Divergent respiration-trait relationships reveal a unique economic strategy in ferns

Leaf respiration is a key metabolic process controlling leaf carbon balance. However, empirical data on leaf respiration, particularly in the light (R_L), remain scarce for ferns, and no study has systematically compared respiration-trait relationships between ferns and seed plants.

We measured seven leaf traits, including leaf respiration in the dark (R_D), R_L, light-saturated photosynthetic rate (A_sat), leaf N and P content, leaf dry mass per unit area (LMA), and leaf dry mass content (LDMC) in 19 ferns. These fern data were analyzed in comparison with seed plant data from the Glopnet data set.

Light inhibition of leaf respiration averaged 37%. Ferns had significantly lower LMA, A_sat, and R_D than seed plants. Standardized major axis analyses (SMA) revealed the slopes of R_D vs. LMA, A_sat, and P differed significantly between ferns and seed plants, while R_D-N relationships shared common slopes, but their intercept differed markedly. Generalized linear models identified LMA and R_D as the strongest predictors of R_L, jointly explaining 70.6% of its variation.

The terrestrial understory ferns employ a distinct "low-cost, slow-cycling" strategy-characterized by a unique combination of low leaf construction cost (LMA) and low metabolic rates (photosynthesis and respiration), which deviates from the traditional fast-slow leaf economics spectrum. The divergent respiration-trait relationships suggest that applying trait-based models parameterized solely for seed plants to ferns may introduce uncertainty into ecosystem models. This highlights the value of incorporating fern-specific trait relationships to refine projections of the global carbon cycle.