Silicon concentration and leaf longevity: is silicon a player in the leaf dry mass spectrum?

1. Multiple functions of plant silicon are known from agricultural species where silicon fertilizer alleviates biotic and abiotic stress impacts. In contrast, the function of plant silicon in natural ecosystems is often overlooked and is relatively poorly understood. 2. We investigated a potential integration of silicon with the leaf dry mass economics framework. We examined the relationship between leaf longevity, a trait considered to optimize leaf carbon use, and leaf silicon concentration across plant functional types and phylogenetic groups, testing the hypothesis that short-lived leaves have higher silicon content to maximize carbon allocation to growth. We considered leaf longevity a continuous trait (months, n = 155 species) and a binary trait (annual vs. perennial, n = 602 species). 3. A significant negative correlation was found across all species between leaf longevity (months) and relative silicon concentration, indicating that across functional types and plant families, leaves with shorter life spans contain higher concentrations of silicon. A similar significant relationship was also found within deciduous angiosperm leaves. Silicon concentration was significantly higher in annual vs. perennial species across most plant functional types. Within order and family, no significant difference in silicon concentration was found between annual and perennial leaves except in the Poales and the Poaceae, where annual leaves had higher silicon concentration. Relative leaf/shoot silicon concentration varied much more than leaf longevity across the phylogeny, and there was a stronger phylogenetic signal from silicon concentration than from leaf longevity. Evolutionary divergence analysis showed that divergences between relative silicon concentration and leaf life span (months) were not significantly correlated; however, using a larger data set, leaf life span (as an annual/perennial binary trait) and relative silicon concentration were significantly correlated, implying that across a larger data set, shifts to higher relative leaf/shoot silicon concentration are consistently associated with shifts to annual foliage. 4. We suggest that in shorter-lived leaves, silicon could be a metabolically cheaper alternative to carbon, allowing a more favourable leaf carbon balance over short periods. Silicon could be a less versatile, but more disposable resource, with the potential to function in structural, stress alleviation and defensive roles by substituting for carbon that could be allocated to further growth and reproduction.