It is widely accepted in ecology and biogeography that biotic interactions are important for the southern distribution limits of a species, while northern range extents tend to be limited by the abiotic environment. Despite this being understood since the time of Darwin, there has been a lack of research investigating the strength of biotic interaction's effects on southern range limits. Slowly, but surely, attention is being turned to this understudied aspect of species distributions. Perhaps one of the earlier investigations on biotic limiters is from Loehle's (1998) paper, 'Height growth rate tradeoffs determine northern and southern range limits for trees' which considers how biotic interactions affect inter- and intra- specific differences in trees.
Loehle approaches the question of range limits from a tree energetics perspective. Data from many tree species clearly indicates that those native to southern latitudes are capable of greater growth rates in terms of height in the same time period than poleward natives. Not only are there distinct interspecific differences in height, but within the same species, provenance studies have shown that individuals from populations further south in latitude grew taller at rates higher than those from northward populations. The explanation provided for the contrast in northern and southern tree growth rates is that energetic tradeoffs are made by northern species in order to protect from freezing temperatures. As a consequence, the more energy a tree puts into freezing tolerance, the less energy is available for it to put into growth. Southern species do not have to do any of this additional energy budgeting and are therefore free to put all of that energy into growth.
At the time of publication of this paper, heat was used in models as the primary determinant of southern range limits of northern hemisphere tree species. However, Loehle points out a number of studies which show that, physiologically, many northern tree species have no issues growing in the south. This is particularly so in the eastern US (where, uncoincidentally, the study species of this paper are native to), which is a case where there are no major latitudinal shifts in precipitation. Therefore, temperature and drought-stress can largely be ruled out as factors preventing northern distributed species from migrating south. If abiotic conditions are not the limiting factors, then the most likely explanation would be biotic interactions. Loehle points to the advantage in competition trees with high growth rates would have over those that have to put energy into frost tolerance. While competition is the oft studied biotic interaction, predation, facilitators, diseases, and parasitism are also important potential explainers.
This paper provides substantial evidence that biotic interactions are an important factor to consider when explaining a tree species' southern distribution limit. Several reviews have since been written considering evidence of biological limitations of southern ranges and further studies are still needed to determine the role biotic interactions play in this regard.
Paper reference:
Loehle C. (1998). Height growth rate tradeoffs determine northern and southern range limits for trees. Journal of Biogeography. 25: 735 - 742.
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