Urban Restoration Ecology: Investigating environmental change, ecological function, and enrichment planting

Ecosystems worldwide are being degraded and destroyed by human actions on an unprecedented level. This continues despite growing evidence that intact, functioning ecosystems are critical to human health and well-being. The field of restoration ecology has rapidly developed as a response to ameliorate the damage by investigating how to re-build ecosystems. This branch of science generally posits that replacing lost structure, i.e. re-vegetation, will re-create ecosystems, but there is little empirical evidence supporting this assumption. In ecosystems with long development timelines, such as forests, it is unclear how dynamics change after planting and over the long-term, and hence what best management practices should be used in the decades following initial plantings. This thesis addresses these knowledge gaps through three separate studies on restored urban forests. The first study investigated whether planted trees will eventually grow into self-regenerating forests that provide suitable conditions for establishment of late-successional plant species. Further, what timeline does this occur along and what specific conditions drive native plant regeneration? To answer these questions a chronosequence of restored urban temperate rainforests aged 3 to 70 years was used in the New Zealand cities of Hamilton and New Plymouth. Various ecosystem properties were measured in the restored forests and compared with the same in remnant and unrestored forests. Structural equation modelling was used to determine which properties were significant drivers of plant regeneration and break point analysis was used to identify thresholds in ecosystem property development over time. Results indicated that unrestored forests had marginally fewer regenerating late-successional plants than remnant forests did. This indicates that restoration actions must take place in these areas to ensure regeneration densities reach natural, desirable levels. The final model indicated it is possible for restored forests to provide suitable conditions for late-successional plant regeneration at 20 years after planting, when basal area has reached ≥ 27 m2/ha. Further, there are key ecosystem properties that drive native plant regeneration, including formation of a forest canopy that reduces competition from herbaceous exotic weeds and stabilizes the microclimate. The second study explored the connection between ecological function and forest structure. Specifically, it investigated whether nutrient cycling in the forms of decomposition and denitrification were related to restored forest structural properties and if so, what properties exactly? This is especially important in the New Zealand urban forest, where exotic deciduous trees shed leaves each winter, allowing drastic annual swings in sunlight reaching the forest floor and in leaf litter inputs. This, together with horticultural runoff that causes nitrogen enrichment, disturbs normal nutrient cycling. To understand drivers of decomposition rates and denitrification potential, various ecosystem properties were measured in 27 restored urban temperate rainforests. Structural equation modelling was used to determine whether forest structural attributes were related to the decomposition and denitrification. We found that decomposition rates were indirectly related to the forest canopy but denitrification potential was completely uncoupled from forest structure and instead was driven by edaphic and landscape qualities such as soil texture and drainage patterns. The third study investigated methods for establishing late-successional tree species under restored urban forest canopies which are invaded by exotic herbaceous weeds. Urban areas are prone to invasion by exotic plants. Throughout much of New Zealand the herbaceous groundcover species Tradescantia fluminensis Vell. (Commelinaceae) has invaded forest remnants, forming mats up to 1 m tall that prevent regeneration of native woody species. Without regeneration of late-successional native trees, an initially planted early-successional tree community will lack long-term diversity and resilience. This study used Beilschmiedia tawa (A. Cunn.) Kirk (tawa) as a model late-successional tree species to enrich early-successional tree plantings. Seedlings of heights 0.5 m and 1 m were planted into 11 replicate blocks infested with T. fluminensis throughout the city of Hamilton, New Zealand. Weeding and mulching were combined in a full factorial design to determine impacts on survival and growth of B. tawa. Environmental conditions were also measured to investigate their relationships with B. tawa growth and survival. Using ANOVA it was determined that weeding has no impact on B. tawa growth and mulching reduces its growth rate. Mulching is typically helpful in early-successional plantings in exposed landscapes but here did not aid tree growth, perhaps because soil moisture was not a limiting factor. Height partially determined growth rates where seedlings that were at least 1 m tall at planting grew faster and were not overgrown by T. fluminensis, but shorter seedlings were overtopped and had their growth rates hampered by T. fluminensis. Environmental conditions positively related to seedling growth were canopy openness, soil temperature, and air temperature. These results suggest that for maximum establishment success in the presence of aggressive exotic weeds, enrichment trees should be ≥ 1 m tall and planted when the developing forest understory microclimate is suitable. These studies contribute theoretical and practical advancements to the field of restoration ecology by demonstrating how planted urban forests develop, their best management after initial planting, and relationships between nutrient cycling and forest structure. Results indicate that there are some specific ecosystem properties that are disproportionately key in restored forest succession and nutrient cycling, such as exotic herbaceous weeds and the microclimate. These properties affect a critical goal in restoration, the regeneration of late-successional native plants. Analyses demonstrated that formation of the forest canopy is a key indirect driver of herbaceous weeds, the microclimate, and of decomposition rates. This information is important to ensure initial planting efforts and follow-up management are successful in providing long-lived, resilient restored forests