Tree cover and shade tolerance shape plant species distributions

Submitted by editor on 20 November 2014. Get the paper!
Shade understory and light penetrating through the tree canopy. Photo by Diego Nieto Lugilde.

By Diego Nieto-Lugilde, Jonathan Lenoir and Jens Christian Svenning

If you have already experienced a walk in a forest stand during a sunny day, you have certainly noticed the diffused light conditions in the understorey and how this shapes the distribution and community composition of lower stature plants like herbs and shrubs. The importance of such a “size-asymmetric” plant-plant interaction effect for species distribution and community composition is pretty well-known at local scales and involves shade tolerance as “a key plant feature of complex nature and consequences” (Valladares and Niinemets 2008). In that respect, trees can thus be seen to as biotic modifiers (Linder et al. 2012) because of their abilities to not only modify light conditions in the understorey but also microclimatic conditions (Pinto et al. 2010) and soil properties (Thomas and Packham 2007).

Circaea luteciana L. a shade tolerant species.Photo by Jonathan Lenoir.

But, to our knowledge, the importance of taller plant species (such as trees) as biotic modifiers driving the broad-scale distribution of herb and shrub species is still very much unknown. And yet, the effects of such a “size-asymmetric” plant-plant interaction acting not only at fine spatial grains, but potentially propagating to coarser spatial grains may challenge predictions from coarse-resolution models of species distribution and diversity which do not account for biotic interactions (Wisz et al. 2013).

Few years ago, Jonathan, Jens Christian and I discussed this idea to test the importance of tree cover on the distribution of low-stature plants (herbs and shrubs) based on an exceptional, large, high-quality database on vascular plant communities across the European Alps (37 513 vegetation plots). Combining presence-absence data on species distribution from this unique database together with field measurements as well as remote sensing surveys of tree cover into species distribution models for 990 low-stature plants, we found that tree cover is an important biotic modifiers in determining broad-scale low-stature plant species distributions across the Alps, being even more important than soil conditions at fine spatial grains. Interestingly, we showed that landscape-grain tree cover assessed from remotely-sensed images also has a significant effect in herb and shrub species distributions, and that this effect is partially independent from the effect of fine-grain tree cover assessed from field surveys, probably reflecting metacommunity dynamics. Unsurprisingly, the effects of tree cover on species response curves at both local and landscape scales interacted with species’ shade tolerance.

One important implication of our findings is that plant-plant interactions should be taken into account not only at fine spatial resolutions but also at coarser resolutions when forecasting plant distribution and diversity under future climate change scenarios. We believe that our study provides insight to the timely topic of plant–plant interactions and environmental change put forward by Rob W. Brooker in 2006: “A major research challenge is to understand when plant–plant interactions play a key role in regulating the impact of environmental change drivers, and the type of role that plant–plant interactions play”.


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