Canopy manipulation as a tool for restoring mature forest conifers under an early‐successional angiosperm canopy

Abstract

Low‐light environments in early‐successional forests that have established after abandonment of farming often restrict the establishment of later successional species resulting in an arrested succession. This 6‐year study tested the potential of different canopy manipulations to facilitate the establishment of a light‐demanding canopy tree species, tōtara (Podocarpus totara), within a regenerating kānuka (Kunzea robusta) stand. Results highlighted the effectiveness of artificial gaps over other methods (ring‐barking and edge‐planting) in accelerating the growth of planted tōtara. Seedlings under gaps grew consistently taller and faster over time indicative of an improved understorey light environment. Ring‐barking did not have a significant effect on tōtara growth because only a portion of the treated trees died, and after 6 years dead trees remained standing with intact branches resulting in insignificant increases in light transmission. At the forest edge sites, tōtara growth was highly variable. Although some seedlings grew as tall as in the gaps, others did not. Survival was also lower in the edge sites than in other treatments, which was likely due to enhanced herbivory from ungulates which impacted some plants at these sites. Gap creation is likely to be an important tool for restoring late‐successional canopy species in regenerating stands both through providing ideal sites for the growth of light‐demanding species such as tōtara and through natural establishment of other future canopy trees into the gaps.

Upscaling restoration of native biodiversity: A New Zealand perspective

“…When dispersal of later canopy dominants is constrained, consideration should be given to either establishing these species with the initial plantings or undertaking subsequent enrichment plantings (Tulod et al . ).…”

“…However, many of the latesuccessional tree species require the shelter of early successional species to establish and grow, especially on exposed sites. When dispersal of later canopy dominants is constrained, consideration should be given to either establishing these species with the initial plantings or undertaking subsequent enrichment plantings (Tulod et al 2018).…”

Restoring mature-phase forest tree species through enrichment planting in New Zealand’s lowland landscapes

“…A study in north Canterbury trialled artificial canopy gaps as a means of addressing light limitation in mature Kunzea robusta (kānuka) forest to assist the growth and restoration of the long-lived native conifer P. totara (Tulod et al 2019). Seedling height growth of P. totara was significantly greater beneath canopy gaps than under a closed canopy, with seedling growth rates in the gaps nearly twice those under the closed canopy.…”

“…Gaps of approximately 3 m radius and 0.6 gap ratio allowed 33% of available light to reach the understorey (gaps where four Kunzea trees were removed). These canopy gaps equated to a 76% increase in transmitted light compared to that measured beneath the intact forest canopy (Tulod et al 2019).…”

Opportunities and limitations of exotic <i>Pinus radiata</i> as a facilitative nurse for New Zealand indigenous forest restoration

“…Interventions to create more heterogeneous light conditions in P. radiata plantations might assist with the recruitment of missing mature forest tree species (Onaindia et al 2013). There are grounds to expect canopy interventions (creation of canopy gaps or thinning) or understorey manipulations would benefit podocarp establishment and seedling growth in mature P. radiata plantations (Tulod et al 2018).…”