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1. Ungulates have reached such high densities in some natural ecosystems that culling is frequently used to reduce their impacts on vegetation. However, much is still unknown about the outcomes of landscape-level control, in part because monitoring vegetation recovery requires decades. 2. We report long-term vegetation changes in permanent plots located in forest, shrubland and grassland communities across a mountain range in southern New Zealand. We test whether c. 92% reduction in the population of invasive non-indigenous red deer Cervus elaphus since 1964 has led to the recovery of deer-preferred species. 3. Tree seedlings, saplings and the number of seedlings per adult tree increased over time. There was lower recruitment, however, of highly palatable forest species compared with less palatable species, and the recruitment of saplings was lower in browsed forest plots compared with deer exclosures. 4. The total number of occurrences and absolute number of palatable species per plot increased over time in shrublands and grasslands respectively. The height of both shrublands and palatable grassland snow tussocks Chionochloa spp. increased, although the occurrences of most individual species remained unchanged over time. 5. Vegetation recovery at our site in response to long-term and significant herbivore reductions may be limited by several factors, including the slow growth rates of New Zealand species, density-dependent diet switching by deer, altered successional trajectories and below-ground processes. 6. Synthesis and applications. Our results suggest that after nearly four decades, even low densities of introduced herbivores may restrict ecosystem recovery, and therefore, restoring herbivore-disturbed ecosystems by solely manipulating herbivore population numbers may require a long-term perspective. Management strategies can accelerate recovery by protecting existing palatable plants within deer-exclosures, and planting or seeding palatable species within these refugia. However, in addition to increasing seed sources, restoration may only become apparent following large-scale disturbance events and canopy turnover.

1 Angiosperm trees often dominate forests growing in resource-rich habitats, whereas conifers are generally restricted to less productive habitats. It has been suggested that conifers may be displaced by angiosperms except where competition is less intense, because conifer seedlings are inherently slow growing, and are outpaced by faster-growing angiosperm species. Here we investigate whether competition with ferns and deeply shading trees also contributes to a failure of conifers to regenerate in resource-rich habitats. 2 We examined how changes in soil nutrient availability and drainage affected vegetation along the retrogressive stages of a soil chronosequence in southern New Zealand. Vegetation composition shifted from angiosperm-tree dominance on 'recent' alluvial terraces (< 24 ky), via coniferous-tree dominance on older marine terraces (79-121 ky), to coniferous-shrub dominance on the oldest marine terraces (291 ky). Soil drainage deteriorated along the sequence, and N : P(leaves) and N : P(soil) indicate increasing P-limitation. Conifer species appear to be adapted to persistence on infertile and poorly drained soils. 3 The floor of the relatively fertile alluvial forests was deeply shaded (approximately 1% light transmission) by dense groves of tree-ferns and ground-ferns, and by large-leaved subcanopy trees. Few seedlings of any type were found on the forest floor, even in tree-fall gaps, and establishment was restricted to rotting logs and tree-fern trunks. Angiosperms were particularly successful at colonizing these raised surfaces. 4 Less shade was cast by the conifer-dominated forests on infertile marine terraces (approximately 5% light transmission), which lacked tall ferns. There were many opportunities for conifer establishment, with high seedling densities recorded on the forest floor and on logs. By contrast, angiosperm seedlings were mainly restricted to logs. 5 Our results suggest that several mechanisms act in concert to reduce regeneration opportunities for conifers in productive habitats. In particular, we suggest that tall ferns and deep shade are responsible for a restriction of regeneration opportunities in relatively productive forests in New Zealand, diminishing the opportunities for conifers to escape the competitive effects of fast-growing angiosperms. Thus 'crocodiles' may alter the outcome of the race between 'hares' and 'tortoises'.

Tree species have been planted widely beyond their native ranges to provide or enhance ecosystem services such as timber and fibre production, erosion control, and aesthetic or amenity benefits. At the same time, non-native tree species can have strongly negative impacts on ecosystem services when they naturalize and subsequently become invasive and disrupt or transform communities and ecosystems. The dichotomy between positive and negative effects on ecosystem services has led to significant conflicts over the removal of non-native invasive tree species worldwide. These conflicts are often viewed in only a local context but we suggest that a global synthesis sheds important light on the dimensions of the phenomenon. We collated examples of conflict surrounding the control or management of tree invasions where conflict has caused delay, increased cost, or cessation of projects aimed at invasive tree removal. We found that conflicts span a diverse range of taxa, systems and countries, and that most conflicts emerge around three areas: urban and near-urban trees; trees that provide direct economic benefits; and invasive trees that are used by native species for habitat or food. We suggest that such conflict should be seen as a normal occurrence in invasive tree removal. Assessing both positive and negative effects of invasive species on multiple ecosystem services may provide a useful framework for the resolution of conflicts.

Background and aims Models of retrogressive succession have emphasised the role of phosphorus (P) depletion in driving biomass loss on surfaces of increasing geologic age, but the influence of impeded drainage on old surfaces has received much less attention. We tested whether poor drainage contributed to changes in ecosystem properties along a 291,000-year chronosequence in New Zealand (the Waitutu chronosequence). Methods Soil and ecosystem properties were measured at 24 evenly distributed points within each of eight 1.5 ha plots located on young, intermediate and old surfaces. Regression analyses tested whether drainage, in addition to P, affected ecosystem functioning. A complementary fertilization experiment tested whether P was indeed limiting on the most nutrient-depleted sites. Results Most phosphorus depletion occurred in the early stages of pedogenesis (within 24,000 years), and the older surfaces were similar in soil-P contents, whereas drainage was initially good but became increasingly impeded with surface age. In the fertilizer experiment, species showed positive responses to both nitrogen (N) and P addition on the oldest surfaces, supporting Walker and Syer's model. However, water table depth was also found to be strongly correlated with plant species composition, forest basal area, light transmission, and litter decomposition when comparisons were made across sites, emphasising that it too has strong influences on ecosystem processes. Conclusions Poor drainage influences the process of retrogressive succession along the Waitutu chronosequence. We discuss the implications of our work with regard to other chronosequences, suggesting that topography is likely to have strong influences on retrogressive processes.

1. Many grasslands worldwide are undergoing succession to woody vegetation, causing complex effects on carbon (C) sequestration, nutrient cycling and biodiversity. Land managers are frequently tasked with maximizing ecosystem services and biodiversity. Nonetheless, there are few studies quantifying trade-offs between ecosystem services and biodiversity during early woody succession. 2. We assessed the consequences of woody succession for C stocks, above- and below-ground taxa richness (plants, nematodes, mites, microbes, fungi), and soil ecosystem function at one site with a native tree, Kunzea ericoides, and one site with a non-native tree, Pinus nigra, both establishing in conservation grasslands. 3. Woody succession at both sites was associated with large gains in above-ground C stocks and, under P. nigra, losses from the mineral soil-C pool. 4. Taxa richness responses were complex, nonlinear and incongruent. While some taxa showed initial increases in richness with woody succession (e.g. plants), other taxa had rapid declines (e.g. plant-feeding and plant-associated nematodes, oribatid mites). 5. Below-ground ecosystem functioning shifted towards increased bacterial energy channels with woody succession, despite no change in bacterial or fungal biomass or fungal hyphal lengths. Most other soil measures were consistent with literature expectations (increased C:N ratios, release of recalcitrant phosphorus). 6. Synthesis and applications. Our gradient-based measurements of woody succession effects on ecosystems did not follow expectations based on comparing end-points of grasslands to homogeneous mature forest. The discordance of biodiversity responses across taxonomic groups suggests that managers cannot rely on the indicator-species concept to ensure conservation of cryptic biodiversity. Carbon sequestration and biodiversity followed non-congruent patterns, with significant losses of taxa richness from some functional groups during woody succession. Management to maximize individual ecosystem services such as carbon sequestration may therefore result in significant negative effects on biodiversity of some, but not all, taxa.

Background: The tree stem density which optimises merchantable timber yield (volume per unit area) is unknown for most of New Zealand’s indigenous tree species. While moderate thinning of even-aged stands can promote yield, intense thinning may decrease yield by creating space that cannot be filled by residual trees, increasing tree mortality or reducing tree height. We quantified the effects of density on silver beech (Lophozonia menziesii (Hook.f.) Heenan & Smissen) tree growth, height and mortality, identified the density leading to optimal merchantable yield and assessed if this density varied with stand age. Methods: Tree stem diameter growth, height, and mortality responses to density were determined using tagged individuals monitored over time on a long-term thinning trial combined with flexible, multilevel, non-linear models. Empirical stand yield responses to density were determined and compared to yield–density relationships in simulated stands. The stand simulations projected beyond the monitored stand ages using the tree-level responses fitted to empirical data. Results: Low densities (≤400 stems ha-1) sustained fast tree growth for longer than high densities (≥700 stems ha-1) after thinning, but density did not consistently affect merchantable tree heights. The probability of tree mortality increased after intense thinning, but only temporarily, and never exceeding c. 0.01 year−1. A regression of yield–density relationships identified an empirical optimum of c. 570 stems ha–1 for stand ages of 48 and 58 years. At this density, merchantable yield at 58 years was seven-fold greater than that in unthinned stands. The simulations suggested moderately higher densities for optimal yield than our empirical optimum, a moderate increase in optimal densities with stand age, and that c. 90 % of potential cumulative yield was attained at 80 years. Conclusions: Because thinning increased tree growth, but had minimal effect on tree mortality, our results alleviate concerns about the stability and productivity of thinned stands. Densities that optimise yield are about two-fold greater than those previously recommended for silver beech and they remain relatively stable as stands age. This suggests that a single density will be adequate for a range of harvest ages, although harvest should take place before a stand age of 80 years. Such conclusions are relevant to managing regeneration within coupes harvested under existing legislation and to areas planted with silver beech.

Seedling regeneration on forest floors is often impaired by competition with established plants. In some lowland temperate rain forests, tree fern trunks provide safe sites on which tree species establish, and grow large enough to take root in the ground and persist. Here we explore the competitive and facilitative effects of two tree fern species, Cyathea smithii and Dicksonia squarrosa, on the epiphytic regeneration of tree species in nutrient-rich alluvial forests in New Zealand. The difficulties that seedlings have in establishing on vertical tree fern trunks were indicated by the following observations. First, seedling abundance was greatest on the oldest sections of tree fern trunks, near the base, suggesting that trunks gradually recruited more and more seedlings over time, but many sections of trunk were devoid of seedlings, indicating the difficulty of establishment on a vertical surface. Second, most seedlings were from small-seeded species, presumably because smaller seeds can easily lodge on tree fern trunks. Deer browsing damage was observed on 73% of epiphytic seedlings growing within 2 m of the ground, whereas few seedlings above that height were browsed. This suggests that tree ferns provide refugia from introduced deer, and may slow the decline in population size of deer-preferred species. We reasoned that tree ferns would compete with epiphytic seedlings for light, because below the tree fern canopy photosynthetically active radiation (PAR) was about 1% of above-canopy PAR. Frond removal almost tripled %PAR on the forest floor, leading to a significant increase in the height growth rate (HGR) of seedlings planted on the forest floor, but having no effects on the HGRs of epiphytic seedlings. Our study shows evidence of direct facilitative interactions by tree ferns during seedling establishment in plant communities associated with nutrient-rich soils.

Questions: How do forest types differ in their distinctiveness among islands in relation to environmental and anthropogenic disturbance gradients? Are biogeographic factors also involved? Location: Tonga, ca. 170 oceanic islands totalling 700 km2 spread across 8° of latitude in Western Polynesia. Method: Relative basal area was analysed for 134 species of woody plants in 187 plots. We used clustering, indirect gradient analysis, and indicator species analysis to identify continuous and discontinuous variation in species composition across geographical, environmental and disturbance gradients. Partial DCA related environmental to compositional gradients for each major forest type after accounting for locality. CCA and partial CCA partitioned observed compositional variation into components explained by environment/disturbance, locality and covariation between them. Results: Differences among forest types are related to environment and degree of anthropogenic disturbance. After accounting for inter-island differences, compositional variation (1) in coastal forest types is related to substrate, steepness and proximity to coast; (2) in early-successional, lowland rain forest to proximity to the coast, steepness and cultivation disturbance; (3) in late-successional, lowland forest types to elevation. For coastal/littoral forests, most of the compositional variation (71%) is explained by disturbance and environmental variables that do not covary with island while for both early and late-successional forests there is a higher degree of compositional variation reflecting covariation between disturbance/environment and island. Conclusions: There are regional similarities, across islands, among littoral/coastal forest types dominated by widespread seawater-dispersed species. The early-successional species that dominate secondary forests are distributed broadly across islands and environmental gradients, consistent with the gradient-in-time model of succession. Among-island differences in early-successional forest may reflect differences in land-use practices rather than environmental differences or biogeographical history. In late-successional forests, variation in composition among islands can be partly explained by differences among islands and hypothesized tight links between species and environment. Disentangling the effects of anthropogenic disturbance history versus biogeographic history on late-successional forest in this region awaits further study. Abbreviations: GA = Group averaging; MRPP = Multi-response Permutation Procedure; NMS = Non-metric Multidimensional Scaling; pCCA = Partial CCA. Nomenclature: Smith (1979, 1981, 1985, 1988, 1991); for species not treated by Smith: Yuncker (1959), Whistler (1991), Wagner et al. (1999).

Forest restoration is an activity that can be readily undertaken to address both the climate and biodiversity crises. In Aotearoa New Zealand, aspirations for large-scale native forest restoration are growing across governmental and private sectors and a considerable focus to date has been on forest establishment by actively planting native trees. In contrast to actively planting trees, considerable proportions of Aotearoa New Zealand have a demonstrated potential for passive tree establishment through natural regeneration processes, subsequent to land use change away from pastoralism or exotic forestry. At a policy and land manager level, knowledge is lacking over the main considerations that should determine whether native restoration will most efficiently be achieved by active tree planting or by natural regeneration. Whether restoration follows active or passive establishment methods (or an intermediate point along the active-to-passive continuum), adequate forest management is essential to achieve high levels of native forest health, functionality, and permanence. We describe a step approach for assessing at a site scale whether forest restoration can most efficiently be achieved via active or passive methods, or combinations of the two. Our assessment covers the main biotic and abiotic factors which explain the probability of native tree establishment. These factors are mean annual rainfall, mean annual air temperature, proximity and composition of adjacent seed sources, landform type, slope aspect, slope, topographic exposure, and the presence of existing woody cover. We then describe the main management interventions that will be required to support successful natural regeneration outcomes and highlight the importance of strategic natural regeneration for achieving large scale restoration for the betterment of both our climate and biodiversity.