160
M.T .K. Arroyo et al.
be easily tested with simple transplant experiments and the determination
of ecophysiological tolerances.
A third effect worthy of note in relation to the latitudinal speciesrichness pattern concerns the nature of the ecotone between rainforest
and mediterranean-type scrublands on the west coast of Chile. Numerous
species of vines and trees considered part of the rainforest flora in southern
South America persist well north of the rainforest zone in mediterraneantype climate latitudes (Arroyo et al., 1995; San Martin, Troncoso, &
Ramirez, 1986; Troncoso & San Martin, 1988). These species tend to
segregate out from the more xeric elements in mediterranean-type climate
scrub over local humidity gradients (San Martin, Figueroa, & Ramirez,
1984). Included here are low-elevation dominant rainforest trees, as
Aextoxicon punctatum, existing under 2000mm rainfall in southern Chile,
which can be still found in cool coastal ravines with around 500mm
rainfall at 35°S and a long period of summer drought, and even further
north (30°S), where there is coastal fog. Hydrangea serratifolia and
Elytropus chilensis, two important rainforest vines, extend well up the
coast in discontinuous populations. On the Chilean coast, there is very
gradual turnover along the latitudinal gradient. Such a gradual ecotone
should have facilitated floristic migration in both latitudinal directions,
with many "mediterranean-type" trees occurring in the seasonal rainforest.
Finally, it is perhaps surprising that only two of the regionally endemic
genera occurring in rainforest are strictly confined to rainforest. This
situation also has to do with the very gradual ecotone between rainforest
and mediterranean scrublands just referred to in central Chile. Additionally noncorrespondence between floristically and climatically defined
rainforest comes into play. Most notably, floristically defined Magellanic
rainforest extends far eastward of the climatic boundary to include areas
with as little as 500mm rainfall on the southern coast of Tierra del Fuego
(Pisano, 1977, 1989). The floristic limit of theValdivian rainforest lies at
37°45' S, to the north of the rainforest limit. Under a floristic definition
Philesia, Laureliopsis , and Lebetanthus would correspond to rainforest
endemic genera . However an untold number of "typical " monotypic
rainforest elements , such as Asteranthera, Elytropus, Mitraria, extend
well north of both rainforest limits.
Rapoport's Rule
Rapoport (1975) drew attention to a trend for the range size of organisms
to increase with latitude. This trend has now been corroborated for the
Northern Hemisphere for tree species, marine mollusks, fresh water and
coastal fishes, reptiles , and amphibians by Stevens (1989). An analagous
M.T .K. Arroyo et al.
be easily tested with simple transplant experiments and the determination
of ecophysiological tolerances.
A third effect worthy of note in relation to the latitudinal speciesrichness pattern concerns the nature of the ecotone between rainforest
and mediterranean-type scrublands on the west coast of Chile. Numerous
species of vines and trees considered part of the rainforest flora in southern
South America persist well north of the rainforest zone in mediterraneantype climate latitudes (Arroyo et al., 1995; San Martin, Troncoso, &
Ramirez, 1986; Troncoso & San Martin, 1988). These species tend to
segregate out from the more xeric elements in mediterranean-type climate
scrub over local humidity gradients (San Martin, Figueroa, & Ramirez,
1984). Included here are low-elevation dominant rainforest trees, as
Aextoxicon punctatum, existing under 2000mm rainfall in southern Chile,
which can be still found in cool coastal ravines with around 500mm
rainfall at 35°S and a long period of summer drought, and even further
north (30°S), where there is coastal fog. Hydrangea serratifolia and
Elytropus chilensis, two important rainforest vines, extend well up the
coast in discontinuous populations. On the Chilean coast, there is very
gradual turnover along the latitudinal gradient. Such a gradual ecotone
should have facilitated floristic migration in both latitudinal directions,
with many "mediterranean-type" trees occurring in the seasonal rainforest.
Finally, it is perhaps surprising that only two of the regionally endemic
genera occurring in rainforest are strictly confined to rainforest. This
situation also has to do with the very gradual ecotone between rainforest
and mediterranean scrublands just referred to in central Chile. Additionally noncorrespondence between floristically and climatically defined
rainforest comes into play. Most notably, floristically defined Magellanic
rainforest extends far eastward of the climatic boundary to include areas
with as little as 500mm rainfall on the southern coast of Tierra del Fuego
(Pisano, 1977, 1989). The floristic limit of theValdivian rainforest lies at
37°45' S, to the north of the rainforest limit. Under a floristic definition
Philesia, Laureliopsis , and Lebetanthus would correspond to rainforest
endemic genera . However an untold number of "typical " monotypic
rainforest elements , such as Asteranthera, Elytropus, Mitraria, extend
well north of both rainforest limits.
Rapoport's Rule
Rapoport (1975) drew attention to a trend for the range size of organisms
to increase with latitude. This trend has now been corroborated for the
Northern Hemisphere for tree species, marine mollusks, fresh water and
coastal fishes, reptiles , and amphibians by Stevens (1989). An analagous