Monday, October 31, 2016

This is the forest primeval: each tree an evolution

This is the forest primeval. 
The murmuring pines and the hemlocks,
Bearded with moss, and in garments green,
indistinct in the twilight,
Stand like Druids of old,
with voices sad and prophetic,
Stand like harpers hoar,
with beards that rest on their bosoms.
Loud from its rocky caverns,
the deep-voiced neighboring ocean Speaks,
and in accents disconsolate
answers the wail of the forest.

These famous lines, from Longfellow's 1847 epic poem Evangeline, spoke of a sad human tale in the days of early European settlement in the New World. The story was about people, but there is much to tell about the Druids of old, the lives and evolution of treest can be quite surprising.

This post was motivated by a recent trade book The Hidden Life of Trees, by Peter Wohlleben, a German forester, who described what his life in the woods has taught him about trees, their nature, evolution, and biology.  It's written at a pop sci level, and is often quite subjective and evocative, but it's laden with important facts when it comes to trying to understand the evolution of these terrestrial beasts. And, in a sense, these facts generalize in many ways.

The author discusses all sorts of observations that have been made about the responses of different parts of trees (bark, vessels, wood, leaves, roots) to their environment (sunlight, presence of trees of their own species, or of other species, of insect, fungal and other parasites), even going so far as to describe the sociology of trees and their responses to being isolated vs being in a forest of their friends and relatives. Trees interact with their own detected relatives, connected via communication through the air and underground via fungal networks, to the point that they even assist each other, when in trouble, with nutrients. It is a remarkable picture of interactions between organisms in organized, positively coordinated ecosystems.

The book is very selectionist, in that every trait is described as an adaptation to this or that condition, but trees that seem very similar can be different in these respects, so there is the assumption (very hard to prove, if even possible) that each trait evolved 'for' its current function. This is a more deterministically selectionist or even determinist viewpoint than we think is justified by actual fact, even if the functional aspects are as described (which we have no reason to doubt). Indeed, many examples are given of ways trees respond differently to different environments, and hence are not rigidly programmed to live in one particular way.

In any case, our point here aside from recommending an interesting and informative book, is to muse over some we think rather widely missed aspects of trees, their lives, and how they manage to survive and evolve.

While the author is a very strong selectionist when it comes to explaining who does what among trees or among woodsy species, I think he--and for all I know the vast majority of botanists--overlooks what is likely a very major aspect of arboreal evolution.

One major problem that seems to need to be more widely considered (maybe it is by botanists, but we haven't seen much that refers to this particular issue) relates to the implications of time scales (a matter that Wohlleben discusses in detail). Trees can live for decades, centuries, or even millennia.
Wohlleben very clearly and repeatedly stresses the fact that trees live on such a different time scale compared to us, that it can be hard for us to fathom how their lives evolve--and evolve is the appropriate word. If trees are, so to speak, rooted in their origins for hundreds or even thousands of years, while insects, fungi, and other plants and animals (not to mention microbes) have generations in years or even minutes, how can trees ever adapt or survive? By the time a tree has reached a venerable age, hasn't it been out-evolved by almost every other species that lives in or that is blown into its neighborhood?  By the time it dies, when any of its seeds germinate they must already be obsolete, ready to fight the last war-or the last war minus 10 or 100 or 1000.

One answer, in my view, is the largely overlooked fact of the evolution of tree--of each individual tree--during its lifetime.

The evolution of tree (not trees)
Unless my feeble knowledge of botany totally fails me, I think there is a lot going on even at the normal pace of things, within an individual tree. That is that each tree is a remarkable micro-example of evolution in itself.

Each tree starts life as a single fertilized egg (its seed). During its life, that little cell divides into billions, probably trillions, of descendant cells. These make up its roots and, important for us, its trunk, branches, leaves, and flowers. While there are various aspects of communication among these cells, they are essentially independent.

Each cell division along the way from the root tip to the branch tip (or 'meristem'), mutations will occur. This happens in humans, too; such mutations are called somatic because they don't occur in the individual's germ line (that is, the cell lineage that leads to sperm or egg), and hence while the mutation carried by the original cell and its descendants may affect the local tissue, the change isn't inherited by the next generation. Only mutations in the germ line are, and indeed that's where the idea of 'mutation' historically arose. Most somatic mutations will have no effect on the gene-usage of the cell involved, but if they do it might be negative and the cell will die or just misbehave in a way that has no consequences because it's surrounded by countless healthy cells. Sometimes, such as with cancer, somatic mutations can be devastating.

Trees are different. They have no separated somatic and germ lines. Mutations occurring from the seed to the roots and limbs may lead to dead cells, or do nothing, or they may be screened for their 'fitness', their ability to generate the bark, vascular, leave of other tissues in their local time and place. They are, relative to other cells in the tree, removed by what we could call a version of natural selection. Those mutations that survive will be passed down the line or, rather up the line as the trunk, branches, and leaves grow.

Here is a photo of an oak tree and (metaphorically) its single starting genome:

At the end of the countless stems in a tree, over its long lifetime, would be meristem cells each carrying a wide but individually unique variety of mutational differences from what was in the founding acorn. At the meristem, in the appropriate time of year, cells differentiate into pollen and ovule cells. These are many generations of selection away from their founding acorn, and on a given tree there must be a great variety of genotypes, whose sequences would form a tree (a phylogeny), much as we find when we compare DNA sequences from dog species, or from individual humans.

A single tree is a very large evolutionary 'experiment'. Branches affected by harmful mutation, simply aren't there, so to speak. They and their genomic lineage are 'extinct'. A single tree, and its lifetime, comprise such a large 'experiment' that they are comparable in numbers to whole species of shorter-lived, germ-line-dependent organisms.

Here is a photo of a tree from our yard that may illustrate the point. Why are only the leaves on this one branch turning to fall colors so much earlier than the others on the same tree? There may be local environmental reasons, such as different sunlight or water supply or parasite effects, but this seems rather unlikely because other branches in similar positions, even on this same tree, are still green.

And now here is another photo, of a different tree in our front yard that we think illustrates the points we're making. This red oak loses its leaves in the usual way....except for the one major branch shown. Its leaves do not fall until the following spring, but the remaining branches on this tree drop in fall as would be normal. This happens every year and is not a fluke of some particular season.

A forester might have a local explanation, that there is some connection between the location of roots supplying these particular branches, relative to the underground water or soil conditions, but one possible explanation is somatic mutation. That is, some mutational effect, arising when the branch was early in its formation, led to a difference in the abscission  layers of the leaves to be produced by that branch, that retained those leaves through the winter.   If the explanation is local physical conditions, of course, that means the tree cannot be predicted from its founding acorn's sequence. But it is rather difficult to believe that somatic mutation doesn't have at least the kinds of effects seen.  A good experiment would be to take an acorn from this part of the tree and plant it next to one from another part of the tree and see what happens. Unfortunately, the answer wouldn't be available for many years....

Our point here is that among the countless cells in a tree's life, between its origin as a single cell and the also countless generations of its own acorns from its founding genome through its long live, there simply must have been countless somatic mutations, occurring all along the roots and trunk and branches, cell division by cell division.  Their descendants, down the root network, and up the trunk and into the branches must have been screened for the viability of any phenotypic effects, which many must have had.  If insects or bacteria attack or animal predators or the climate change, parts of a tree may be better able to survive than others.  Cells in the trees' future lives will have the benefit of these changes.  They may be small, but they may accumulate over the decades.  The branches affected by less helpful changes would flower less, or lead to branches that die or fall to predators, and so on--ones we never see later on, when we look at the tree.  Among the countless meristems every generation will be a population of differing genotypes to be passed on to its season's thousands and thousands of seeds.

In this way, by working through meristems everywhere (above ground) on the tree are cells with new genotypes screened for suitability in its environment at each time during the tree's life.  A tree is not a single organism, but a population of descendants of a founder.  The acorn was primeval perhaps, but not the forest.  It is this kind of within-life evolution that may, or perhaps must, explain how a single, immobile organism can survive for so long in the dynamics of local ecosystems.

That is, it's the tree itself, in its ever-renewing parts from root to twig, not just its evolving population of annual seeds, that must be evolving.  Decades, centuries, or millennia must often encompass changes in the biota around each primeval individual, and would destroy it, if it, too, were not evolving.  Otherwise, it would seem like asking for doom to be fixed in a given location for hundreds or thousands of years, surrounded by junior, dynamically evolving predators and competitors.  

The forest is always primeval: Each individual tree, in this view, is an evolving population, always adapting in its unchanging location to its locally changing conditions.

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