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Understanding and predicting the relationship between leaf temperature ( T leaf ) and air temperature ( T air ) is essential for projecting responses to a warming climate, as studies suggest that many forests are near thermal thresholds for carbon uptake. Based on leaf measurements, the limited leaf homeothermy hypothesis argues that daytime T leaf is maintained near photosynthetic temperature optima and below damaging temperature thresholds. Specifically, leaves should cool below T air at higher temperatures (i.e., > ∼25–30°C) leading to slopes <1 in T leaf / T air relationships and substantial carbon uptake when leaves are cooler than air. This hypothesis implies that climate warming will be mitigated by a compensatory leaf cooling response. A key uncertainty is understanding whether such thermoregulatory behavior occurs in natural forest canopies. We present an unprecedented set of growing season canopy-level leaf temperature ( T can ) data measured with thermal imaging at multiple well-instrumented forest sites in North and Central America. Our data do not support the limited homeothermy hypothesis: canopy leaves are warmer than air during most of the day and only cool below air in mid to late afternoon, leading to T can / T air slopes >1 and hysteretic behavior. We find that the majority of ecosystem photosynthesis occurs when canopy leaves are warmer than air. Using energy balance and physiological modeling, we show that key leaf traits influence leaf-air coupling and ultimately the T can / T air relationship. Canopy structure also plays an important role in T can dynamics. Future climate warming is likely to lead to even greater T can , with attendant impacts on forest carbon cycling and mortality risk. 

Abstract Nonstructural carbohydrates (NSCs) play a critical role in plant physiology and metabolism, yet we know little about their distribution within individual organs such as the stem. This leaves many open questions about whether reserves deep in the stem are metabolically active and available to support functional processes. To gain insight into the availability of reserves, we measured radial patterns of NSCs over the course of a year in the stemwood of temperate trees with contrasting wood anatomy (ring porous vs diffuse porous). In a subset of trees, we estimated the mean age of soluble sugars within and between different organs using the radiocarbon (14C) bomb spike approach. First, we found that NSC concentrations were the highest and most seasonally dynamic in the outermost stemwood segments for both ring-porous and diffuse-porous trees. However, while the seasonal fluctuation of NSCs was dampened in deeper stemwood segments for ring-porous trees, it remained high for diffuse-porous trees. These NSC dynamics align with differences in the proportion of functional sapwood and the arrangement of vessels between ring-porous and diffuse-porous trees. Second, radial patterns of 14C in the stemwood showed that sugars became older when moving toward the pith. The same pattern was found in the coarse roots. Finally, when taken together, our results highlight how the radial distribution and age of NSCs relate to wood anatomy and suggest that while deeper, and likely older, reserves in the stemwood fluctuated across the seasons, the deepest reserves at the center of the stem were not used to support tree metabolism under usual environmental conditions. 

Leaf optical properties impact leaf energy balance and thus leaf temperature. The effect of leaf development on mid‐infrared (MIR) reflectance, and hence thermal emissivity, has not been investigated in detail.

We measured a suite of morphological characteristics, as well as directional‐hemispherical reflectance from ultraviolet to thermal infrared wavelengths (250 nm to 20 µm) of leaves from five temperate deciduous tree species over the 8 wk following spring leaf emergence.

By contrast to reflectance at shorter wavelengths, the shape and magnitude of MIR reflectance spectra changed markedly with development. MIR spectral differences among species became more pronounced and unique as leaves matured. Comparison of reflectance spectra of intact vs dried and ground leaves points to cuticular development – and not internal structural or biochemical changes – as the main driving factor. Accompanying the observed spectral changes was a drop in thermal emissivity from about 0.99 to 0.95 over the 8 wk following leaf emergence.

Emissivity changes were not large enough to substantially influence leaf temperature, but they could potentially lead to a bias in radiometrically measured temperatures of up to 3 K. Our results also pointed to the potential for using MIR spectroscopy to better understand species‐level differences in cuticular development and composition.