First snows

some tracking discoveries and other observations

One of my favorite activities in winter is to go out looking for animal tracks in the snow. In early-mid November this year, the snow was perfect: not a lot of it, but soft enough to register animal passage and firm enough to hold the tracks’ shapes.

So, off to Eaglecrest I went, with two good friends who like these little explorations too. We found lots to look at. Porcupines had plodded in and out among the trees, in some cases making small highways of repeated use. A few red squirrels had ventured out of their burrows. A weasel had covered a lot of ground, bounding with shorter leaps when it went uphill. It investigated many a fallen log and stump in hopes of nice lunch. Weasels have to eat a lot, just to keep warm and feed their active metabolism.

Voles (or maybe mice – it’s often hard to tell which) had run over the snow from one grass tussock to another or from log to bush and back again. These were the most common tracks, often right out in the open meadows, where they might be easy marks for predators. But we saw no signs of lethal events.

Near the road, we found a spot where an indisputable mouse had hopped across. On either side of its trackway were marks of a tail flick. It couldn’t have been a vole, whose tails are very short, so it had to be a mouse. Why it had flipped its tail from side to side was not clear, however; we speculated that perhaps it was slightly off balance on the coarse cobbles at the edge of the road and used its tail to restore an even keel.

mouse-tracks-kmh
Photo by Katherine Hocker

We found a few lines of tiny tracks that were made by shrews. Emerging from one dime-sized hole, crossing over the snow to an equally minuscule hole, occasionally they tunneled just barely below the snow surface.

 

Every so often, we looked up instead of down and noted that quite a few trees had long-dead tops. No mystery there, given the howling gales that sometimes whip through this area. But none of the lower, lateral branches had grown upward to replace the missing tops. We’ve all seen conifers whose original ‘leader’ at the top of the tree has been killed but a lateral branch just below it has taken over as leader, creating a kink in the trunk. We puzzled over why this hadn’t happened on the trees in which the entire top was dead.

 

An answer might lie in the way that hormones control growth. Normally, the leader at the top of a conifer suppresses the growth of lower branches; this is known as apical dominance. But the effects of apical dominance diminish as the distance from the leader increases. So, perhaps, when the entire top of a tree is killed, the distance from the leader was so great that there was no dominance exerted on the remaining branches. Thus, the lower branches had not been suppressed and they did not respond to the loss of the tree top.

 

A few days later I walked out into the Mendenhall Glacier Recreation Area near Crystal Lake. Tracking was still good and there had been lots of activity. A porcupine had trundled across the ice on the lake, and a weasel (I think) had walked (not bounded) along the footpath. Squirrels and snowshoe hares had crossed the path.

 

The most interesting marks were made by a bird, whose wingspan exceeded five feet—surely an eagle. Its wing tips brushed the snow in several places around a patch where the snow had been disturbed. Here I could see some heavy-duty bird tracks, confirming the presences of an eagle. All around this area were raven tracks too. But there was no clue about what the eagle was after—unless it might have been a raven (eagles do capture ravens sometimes). It seemed unusual for an eagle to be hunting in a wooded area where the only open ground, where an eagle could spread its wings, was the path itself.

 

Lots of stories in the snow, so winter was off to a good start for me!

Apical dominance

which leader will lead?

Did you ever wonder how it happens that spruce trees typically have such nice, conical tops? The uppermost shoot, called the leader, produces a particular hormone that suppresses growth in the branches below, most effectively in the branches nearest the leader. The effect dwindles to negligible on the lowermost branches. Voilá! –a conical top to the tree.

If the leader is damaged—chewed by a porcupine, or invaded by an insect, or cut off, the next-lower branches rapidly start to grow, and the damaged tree-top may now display two or three new leaders, until one of them might eventually take over. The dominance of the leader at the apex is disrupted, allowing lower branches to grow more. A similar but usually smaller effect may occur on the end of branches.

The effect of that suppressive hormone is countered by another hormone, one that encourages growth. The balance between the two hormones differs among species of plants, so not all plants grow with conical tops. But one can see the same phenomenon in other trees: for example, look at the cottonwood trees that have been decapitated to prevent them from growing up into the power lines. The remaining branches grow well and begin to reach upward more than before the top was removed.

The strength of apical dominance and the effects of release from that dominance vary among species, although very few general patterns have been discerned (such as effects of habitat, geography, climate, or life history). And the evolutionary pressures that govern both the strength of apical dominance and the effects of its release have been little studied. So I’ll just present a few examples here, to illustrate some of the variation and complexities.

Herbivory, by grazing and browsing animals, often crops off the tops of a plant, ending the dominance effect and leading to branching, which frequently increases the production of flowers and fruits (this is why gardeners commonly pinch off the tops and ends of branches, making a plant bushier and potentially more fruitful). But this begs the obvious question: if branchi-ness is good for the reproductive output of the plant, why was apical dominance so strong, suppressing the branches? Presumably, there are other advantages associated with apical dominance. For example, if the plant allocates resources not to branches but to good vertical growth, this can be advantageous by reaching more light and increasing the survival of the plant, which would eventually lead to greater lifetime reproduction. That option might over-ride the advantages of short-term increased fruit production in most circumstances. Nevertheless, the ability to respond to release, if herbivory occurs, is itself a useful trait. In other words, the evolutionary fitness of a plant may be better with strong apical dominance in the absence of herbivory, but better with release when herbivory occurs.

The club moss (Lycopodium) grows along the ground, sending up short vertical shoots. If the main upright shoot dies, dominance is reduced, and lateral spread increases. The horizontal growth then eventually reaches a new location and a new upright shoot forms there. A living, dominating shoot is successfully exploiting a good site, but when it fails and releases lateral growth, the individual plant as a whole may survive by reaching a new site. In short, the dominance is advantageous in certain circumstances but not in all.

In a species of fireweed (Epilobium in some taxonomies), strong apical dominance leads to good seed production and good seed dispersal (by wind) in areas where competition for light is severe, giving an advantage to good vertical growth and reduced branching. However, this only works where soil nutrients are sufficient to support such growth; in poor sites, which cannot support good growth, the potential advantages of apical dominance may be negated. Again, the advantage of apical dominance is seen in certain circumstances, in this case related to resources.

The relationship between resource availability and the effects of apical dominance are seen also in bearberry (Arctostaphylos uva-ursi). Here (in contrast to fireweed), apical dominance is weak in rich habitats, the plants are much-branched, and the plant is exploiting its present location to the fullest possible extent. In resource-poor habitats, apical dominance is strong, as the plant allocates its limited nutrients to growing in situ: release from apical dominance, perhaps by herbivory, leads to increased branching, increasing the chances of the plant stretching out to find a better site.

These few examples serve to emphasize that multiple factors interact to determine the strength of apical dominance and the consequences of release from that dominance. No wonder that researchers have yet to find general patterns in the ecology of apical dominance. The physiology is well understood; now to learn the whys and wherefores!