Data from: 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.

森林砍伐与片段化会影响热带森林剩余斑块的微气候、植被结构与群落组成。在农田扩张前沿的亚马孙南部地区，森林被转化并片段化，仅保留由巴西《国家森林法》规定尺寸的河岸森林（riparian forest）。此类受干扰的河岸森林与已有大量研究的旱地森林片段具有诸多相似特征，但存在两点差异：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盖率降低会抬升地下水位并增加溪流径流量，其土壤湿度可能更高。本研究选取4处未受干扰的河岸森林，以及自20世纪80年代初起被农田环绕的窄、中、宽三类受干扰河岸森林（每类各4处），对其森林更新、结构、群落组成与多样性进行了对比分析。研究发现，相较于未受干扰的河岸森林，受干扰河岸森林的幼苗丰度最多降低64%，幼树丰度最多降低67%。受干扰与未受干扰森林间的差异最为显著之处位于森林边缘，以及受干扰河岸森林的最窄区段内。与未受干扰的河岸森林相比，受干扰森林的木本植物群落组成存在差异，且物种多样性有所降低。然而，尽管已被片段化数十年，受干扰与未受干扰河岸森林在大型木本植物生物量与碳储量、存活/死亡大型木本植物数量、死亡率以及木本植物径级分布上均无显著差异。因此，即便在这些边缘面积比偏高的相对狭窄的森林中，我们并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。然而，由于物种群落组成发生改变且更新过程受损，目前尚不清楚这种相对稳定的状态能够维持多久。此外，巴西近期修订了《国家森林法》，允许在修复区域设置比本研究中更窄的河岸缓冲带，这可能会对这些河岸森林的长期可持续性造成影响。