Abstract:
Transpiration is an important aspect of the water cycle. However, the nocturnal component of plant water loss is often neglected. Recent studies have shown in some species and under a variety of conditions, nocturnal transpiration can contribute a considerable portion to daily water loss. Importantly, night-time can be a period of replenishment for plant water stores, which are depleted as a result of the higher evaporative demand and temperatures of the day. Therefore, I calculated the daily and seasonal patterns of nocturnal water loss in Agathis australis, and the average daily pattern of water storage refilling and withdrawal in the summer. To calculate water storage flux, past studies used the differential between base-of-crown and basal stem sap flow measurements, as base-of-crown sap flow is assumed to equal transpiration. Large branches, as are seen in A. australis, can be a significant water store, thereby decoupling base-of-crown sap flow and transpiration at the leaf-air interface. Therefore, instead of base-of-crown sap flow, I used the Penman-Monteith equation, an analytical model, which estimates transpiration based on meteorological and leaf-scale physiological data. Water storage refilling occurred partially in the evening (45 %), but mostly at night (55 %), with withdrawal occurring during the day, peaking around noon. Daily transpiration was higher in the summer than winter, although the daily sum of nocturnal transpiration was relatively similar year-round. This implies the rate of nocturnal transpiration was higher in summer, as the shorter nights yielded similar levels of nocturnal water loss. Additionally, the drought summer (2012/13) had significantly more potential transpiration both diurnally and nocturnally. Finally, the overall contribution of nocturnal transpiration to daily water loss was on average 15.6 %, being higher in winter (27.5 %) than summer (11.4 %). The large water storage within A. australis stems leads to a longer refilling period, extending all the way to pre-dawn. Compared to other plants, where refilling occurs almost exclusively in the evening, night-time has added importance to A. australis, especially as water storage is an important component of its drought avoidance strategy. As forest managers, we also need to consider the evaporative conditions of night, as this will scale directly with a trees ability to refill depleted water storage tissue. As well, nocturnal evaporative demand was shown to be far higher in a drought summer, which compromises the ability of A. australis to cope with drought. Worryingly, drought summers are expected to increase in frequency and intensity for much of the current A. australis geographic distribution. However, refinement of the PM-model, in terms of its stomatal conductance input, is required for better estimation of canopy transpiration under lowered soil moisture conditions. The functionality of nocturnal stomatal conductance, both its patterns and magnitude, were ii also explored via shade-house experimentation. This provided further clarification to the proposed functionality of the commonly observed increase in stomatal conductance before dawn, where it is likely in anticipation of the favourable photosynthetic conditions of the morning. Why nocturnal stomatal conductance exists at all was also questioned, with focus on the leaky stomata hypothesis. Our results were consistent with what would be expected under this hypothesis, in that the plant species with less regulation over stomata also had higher nocturnal stomatal conductance, even when under drought conditions. However, there may also be functional explanations behind the existence of nocturnal stomatal conductance, other than the idea that stomata are leaky. For example, alleviation of intercellular CO2 build-up or for sustained nocturnal uptake of soil nutrients. These should certainly be tested, as the functional explanations of nocturnal stomatal opening can help inform mechanistic stomatal conductance models. This thesis counters the idea that night-time is a period of dormancy in plants. Although most water loss and carbon uptake occurs during the day, night is when important storage tissues are refilled (especially trees with large water stores) and plants prepare for the succeeding day. Therefore, understanding nocturnal ecophysiology is of importance to forest management and developing complete water budgets for ecologically significant species. However, there is still much work to be conducted on the nocturnal water relations of plants, specifically on the functional significance of nocturnal stomatal opening and its drivers.