Leaf mass per area (LMA) increases from the bottom to the top of tree canopies in all forest types. This universal pattern has historically been attributed to increasing light availability, based on classic experiments where leaves exposed to high light conditions developed thicker palisade mesophyll layers. Recently, however, hydraulic limitation has been implicated as the primary driver of this vertical gradient of leaf morphology. The theorized mechanism is that both leaf turgor pressure and xylem water potential decrease with height, which may inhibit cell expansion, leading to denser leaves with less intercellular air space (thus higher LMA). We hypothesized that water stress, rather than light, was the primary driver of leaf morphological traits of sugar maple (Acer saccharum). We expected to find a linear relationship between LMA and height driven by gravity-induced changes in leaf density and intercellular air space, rather than thickness. Using a cable zip-line system and arborist-style climbing techniques, we sampled leaves from 123 points along nine vertical transects up to 23 m of a closed-canopy sugar maple forest in Upper Peninsula, Michigan. We measured light transmittance (%TRANS), leaf water potential (ΨL), leaf dry mass, area, thickness, density, LMA, and mesoporosity, an index of intercellular air space.
Contrary to our expectations, we found light to be the primary driver of changes in leaf morphology. We observed a strong exponential relationship between LMA and height which appeared to be caused by changes in leaf and palisade layer thickness, which both increased exponentially with height (thickness * density =LMA). Relationships between %TRANS, LMA, leaf thickness, and palisade layer thickness were linear but were not as strong compared to those with height. We speculate that weaker relationships between these leaf parameters and light likely reflect difficulties in obtaining precise measurements of light environment. These results contrast with those found in both conifer and tropical trees where hydraulic limitation drove the increase in LMA, suggesting that sugar maple may have different mechanistic responses to environmental gradients. While the negative correlation between mesoporosity and height supported our hypothesis, the tight correlation between leaf thickness and height was unexpected. Additional experiments will be required to fully tease apart the effects of water stress and light. Application of the height-LMA relationship will allow future research to efficiently model 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.