Alleviation of summer drought boosts establishment success of Pinus sylvestris in a Mediterranean mountain: an experimental approach

Published source details Castro J., Zamora R., Hodar J.A. & Gomez J.M. (2005) Alleviation of summer drought boosts establishment success of Pinus sylvestris in a Mediterranean mountain: an experimental approach. Plant Ecology, 181, 191-202.
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Summary

In this study, an assessment was made of the effect of the alleviation of summer drought on Scots pine Pinus sylvestris seedling establishment at the southern limit of the species' range on a mountain in southeast Spain. Watering was undertaken to simulate a wet summer (a common occurrence in its main central and northern European range), a rare event in the Mediterranean area at the extremities of its range. Seedling growth and survival in several different microhabitats were assessed.

Study area: The study was carried out during 1997 and 1998 on Trevenque mountain (37º05'N, 3º28'W; 1,700 m a.s.l.) in the National Park of Sierra Nevada, southeast Spain. Here, P.sylvestris reaches the southern limit of its range. Tree canopy cover is typically around 30%, with an understory of several shrub species and patches of bare soil.

Experimental design: In a 3 ha area, three microhabitats that covered most of the understory were considered:

i) open areas of bare ground large enough to avoid shading by surrounding vegetation;

ii) under the canopy of deciduous, spiny shrubs (Berberis hispanica and Prunus ramburii).

iii) under the canopy of adult P.sylvestris trees;

In each microhabitat, 20 sampling stations, each with two paired sampling plots c.75 cm apart, were established. On 19 March 1997, plots were sown with 25 seeds in a 20 x 20 cm area (seeds 4 cm apart, planted 1 cm deep). One of the plots of each pair was randomly assigned as a control and the other was watered, a total of 3,000 sown seeds (3 microhabitats; 2 irrigation levels; 20 stations). The plots were protected with a wire mesh cage.

In 1997, watered plots were sprinkler irrigated 12 times (2 l of water on each occasion, equivalent to 32 mm) at c.10-day intervals from the onset of seedling emergence (12 May) to the end of summer drought (15 September, when the first major rain fell). The amount of water added simulated that which might be expected during summer storms that occur in some years. Watered plots in open areas exhibited an increase in herb cover, and were thus regularly weeded to levels comparable to control plots (nearly bare ground).

Seedling monitoring: Seedling emergence, survival, growth, and cause of mortality (where possible) were recorded for 2 years. After sowing, plots were visited weekly until first seedlings were detected, and thereafter at around 10-day intervals in the summer. In the second year, survival was recorded at the beginning and at the end of the growing season. At the end of the first growing season (October 1997), the length of the shoot and the number of leaves on surviving seedlings were noted; a subsample of 21 randomly selected plots were dug up and the root and shoot dry mass of the seedlings weighed. At the end of the second growing season (October 1998), all remaining seedlings were harvested and shoot dry mass weighed (roots were not measured as they could not be removed intact from the soil).

Summer drought was the main mortality factor of P.sylvestris seedlings in all the microhabitats investigated. Watering increased emergence and doubled seedling survival compared to non-watered controls. There was an overall recruitment of 22.4% in watered plots but only 7.9% in control. Watering increased growth in bare soil and under shrubs, but had little effect on seedlings under established pines. The positive effects of watering in the first year were also apparent on growth parameters in the second year, even though no watering took place.

In this forest (and probably in other forests in Mediterranean areas at the limit of the southern distribution) summer drought appears to limit P.sylvestris establishment, by reducing seedling recruitment and growth. These relict populations under current regional increase in dryness and rainfall variability associated with global warming may thus be expected to gradually retreat as recruitment declines.

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