Ecological Thresholds as Constraints to the Growth and Survival of Woody Tree Species in Degraded Grassland in the South Island's Dryland Zone

Posted December 2013

Location: New Zealand

Organization: School of Forestry, University of Canterbury

Photo 1 (top left): Herbicide plot (the Y-posts to hold the shade cloth once native seedlings were planted)
Photo 2 (top right): Native plants under the Herbicide + Shade treatment at the end of the experiment


Photo 3 (bottom left): Setting up the Irrigation trial (pre-planting)
Photo 4 (bottom right): Native plants (Ozothamnus leptophyllus) in the irrigated plot at the end of experiment, where we can also see some native tussock grass growing.

New Zealand’s dryland zone contains some of the country’s most endangered ecosystems and is also the least well protected. Natural regeneration of native forest and shrubland species is often limited by several environmental and anthropic factors that affect the establishment and growth of seedlings. This research is focusing on better understanding some of these factors and how they can be manipulated to enhance restoration success in New Zealand’s South Island’s dryland zone, all of which were previously farmed and are now dominated by a rank growth of exotic grasses, weeds, alien mammals that browse on native plants, and have extremely dry and compacted soils likely caused by decades of intensive farming and grazing.

A combination of grass removal (cultivation, mulch, and herbicide) and shade treatments were established at the Rank Grass sites (two in the north Canterbury and one site in the Mackenzie Basin). Two other different trials were established in the Mackenzie Basin, one combining irrigation and shade, and another testing grazing and shade effect on the degraded short tussock landscape. The experiments’ objective was to assess the environmental factors that may be limiting the growth and survival of native woody seedlings in these areas.

Seedling survival, growth (Relative Height Increment, RHI) and physiological responses were followed for two growing seasons (summers of 2012/2013 and 2013/2014) along with soil physical and chemical properties. Large seedlings (20-40 cm tall) had higher survival rates than small seedlings (< 5 cm) at all study sites. Number of survivors was above 75% for large seedlings and below 15% for small seedlings at the end of the experiment. Though the plants’ responses to the treatments varied among the four species chosen for the experiment, in general, the shaded treatments had the highest survival rates and RHI in all five trials. Physiological measurements corroborated the RHI values, indicating that the plants in the shaded plots were also more photosynthetically efficient (mean Y-values > 600). These results suggest that soil-water deficit is the major ecological threshold preventing the establishment of native tree seedlings in those areas, which is aggravated by direct solar radiation, consequently high evaporation levels and soil compaction. Competition between native and exotic plants for this resource aggravates soil-water deficit.

For further details and information, please contact: 

Anna Paula Rodrigues1, Prof. David Norton2,  Prof. Matthew Turnbull3

Forestry PhD Candidate, School of Forestry, University of Canterbury ([email protected])

School of Forestry, University of Canterbury
School of Biological Sciences, University of Canterbury

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