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Providing evidence to improve practice

Action: Use fertilizer Forest Conservation

Key messages

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Supporting evidence from individual studies


A controlled study in 1991-1997 in temperate coniferous forest in Louisiana, USA (1) found that fertilizing increased herbaceous plant biomass but did not affect longleaf pine Pinus palustris growth. Annual herbaceous productivity was higher in fertilized (dry biomass: 472-1795 kg/ha) than in unfertilized plots (452-1088). Average diameter at breast height (30 cm), total height (22 m) and basal area (23-24 m2/ha) of longleaf pine were similar between treatments. Data were collected in four replicates of 0.64 ha treatment plots: fertilized (50 kg/ha N and 56 kg/ha P applied in April 1991 and May 1997) and unfertilized. Longleaf pine were sampled in February 1996 in four 0.09 ha plots within each treatment. Herbeceous weight was sampled in July 1997 in 12 quadrats (0.02 m2) within each treatment.



A controlled study in temperate montane forest in 1995-1998 in Switzerland (2) found no effect of fertilizing on the growth rate of Norway spruce Picea abies. Annual increase in height (unfertilized: 11-12 mm; nitrogen addition: 15-16 mm) and diameter (unfertilized: 5 mm; nitrogen addition: 3 mm) was similar between treatments. Monitoring was in 1996-1998 on four trees inside a 1,500 m2 plot with nitrogen fertilizer added (30 kg N/ha/year starting in 1995) and on five trees in the unfertilized surroundings.



A replicated, randomized, controlled study in 2001-2002 in subtropical Araucaria forest in Brazil (3) found no effect of fertilizing on species richness and abundance of new tree seedlings. Species richness (fertilized: 0.2-1.9; unfertilized: 0.4-2.0/m2) and abundance (fertilized: 0.2-2.7; unfertilized: 0.4-2.5/m2) were similar between treatments. Data were collected in 2002 in two fertilized (nitrogen: 40 kg/ha; phosphorus: 130 kg/ha; potassium: 30 kg/ha) and two unfertilized plots (3 × 3 m) in each of ten blocks randomly located inside a 2 ha area.



A replicated, randomized, controlled study in 1999-2003 in Piedmont forest in North Carolina, USA (4) found that applying fertilizer increased the height and diameter of young trees. At one site trees in fertilized plots were taller (fertilized: 133-137 cm; unfertilized: 103-119 cm) and had greater diameters (fertilized: 13-16 mm; unfertilized: 10-12 mm) than unfertilized plots three years after clearcutting. At the second site there was no difference in tree height (fertilized: 63-71 cm; unfertilized: 63-77 cm) or diameter (fertilized: 9 mm; unfertilized: 9-10 mm) in fertilized and unfertilized plots three years after clearcutting.  However, after five years, height (fertilized: 205-212 cm; unfertilized: 154-155 cm) and diameter (fertilized: 21-23 mm; unfertilized: 18-19 mm) were higher in fertilized plots. Data were collected in 2000-2003 in 16 fertilized (phosphorus and potassium at 100 kg/ha each in 1999 and 2001) and 16 unfertilized plots (10 m2) at each of two sites. The first site was clearcut in 1998-1999, the second in 1996-1997.



A replicated, randomized, controlled study in 2003-2005 in temperate coniferous forest in Washington State USA (5) found that fertilization increased the cover of plant species that were seeded artificially but did not affect total plant cover. Seeded species cover was higher in fertilized (12%) than in unfertilized plots (8%). Total plant cover was approximately 55% under both treatments. In 2002-2003, fertilized (ammonium nitrate and ammonium sulphate) and unfertilized treatments were randomly assigned to 8-16 plots (6 × 8 m) established at each of four sites. Each site had first been covered with a mixture of perennial grass and herbaceous seeds. The area had been burnt by wildfire in summer 2002. Plants cover was measured in summer 2005.



A replicated, randomized, controlled study in 2000-2002 in boreal forest in Finland (6) found that fertilizing increased the relative weight of grasses (out of total weight of all plant) but not the total weight. Relative weight of grass was higher in plots treated with 40 and 80 kg/ha of nitrogen (N) than unfertilized plots (unfertilized: 5%; 20 kg N/ha: 7%; 40 kg N/ha: 15%; 80 kg N/ha: 16%). Relative weight of evergreen shrubs (25-40%), deciduous shrubs (53-67%) and herbaceous species (1-7%) and total above ground weight of all plants (12-190 g/m2) were similar in all treatments. Data were collected in 2002 in eight replicates of four treatments (3 × 3 m):  unfertilized, 20, 40 and 80 kg N/ha in a year in 1998-2002, in each of two sites.



A replicated, randomized, controlled study in 2005-2004 in temperate coniferous forest in Washington State, USA (7) found that fertilizing after wildfire increased plant cover. Plant cover was higher in plots with low (34-40%) and high (38-45%) input of fertilizer than in unfertilized plots (28-31%). Data were collected in 2005-2006 in 24 plots (4 × 10 m) of each treatment: unfertilized, low fertilizer input and high fertilizer input (0, 56, 112 kg N/ha respectively) in each of eight sites. All sites were burned by wildfire in 2004 and were seeded with different seed mixtures.



A replicated, controlled, randomized study in 1995–2007 in a limestone quarry in Western Australia (8) found that applying fertiliser over the ground, along with a range of other soil enhancers, did not increase the number of naturally regenerated tree seedlings. After 12 years, neither fertiliser nor the three soil enhancers increased the number of seedlings in the two experiments (no data provided). Experiment one consisted of four blocks, containing six plots (6 × 10 m). Experiment two consisted of four blocks with four plots (5 × 6 m). Half of the plots in both experiments received fertiliser once (superphosphate: 400 kg/ha and potassium chloride: 100 kg/ha).  The plots treated with soil enhancers received all but one of the following treatments: fertiliser tablets, added topsoil, sewage sludge and micronutrients (see paper for details). At the end of the experiments, the number and species of naturally recruited seedlings were recorded for each plot.


Referenced papers

Please cite as:

Agra H., Schowanek S., Carmel Y., Smith R.K. & Ne’eman G. (2019) Forest Conservation. Pages 331-347 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.