Understory microclimate and photosynthetic response of saplings in an old-growth eastern hemlock (Tsuga canadensis L.) forest

Abstract

I measured microclimate and photosynthesis of eastern hemlock (Tsuga canadensis L.) saplings in the understory of an old-growth eastern hemlock forest. About 14% of the canopy trees were deciduous. Daily maximum air temperature in the understory was 3-4°C lower than above the canopy in July through September, but only 1-2°C lower from October through June. Photosynthetic photon flux density (PFD) in the understory was typically less than 20 µmol m−2 s−1, or about 1% of full sunlight, and exceeded 200 µmol m−2 s−1 during < 1.2% of the daylight hours in July through September. Average PFD was higher in spring, with May having about twice as much total daily PFD as late June and early July. At 22°C, net photosynthesis by hemlock sapling foliage was 0 at ≈10 µmol m−2 s−1 PFD and reached light saturation (≈ 5 µmol m−2 s−1) at ≈ 350 µmol m−2 s−1 PFD. Net photosynthesis near light saturation increased slightly as air temperature changed from 11 to 15°C, but did not change significantly between 15 and 30°C. These microclimatic and physiological measurements indicate that net photosynthesis by hemlock saplings is probably little affected by temperature from late spring through early fall. Late spring is probably the time of greatest photosynthesis by eastern hemlock saplings in hemlock forests with deciduous trees in the canopy.