Forest filled with gaps Creating experimental gaps − 16 − Chapter 2: Study area & theory 2 STUDY AREA, THE PIBIRI GAP EXPERIMENT AND ASPECTS OF MICROCIMATE, WATER AND NUTRIENT CYCLING IN TROPICAL RAIN FOREST LOGGING GAPS Abstract. This chapter describes the set-up of the Pibiri Gap Experiment and provides background information on the physical environment, vegetation and climate of the study area. Some theoretical aspects of microclimatic conditions, hydrology and nutrient cycling of the tropical rain forest of the region and of logging gaps in particular are discussed. THE PIBIRI GAP EXPERIMENT Objectives As was explained in the introduction chapter, there is a lack of knowledge as to the effects of logging gaps of different size on edaphic conditions, microclimate and regeneration in the gap and the forest surrounding the gap. The question concerning what form of inevitable disturbance that accompanies forest exploitation is most conducive to a desired regeneration of the forest is addressed in a broad range of projects of the Tropenbos-Guyana programme. The Pibiri Gap Experiment (PGE) forms the nucleus of this research topic. One of the main timber species of Guyana is Chlorocardium rodiei (Greenheart). Greenheart is a heavy seeded tree species with a limited dispersal distance from the parent tree, usually within the crown zone (ter Steege 1990). Notably, Greenheart dominated forest has a clumped spatial distribution. Exploitation of this species ultimately results in areas of variable sizes being opened up. Both light demanding and shade tolerant species are thereby exposed to a multitude of environmental conditions. The ecological constraints of these various gap sizes on the regeneration potential of the forest in general and commercial tree species in particular is not well understood. The PGE aims at generating knowledge about the ecological interactions in logging gaps that can contribute to the definition of a maximal or optimal gap size for the regeneration of the forest in general and of desired tree species in particular. This information can be used for policy makers and forest managers to set-up guidelines for forest management. The PGE is part of the Gap Study programme of Tropenbos-Guyana, which encompasses a range of gap related studies (see Brouwer 1996: nutrient cycling, Ek 1997: botanical diversity, Jetten 1996a: hydrology, Thomas 1999: forest productivity, van der Hout 1999: reduced impact logging, Zagt 1997: tree demography). The study presented in this thesis forms an integral part of the PGE. Other studies within the PGE are: plant population biology (Rose 2000), plant ecophysiology (Houter in prep) and plant-animal relations (Hammond in prep). The set-up of the Pibiri Gap Experiment is summarized below, but additional information can be found in a Tropenbos- Guyana Interim report (van Dam et al. 1999). Research area The study as presented in this thesis was carried out in three study areas, which were located within the Demerara Timbers Ltd. forest concession, approximately 250 km south of the capital − 17 − Forest filled with gaps Georgetown (Figure 2.1). The main research activities took place in the Tropenbos Pibiri reserve, located 50 km South of the Mabura Hill township (5°17’N, 58°42’W). The PGE took place in this reserve (5°02’N, 58°38’W) and the study area was located between the study areas of van der Hout (1999) and Ek (1997). The area contained 200 ha of undisturbed mixed Greenheart forest (Chlorocardium rodiei) on one soil type and an almost flat topography. Furthermore, the area had the logistical advantages of the Pibiri field camp of previous researches. The second study area was located in the Forest Reserve Mabura Hill (FRMH), which was located at 15 km south of Mabura (5°09’N, 58°42’W). The research activities in the FRMH area took place in two experimental gaps, where previous research on nutrient cycling had taken place (Brouwer 1996). A third study area called 2K, was located at 3 km south of Mabura Hill (5°16’N, 58°42’W). At 2K, two logging gaps were selected with known age and their original size (Ek 1997). Figure 2.1 Location of the research area in Guyana (A) and in the Pibiri research site (B). Numbers in map B refer to the research sites of van der Hout (1999) & Ek (1997). PGE methodology: selection of gap sizes and gap definition The effect of gap size on the regeneration potential of selected tree species was addressed in an experimental set-up in which a gradient of gap sizes was created in undisturbed mixed Greenheart forest. Gap size ranged from small single tree fall gaps to large multiple tree fall gaps. In logging gaps, conditions are very variable due to remaining trees and saplings, localised skidder operation and crown zones of the felled trees, tree stems and stumps. Since the number of gaps that could be created was limited, it was decided to create a situation in which variability in growth conditions and species composition between gaps was reduced as much as possible except for the gap size variable. The selection of the gap sizes for the experiment was based upon calculations of potential radiation in the centre of perfect circular gaps that were surrounding by trees of 30m height. For these calculations, a simplified above-canopy solar − 18 − Chapter 2: Study area & theory projection at the equinox was used, with standard solar equations (Kreith and Kreider, 1975; Gates, 1980 see also Chapter 4). The variation in radiation showed a sigmoidal relation with a logarithmic change in canopy gap size (Figure 2.2). The largest increase in radiation occurs between 50 and 5000m2. The size of experimental gaps was selected within this range, namely 50, 100, 200, 400, 800, 1600 and 3200m2. This logarithmic increase of gap size gave an almost linear increase of 1.8 MJ.m-2.d-1 (or 4.3 mol.m-2.d-1 in Photosynthetic Photon Flux Density PPFD units) in the centre of the gaps. The range of gap sizes also covers the full range of gap sizes from natural tree fall gaps to gaps created during normal felling operations. In the initial set-up, each gap size was replicated three times. It was foreseen that this would be difficult to achieve in practice and a continuum in sizes would be the result. However, after measuring the actual gap sizes, gaps could still be grouped together within a certain range. 25.0 ) Hypothetical -1 50.0 ) -1 .d 20.0 light curve -2 .d Aimed gap 40.0 -2 15.0 sizes 30.0 10.0 20.0 5.0 10.0 PPFD (mol.m Radiation (MJ.m 0.0 0.0 1 10 100 1000 10000 100000 Gap size (m2) Figure 2.2 Hypothetical light curve in increasing circular gaps (potential radiation on 1 April at 5°latitude and 50m altitude). Gap definitions In an undisturbed forest, falling trees or branches create gaps in the canopy. The occurrence and size of these natural gaps is referred to as gap dynamics. As was explained in the previous chapter, these gaps trigger the regeneration of the forest (Bazzaz 1991, Brokaw 1985b, Brown 1990, Pickett 1983). A widely tested hypothesis states that different tree species partition different light climates and thus occur in different sized gaps (Denslow 1987, Poulson and Platt 1989). This hypothesis assumes that climax species perform better in the low light environment of small gaps (approximately < 100m2) and pioneer species have the highest grow rate in large gaps (approximately > 250m2). Recent studies however did not find a correlation between gap size and species dominance, but suggest that regeneration is a stochastic process and that the simple presence of a species at the time of gap creation (Hubbell et al. 1999) or the height of the seedlings and saplings (Rose 2000) is more important. Knowledge about the variation of the amount and size of naturally created gaps is needed, when the effects of loggings are to be assessed. Unfortunately, ‘a gap’ is not clearly defined and consequently, many definitions can be found in literature (Table 2.1). In most studies however, the definition by Brokaw (1982a) is applied. A slightly adapted gap definition of Brokaw (1982a) has been used in this study (see also van Dam and Rose 1997; van Dam et al. 1999): A gap is a hole in the canopy extending through the layer of the crowns of the trees that form the canopy. A gap is not limited by a minimum area, but for practical reasons canopy openings smaller than 5m2 are within the natural variation of a closed canopy and are not considered a gap. A gap is not limited by maximum seedling height, but it is assumed that when the crowns of the trees within the original canopy opening reach a height by which they are becoming part of the main canopy layer, the gap is closed. − 19 − Forest filled with gaps Table 2.1 Definitions of a ‘Gap’. Reference Definition Addition constraints Oldeman 1978 “Chablis”: the fallen tree and the resulting destruction. Runkle 1981 Canopy gap expanded to the bases of the canopy trees surrounding the gap. Brokaw 1982a Hole in the forest extending through Maximum average height vegetation all levels in gap 2m. Brandani et el. 1988 Perpendicular projection of opening Including zone where tree crown fell, or disturbance. even if this zone lies under closed canopy. Popma et al. 1988 Projected canopy opening: sensu Total gap effected area: projected Brokaw 1982a canopy opening including the area with young pioneer over 0.5m. In the gap definition a of this study, the height constraint of the vegetation in the gap (2m) by Brokaw (1982a) was dropped.