Global warming likely to enhance black locust (Robinia pseudoacacia L.) growth in a Mediterranean riparian forest
Abstract
Mediterranean riparian forests are comparably humid environments that provide shelter for several broadleaved deciduous tree species at their southernmost distribution margin. The stability of these communities, however, is threatened by climate change as well as invasive tree species, such as black locust (Robinia pseudoacacia L.). So far, black locust's European distribution appears to be mostly limited by low temperatures, but global warming might enhance its growth in colder areas. Moreover, R. pseudoacacia can better access water from the phreatic level than some native non-phreatophytic tree species such as European ash (Fraxinus excelsior L.). In this study, we compare the performance of European ash, a native deciduous tree species at its southernmost distribution border, with the invasive black locust, under a range of climate change projections, in a stand located at N.E. Spain. We first use Bayesian inference to calibrate the GOTILWA + vegetation model against sap flow data for both tree species. We then project each tree species’ performance under several climate change scenarios. Our results indicate that increasing temperatures will enlarge black locust's vegetative period, leading to substantially increased annual productivity if the phreatic water table keeps reachable. For European ash, we project a slight increase in productivity, but with higher uncertainty. Our findings suggest that black locust will profit more from global warming than the native European ash, which is concerning because of the already detrimental impact of black locust for the local ecosystems. We conclude that climate change has the potential to stimulate black locust growth on Mediterranean riparian forests. Forest management should therefore include mechanisms to avoid black locust establishment, such as avoid clear-cutting and maintaining closed riparian forest canopies.
What the paper shows and why it matters
(AI-generated)
Climate change and the arrival of non-native species are two pressures on forests that can reinforce each other, and the paper takes Mediterranean riparian forests as an example. There, native trees such as European ash find shelter at the southern edge of their range, while the invasive black locust, which appears to be held back in Europe mainly by cold, can also draw on groundwater. Warming could favour the invader. Earlier projections relied on statistical distribution models, which the paper describes as limited for changing conditions and for climates without a present-day equivalent, while process-based models lacked reliable estimates of black locust’s water use and growing-season timing. What is missing is a mechanistic comparison, with quantified uncertainty, of invader and native under future climates.
The paper argues that, in a riparian forest stand in north-eastern Spain, the invasive black locust would profit more from climate change than native European ash, because, under warming and rising CO2, its growing season lengthens and its daily carbon uptake rises. The study fitted a process-based forest model (GOTILWA+) to sap-flow measurements, which record water moving through tree stems, from both species. Bayesian inference, which yields ranges of plausible parameter values, made the projections’ uncertainty explicit. Under a baseline of constant climate with no further forcing, ash is the more productive species. In 2071–2100 the gain is larger for black locust: gross primary production (the carbon fixed by photosynthesis) under a moderate warming scenario is 27% above that baseline for black locust and 18% for ash. The ash projections are more uncertain, and the gains of black locust are likely to hold only if groundwater stays reachable.
The paper reads these results as evidence that climate change could raise the invasion potential of black locust in Mediterranean riparian forests. Because black locust is hard to remove once established, yet regenerates poorly under a closed canopy, the paper advises prevention: not planting it, and avoiding thinning or clear-cutting that opens gaps. It names its limits: one year of calibration data without a serious summer water deficit, productivity assumed to indicate how well a species does, and no modelling of interactions between the species. The paper therefore presents the results as pointing to better growth of black locust, with considerable uncertainty.