Individual study: Fertilizer application during primary succession enhances plant establishment and invertebrate herbivore communities at Penrhyn slate quarry, Gwynedd, Wales
Rowe E.C., Healey J.R., Edwards-Jones G., Hills J., Howells M. & Jones D.L. (2006) Fertilizer application during primary succession changes the structure of plant and herbivore communities. Biological Conservation, 4, 510-522
Rock quarry waste tips are often hostile environments for plant and invertebrate colonization. Large areas of bare rock waste may remain sparsely vegetated for many decades. The main factors limiting plant establishment are frequently lack of soil and nutrients; nitrogen is usually the limiting nutrient.
In this study, fertilizer was applied to areas of nutrient-poor slate waste at a site in North Wales that had been partly colonized by woodland and heathland species. The aims were to assess if improved nutrient availability led to: i) increased plant cover, biomass and species richness; ii) increased abundance of plants typical of more fertile habitats and decreases in species of infertile habitats; iii) increased quality of leaves as invertebrate food; and iv) increased foliar invertebrate abundance and diversity.
Study site: The study was carried out at Penrhyn slate quarry (53°9′ N, 4°4′ W). The site was a mosaic of bare rock waste, heath, isolated trees and low-growing woodland, grazed by sheep at low density (approx 0.1 ewes/ha). Two natural hybrid trees (20 of each) were studied; birch (Betula pendula × B.pubescens = S. × aurata) and willow (Salix caprea × S.cinerea = S. × reichardtii); both are common colonisers of slate waste tips in the UK. At the start of the experiment, willows ranged from 179-410 cm in height and 6-35 cm² in stem basal area; birches from 62-224 cm height and 6–81 cm² basal area. A plot of radius 1.5 m was established around each tree. Twenty plots with 50–100% heather Calluna vulgaris cover were also delineated. The plots (>50 m from adjacent woodland) encompassed a range of tip ages (40–100 years since creation) and environmental characteristics.
Fertilizer application: Fertilizer was applied to 10 each of the tree and heathland plots. On 23 May 2000 and 2001, fertilizer was spread at a rate equivalent to 175 kg N/ha, 53 kg P/ha and 188 kg K/ha. Half was applied in soluble form (‘Phostrogen’, N:P:K 15:4:22 with trace elements) and half as slow-release prills (‘Osmocote’, N:P:K 15:4:8 with trace elements).
Tree growth: Trees were measured before fertilizer application, and after autumn leaf drop in 2000 and 2001. The following were recorded: annual shoot extension; tree height; stem basal diameter; and crown diameter. Stem basal area and crown area were calculated.
Ground flora: Ground flora was assessed by recording species within the central of a plot (1 m radius) in June 2000 and August 2001.
Invertebrate abundance: Foliar invertebrates of the 20 birches were sampled in May (before fertilizer application), June and July 2000 and 2001, and in 2001 from the 20 quarry willows. Ten birch and 10 willow trees in an area of woodland (40 ha) adjacent to the quarry were sampled for comparison in both years. For analyses, invertebrates were grouped into 10 taxa: Aphididae, Cicadellidae, Psyllidae, other Hemiptera, Coleoptera, Hymenoptera, Lepidoptera, Psocoptera, Arachnida, and miscellaneous. These were also grouped according to whether they feed only by sap-sucking (Aphididae, Cicadellidae, Psyllidae, other Hemiptera, Psocoptera) or by other feeding methods (the other groups).
Fertilizer addition increased tree growth (i.e. stem basal area: 129% fertilized, 54% unfertilized; relative crown area increment: 96% fertilized, 24% unfertilized; relative height growth rate: fertilized 38%, 19% unfertilized; extension of longest shoot: fertized 58cm, unfertilized 35 cm). Cover of several dominant ground flora species e.g. Teucrium scorodonia, Sedum anglicum, Deschampsia flexuosa and Agrostis tenuis increased in fertilized plots; bryophyte cover was unaffected; some less-competitive ruderals were adversely affected.
Tree foliar invertebrates were less abundant on the trees on slate waste than on trees in the adjacent woodland. However, fertilizer addition made tree leaves more palatable and the abundance of sap-sucking invertebrates was consistently higher; foliar invertebrate diversity was similar between treatments.
Overall, fertilizer addition was beneficial for plant and insect biodiversity; it increased abundance of dominant plant species and foliar invertebrate herbivores, and few taxa were adversely affected. Due regard should be taken of plants characteristic of open and infertile habitats and their obligate invertebrate herbivores that are likely to decrease in abundance in response to fertilizer addition.
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