As you might already know, we’ve been making it our goal in iLand to “make every tree count”, i.e. to model individuals and their interactions and responses explicitly. Some of our recent progress in model development has focused on implementing a process-based, spatially explicit regeneration module in iLand, and in this regard I briefly want to revisit this “no tree left behind” strategy once more.

But before I do so, let me briefly put this into a greater ecological context. A hotly debated issues in forest ecology over the last years has been the potential migration rate of tree species. This is no surprise, given the mounting pressure from climate change, and the concerns about how trees will be able to cope with this rapid (relative to a trees lifetime) changes. Answering the question how fast trees can potentially migrate turns out to be more complex than it might seem at first glance: In a nutshell, migration rates derived experimentally (in the order of several 10s of meters per year on average) differed substantially from those derived from pollen records documenting the re-invasion of trees since the last glacial maximum (in the order of several 100 to even 1000m per year). Essentially, analyzing the data available from the most recent large scale tree species migration wave after the last ice age resulted in migration rates differing significantly from that inferred from contemporary trees.

A lot has been said on the possible reasons for this divergence, and I’m not attempting to give a complete overview here (see for instance here for more details). I want to highlight one point though: advances in genetics have changed our view of post-glacial vegetation development quite drastically in recent years. From such analyses it gets more and more clear that this large vegetation migration was much less a homogeneous wave as previously pictured, and relied heavily on (relatively small) local refugia. I.e. it gets more and more apparent that local remnants, surviving in sheltered and/or climatically favorable microhabitats contributed significantly to the re-colonialization. Finding such as those described here thus also reconcile the differences between the migration rates found in the literature.

The moral of the story, in the context of modeling forest ecosystems, is: The local variability in climate, in combination with ecological legacies (e.g. trees left after disturbances, see photo, showing disturbance legacies in the Biscuit Fire), can have a distinct influence on the trajectory of vegetation dynamics. And it thus might be important to capture those aspects in our models, particularly if we’re interested in projecting vegetation dynamics under changing environmental conditions. Below are two series captured from the iLand GUI illustrating this point: On the top, migration of trees occurs only from a (hypothetical) seed source to the west of the simulated transect (1400m x 300m), while in the lower panel I also simulated a disturbance legacy of 10 mature Douglas fir trees (in the east part of the transect). Although a rather simplified simulation experiment (for demonstration purposes e.g. climatic heterogeneity was not considered here) the effect of just a small legacy population on vegetation development (the colors indicate a competition index as proxy of vegetation density here) gets quite apparent. Which brings me back to the title of the post: every tree counts!