Lecture 4: Tropical forest structure and dynamics Flashcards
1
Q
Key Structural characteristics of Tropical Forest Types
A
- Want to know what type/forest it is and what stage of succession forests are
o Because any of these forest types can be disturbed by disturbances
2
Q
Secondary Forests
A
- Any of those types of forests (from table in previous slide) can be cleared and starting to regenerate
- Are large areas of secondary forests in tropics
- Often very close to where people are traveling
3
Q
Large Increases in Secondary Degraded Tropical Forests
A
- Remote green areas in western Amazon = where there is primary forest left
- But most accessible areas are already secondary forests or completely cleared
- Even in largest blocks of tropical forest in world will see more secondary forest
4
Q
Disturbance and succession
A
- Disturbance = removal of vegetation by natural or human means
o EX: Fire, windthrow - Succession = recovery of system after disturbance
o Emphasis on species composition changes
o Changes in vegetation structure
5
Q
Changes in species composition and structure following disturbance
A
- Often have set of species that dominate after disturbance
- Completely different set of species that come in late after succession
- This results in change in forest structure over time:
o Immediately after disturbance have things growing (small statured seedlings and saplings) - 4-stage categorization:
o (1) Stand initiation stage
o (2) Stem exclusion stage
Canopy closed
Dominance by some plants
Then density dependence and exclusion
o (3) Understory reinitiation
More trees dying off
Light getting into forest
o (4) Old Growth
Trees completing lifecycle and forming gaps in forest that then are colonized
= Gap-phase dynamics
6
Q
Characteristics of secondary forests
A
- Species that can grow rapidly (if they have lots of light)
- But tend to have much lower biodiversity than primary forests
o Small amount of tree species
o But some species that are really specialized on secondary forests
EX: Red-tailed monkeys common in secondary forests - Can also be good habitat for large mammals
o Elephants (if not hunted) - Dominance by early-successional “pioneer” species
7
Q
Often easy to distinguish primary from secondary forces by canopy structure
A
- Smooth canopy layer in secondary forest
- Especially at stem exclusion stage
- Aerial photo: Similar views all over forest
- Recognize primary forest in the back:
o Broccoli structure with some gaps
o Lumpy
o More heterogeneous - Secondary forest more smooth
o Fewer species in primary forest
8
Q
Arrested Succession
A
- Things can go awry
- Certain species take over and prevent further regeneration
- Most extreme example:
o In SE Asia: Non-native grass (Imperata) forms dense mats and is fire-promoting
o During dry season, these stands burn off
o Hot fire sweeps through
o Management nightmare
o Monoculture of imperata
9
Q
Tropical forest gaps: signature feature of primary (old-growth) forests
A
- Gaps more frequent in old-growth forests
- When a tree falls in the forest creates a gap
10
Q
Anatomy of a Gap
A
- Trees fall over because of
o Root disease
o Or wind-throw - Often tree completely uprooted
- Get tip-up mound and pit formed
o Pit contains exposed mineral soil (low nutrient status)
o Crown of tree
Most of nutrients of tree are in crown and nutrients have been displaced
Now nutrients are where the crown is may 40 m away
o Mound at root ball - Characteristic type of disturbance
- = complicated mess with particular structure
11
Q
Gaps more important in tropics
A
- Gaps prevalent in any forest
- But more important as structural element in tropics b/c sunlight can be directly overhead
- In temperate, often have sun coming in at maximum angle (temperate)
o So unlikely to have full direct sun in middle of day - Whereas in forest, will have sunlight directly above
o = more light and stronger signal
12
Q
Tip-up mound
A
- Formed by trees uprooting
- Results:
o Get tip-up mound w/ distinct microhabitats
o Exposure and mixing of mineral soil
o Over time have tip-up mounds forming, get pit-mound topography
= lumps and troughs through forest
Legacies of individual tree death - Secondary forests (recovered from agriculture) where there haven’t been trees in long time will be
level - But primary forests will have uneven forest floor topography
13
Q
Gap phase dynamics
A
- In primary forest have gap-phase dynamics
o Young gaps that are bigger then get closed up
o Idealized structure could be over-simplified - Remote-sensing map
o Completely open areas down to forest floor are red
o But all gaps are heterogeneous
14
Q
Characteristics of secondary and primary tropical forests
A
- Primary forest o Forest that’s never been logged o High Basal area Area of stem per area of ground is high Because have large trees/large stem diameters o Large seeds, animal dispersed o Have really big lianas o Epiphytes Takes time for them to be deposited o Presence of downed-woody debris o Pioneer species present maybe in gaps - Secondary forest o Forest with small trees = secondary o Small seeds o Typically have non-woody vines Have smaller lianas o Less epiphytes here o Not much downed-woody debris o Pioneers dominant in forest - General diagnostic trees
15
Q
Secondary or old-growth?
A
- A lot of pretty small trees
- No big lianas
- Many seem like similar species
- So probably secondary
