Posted by Tynan Burkhardt @TynanBurkhardt

Nocturnal transpiration is often ignored when studying the water relations of plants, with the assumption that stomata (small pores on the bottom of leaves) close at night, leading to negligible water loss. Although transpiration is far lesser at night than during the day, it can contribute a considerable component of daily water loss. For kauri, I have found nocturnal water loss to make up around 15 % of yearly canopy transpiration. However, for other species, nocturnal transpiration can contribute up to 30 % of daily water loss!

For many plants, night-time is a period of replenishment, where the stem water storage is refilled, after being depleted during the day. However, the importance of night time in the refilling process differs between species. In South American rain forests, where water is readily available year-round, water storage is small and very little refilling occurs at night, with most occurring in the evening. In comparison, kauri have extensive water stores, which are held within their iconic large stems and branches. Refilling of these stems and branches extends almost all the way to sunrise (Figure 1), demonstrating the importance of the nocturnal replenishment period for kauri.

Figure 1 – Daily pattern of withdrawal and refilling for kauri water storage, showing diurnal withdrawal followed by evening and night-time refilling.

Kauri rely heavily on night-time refilling in their water use strategy, with water storage buffering trees from the high evaporative demand and temperatures of summer. Night-time water loss limits a plant’s ability to refill water stores and increases in drought summers. For example, most nights of the 2012/13 drought summer had a considerable amount of transpiration, compared to last summer (2017/18), where most nights had very little transpiration (Figure 2). Worryingly, drought is expected to increase in frequency and severity for many regions where kauri is present.

Figure 2 – Frequency of nights at different levels of nocturnal transpiration (E_n) for kauri canopies in a ‘normal’ summer (2017/18) and a drought summer (2012/13).

But what does this mean for kauri if drought does become a common summer condition? Clearly, they will be less able to refill their water stores, perhaps leading to a water deficit as the drought progresses. However, water storage is not the only defense kauri have against drying soils. They have also been observed to drop leaves in drier summers and close their stomata when under even mild water stress. Therefore, the reduced ability to refill water storage does not necessarily mean there will be large scale kauri die offs when drought does occur, but it is one of the pathways in which kauri stands may become more water stressed.

Tynan is a Masters student at the University of Auckland’s Ecology Ngatahi lab group. He is studying Nocturnal Transpiration in kauri trees and is supervised by Cate Macinnis-Ng.
email: tbur187@aucklanduni.ac.nz