Short-term effects and long-term after-effects of fertilizer application on the flowering population of green-winged orchid Orchis morio

Published source details Silvertown J., Wells D.A., Gillman M., Dodd M.E., Robertson H. & Lakhani K.H. (1994) Short-term effects and long-term after-effects of fertilizer application on the flowering population of green-winged orchid Orchis morio. Biological Conservation, 69, 191-197.
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Summary

Green-winged orchid Orchis morio is one of many plants that have dramatically declined in England due to loss of habitat through agricultural improvement of hay meadows. In this present study, various fertilizer treatments were applied over a 6-year to plots in an old hay meadow, Bratoft Meadows, (National Grid ref. TF 484640) in Lincolnshire, eastern England, to determine any effects on the number of O.morio flowering spikes.

Bratoft Meadows (2.2. ha) is cut for hay at the end of July and aftermath cattle-grazed in autumn. No inorganic fertilizers (or herbicides) had previously been applied.

In June 1970, a block of 64 (1.83 x 9.14 m) plots was established. From 1970 to 1976 they were cut for hay as usual, but fenced to exclude cattle and a second cut made at the end of October. In 1970 and 1971, the inorganic nutrient contents of hay samples were analysed. The average amount of nutrients removed in the hay in 1970 and 1971 was: 44 kg/ha N, 4 kg/ha P, 65 kg/ha K and 6 kg/ha Mg.

In 1972, seven fertilizer treatments and a control (no fertilizer) were randomly allocated. Application rates were chosen to return the inorganic nutrients removed to different degrees (N from 0-200% return; P, 0-1,000%; Mg, 0-1,000%; and K, 0-123%) in various combinations. All treatments were applied each spring (1972-1977), except Treatment 5 (1,000% P addition) applied once in 1972 and 1975.

An application of well-rotted manure was made in 1981, excluding the experimental area. Orchid spikes per plot (and across the meadow) were counted annually between 15 May and 1 June 1970-1992.

The average number of spikes per plot (1972-1992, adjusted for initial spike numbers/plot before fertilizer addition) varied from about 28 in the control; 18-24 in fertilizer treatments, excluding treatment 5; and treatment 5 (equivalent to 40 kg/ha P), five spikes (effect of P was greatly out proportion to effect on hay yield, suggesting possible toxicity to the orchid).

Fertilizer treatments, overall, significantly decreased flowering spike numbers. Except for treatment 5, decreases were closely correlated with hay yield increases following fertilization, suggesting that competition between O.morio and other vegetation was responsible.

In retrospect the authors conclude that fertilizer effects would have been easier to detect if all O.morio plants had been counted (not just flowering individuals). Orchid density varied greatly across plots in 1970 and 1971 (before treatment) and in the two years after treatment ceased, suggesting that initial orchid numbers per plot might mask treatment effects. Even so, detrimental effects of levels of fertilizer applied (generally low by agricultural standards) were apparent, thus fertilizer application should be avoided where O.morio conservation is an objective.

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