Dating the forest

Determining stand ages in a tropical humid forests remains a challenge, mainly due to the lack of (annual) growth rings in the wood of many tropical trees. A few months ago I was offered the opportunity to date wood core samples using radio carbon dating at the UC Irvine Keck Carbon AMS facilities, resolving the issue of a lack of visible characteristics to determine the age of a tree. I’m indebted to Prof. dr. Trumbore for providing this generous offer as these tests are expensive.

Three samples along a wood core for two tropical humid forest species were selected. One species consisted of a dominant canopy species Scorodophleus zenkeri and a common mid-sized / understory species Panda oleosa.

Only the  youngest of the three samples for each species was valuable in this exercise. Other samples fell within a region which is hard to date using radio carbon dating as the calibration curve for this region is relatively flat (~1700 – 1950) resulting in large uncertainties when relating 14C-age to calendar years (see examples Figure 1).

Figure 1. – Relationship between 14C-age and calendar years. Example 1 shows a clearly defined date for a given 14C-age. Example 2 shows multiple possible calendar years matching a given 14C-age, complicating accurate dating (courtesy of Utrecht University)

 

Although the data is limited to two data points per species, the first being the time at which the wood core sample was extracted and a second the dated sample as determined by radio carbon dating, we could get a rough estimate of the growth rates for both species during the last few decades.

Ages of the most recent samples were determined to be 1962 / 1985 and 1966 translating into growth speeds of~0.8 / 1.3 mm/yr and 0.7 mm/yr for Scorodophleus zenkeri and Panda oleosa respectively. These values fall well within the ranges as defined by literature but on the lower end of the range.

The slow growth rate of Panda oleosa is not surprise as it is often light limited, being a mid-sized species. The stature of these understory trees pales in comparison to emergent trees such as Petersianthus macrocarpus (Figure 2), their age however might not. Take home message: don’t judge a tree’s age by it’s stature.

Figure 2 – Petersianthus macrocarpus, a potentially emergent tree reaching ~45m height with a diameter of ~150cm (courtesy http://liberianfaunaflora.org)