Ultraviolet-B radiation and its effects on New Zealand trees

Abstract

This thesis explores the physical attributes of the ultraviolet-B (UV-B; 280-320 nm) environment in New Zealand and the effects of UV-B on the growth and physiology of native tree species.

The role that clouds play in modifying the UV-B environment was quantified by measuring biologically effective UV-B irradiance and photosynthetically active radiation (PAR) over a year at a timberline site in Marlborough, New Zealand. During the year of measurement, clouds reduced annual clear-sky UV-B by 30% and daily UV-B dose by up to 90%. The UV-B:PAR ratio reached a maximum at midday under both clear-sky and completely-cloudy conditions. Due to clouds, whose occurrence is quite irregular in New Zealand's maritime climate, UV-B was highly variable on sub-hourly and daily bases with maximum changes in daily UV -B dose of 4.5 kJ m⁻² d⁻¹ occurring around the time of tree budbreak.

The effects of a step increase in UV-B irradiance on six native tree species were determined in a laboratory experiment using measurements of leaf chlorophyll a fluorescence to estimate photoinhibition. A wide range of responses were observed, including a 20% increase in photoinhibition for two shade tolerant species and a 10% increase for the shade intolerant red beech (Nothofagusjusca (Hook. f.) Oerst.). The other three species were not affected. Mountain beech (N. solandri var. cliffortioides (Hook. f.) Poole) was particularly tolerant of high UV-B doses. Leaf and epidermal thickness, and UV -B absorbing compounds did not correspond well with species response to UV-B.

The way that nutritional (nitrogen) stress modifies UV-B sensitivity was ascertained for a sensitive species: cucumber (Cucumis sativus L. cv. Poinsett). Plants were grown in pots with four levels of applied nitrogen (N, 0.5, 2, 5, 10 mol N m⁻³) under ambient and ambient + 25% levels of biologically-effective UV-B. Plants grown under the highest level of N were deleteriously affected by increased levels of UV-B, resulting in decreased leaf area (-24%), height (-28%), and total biomass (-20%). Concentration of UV-B absorbing compounds increased in plants grown under supplemental UV-B; however, there was a greater increase when N was limiting (+72%). Therefore, the response of the plant to increased UV-B depended on the foliar N status.

Seedlings of two tree species, mountain beech and broadleaf (Griselinia littoralis Raoul), were grown in the field for 125 days during summer, to determine if the present-day level of UV-B irradiance affects biomass increment and physiology. Plants were covered with either UV-B transmitting or UV-B absorbing filters. Without UV-B, height increased (23%) and the number of new leaves decreased (-21 %) in beech, but broadleaf was unaffected. These effects became prominent during the second-flush of leaves, suggesting differential responses of leaves initiated in different seasons and under different UV-B regimes. In the absence of UV-B, the transmission of PAR through leaves and UV-B absorbing compounds of both species decreased, and foliar N concentration increased. In the youngest leaves of beech, but not of broadleaf, removal of UV-B reduced midday photoinhibition. Although these results (Le. reduced photoinhibition in beech with the removal of UV-B from the spectrum) are contrary to the laboratory study, this may simply reflect differences in leaf age, total UV-B dose and dose-rate history. Results indicate that present-day levels of UV-B are high enough to modify biomass allocation and the physiology of beech and broadleaf seedlings. Consequently, although future changes in UV-B levels may be small, there are likely to be important cumulative biological and ecological ramifications.