pyatt often
- Repetitive in presenting and interpreting results
- When they examined the effects of increased stress on shoot lengths they marked the
alpine trees for remeasure but not the subalpine trees so the same trees weren’t
necessarily sampled the next year - may create error
- Underemphasized interacting factors in terms of the threats to P. albicaulis
- No mention of the necessity of Ribes spp. for C. ribicola
- Hardly touched on the effects of climate change pushing the ideal climatic envelope for
whitebark pine past the peaks of some ranges they are currently in
Interactions: The impact of Cronartium ribicola on Rocky
Mountain treeline communities
Fig. 1 taken from Loehman et al. 2016
Discussion: Restore, Assist, Intervene?
Should we be focusing restoration efforts on the current species range or looking
into projected species range?
Assisted migration?
Is this a situation where genetic engineering is an option that needs to be
considered in terms of disease resistance and conservation?
Is it realistic?
Current Range of
Whitebark Pine
Discussion: Future Scenarios
It has been shown that beetle attack on whitebark pine is more prevalent on trees
that have been damaged by blister rust (Bockino 2008). Do you think that it is
more likely in the future that mountain pine beetle would preferentially remove
blister-rust infected trees and this selection could improve disease resistance in
regenerating stands?
Or
That mountain pine beetle will wipe out so many mature whitebark pines that
Clark’s nutcrackers would need to abandon the site and that seed from resistant
trees would not be dispersed and that whitebark pine resistance will decrease?
References
Bockino, N. K. (2008). Interactions of white pine blister rust, host species, and mountain pine beetle in whitebark pine ecosystems in the Greater
Yellowstone. University of Wyoming.
Loehman, R. A., Keane, R. E., Holsinger, L. M., & Wu, Z. (2016). Interactions of landscape disturbances and climate change dictate ecological
pattern and process: spatial modeling of wildfire, insect, and disease dynamics under future climates. Landscape Ecology, 1-13.
McLane, S. C., & Aitken, S. N. (2012). Whitebark pine (Pinus albicaulis) assisted migration potential: testing establishment north of the species
range. Ecological Applications, 22(1), 142-153.
Global change is projected here to increase
the rusts’ ability to generate new virulence
combinations: (1) host plant homogenization
leads to greater numbers of susceptible
individuals, allowing higher numbers of
recombinants to beproduced (Park &
Wellings, 2012); and (2) the bringing
together of hitherto separate rust genotypes
through global movement of pathogens
provides the opportunity for novel hybrid
species to be generated. While many of
these new recombinations are expected to
be less aggressive than their parent strains,
occasional more highly adapted strains are
likely to emerge and, given the availability of
susceptible host plants,may cause
unprecedented epidemics (Kerr &
Keane,1997).
Where disease escape has currently been
maintained because of unfavourable
environmental conditions, climate change is
now a relevant factor which may be
instrumental in the widening or narrowing of
the range of rust fungi