130 Egbert G. Leigh, Jr
regeneration from a clearing consists primarily of
stump sprouts, whereas in nearby dipterocarp for-
est on better soil, regeneration is driven by seed
fall and seedling growth (Riswan and Kartawinata
1991).
Finally, evidence from other sources suggests
that, as predicted, leaf turnover is lower on poorer
soil. In montane forest of Jamaica, leaf turnover
(dry biomass of leaves divided by dry biomass of
annual leaf fall) is 20% lower on a mor ridge
than on a nearby, more fertile, mull ridge (Tan-
ner 1980a,b). In Venezuelan Amazonia, leaves
are longer lived on poorer soils (Reichet al. 2004,
pp. 18–19). In sum, poor soil promotes nutrient
conservation.
How conserving nutrients affects forest
characteristics
The primary impacts on a forest of conserving
nutrients arise from making longer-lived leaves.
To live long, a leaf must avoid being eaten. To
deter herbivores, leaves must be thick and tough,
with low N concentration (Coley 1983, Waterman
etal. 1988). In Venezuelan Amazonia, longer-lived
leaves are tougher, and have lower N concen-
tration (Reichet al. 1991), as this argument
predicts. The relation between soil quality and
leaf anti-herbivore defenses can become a vicious
circle: defensive compounds in long-lived leaves
slow their decomposition when they fall, further
diminishing soil quality (Bruening 1996, p. 23).
To liv elong, a l eaf must also avoid drying
out. To do so, leaves must limit transpiration,
and therefore stomatal conductancegs(rat eof
water loss per m^2 leaf area per kPa vapor pres-
sur ed eficit outsid eth el eaf, kg m−^2 s−^1 kPa−^1 )
(Givnish 1984). Because leaves must release water
vapor to let in CO 2 , trees whose leaves have
lower maximum stomatal conductancegs max
must hav elow erAmax, lower photosynthetic
capacity per unit area (Reichet al. 1991, Tyree
2003, Santiagoet al. 2004). In turn, trees with
less conductive leaves can make do with lower
hydraulic conductance in their wood. As pre-
dicted, trees with lowerAmaxhave leaf-bearing
branches with lower leaf-specific hydraulic con-
ductancekL(kg water moving through the xylem
per second per m^2 leaf in response to a unit change
in water potential, MPa, per meter of stem), in
both th eOld and N ew World tropics (Brodribb and
Feild 2000, Brodribbet al. 2002, Santiagoet al.
2004). Katulet al. (2003) derived mathematical
theory predicting such relationships amongAmax,
gs max, andkL.
To keep their long-lived leaves from drying
out, trees on poor soil also design leaves and
crowns to restrict transpiration by limiting both
leaf temperature and turbulent airflow around
leaves (Givnish 1984). In nutrient-poor forests of
heath, peat swamp, and white sand in Borneo
and Venezuelan Amazonia, canopy leaves avoid
overheating by being smaller, more reflective, and
more nearly vertical than their counterparts on
better soil (Brunig 1970, 1983, Medinaet al.
1990). On th es epoor soils, th ecanopy is far
smoother than on nearby oxisols (Brunig 1983),
reducing turbulent airflow (Bruenig 1996), and
allowing thick, transpiration-limiting boundary
layers to build up around leaves and whole tree
crowns (Meinzeret al. 1993). These character-
istics were considered adaptations to occasional
water shortage in sandy, easily drained soils
(Brunig 1983). Perhaps because of these features,
heath forests and Amazon caatinga are no more
sensitive to water shortage than adjacent forests
on oxisols (Coomes and Grubb 1998).
Similarly, if a tr e eis to liv elong, its wood
should b estrong and d ens e(Putzet al. 1983),
but trees with denser wood tend to have narrower
vessels and lower hydraulic conductance, which
limits their leaves’Amax(Santiagoet al. 2004).
In denser-wooded trees, however, lower water
potentials are needed to make leaves wilt (Gartner
and Meinzer 2005) or to cause embolism in the
xylem (Santiagoet al. 2004, Hackeet al. 2005).
Thus trees with denser wood can photosynthe-
size under drier conditions (Santiagoet al. 2004).
Furthermore, in leaves with lower maximum
stomatal conductanc eand low erAmax, stomatal
conductance is less sensitive to increasing vapor
pressure deficit (Orenetal. 1999). Therefore, aver-
ag etranspiration is much th esam ein a matur e
Bornean heath forest as in a nearby mature for-
est on better soil (Becker 1996): the heath forest’s
more sustained transpiration and photosynthe-
sis makes up for its neighbor’s episodes of high
photosynthesis.