Tree distribution limits from an energy investment perspective

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.

The biotic side of species range limitations

Very often, when the environmental factors limiting a species' distribution are discussed, climatic variables dominate the conversation. However, going as far back as Darwin's 'On the Origin of Species' it was recognized that the latitudinal extent (north or south) of a species' distribution being considered affected the importance of limiting factors. Darwin himself recognized that where environments were stressful (e.g. far northern latitudes) abiotic environmental conditions were almost exclusively the determining factors limiting that extent of the species distribution. Conversely, where abiotic conditions posed little to no challenges for most species (e.g. tropical/sub-tropical climates) it was recognized that biotic interactions were the dominant limiting factors for a species' distribution. In their 2015 paper 'Where and When do Species Interactions Set Range Limits?' Louthan et al. take on the long overdue task of formalizing a hypothesis to explain Darwin's (and many, many others since) observations on species range limitations.

The authors refer to their hypothesis as the 'Species Interactions- Abiotic Stress Hypothesis,' abbreviated as SIASH. SIASH in its simplest form predicts that the more stressful abiotic conditions are for species, the less important biotic interactions will be for determining range limitations and vice versa. Of course, there are known exceptions to SIASH. Many ecologists have adopted the mantra of 'it depends' as exceptional cases occur throughout many natural patterns. An example contrary to SIASH given in this paper are that high-latitude invasive species tend to have larger ranges compared to lower (but not tropical) latitude invasive species.

As mentioned earlier, a large portion of the literature has focused solely on the abiotic dimensions limiting distributions. However, according to SIASH, we may be making erroneous assumptions, particularly in the southern latitudes, if we are not also considering the biotic dimensions when determining the primary limiting factors of a distribution. Very little work has been done to consider the biotic and abiotic environment in tandem as limiting factors, and even fewer, if any, studies have considered both environmental dimensions at both latitudinal extents of a species. This is likely because it would be quite a task to conduct the necessary field work at two latitudinal range extents, which in many cases could be hundreds to thousands of miles apart! However, elevational as opposed to latitudinal range extents could be used to limit the distance between the abiotic-stressful and non-stressful extents of ranges. The University of Georgia sits at an ideal location where many temperate tree species find their southern (non-stressful) limits at or around the latitude of Athens, whilst being a short drive to high-stress elevations in the Southern Appalachians where the same tree species are also (but, due to abiotic stress) limited! Sounds like a research project to me!

Paper reference:

Louthan A.M., Doak D.F., Angert A.L. (2015). Where and When do Species Interactions Set Range Limits. Trends in Ecology and Evolution. 30(12): 780 - 792.

How are tree species' distributions responding to climate change?

Perhaps one of the oldest questions asked by ecologists is why do species occur in the environments they do? This question gets at the core of ecology and requires an understanding of the various interactions every species has with the biotic and abiotic environment. To add further complexity to this question, it must also be considered that environments are dynamic - some more dynamic than others. The changes in environments can occur on a daily basis (e.g. temperature fluctuations in a desert), seasonally (e.g. wet and dry season in the Mediterranean), or on a geological time scale. Most species are adapted to these sorts of changes in the environment. However, when rapid changes occur which are out of the norm for an environment - for example, global climate change - a species is faced with 3 options: move, adapt, or die. For some species, rapid adaptation or migration isn't out of the question. However, for sessile species with longer generation times, rapid adaptation and migration are much more of an issue. Trees are a primary example of such a group of organisms expected to struggle with rapid global climate change. In their paper 'Failure to migrate: lack of tree range expansion in response to climate change' Zhu et al. (2012) assess how tree species ranges are changing as a result of rapid climate change.

The literature prior to this paper suggested that as temperatures increase species will migrate northward in latitude in response. This assumption was evidenced by data showing that tree species have been migrating upwards in elevation as temperatures increased. The key flaw in this assumption comes from the attempt to generalize elevational tree responses to latitudinal responses. The primary difference between the two responses being that the distance of migration required to reach cooler temperatures is vastly smaller when moving up an elevation gradient than a latitudinal gradient.

The authors here instead focus on latitudinal shifts of tree species in response to climate change. A depressing 58% of the 92 tree species studied were found to be undergoing an overall contraction in range at both latitudinal extents. Of the 92 tree species ranges, 20% were following the previously expected pattern of expanding northward and contracting at southern latitudes while 16% of species were actually expanding southward whilst contracting in the north. The remaining 4% of tree species ranges were undergoing an overall expansion at both latitudinal extents.

This study suggests that most US tree species simply are not migrating poleward fast enough to keep up with temperature increases. This makes the situation for tree species (and the multitude of other species which rely on trees) all the more dire as climate change continues. While conservation efforts such as assisted migrations may be able to somewhat ameliorate the effects of climate change on tree species distributions, there will undoubtedly be major ecological shifts for tree species over the course of the 21st century.

Paper reference:

Zhu K., Woodall C.W., Clark J.S. (2012). Failure to migrate: lack of tree range expansion in response to climate change. Global Change Biology. 18(3): 1042 - 1052.