Species groups based on leaf characteristics

The 61 species studied at Los Tuxtlas (Popma et al. 1992), were divided into three ecological groups based on their apparent light requirement for regeneration. The groups were 'obligate gap' species that were only ever found in gaps, 'gap-dependent' species that can survive in canopy shade as juveniles but need gaps to grow to large size, and 'gap-independent' species that can complete their life cycle in the shade (Popma et al. 1992). The obligate gap species had larger leaves than the other two groups (Table 2.9). The gap-dependent species had the highest LMA and more N and P per unit area than the other two groups. They also showed the greatest plasticity in leaf form between sun and shade leaves. The gap-independent species had more elongated leaves with fewer, but larger, stomata per unit area.

Analysis of published data on leaf photosynthetic rates (Fig. 2.29) for different tree species shows a pattern of increased carbon dioxide uptake rates

Shade-intolerants

Light-demanders

Shade-tolerants

Figure 2.29 Box-and-whisker diagram of maximum photosynthetic rates (Amax) of leaves of tropical tree species divided into three ecological classes. Extreme outliers shown as asterisks. Species as follows: Shade-intolerants: Adinandra dumosa12, Annona spraguei1, Antirrhoea trichantha2, Bellucia grossularioides3, Castilla elastica2, Cecropiaficifolia3, Cecropia longipes2A, Ceibapentandra11, Clidemiasericea3, Dillenia suffruticosa12, Ficus insipida5, Ficus obtusifoliaA, Glochidion rubrum13, Macaranga heynei12, Macaranga hypoleuca 13, Mallotuspaniculatus 12, Melastomamalabathricum 12, Piper um-bellatum6, Schefflera morototonf, Solanum straminifolium3, Trema tomen-tosai2, Urera caracasana2, Vismia japurensis3, Vismia lauriformis3. Light demanders: Anacardium excelsum2A, Chisocheton macranthusi3, Dialium pachyphyllum1, Dipterocarpus caudiferusi3, Dryobalanops aromaticas, Dryobalanops lanceolatai3, Licania heteromorpha3, Luehea seemannii2A, Ocotea costulata3, Parashorea tomentellai3, Pentace adenophorai3, Pentac-lethra macroloba9, Piper auritum6, Protium sp.3, Shorea seminisi3, Shorea xanthophyllai3. Shade-tolerant species (understorey conditions): Desmopsis panamensisi, Ouratea lucensi, Piper aequale6, Piper lapathifolium,6, Psycho-triafurcatai°, Psychotria limonensisi0, Psychotriamarginatai0, Xylopiamic-ranthai.

Data from iKursar & Coley (1992c), 2Kitajima et al. (1997) (data for pre-dry-season leaves), 3Reich etal. (1995), 4Hogan etal. (1995), 5Zotz etal. (1995) (dry season average values), 6Chazdon & Field (1987), iKoch et al. (1994), 8Ishida et al. (1996), 9oberbauer & Strain (1986), iOMulkey et al. (1993), iiZotz & Winter (1994), ^Tan et al. (1994), "Eschenbach et al. (1998).

for more light-demanding species. Fast-growing, shade-intolerant species are generally found to have maximal rates of more than 10 |imol m"2 s_1, often in the range 15-25 |imol m"2 s"1 Light-demanding species, which usually have relatively shade-tolerant juvenile stages, have mature leaves with a maximal net assimilation rate averaging 5-12 |imol m"2 s"1 Shade-tolerant species in the shade frequently photosynthesise at less than 5 |imol m~2 s"i, but growing in the higher irradiances of canopy gaps they can probably improve to nearly 10 |imol m~2 s"i. The degree of acclimation to light environment is likely to be less in the most shade-tolerant species than the more light-demanding ones. A similar relative pattern among the groups is seen for mass-based and N-based photosynthetic rates. Maximum instantaneous rates of carbon dioxide uptake can be related directly to total daily uptake across a wide range of leaf performance (Fig. 2.30).

The relative position of a species on the axis of shade tolerance is often referred to via its time of appearance in succession. Early-successional species are considered light-demanding, late-successional species are more shade

0 5 10 15 20 25 30 35

Figure 2.30 Relation between maximal rate of net carbon dioxide uptake (Amax) and diurnal carbon gain (AL) for the leaves of five tropical tree species. After Zotz et al. (1995).

Rainforest Images
Figure 2.31 Potential photosynthetic N-use efficiency (defined as instantaneous net photosynthesis per unit leaf N) in relation to N resorption efficiency (p < 0.001, r2=0.80) for ten Amazonian species. After Reich et al. (1995).

tolerant. Reich et al. (1995) analysed a subset of 13 of the 23 species studied in the San Carlos de Rio Negro region of Venezuela. There was a clear correlation between successional status and leaf longevity, with net assimilation rate and foliar nutrient concentrations (particularly Ca and Mg) decreasing and LMA increasing as leaf longevity increased with successional stage. Early-colonising species had higher foliar concentrations of nitrogen and phosphorus than late-successional and primary-forest species. All the species retranslocated more P (average of 62%) than N (43%). The early-successional species showed the greatest retranslocation rates. There was a positive correlation between N-resorption efficiency and the photosynthetic rate per unit foliar nitrogen across the species for which data were available (Fig. 2.31). For a given nitrogen concentration, early-successional species had a higher mass-based photosynthetic rate than late-successional species. Early-suc-cessional species showed greater variation in leaf N concentrations and a greater change in leaf photosynthetic rates for a given change in N concentration. Raaimakers et al. (1995) found a similar greater responsiveness in photosynthetic rates of early-successional species to changes in foliar nitrogen concentration at a site in Guyana. They also showed the same pattern

Table 2.10. Assimilation rates in relation to foliar nutrient concentrations for species from Guyana

Values are mean ± standard error.

Table 2.10. Assimilation rates in relation to foliar nutrient concentrations for species from Guyana

Values are mean ± standard error.

(mmol CO2 mol"i Ps"i)

All species

84 ±3

5.4 ±0.2

Pioneer species

118 ± 5

7.8±0.3

Cecropia obtusa

133 ± 9

8.1 ±0.5

Tapirira marchandii

121 ±6

7.2 ±0.3

Goupia glabra

100 ±7

8.1 ±0.5

Climax species

66 ±2

4.2 ±0.2

Peltogyne venosa

80 ±5

4.9 ±0.3

Dicymbe altsonii

74 ±6

4.7 ±0.4

Mora excelsa

72 ±4

3.3 ±0.2

Eperua falcata

74 ±6

4.5±0.4

Eschweilera sagotiana

59 ±3

4.6±0.3

Chlorocardium rodiei

39 ±3

3.1 ±0.3

Data from Raaimakers et al. (1995).

Data from Raaimakers et al. (1995).

with leaf phosphorus. At San Carlos de Rio Negro, leaf calcium concentrations were very low in the late-successional species. It is possible that a shortage of calcium was depressing photosynthetic rates in these species. There was no evidence of greater instantaneous nutrient use efficiency in the late-successional species. In Guyana, quite the opposite was the case, with the 'pioneer' species having significantly higher instantaneous nitrogen- and phosphorus-use efficiencies than the 'climax' species (Table 2.10). The only way in which late-successional species could have been using nutrients more efficiently was through longer residence times in the leaves.

Fredeen & Field (1991) measured the dark respiration rates of leaves of six species of the genus Piper growing at Los Tuxtlas, Mexico. The gap specialists Piper auritum and P. umbellatum had area-based respiration rates about twice those of the shade-tolerant species. Leaf dark respiration rate was positively correlated with daily total photosynthetic photon flux density and negatively to mean leaf longevity.

I believe it is possible to summarise the likely characteristics of species of differing shade tolerance, as given in Table 2.11. I have chosen to recognise three divisions of the shade-tolerance axis, referring to the least shade-tolerant species as shade intolerants, the most shade-tolerant as shade tolerants and the intermediate group as light-demanding. I would envisage the regeneration characteristics of the three groups to be as follows: shade intolerants require gaps to establish and grow to maturity and die relatively quickly if strongly shaded. Shade-tolerant species can persist readily in deep

Table 2.11. Generalisations about leaf form among three ecological groups of tropical rain-forest tree species

Shade intolerants

Light demanders

Shade tolerants

Leaf size

big

small

small

LMA

low

high

low

Lamina thickness

thin

thick

thin

NPPR

low

intermediate

high

Stomatal density

high

high

low

Nutrient/mass

high

low

lowa

Nutrient/area

low

high

lowa

Assimilation/mass

high

low

low

Assimilation/area

high

high

low

Leaf longevity

short

long

long

Leaf production

continuous

rhythmic

rhythmic

Leaf expansion

rapid

slow

rapid

Delayed greening

infrequent

infrequent

frequent

Chemical defence

weak

strong

strong

Leaf toughness

low

high

high

Ant defence

common

infrequent

rare

Plasticity of leaf form

low

high

low

"Often high for K.

"Often high for K.

shade and may be able to complete their full life cycle in such conditions. However, most species probably do benefit from the increased irradiance in gaps, and may need such conditions for reproduction. The intermediate group can persist in shade, but probably not as successfully as the shade tolerants, but only make headway with growth if in, or near, a canopy gap. I do not set any formal boundaries on the groups, merely indicate that a three-way contrast results in a sufficient degree of repetition in suites of traits among species of similar regeneration ecology to allow recognition of certain generalisations. However, probably inevitably, there are species that do not conform to some, or even all, of these generalisations.

The leaves of shade intolerants can be summarised as short-term highperformance photosynthetic units. Fast growth is of paramount importance to these species, and this is achieved by a rapid turnover of large thin leaves with high assimilation rates, particularly per unit dry mass. Leaf respiration rates are high and an inability to down-regulate both respiration and leaf-production rate appears to be the proximate cause of the lack of shade tolerance in these species (see later chapters). Leaf defences, particularly physical ones, are poor, probably because their capital and maintenance costs would detract from fast growth. At the other end of the spectrum, shade-tolerant species also have thin leaves, but they have much lower metabolic rates. Conservation of resources and persistence are much more important in the resource-poor environment of the forest understorey. The intermediate group is the most flexible, producing high-investment, high-return leaves in favourable conditions.

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