Simulating migration in dynamic vegetation models efficiently with LPJ-GM

Abstract

Abstract. Dynamic vegetation models are a common tool to assess the effect of climate and land use change on vegetation. While the current development aims to include more processes, e.g. the nitrogen cycle, the models still typically assume an ample seed supply allowing all species to establish once the climate conditions are suitable. A number of species have been shown to lag behind in occupying climatological suitable areas (e.g. after a change in the climate) as they need to arrive and establish at the newly suitable areas. Previous attempts to implement migration in dynamic vegetation models have allowed simulating either only small areas or have been implemented as post process, not allowing for feedbacks within the vegetation. Here we present two novel methods simulating migrating and interacting tree species which have the potential to be used for continental simulations. Both distribute seeds between grid cells leading to individual establishment. The first method uses an approach based on Fast Fourier transform while in the second approach we iteratively shift the seed production matrix and disperse seeds with a given probability. While the former method is computationally marginally faster, it does not allow for modification of the seed dispersal kernel parameters with respect to terrain features, which the latter method allows. We evaluate the increase in computational demand of both methods. Since dispersal acts at a scale no larger than 1 km, all dispersal simulations need to be performed at least at that scale. However, with the current available computational power it is not feasible to simulate the vegetation dynamics of a whole continent at that scale. We present an option to decrease the required computational costs, reducing the number of grid cells where the local dynamics is computed by simulating it only along migration transects. Evaluation of species patterns and migration speeds shows that although the simulation along transects reduces the migration speed slightly, both methods are reliable. Furthermore, both methods are sufficiently computationally efficient to allow large scale DGVM simulations with migration on entire continents.

Implementing spatially explicit wind-driven seed and pollen dispersal in the individual-based larch simulation model: LAVESI-WIND 1.0

“…This could be achieved by comparing the migration rate in a continuous landscape in LAVESI-WIND, which covers grid cells of the DGVM to achieve a better representation of processes constraining or enhancing the spread of a plant species (cf. Lehsten et al, 2018). With this new model version, we can approach novel research questions, such as "do wind regime shifts explain faster or slower migration rates in past climate changes?"…”

“…Seed dispersal processes are well studied (Nathan et al, 2011a;Nathan and Muller-Landau, 2000) and are sometimes implemented in vegetation models but rarely coupled with wind speed and direction (e.g. Lee, 2011;Levin et al, 2003;Snell, 2014). Also wind patterns might change over time, as the pressure levels vary in a changing climate (Trenberth, 1990), or are directed (Lisitzin, 2012) so that an implementation of wind-dependent dispersal would enable a more realistic simulation of migration (cf.…”