Growth and Photosynthetic Response to Light and Nutrients of Flindersia brayleyana F. Muell., A Rainforest Tree With Broad Tolerance to Sun and Shade

Abstract

Seed from four species of rainforest trees with widely contrasting sunlight requirements for growth and development were sown within disturbance gaps amidst mature forest on the Herberton Range in North Queensland. Observations on seedling persistence plus comparative growth of young trees of Acacia aulacocarpa, Toona australis, Flindersia brayleyana and Darlingia darlingiana (species ranked according to adaptation from full sun to deep shade) confirmed a broad tolerance of Flindersia to sunlight under all conditions, from wide to narrow gaps (minimum 0.6% full sun equivalent). Photosynthetic attributes which underlie such broad tolerance were subsequently inferred from single leaf gas exchange, plus foliar analyses of nitrogen, phosphorus and chlorophyll on tree seedlings held for 180 days under two nutrient × three irradiance levels adjusted to represent natural irradiance incident upon the forest floor (low), mid-canopy (medium) and emergent crowns (high irradiance treatment). Medium irradiance plus high nutrients proved optimal for leaf expansion, chlorophyll content and photosynthesis in air. Growth under low irradiance was characterised by thinner leaf palisade tissue, lower rates of dark respiration, increased leaf chlorophyll per unit nitrogen and lower light compensation point for photosynthesis. Such leaves retained a relatively high photosynthetic capacity despite these other shade-leaf attributes. High irradiance plus low nutrients proved supraoptimal for leaf expansion and expression of photosynthetic activity. Chronic photoinhibition appeared to prevail because apparent quantum yield was reduced, while photosynthetic processes on a nitrogen basis were substantially impaired. Nitrogen use efficiency, as inferred from leaf chlorophyll content, light saturated CO2 assimilation rate, electron transport rate and carboxylation rate on a nitrogen basis declined with increasing growth irradiance. Some ecological implications for the establishment and growth of these rainforest tree species in disturbance gaps are discussed.

Biotic and abiotic consequences of differences in leaf structure

“…Sun leaves are markedly thicker and can have additional palisade layers compared with shade leaves (Nobel & Hartsock, 1981 ;Thompson et al, 1988). In terms of resource use, development of the mesophyll (palisade plus spongy mesophyll) represents foremost the investment in N and in total construction costs.…”

Nutrients limit photosynthesis in seedlings of a lowland tropical forest tree species

“…Furthermore, nutrient addition causes an increase in A max in West African tree seedlings (Riddoch et al 1991). Although the studies by Thompson et al (1988Thompson et al ( , 1992 and Riddoch et al (1991) showed similar results to our data, their low light treatments (25-35 lmol m -2 s -1 PFD) likely had greater light availability than our study plants, whose ambient irradiance during our measurements, as recorded by the external PFD sensor on our infrared gas analyzer, was 6.7 lmol m -2 s -1 ± 0.5 (±1SE). Another difference between the studies by Thompson et al (1988Thompson et al ( , 1992 and Riddoch et al (1991) and the current study is that they added all elements together in nutrient treatments, limiting the potential to determine which elements affected photosynthetic processes.…”

“…The response of A max to nutrient addition is consistent with other studies investigating fertilization effects on tropical tree seedlings in low light. For example, seedlings of Flindersia brayleyana, an Australian tropical rainforest tree species with broad tolerance to sun and shade, increase A max and PFD sat in response to nutrient addition (Thompson et al 1988). In addition, nutrient addition causes an increase in A max in seedlings of two out of three shadetolerant tree species and one shade-intolerant tree species (Thompson et al 1992).…”

Nutrient limitation of eco-physiological processes in tropical trees

“…In an experiment with a combined N-P-K nutrient treatment the understory in Sabah, Malaysia, seedlings of Dryobalanops lanceolata and Shorea johorensis increased photosynthetic induction time with nutrient addition, whereas only S. johorensis increased A max with nutrient addition (Bungard et al 2002). In upland rainforest in Queensland, Australia, seedlings of Flindersia brayleyana, an Australian tropical rainforest tree species with broad tolerance to sun and shade, increase A max and the light level at which photosynthesis saturates, in response to a combined macroand micro-nutrient addition treatment (Thompson et al 1988). In a later pot study on four species from Queensland rainforests, nutrient addition caused an increase in A max in seedlings of two out of three shade-tolerant tree species and one shade-intolerant tree species in low (30 lmol m -2 s -1 ) light (Thompson et al 1992).…”