Environmental Engineering Reference
In-Depth Information
(a) Target species
(b) Nurse shrubs
(c) Altitude
3.0
3.0
3.0
2.5
2.5
2.0
2.0
2.0
1.0
1.5
1.5
0
1.0
1.0
-1.0
0.5
0.5
-2.0
0
0
- 0.5
- 0.5
-3.0
Evergreen Deciduous
Pines
Shrubs
Legumes
Small
shrubs
Spiny
Rockroses
Low
High
(d) Aspect
(e) Year
2.0
3.0
2.5
1.6
2.0
1.2
1.5
0.8
1.0
0.4
0.5
0
0
- 0.4
- 0.5
Sunny
Shady
1997
2000
2001
Fig. 8.7
The mean effect size (
d
95% confi dence limits) of the presence of pioneer 'nurse' shrubs for survival of seedlings
of target mid- and late-successional Mediterranean forest species. Effect size is the difference in survival probability after 1
year in the presence and absence of nurse shrubs divided by the mean survival probability in their absence. Thus positive
values indicate
facilitation
and larger positive values indicate a stronger facilitative effect. A negative value indicates the
opposite of facilitation - survival is reduced in the presence of the (in this case inappropriately named) nurse plant. (After
Gómez-Aparicio et al., 2004.)
±
most other shrubs, no doubt provide another benefi t by reducing the effects on the
target seedlings of the scorching Mediterranean summer sun. Rockroses (
Cistus
spp.) were unique in providing no 'nursing' benefi t, and this refl ects their production
and release of 'allelopathic' chemicals into the soil, substances that have evolved to
provide a competitive benefi t by reducing success of neighboring plants.
The size of benefi t provided by nurse shrubs also depended on local abiotic
conditions. The advantage was highest at low altitudes and on sunny slopes,
where lower rainfall and higher temperatures result in intense summer droughts
that, in the absence of nurse plants, strongly limit seedling survival and growth
(Figure 8.7c, d). Finally, it is clear that the facilitation effect of nurse shrubs was
least noticeable in 1997 - a year that was uncharacteristically wet and thus provided
conditions amenable to seedling survival even in the absence of nurse plants.
When pioneer species are facilitators of successional change the management
prescription is to leave them in place, the converse of the case described in Section
8.2.3 where early species slowed succession to climax vegetation.
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