As good as the neighbors? Photosynthetic light response of Lonicera maackii in heavy shade

Background/Question/Methods

Amur honeysuckle (Lonicera maackii) invades forests throughout the Northeast and lower Midwest USA. Dense populations cause substantial changes in species diversity, community dynamics, wildlife habitat, and ecosystem functions. It is purported that honeysuckle succeeds in forests largely because it produces functional leaves well before canopy closure and maintains leaves after canopy senescence. It is asserted that honeysuckle is only semi shade-tolerant, suggesting that carbon gain during summer is very limited. Were this the case, honeysuckles across the forest light gradient would depend on early and late season photosynthesis. They should produce similar numbers of flowers, and depending on pollination success, similar numbers of fruits and seeds. Several authors have documented reductions in flower, fruit, and seed production in shaded shrubs, which implies that spring and fall photosynthesis does not entirely offset limits caused by the tree canopy in summer. In a field study we compared the late-summer photosynthetic the light response of 10 honeysuckle to adjacent pairs of two native shade-tolerant understory species, spicebush (Lindera benzoin) and pawpaw (Asimina triloba). Individuals were naturally established under a canopy of seedling sugar maple (Acer saccharum) (mean PPFD: = 5.1, s = 1.2 mol m^-2 d^-1). Light response curves were measured over the range 0-800 µmol m^-2 s^-1 at CO₂ = 400 ppm, T_leaf = 30 C, and VPD ~ 1.1 kPa. Leaves were harvested to determine chlorophyll and specific leaf mass.

Results/Conclusions

Area and mass-based photosynthetic light response curves (0-500 µmol m^-2 s^-1) were very similar between species (p = 0.22 and 0.07, respectively). Area-based maximum photosynthetic rate (A_max) tilted (p = 0.11) slightly higher in spicebush. Mass-based A_max was 2x greater for native species than honeysuckle (p < 0.0001). Area and mass-based dark respiration did not differ among species (p = 0.54 and 0.08, respectively). Chl a:b ratio, and area-based total chl did not differ among species, however mass-based chl was 2x higher in native species (p < 0.0001) and specific leaf weight in native species was half that in honeysuckle (p < 0.0001). We suggest that honeysuckle has substantial light acclimation capacity based on physiological measurements and mere fact that it persists under sugar maple. However, in terms of efficient use of energy allocated to leaf mass under shade, it does not compare to natives spicebush and pawpaw. The costs of leaf support and rootmass remain to be compared.