Artificial canopy gaps accelerate restoration within an exoticPinus radiataplantation

Abstract

We created small-scale artificial canopy gaps to accelerate the growth of mature indigenous forest canopy species for restoration of an 18-year-old exotic Pinus radiata plantation forest, in the Marlborough Sounds, New Zealand. Small and large circular gaps were formed by felling. Seedlings of two indigenous forest canopy species, Podocarpus totara (Podocarpaceae) and Beilschmiedia tawa (Lauraceae), were planted within artificial gaps and undisturbed plantation canopy. Seedling height growth, mortality, and occurrence of animal browse were monitored at approximately 6-month intervals over 17 months. Both P. totara and B. tawa differed significantly in height growth and in animal browse occurrence among artificial gap treatments. Growth of the light-demanding P. totara was better under large canopy gaps, whereas growth of the shade-tolerant B. tawa increased under gaps of any size but was most consistent under small gaps. For P. totara, any significant restoration benefit of gap formation on height growth was lost when browsed seedlings were taken into account. Animal browse significantly limited B. tawa height growth in large but not in small gaps. Small-scale canopy gap creation is an effective method of modifying light transmission to the plantation understorey and accelerating seedling growth rates. Canopy gap size can be used to optimize understorey illumination according to species-specific light requirements. The increased occurrence of animal browse in gaps requires consideration. Artificial canopy gaps within planted monocultures create structural heterogeneity that would otherwise take an extended period of time to develop. These results further support the role of plantations as indigenous forest restoration sites.

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

“…Although some seedlings grew as tall as or taller than in the gap treatment, others did not. Survival was also lower in the edge sites, which was likely due to enhanced herbivory (most likely by feral red deer; D. Norton 2017, University of Canterbury, personal communication) which impacted some plants at these sites similar to the effect of herbivory on tōtara seedlings planted into Pinus canopy gaps (Forbes et al ). While our study did not account for this browsing damage, herbivory by introduced mammals is a common problem in New Zealand forests and has been observed to restrict establishment of many indigenous tree species across a range of light levels (Husheer et al ).…”

“…In this study, total light transmission under gaps was 33% of open sky conditions, similar to the 30% total light transmission under large, naturally formed, canopy gaps in dense‐mixed forest in central North Island, New Zealand, that was found to result in the best growth of tōtara seedlings in a natural setting (Ebbett & Ogden ). In a study looking at two gap sizes in an exotic Pinus radiata plantation, in central New Zealand, Forbes et al () found significantly higher tōtara growth in gaps with at least 50% light transmission, although larger canopy gaps with 84% light transmission were more favorable for planted tōtara seedlings. Similar growth response with increasing light transmittance has been reported in other gap studies (Ramos & del Amo ; Coates & Burton ; Fahey & Lorimer ).…”

“…While our study did not account for this browsing damage, herbivory by introduced mammals is a common problem in New Zealand forests and has been observed to restrict establishment of many indigenous tree species across a range of light levels (Husheer et al ). Increased browsing by ungulates has been observed in large canopy gaps compared to small gaps (Forbes et al ). Thus, in sites where herbivory may impact seedling growth, small‐size gaps may be sufficient to improve understorey light conditions and achieve positive growth rates compared to creating larger clearings which are favored by ungulates.…”

“…Increased mammalian herbivory in canopy gaps or improved light levels has been reported in other studies (Howe ). Although tōtara is not that palatable, Forbes et al () noted that it can be attractive at times to deer browsing especially when the nutrient contents of its young shoots are seasonally high.…”

Browsing evidence of the native and near-threatened Pudu puda deer in restoration plantings on Chiloé Island, Chile

“…This pattern confirms our second hypothesis that in degraded North-Patagonian forests, N. nitida planted seedlings present higher browsing rates in places with higher %PPFD and/or lower residual basal area. For both removal of the understory and reduced (degraded) tree overstory, higher browsing rates have been found for many species and forest types worldwide (H orsley et al 2003, Tremblay et al 2007, Kuijper et al 2009, Churski et al 2016, Forbes et al 2016, Charron & Hermanutz 2017, Bose et al 2018. However, this pattern has shown controversial results for different species worldwide and thus, is a widely discussed issue (C asabon & Pothier 2007, Tremblay et al 2007, Kuijper et al 2009, Gerhardt et al 2013.…”