Seasonal patterns of leaf gas exchange and water relations in dry rain forest trees of contrasting leaf phenology

Abstract

Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.

Environmental drivers of leaf phenology in trees of the tropical species Ficus obtusifolia

“…The increase in y T observed in mature leaves before shedding could have been due to the water conservation exerted through low transpiration rates, suggested by low V. Increases in y T as well as V observed after foliar exchange were evidently a consequence of rains. In deciduous species, water potential measured under drought remained close to or above the values of the rainy season due to the joint diminution of stomatal conductance and total leaf area, which revealed a poor or null relation between leaf fall and water status (Choat et al, 2006).…”

Role of Wood Water Properties and Leaf Dynamics in Phenology and Response to Drought in Evergreen Himalayan Tree Species

“…In 2001 W c decreased from January when vegetative buds were fully developed and a further decrease was observed in the next month when leaf emergence started thus indicating the withdrawal of stem moisture for bud break. In Cochlospermum (with high water storage and low wood density), Choat et al (2006) found senescence early in the dry season to avoid large decrease in leaf Ψ.…”

Deciduousness in a seasonal tropical forest in western Thailand: interannual and intraspecific variation in timing, duration and environmental cues

“…Several recent studies have investigated aspects of the physiological controls and constraints on foliar phenology. For example, studies of trees in seasonal dry tropical forest have investigated water relations and gas exchange (Choat et al 2006), hydraulic architecture (Choat et al 2005), and relationships between hydraulics and photosynthesis (Brodribb et al 2002) across seasons and in species with differing foliar phenologies. An extensive field survey such as ours can provide the necessary context for species selection to target specific phenological comparisons or contrasts that may be informative and help bridge the gap between correlative and physiological analyses.…”