Individual study: Early establishment of planted Spanish broom Retama sphaerocarpa seedlings under different levels of light, water and weed competition on abandoned cropland at the El Encín experimental estate, Madrid, Spain
Benayas J.M.R., Lopez-Pintor A., Garcia C., de la Camara N., Strasser R. & Sal A.G. (2002) Early establishment of planted Retama sphaerocarpa seedlings under different levels of light, water and weed competition. Plant Ecology, 159, 201-20
To formulate guidelines for reclamation of abandoned cropland in central Spain, the effects of shading, irrigation and removal of weed competition on the performance of seedlings of the leguminous shrub, Spanish broom Retama sphaerocarpa, was assessed. R.sphaerocarpa was chosen as the study species as is of interest for revegetation due to its drought tolerance and as it is a major component of the native shrubland plant community in the region.
Study site: The study was undertaken on a 1 ha former cropland plot previously cultivated for many years in the 'El EncÃn' experimental estate, AlcalÃ¡ de Henares, central Spain (40º35'N, 3º25'W). In the experimental year (1998) the late spring and summer were drier than usual.
Experimental design: Four replicate blocks each containing four main(7.5 × 7.5m) plot treatments were established. The main plot treatments were combinations of artificial shading (shaded vs. full-light) and summer irrigation (irrigated vs. non-irrigated). Each main plot was split into two sub-plot treatments, clipping vs. no clipping of weeds.
In the winter of 1998, 12, 1-year-old R.sphaerocarpa seedlings were planted (at least 1.5m apart) in each of the 32 sub-plots. All seedlings which died within the first 4 weeks were replaced. Although the plot was fenced to exclude medium to large vertebrate herbivores, severe rabbit Oryctolagus cuniculus damage to seedlings was evident at the end of summer.
Treatments: The following treatments were applied:
i) Artificial shading - black polyethylene net placed 1.8 m above the ground, reducing incident radiation by 68%;
ii) Irrigation (simulating likely irrigation practice) - watered three times around the seedlings: 15 litres/mÂ² in mid June; 30 l/mÂ² at the end of June; 30 l/mÂ² in late July (peak of the dry season);
iii) Clipping weeds around broom seedlings - weeds were clipped three times (April, May and June) to reduce competition within a 0.5 ×0.5 m area centred on each seedling.
Monitoring: The following variables were recorded:
i) Seedling survival - assessed three times: early summer (beginning of July), October, and 1 year after planting.
ii) Growth - estimated in living seedlings by measuring: seedling height, stem diameter (2 cm above ground level) and seedling volume, measured immediately after planting (before treatments applied) and at the end of the growing season in October.
iii) Effects on photochemical efficiency - the average of two fluorescence measures (using a Plant Efficiency Analyser - see original paper for details) per live seedling were taken in early summer.
iv) Soil moisture - measured at 0-20cm depth in early spring (May), late spring (June) and mid summer (end of July).
v) Above ground weed biomass - weeds clipped, dried and weighed in one third of 0.5 × 0.5 m quadrats.
Artificial shading was the most effective treatment, followed by removal of weeds. After summer, 34% of the seedlings survived in the most favourable plots (shaded with weeds removed), compared to 1% in full-light plots with no weed control. Irrigation enhanced seedling cover and height in shaded plots. Analysis of photochemical efficiency indicated that competition between seedlings and weeds was primarily for water rather than for light.
For broom seedling growth, the only significant effect found was that irrigation increased volume under shaded conditions (−0.48 in irrigated plots vs. − 0.57 in non-irrigated plots). Only one significant interaction effect was apparent: weed removal and irrigation increased broom stem diameter growth in shaded plots; these growth parameters were infact slightly negative, i.e. seedling volume and stem diameter shrank rather than expanded, meaning that seedling performance was extremely poor even under the most favourable treatments.
A marginally negative effect of irrigation on seedling performance was that fewer seedlings survived in irrigated plots at mid-summer, this was due to increased weed competition.
Soil moisture increased very slightly where weeds were removed. Total weed biomass production also increased in shaded plots (64 g/m² in full light plots, 84 g/m² in shaded plots).
Conclusions: Whilst artificial shading improved seedling performance, this technique might be inhibitive because of its high cost. Weeds clearly competed with the broom seedlings and weed control by clipping is considered feasible and would improve seedling survival. The authors consider that seedling performance might be considerably improved with higher irrigation than was used (75 l/m² per growing season), combined with more intensive weed control. The importance of restricting grazing is also highlighted by the severe rabbit damage to seedlings.
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