Unpalatable plants facilitate tree sapling survival in wooded pastures

Summary

In wooded pastures, established tree saplings are frequently found in association with protective ‘structures’, including plant species unpalatable to grazing animals. In this study the possible nurse effects of unpalatable plants on native Norway spruce Picea abies sapling performance was investigated in a cattle and horse grazed wooded pasture in Switzerland. Saplings were planted with and without unpalatable plants (dwarf thistle Cirsium acaule with spiny foliage; yellow gentian Gentiana lutea with toxic foliage) and performance followed over two grazing seasons.

Study area: The study was conducted in a 12 ha extensively grazed wooded pasture in the Pré-aux-Veaux (Swiss Jura Mountains). The pasture is a species-rich grassland with scattered native Norway spruce Picea abies trees and a tree cover of about 40%. The area is typically grazed from early June until early October, with three to four grazing rotations per year, each lasting around 10 days. The average herd consists of around 58 heifers, occasionally with 5-10 horses. Naturally occurring large herbivores in the site are rarely seen but include roe deer Capreolus capreolus and mountain hare Lepus timidus.

Transplant experiment: Two unpalatable perennial herbs common in the wooded pasture were selected: Gentiana lutea (60-120 cm tall) contains toxins; and Cirsium acaule (10-30 cm in height) are spiny. Cattle generally avoid them. Naturally established P.abies saplings have been found to be associated with both species, indicating nurse effects. An equal number of plots (200, 600 in total; at least 1.5 m apart) were randomly selected with Gentiana and Cirsium or without any unpalatable plant within a 1.5 m radius around the plot. Gentiana and Cirsium plots contained more than one shoot with patch sizes measuring about 30 cm and 20 cm, respectively. Half of these plots were randomly selected for a clipping treatment (i.e. stems cut at soil level leaving surrounding vegetation intact), repeated periodically during the season. In May 2003, Picea saplings (600, 1- and 2-year-old saplings of local stock) were transplanted in the plots:

1) near either uncut Gentian or Cirsium at ±10 cm from their stem;

2) near cut Gentiana or Cirsium;

3) not amongst unpalatable plants.

Saplings that died within two weeks of transplanting (29) were replaced. At the start of the experiment (31 May) average height of the 1- and 2-year-old saplings was 4.9 cm and 10.1 cm  respectively. Average vegetation height during the grazing seasons was 5 cm so it was assumed that the 1-year-old saplings (hereafter small) would be less apparent to grazers than the 2-year-olds (hereafter large).

Measurements: Grazing intensity was measured after the first grazing rotation in 2003 (estimating percentage cover of grazed shoots in a 40 cm circle). Sapling height and survival was recorded in 2003 and 2004 (once before the first cattle arrival; after each of the three rotation periods of the first season; once before cattle arrival for the second season; and once at the end of the second grazing season).

Grazing intensity: Grazing intensity was generally lowest in plots with unpalatable plants, of middling intensity in clipped plots and highest in plots with neither unpalatable plant present. Grazing intensity was significantly higher in plots with uncut Cirsium than in plots with uncut Gentiana.

Sapling performance: The number of surviving saplings declined from 600 at the start  to 451 (74.1%) and 63 (10.6%) after 52 and 498 days, respectively. Most saplings (194) disappeared after the third and longest grazing rotation period. Survival was consistently highest in the plots with unpalatable plants. Overall, survival was significantly higher for large saplings (26%) than for small saplings (17%).

After the first grazing season survival was highest in plots with unpalatables (33%), intermediate in plots with cut unpalatables (19%) and lowest in plots without unpalatables (14%). There was significantly lower survival of small saplings in Cirsium plots compared with Gentiana plots, while survival of large saplings did not differ between species.

After two grazing seasons, again survival was higher for large saplings (14%) compared with the small (7%), and higher in plots with unclipped unpalatable plants (18%) compared with clipped unpalatables (9%) and without unpalatable plants (6%). Sapling survival was lower for small saplings in Cirsium than in Gentiana plots, but this difference was not apparent for the large saplings. Sapling survival was significantly higher in plots with uncut Gentiana than in plots with uncut Cirsium.

Sapling growth: On average, sapling height increases after 1 and 2 years was 3% and 11%, respectively, but there were no significant effects of species, treatment or their interaction.

Conclusions: Grazing intensity was highest in plots without unpalatable plants, but higher in plots with dwarf thistles Cirsium acaule (spiny foliage) present than those with yellow gentians Gentiana lutea (toxic foliage). Sapling survival was significantly higher near unpalatable plants, and significantly higher in plots with Gentiana than with Cirsium. Large saplings survived better than small ones and depended less on the unpalatable plants for survival. Except for those near Gentiana, sapling removal by cows and horses was the major cause of death. Growth in sapling height was unaffected by the treatments.This study indicates that the plants generally unpalatable to large vertebrate herbivores (Cirsium acaule and Gentiana lutea) may enhance tree regeneration in wooded pastures by acting as nurse crops for tree saplings. Transplanting tree saplings near unpalatable plants could be an alternative reforestation technique in intensively grazed wooded pastures to assist tree regeneration.

Note: The compilation and addition of this summary was funded by the Journal of Applied Ecology (BES). If using or referring to this study, please read and quote the original paper, this can be viewed at: http://blackwellpublishing.com/submit.asp?ref=0021-8901