Sapling communities in Amazonian white-water forests
Structure and floristic composition of forest regeneration (trees between 1 and 10 cm diameter at breast height (d.b.h), ≥ 1 m growth height) was described in three forest types of Amazonian white-water forests (várzea), in order to analyse whether floristic composition of saplings is related to the successional stage of the forests, whether it differs in comparision with the mature flora, and if there exists a zonation of sapling species along the gradients of flooding and irradiation. Location
Mamirauá Sustainable Development Reserve, Western Brazilian Amazon. Methods
The investigated forests were of the low and the high várzea type, on an annual average flooded around 4 and 1 months, respectively. The two low-várzea forests belonged to the secondary and to the late-successional stage, the high-várzea forest also to the late-successional stage. A total of 24 circular sample plots covering 1885 m2 were installed. They were nested within three rectangular 1 ha permanent sample plots where individuals ≥ 10 cm d.b.h. were formerly inventoried. Average inundation and radiation [relative photosynthetically active radiation (rPAR)] at the forest floor was recorded in all inventoried plots. The Mean spatial Distribution Center (MDC; Ebdon, 1998) for all sapling species was calculated. Sapling species were grouped into associations with respect to their light-demand and their location along the flood-level gradient. Results
Average flood height and average rPAR at the forest floor was highest in the secondary stage of the low várzea and lowest in the high-várzea forest. Overall average density and basal area of the saplings averaged 2250 individuals and 2.3 m2 ha−1 in the secondary stage, 2330 individuals and 2.6 m2 ha−1 in the late-successional stage of the low várzea and 5000 individuals and 4.8 m2 ha−1 in the high várzea. In all forest types, species richness of saplings was lower than species richness of trees ≥ 10 cm d.b.h., and amounted to 25 species in the secondary stage, to 35 species in the late-successional stage of the low várzea and to 88 species in the high-várzea forest. The amount of ‘immigrants’ ( Bazzaz, 1991) at the sapling level increased with proceeding forest succession and amounted to 24 and 29% in the low várzea and to 31% in the high várzea. Sapling species distribution, species richness and individual density was linked to both, the gradient of flooding and the gradient of irradiation. Species richness and individual density was highest in the high várzea and decreased with increasing influence of flooding. The high várzea was predominated by pronounced shade-tolerant sapling species, whereas in the low várzea occured both, light demanding and shade-tolerant species groups. Main conclusions
Influence of flooding seems to be the main factor triggering species composition and structure of sapling communities in várzea forests. Proceeding forest succession reduces the impact of flooding because of the biogenical induced silting up of the forested sites. However, forest succession also alters forest architecture of the overstory and such the light conditions at the sapling level. Therefore, radiation is an important factor influencing species composition of sapling communities in várzea forests.