Environmental factors influencing survival of poplar material planted for erosion control on hill country farms in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand
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2023
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Massey University
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Abstract
New Zealand has naturally high erosion rates, and space planting trees (most commonly poplars and willows species) in hill country landscapes is a widely used mitigation tool (Hicks & Anthony, 2001; Basher, 2013; Marden & Rowan, 1993; Hancox & Wright, 2005; Marden, 2012). The effectiveness of planting is, foremost, dependent on the survival and establishment of the trees. Much existing information on tree survival and the factors that influence survival is unpublished, with few studies having quantitively investigated survival rates and multiple potential explanatory variables. Moreover, there has not been a study to understand how different planting practitioners view the importance of survival and what influences their decisions of where, how, and what to plant in different conditions. To explore these issues, this research involved a set of interviews which captured the current knowledge and views about tree planting and survival within a cross-section of New Zealand’s North Island regional and district councils and scientists (Chapter 4). The interviews revealed a general consensus on the importance of trees in erosion mitigation, but mixed views regarding the importance of certain survival variables (e.g., where on the stems the poles are cut, duration of time between harvest and planting, or wind exposure). Differences likely reflect the different regional climate conditions and practitioner experience. To enrich the quantitative data on tree survival in New Zealand hill country, a poplar planting trial was performed on three hill country farms in the North Island (Chapter 4). With climate considered to be one of the most important factors to affect survival, the trial tested the impact of different climatic conditions on plant survival over one planting season. Data from on-site climate stations and nearby NIWA (CliFlo) climate monitoring stations were used to analyse rainfall, temperature and solar radiation. The trial also tested the impact of morphometric variables (e.g., slope aspect, profile curvature, slope gradient and topographic position index), considered to affect the microclimate and soil conditions. At each farm, a north- and a south-facing slope susceptible to landslide erosion, was selected and each planted with 90 trees made up of three plant types; 30 3-m poplar poles planted using a pole driver, and 30 unrooted and 30 rooted poplar wands planted using a spade. Overall survival across all three plant types (poles, unrooted wands, rooted wands) at all trial sites combined was high (90.4%). The highest survival across all plant types occurred at the Hawke’s Bay site (93%) and the lowest occurred at the Taumarunui site (89%). South-facing
aspects had slightly higher survival than north-facing aspects, of 90% and 89% respectively. The Wairarapa had no pole deaths which was attributed to experienced pole planting contractors, but the lowest rooted wand survival (76.5%). The Hawke’s Bay site had the highest unrooted wand (98.5%) and rooted wand survival (93%). South-facing aspects had higher pole survival (98%) than north-facing aspects (92.3%), but lower unrooted and rooted wand survival (90% and 84.3% respectively) than north-facing slopes (91% and 89% respectively). The results suggest there was no significant relationship between poplar deaths and morphometric or climatic variables. However, there was a statistically significant difference in
growth rates between the north and south facing aspects, suggesting that microclimate may affect long-term survival. It was concluded that higher rainfall throughout the trial period potentially contributed to higher survival. Therefore, suggesting that in years of higher rainfall (i.e., East Coast during La Niña phases of the El Niño Southern Oscillation (ENSO) and positive Southern Annular Mode (SAM)), targeting planting in areas of higher non-survival risk should be completed to increase survival rates. Further research under more typical seasonal conditions is necessary to fully explore the morphometric or climate variable impacts. This research has captured the opinions and knowledge of experienced industry prractitioners which highlights the importance of several aligned variables that influence survival. It has provided a valuable systematic analysis of the cause of death of newly planted poplars and quantitatively assessed early establishment growth. These findings will be valuable to regional council planners, land management advisors and farmers and will potentially lead to more positive planting and reduced soil erosion outcomes for New Zealand hill country.
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