How can urban forest fragments help water resource management? Using GLMMs to assess the water retention capacity of litter in floodplain and upland forest ecosystems in the Belém Environmental Protection Area, Belém, Brazil

Urban sprawl is a growing concern worldwide as it reduces the prevalence of vegetation and expands impervious surfaces. These changes can disrupt the water cycle, increase flood risk and contribute to water insecurity. With nearly 70% of the world’s population expected to live in cities by 2050, controlling urban expansion and protecting forest fragments within cities can improve water resource management and help to reduce rainwater runoff through stemflow, soil infiltration and water storage by litter. Litter plays a critical role in storing moisture, intercepting rainfall, reducing soil erosion, and maintaining favorable microclimatic conditions. We assessed the maximum water storage capacity (WSC) of the litter using the gravimetric method. This study was conducted in a protected urban Amazonian forest fragment in Belém, Brazil, which comprised two ecosystems (floodplain and upland) evaluated in two seasons (wet and dry). The fragment is part of the Belém Environmental Protection Area. We used generalized linear mixed models to test differences in litter WSC between ecosystems and seasons to generate information about where and when litter WSC can help to improve urban water resources conservation. Our findings showed that a wet period and higher-leaf-percentage litter can store more water, with no differences observed between floodplain and upland ecosystems. Conversely, the models pointed out that dry season has 22% more leaf content than wet season litter. These results may be explained by the litter decomposition stage. In spite of the lower leaf content, the higher decay of wet season litter, caused by higher moisture and enhanced microbial activity, may favor higher WSC. This increase in WSC may be due to changes in the physical and chemical properties of litter during decomposition, such as bulk density and Carbon–Nitrogen ratio. Our work highlights the importance of urban forest fragment conservation for water storage management, as water retained in litter can help maintain soil moisture throughout the year. Furthermore, knowing the litter WSC can provide a more complete understanding of urban forest’s hydrological ecosystem services and its loss due to urbanization processes. This is especially imperative in the current climate change scenario worldwide and in the Amazonian context, where dry seasons are becoming more and more prominent.