Intracanopy adjustment of leaf-level thermal tolerance is associated with microclimatic variation across the canopy of a desert tree (Acacia papyrocarpa)

Abstract

Tree crowns are spatially heterogeneous, sometimes resulting in significant variation in microclimate across the canopy, particularly with respect to temperature. Yet it is not known whether such localised temperature variation equates to intracanopy variation in leaf-level physiological thermal tolerance. Here, we studied whether microclimate variation across the canopy of a dominant desert tree equated to localised variation in leaf thermal thresholds (T₅₀) among four canopy positions: upper south, upper north, lower south, lower north. Principal component analysis was used to generate a composite climatic stress variable (C_STRESS) from canopy temperature, vapour pressure deficit, and relative humidity. We also determined the average number of days that maximum temperatures exceeded the air temperature equating to this species’ critical threshold of 49 °C (AT₄₉). To estimate how closely leaf temperatures track ambient temperature, we predicted the thermal time constant (τ) for leaves at each canopy position. We found that C_STRESS and AT₄₉ were significantly greater in lower and north-facing positions in the canopy. Differences in wind speed with height resulted in significantly longer predicted τ for leaves positioned at lower, north-facing positions. Variation in these drivers was correlated with significantly higher T₅₀ for leaves in these more environmentally stressful canopy positions. Our findings suggest that this species may optimise resources to protect against thermal damage at a whole-plant level. They also indicate that, particularly in desert environments with steep intracanopy microclimatic gradients, whole-plant carbon models could substantially under- or overestimate productivity under heat stress, depending on where in the canopy T₅₀ is measured.