PARENT SESSION
Contributed Oral Session 16: Photosynthesis and Water Relations: Conductivity; Stress
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 524 A, Level 5, Palais des congrès de Montréal

Effect of nutrient addition on carbon gain in relation to water-use: Implications for responses of mangroves to eutrophication.

Martin, Katherine^*,1, Bruhn, Dan^{1, 2}, Lovelock, Catherine³, Feller, Ilka⁴, Ball, Marilyn^{1, 2}, ¹ The Australian National University, Research School of Biological Sciences, Canberra, ACT, Australia² The Australian National University, Cooperative Research Centre for Greenhouse Accounting, Canberra, ACT, Australia³ University of Queensland, Centre for Marine Studies, Brisbane, QLD, Australia⁴ Smithsonian Environmental Research Center, Edgewater, MD, USA

ABSTRACT- Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing in a mono-specific stand along a natural salinity gradient in southeastern Australia. Tree height varied across the gradient in salinity; a fringe zone of tall trees characterised areas where seawater salinities prevailed, while a zone of dwarf trees dominated landward hyper-saline areas. Trees were fertilized annually from 2001 with nitrogen (N) or phosphorous (P), or not fertilised (controls). P-fertilisation had no significant effect on growth. However, N-fertilisation enhanced canopy development, particularly in dwarf trees. This N-induced growth response was not the result of increased CO₂ assimilation rates per unit leaf area when measured under standard conditions; the assimilation rates of dwarf N-plants did not differ from those of plants in other treatments, but were achieved at greatly reduced rates of transpiration, stomatal conductance and lower intercellular CO₂ concentrations. The maintenance of comparable assimilation rates at lower stomatal conductance implies dwarf N-plants had higher carboxylation efficiency. This is consistent with dwarf N-plants having greater N contents per unit leaf area and therefore more photosynthetic apparatus. Thus, N-fertilisation induced changes in leaf function that resulted in greater carbon gain per unit water transpired (water-use efficiency, WUE), but lower nitrogen-use efficiency (NUE). Likewise, dwarf plants generally had greater WUE and correspondingly lower NUE than fringe plants. Less negative foliar ¹³C values for both N-fertilised plants and plants growing in the dwarf zone were consistent with increased WUE predicted from gas exchange measurements. Hydraulic conductivity data show that plants with greater WUE may also be those most subject to cavitation. These results indicate that the increase in plant canopy in response to N-fertilisation was accompanied by a more conservative water use under highly saline conditions. Coastal eutrophication may significantly alter the structure and function of mangrove forests along a salinity gradient.

Key words: water use efficiency, photosynthesis, hydraulic conductivity, eutrophication