Logging effects on liana diversity and abundance in Central Guyana

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Introduction natural processes is too limited for the identification of simple yet robust, sensitive and widely applicable indicators and their associated norms. 1.3.2 Use of species as indicators Species or species groups are frequently proposed as suitable indicators describing some aspect of the condition of the forest ecosystem, such as biodiversity or measures of ecosystem “health” (e.g. as a theme within the NWO priority programme on disturbed ecosystems, www.nwo.nl). This is based on the assumption of a strong correlation between the ecological behaviour of one species (group) and that of other species groups or of ecosystem characteristics. The value of using indicator species for assessing tropical forests is disputed. Generally, little is known about the requirements of indicator species; there is uncertainty about the validity of extrapolation of indicative power from one area to another area with potentially different species communities and about the relation between indicator species abundance and biodiversity of all other taxa. Reliably measuring changes in abundance of the indicator species itself is often a problem. Comparison of patterns of change in a number of animal groups over a gradient of habitat disturbance in African forest led Lawton et al. (1998) to conclude that “attempts to assess the impacts of tropical forest modification and clearance using changes in the species richness of one or a limited number of indicator taxa to predict changes in richness of other taxa may be highly misleading.” This was the reason why the indicator species concept was abolished when developing criteria and indicators for biodiversity in the CIFOR framework (Stork et al.1997, CIFOR C&I team 1999). 1.3.3 Requirements for indicator species Ecological indicators (including indicator species) should satisfy at least the following criteria: • There should be a direct and measurable relation between the indicator and the underlying variable of interest; • The species should be sensitive in presence or abundance to changes in the variable of interest; • Easy methods should be available to reliably measure presence and abundance of the species • Identification of the indicator species should be straightforward • The ecological behaviour of the indicator species (group) should be predictable and constant over a considerable spatial and temporal scale • The relationship between indicator value (species abundance) and the value of the response variable (level of damage) should be unambiguous, reciprocal and preferably linear, i.e., a high level of damage should always be associated with a high abundance of the indicator species, but, conversely, a high abundance of the indicator species should always be associated with a high level of logging damage. Preferably, an indicator provides an integrative measure or summary over space and time of the state of the relevant criterion. “Keystone” species (Terborgh 1986) are often suggested to be suitable indicator species with a high integrative value as their abundance in a community will influence the abundance of many other members in that community. 1.3.4 Criteria addressed in this study In this study the criteria or response parameters of interest are biodiversity (taken as liana biodiversity), logging damage and the abundance of lianas that present a problem for forestry. The precise formulation of suitable criteria is not addressed here. 7
<strong>Logging</strong> <strong>effects</strong> on liana diversity and abundance in Central Guyana <strong>Logging</strong> damage Many aspects of the health and functioning of managed forest ecosystems are heavily dependent on the amount of damage inflicted by logging, e.g. growth and survival of trees (Sist & Nguyen-Thé 2002), forest structure, accessibility of the forest, composition of animal and plant populations etc. (Putz et al. 2000). Therefore, the reliable assessment of logging damage is a critical element of any certification system. <strong>Logging</strong> affects liana communities in two different ways: direct and indirect. Direct logging damage occurs when falling trees and machinery physically kill or damage lianas, or when lianas are cut prior to logging to avoid problems during the cutting of trees. Indirectly, logging leaves a mosaic of habitat types, which differ in the type and severity of damage inflicted by the harvesting operations. Variation in extent and severity of damage to the canopy (creating gaps) and to the soil (principally on skid trails) determines the conditions for germination, establishment, growth and survival of plants including lianas. The composition of the liana community will therefore be determined by the damage patterns created during logging. In forest-interiors (no damage) the composition will be very similar to the pre-harvest species composition, while on skid trails leading through gaps (complete removal of the canopy and of the topsoil) an entirely new community will emerge that will contain many early successional species. One of the challenges presented by logging is the large spatial variation in damage patterns: locally many trees are cut and/or damaged, whereas elsewhere in the same forest damage may be very limited. For this reason it is difficult to quickly obtain a reliable measure of logging damage at the scale of hectares or more. If logging damage is closely associated with the growth conditions of certain species, then the occurrence of such species may serve as an indicator of (certain aspects of) logging damage. It is quite likely that soon after logging direct assessment of logging damage is the preferred alternative to applying indirect measures, such as the occurrence of indicator species. However, once secondary vegetation has developed into dense thickets, direct assessment might be hampered by poor visibility and indirect indicators may provide suitable means of damage assessments. In addition, the abundance of unwanted species may still be a useful parameter even if the opportunity exists to measure logging damage in a direct way. After all, the objective of sustainable forest management is to limit the actual occurrence of negative consequences of inappropriate logging, regardless whether planned targets of logging damage are met or not. The information obtained can be fed back into the management system in order to adjust and refine operational procedures. Biodiversity The use of liana species as indicators for biodiversity (at the community level) might present a larger problem. Even though many species or species groups are known to respond to logging by shifts in abundance, correlations between trends in species richness of different groups are often low (Lawton et al. 1998). This will not be attempted in this report. The remaining issue is whether any liana taxon can be used as an indicator of diversity within the liana guild. While in most cases it will be possible to demonstrate clear causal relationships between logging and changes of abundance and distribution of individual species, these relations are less clear for the rather intangible concepts of diversity or even species richness. Detrimental <strong>effects</strong> of lianas In spite of their obvious ecological value in tropical rain forests, lianas as a group enjoy a bad reputation with foresters. On the positive side, lianas are species rich components of forest communities and their fruits and leaves are important components of primate diets. Morrelato and Leitão-Filho (1996) showed phenological patterns of climbers that were complementary to those of trees, suggesting that lianas could span periods of lower tree flower and fruit availability for dependent animal communities. This resulted in a constant availability of flowers and fruits throughout the year. 8
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Logging effects on liana diversity and abundance in Central Guyana

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