The North Bayo Bench Trail

© Tom Jervis

It is always a good time to think a little about the forest that surrounds our community and why it looks the way it does. The North Bayo Bench trail, starting from the intersection of Diamond Dr. and San Ildefonso Rd. provides a good opportunity to observe the forest structure that is developing around town. What follows is a kind of field guide to that trail, with an emphasis on the forest.

Before starting out on the trail, it is worthwhile to walk a ways north up San Ildefonso towards Barranca Mesa. From about half way up the hill, you get a good view of the treetops. Looking over the canopy, take note of the trees that stick up out of the canopy. These trees are of course much bigger than the others, but they also show another characteristic of advancing age; they are not as pointed on top, having flatter, more spread out crowns characteristic of mature Ponderosa Pines. What is particularly noteworthy is how few of these trees there are in contrast to the denser young ones. From a bit further up the hill you can also see a few dead snags sticking up on the top of North Mesa.

Returning to the bottom of the hill and setting off down the trail across the meadow, you soon enter a very dense stand of small trees. A few dozen yards into the woods is a single larger tree on the right side of the trail. It is easy to pick out from its red bark and its larger size, about 25” diameter at breast height (dbh). This “yellow-belly” is well over 150 years old, having germinated some time in the early 1800s. This tree is not at all old by Ponderosa Pine standards, but it is still not as large as one might imagine it should be, particularly since it is situated in fairly deep soil on a north-facing slope where it should get more moisture.

Information about the history of an individual tree can be obtained by extracting a thin core of wood through the center of the tree. These cores allow the examination of the annual growth rings of the tree without cutting it down. Looking at a core taken from this tree reveals why it is no larger than it is. For most of its life, it grew at a rate of about a tenth of an inch (in radius) a year so it should have a dbh of about three feet. However, about 60 years ago, the growth rings seen in the core become abruptly closer together, indicating a growth rate only a fifth of that seen earlier. The cause of this slowdown in growth is everywhere you look from here. What you see is many “blackjacks”, smaller Ponderosa pines with black bark characteristic of younger trees. Some of them are very puny, 4” dbh or less and some, particularly those below the trail on flatter ground are much larger (10” dbh) and seemingly healthy.

The surprise comes when you begin to core a number of these smaller trees. Despite the difference in size, they are all the same age! All of the blackjacks in the area germinated in the 1920s. From the density of the big trees (and some snags and stumps), it is apparent that this area must have been a mostly open meadow at that time. The relatively moist years of the late 1920s contributed to good germination and a lot of little trees got their start. During the dry 1930s, the area probably looked a lot like areas in Rendija Canyon today, where lots of little trees are coming up in what is mostly a grassland with a few larger trees.

As these trees got established and started to grow, they began to compete with the older residents, and the competition for water and nutrients slowed the growth of all. Even these vigorous young trees could not grow at the rate that the big tree sustained through most of its life, and the vagaries of soil depth and location had a greater effect on their growth, resulting in a wide variation in size despite the common age. As the younger trees get bigger, they are fighting to the death for available nutrients, and indeed some are beginning to succumb. But this is not a healthy competition, it is one in which all of the combatants are barely hanging on to life. They are susceptible to disease and parasites like Dwarf Mistletoe, and more importantly, their density makes them very susceptible to fire.

This scenario is also what led to those snags seen up on North Mesa. Intense competition from a large number of smaller trees on a somewhat drier site on the mesa top pushed them over the edge, and they died within the last decade, after a life of only about 150 years. In years before widespread grazing and fire suppression, low level fires would have burned in the grass and thinned out those small trees so that the survivors could grow more vigorously and older.

This pattern is seen all over Los Alamos. If you stop almost anywhere in the woods and look around, you see a very few of the large yellowbellies surrounded by many blackjacks. This is perhaps the predominant feature of our forests, and it makes them a significant fire hazard.

A hike along the North Bayo Bench Trail also illustrates the effects that exposure and slope have on the growth and composition of the trees in the forest. About a third of a mile from the parking lot, the trail crosses to the north side of the canyon and the difference in forest structure is immediately apparent. Compared with the dense and relatively young forest at the beginning, the trees are more widely spaced, and there is a good cover of grasses. Shrubs, particularly oaks, mountain mahogany, and a few junipers are also present. While there are some small, young trees, there is not the high density found in the canyon bottom and on the north facing slopes near the beginning of the trail.

The difference is primarily due to the south-facing aspect. Although the same rain and snow falls on both north and south-facing slopes, it evaporates more readily here so conditions are more severe on the south-facing sides. Another feature is that the south-facing slopes are much steeper so rain runs off more quickly. Differential effects of weathering result in cliffs and steep hillsides on the south-facing slopes and more gradual slopes and deeper soils on the north-facing slopes.

As a result, the forest here looks much as it might have a hundred or more years ago, before the advent of grazing and fire suppression did so much to change the character of the forest elsewhere in the County. The broad south-facing bench has only scattered trees, and nearly all show the flattened crowns and deep red bark of older Ponderosa Pines. In some areas there are indeed a few “blackjack” Ponderosa that are about 50 years old, but they are the exception.

Although most of these trees are not particularly impressive in size--most are no more than 18” diameter breast height—their longevity certainly should earn our respect. Core samples taken from them allow the annual growth rings to be counted to determine their age. The larger of these trees are over 250 years old and typical ones are well over 200. Cores taken from fallen logs lying in the area, which could have died 100 years ago, also show many closeset rings indicating that slow growth has been a feature of this bench for a long time.

Because these trees are growing in what is essentially a marginal environment for them, slight changes in the microclimate can have a substantial effect. Trees growing up the slope where the soil is slightly deeper tend to be larger, those growing at the bottoms of side drainages likewise are larger. Continuing out the trail there are few real giants, although one, on the south side of the trail with a large side branch fairly low down, shows that even in this difficult exposure, trees can get quite large. It is difficult to get an accurate age for this tree, but it is well in excess of 250 years, probably close to 400 years.

The location of the sewer line along this bench also provides an insight. The deeper soil resulting from the excavation and the fact that the trench for the line catches some moisture has led to a dense growth of small pines. This indicates that the sparseness of the trees is not due to a lack of seed or moisture but to the overall harshness of the site.

As one nears the point at the end of the trail, the Ponderosa Pines give way almost completely to Piñon Pine and One-seed Junipers. This is the ecotone or transition zone between Ponderosa Pine and Piñon-Juniper forest types. Looking into and across the canyon at this point demonstrates how important exposure is in determining which trees grow and how vigorously. In the canyon bottom there are large Ponderosa Pines taking advantage of the deep soil and adequate moisture. Across the canyon are Douglas Fir and Western White Pine, mixed with Ponderosa on the cooler north-facing slope.

Elevation, exposure, and soil conditions, as well as fire and other disturbance have shaped the forests that surround our community. Some, like those on the dry slopes, are quite healthy. Others are densely overgrown and are cause for concern, both because of the possibility of fire danger to our community and for the loss of wildlife that results from these unnatural forests.