Environmental Engineering Reference
In-Depth Information
Table 3.1 Ecological
traits of forbs (nongrass
herbaceous plants) that
showed a signifi cant
relationship with
successful performance
of the plants in years
1-4 after sowing in
grassland restoration
experiments. (After
Pywell et al., 2003.)
Trait
N
Year 1
Year 2
Year 3
Year 4
Ruderality (colonization ability)
39
+
*
Autumn germination
42
+
*
% germination
43
+
**
+
*
+
*
Seedling growth rate
21
+
*
+
**
+
*
Competitive ability
39
+
*
+
**
+
***
+
***
Vegetative growth
36
+
**
+
*
+
*
+
*
Seed-bank longevity
44
+
*
+
*
+
*
+
*
Stress tolerance
39
**
**
***
***
Generalist habitat
45
+
**
+
**
+
**
+
**
N , number of species in analysis.
+
, negative relationship. A positive sign means that the trait in question is associated with
good performance in grassland restoration.
Asterisks indicate the level of statistical signifi cance of the results for each trait in each year - more asterisks
mean a greater level of confi dence in the result: * p
, Positive relationship;
0.001, blank not signifi cant. Thus,
for example, species with good competitive ability are highly likely to be successful in restoration experiments
(particularly in years 3 and 4). Stress tolerators, on the other hand, do very poorly.
<
0.05, ** p
<
0.01, *** p
<
ity. Now the areas have been invaded by tall, impenetrable stands of the exotic
grass Saccharum spontaneum , a serious invader in places as far apart as India, the
Philippines and Puerto Rico.
As was the case with native grass seeds in Section 3.2.1, the lack of naturally
arriving tree seeds is a major barrier to unassisted regeneration in Panama. However,
it would be wasteful simply to throw seeds in an indiscriminate manner into these
areas. Hooper et al. (2002) tested the potential of seeds of 20 native trees to suc-
cessfully germinate, survive and grow within the Saccharum -dominated community
(Figure 3.1). Two kinds of species trait have particular signifi cance in determining
tree seed success in the Saccharum grassland - seed size (small:
<
0.15 g; medium:
0.15-1.0 g; large:
1.0 g) and shade tolerance (low, medium and high; determined
from separate germination experiments in a greenhouse where light levels were
varied). To demonstrate the major patterns, I present results separately according
to these different traits.
Species with the smallest seeds generally perform most poorly. Those considered
'pioneer' species, so named because they are characteristic of open and disturbed
locations (and, thus, have low shade tolerance), do a little better than those with
intermediate or high shade tolerance, but nowhere near so well as species with
medium seeds (in the intermediate shade tolerance class) or large seeds (in both
medium and high shade tolerance classes) - just compare their respective perform-
ance indexes in Figure 3.1.
You might suppose, from fi rst principles, that the r -selected, small-seeded pioneer
trees (which include Tre m a mi crantha and Jacaranda copaia ) would be ideal candi-
dates for restoration, because of their good powers of colonization and rapid growth
in disturbed situations. However, in Panama at least, they do not tolerate the
constraints imposed by the Saccharum grass.
At the other end of the scale, species with large or very large seeds (2.9 to a
massive 50.4 g each) include the moderately shade tolerant Dipteryx panamensis
and the very shade tolerant Calophyllum longifolium , Carapa guianensis and Virola
surinamensis . The latter three germinate immediately upon planting in the wet
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