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Shade tolerance: an additional factor affecting the distribution of mountain beech and silver beech in New Zealand?

Bayeur, NicoleM., Carpenter, KaylynL., Hughes, NicoleM.
Trees 2018 v.32 no.2 pp. 539-547
Nothofagus menziesii, canopy, carbon, cloud cover, drought tolerance, gas exchange, geographical distribution, habitats, leaching, photosynthesis, shade tolerance, soil nutrients, solar radiation, temperature, understory, New Zealand
KEY MESSAGE: Increased shade tolerance and a lower thermal optimum for photosynthesis may give silver beech a photosynthetic advantage over mountain beech in cloudy regions of New Zealand, and possibly the understory. The geographic distributions of silver beech (Lophozonia menziesii) and mountain beech (Fuscospora cliffortioides) in New Zealand roughly correspond with precipitation gradients, with F. cliffortioides dominating in sunny and dry regions, and L. menziesii either dominating or co-dominating with F. cliffortiodes in regions that are more cloudy and moist. Previous research suggests that greater drought tolerance may account for dominance of F. cliffortioides in sunny/dry regions, while greater tolerance of low soil nutrients may render a competitive advantage to L. menziesii in wetter habitats vulnerable to nutrient leaching. In this short communication, we propose and provide preliminary evidence for an additional hypothesis—that greater shade tolerance and a lower thermal optimum for photosynthesis confers a photosynthetic advantage to L. menziesii relative to F. cliffortioides under heavy cloud cover, and possibly in the forest understory. Sunlight measurements were recorded under clear sky, partly cloudy, overcast, and understory conditions at four field sites in New Zealand where the species co-occur, and integrated into previously published photosynthetic light response curves to estimate predicted carbon gain at 10, 20, and 25 °C. Photosynthetic gas exchange measurements were sampled in the field to corroborate expected results. Consistent with our hypothesis, predicted carbon gain was significantly greater in L. menziesii under cool (10–20 °C), low-light (< 180 μmol m⁻² s⁻¹) overcast and understory conditions. In the field as well, L. menziesii exhibited significantly (3.5×) greater photosynthesis under cool overcast conditions, and 0.5 to 6× greater photosynthesis under cool understory conditions compared to F. cliffortioides. No differences were observed under either of these low-light conditions when temperatures were > 20 °C. We conclude that relative tolerance of cloud and canopy shade may be an additional physiological factor, in combination with those previously described in the literature, affecting geographic and ecological distribution of mountain and silver beech in New Zealand.