Ecological thresholds and indicators during 14 years of urban forest development

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Kiri Joy Wallace, Bridgette Farnworth, & Bruce D. Clarkson

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Successfully reconstructing functioning forests from early-successional tree plantings is a long-term process requiring continual monitoring to gauge progress and hone management. However, many projects lack quantitative follow-up, sometimes leading to failure. Here we present a 65-ha urban restoration project in Aotearoa New Zealand where forest has been undergoing reconstruction on public retired pasture land since 2004. Such projects are becoming common due to emphasis on restoring urban nature, so by studying them we improve management and develop urban restoration theory. We measured key components of forest development in 25 plots (100m2 each) across a 14-year chronosequence including: canopy openness, native tree basal area, non-native herbaceous ground cover, leaf litter, dead trees, native tree seedling abundance and richness, and epiphyte colonisation. Linear regression models revealed statistically significant relationships between all these ecosystem attributes and forest age. Using breakpoint analysis we also identified several important ecological thresholds after planting, when canopy closure occurred (9.6 years) reducing understorey light, and a subsequent drop in nonnative herbaceous weeds (100% to 25%; 10.9 years). With these ecosystem changes, important indicators of functioning forests appeared, like leaf litter accrual (0% to 95% cover), increased tree deaths (0 to 15), and juvenile native plant recruitment. For example, spontaneous native tree seedling regeneration increased both in abundance (0 to ~150 per 4m2) and species richness (0 to 13), and epiphytes colonised (0 to 3 individuals). These results contribute to ecological theory regarding forest developmental thresholds and ecological indicators while also highlighting timelines and informing future restoration management.

Resource Type:
Conference Presentation, SER2021

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