Bamboo-Dominated Forests of the Southwest Amazon: Detection, Spatial Extent, Life Cycle Length and Flowering Waves

Abstract

We map the extent, infer the life-cycle length and describe spatial and temporal patterns of flowering of sarmentose bamboos (Guadua spp) in upland forests of the southwest Amazon. We first examine the spectra and the spectral separation of forests with different bamboo life stages. False-color composites from orbital sensors going back to 1975 are capable of distinguishing life stages. These woody bamboos flower produce massive quantities of seeds and then die. Life stage is synchronized, forming a single cohort within each population. Bamboo dominates at least 161,500 km2 of forest, coincident with an area of recent or ongoing tectonic uplift, rapid mechanical erosion and poorly drained soils rich in exchangeable cations. Each bamboo population is confined to a single spatially continuous patch or to a core patch with small outliers. Using spatial congruence between pairs of mature-stage maps from different years, we estimate an average life cycle of 27–28 y. It is now possible to predict exactly where and approximately when new bamboo mortality events will occur. We also map 74 bamboo populations that flowered between 2001 and 2008 over the entire domain of bamboo-dominated forest. Population size averaged 330 km2. Flowering events of these populations are temporally and/or spatially separated, restricting or preventing gene exchange. Nonetheless, adjacent populations flower closer in time than expected by chance, forming flowering waves. This may be a consequence of allochronic divergence from fewer ancestral populations and suggests a long history of widespread bamboo in the southwest Amazon.

Impact of pre-Columbian “geoglyph” builders on Amazonian forests

“…Wetter-than-previous climatic conditions characterized the late Holocene (39), which would have made the vegetation less naturally flammable, whereas archaeological dates attest to people in the landscape from at least 4400 BP (28); thus, we can be confident that fire activity in these levels was human-rather than naturally-driven. It is likely that these cultures took advantage of the bamboo life cycle to facilitate deforestation (36), as Guadua bamboo undergoes periodic mass die-offs every 27-28 y across areas averaging 330 km 2 (35). The resulting dead vegetation is flammable in the dry season, which favors clearance using fire, rather than laborious tree felling with stone axes.…”

“…forests (Fig. 1B), which cover roughly 161,500 km 2 of southwest Amazonia (35). Was bamboo forest also dominant before the geoglyphs, as some have suggested (36)(37)(38)?…”

Post-Fire Changes in Forest Biomass Retrieved by Airborne LiDAR in Amazonia

“…Several authors associate the reduced biomass of western Amazonian forests to the presence of bamboo [64][65][66]. Bamboo tends to reduce density and basal area of trees [67,68], restraining forest biomass development due to a negative feedback with the recruitment, growth, and survival of other individuals [69,70]. Tree recruitment is suppressed by the low density of seedlings and saplings [71][72][73][74].…”

Large-scale climatic and geophysical controls on the leaf economics spectrum

“…The N hotspot associated with the Fitzcarrald Arch is geographically aligned with geologic uplift and the occurrence of vast swaths of canopy bamboo (Guadua spp.) (19), which maintains high foliar N concentrations of 3.1-3.4% (20). Elevated foliar N in both Andean valley and lowland floodplain forest canopies is associated with highfertility sediment deposition from montane sources (21).…”