84
T.J. Givnish
Most woody species in moist tropical forests are understory shrubs or small trees
(Whitmore 1975; Gentry 1982; Condit et al. 1996). Greater availability of moisture
on rainier, less seasonal sites (and of nutrients on more fertile sites) should reduce
whole-plant compensation points, increase shade tolerance (Givnish 1988, 1995;
see also Wright 1992; Burslem et al. 1996) and permit more individuals - and,
thus, species - to persist in the understory. In fact, there is roughly a doubling of
woody stem number along the rainfall gradient in Neotropical forests, almost entirely involving stems < 2.5 cm d.b.h. (Givnish 1999). Given the regular relationship within a given region between stem number and tree species richness (Condit
et al. 1996), the observed increase in stem number across the rainfall gradient accounts for only 17% of the observed 8.3-fold increase in tree species richness per
0.1 hectare (Givnish 1999). Both rainfall (a proxy for density-dependent tree mortality) and tree density have significant effects when each is included in a model for
tree species richness:
In species richness = 0.361 In rainfall + 1.11 In individual density - 4.39
(r2 = 0.735, P < 0.0001 for 48 d.f.). The high density of small-diameter stems in
tropical moist and wet forests should account for their high diversity of smallstatu red trees and tree lets per unit area (see Whitmore 1975; Gentry 1982; Condit
et al. 1996). The lognormal relationship between species number and stature in a
lineage of tropical wet-forest plants such as Cyanea (Fig. 6) should be fairly general, and reflect the balance of three processes. First, competition for light should
favor the evolution of taller and taller forms, which would tend to extend the species distribution to the right in Fig. 6. Second, shorter species have greater population densities (favoring low extinction rates) and may have lower dispersal capacity
(favoring high speciation rates), favoring an accumulation of small-statured species at the left of the species-abundance curve. Strong herbivory pressures caused
by high humidity, low seasonality, and high stem abundance among short species
of rain-forest understories may select for divergent anti-herbivore defenses, and
12
en 10
IV
.~ 8
0en
'0 6
Qj
.0
E 4
::J
Z
2
o
Cyanea
3 4 5 6 7 8 9 10 11 12
Maximum plant height (m)
Fig. 6. Histogram of maximum plant height in Cyanea
in the Hawaiian Islands (after Givnish 1997)
T.J. Givnish
Most woody species in moist tropical forests are understory shrubs or small trees
(Whitmore 1975; Gentry 1982; Condit et al. 1996). Greater availability of moisture
on rainier, less seasonal sites (and of nutrients on more fertile sites) should reduce
whole-plant compensation points, increase shade tolerance (Givnish 1988, 1995;
see also Wright 1992; Burslem et al. 1996) and permit more individuals - and,
thus, species - to persist in the understory. In fact, there is roughly a doubling of
woody stem number along the rainfall gradient in Neotropical forests, almost entirely involving stems < 2.5 cm d.b.h. (Givnish 1999). Given the regular relationship within a given region between stem number and tree species richness (Condit
et al. 1996), the observed increase in stem number across the rainfall gradient accounts for only 17% of the observed 8.3-fold increase in tree species richness per
0.1 hectare (Givnish 1999). Both rainfall (a proxy for density-dependent tree mortality) and tree density have significant effects when each is included in a model for
tree species richness:
In species richness = 0.361 In rainfall + 1.11 In individual density - 4.39
(r2 = 0.735, P < 0.0001 for 48 d.f.). The high density of small-diameter stems in
tropical moist and wet forests should account for their high diversity of smallstatu red trees and tree lets per unit area (see Whitmore 1975; Gentry 1982; Condit
et al. 1996). The lognormal relationship between species number and stature in a
lineage of tropical wet-forest plants such as Cyanea (Fig. 6) should be fairly general, and reflect the balance of three processes. First, competition for light should
favor the evolution of taller and taller forms, which would tend to extend the species distribution to the right in Fig. 6. Second, shorter species have greater population densities (favoring low extinction rates) and may have lower dispersal capacity
(favoring high speciation rates), favoring an accumulation of small-statured species at the left of the species-abundance curve. Strong herbivory pressures caused
by high humidity, low seasonality, and high stem abundance among short species
of rain-forest understories may select for divergent anti-herbivore defenses, and
12
en 10
IV
.~ 8
0en
'0 6
Qj
.0
E 4
::J
Z
2
o
Cyanea
3 4 5 6 7 8 9 10 11 12
Maximum plant height (m)
Fig. 6. Histogram of maximum plant height in Cyanea
in the Hawaiian Islands (after Givnish 1997)