Hügelkultur at Wairarapa Moana: Exploring the effects of buried coarse woody debris on revegetation efforts in a high-stress growing environment

In partnership with local government, the Featherston-based community restoration group Pai Tū Mōkai o Tauira led a revegetation trial at Lake Wairarapa, New Zealand. Using Hügelkultur, a technique that involves burying wood to improve soil fertility and moisture retention, this study examined the effects of the buried wood on the survival and growth of 800 native seedlings of eight species planted in a high-stress growing environment. Differences in soil properties and leaf traits were also assessed on and off Hügelkultur mounds.

Plants within the Hügelkultur area exhibited a 71% survival rate, 43% higher than those planted in the control plots without buried wood. However, survival patterns during the first five months diverged significantly from those observed over the subsequent two years. Among the plants that did survive, those in the Hügelkultur plots showed a remarkable 140% higher average growth compared to their counterparts in the control plots. Curiously, the survival of Cordyline australis and the relative growth of Leptospermum scoparium were not significantly influenced by the Hügelkultur beds. When considering an overall performance score that combines growth and survival, Phormium tenax, Cordyline australis, and Austroderia fulvida performed relatively well in control plots as well as across the study. Muehlenbeckia astonii showed strong performance, specifically in Hügelkultur conditions, but not in the controls.

When examining only the Hügelkultur plots without control plants as the reference, plants situated directly above the buried wood continued to demonstrate significantly higher survival rates. This effect was particularly pronounced for Leptospermum scoparium and Phormium cookianum. However, within the Hügelkultur plots, the presence of buried wood had no significant impact on overall relative growth rates. Conversely, Phormium tenax and Phormium cookianum did exhibit significantly higher growth when located on or near the buried wood.

Initial soil tests in 2021 verified the soil's poor quality. Despite the addition of substantial amounts of buried wood and wood chip, further tests in 2023 showed a significant decline in soil conditions, with an especially pronounced decrease in organic matter at the shallow sampling depth. The buried wood appeared to have a significant impact only on potassium and pH levels. These were also the two soil properties significantly associated with both plant growth and survival rates. Austroderia fulvida and Cordyline australis showed growth rates sensitive to soil conditions, suggesting their potential as soil quality indicators.

Coprosma robusta and Myoporum laetum exhibited extreme responses to the buried wood treatments: both species had poor overall survival but high growth rates among surviving individuals. This "grow or die" strategy was particularly evident in Myoporum, the only species to have an overall higher relative growth rate than survival rate.

Coprosma robusta had a significant inverse relationship between specific leaf area (SLA) and relative growth rate (RGR), suggesting intraspecific variation. Along with a strong negative relationship between SLA and 2023 plant height, results hint at Coprosma potentially being highly adaptable when differentiated sunlight amount is not a factor. Mantel tests found no significant spatial patterns in plant survival or growth, and overall, edge effects were generally not evident across the study. This suggests that other, non-spatial variables likely drive these outcomes. However, Coprosma robusta exhibited a unique sensitivity to edge effects, enjoying higher survival rates away from the edges. Unmeasured variables like soil moisture and weed competition are suggested as likely proximate drivers of the main effects of the Hügelkultur beds observed in the data. Given the slow decay rate of wood, the study captures just a brief initial snapshot of a long-term process. The study has expanded current understanding, leading to new insights while laying a good foundation for further study.