There is a lot of spotlight on the planting of pine plantations in our rural landscapes, and one lens that we should be looking at this through is the impact on water yield within our catchments.
Catchments characterised by indigenous tussock grassland in the Otago uplands of New Zealand have been identified as having greater water yields compared to those of other vegetation cover (Mark and Rowley, 1976; Fahey and Watson, 1991; Fahey and Jackson (1997). The Otago region has a large amount of tussock grasslands, primarily consisting of Chionochloa rigida (snow tussock) and Chionochloa rubra (red tussock). These tussock grassland ecosystems are resistant to wind and snow, making them a hardy and robust land cover that is able to cope with the extremities of the Otago climate (Mark and Dickinson, 2008). In addition to their hardiness, the morphology of these tussocks also allows for conservative use of water through long, needle-shaped foliage with rolled leaves, and the ability to shut their stomata when atmospheric demand for water vapour increases (Davie et al. 2006). In addition to conservative use of water, tussock vegetation is characterised by foliage that has a high length-to-width ratio and needle-like structure, allowing these plants to be effective at intercepting fog water via promoting the coalescence of fog droplets on tussock vegetation (Mager et al. 2016).
Studies focused on understanding the differences of catchment water yields with differing land cover in Otago have largely focused on comparisons of unmodified tussock grasslands, pine plantations and/or treated tussock grasslands. One such study is the Glendhu experimental catchment study in upland east Otago, which recorded hydrological observations of the impacts of converting tussock grasslands to pine plantation over an observation period of 34 years. This study was designed to determine what effect that afforestation of tussock grassland might have on water yield, and it was seen as a study of importance due to the presence of down-stream hydro-electric-generating plants. This study is also unique as it presents results over a time period that includes a complete forest rotation from planting to just prior to harvesting.
This blog takes a look at key findings from a research paper by Barry Fahey and John Payne, published in 2017, titled: The Glendhu experimental catchment study, upland east Otago, New Zealand: 34 years of hydrological observations on the afforestation of tussock grasslands.
Study Area
The Glendhu study is located in the former Glendhu State Forest in the upper Waipori Basin, in the southern Lammerlaw Range. This is approximately 60 km due west of Dunedin. Glendhu’s climate is dominated by cold fronts moving across the area in the prevailing south-westerly air stream, with anticyclones that are more common in the warmer months. The closest climate station is at Lake Mahinerangi, 20 km east of the study catchment. Mean annual temperature at this climate station is 8.6 oC, with a mean annual rainfall of 980 mm and potential evapotranspiration of 658 mm.
How the study was conducted
The Glendhu study was a paired-catchment study, meaning that two catchments side-by-side were studied. One catchment was a control with 100% tussock vegetation cover, covering 216 ha. The tussock catchment was also referred to as GH1. A second catchment was planted with 67% pine vegetation cover, with the catchment covering a total of 310 ha. The planted catchment was referred to as GH2.
The planted catchment (GH2) was contour ripped in December 1981 by bulldozers to a depth of 0.8 m in preparation for planting. The ripping lines were 3.5 m apart. Planting occurred in June 1982 with Pinus radiata at a rate of 1,280 stems/ha, meaning a total area of 207 ha was planted with trees (Fahey and Payne, 2017). Harvesting of the trees commenced in 2014.
The study sought to understand the differences in water yield between the two catchments. In order to understand this, rainfall data was collected from the head of the two catchments, with streamflow measured at the base of the two catchments through v-notch weirs. These two measurements allowed for total evaporation to be calculated by subtracting total streamflow from total rainfall. Water yield was therefore defined as the total streamflow that went through the v-notch weir.
Results
Canopy closure of the planted catchment occurred in 1991, so changes in water yield were observed between the two catchments from this point in time. Before planting of one of the catchments in pines, there was little difference in water yield between the two catchments. However, the average annual water balance for the period of 1991-2013 (canopy closure to before harvest) showed a very different story. While both catchments had an average annual rainfall of 1,370 mm, water yield averaged 33% lower in the pine planted catchment (GH2) than that of the tussock catchment (GH1) (Fahey and Payne, 2017). Average annual water yield for the tussock catchment (GH1) was 832 mm, compared to only 559 mm for the planted catchment (GH2). This accounted for an average reduction in water yield for the pine planted catchment of 273 mm.
This reduction in water yield is comparable to that of other studies in New Zealand that have investigated the percentage reduction in water yield following conversion from pasture to pine plantation forest (Fahey and Payne, 2017). One such study was that of Smith (1987) which compared streamflow from two catchments in Pinus radiata and two in pasture at Berwick Forest, 30 km east of the Glendhu site. This study found that water yield in the forest catchment was on average 43% lower than that of the pasture catchments (Smith, 1987).
As the Glendhu catchment was only 67% planted in pines, it is believed that if this catchment was 100% planted in pines that the reduction in water yield would have been closer to 50% in comparison to the tussock catchment (Fahey and Payne, 2017).
This difference in water yield is attributed to the differences in total evaporation. Average annual total evaporation for the tussock catchment (GH1) was 498 mm, compared to 771 mm for the planted catchment (GH2). The study suggests that the difference is due to increased transpiration by the maturing tree crop, compared to that of tussock grasslands which are conservative in their use of water (Fahey and Payne, 2017).
It is clear from this research that there are implications of planting pine plantations on water yield within catchments. At the same time, the results speak to the importance of tussock grassland ecosystems for increased water yield in Otago’s catchments.