Comparison of the LMA-height gradient in a canopy gap and closed-canopy in a sugar maple (Acer saccharum) forest

Background/Question/Methods

Leaf mass per area (LMA) is arguably the most useful plant trait for predicting leaf physiological function and extrapolating canopy processes across larger spatial scales.  The universal pattern of increasing LMA from the bottom to the top of tree canopies has historically been attributed to increasing light availability, based on classic experiments where leaves exposed to high light developed thicker palisade mesophyll layers.  Recently, however, hydraulic limitation in tall trees has been implicated as the primary driver of this vertical gradient in LMA. Consistent with the classical studies, our preliminary results show light to be driving the changes in LMA with height in a closed-canopy sugar maple (Acer saccharum) forest.  To further test this hypothesis, we sampled leaves within a canopy gap (greater light conditions) to compare with leaves of the same height within closed-canopy conditions.  In a sugar maple forest in Upper Peninsula, Michigan, we used a cable zip-line and arborist-style climbing to sample leaves along ten vertical transects up to 30 m within the closed-canopy, and a 23 m tower to sample leaves within a canopy gap (162 sample points total).  We measured light conditions as diffuse non-interceptance, predawn water potential (Ψ_Pre), leaf thickness, density, and LMA.

Results/Conclusions

We developed a model to predict LMA and to identify the relative contributions of hydraulic limitation (Ψ_Pre) and light on leaf morphology.  Within and across the two canopy types, light was consistently the most influential variable.  For a given height, LMA, leaf density, and thickness were greater in the canopy gap than closed-canopy, especially at the highest heights, supporting our hypothesis that light is driving leaf morphology in sugar maples.  This hypothesis was further supported by higher light levels in the canopy gap, but no apparent differences in  Ψ_Pre between canopy types.  LMA was more strongly correlated with leaf thickness than density, supporting previous findings that leaves exposed to higher light are thicker.  Our results suggest that greater leaf density may not be exclusively linked to water-stress, and light may play a major role in determining LMA, leaf thickness, and leaf density in broad-leaved deciduous species.  Our results have identified a simple parameter (canopy gap vs. closed-canopy) that largely explains spatial variation in height-LMA relationships.  This will be useful for modeling canopy function over larger spatial scales with remote sensing, because LMA is highly correlated with maximum photosynthetic rate, leaf respiration, and leaf nitrogen within canopies.