Interactive effects of salinity and nutrients on mangrove physiology : implications for mangrove forest structure and function

The mangrove habitat is characterised by temporal and spatial variation in water and nutrient availability, and exposure to highly variable evaporative conditions. This thesis examines effects of salinity and nutrients on leaf gas exchange characteristics and water relations of the grey mangrove, Avicennia marina, growing in a monospecific stand along a natural salinity gradient in south-eastern Australia. Tree height varied across the salinity gradient; a fringe zone of tall trees characterised areas where seawater salinities prevailed, while a zone of scrub trees dominated landward hyper-saline areas. Trees were fertilised annually from 2001 with nitrogen (N) or phosphor/us (P), or not fertilised (controls), as part of a global project established by J ^^ the Smithsonian Institute. recui. PR.CFEFWORA'I Salinity had a minor influence on leaf gas exchange characteristics compared to nutrients, with scrub plants growing in more variable, hyper-saline conditions maintaining greater CO2 assimilation per unit water transpired (water-use efficiency, WUE) and correspondingly lower nitrogen-use efficiency (NUE; CO2 assimilation rate per unit leaf nitrogen). Addition of phosphorous had no significant effect on growth, whereas N-fertilisation enhanced canopy development, particularly in scrub trees. When measured under standard conditions, the CO2 assimilation rates of N-fertilised plants generally did not differ from those of plants in other treatments, but were achieved at lower rates of transpiration, stomatal conductance and intercellular CO2 concentrations. The maintenance of comparable assimilation rates at lower stomatal conductance requires greater RuBisCO activity, consistent with greater N content per unit leaf area in N-fertilised plants compared to other treatments. Thus, N-fertilisation induced changes in leaf function that resulted in greater WUE, but lower NUE. Instantaneous estimates of WUE were supported by less negative foliar values for control plants in the scrub zone and N-fertilised plants in both zones compared to other treatments. There was no difference in hydraulic conductivity between stems of plants growing in scrub and fringe zones. Fertilisation with N and P enhanced stem hydraulic conductivity in relation to sapwood area (Ks) and downstream leaf area (KL) in A. marina growing along a natural salinity gradient. Seasonal changes in Ks and KL occurred with changes in the Huber value (ratio of sapwood area to downstream leaf area). The higher KL of N-fertilised plants enabled maintenance of a given transpiration rate at lower pressure gradients, and it was predicted that increased vulnerability to cavitation would underlie the low transpiration rates (and high WUE) in N-fertilised plants. However, addition of N had no effect on the vulnerability of stenns to cavitation, which increased markedly when stem water potentials declined below -5 MPa, particularly in plants in the scrub zone. There was no significant correlation between gas exchange characteristics and stem hydraulic conductivity; however, lower stomatal conductance at midday in all treatments was associated with increased embolism and more negative stem and leaf water potentials. Leaf water potentials declined to nadir values of -4.5 and -4.8 MPa in fringe and scrub plants, respectively, by mid-morning and were sustained through the afternoon. It appeared that stomata adjusted such that water potentials were maintained near the cavitation threshold. Nevertheless, tension-induced embolism averaged 70 and 50% in scrub and fringe trees, respectively. Further, there was no correspondence between diurnal patterns in percent loss of stem hydraulic conductivity (PLC) and stomatal conductance when measured on similar days. Although measurements were temporally separated, plants appeared to sustain relatively high transpiration rates with significant stem PLC, implying greater relative resistance of the roots. A complementary series of experiments was conducted on N-limited scrub A. germinans in eastern Florida to test the generality of responses to N-fertilisation in Nlimited systems. The Florida site was similar in salinity to the Australian site, but differed in temperature and relative humidity with the Australian site being cooler and drier. In Florida, greater photosynthetic capacity enabled N-fertilised A. germinans to maintain higher assimilation rates and stomatal conductance with the same instantaneous WUE as control plants. Fertilisation did not influence hydraulic conductivity or diurnal patterns of cavitation, and levels of native and midday embolism were lower in A. germinans at the more humid Florida site than in A. marina at the drier Australian site. While inter-specific differences cannot be dismissed, the lower stomatal conductance in N-fertilised A. marina relative to controls was consistent with apparent requirements to limit the extent of PLC at the drier site in Australia. Responses to N-fertilisation in N-limited systems contrasted with effects of Pfertilisation on the water relations of A. germinans in a P-limited system in Belize. Here, there was a dramatic increase in hydraulic conductivity following fertilisation with P, attributed to large increases in the conductive area of the stem. However,