In the spring season, because cambium is very active and produces a large number of xylary elements having vessels / tracheids with wide lumen.

Annual Rings
(i) The activity of Vascular cambium is under the control of many physiological and environmental factors.
(ii) In temperate regions, the climatic conditions are not uniform throughout the year.
(iiii) In the spring season, cambium is very active and produces a large number of xylary elements having Vessels/tracheids with wide lumen.
(iv) The wood formed during this season is called spring wood or early wood.
(v) The tracheary elements are fairly thin walled.
(vi) In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels tracheids and this wood is called autumn wood or late wood.
(vii) The tracheary elements are with narrow lumen, very thick walled.
(viii) Spring wood is lighter in colour with lower density.
(ix) Autumn wood is darker and has higher density.
(x) Sometimes Annual rings are called growth rings but all growth rings are not annual rings.
(xi) In some trees more than one growth ring is formed within a year due to climatic change.
(xii) Additional growth rings are developed within a year due to adverse natural calamities like drought, frost, defoliation, flood, mechanical injury and biotic factors during the middle of a growing season, which results in the formation of more than one annual ring. Such rings are called pseudo or false-annual rings.
(xiii) Each annual ring corresponds to one year's growth and on the basis of these rings, the age of a particular plant can easily be calculated.
(xiv) The determination of the age of a tree by counting the annual rings is called dendrochronology.