Disequilibrium between Tree Species Distributions and Regional Temperatures

by Cortland Henderson

Correlations between geographic distributions of plant species and the current climate have been identified, suggesting that species ranges will shift upwards if global temperatures rise. These links, however, are based on models that do not establish whether or not plant species are at equilibrium with the current climate, and are incapable of differentiating between naturally occurring shifts and climate-induced shifts. García-Valdés et al. (2013) examine the ten most common tree distributions throughout the Iberian Peninsula by creating a new species distribution model that relaxes built-in assumptions that tree species and climate are currently at equilibrium. Their model successfully removed previous biases and found that tree species are not at equilibrium with modern climate, revealing that distributions are likely to shift over the next 100 years without any additional global warming. Incorporating these findings into models with projected climate change, the authors were able to identify vulnerability that each tree species faces over the next 100 years. These results suggest that a majority of tree species in Spain is greatly out of equilibrium. Species-specific reactions to climate change will result in exacerbated, decreased, or stabilized shifts in distributions.

García-Valdés et al. conducted their study in continental Spain on the Iberian Peninsula. The peninsula is an effective study site due to its high environmental variability and its strong north–south aridity gradient. Data for the ten most common tree species in continental Spain were extracted from the Spanish Forest Inventory (SFI) between 1986 and 2007. Two subsequent surveys were conducted at each of the 46,596 plots with an average interval of ten years. This interval allowed for visible colonization and extinction events at the local level. The observed changes in occupancy of trees were then used to model potential reactions against future projections of climate change. Building upon previous programs called “spatially explicit patch occupancy models” (SPOM), the authors ran multiple runs until the trees reached a state of pseudo-equilibrium. All of the extinction and colonization events were created such that they were dependent on temperature and rainfall; however, the underlying mechanisms differ between the two extremes. Opting to use nine model simulations for all ten tree species, the authors had to develop criteria for each tree to determine which parameters to use on a tree-by-tree basis. Determining these parameters allowed the authors to calibrate the models so each tree species was best represented.

The extensive number of simulations found that temperature and precipitation were good explanatory variables to explain tree ranges. In nine of the ten species, the models found that the average frequency would keep increasing for the whole 21st century without additional warming. Among the nine species, eight showed a constant increase in frequency with no signs of slowing or reaching an asymptote. These simulations reveal that the dominant tree species in mainland Spain are currently far out of equilibrium with the current climate conditions. Furthermore, they are likely to remain out of equilibrium for at least the next century. Models that continued expected rates of climate change were then run to help identify the vulnerability of each of the tree species to global warming. Trees that displayed a low vulnerability had similar projections for the next century with or without climate change. High vulnerability was attributed to trees whose ranges were expected to shift polewards without climate change, but became less pronounced after increases in temperatures. The third category, favored trees, was given to species whose ranges are projected to shift northwards without climate change, but are expected to shift even more with warming. Much of the effects of climate change were attributed to differences in drought tolerance between tree species.

The authors suggest that trees in Spain are out of equilibrium either because forests may have become constricted to small sites as humans have created farmlands and manage otherwise suitable land that could have been used for migration or the trees may not have had enough time since the last glaciation to sufficiently rebound to an equilibrium point. Subsequent research will need to include this idea of current disequilibrium to better model underlying forest dynamics and climate change effects. Future models will need to address the effects of biotic interactions among species and possible temporal scaling issues.

García-Valdés, R., Zavala, M.A., Araújo, M.B., Purves, D.W. 2013. Chasing a moving target: projecting climate change-induced shifts in non-equilibrial tree species distributions, Journal of Ecology, DOI: 10.1111/1365-2745.12049. Full paper: http://bit.ly/1r7JSXy

One thought on “Disequilibrium between Tree Species Distributions and Regional Temperatures

  1. Hi Cortland. Thanks for your interest in this paper! You’re right to highlight that the ‘disequilibrium’ issue was the main one that it addressed, and I like your summary of our findings. Interestingly, this might be one area where advancing the science (to take this issue into account) might help to bridge the gap to the public understanding of the potential effects of climate change. The public is smart enough to know that things would change anyway, with or without climate change. But we show here that doesn’t mean that climate change doesn’t matter or that we can’t understand its effects. Rather, we can being to understand the effects of climate change in altering the future direction of change, i.e., by comparing what would happen with, and without, climate change, and taking the difference. People are calling for this ‘counterfactual’ approach in other areas too, like conservation. E.g., you could judge a conservation policy by estimating what you think would have happened without the policy being implemented, vs what did happen. It’s this difference that counts. The results could really change the way we think about things, e.g., species might have declining, declined less than they would have otherwise. This is, arguably, a much more reasonable approach to many problems, than pretending there’s always a fixed, static, equilibrium baseline (even if this approach makes sense in some cases). Anyhow, thanks again for the interest!

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