GAP PHASE REGENERATION

(Adam Ch. 5 pp.103-36;Whitmore Ch. 2 pp. 23-25, Ch. 7; good refs in bibliography)

 

Gap disturbances provide the main or only means by which most tree species can maintain their representation in closed-canopy forests.

An understanding of gap-phase regeneration is fundamental to the understanding of the structure and dynamics of closed forest communities.

Gaps make up 10-15% of the area of forest in many forest types.

Gap-phase regeneration is a complex process involving:

  1. Biology
  2. Chance } the location of the next treefall gap is unpredictable,
  3. History} so the success of viable seeds or suppressed saplings depends on being in the right place at the right time.

A. Gap Structure

B. Gap Creation

Gaps are diverse in structure because there are many causes and circumstances of creation:



PHASES IN FOREST GROWTH
- Whitmore (1978) recognizes 3 phases:
GAP ---> MATURE ---> BUILDING

C. Gap Environment

Structural diversity leads to a diversity in gap microclimates.

Gap size, shape, compass orientation, and height of surrounding vegetation, as well as vegetation and debris in the gap itself, all influence gap light, temperature, moisture, humidity, wind regimes.

D. Regeneration Guilds (Whitmore pp. 102-8)

  1. PRIMARY (MATURE) FOREST TREE SPECIES
  2. EARLY PIONEERS - 10% of 800 tree spp.

    3. - Very fast growth rates. Growth responds more dramatically to increased light (Macaranga tanarius can grow 8 m in 2 y). Balsa often grows 5-6 m y-1. The world record is for Paraserianthes (Albizia) in a Malaysian plantation - 9.91 m y-1.

    - simple branching architecture. Typically show rapid elongation of a monopodial shoot.

    - mature rapidly; relatively short life-spans (1-15-30 y).

    - much less shade-tolerant, juveniles are found mainly in gaps.

    - low wood density {e.g. Ochroma (Balsa), BOMBACACEAE}.

    - high maximal photosynthetic and growth rates.

    - produce large numbers of small seeds, often in succulent fruits, which are widely dispersed by birds and bats.

    - frequently persist as dormant seeds in soil seed banks.

    - require specific gap disturbance cues to germinate.

    (i) Photoblastic - phytochrome triggered by light with R>FR - high red:far red light stimulates germination of Cecropia (MORACEAE/URTICACEAE?) {R:FR 1.2 in open, 0.99 in sunflecks, 0.42 in shade.}

    (ii) Thermoblastic - germination triggered by high temperatures or alternating high and low temperatures.

    - r-adapted species.

  3. LATE SECONDARY SPECIES

{Summary table of regeneration guilds}

E. Gap Regeneration

- regrowth from 3 sources

(a) seeds - soil seed bank --> 200-300 seedlings/m²

- dispersal by birds and mammals

(b) seedling banks - plants established prior to gap formation ("advance regeneration").

(c) coppicing and lateral ingrowth of branches from trees on periphery.

Large gaps - pioneers and late 2 spp. from seed grow rapidly and overtop slower-growing established juveniles of surviving 1 spp.

- natural treefall gaps are usually filled by "advance regeneration" from 1 spp, cf. logged gaps(?)

Hopkins & Graham (1987 - Aust. J. Ecol.)

- buried seeds of 50 spp. and tested viability for 2 y.

- 1 spp. generally had large fleshy fruits with a single seed, while pioneer species had dry or fleshy fruits with smaller, multiple seeds.

- many pioneer and 2 spp. remained viable for 2 y.

1/3 had dormancy enforced by burial.

- 1 spp. had a mean viability of 10% after 6 mo. burial.

- burial did not enforce or induce dormancy.

- some spp. have seeds which can sustain seedlings for 1 y, e.g. Castanospermum - 2 y.

{Pioneers may suffer heavy mortality in small gaps; their growth rate increases with gap size.

Building phase - survivors grow into larger sub-canopy sub-adults ("trees of the future").

- replace short-lived pioneer spp. as they die, becoming "trees of the present".

Larger gaps - persistent late 2 spp. in developing canopy may continue to suppress 1 spp. for longer.

Growth may be episodic as juvenile canopy trees are alternately suppressed and released from shade competition.

HYPOTHESES CONCERNING GAP-PHASE REGENERATION

Are forest communities in species-equilibrium?

Are there forces which tend to stabilize particular taxonomic assemblages of tree spp., and to resist invasion by other spp.?

e.g. specialization for conditions of regeneration.

Explicit microsite requirement hypothesis

Ricklefs (1977) - high tree diversity in tropical forests because gaps have a greater range of microclimate in gradient from the gap centre to gap edges of large gaps, allowing niche specialization.

Brokaw, Denslow, Hartshorn, Whitmore - size, and possibly seasonal timing, of gaps may influence the establishment of particular tree species.

Variable recruitment hypothesis

Specialization into regeneration niches may involve episodic, asynchronously-cued reproduction among tree spp.

Each species has a time period during which recruitment is greater than the average recruitment for all other spp. in the community.

- if space is limited (resources finite) this results in a frequency-dependent reproductive advantage for rare spp.

Denslow (1980) - most spp. in forest will be adapted to the prevalent gap disturbance regime ---> shifting balance in representation of major regeneration guilds in the canopy tree community as the gap disturbance regime changes.}

 

 

 

Page last revised August 2002/ Bob Congdon