Tree growth reactions to warming in the Mediterranean

With the continuing disturbance of climate change, there has been a rush amongst ecologists to understand how the predicted changes will affect species. This is because the better we understand how species will be affected, the better we can predict and plan for our future. While climate change did not come into the mass consciousness until relatively recently, it has been affecting ecology for quite some time. Jump et al. (2006) consider how increasing temperatures have been affecting trees at their southern range limits in their paper 'Rapid climate change-related growth decline at the southern range edge of Fagus sylvatica''.

Fagus sylvatica, the European beech, ranges throughout western Europe and finds its southern limit along the Mediterranean. This tree species is not drought tolerant and only grows at elevations higher than 600 meters at its southern limits. In the Mediterranean, temperatures have been rising whilst precipitation patterns have remained stable in the same period. At the time of publication, reports indicated that decline in low elevation populations was occurring, which spurred on this investigation by Jump et al. The authors showed that since 1975, there have been declines in European beech growth in its southern range. The reason for this decline being that increasing temperatures coupled with stable precipitation amounts have induced a drought response in populations. Higher temperatures require trees to imbibe more water. If a species is in an environment where precipitation is close to being a limiting resource, increasing temperatures can push precipitation into a limiting function.

This paper is a case where, in the southern range limits, previously non-stressful abiotic conditions can become stressful as the climate rapidly shifts. The phenomenon of induced drought response as a consequence of increasing temperatures could be a widespread issue for plant species as precipitation patterns are not necessarily expected to follow the increases in temperatures. In environments like the one in this paper, drought tolerant species will be selected for, which will undoubtedly shift the ecology of these environments.

Paper reference:

Jump A.S., Hunt J.M., Penuelas J. (2006). Rapid climate change-related growth decline at the southern range edge of Fagus sylvatica. Global Change Biology. 12(11): 2163 - 2174.

What are the causal mechanisms of cold range limits in temperate trees?

Northern latitude/high-elevation distribution limits have long been associated with the cold temperatures that occur in these environments. Ecological niche models have been employed countless times and consistently show that temperature is a determining factor when it comes to cold range limits in temperate tree species. The issue with using ecological niche models is that they only establish correlative relationships between predictors and responses, which can mislead researchers when trying to understand the drivers of distributions. It is difficult, if not impossible, in ecological niche models to tell whether a predictor actually mechanistically affects a species or if it is simply a proxy for a causal mechanism. In their article 'Where, why and how? Explaining the low-temperature range limits of temperate tree species' Körner et al. (2018) searched for the mechanism(s) that cause cold range limits in trees.

Körner et al. took an exhaustive approach to establishing causal mechanisms by using a combination of latitudinal- and elevation-based field work, lab experiments, and common garden provenance studies. A remarkable finding, counter to the hypotheses of the authors, was that reproductive potential was not reduced at the cold edge of populations. Along with recruitment issues, soils and limited carbon were likewise ruled out as causal mechanisms of cold range limits. Summer temperatures are clearly not the limiting factor for trees, as this is when the majority of growth and production are taking place. Autumn weather was also eliminated as the potential mechanism, since tissues are produced by late summer and fall temperatures only control tissue maturation and entry into dormancy. The authors also show that winter temperatures do not limit distributions, as the dormant state of temperate trees largely prevents temperature-caused mortality during this period. Spring temperatures however, were the determining factor for cold-range limits. Specifically, freezing resistance in early spring was the suspected mechanism, as determined by the timing of leaf flushing. An evolutionary interplay at the cold range limit of temperate trees thereby becomes the decisive factor, whereby a balance must be stricken between the risk of leafing out early and the loss of growing season duration caused by leafing out late.

While this study confirms the broad assumption that cold temperatures are important limiting factors at northern/high-elevation range limits, having the specifics of the mechanisms involved and the timing of effective cold temperatures is monumental. Spatial predictions, like those made in ecological niche models, can now be made more accurate by using cold extremes of the spring as a predictor as opposed to mean temperatures. Further, the results from this study can be implemented in process-based phenological models to improve their accuracy. In a time of rapid climate change, information like that presented in this study greatly improves our ability to predict species reactions to the aforementioned changes.

Paper reference:

Körner C., Basler D., Hoch G., Kollas C., Lenz A., Randin C.F., Vitasse Y., Zimmerman N.E. (2016). Where, why and how? Explaining the low-temperature range limits of temperate tree species. Journal of Ecology. 104: 1076 - 1088.

Using ecological niche models to investigate southern distribution limits

Ecological niche models make predictions about where a species does or could possibly occur. Besides the obvious utility of knowing where a species occurs, ENMs can also be used for hypothesis testing. This can be used to answer questions about past and future distributions of species or to test biogeographic hypotheses. In their 2008 paper "Competitive interactions between felid species may limit the southern distribution of bobcats Lynx rufus" Sánchez-Cordero et al. take an interesting approach of using ENMs across multiple species to test a biogeographic hypothesis.

The primary focal species modeled was the bobcat, however, several other felid species known to co-occur with bobcats were also concomitantly modeled in Mexico. The bobcat ranges from southern Canada to southern Mexico, and exists in a myriad of ecosystem types. In southern Mexico, bobcat distribution has a cut off at the Isthmus of Tehuantepec. Interestingly, no abiotic environmental changes occur at this cutoff point that would prevent the vagile bobcat from entering or surviving in this area east of the isthmus. Further, potential distribution models (which did not account for biotic predictor variables) for the bobcat show suitable habitat occurring south and east of the Isthmus of Tehuantepec, providing additional evidence that abiotic factors are not preventing the bobcat from living in these areas. Amazingly, when the other members of the felid community of Mexico were modeled, four of them show what visually appears to be an inverse distribution compared to the bobcat (Figure 1). Notably, these same four felids also occur south and east of the Isthmus of Tehuantepec. This visual evidence (which was also further backed by statistics) strongly suggests a competitive interaction of some sort between the bobcat and the other felids.

While the ENMs in this study can't be used to elucidate the nature of the competitive interactions occurring between the studied felids, they do provide robust evidence of some interaction occurring. The results further suggest that biotic interactions become increasingly important at southern distribution limits. This study also demonstrates the importance of having expert knowledge of the ecology of the focal species as well as the geography in which they exist. The authors were able to extract much more information from their models because of their knowledge around the relevant environment. Further, this study shows that ENMs can be used to test hypotheses and generate new and directed hypotheses before even stepping foot into the field. Utilizing the methods in this study will, no doubt, make studies of the same kind much more efficient in their execution.

Paper reference:

Sánchez-Cordero V., Stockwell D., Sarkar S., Liu H., Stephens C.R., Giménez J. (2008). Competitive interactions between felid species may limit the southern distribution of bobcats Lynx rufus. Ecography. 31(6): 757 - 764.

Figure 1 (unedited from Sánchez-Cordero et al. 2008)