ARE MODERN FERN LEAF ECONOMICS COHERENT ENOUGH TO USE AS A PROXY FOR FOSSIL INTERPRETATIONS?
Why LMA? Leaf mass per area indexes a species’ position along a multi-trait, “leaf economic” spectrum that describes key features of the dry mass and nutrient economics of carbon gain. Typically, species near the low-LMA end of the spectrum have high leaf nutrient concentrations and photosynthetic capacity, and are relatively fast growing, but also tend to be highly palatable to herbivores and have short leaf lifespans. Species towards the high-LMA end of the spectrum tend to show the opposite suite of traits, consistent with far more conservative nutrient-use strategies.
Previously both leaf mass and LMA of extant species have been shown to be reliably correlated with the width of the petiole (both for biomechanical and hydraulic reasons), allowing estimation of LMA for fossil gymnosperm and angiosperm leaves. Here we investigate the potential of using a similar approach for fossil ferns.
Petiole and leaf traits were measured on ~120 extant species in order to generate predictive regression equations for LMA. LMA could be predicted well from petiole dimensions within some fern families and orders, but not in others. Whether a family belonged to a monophyletic or paraphyletic clade had no bearing on this, although growth form potentially plays a role, for as yet unknown reasons. Overall the relationship between petiole dimensions and LMA was considerably weaker for ferns than for previously-tested plant groups, and the variance within ferns was considerably larger. Thus, although measurements made on fossil petioles can indeed aid in reconstructing LMA for species in some plant groups, at this point this particular method appears less generally applicable for fossil fern species. That said, for fossil species in at least some clades this approach looks likely to greatly aid investigation of their ecology and evolutionary history.