THE LEAF ECONOMICS OF SEED FERNS IN PENNSYLVANIAN MIRES: NEUROPTERID FRONDS WERE CHEAP AND SHORT-LIVED RELATIVE TO MOST ALETHOPTERID FRONDS

In modern plants, leaf mass per area (LMA) predicts leaf economics. Leaves with high LMA have low photosynthetic rates per unit mass and long leaf life spans. These leaves are metabolically expensive to grow, and have low concentrations of N and P per unit area and slow rates of dark respiration. They tend to have defenses against herbivory. Leaves with low LMA have high photosynthetic rates per unit mass and short leaf life spans. These leaves are metabolically cheap, and have high N and P per unit area and fast rates of dark respiration. They have few defenses against herbivory.

Thick leaves with high LMA require robust petioles for support. In dicotyledonous angiosperm, the area of the leaf blade and the width of the petiole predict LMA and leaf economics. This relationship cannot be used to investigate the leaf economics of Paleozoic plants with compound leaves: 1, Compound leaves break during fossilization; thus their petioles and pinnules are seldom preserved together and the size of the photosynthetic surface and petiole remain unknown for most species. 2, Fern and seed-fern pinnules are broadly attached; the pinnule lamina is supported by the midrib and ultimate rachis; thus midrib width does not predict the weight of the pinnule; 3, The midrib and ultimate rachis of tree fern fronds must support sporangia as well as the pinnule lamina. However, in permineralized seed fern fronds from Pennsylvanian mires, the ratio of pinnule width to pinnule thickness indicates relative LMA, and leaf economics.

Most Alethopteris populations in Pennsylvanian mires have thick photosynthetic laminae (350 – 635 μm) relative to their width (3 – 10.5 mm) resulting in width/lamina ratios of 8 – 18, suggesting relatively long-lived fronds. Most also have anatomical features associated with sun leaves: multiple mesophyll layers, prominent hypoderm, or upright leaves. Most neuropterid pinnules in Pennsylvanian mires have thin photosynthetic laminae (150 – 240 μm) relative to their width (6 – 20 mm) resulting in width/lamina thickness ratios of 34 – 83, suggesting relatively short-lived fronds. Many have anatomical features of shade leaves, e.g. hydathodes. Most mire Alethopteris apparently had long-lived fronds and grew in sun, perhaps as canopy trees. Most mire neuropterids may have been shade plants with relatively short-lived fronds.