Texas Hill Country Trees, comparing size and shape to age

I have been gathering tree ring data on the first of four or more species of common hill country trees.  Ultimately I hope to have size and shape relationships for each species to post here, along with maybe tracings or photos of the shapes at different ages. 

 I've started on Ashe Junipers.  I have crossectioned or cored 72 trees so far from eighteen sites within Comal County.  See the map.  The map is not up to date but shows most of the sample sites.  Some of the sites are clumped near others so do not show as seperate sites on this county map.

I did a quick look at One-seed Junipers in the Texas panhandle also.  Click here to go over to that area, or just continue below for hill country junipers.

The first stage was to get the growth profile under different conditions.  I measured the trunk diameter, height and  shape for each tree.  I also recorded a moderately detailed micro-site description, GPS lat-lon, and photo for each tree.  Growth rates within the county varied mainly with the soil depth, amount of sun, and the slope of the ground.  Minor variations occurred also with soil composition.  I broke the sampled trees into 16 groups to account for these growth factors.  The results are posted on these composite graphs.  Several categories are not finished yet but most trees will fit somewhere in one of the categories shown.  I made a composite chart which uses both height and trunk size for nearly all of the 16 growth profiles. The chart is much easier to use.  Based on the 72 trees it was made from, its intrinsic error is six percent for most categories, 12 percent for the slowest  growth categories.  See the chart here.

The correlation coefficient for the chart age estimate to ring count age of 67 of the 72 trees is 0.87 so the age is 75% attributable to the factors which went into the chart.  The remaining five trees had rottten cores or otherwise incomplete cores not suitable for reliable ages.

 The current 100% canopy coverage for junipers can be seen on this false color satellite image.  Juniper woods (cedar brakes) are the dark green or dark olive green on the image. Areas with less than about 70 percent coverage are not apparent at this scale and satellite resolution. The next step in ground cover is to quantify the cover from ground checks.  This image shows the juniper-oak extracted from the satellite image overlain with some ground checked values.  These and additional ground checked cover values will eventually allow the ground cover to be transformed from the qualitative image to a quantitative view.

Following ground cover study will be a little study of stand age profiles in different parts of the county comparying the age spreads with a few control points on virgin stands. 
Conversations with a few landowners indicate it was vastly different in 1942 when the CCC finished cutting cedar and much different than that prior to 1933.   I may do a change detection approach from an older satellite image to see if the juniper growth is outstripping the cutting for fence posts.  That may be it for the Junipers.

After that it will be time to try other trees, if there is time.


I will add a few more pictures, especially of older trees to illustrate their characteristic shape soon.  Meanwhile here are a few pictures.
Here is a photo of a
crossection from a 2 inch diameter sixteen foot 45 year old juniper  which grew in very dense shade under canopy.
Here is a photo of a 1 inch 38 year old tree sprouting from bare rock, also in shade.
Here is its crossection.  The pencil point is for scale.  Twenty years of growth occur between the pith and the pencil mark just under the pencil point.  Many false rings can be seen as well as some missing rings, even in this half inch of tree.  Most of these are identifiable under the microscope and aging is not as difficult as it first appears.  The draught years of the 1950s usually appear within three rings of where they would be expected if the tree is old enough to have been around then.  This would imply a 6% accuracy without cross-correlation
Here is a 175 year old tree.  It is growing on a ridgetop covered with very shallow soil.  Not visible in the photo, masked by the much younger junipers,  are many older junipers just across the fence.  This very old ridgetop grove must have been the source for most of the junipers now covering those particular hills.  Originally only the creekbeds had junipers.  By 1930 the junipers had spread everywhere.  Between 1930 and 1950 junipers were cut back from the lowlands but the ridgetops were commonly left as the grass doesn't grow well for cattle grazing there anyway.
Here is a crossection of a one inch diameter five foot high tree which was growing in the sun on bare ground.