Effects of widespread non-native trees on regulating ecosystem services

Tree taxa are often planted beyond their native range to increase the provision of some ecosystem services. Yet, they can disrupt ecosystem processes in their new ranges, causing changes in the provision of other services. Here we review the effects of five widespread tree taxa (Acacia, Ailanthus, Eucalyptus, Pinus and Robinia) on six regulating ecosystem services in areas where they are non-native. We conducted a literature search for pair-wise comparisons between sites dominated by any of the selected taxa and sites with native vegetation. An array of variables were used as indicators for each ecosystem service. Data were analysed using multi-level meta-analyses to compare effects of taxa on each ecosystem service, and effects of the same taxa across contexts. We compiled 857 case studies from 107 source papers. Several taxa tended to increase climate regulation, mostly Eucalyptus. Acacia decreased fire risk prevention. Robinia, Acacia and Ailanthus increased soil fertility, while Eucalyptus and Pinus, tended to decrease it. Soil formation was enhanced by Robinia and Ailanthus. Acacia promoted the increase of water in land pools, while Eucalyptus tended to decrease them. All effects show a large heterogeneity across case studies. Part of this heterogeneity could be attributed to gross climatic differences (i.e. biome), to species differences within each genus, to the structure of the recipient ecosystem, and/or to human management. Managers and policy-makers should consider the context-dependency and the potential effects of non-native trees on a wide range of services to ground their decisions. Our analyses also revealed important gaps of knowledge (e.g. on fire risk prevention, erosion control or water cycle regulation) and some potential publication bias. The methodology used here easily allows for future updates as new information will become available.

Biological Control of Salvinia molesta (D.S. Mitchell) Drives Aquatic Ecosystem Recovery

Salvinia molesta D.S. Mitchell (Salviniaceae) is a damaging free-floating invasive alien macrophyte native to South America. The biological control programme against S. molesta by the weevil Cyrtobagous salviniae Calder and Sands (Erirhinidae) has been successful in controlling S. molesta infestations in the introduced range, however, there is some debate as to how biological control success is measured. This study measured the response of epilithic algae and aquatic macroinvertebrate communities in a S. molesta-dominated state and subsequently where the weed had been cleared by biological control, as a proxy for ecosystem recovery in a before–after control–impact mesocosm experiment. The restored treatment (S. molesta and C. salviniae) demonstrated epilithic algae and aquatic macroinvertebrate recovery during the “after” biological control phase, defined as similar to the control treatment. Comparatively, the impacted treatment (100% S. molesta) showed a drastic decline in biodiversity and shifts in community assemblages. We conclude that the biological control effort by C. salviniae facilitated biodiversity recovery of the impacted treatment. Furthermore, epilithic algae and aquatic macroinvertebrate communities were reliable biological indicators for measuring ecological impacts of invasion and ecosystem recovery following biological control, and thus represent potential tools for evaluating biological control success and ecological restoration.

Global effects of non-native tree species on multiple ecosystem services

Non-native tree (NNT) species have been transported worldwide to create or enhance services that are fundamental for human well-being, such as timber provision, erosion control or ornamental value; yet NNTs can also produce undesired effects, such as fire proneness or pollen allergenicity. Despite the variety of effects that NNTs have on multiple ecosystem services, a global quantitative assessment of their costs and benefits is still lacking. Such information is critical for decision-making, management and sustainable exploitation of NNTs. We present here a global assessment of NNT effects on the three main categories of ecosystem services, including regulating (RES), provisioning (PES) and cultural services (CES), and on an ecosystem disservice (EDS), i.e. pollen allergenicity. By searching the scientific literature, country forestry reports, and social media, we compiled a global data set of 1683 case studies from over 125 NNT species, covering 44 countries, all continents but Antarctica, and seven biomes. Using different meta-analysis techniques, we found that, while NNTs increase most RES (e.g. climate regulation, soil erosion control, fertility and formation), they decrease PES (e.g. NNTs contribute less than native trees to global timber provision). Also, they have different effects on CES (e.g. increase aesthetic values but decrease scientific interest), and no effect on the EDS considered. NNT effects on each ecosystem (dis)service showed a strong context dependency, varying across NNT types, biomes and socio-economic conditions. For instance, some RES are increased more by NNTs able to fix atmospheric nitrogen, and when the ecosystem is located in low-latitude biomes; some CES are increased more by NNTs in less-wealthy countries or in countries with higher gross domestic products. The effects of NNTs on several ecosystem (dis)services exhibited some synergies (e.g. among soil fertility, soil formation and climate regulation or between aesthetic values and pollen allergenicity), but also trade-offs (e.g. between fire regulation and soil erosion control). Our analyses provide a quantitative understanding of the complex synergies, trade-offs and context dependencies involved for the effects of NNTs that is essential for attaining a sustained provision of ecosystem services.