Formulae:

These (and many more) can be found in: Evans, G.C. (1972) The Quantitative Analysis of Plant Growth, Univ. California Press, Berkeley.

T₁ = time 1, T₂ = time 2, TW_x = total dry weight at T_x, LW_x = leaf dry wt at T_x, A_x = leaf area at T_x

• Leaf area ratio (LAR) = A_x / TW_x
• Specific leaf area (SLA) = A_x / LW_x
• Relative growth rate (RGR) = (ln TW₂ - ln TW₁) / (T₂ - T₁)
• Net assimilation rate (NAR) = (TW₂ - TW₁) * (ln A₂ - ln A₁) / ((T₂ - T₁) * (A₂ - A₁))
• As an approximation NAR = 2 * (TW₂ - TW₁) / ((T2 - T1) * (A1 + A2))

ln is the natural logarithm (LOG(number) in Excel)

Evans argues that unit leaf rate (ULR) is a better term than NAR but most people have stuck with NAR.

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