Recovering from large disturbances, such as deforestation or forest fires, is a lengthy process. A clearing goes through multiple steps, with different plants starting to regrow in a process called succession. Smaller, weedier plants typically grow first, priming the area for larger, more long-lived ones (like shrubs and trees). This type of multi-step succession occurs in Mediterranean montane forest environments as well, with “nurse” shrubs providing protected conditions for larger trees.
These nurse shrubs prepare the area in two ways: by creating a shaded environment and by depositing higher concentrations of potassium into the soil. The shaded environment promotes tree seedling growth through the canopy effect: a group of effects including lower air temperatures, lower soil temperatures, and higher water levels. All of these effects together help create the perfect environment for seedlings.
Second, nurse shrubs increase soil potassium concentrations by trapping windblown particles in their branches and by generating organic litter such as broken twigs and fallen leaves. During the dry season, when water is scarce, higher levels of potassium help protect the new tree saplings from drought by reducing transpiration rates (the plant equivalent of sweating). This accumulation of potassium reduces transpiration losses by affecting the osmotic gradient (the comparable concentrations of two solutions separated by a semipermeable membrane). By increasing the potassium levels within the plant, the gradient is increased, causing water to diffuse into the plant as it tries to maintain equal concentrations on each side of the membrane. In essence, by increasing its own potassium concentration, a plant can hold on to water better. In such a way, these nurse shrubs play a key role in ecosystem succession.
This summary was contributed by Thomas McAuley-Biasi.Edit Summary
“Is the facilitative effect of nurse shrubs on early recruitment of trees mediated by a ‘canopy effect’ (micro-climate amelioration and protection from herbivores), a ‘soil effect’ (modification of soil properties), or both?…Both effects benefited seedling performance. However, microclimatic amelioration due to canopy shading had the strongest effect, which was particularly pronounced in the drier site. Below-ground, shrubs did not modify soil physical characteristics, organic matter, total N and P, or water content, but significantly increased available K, which has been shown to improve seedling water-use efficiency under drought conditions. Conclusions: We propose that in Mediterranean montane ecosystems, characterised by a severe summer drought, pioneer shrubs represent a major safe site for tree early recruitment during secondary succession, improving seedling survival during summer by the modification of both the above- and below-ground environment.” (Gómez-Aparicio et al. 2005:191)
“The canopy effect due to microclimate amelioration was the main mechanism enhancing seedling survival and growth (see also Valiente-Banuet & Ezcurra 1991; Valiente-Banuet et al. 1991; Greenlee & Callaway 1996; and Maestre et al. 2001). Lower air and soil tempera- tures, radiation and vapor pressure deficit measured below shrub canopies in comparison to open areas re- duced leaf temperature and transpiration losses, conse- quently inducing a more favourable water balance in tree seedlings.” (Gomez-Aparicio et al. 2005:197)
“Shade appears to be a necessary condition for seedling establishment of several Mediterranean woody species (Gómez 2004; Castro et al. 2004a; Gómez-Aparicio et al. 2005).” (Gomez-Aparicio et al. 2005:192)
“However, nurse shrubs effectively modified one soil property, available K [potassium], being in all cases higher under shrubs than in open interspaces. Potassium seems to increase plant resistance to drought, mainly due to os- motic adjustments and a reduction in transpiration rates, which in turn involves a higher water-use efficiency (Bradbury & Malcom 1977; van den Driessche 1991; Egilla et al. 2001).” (Gomez-Aparicio et al. 2005:196)