Tree growth in the forest

Most studies of the growth of tropical forest trees have found that the vast majority of trees grow very slowly. For instance, 64% of trees in plots at Bukit Lagong and Sungei Menyala in Peninsular Malaysia had diameter growth rates averaging around 1 mm yr~ (Fig. 3.3) over the 20-30-year

Walking With Mammals Temperature Graph

Figure 3.2 Fraction of plants surviving over the intercensus interval 198285 as a function of diameter class for sample tree species in the Barro Colorado Island plot. Many shade-tolerant species showed high and relatively constant rates of survival at all size classes except the largest. Examples of such species are Prioria copaifera, Drypetes standleyi, QQuararibea aster-olepis, Alseis blackiana and Trichilia tuberculata. In contrast, shade-intolerant species such as Luehea seemannii, Spondias mombin and Cecropia insignis all show increasing survival with increasing diameter, approaching the high survival rates of the shade-tolerant species once the shade-intolerant species reach the high-light environment of the canopy. After Hubbell & Foster (1990).

Figure 3.2 Fraction of plants surviving over the intercensus interval 198285 as a function of diameter class for sample tree species in the Barro Colorado Island plot. Many shade-tolerant species showed high and relatively constant rates of survival at all size classes except the largest. Examples of such species are Prioria copaifera, Drypetes standleyi, QQuararibea aster-olepis, Alseis blackiana and Trichilia tuberculata. In contrast, shade-intolerant species such as Luehea seemannii, Spondias mombin and Cecropia insignis all show increasing survival with increasing diameter, approaching the high survival rates of the shade-tolerant species once the shade-intolerant species reach the high-light environment of the canopy. After Hubbell & Foster (1990).

Figure 3.3 Frequency of trees (> 10cmdbh) in growth rate classes in three primary dipterocarp forests in Peninsular Malaysia. After Manokaran & Kochummen (1994).

measurement period (Manokaran & Kochummen 1994). Similarly, saplings of 1-4cmdbh on Barro Colorado Island showed a negative exponential distribution of growth rate frequency for three years of data (Welden et al. 1991). The modal class for growth was zero. In conclusion, most trees in the rain forest are hardly growing, and some even shrink. Being hit by fallen debris, broken off by large mammals or attacked by pests or disease can all cause leader shoot loss and a reduction in size, with young trees being the most susceptible (Clark & Clark 1991). Large trees generally grow faster (Fig. 3.4), even when growth is expressed on a basis relative to original size, probably because larger individuals are usually in more brightly lit conditions.

The growth rates of individual species in the rain forest vary considerably, with average diameter growth rates generally in the range 0.5-6 mm yr~ , with maximal rates reaching 15 mm yr~ (Fig. 3.5). There tends to be strong autocorrelation between successive increment measurements for forest trees (Swaine et al. 1987). Slow growers remain stagnant in growth, whereas leading trees maintain high growth rates. There are several possible reasons for this. The most obvious is that the fast-growers are occupying particularly favourable sites in the forest whereas the slow-growers are in the generally unfavourable milieu of the forest interior. Trees need adequate supplies of light, water and nutrients to grow quickly. Soil water and nutrient availability do exhibit both temporal and spatial patchiness on the forest floor, but not with such a consistent and large variation in supply rate as light. In the dimly lit forest understorey where most tree individuals are situated there is insufficient light for fast growth. Only individuals with crowns near the top of the canopy, or located in gaps, are likely to receive sufficient light to achieve high rates of growth. Trees in the deep shade have strongly suppressed growth rates. It is possible that they lose the ability to respond if more light does become available, reinforcing the temporal autocorrelation of growth rates. There has been little research into long-term effects of shade suppression on tree physiology. Intrinsic growth rates of trees are also under a degree of genetic control and lead trees may be genetically predisposed to fast growth. Identifying such trees has long been the ambition of foresters. However, lead

Diameter class (dbh, cm)

Figure 3.4 Mean annual percent growth rates for all individuals of canopy tree species in the 50 ha plot on Barro Colorado Island. These means are unweighted: the total growth increments divided by the total number of individuals. Some plants decreased in diameter due to stem breakage; these plants were excluded. Vertical lines indicate 95% confidence limits. Limits for the 1-2 and 2-4 cm dbh classes are too close to the circles to be visible (sample sizes were more than 17 000 individuals in these size classes). After Condit etal. (1992a).

Figure 3.5 Average diameter increment against maximal increment for tree species from seven tropical-forest sites. Trees are > 10 cmdbh with n > 20. Data from Manokaran & Kochummen (1994), Ashton & Hall (1992), Lieberman et al. (1985a) and Korning & Balslev (1994).

trees need not necessarily be superior. Fast growth, for instance, may come at the cost of reduced resistance to pests and diseases.

Tree performance in relation to light climate

It is not easy to obtain quantitative data on the relationship between survival or growth rate and incident irradiance for large tropical trees because of the technical difficulties of obtaining long-term climatic data above tall trees. An approach used to get round this has been to employ semi-quantitative or qualitative measures of relative light availability to trees and use these in comparing individual growth rates. Various crown illumination indices are available (Clark & Clark 1992). These involve one or more fieldworkers in visually assessing the amount of light received by the crown of each tree in the study on a predetermined scale ranging from 'crown completely unobscured from the sun in all directions' to 'crown beneath continuous canopy'. Of the six large-tree species studied in detail by Clark & Clark (1992) at La Selva, all showed an increase in crown illumination index with increasing size of individual (Fig. 3.6). The biggest difference between the species was the failure of Minquartia guianensis to reach high crown-illumination-index scores. This species does not reach such large sizes as the others and hence cannot grow big enough to obtain a crown unobscured in all directions.

Lieberman et al. (1995) used a geometrical analysis of crown heights and distances from focal trees to generate an index of canopy closure. They regressed the closure index against 1000/dbh to obtain a linear relation, and

Figure 3.6 Crown illumination index for successive size classes for six large-tree species at La Selva, Costa Rica. Data from Clark & Clark (1992).
Figure 3.7 Frequency distribution of the mean residual of canopy closure for the 104 most abundant tree species in the study of Lieberman et al. (1995).

then used the residual from the linear regression as a size-compensated estimate of relative illumination, which could then be averaged for all the individuals of each species. The frequency distribution of the average residual for the 104 species studied is given in Fig. 3.7. Nine species, including Hampea appendiculata and Cecropia obtusifolia, had residuals significantly higher than those of all other species combined. In other words, they were found in sites significantly brighter than average. A smaller group of five species occupied sites shadier than average, with Faramea terryae being the most extreme shade-lover.

In the 50 ha plot on Barro Colorado Island, the forest was classified into high- and low-canopy ( 10m tall) sites (12.7% of the total) on a 5 m x 5 m grid (Welden et al. 1991). This very rough classification of forest light climates has been employed to investigate the effects of shade on the survival, growth and recruitment of saplings of the commoner species (Table 3.2). Relatively few species showed a significant difference in survival between high- and low-canopy sites. Growth showed a much stronger effect, with all the species that displayed a significant difference between site types having faster growth in low-canopy sites. Recruitment to the 1 cm dbh class exhibited a similar pattern, with only two species (Drypetes standleyi and Rheedia acuminata) having

Table 3.2. Sapling survival, growth and recruitment in relation to canopy height on Barro Colorado Island over the period 1982-85

Tabulated are the number of species showing significantly greater survival, growth or recruitment in low-canopy (no foliage above 10 m height) sites, high-canopy sites, or with no difference between sites (indifferent). Saplings are 1-4 cm dbh, except in shrub species (adult height 4 m), where saplings were classed as 1-2 cm dbh.

Tabulated are the number of species showing significantly greater survival, growth or recruitment in low-canopy (no foliage above 10 m height) sites, high-canopy sites, or with no difference between sites (indifferent). Saplings are 1-4 cm dbh, except in shrub species (adult height 4 m), where saplings were classed as 1-2 cm dbh.

Low-canopy sites

High-canopy sites

Indifferent

no. spp.

% ofspp.

no. spp.

% of spp.

no. spp.

% of spp.

Survival

6

4.0

19

12.8

123

83.1

Growth

66

57.4

0

0

49

42.6

Recruitment

70

44.9

2

1.3

84

53.8

Data from Welden et al. 1991.

Data from Welden et al. 1991.

better recruitment in high-canopy sites. A large proportion of the species was indifferent to canopy type in one or more of the demographic measures. Welden et al. (1991) argued that this implied most species were 'gap-neutral' in their regeneration ecology. The poor level of accuracy and discrimination in light climate possible from the method used for its estimation might be a readier explanation for the relative rarity of 'gap-positive' species.

Mortality, growth and adult size

On Barro Colorado Island, the species were divided into four stature classes based on maximal height. These groups were 'shrubs' ( 4 m adult height), 'treelets' (4- 10 m), mid-sized trees (10- 20 m) and large trees ( 20 m adult height). The 'shrub' species showed mortality rates of about twice those on average of the trees and 'treelets' for individuals 1-9.9 cm dbh (Condit et al. 1995) (Table 3.3). 'Shrubs' tended to occupy the lower end of this size range, and being generally smaller may be more susceptible to death by falling debris. In addition, the reproductive activity of the 'shrubs' may deplete their resources and render them more liable to mortality. The saplings of the tree species were more likely to exhibit a positive growth and recruitment response to low-canopy sites than those of the two smallest size classes (Welden et al. 1991). For instance, 66% of the tree species showed a significant positive response in sapling growth to low-canopy sites, but only 41% of the 'treelet' and 'shrub' species did the same. The probable smaller average size of the 'treelet' and 'shrub' saplings may mean that the difference in light climate experienced between the two canopy-height classes is less marked. Another possible explanation is the tendency of understorey species to reproduce rather than grow more when in gaps.

Table 3.3. Annual mortality rates (%) averaged across species for trees of different stature on 50 ha at Barro Colorado Island, Panama n, Number of species.

Table 3.3. Annual mortality rates (%) averaged across species for trees of different stature on 50 ha at Barro Colorado Island, Panama n, Number of species.

Stature class Stems 10-99 mm dbh

n

Stems > 100 mm dbh

n

20 m adult height 3.2 ± 0.4

71

1.9 ±0.2

63

10- 20 m adult height 2.7 ±0.4

54

3.3 ±0.2

49

4- 10 m adult height 2.9 ± 0.6

41

2.9 ±0.8

16

4 m adult height 6.3 ± 0.9

28

all 3.5 ± 0.3

194

2.6 ± 0.2

128

Data from Condit et al. (1995).

Data from Condit et al. (1995).

Table 3.4. Annualised mortality rates for different stature classes for three sites in Malaysia

Stature class

Bukit Lagong

Sungei Menyala Pasoh

emergent

1.11

1.44 1.65

main canopy

1.33

1.84 1.90

understorey

1.45

2.58 2.35

Data from Manokaran & Swaine (1994).

Data from Manokaran & Swaine (1994).

Large-tree species showed significantly higher rates of survival among stems of 10 cm dbh or greater on Barro Colorado Island than those of species of smaller adult stature (Condit et al. 1995) (Table 3.3). At La Selva, the six emergent species studied by Clark & Clark (1992) showed low annual mortality for trees > 10 cm dbh (0.44%) in comparison to the forest average (2.03%). The same pattern was seen in studies of lowland dipterocarp forests in Malaysia (Manokaran & Kochummen 1987) (Table 3.4), and trees in Ecuador (Korning & Balslev 1994). Emergent species also exhibit some of the highest maximal growth rates in forests (Manokaran & Kochummen 1994). The data from Figure 3.5 have been re-plotted indicating the stature of the species concerned (Fig. 3.8). Clearly, large trees tend to have higher average and maximal growth rates, particularly when compared with the understorey species. Subcanopy species tend to be more variable, with some having very rapid growth. Interestingly, the average rates for canopy-top species estimated from periodic increment data on individual trees in the forest are similar to the long-term averages (1-6 mm yr~ ) calculated for 20 large trees aged by using C-dating in the Amazon (Chambers et al. 1998). Maximal growth rates of large trees (5-15 mm yr~ ) in the forest still fall short of the rates of similar species in arboreta or plantation stands, where diameter increments greater than 10 mm yr~ are common, and over 20 mm yr~ are known (see for example, the data of Ng & Tang 1974).

The maximum projected lifespan estimates for trees from La Selva, Costa Rica, (Lieberman et al. 1985a) and Ecuador (Korning & Balslev 1994) show a trend of increasing longevity with increasing stature (Table 3.5). Only in the data from La Selva, are there statistically significant differences between stature classes. Here the understorey species have significantly shorter lifespans than the other two. However, the range of values is large for all the stature classes, with each height group possessing some fast-growing shortlived species.

The use of growth and mortality data to recognise species groups

A frequent criticism of the functional classifications of tropical tree species is that they are largely based on ad hoc impressions of the species concerned rather than on detailed quantitative information. There have however, been several attempts to classify species by using growth and mortality data.

The 50 ha plot study on Barro Colorado Island is the largest data set

Figure 3.8 The same plot as Fig. 3.5, but here the symbols indicate the stature class of each species.

Table 3.5. Maximum projected life span (10 cm dbh to maximum diameter) for tree species of different stature classes from Costa Rica and Ecuador

Table 3.5. Maximum projected life span (10 cm dbh to maximum diameter) for tree species of different stature classes from Costa Rica and Ecuador

Stature class

Costa Rica

Ecuador

understorey

126 ± 56

199 ± 103

(52-221)

(69-348)

13

7

subcanopy

242 ± 81

270 ± 175

(78-338)

(54-529)

11

10

canopy

206 ± 102

299 ± 40

(78-442)

(250-353)

21

5

Data from Lieberman et al. (1985a) and Korning & Balslev (1994).

Data from Lieberman et al. (1985a) and Korning & Balslev (1994).

employed for such a purpose (Condit et al. 1996a). Five parameters were used to perform principal components analysis (PCA) for 142 species. The five vital statistics employed were: annual mortality (avoiding the drought years) for 1-9.9cmdbh and > 10cmdbh classes; mean growth rate for 1-2cmdbh and 10-20 cm dbh size classes; and the colonising index (the proportion of recruitment that occurred in low-canopy sites). The understorey species had to be dealt with separately from the others because they did not have sufficient data for the larger tree sizes. The two resulting PCAs produced similar results, particularly for the first axis. Most of the species occurred in a tight knot in the range -1 to 1 on the axis, with a scattering of species stretching from 1 to 6. These were species with a high mortality, high growth rates and tendency to recruit in gaps. This group was arbitrarily divided into 'pioneers' with first-axis scores of more than 3 and 'building-phase' species in the range 1-3. The pioneers included Cecropia insignis and Zanthoxylum belizense among the large-statured species and Croton billbergianus and Palicourea guianensis from the understorey group. An important point to note is that the species showed no marked discontinuity of distribution along the shade-tolerance axis, and that the groupings proposed by the authors were defined at arbitrary intervals.

At La Selva, Lieberman et al. (1985a) used their growth trajectory simulation technique to estimate maximum potential lifespan of tree species from 13 years of enumeration data on 12.4 ha of forest. They identified four species groups on a three-dimensional plot of maximum dbh (as measured on the plot), maximum potential lifespan (as projected from the slowest growth simulation) and maximum growth rate (as estimated from the fastest growth simulation). These were:

Group I: understorey species with slow maximum growth rates and short lifespans.

Group II: shade-tolerant subcanopy species that lived up to twice as long as the understorey species but had similar maximum growth rates.

Group III: canopy and subcanopy species that were shade tolerant but could grow fast in high light conditions and were long-lived.

Group IV: shade-intolerant species with fast growth and short life spans.

The longest-lived species were either relatively shade-tolerant mid-canopy trees or large canopy-top species. The distinction between groups I and II, and between III and IV, seems rather arbitrary to the external observer. That between the two pairs of groups appears clearer.

These two studies, and the more empirical observations of other tropical ecologists, lead to the conclusion that groups of species based on growth and survival, particularly in relation to irradiance regime, mature stature and probable longevity can be recognised among the tropical rain-forest tree community. These groups are not discrete, but better considered as regional references for locations within the character space available. The four groups of Lieberman et al. (1985a) seem to be reasonable and can be characterised as follows.

Understorey species: small-statured at maturity with relatively low maximal growth rates and quite high mortality (though not as high as that of pioneers). Generally shade tolerant with limited growth responses to increased irradiance.

Subcanopy species: intermediate in stature between understorey and canopy species. Often shade-tolerant with a relatively low mortality rate. Some species long-lived.

Canopy species: the largest trees, usually with high juvenile survival and a large increase in growth rates when exposed to high light.

Pioneer species: fast-growing, with high mortality rates, particularly in the shade and among juvenile stages. Relatively short-lived.

These groups, and possible subdivisions of them, are discussed in greater detail in Chapter 6.

Relative performance of species of similar life history

Clark & Clark (1992) investigated the growth and mortality of six species in detail on a 150 ha area of forest at La Selva, Costa Rica. Each of the six would be placed in the canopy group of the scheme above. All individuals encountered greater than 50 cm tall were included in the survey. These were monitored annually for survival and growth and were assessed for crown illumination class and forest growth phase. Using these data, and some extra observations for Simarouba amara and two species of Cecropia, the authors identified four species groups as follows.

Group A: found in low crown illumination classes and with a high proportion of juveniles in the mature phase of the forest [Lecythis ampla and Minquartia guianensis].

Group B: steady increase in illumination class and proportion in early phases with increasing size [Dipteryx panamensis and Hy-menolobium mesoamericanum].

Group C: a tendency to be found in low illumination classes as middle-sized juveniles, interpreted by Clark & Clark as a requirement for gaps as saplings but with a tendency to become overtopped in the building phase and then waiting for a new gap event to grow on [Hyeronima alchorneoides and Pithecellobium elegans].

Group D: occupying the brightest sites at all size classes [Cecropia spp.]

The Group A species showed no significant relationship between sapling mortality and crown illumination. The other species did show an improved chance of survival for saplings growing in brighter conditions. However, there was no great difference in the maximal growth rate of the six canopy-top/emergent species. This led Clark & Clark to question the view that there is a tradeoff between ability to persist in the shade and maximal growth rate in high light. We will return to this subject in a later section. In general, there was relatively little difference in the growth and survival responses to forest environment between the species, particularly the five emergents. As in the study of Welden et al. (1991), this might be attributable to methodological problems including low sample sizes for some diameter classes and the accuracy of the light regime estimates. However, it may also reflect reality. Tropical tree species of similar adult stature and shade tolerance may be very similar.

What limits tree growth?

It is interesting to consider what factors actually limit the growth of individual trees in the forest. The answer, as already indicated, in most cases is light availability. Shading results in most trees in the forest growing at rates well below their potential maximum. Clark & Clark (1994) found large interyear variations in growth of six species at La Selva, Costa Rica. Over an eight-year period, diameter increments for individuals from more than 50 cm tall to 1 cmdbh were 3-10 times greater in the year of fastest growth compared with that of slowest (Fig. 3.9). The six species showed a strong concordance in performance over the observation period, i.e. stand productivity was varying relatively uniformly over time. The year of best growth was the one with the lowest rainfall. It seems improbable that poor growth in wet years was due to waterlogged soils. More likely was that wet years were much cloudier and had substantially lower total photosynthetically active radiation (PAR) received than dry years.

22 40

Year

Figure 3.9 Annual variation in mean diameter growth rates of adult tree ( 30 cm in diameter) of six canopy and emergent species in primary forest at La Selva, Costa Rica (after Clark & Clark 1994). Data for each species are each year's mean annual adjusted increment (averaged over all individuals after detrending or subtracting the 8-year mean diameter increment from each tree's annual increment), given as a percentage deviation from the species' overall mean diameter increment over the 8-year period. Abscissa labels indicate the calendar year beginning each measurement period. MG, Minquartia glabra, LA, Lecythis ampla, HM, Hymenolobium me-soamericanum, DP, Dipteryx panamensis, PE, Pithecellobium elegans, HA, Hyeronima alchorneoides.

In seasonally dry forests, growth is probably reduced owing to water shortage during the dry season, unless the trees can tap water deep in the soil. At La Selva, Costa Rica, a detailed study of diameter growth patterns on a day-to-day basis (Breitsprecher & Bethel 1990) showed an annual periodicity in growth in a majority of species on well-drained soils. Most species showed reduced growth in the mild dry season at La Selva. During a severe drought in 1983 on Barro Colorado Island, there was considerably higher mortality among trees than normal, with large-diameter stems suffering the greatest increases in mortality.

Herbivory

The loss of material to herbivores can be substantial, even in tropical trees. Complete defoliation of trees has been reported quite frequently. It is likely that such an event will influence the performance of a tree negatively. The studies by Marquis (1984, 1987) on Piper arieianum referred to earlier showed this to be the case. The best-studied group in terms of the influences of defoliation is understorey palms (Mendoza et al. 1987; Oyama & Mendoza 1990; Chazdon 1991; Cunningham 1997). Presumably their large, and relatively few, leaves make observations easier. All the neotropical palms studied proved to be remarkably resilient to leaf loss. Indeed, Chamaedora tepejilote showed increased reproduction after being defoliated (Oyama & Mendoza

1990). Neither leaf nor ramet removal showed any influence on leaf size or stem diameter in Geonoma congesta three years after defoliation (Chazdon

1991). It is probable that mobilisation of stored carbohydrate is important in buffering palms from premature leaf loss, but even so, Cunningham (1997) could not find any significant effect of defoliation on stored carbohydrate in Calyptrogyne ghiesbreghtiana.

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