8. Phytogeographic Relationships and Regional Richness Patterns
157
final conclusion in this connection must await availablity of data for areas
more comparable in size.
Latitudinal Trends in Species Richness
Along the 17.so of latitude covered by the rainforest in southern South
America , climate becomes progressively cooler , and the land mass with
sufficient rainfall for rainforest narrows (Figure 8.4). Very strong glacial
effects are evident on the landscape in the far southwest. A more detailed
examination of the latitudinal species gradient shows that there are some
striking differences for tree species in relation to western North America
(Figure 8.6). Some of these differences were alluded to earlier by Alaback
(1991).
Alaback (1991) showed tree-species richness increases monotonically
from only 5 species per degree of latitude in the Alaskan rainforest
to around 17 species at latitude 40° N. Based on preliminary data for
southern South America, he detected an increase in richness north of the
principal ice fields. With access to more complete data , the trend for
South American trees can now be refined considerably, with evidence of
an even a greater difference between the two continents than originally
seen (Figure 8.6a) . There is indeed a suggestion of some reduction in
species richness around the ice field region (47°S) to a final 7 species at
55°S. Progressing northward, a major increase in trees is now perceived
around latitude 43° S to a total of 43 species at latitude 40°S (Figure
8.6a). This is unlikely to be a collecting artifact , as the level of the island
of Chiloe , where the change is expressed, has been very well studied.
Thes e 43 tree species must be compared with about 17 trees species at
latitud e 40°N in western North America.
Three factor s seem relevant in relation to these contrasting patt ern s for
trees. The dramatic increase in tree species in South America to some
extent parallels the tran sition to seasonal rainforest with a high angiosperm
component. It could reflect the presence of many angiosperm trees
(weighing heavily in the species count), which are less tolerant of cold
summer temperatures farther poleward than gymnosperms in western
North America. Some support for this is seen in the rather contrasting
monotonic drop off in species richness for Nothofagus and gymnosperm s
(Figure 8.6a). Nothofagus is a cold-hardy genus (Alberdi, Romero , Rios ,
& Wenzel , 1985). Arguing against this interpretation , however , is the
very strong latitudinal reduction in species richness in ferns, a group of
plants well adapted to cold conditions, which practically parallels that
seen in tree species in general (Figure 8.6c).
Contrasting Pleistocene histories , already referred to , are perhaps
anoth er factor contributing to this striking interhemispheric difference.
157
final conclusion in this connection must await availablity of data for areas
more comparable in size.
Latitudinal Trends in Species Richness
Along the 17.so of latitude covered by the rainforest in southern South
America , climate becomes progressively cooler , and the land mass with
sufficient rainfall for rainforest narrows (Figure 8.4). Very strong glacial
effects are evident on the landscape in the far southwest. A more detailed
examination of the latitudinal species gradient shows that there are some
striking differences for tree species in relation to western North America
(Figure 8.6). Some of these differences were alluded to earlier by Alaback
(1991).
Alaback (1991) showed tree-species richness increases monotonically
from only 5 species per degree of latitude in the Alaskan rainforest
to around 17 species at latitude 40° N. Based on preliminary data for
southern South America, he detected an increase in richness north of the
principal ice fields. With access to more complete data , the trend for
South American trees can now be refined considerably, with evidence of
an even a greater difference between the two continents than originally
seen (Figure 8.6a) . There is indeed a suggestion of some reduction in
species richness around the ice field region (47°S) to a final 7 species at
55°S. Progressing northward, a major increase in trees is now perceived
around latitude 43° S to a total of 43 species at latitude 40°S (Figure
8.6a). This is unlikely to be a collecting artifact , as the level of the island
of Chiloe , where the change is expressed, has been very well studied.
Thes e 43 tree species must be compared with about 17 trees species at
latitud e 40°N in western North America.
Three factor s seem relevant in relation to these contrasting patt ern s for
trees. The dramatic increase in tree species in South America to some
extent parallels the tran sition to seasonal rainforest with a high angiosperm
component. It could reflect the presence of many angiosperm trees
(weighing heavily in the species count), which are less tolerant of cold
summer temperatures farther poleward than gymnosperms in western
North America. Some support for this is seen in the rather contrasting
monotonic drop off in species richness for Nothofagus and gymnosperm s
(Figure 8.6a). Nothofagus is a cold-hardy genus (Alberdi, Romero , Rios ,
& Wenzel , 1985). Arguing against this interpretation , however , is the
very strong latitudinal reduction in species richness in ferns, a group of
plants well adapted to cold conditions, which practically parallels that
seen in tree species in general (Figure 8.6c).
Contrasting Pleistocene histories , already referred to , are perhaps
anoth er factor contributing to this striking interhemispheric difference.