11. Patterns of Tree Species Diversity
171
2.5 Environmental Variability over Time
Connell (1978), on the evidence of benthic coastal communities, showed that a
peak distribution of species richness can also occur along a gradient of canopy
disturbance. In calm lagoons, and at depth, canopy closure by climax species leads
to restriction of pioneers to scattered gaps which, as in an archipelago, experience
low immigration and high extinction rates. Species richness is depressed therefore
both by impoverishment of the pioneer community, and by a few dominant climax
species which create a uniform and unfavorable light climate beneath their dense
even canopy. Under the turbulent conditions of the wave-line, climax species cannot establish and the community is dominated by pioneers and is thus also impoverished; but under intermediate, moderately low disturbance regimes behind the
reef edge, conditions favor a mixed community in which species diversity reaches
its zenith. Each of these communities, in the classic case of coral reefs, is characterized by a distinct coral fauna.
Connell (1978) drew an analogy between coral reefs and tropical forests, but
there are major differences. Canopy disturbance by wind, that which is most closely
analogous to wave action, probably never differs in intensity on local spatial scales
sufficiently consistently over long enough periods for communities of differing species richness and composition to evolve as a consequence. Examination by means
of plots smaller than the scale of the community mosaic as a whole, of differences
in species richness in relation to differences in disturbance intensity, precludes
distinction between real differences and artefacts. Very large samples are required
to capture the full array of pioneer species in a community where gaps are small
and scattered in a matrix of the mature phase as is usual in tropical forest (Whitmore
1984), and even more so for climax species in severely disturbed forest. But both
climax and pioneer species will be more easily captured by small plots in moderately disturbed forests, thereby artificially increasing the relative species richness
of these communities.
The severe effects of typhoons (hurricanes) appear to intluence forest structure,
and possibly richness, in ways different from wave impact on a coral reef. Occasional typhoons cause catastrophic canopy damage and successional stages may
patchily dominate the landscape for long periods (Wyatt-Smith 1954; Whitmore
1984; Ashton 1993). If such typhoons occur more frequently than the time required
for the mature phase to reestablish and reproduce, extinction of climax species
might be expected to result, but we have no example of such a case. By contrast,
forests subject to frequent typhoons become adapted. The typhoons remove leaves
and many twigs and the canopy is therefore diffuse. In extremely exposed southeast Taiwan, stunted closed-canopy forest on windward slopes is less rich in woody
species than that in coves (Sun et a1. 1998). Nevertheless, in Luzon heliophytes
including some pioneers may persist in the understory (personal observation). A 16
ha tree demographic plot is under construction in such forest at Palanan on the
windward eastern coast of Luzon, but detailed results are still awaited. We anticipate that here, in contrast to the coral reefs, species richness may be even greater in
171
2.5 Environmental Variability over Time
Connell (1978), on the evidence of benthic coastal communities, showed that a
peak distribution of species richness can also occur along a gradient of canopy
disturbance. In calm lagoons, and at depth, canopy closure by climax species leads
to restriction of pioneers to scattered gaps which, as in an archipelago, experience
low immigration and high extinction rates. Species richness is depressed therefore
both by impoverishment of the pioneer community, and by a few dominant climax
species which create a uniform and unfavorable light climate beneath their dense
even canopy. Under the turbulent conditions of the wave-line, climax species cannot establish and the community is dominated by pioneers and is thus also impoverished; but under intermediate, moderately low disturbance regimes behind the
reef edge, conditions favor a mixed community in which species diversity reaches
its zenith. Each of these communities, in the classic case of coral reefs, is characterized by a distinct coral fauna.
Connell (1978) drew an analogy between coral reefs and tropical forests, but
there are major differences. Canopy disturbance by wind, that which is most closely
analogous to wave action, probably never differs in intensity on local spatial scales
sufficiently consistently over long enough periods for communities of differing species richness and composition to evolve as a consequence. Examination by means
of plots smaller than the scale of the community mosaic as a whole, of differences
in species richness in relation to differences in disturbance intensity, precludes
distinction between real differences and artefacts. Very large samples are required
to capture the full array of pioneer species in a community where gaps are small
and scattered in a matrix of the mature phase as is usual in tropical forest (Whitmore
1984), and even more so for climax species in severely disturbed forest. But both
climax and pioneer species will be more easily captured by small plots in moderately disturbed forests, thereby artificially increasing the relative species richness
of these communities.
The severe effects of typhoons (hurricanes) appear to intluence forest structure,
and possibly richness, in ways different from wave impact on a coral reef. Occasional typhoons cause catastrophic canopy damage and successional stages may
patchily dominate the landscape for long periods (Wyatt-Smith 1954; Whitmore
1984; Ashton 1993). If such typhoons occur more frequently than the time required
for the mature phase to reestablish and reproduce, extinction of climax species
might be expected to result, but we have no example of such a case. By contrast,
forests subject to frequent typhoons become adapted. The typhoons remove leaves
and many twigs and the canopy is therefore diffuse. In extremely exposed southeast Taiwan, stunted closed-canopy forest on windward slopes is less rich in woody
species than that in coves (Sun et a1. 1998). Nevertheless, in Luzon heliophytes
including some pioneers may persist in the understory (personal observation). A 16
ha tree demographic plot is under construction in such forest at Palanan on the
windward eastern coast of Luzon, but detailed results are still awaited. We anticipate that here, in contrast to the coral reefs, species richness may be even greater in