Water and nutrient limitations to tree establishment on slate waste Edwin Rowe, Julie Williamson, Davey Jones, John Healey University of Wales, Bangor e.c.rowe@bangor.ac.uk

Introduction Objective: to restore slate waste tips to woodland Trees do establish naturally, but cover is sparse even after > 100 years

Introduction What limits the development of tree cover? grazing? competition with other plants? dispersal? water shortage? nutrient shortage? or a combination of these factors?

Aims To examine the effects on tree establishment and growth of soil amendments designed to improve water supply nutrient supply To assess soil amendment requirements of different tree species in relation to their ability to fix nitrogen seed size

Hypotheses Null 1. Without soil amendments, trees will establish on flat sites protected from sheep grazing Water-holding 2. Low water availability is the major cause of failure in plant establishment

Hypotheses (continued) Nutrient availability 3. Low nutrient availability is the major cause of failure in tree establishment Combined nutrient and water availability 4. Improving nutrient supply has a greater effect on tree establishment than improving water-holding capacity

Hypotheses (continued) Species 5.1 Nitrogen fixers grow faster than non-fixers 5.2 Nitrogen fixers can establish without fertilisers 5.3 Natural establishment of large-seeded trees is limited only by dispersal 5.4 Small-seeded trees respond more strongly to fertiliser application than large-seeded trees

Design: 3-way split-plot factorial Main plots: 3 water-holding treatments (x 3 reps) None Clay Polyacrylamide gel Split plots: 6 “tree” species Alder Birch Gorse Oak Rowan Willow (i.e. 2 N-fixers, 4 non-fixers; 3 small-seeded, 3 large) x 3 nutrient supply treatments None Sewage cake Osmocote (NPK + trace) + paper slow release fertilizer in 2 randomised complete blocks Sheltered, 275 m a.s.l. Exposed, 330 m a.s.l. Significance of effects and interactions tested by ANOVA

Method Set up April 2000 Flat areas of slate waste fenced against sheep Willows placed as cuttings (2-3 cm diameter x 20 cm length) Other species planted as nursery-raised 1 year old seedlings (NB “seed size” indicates type of tree - trees not sown in situ) Standard 3 L hole dug for all treatments 5 plants per plot, in quinqunx arrangement (50 cm spacing)

Method (continued) Trees watered (0.2 L per plant) on 2 occasions during the first 2 months after planting Trees measured winter 2000 and winter 2001: Height Stem basal diameter Crown diameter Some trees destructively sampled, dried and weighed to establish allometric relationships for biomass estimation Analyses on absolute growth data Rainfall, soil and air temperature, and PAR logged

Results: Rainfall Year total: 2725 mm

Results: Mortality in first summer

Results: Allometry for estimating biomass A = alder B = birch G = gorse O = oak R = rowan W = willow

Results: Estimated shoot biomass, end of second season - effects of fertiliser and water-holding treatment Source of variation P W < 0.05 F < 0.001 W.F ns

Results: Estimated shoot Source of variation P S < 0.001 (Fixer ?) < 0.05 (Seed size) < 0.001 F.S < 0.05 F.(Fixer ?) < 0.05 F.(Seed size) < 0.001 Results: Estimated shoot biomass, end of second season - effects of species and fertiliser treatment

Results: Estimated shoot biomass, end of second season - effects of species and water-holding treatment Source of variation P W.S < 0.001 W.(Fixer ?) < 0.001 W.(Seed size) ns

Results: Leaf element concentrations

Results: Recovery of applied Nitrogen N per plant estimated as Shoot Biomass x Leaf N Proportion

Old experiment described in Bradshaw and Chadwick (1980) after > 25 years, both N2 fixers and non-fixers have only grown to 3-4 m Alnus incana Alnus glutinosa Betula pubescens

Hypothesis testing Null 1. Without soil amendments, trees will establish on flat sites protected from sheep grazing True for N fixers Water-holding 2. Low water availability is the major cause of failure in plant establishment Some failures, but poorly tested due to wet summers Boulder clay reduced mortality, polyacrylamide gel did not

Hypotheses (continued) Nutrient availability 3. Low nutrient availability is the major cause of failure in tree establishment True (for flat, ungrazed site) Fertiliser amendments improved tree growth Combined nutrient and water availability 4. Improving nutrient supply has a greater effect on tree establishment than improving water-holding capacity True

Hypothesis testing (continued) Species 5.1 Nitrogen fixers grow faster than non-fixers True 5.2 Nitrogen fixers can establish without fertilisers True, though growth may not be sustained 5.3 Natural establishment of large-seeded trees is limited only by dispersal Not true 5.4 Small-seeded trees respond more strongly to fertiliser application than large-seeded trees True

Conclusions Polyacrylamide gel appears to be of little benefit (although its utility during sustained periods of drought remains to be tested) Nitrogen fixers establish rapidly on slate waste even without added fertiliser Sustained growth, even of nitrogen fixers, depends on an adequate and sustained supply of nutrients (particularly P)