Coffee and the Leaf Economics Spectrum: size matters

Leaf size and other traits can tell a lot about a plant’s growth and condition. These traits are often compared between different species or crop varieties, grown in different environments. In 2004, scientists collected the leaf measurements of 2,548 plant species and proposed the global Leaf Economics Spectrum (LES) where there are trade-offs between chemical, structural and physiological traits. 

Drs Adam Martin and Marney Isaac from the University of Toronto, investigated how eight key leaf traits follow a “within-coffee-cultivar LES” that relates to plant size and berry production. The scientists found strong cultivar-level constraints across LES and leaf traits shifted to “resource-conservation” as the plants grew larger and started producing berries. Drs Adam Martin and Marney Isaac previously examined the LES in coffee grown in Costa Rica and Nicaragua.

The researchers grew 72 coffee plants (Coffea arabica, variety Caturra) at CATIE in Costa Rica under different fertiliser (e.g. moderate and high) and light conditions. Some plants were grown as a monoculture whilst others were grown beneath shading nitrogen-fixing trees, Erythrina poeppigiana in an agroforestry system. Scientists grouped the plants into six size categories based on their diameter at 15cm aboveground (D15). 

Plant height, crown diameter, reproductive status (e.g. presence of berries) and eight leaf traits (e.g. leaf dry mass, leaf thickness, photosynthetic capacity, stomatal conductance) were measured. Based on the plant measurements and reproductive status, the researchers calculated the critical tree size at when berries are produced and investigated how their measurements compared to global LES.

Martin and Isaac found that six out of eight leaf measurements were overall different based on plant size (D15) but only the leaf thickness trait increased linearly with plant size. Traits associated with physiological functions decreased as the plant size increased, suggesting a shift to “resource-conservation”. Three traits (e.g. photosynthetic capacities, leaf mass per area) were different under different light and fertiliser treatments. The probability of berry production increased with plant size and the tree diameter at reproductive onset was approximately 1.18 cm.

This study has shown that leaf traits are plant size- and environment-dependent and trade-offs between the traits change once the plants are producing berries. The scientists found that the magnitude of these trait changes are limited to a (potentially) cultivar-specific LES. Compared to the global LES, Martin and Isaac did not find correlation between leaf mass per area (LMA) and leaf N content within one coffee cultivar. This was unexpected as the correlation between these two traits (e.g. LMA and leaf N) is said to be one of the central trade-offs that defines plant resource-use strategies. 

“[T]here remain outstanding questions surrounding how deeply constrained the “shape” of within genotype or intraspecific Leaf Economics Spectra may be: are patterns of trait covariation and tradeoffs that occur within species 1) constrained along a single species- or genotype-specific LES? Or rather, 2) do multiple Leaf Economics Spectra exist within species or genotypes, which are related to environment, plant ontogeny, both, or other factors?”

“Our findings indicate the former, though whether or not this is unique to crops that have experienced extensive artificial selection – which has likely influenced the strength of constraints on leaf trait covariation or trade-offs within plants – remains to be tested.”