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

Abstract

Background: We investigated the long-term potential of non-harvest Pinus radiata plantations for the facilitation and restoration of a natural forest community dominated by indigenous woody species. We investigated the relationship between indigenous regeneration and light levels and the hypothesis that proximity to indigenous seed sources is critical. We studied nine Pinus radiata stands of different ages located within Kinleith Forest, which is a large (ca. 66 000 ha) commercial exotic plantation forest located in New Zealand’s central North Island.&#x0D; Methods: We constructed a chronosequence of P. radiata plantation stands aged 2–89 years to represent long-term natural forest regeneration following plantation establishment. We surveyed structural, compositional and contextual aspects of this secondary succession and compared these results with an old-growth indigenous forest reference site located within the study area.&#x0D; Results: The exotic P. radiata canopy facilitated a regeneration trajectory characterised by shade-tolerant indigenous forest species. We found that the structure and composition of P. radiata understories were strongly influenced by stand age and proximity to indigenous forest. Stand age was important from the perspective of creating shaded conditions for the establishment of shade-tolerant woody forest species. Our results suggest that proximal indigenous forest was required for the consistent natural establishment of larger-fruited, bird-dispersed mature forest canopy species in P. radiata plantations.&#x0D; Conclusions: Our results showed that, even at ecologically isolated sites, the microclimate conditions created by plantation Pinus radiata stands supported a suite of readily-dispersed indigenous forest plants. Based on these results we suggest that non-harvest P. radiata stands provide an important opportunity for the restoration of indigenous forest communities in New Zealand’s production landscapes. Where restoration of forest composition similar to old-growth is the restoration objective, however, interventions might be necessary to direct and accelerate the secondary forest succession. Further replicated study is required into the relationship between native forest proximity and understorey regeneration patterns.

Effects of competition and habitat heterogeneity on native‐exotic plant richness relationships across spatial scales

“…Adjacent exotic vegetation provides propagules of exotic plant species explaining the positive relationship (Iannone et al, 2015; Wiser & Allen, 2006). However, pine forests adjacent to native forest in New Zealand were shown to have more native than exotic species in their understorey, especially in older stands (Brockerhoff et al, 2003; Forbes et al, 2019). Adjacent exotic woody vegetation may promote both exotic and native richness, but its effect may be overridden by exotic grassland with increasing scale as exotic grasslands cover greater areas (10.6 Mha) in New Zealand than exotic forests (2.1 Mha) (Brockerhoff et al, 2003; Stats NZ, 2022).…”

Understory vegetation provides clues to succession in woody weed stands

“…However, as the stand ages and thins, light levels increase, and native seedlings may be better able to recruit (Wilson 1994;Lugo 2004;Carswell et al 2013;Geldenhuys 2013). This process has been observed in exotic tree plantations in New Zealand (Allen et al 1995;Ogden et al 1997;Brockerhoff et al 2003;Forbes et al 2019) and South Africa (Geldenhuys 1997). High-density populations of exotic pest mammals and domestic livestock can also severely restrict native plant establishment in New Zealand (Wardle et al 2001;Dodd et al 2011;Perry et al 2015).…”

“…McAlpine et al 2018reported 14 woody weed species that had a native understory of > 50% cover and no conspecific weed regeneration at one or more sites (including unmanaged plantations): silver wattle (Acacia dealbata), blackwood (Acacia melanoxylum), sycamore, alder (Alnus glutinosa), common barberry (Berberis glaucocarpa), willow-leaved hakea (Hakea salicifolia), larch (Larix decidua), black pine (Pinus nigra), cluster pine (Pinus pinaster), radiata pine (Pinus radiata), white poplar (Populus alba), Douglas fir (Pseudotsuga menziesii), silver willow (Salix alba) and grey willow (Salix cinerea). Additionally, radiata pine plantations in New Zealand are commonly recorded as having a native understory (Ogden et al 1997;Brockerhoff et al 2003;Forbes et al 2019).…”

“…Although most evidence comes from exotic plantation pine (Pinus species) forests, it should be applicable to wild pine populations and to other invasive conifers with similar regeneration ecology. Where exotic conifers occur in historically forested sites with adequate rainfall and a nearby seed source of shade-tolerant native species, a dense, speciesrich native understory can develop (Henry 1954;Norton 1989;McQueen 1993;Porteous 1993;Brockerhoff et al 2008;Forbes et al 2019). In the absence of disturbance, such sites appear to have the potential to return to native forest naturally.…”

The Who or the How? Species vs. Ecosystem Function Priorities in Conservation Ecology

“…However, R. pseudoacacia , for example, is simultaneously regarded as valuable for the restoration of degraded croplands ( Papaioannou et al, 2016 ). Similarly, exotic pine plantations in New Zealand can support a diverse native flora in their understorey ( Forbes et al, 2019 ). However, as monospecific plantations typically provide less non-production-related ecosystem services on all but least degraded sites ( Bauhus et al, 2010 ), future planted forests may be designed, and management measures adapted, to integrate ineradicable alien species – maximizing multifunctionality ( Liu et al, 2018 ; Messier et al, 2021 ).…”