Stressful climate reduces species richness, but not the diversity of tree strategies

Figure 1. Shifts in tree strategies along precipitation and temperature gradient in western USA. From left to right: Saguaro cacti (Carnegiea gigantea) in Arizona desert; cottonwoods (Populus fremontii) in Escalante (Utah);  redwoods (Sequoia sempervirens) in Californian coast; Sitka spruce (Picea sitchensis) and bigleaf maple (Acer macrophyllum) in Hoh rainforest (Washington). Photos by Irena Šímová.

by Irena Šímová and David Storch

The number of species of most of the higher taxa steeply decreases from the equator towards the poles. Although this pattern has been thoroughly described, it has not yet been satisfactory explained. One possibility of how to understand the mechanism behind the latitudinal diversity gradient is focusing on another biodiversity aspect, the number and variation of functional traits. A classical hypothesis proposes that warm, humid and climatically stable environments allow coexistence of more ecological strategies either due to the higher number of available resources or longer time for species evolution. The greater space for viable ecological strategies could consequently allow the coexistence of more species. We addressed this by focusing on the species richness of North American trees per 100 × 100 km2 grid cells and on the surrogates of the variation of their ecological strategies – the variance in the values of functional traits.

Contrary to our expectation, different regions varied only weakly in their trait diversity and this variation differed for particular traits. Moreover, spatial trends of trait variation were neither associated with warm, wet or stable climate nor with the number of species. Instead, we found strong evidence for the shifts in assemblage trait means, largely driven by climate. This means that stressful environmental conditions select for different optimum strategies rather than that it would constrain their variability.

When hearing the term biodiversity, most people imagine the number of species, which is indeed in central focus of nature conservation. Nevertheless, the variability of organismal traits is another aspect of biodiversity which has been largely overlooked until recently. It is thus important to keep in mind that these two biodiversity aspects could be independent of each other.

Figure 2. Species–rich oak–hickory (Quercus–Carya) forest in North Carolina. Contrary to high species richness, the variance in functional traits here is relatively low comparing to the other North American forests. Photo by Colby Sides.

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