Download Southern Hemisphere forests

Southern Hemisphere forests
Main goal: Review history of ideas regarding
population dynamics of conifers and coexistence
with angiosperms southern forests.
Are southern hemisphere conifers relicts, slowly
dying out and being outcompeted by
angiosperms?
Southern Hemisphere forests
3 southern conifer families:
1. Araucariaceae (exclusively S)
2. Podocarpaceae (primarily S)
3. Cuppressaceae (mixed S & N)
Red & Silver Beech (Nothofagus), New Zealand
Podocarp forest, New Zealand
Athrotaxis subalpine and mixed-eucalypt forest,
Tasmania
Conifers of Patagonia
Angiosperms of Patagonia
Background
Background
• Northern hemisphere conifers widely distributed,
successfully regenerating, often in
cold/unproductive environments. Relatively few
coexist in angiosperm dominated forests.
• Southern hemisphere conifers coexist in highly
productive, angiosperm dominated forests
• Spatially synchronous long-term regeneration
gaps of southern conifers
Are southern conifers relict species!!??
Frequency
“Relict” conifers of New Zealand
(Wardle 1963)
• Most sites lacked regeneration
for hundreds of years
• Regeneration failure most severe
in drier sites
 conifers restricted to humid
“refugia”
• Competition with angiosperms
highest in humid areas
 ecological window for
conifers limited
Podocarpus totara
Agathis
Athrotaxis
Fitzroya
Pilgerodendron
Araucaria
Background
Remember Clements (1916)
 Ideas very influential during middle 20th century
 Directional succession to climatic climax
How to interpret widespread regeneration gap
in productive southern hemisphere angiosperm
forests?
 Competitive exclusion of conifers by angiosperms
under current climatic conditions?
Tortoise (conifer) and the hare (angiosperm)
• Regeneration niche (slow seedling) hypothesis
Growth
Angiosperm advantages
EG - Evergreen
DD - Deciduous
Bond 1989
•Higher photosynthetic
rates
•Greater water supply
(xylem conduits) and
distribution (venation)
•Flexible canopy
architecture (leaf size,
arrangement)
Slow Seedling Hypothesis
Enright et al. 1995
Conifers are not relicts in southern forests!
Mechanisms of coexistence?
Enright et al. 1995
Conifers are not relicts in southern forests!
Mechanisms of coexistence
• Some conifers are disturbance dependent
-Disturbance ecology explains discontinous regeneration
-Fine vs. coarse scale disturbances
• Conifers able to compete with angiosperms
-Longevity (critical temporal component of niche)
-Nutrient use efficiency
-Additive basal area (competition avoidance )
Veblen & Stuart 1982: Studied size
distributions and spatial association of
several L. bidwillii stands
Results
1. Gap phase regeneration in
association with senescent
trees or fine-scale
disturbance
2. Episodic, even-aged
regeneration associated
with course-scale
disturbances (floods, large
wind events, vulcanism,
mudflows)
Gap-phase, multi-age
Episodic, even-aged
Kalela’s hypothesis
• Equilibrium perspective of regeneration
• Arid steppe forests of Austrocedrus
chilensis moving westward in response to
climatic drying and replacing Nothofagus
dombeyi forests
• Evidence
– Regeneration at steppe ecotone lacking
– Widespread subordinate A. chilensis in
understory of mesic Nothofagus dombeyi
forests
– Nothofagus dombeyi not regenerating
– A. chilensis “invading” moister, westward
forests of Nothofagus dombeyi
Cumulative radial growth
Veblen & Lorenz 1987: Studied age
distributions & growth in Austrocedrus
chilensis-Nothofagus dombeyi stands
Age (years)
Individuals are same age, A. chilensis just grows slower
• N. dombeyi fits model of many conifers
• Shade intolerant
• Long regeneration gaps or small scale gap dynamics
• Shade tolerant conifers (Saxegothaea) and angiosperms
(Laureliana) would replace it without disturbance.
Disturbance-mediated regeneration pulses explain
regeneration niche and maintenance of N. dombeyi in
Valdivian rainforest.
“The structure of the old-growth forests of the midelevations of the Valdivian Andes is consistent with the
hypothesis that in the absence of catastrophic
disturbance the emergent Nothofagus would be
replaced in great part by Laurelia philippiana and
Saxegothaea conspicua.”
“…most of the emergent N. dombeyi and N. alpina in the
mid-elevation forests became established as a
consequence of…landslides triggered largely by
earthquakes and volcanic eruptions.”
Veblen et al 1980
Regeneration niche & disturbance
• Main points:
– Non-equilibrium regeneration dynamics mediated
by disturbance
– Equilibrium view led to confusion about
regeneration dynamics of conifers, but also even
some angiosperm species
Longevity (max ages)
• Conifers tend to be longer lived by hundreds
of years
Nutrient Use Efficiency
• Measure of biomass produced per nutrient utilized
(nutrient concentration in senesced leaf biomass)
•High NUE an adaptation to infertile conditions
• NUE of southern
conifers moderate
• Weak advantage,
but not main
factor
Additive Basal Area
GFM independent of KS  competition avoidance
Mechanism:
• early post-disturbance conifer
• emergent canopy position
Hill & Brodribb 1999: evolution of shade tolerant
foliage in southern conifers
•Flattened leaves, upturned to light
•Similar vein density:leaf area ratio as
angiosperms
Key Points
• Non-equilibrium regeneration strategies
– Clementsian (equilibrium) view problematic
• Long lifespan of conifers
• Disturbance is a key factor enabling coexistence
between southern conifers & angiosperms
• However, many southern conifers can compete
with angiosperms in absence of disturbance
– Shade tolerance
Southern conifers & climate change
Climate change suggests increasing dryness
• Podocarps are a tropical moist family
– Susceptible to increasing aridity
Climate change suggests increasing fire frequency
– Effects on regeneration?
– Case study: Athrotaxus