Variation in the degree of coupling between delta13C of phloem sap and ecosystem respiration in two mature Nothofagus forests.

Day-to-day variability in the carbon isotope composition of phloem sap (delta13Chd) and ecosystem respiratory CO2 (delta13CR) were measured to assess the tightness of coupling between canopy photosynthesis (delta13Chd) and ecosystem respiration (delta13CR) in two mature Nothofagus solandri (Hook. f.) forests in New Zealand. Abundant phloem-tapping scale insects allowed repeated, nondestructive access to stem phloem sap 1-2 m above ground. delta13Chd was compared with delta13C predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO2 were used to estimate delta13CR. By including a lag of 3 d, there was good agreement in the timing and direction of variation in delta13Chd and predictions by the canopy photosynthesis model, suggesting that delta13Chd represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. Significant day-to-day variability in delta13CR was recorded at one of the two forests. At this site, delta13CR reflected variability in delta13Chd only on days with <2 mm rain. We conclude that the degree of coupling between canopy photosynthesis and ecosystem respiration varies between sites, and with environmental conditions at a single site.

Seasonal and temperature dependence of photosynthesis and respiration for two co-occurring broad-leaved tree species with contrasting leaf phenology.

We measured the seasonal and temperature responses of leaf photosynthesis and respiration of two co-occurring native New Zealand tree species with contrasting leaf phenology: winter-deciduous fuchsia (Fuchsia excorticata J. R. Forst & G. Forst) and annual evergreen wineberry (Aristotelia serrata J. R. Forst & G. Forst). There was no difference in the amount of nitrogen per unit leaf area (Narea, range 40-160 mmol m-2, P = 0.18) or specific leaf area (S, range 8-27 m2 kg-1, P = 0.87) in summer leaves of wineberry or fuchsia. The amount of nitrogen per unit leaf area and S varied significantly with height of leaves in the canopy for both species (r2 range 0.61-0.87). Parameters describing the maximum rates of rubisco carboxylation (Vcmax) and electron transport (Jmax) were related significantly to Narea, and were 60% higher on average in spring and summer leaves than in autumn and winter leaves for both species. The seasonal effect remained significant (P < 0.001) when Narea was included in a regression model, indicating that seasonal changes were not only due to changes in Narea. Values for Vcmax and Jmax were 30% lower in wineberry leaves than in fuchsia leaves on average, although the difference ranged from 15% in summer leaves to 39% in autumn leaves. Activation energies describing the temperature dependence of Vcmax and Jmax in wineberry were 111 and 114% of corresponding values for fuchsia (Ea (Vcmax) = 39.1 kJ mol-1, Ea (Jmax) = 32.9 kJ mol-1). Respiration at night was the same (P = 0.34) at 10 degrees C for both species (R10 = 0.7 micromol m-2 s-1), although activation energies (E0) were higher in wineberry than in fuchsia (47.4 and 32.9 kJ mol-1 K-1, respectively). These results show that rates of photosynthesis are higher in winter-deciduous fuchsia than in annual evergreen wineberry.

Cold-induced photoinhibition and winter leaf-loss in the broad-leaved tree Aristotelia serrata (Elaeocarpaceae).

Relationships between cold-induced photoinhibition of photosynthesis and winter leaf-loss were investigated for leaves of a wineberry (Aristotelia serrata J. R. Forst. and G. Forst.) tree. Previous work identified consistent differences in leaf loss between shaded and exposed parts of wineberry trees, possibly related to the adverse effects of cold temperatures and high irradiance during winter frosts. Differences in leaf temperature and irradiance between shaded and exposed parts of a wineberry canopy were quantified, together with their relationships to decline and recovery of cold-induced photoinhibition of photosynthesis (measured by the ratio of variable to maximal chlorophyll fluorescence; Fv / Fm). These data, together with measurements of leaf loss from selected shoots, were used to test the hypothesis that leaf loss is greater from open than from shaded parts of wineberry canopies, and increases with decreases in Fv / Fm following winter frosts. Fv / Fm of leaves on exposed shoots was always significantly (P<0.001) lower than leaves on shaded shoots. The absolute difference in Fv / Fm ranged from 0.11 to 0.57. Fv / Fm declined markedly following periods of freezing leaf temperatures (Tl), but recovered after several days of non-freezing Tl. A multiple regression model, including terms for integrated irradiance (Qi), minimum Tl, Tl at the time of Fv / Fm measurements, and the value of Fv / Fm, on the previous day explained 95% of the variability in daily Fv / Fm. Leaf loss was greater from exposed shoots (0.22 leaves shoot-1 d-1) than from shaded shoots (0.07 leaves shoot-1 d-1; P<0.001) and increased following frost-induced decreases in the value of Fv / Fm. Measured rates of leaf loss from exposed shoots were significantly related to Fv / Fm measured 14d previously (r2=0.89, P<0.01), supporting our hypothesis.

Investigating leaf lifespans with interval-censored failure time analysis.

• Here leaf lifespans were investigated using interval-censored failure time analysis. The utility of this relatively novel approach is illustrated by comparing the leaf lifespan of winter-deciduous fuchsia (Fuchsia excorticata) and annual-evergreen wineberry (Aristotelia serrata). • Leaf emergence and mortality were first described using growth equations. Individual leaf lifespans were investigated using a parametric failure-time regression model that accounted for the interval-censored nature of leaf lifespan data. • Bud burst began 7 d earlier, the rate of leaf emergence was 0.06 leaves per shoot d-1 faster, and emergence ended 44 d earlier in fuchsia than in wineberry. The rate of leaf mortality was greatest in mid-summer in fuchsia, but nearly constant over the year for wineberry. There was a highly significant effect of the date of leaf emergence on leaf lifespan. Leaves emerging earlier in the growing season had shorter lifespans relative to later emerged leaves in fuchsia, whereas the reverse was true for wineberry. There was no significant effect of shoot height, branch order, or the number leaves of per shoot length on leaf lifespan. • These results highlight the utility of studying phenology at the individual leaf level using failure time analysis. Although growth equations precisely describe leaf emergence and mortality in wineberry and fuchsia, they obscured key differences between the species that were identified using failure time analysis.