糖心TV

CO904 Stat Mech

Lectures by Ellak Somfai, shared with PX439

Title: Scaling laws at vegetation boundaries - a physical problem with ecological implications

Abstract: Several ecological studies suggest that sharp vegetation boundaries can emerge even across smooth environmental gradients. A typical example is a treeline across altitude or latitude. We applied the theory of critical phase transitions to study the transition zone from high-density, connected to low-density, fragmented state in the vegetation cover. Spatially explicit simulations were applied to model the dynamics of colonization and extinction, by which the boundary was shaped. Connectivity was characterized from the perspective of various species which differ in step length, i.e. dispersal or migration distance. At every step length and every gradient (slope), the edge of the connected patch was found to be fractal with dimension 7/4.  The length and width of this “hull edge” scaled as powers of the gradient with exponents that could be directly predicted from percolation theory.  These results were the same for different spatial models suggesting that there are universal laws in the geometry of ecotones. We propose to use the hull edge for delineating the boundary of a vegetation type unequivocally, and detecting boundary shifts due to climate change.