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The nitrogen-fixing mimosid Leucaena leucocephala continues to be used for afforestation in degraded tropical forests. Yet, fast-growth and high drought stress tolerance enables L. leucocephala to outperform native species and L. leucocephala has been identified as one of the 100 most invasive species globally. This warrants development of effective control measures, including bio-controls, to prevent the spread of this species particularly across tropical islands. Here, we compare differences in key functional traits between L. leucocephala and eight dominant native species ( Bridelia tomentosa, Radermachera frondosa, Lepisanthes rubiginosa, Rhaphiolepis indica, Pterospermum heterophyllum, Fissistigma oldhamii, Psychotria rubra and Cudrania cochinchinensis) in L. leucocephala invaded tropical forests of Hainan Island, China. Functional traits related to growth (photosynthesis rate, stomatal conductance and transpiration rate) and drought stress tolerance (leaf turgor loss point) were measured in wet and dry seasons to investigate whether these functional traits differed between L. leucocephala and the eight dominant native species. Our results demonstrate that L. leucocephala has significantly increased growth rates (at least two-fold) in both wet and dry seasons. Additionally, L. leucocephala shows significantly higher drought stress tolerance (lower TLP) in the dry season. These results indicate that L. leucocephala would almost certainly outperform the eight dominant native species and might successfully invade Hainan tropical forests. There is an urgent need to identify native species that have similar growth and drought stress tolerance traits to enable the development of effective strategies to control L. leucocephala on Hainan Island.

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Deforestation continues to be extensive in the tropics, resulting in reduced soil water content. Reforestation is an effective way to recover soil water content, but the recovery depends on the type of reforestation efforts that are implemented. Monoculture of fast-growing species is a common reforestation strategy, because it is an effective means of preventing landslides resulting from the frequent typhoons and heavy rains in the tropics and easy to implement. To quantify whether monoculture plantings can help recover soil water content, we initiated a reforestation project within a 0.2 km2 area of an extremely degraded tropical monsoon forest. We hypothesized that much higher transpiration rate of fast-growing tree species would deplete soil water more than the dominant slow-growing species in the adjacent secondary tropical rain forest during both wet and dry seasons, thereby resulting in much lower soil water content. To test this hypothesis, we compared transpiration rates and key functional traits that can distinguish transpiration rates between fast-growing and dominant slow-growing species in both wet and dry seasons. We also quantified whether soil water content around these species differed. We found that fast-growing species had transpiration rate and transpiration-related trait values that were 5–10 times greater than the dominant slow-growing species in both seasons. We also found that soil water content around dominant slow-growing species was 1.5–3 times greater than for fast-growing species in both seasons. Therefore, reforestation based on monoculture plantings of fast-growing species seems difficult to effectively recover the soil water content. We also provide a simple method for guiding the use of reforestation efforts to recover soil water content in extremely degraded tropical rain forests. We expect that this simple method can be an effective means to restore extremely degraded tropical rain forests in other parts of the world.

In experiments that test plant diversity-productivity relationships, the common practice of weeding unsown species and disallowing species colonization may have the unintended consequence of favoring priority effects that maintain niche complementarity in determining productivity. However, in naturally assembled communities where colonization occurs, resource competition may favor dominant traits, which eventually have the greatest influence on productivity. Here, in naturally developed long-term subalpine meadows (from 4-year to at least 40 years meadows) in the Qinghai-Tibetan Plateau, we investigated the relationships between species richness and productivity to testify whether positive diversity-productivity relationships can still exist in naturally developed long-term communities. We also measured five functional traits (specific leaf area, photosynthesis rate, leaf proline content, seed mass and seed germination rate) to calculate two functional diversity indices: community-weighted mean trait values (CWM) and Rao's quadratic entropy (RaoQ) which are highly correlated to functional traits of dominating species and variety of functional trait among all species. Finally, we quantified the relative contribution of species diversity, functional traits of dominating species and functional diversity among all species to productivity along the succession. We demonstrated strong positively diversity-productivity relationships in the natural sub-alpine meadow communities across time. The five traits of dominating species explained a large proportion (54-80%) of the variation in productivity during succession, whereas species diversity and functional diversity (FD) for each of the five traits explained much less (24-48% for species richness and 0-40% for FD for each of the five traits respectively). We found unequivocal evidence that significantly positive diversity-productivity relationships in the natural sub-alpine meadow communities across time are up to superior performers (dominant traits) in naturally developed communities where colonization occurs. As a result, understanding diversity-productivity relationships under the full range of community assembly processes therefore merits further investigation. Global biodiversity is declining sharply 1 with the potential to impair ecosystem functioning in the near future, but the mechanisms that connect biodiversity to ecosystem function are not well understood 2. In diversity-function relationships, the connection between plant diversity and productivity is considered particularly important 3. Although the classic diversity-productivity relationship is thought to be hump-shaped, with species richness highest at intermediate levels of productivity 4 , experimental studies have, however, mostly yielded linear positive diversity-productivity relationship 5-9. In long-term experiments, the positive diversity-productivity relationships