Benefits of using shrubs as nurse plants for reforestation in Mediterranean mountains: a 4-year study

Summary

In the Mediterranean basin, harsh environmental conditions, mostly summer drought and livestock browsing, hamper establishment and survival of planted tree seedlings. Reforestations often suffer heavy losses as a consequence. In some cases a shrub nursery-crop can facilitate enhanced establishment of understory seedlings. An experiment was set up in the Sierra Nevada mountains (southeast Spain) to test the use of shrubs as nurse plants as an alternative reforestation technique for two conifers, Sierra Nevada Scot's pine Pinus sylvestris nevadensis and Salzmannii's black pine Pinus nigra salzmannii.

Study area: The planting trials were undertaken on the northwestern side of Loma de los Panaderos (1,800–1,850 m a.s.l.) in the Sierra Nevada mountains, southeast Spain. Existing plant cover was patchy with 28% Spanish sage Salvia lavandulifolia (a small evergreen shrub reaching 25 cm in height) cover, and 10% deciduous spiny shrubs, mostly Prunus ramburii, hawthorn Crataegus granatensis and Spanish berberis Berberis hispanica (reaching around 1.5 m in height) with roughly 36% bare soil. Average annual rainfall is 830 mm, rainfall in 1997, 1998, 1999 and 2000 was 1,283, 632, 755 and 1,045 mm, respectively. In these areas, native forests are dominated by the Sierra Nevada Scots pine Pinus sylvestris nevadensis and Salzmannii's black pine Pinus nigra salzmannii.

Experimental design: In March 1997, 2-year-old Scot's and black pine seedlings were planted in different microhabitats (treatments):

1) Open, seedlings planted in areas of bare soil (the reforestation method usually employed in the region).

2) Salvia (seedlings planted under the canopy of individuals of S.lavandulifolia).

3) Shrub N (seedlings planted just below the northern edge of the canopy of spiny shrubs (P. ramburii, C.granatensis or B. hispanica).

4) Shrub S (seedlings planted just below the southern edge of the canopy of spiny shrubs).

Planting points were within three 6,000 m² plots, about 200 m apart. Half of each plot was protected from ungulates (Spanish ibex Capra pyrenaica) by a 1.5 m high fence. Fenced areas were used to analyze the effect of herbivores and their interaction with the microhabitat on reforestation success. Seedlings of each pine species were planted inside and the same number outside the fence (a total of 1,200 experimental seedlings per pine species: 50 sampling points × 3 plots × 2 herbivory levels × 4 microhabitats), sampling points being randomly assigned. At the end of June 1997, before the onset of summer drought, pines were examined, excluding those that had died due to transplant shock (<5%). The resulting number of monitored seedlings was 1,115 for Scots pine and 1,146 for black pine. and monitored until the fourth growing season.

Pine measurements: For each pine seedling the following was recorded: 1) survival (sampled in October and May from 1997 to October 2000); 2) cause of mortality; 3) growth (estimate of length increase of the leader shoot after each growing season; 4) risk of herbivory (only for seedlings outside the fences), percentage of pines that suffered at least one event of ungulate damage over the study period; 5) damage intensity (for seedlings suffering herbivory); and (6) type of damage, distinguishing herbivory in the leader shoot from lateral shoots.

Grazing damage: Herbivore damage was low, 8.1% of Scot's and 12.7% of black pine suffered grazing damage but with no differences among microhabitats. Damage intensity was 16.8 (± 4.2) for Scots pine (i.e. those suffering herbivory lost 16.8% of their shoots) and 21.4 (± 4.0) for black pine, with no significant differences among microhabitats for either species. When a pine was damaged, the leader shoot was consumed in 89.7% of cases, with the rest of the damage to lateral shoots. Overall, grazing did not affect seedling survival. Therefore, data from the fenced and unfenced area of each plot were pooled for the analysis of survival.

Pine mortality: Overall mortality was 83% for Scot's pine and 57% for black pine after four growing seasons (all microhabitats pooled). Drought was the main cause of mortality (86% of dead seedlings), and second frost heave (14%). Only 19 seedlings died because of other causes (vole tunnels, trampling and ungulate herbivory). The highest mortality occurred during the first summer (Scot's pine 47%; black pine 29%). The microhabitat had a significant effect on the pine survival.

After four growing seasons, pine survival was much higher when planted under individuals of S.lavandulifolia as compared with open areas (2.6 times for Scot's pine and 1.8 for black pine). Those under Salvia showed the highest survival rates (Scot's pine 28%; black pine 64%; all plots pooled). The planted on the north side of spiny shrubs showed intermediate values (Scot's pine 20%; black pine 40%), whereas mortality on the south side (Scot's pine 9%; black pine 30%) was similar to that in open areas (Scot's pine 11%; black pine 36%) had lowest survival.

Growth: The growth of the leader shoot differed among years. For Scot's pine the highest growth occurred in 1999, whereas for black pine there was a steady decrease in leader shoot growth through the years. There were no differences in growth among microhabitats for each of the pine species in any year. The cumulative growth after four growing seasons (1997–2000) was 18.8 (± 0.6) cm for Scot's pine and 26.6 (± 0.4) cm for black pine.

Conclusions: Pine seedling growth was not inhibited when planted with shrubs as compared with open areas. In this pine-planting trial, the use of shrubs as nurse plants to enhance seedling survival in this reforestation trial appeared a viable and low coast technique to increase establishment success in this Mediterranean ecosystem. The reduction of solar radiation by the shrub canopy was considered probably the main factor in facilitating seedling survival, a shrub layer may also have helped reduce frost damage.


Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a www.conservationevidence.com case as this is for previously unpublished work only. The original paper can be viewed at: http://www.blackwell-synergy.com/journal.asp?ref=1061-2971