Dendrochronology basics for timber: The most reliable way to ascertain the age of a tree (and hence the age of the timber source) is to count the visible growth markers known as:

Introduction / Context:
Determining the age of a tree is relevant in forestry, wood science, and conservation. For builders and materials engineers, understanding timber growth patterns helps in assessing properties such as density and ring orientation (quarter-sawn versus plain-sawn).

Given Data / Assumptions:

• We want the best direct indicator of tree age on a cross-section.
• We assume a typical temperate growth pattern with distinct annual rings.

Concept / Approach:
Most trees in climates with seasonal variation form one growth ring per year: a lighter earlywood band followed by darker latewood. Counting these concentric rings from pith to bark gives the number of years of growth (dendrochronology principle). While circumference or radius correlates with age, it is affected by species, site quality, and growth rate; branch count is not a dependable metric.

Step-by-Step Solution:

Verification / Alternative check:
Increment borers (coring tools) are used to extract a narrow sample to count annual rings non-destructively, validating the principle widely used by foresters.

Why Other Options Are Wrong:

• Radius/circumference: growth rates differ; not a precise universal age indicator.
• Branch count: highly variable with species and environment.

Common Pitfalls:

• Mistaking ring anomalies or false rings as separate years; careful observation is needed.
• Assuming uniform ring clarity across all species and climates.

Final Answer:
number of annual rings