Novel selection pressure on plant vascular network as a model of network-mediated hierarchical interactions in the Earth System

<p>Hierarchical interactions across scales can drive macroevolutionary change and interactions between biological and physical domains of the Earth system. A recently discovered example is how the evolution of complexity in vascular organisation of plants allowed them to constrain the spread of drought-induced embolism, which played a key role in their ability to colonise dry land and to increase their stature from mere centimetres to tens of metres. This not only reveals a previously unknown process that contributed to the macroevolution of plants and their role in the Earth System, the adaptation of plant vascular organisation can serve as a microcosm model for similarly structured cross-scale interactions.</p><p>The earliest vascular plants had developed a central cylinder of water-conducting xylem by the Late Silurian period (&#8764;420mya). This simple form diversified into more complex morphologies as both the size and complexity of the plant body increased during and beyond the Devonian terrestrial radiation. Living plants show a variety of complex vascular forms and both extant plant variety and the fossil record shows a well-known relationship between stem size and vascular complexity. We recently demonstrated a novel selective pressure through drought that favours increased vascular complexity with size through the scaling properties of the conduit network. The topology of the network constrains or facilitates the spread of drought-induced embolism, whose systemic spread is lethal to vascular plants. In essence, network topology integrates the cell-scale properties to plant-scale vulnerability, mediating non-linear cross-scale interactions in vascular plant hydrodynamics.</p><p>Topological properties of the conduit network have played a previously underappreciated role in the macroevolution of vascular plants, including in the success of major new taxa as well as in the emergence of novel growth forms over deep time. Moreover, the mathematical properties of these networks and their effects on scaling the biophysical drivers of this selection can be described in a general way that may translate to descriptions of other nonlinear mechanistic cross-scale interactions in the Earth system mediated by network interactions, regardless of physical mechanism and scale.</p>