Structure and composition of altered riparian forests in an agricultural Amazonian landscape

Deforestation and fragmentation influence the microclimate, vegetation structure, and composition of remaining patches of tropical forest. In the southern Amazon, at the frontier of cropland expansion, forests are converted and fragmented in a pattern that leaves standing riparian forests whose dimensions are mandated by the Brazilian National Forest Code. These altered riparian forests share many characteristics of well-studied upland forest fragments, but differ because they remain connected to larger areas of forest downstream, and because they may experience wetter soil conditions because reduction of forest cover in the surrounding watershed raises groundwater levels and increases stream runoff. We compared forest regeneration, structure, composition, and diversity in four areas of intact riparian forest and four areas each of narrow, medium, and wide altered riparian forests that have been surrounded by agriculture since the early 1980s. We found that seedling abundance was reduced by as much as 64% and sapling abundance was reduced by as much as 67% in altered compared to intact riparian forests. The most pronounced differences between altered and intact forest occurred near forest edges and within the narrowest sections of altered riparian forests. Woody plant species composition differed and diversity was reduced in altered forests compared to intact riparian forests. However, despite being fragmented for several decades, large woody plant biomass and carbon storage, the number of live or dead large woody plants, mortality rates, and the size distribution of woody plants did not differ significantly between altered and intact riparian forests. Thus, even in these relatively narrow forests with high edge : area ratios, we saw no evidence of the increases in mortality and declines in biomass that have been found in other tropical forest fragment studies. However, because of the changes in both species community and reduced regeneration, it is unclear how long this relative lack of change will be sustained. Additionally, Brazil recently passed a law in their National Forest Code allowing narrower riparian buffers than those studied here in restored areas, which could affect their long-term sustainability.

森林砍伐与生境片段化会影响热带森林剩余斑块的微气候、植被结构与物种组成。处于农田扩张前沿的亚马孙南部地区，森林被转化并发生生境片段化，留存下受巴西国家森林法(Brazilian National Forest Code)规定尺寸的河岸林(riparian forest)。这类经改造的河岸林与已有充分研究的旱地森林片段具有诸多共性，但存在两处显著差异：其一，它们仍与下游更大面积的森林相连；其二，由于周边流域森林覆被减少导致地下水位上升、溪流径流量增加，其土壤湿度可能更高。我们对比了4片完整河岸林，以及各4片窄幅、中幅、宽幅的经改造河岸林——这些经改造河岸林自20世纪80年代初起便被农田环绕。研究发现，相较于完整河岸林，经改造河岸林的幼苗丰度最多下降64%，幼树丰度最多下降67%。经改造与完整河岸林间最显著的差异出现在林缘附近以及经改造河岸林的最窄区段。经改造河岸林的木本植物物种组成存在差异，物种多样性亦有所降低。然而，尽管已片段化数十年，经改造与完整河岸林间的大型木本植物生物量与碳储量、存活或死亡大型木本植物的数量、死亡率以及木本植物的大小分布均无显著差异。因此，即便在这些边缘面积占比颇高的相对狭窄林带中，我们也未发现其他热带森林片段研究中所报道的死亡率上升与生物量下降现象。但由于物种群落结构与更新过程均发生改变，目前尚不清楚这种相对稳定的状态还能维持多久。此外，巴西近期在其国家森林法中通过了一项法案，允许在修复区域采用比本研究中更窄的河岸缓冲带，这可能会对这些林带的长期可持续性产生影响。