"Measures of climatic extremes statistically significantly add to explain species distribution patterns, and modify the shape of these patterns, meaning that they are important in understanding where a species occurs, and where the climate with its mean and extremes is unsuitable," Nick Zimmermann of the Swiss Federal Research Institute told environmentalresearchweb. "Understanding this relationship should improve our ability to forecast plant responses to climate – in short, where plants are likely to thrive under projected climate change scenarios."
Zimmermann and colleagues from the Swiss Federal Research Institute; the University of Trømso, Norway; Utah State University; Université Joseph Fourier, France; and the University of Lausanne, Switzerland, examined data for 11 species of tree in 12,348 forest plots across Switzerland, where excellent records were available. Using information on climate variability over the last 47 years corrected local overprediction and underprediction, they found.
"Plants especially are only mobile at very low migration rates through seed dispersal and population dynamics and can be expected to be sensitive to varying degrees of extremes at the locations where they grow viable populations," said Zimmermann. "And since climatic extremes are expected to increase in size and frequency according to most IPCC climate scenarios, we felt it is important to study how climate variability modifies or modulates the spatial patterns we deduce from climatic means the traditional approach."
As climate changes, the researchers expect that extremes will become more important for two reasons. "First, extremes will become more frequent because the means are changing, e.g. a 'deviation towards a hot summer' will result in even hotter summers," explained Zimmermann. "And second, most climate models also predict that the variation around means will generally increase, so that even with no change in mean climates we will expect more variation and thus more frequent and stronger extremes."
But Zimmermann says that the biggest difficulty is to make statements about the timing of the plants' response. "When will it be 'too extreme' so that a species disappears?" he asked. "Answering such questions requires us to go further."
When it comes to carrying out this additional research, there are three approaches: studying larger landscapes; combining the analysis with population dynamics to examine tree growth, regeneration and mortality; and targeted field or greenhouse experiments that refine the sensitivity of species to individual climate extremes.
"Some of this research has of course been done worldwide, and many results are already available," said Zimmermann. "Yet they need to be combined to draw a better picture of how species may respond to changing climates. Our approach highlights the general response and the larger scale effect on distribution patterns, but does not give clear physiological reasons for the behaviour of species."
According to Zimmermann, population dynamic analyses are difficult to carry out at large spatial scales, but have the potential to be combined for well-studied species. "Experiments are the least often combined with distribution pattern analyses," he added. "Yet they hold the key for physiological reasoning, and therefore need to be combined more often."
With this in mind, the researchers are involved in several projects that aim to combine these three approaches. These include an investigation with the US Forest Service and US Geological Survey examining how population dynamics respond to climate change in conifers in western North America, and similar projects in Europe, across the European Alps, and in Switzerland.
The team reported its work in PNAS.