Effects of scattered trees on pond ecosystems: experimental evidence from a biodiversity hotspot

Human-driven landscape modifications threaten pond ecosystems worldwide. The conversion of landscapes from forests to pastures can have significant impacts on pond biodiversity and ecosystem processes. Scattered trees are common elements in deforested landscapes and represent keystone structures because they increase biodiversity. Moreover, scattered trees are often associated with ponds, potentially affecting their structure and function. Here, we experimentally investigated how pond ecosystems respond to different landscape elements, including scattered trees. We built an experimental set of 12 ponds in a pasture area with three treatments: ponds near the edge of a continuous forest, those under the canopy of a scattered tree and those in the open pasture. To test the effects of the treatments on the biotic and abiotic characteristics of the ponds, we quantified water temperature, oxygen, dissolved nutrients (NO₃⁻, NH₄⁺, and PO₄³⁻), turbidity, conductivity, allochthonous organic material and phytoplanktonic chlorophyll. Scattered-tree ponds presented relatively high concentrations of dissolved NH₄⁺ and PO₄³⁻, as well as relatively high levels of phytoplanktonic chlorophyll. Edge ponds presented relatively high levels of allochthonous organic material and conductivity, whereas open-pasture ponds presented relatively high temperatures, oxygen concentrations and NO₃⁻ concentrations. Our results provide evidence that landscape elements can have diverse effects on pond ecosystems. Although we expected scattered-tree ponds to resemble edge ponds by buffering the impact of the pasture, the presence of a single scattered tree created a distinct and novel environment. Scattered trees associated with ponds thus represent important elements that contribute to increasing ecological heterogeneity in deforested landscapes.