Ecohydrological processes in tropical rainforests are insufficiently understood, and existing studies yield contradictory results. We investigated relative contributions of different soil depths to tree water uptake of 83 trees and possible species‐specific differences in a 50 × 50 m forest plot at four dates in a tropical montane forest in Kenya using stable water isotopes and the Bayesian mixing model framework MixSIAR. We found distinct individual tree differences (e.g. Drypetes gerrardii taking 75% of its water from <0.5 m, or a rather large shift in uptake patterns based on the climatic conditions, that is the fourth sampling date), but no consistent species‐specific or small‐scale spatiotemporal patterns in water uptake and depth contributions. Soil water δ18O showed a lateral variation of up to 6‰, which was accounted for by a spatial interpolation of soil water isotopes and enabled us to improve allocations of water uptake sources to individual trees. Our results show that ignoring the lateral variability of water isotope signatures in soils complicates the applicability of a mixing model in this context and might be a widespread constraint reducing the validity and comparability of mixing model results. Further research on underlying processes of water fluxes in forest ecosystems is urgently needed and we point out the need for considering large individual differences in water uptake patterns and small‐scale variability of soil water isotopic composition despite homogeneous soil characteristics.
Tag: mountain forests
Spatial distribution and perceived drivers of provisioning service values across an East African montane forest landscape
Increasingly, resource managers and planners seek to manage forested landscapes for the value of the services they provide. This is especially true in the Mau Forest of Kenya, a montane area that harbors some of Kenya’s most important headwaters but has lost a quarter of its forest cover since 1999. While managing for the Mau Forest’s landscape services is a priority, it is critical to understand why and how people value these services differently. Otherwise, land management policies risk exacerbating rather than alleviating conservation and environmental justice problems. This is particularly true of provisioning services, a category of landscape services on which communities directly depend. This research combines participatory mapping and semi-structured interviews to understand how socio-cultural values of provisioning services are distributed across two sites within the Western Mau Forest and analyze linkages between mapped values, their locations, and influencing factors. In total, 55 informants were interviewed. Frequently listed provisioning services were water, firewood, cultivation, grazing, timber, and medicine. Results indicate that four main factors influence the location from where these services were derived: historical and legal arrangements, social relations, economic conditions, and biophysical conditions. How these factors influence where people value provisioning services differ based on the service and community in question. This study demonstrates that communities can use and value provisioning services differently and that the distributions of these services are influenced by the factors mentioned above. Understanding this heterogeneity can enable managers and policy makers to create local land use plans that account for spatially-explicit values.
Agricultural land is the main source of stream sediments after conversion of an African montane forest
In many parts of Africa, soil erosion is an important problem, which is evident from high sediment yields in tropical montane streams. Previous studies in Kenya pointed to a large contribution from catchments cultivated by smallholder farmers. This led to the hypothesis that unpaved tracks and gullies are the main sediment sources in smallholder agriculture catchments of the highlands of Kenya. The aim of this study was to investigate the sediment sources with sediment fingerprinting to generate the knowledge base to improve land management and to reduce sediment yields. Four main sediment sources (agricultural land, unpaved tracks, gullies and channel banks) and suspended sediments were analysed for biogeochemical elements as potential tracers. To apportion the catchments target sediment to different sources, we applied the MixSIAR un-mixing modelling under a Bayesian framework. Surprisingly, the fingerprinting analysis showed that agricultural land accounted for 75% (95% confidence interval 63–86%) of the total sediment. Channel banks contributed 21% (8–32%), while the smallest contributions to sediment were generated by the unpaved tracks and gullies with 3% (0–12%) and 1% (0–4%), respectively. Erosion management strategies should target agricultural lands with an emphasis on disconnecting unpaved tracks form hillslope source areas to reduce sediment yields to Lake Victoria.