Die Tropenstation La Gamba

Die „Tropenstation La Gamba“ in Costa Rica – Wissenschaftlicher Bericht
(5) Timber volume (TV). TV correlated significantly with BA, the greatest TV being found where
BA was greatest, and where the trees grew tallest. Timber volume varied between 433.8m³ in the
Gorge forest, where trees were thick-stemmed but small, and 698.7m³ in the Coastal slope forest,
where trees were thick-stemmed, tall and abundant. The recorded TVs are exceptionally high
values as compared to other neotropical forests, and this may be due to the relatively fertile soils of
the Golfo Dulce region.
(6) Leaf form, size and coverage value. The typical leaf of the lowland forest species was found to
be simple (76%), entire (84.3%) and mesophyllous (49.2% coverage value). Only 24% of all
species had compound leaves. Due to the abundance of palms in the Inland slope forest, the
number of compound leaves was greater here (39.9%).
Leaf size varied significantly within the forest plots. Possible reasons are soil,
microclimate and site conditions. The Gorge forest had a greater number of leaves in the larger size
classes, perhaps due to higher soil fertility and sustained water supply, whereas the drier and
nutrient-poorer Ridge forest had a higher proportion of leaves in the smaller size classes.
(4) Growth forms. The most important growth forms of the recorded woody plants were trees,
followed by palms. The occurrence of large hemiepiphytes, lianas, tree ferns and cycads varied
conspicuously within the plots. The hemiepiphytes were found mainly on deciduous trees. In the
Gorge forest tree ferns proved abundant, replacing ecologically the palms, and exhibit a similarity
with montane forests.
Profile diagrams and forest physiognomy. The profile diagrams of the plots indicate that at least
parts of the Esquinas forest are well stratified. Stratification was more prominent in the Gorge and
Ridge forest than in the two other plots. This is possibly a result of synchronised growth. In the
Inland and the Coastal slope plot, stratification was rather inconspicuous due to the steepness of the
sites and the imbricate arrangement of the crowns. Crown forms varied within the strata and the
ratio of depth to width decreased with the height of the tree. Understorey trees had mainly roundconical
crown forms, while canopy trees had wider or umbrella-shaped crowns. The canopy was
found to consist mainly of species of Moraceae (Brosimum spp.), Leguminosae (Parkia pendula,
Peltogyne purpurea) and Meliaceae (Carapa guianensis). Only in the drier Ridge forest members
of Clusiaceae (Calophyllum spp.) and Vochysiaceae (Qualea paraensis, Vochysia spp.) proved
dominant. Palms (Welfia regia, Iriartea deltoidea, Socratea exorrhiza) were observed to represent
an important family in the understorey, especially in the Inland slope forest, where they became
dominant with 26.9% of the individuals. Palms were scarcely recorded in the Coastal slope forest,
perhaps because of the salinity of the soil. Emergent trees, well overtopping the canopy, were rare
(possibly due to the strong dynamics of the forest) and only found in the Coastal slope forest
(Schizolobium parahyba).
Dynamics of the forest plots. (1) Gap occurrence. In general, gaps occurred frequently and
differed substantially between the sites, with an average of 13.2% of the plot area consisting of
gaps (0-4yr. old). An exceptionally high gap area was found on the alluvial soils of the Gorge
forest (20.3%) and a very low rate in the Ridge forest (7.7%) with its drier soils. The most
important causers of gaps were strong winds, heavy rainfalls, lightning and landslides. Gaps were
mainly formed during the rainy season.
(2) Gap number and size. At sites with high gap occurrence, the average gap size was small. The
largest number of gaps was found in the Ridge forest (43 gaps), where the mean gap size was small
(56.5m²). In the Inland slope forest only seven gaps occurred and mean gap size was 510m².
(3) Tree mortality. Most of the trees died snapping (mean value 51%), became uprooted (31%), or
died standing (18%). Trees in the exposed Ridge forest frequently died standing (32.4%), due to
lightning or branch/crown breakage, or they snapped (45.9%). In the Gorge forest uprooting was
the most common cause of tree death (54.8%), and in the Coastal slope forest trees mainly died by
snapping (77.8%).
Mortality rate was almost even between the size classes, with a slightly higher rate in the
larger size classes. A great variety was found in the annual mortality rate, which varied between
1% and 2.8%. The highest rates were found in the Gorge and Inland slope forest, due to
thunderstorms in 1995 and 1997 which resulted in over-proportional tree mortality. These two
plots were found to be extremely dynamic, with exceptionally rapid turnover rates of 36.3 and 37.6
years. The Ridge and the Coastal slope forest had slower turnover rates of 91.6 and 98 years,
respectively.
Species-richness. With up to 179 species/ha (Ridge forest) and up to 847 ind./ha, the Esquinas
forest was found to be one of the most individual-densest and species-richest lowland forests of
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