While leaves’ photosynthetic responses to light and water availability are well understood, there is still no consensus about the factors driving the vertical gradient of photosynthesis within forest canopies. This has hampered the upscaling from leaf-level photosynthesis measurements to canopy and landscape levels needed for carbon exchange models. Light availability gradients in forest canopies have long been considered in upscaling models, and leaf economic traits have been positively related to it. Water limitation along the canopy has, on the other hand, only recently gained attention, while microclimatical conditions have been focused on even less. The objective of this study was therefore to understand how light, CO2, vapour pressure deficit (VPD) and temperature along the canopy affects resource allocation and photosynthetic gradients.
In spring and summer 2017, we assessed vertical gradients of several leaf traits in the canopy of beech and silver fir trees growing in a mixed forest in Switzerland. We characterised the photosynthetic gas exchange of leaves, water-use efficiency (13C fractionation), chlorophyll (Chl) and nitrogen (N) concentrations, and leaf structural traits at four canopy heights and in the understorey. We measured the microclimate (VPD, temperature, CO2 concentrations, light availability) in the canopy and related leaf economic traits to microclimate and phenology.
Results/Conclusions
Light availability in the canopy of beech and silver fir trees increased up to threefold from bottom to top, but surprisingly, the photosynthetic capacity remained relatively constant in both European beech and silver fir. Later leaf-out in the upper canopy of beech in spring even led to lower photosynthetic capacity with canopy height. Chl concentrations decreased by up to 47% from the lower to the upper canopy, reflecting the reduced need for light harvesting at the top of the canopy. N concentrations remained relatively constant within the canopy, which reveals that the optimal allocation of resources is not driven by light availability alone. Instead, 60% higher VPD in the upper canopy may have led to stomatal closure in order to limit water loss, as indicated by an enrichment of 7‰ leaf-13C compared to the lower canopy. CO2 concentrations were, on the other hand, relatively constant within the canopy.
The photosynthetic capacity in the upper canopy might thus be restricted by water limitations due to microclimatic gradients, hence driving the resource allocation in tree canopies. Consequently, upscaling efforts within ecosystem carbon exchange models may need to include additional microclimatical drivers, in addition to radiation, to accurately describe photosynthesis gradients.