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Diversity in Woody Pioneer Species After the 1997/98 Fires in Kalimantan

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Diversity in Woody Pioneer Species After the 1997/98 Fires in Kalimantan The Balance between Biodiversity Conservation and Sustainable Use of Tropical Rain Forests DIVERSITY IN WOODY PIONEER SPECIES AFTER THE 1997/98 FIRES IN KALIMANTAN Karel A.O. Eichhorn GUILDS OF PLANTS IN THE TROPICAL RAIN FORESTS The tropical rain forests (TRF) in the world are generally very similar in their structure and ecological processes, in spite of their often pronounced differences in composition of species and higher taxa (Whitmore, 1984; Mabberley, 1992). This is well illustrated by the regeneration in gaps in the forest canopy created by falling mature trees. Generally, these gaps are first colonised by herbaceous and woody plants which share a whole set of ecological characteristics, including fast growth, intolerance of shade, short life span, high palatability, and small persistent seeds which germinate only at high light intensities (Ewel, 1980; Whitmore, 1984). These plants are usually referred to as pioneers. After the pioneers have colonised a gap, they are gradually replaced by species which share the opposite set of characteristics. Swaine and Whitmore (1988) reviewed the different classifications of plants according to their role in the regeneration of TRF gaps and concluded that only a few of them are properly explained, and that the terminology used is generally confused by a lack of precise definitions. They themselves recognise only two main groups of tree species observable in TRF: pioneer and climax or non-pioneer species. They define pioneer species as “species whose seeds can only germinate in gaps in the forest canopy open to the sky and in which full sunlight impinges at the ground level for at least part of the day”. They further state: “All pioneers also require full sunlight for seedling establishment and growth. As a consequence, seedlings and young plants are found in openings in the forest (tree-fall gaps, roadsides, landslides, felled areas etc.) and are never found under a closed forest canopy, including their own. Non-pioneer or climax species are able to germinate, establish and survive in forest shade. Young plants of these species are thus commonly found below a canopy, but may also be seen in open environments”. Although pioneer species share a set of ecological characteristics, they can be very different in others. Tropical ecologists have mainly focussed so far on the characteristics in which pioneer species differ from climax species (Davies, 1998). It is only recently that more attention has been paid to the often considerable differences within these groups. Pioneer species include plants of different growth forms, longevity and height at maturity, different crown characters, breeding systems, and photosynthetic rates. In spite of their definition, they even show considerable differences in their requirements for germination, since light intensity, red far-red ratio and temperature can be all triggers for it, either separately or in combination (Vasquez-Yanes and Orozco-Segovia, 1993). Furthermore, closely related pioneer species can be also very different in their ecological characteristics, as has recently been illustrated by different species of the genus of Macaranga (Davies, 1998; Davies et al., 1998). As a consequence of their different morphological and physiological characteristics, pioneer species can differ considerably in their preferences for gap size (Brokaw, 1987) and in their ability to colonise secondary habitats, as has been illustrated by the many species elimination trials for reforestation of Imperata grasslands in Kalimantan. These differences between pioneer species have led some authors to conclude that 131 The Tropenbos Foundation, Wageningen, the Netherlands the dichotomy in pioneers and climax species represents only the two extremes of a continuum (e.g., Alvarez-Buylla and Martinez-Ramos, 1992). Swaine and Whitmore (1988) state that, in contrast to the dichotomy between pioneers and climax species, all the variation within both two groups is continuous. They believe that the recognition of subgroups is acceptable, as long as it is realised that they are arbitrary segments of a continuum. SPECIES OF TROPICAL SECONDARY VEGETATIONS Before human interference, disturbance in TRF was mainly the result of falling mature trees and was consequently small in scale. Large-scale disturbances could be caused by landslides, earth- quakes, hurricanes, animals (elephants, caterpillars), fire and drought (Mabberley, 1992). After the introduction of shifting cultivation, disturbance of TRF continued to be relatively small-scale and in equilibrium with the recovery of the forest (Whitmore, 1984). In modern times, however, human interference has assumed the form of the destruction of huge areas of TRF by logging, fire and mining activities. As a result, these areas have been converted into tropical secondary forests (TSF) dominated by woody pioneers, where climax species are scarce or even completely absent, especially when no remnant trees of the original forest have survived. Although the conversion of TRF into TSF changes most environmental factors in the same direction as the formation of tree-fall gaps in TRF, these shifts can expected to be much more pronounced in TSF: no published direct comparative studies are known, but light intensity, soil and air temperature and drought stress are strongly correlated with gap size (Denslow, 1987; Bazzaz and Picket, 1980). Moreover, the forest floor is often severely damaged in TSF (Whitmore, 1984; Cannon et al., 1998), leading to a loss of nutrients due to erosion. Because pioneer species show considerable differences in their preferred gap size (Denslow, 1980; Brokaw 1985, 1987), only a subset of all pioneers are expected to be fully adapted to habitats resulting from large-scale disturbance. Only these pioneer species should therefore be referred to as secondary species, or species of secondary vegetations, terms often confused with pioneer species. Other pioneer species are less adapted to the circumstances in secondary vegetations and are easily squeezed out by the real secondary species. THE 1997/98 FIRES IN EAST KALIMANTAN Induced by the El Niño Southern Oscillation (ENSO) climatic phenomenon, millions of hectares of land were ravaged by the extensive fires of 1997 and 1998 in Southeast Asia, including both forest and non-forest areas. The fires and accompanying smoke caused serious air pollution, damage to the public health, loss of life, destruction of property and substantial economic losses in many parts of Southeast Asia. The most severely damaged area is East Kalimantan: after moderate damage by the fires in 1997, huge areas were burnt between January and May 1998 (Dennis, 1998). The hotspots were largely concentrated in the central part of the Indonesian province, in the environs of Balikpapan and Samarinda. The burnt areas included grassland, brush land, degenerated forests, secondary forest, and Macaranga stands, with surface fires burning in primary forest. The official estimate of 132 The Balance between Biodiversity Conservation and Sustainable Use of Tropical Rain Forests burnt land stands at about 0.5 million hectares, but recent calculations of International Forest Fire Management Project (IFFM) in Samarinda indicate that the real losses are at least 5 million ha. (pers. comm. A.A. Hoffmann, 1999). Like everywhere in Southeast Asia, small-scale fires are a normal phenomenon in Kalimantan, as both smallholders and owners of larger areas of land use it to clear land for cultivation. However, large-scale logging activities, even when based on sustainable management, make the forests very susceptible to drought and fire (Dennis, 1998; Goldammer, 1999). In the exceptionally dry years induced by ENSO, fires spread easily into the forests and damaged huge areas of it. During the last two decades, millions of hectares of forest have been burnt in this way, especially in 1982/83 and 1997/98, with minor events in 1987, 1991 and 1994. Once a forest has been burnt, it has a much higher risk of being burnt again (Dennis, 1998; Goldammer, 1999). The repeated disturbances by logging and fire convert TRF into secondary forests, shrub-dominated stands, fern-dominated stands and, finally, into alang-alang dominated grasslands, with the prospects for recovery of the original forest becoming lower after each new disturbance event (Uhl et al., 1988). Two years after the fires in East Kalimantan, woody pioneer species dominate the burnt areas, together with many herbaceous species and small lianas. Examples of currently dominant pioneer species belong to the genera of Macaranga, Homalanthus, Trema, Piper, Melastoma and Chromalaena. SCALE DEPENDENT DIVERSITY EFFECTS OF DISTURBANCE ON PIONEER SPECIES Field observations indicate that not all pioneer species profit from large-scale disturbances like the fires of 1997/98 in Kalimantan. Only a subset of all pioneer species seems to be fully adapted to the man-made environments. Other pioneer species seem to be easily forced out by them. Successful secondary species also invade new regions where they did not occur before, thereby out-competing local pioneer species. Consequently, pioneer species diversity may be even lower in TSF than in TRF, and large-scale disturbances may therefore lead to a considerable loss of diversity in pioneer species, in spite of the fact that pioneer species increase considerably in terms of biomass. Ecologists are generally not aware of the loss of pioneer species diversity after large-scale disturbance of TRF, because it is obscured by the experimental
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