• Energy Research

AbstractTropical forests face increasing climate risk1,2, yet our ability to predict their response to climate change is limited by poor understanding of their resistance to water stress. Although xylem embolism resistance thresholds (for example, $$\varPsi $$ Ψ 50) and hydraulic safety margins (for example, HSM50) are important predictors of drought-induced mortality risk3–5, little is known about how these vary across Earth’s largest tropical forest. Here, we present a pan-Amazon, fully standardized hydraulic traits dataset and use it to assess regional variation in drought sensitivity and hydraulic trait ability to predict species distributions and long-term forest biomass accumulation. Parameters $$\varPsi $$ Ψ 50 and HSM50 vary markedly across the Amazon and are related to average long-term rainfall characteristics. Both $$\varPsi $$ Ψ 50 and HSM50 influence the biogeographical distribution of Amazon tree species. However, HSM50 was the only significant predictor of observed decadal-scale changes in forest biomass. Old-growth forests with wide HSM50 are gaining more biomass than are low HSM50 forests. We propose that this may be associated with a growth–mortality trade-off whereby trees in forests consisting of fast-growing species take greater hydraulic risks and face greater mortality risk. Moreover, in regions of more pronounced climatic change, we find evidence that forests are losing biomass, suggesting that species in these regions may be operating beyond their hydraulic limits. Continued climate change is likely to further reduce HSM50 in the Amazon6,7, with strong implications for the Amazon carbon sink.

Aim Climate change causes shifts in species distributions, or ‘migrations’. Despite the centrality of species distributions to biodiversity conservation, the demonstrated large migration of tropical plant species in response to climate change in the past, and the expected sensitivity of species distributions to modern climate change, no study has tested for modern species migrations in tropical plants. Here we conduct a first test of the hypothesis that increasing temperatures are causing tropical trees to migrate to cooler areas. Location Tropical Andes biodiversity hotspot, south-eastern Peru, South America. Methods We use data from repeated (2003/04–2007/08) censuses of 14 1-ha forest inventory plots spanning an elevational gradient from 950 to 3400 m in Manu National Park in south-eastern Peru, to characterize changes in the elevational distributions of 38 Andean tree genera. We also analyse changes in the genus-level composition of the inventory plots through time. Results We show that most tropical Andean tree genera shifted their mean distributions upslope over the study period and that the mean rate of migration is approximately 2.5–3.5 vertical metres upslope per year. Consistent with upward migrations we also find increasing abundances of tree genera previously distributed at lower elevations in the majority of study plots. Main conclusions These findings are in accord with the a priori hypothesis of upward shifts in species ranges due to elevated temperatures, and are potentially the first documented evidence of present-day climate-driven migrations in a tropical plant community. The observed mean rate of change is less than predicted from the temperature increases for the region, possibly due to the influence of changes in moisture or non-climatic factors such as substrate, species interactions, lags in tree community response and/or dispersal limitations. Whatever the cause(s), continued slower-than-expected migration of tropical Andean trees would indicate a limited ability to respond to increased temperatures, which may lead to increased extinction risks with further climate change.

AbstractTropical soils contain huge carbon stocks, which climate warming is projected to reduce by stimulating organic matter decomposition, creating a positive feedback that will promote further warming. Models predict that the loss of carbon from warming soils will be mediated by microbial physiology, but no empirical data are available on the response of soil carbon and microbial physiology to warming in tropical forests, which dominate the terrestrial carbon cycle. Here we show that warming caused a considerable loss of soil carbon that was enhanced by associated changes in microbial physiology. By translocating soils across a 3000 m elevation gradient in tropical forest, equivalent to a temperature change of ± 15 °C, we found that soil carbon declined over 5 years by 4% in response to each 1 °C increase in temperature. The total loss of carbon was related to its original quantity and lability, and was enhanced by changes in microbial physiology including increased microbial carbon‐use‐efficiency, shifts in community composition towards microbial taxa associated with warmer temperatures, and increased activity of hydrolytic enzymes. These findings suggest that microbial feedbacks will cause considerable loss of carbon from tropical forest soils in response to predicted climatic warming this century.

Andean tropical montane forests (TMF) are hotspots of biodiversity that provide fundamental hydrological services as well as carbon sequestration and storage. Agroforestry systems occupy large areas in the Andes but climatic pressures, market volatility and diseases may result inagroforest abandonment, promoting secondary succession. Secondary forests are well-adapted and efficient carbon sinks whose conservation is vital to mitigate and adapt to climate change and to support biodiversity. Little is known, however, about how secondary TMF recover their aboveground biomass (AGB) and composition after abandonment. We established a 1.5 ha plot at 1780 masl on a 30-year old abandoned agroforest and compared it against two control forest plots at similar elevations. Agroforestry legacies influenced AGB leading to far lower stocks (42.3 ± 5.4–59.6 ± 7.9 Mg ha−1 using allometric equations) than those expected after 30 years (106 ± 33 Mg ha−1) based on IPCC standard growth rates for secondary montane forests. This suggests a regional overestimation of mitigation potentials when using IPCC standards. Satellite-derived AGB largely overestimated our plot values (179 ± 27.3 Mg ha−1). Secondary growth rates (1.41–2.0 Mg ha−1 yr−1 for DBH ≥ 10 cm) indicate recovery times of ca. 69 to 97 years to reach average control AGB values (137 ± 12.3 Mg ha−1). This is 26 years above the average residence time of montane forests at our elevation (71 ± 1.91 years) suggesting a non-recovery or far slower recovery to control AGB values. Three variables appear to define this outcome compared to the control plots: lower DBH (15.8 ± 5.9 cm vs 19.8 ± 11.0 cm), lower basal area (12.67 ± 0.7 vs 28.03 ± 1.5 m2 ha−1) and higher abundance of lighter-wood tree genera (0.46 ± 0.10 vs 0.57 ± 0.11 gr cm3) such as Inga, a common shade-tree in Andean agroforests. With 3.2 million hectares committed to restoration, Peru needs to target currently neglected TMF recovery schemes to support biodiversity, water and carbon storage and fulfill its international commitments.

AbstractTerrestrial biogeochemical feedbacks to the climate are strongly modulated by the temperature response of soil microorganisms. Tropical forests, in particular, exert a major influence on global climate because they are the most productive terrestrial ecosystem. We used an elevation gradient across tropical forest in the Andes (a gradient of 20°C mean annual temperature, MAT), to test whether soil bacterial and fungal community growth responses are adapted to long‐term temperature differences. We evaluated the temperature dependency of soil bacterial and fungal growth using the leucine‐ and acetate‐incorporation methods, respectively, and determined indices for the temperature response of growth: Q10 (temperature sensitivity over a given 10oC range) and Tmin (the minimum temperature for growth). For both bacterial and fungal communities, increased MAT (decreased elevation) resulted in increases in Q10 and Tmin of growth. Across a MAT range from 6°C to 26°C, the Q10 and Tmin varied for bacterial growth (Q10–20 = 2.4 to 3.5; Tmin = −8°C to −1.5°C) and fungal growth (Q10–20 = 2.6 to 3.6; Tmin = −6°C to −1°C). Thus, bacteria and fungi did not differ significantly in their growth temperature responses with changes in MAT. Our findings indicate that across natural temperature gradients, each increase in MAT by 1°C results in increases in Tmin of microbial growth by approximately 0.3°C and Q10–20 by 0.05, consistent with long‐term temperature adaptation of soil microbial communities. A 2°C warming would increase microbial activity across a MAT gradient of 6°C to 26°C by 28% to 15%, respectively, and temperature adaptation of microbial communities would further increase activity by 1.2% to 0.3%. The impact of warming on microbial activity, and the related impact on soil carbon cycling, is thus greater in regions with lower MAT. These results can be used to predict future changes in the temperature response of microbial activity over different levels of warming and over large temperature ranges, extending to tropical regions.

AbstractUnderstanding the relationship between photosynthesis, net primary productivity and growth in forest ecosystems is key to understanding how these ecosystems will respond to global anthropogenic change, yet the linkages among these components are rarely explored in detail. We provide the first comprehensive description of the productivity, respiration and carbon allocation of contrasting lowland Amazonian forests spanning gradients in seasonal water deficit and soil fertility. Using the largest data set assembled to date, ten sites in three countries all studied with a standardized methodology, we find that (i) gross primary productivity (GPP) has a simple relationship with seasonal water deficit, but that (ii) site‐to‐site variations in GPP have little power in explaining site‐to‐site spatial variations in net primary productivity (NPP) or growth because of concomitant changes in carbon use efficiency (CUE), and conversely, the woody growth rate of a tropical forest is a very poor proxy for its productivity. Moreover, (iii) spatial patterns of biomass are much more driven by patterns of residence times (i.e. tree mortality rates) than by spatial variation in productivity or tree growth. Current theory and models of tropical forest carbon cycling under projected scenarios of global atmospheric change can benefit from advancing beyond a focus on GPP. By improving our understanding of poorly understood processes such as CUE, NPP allocation and biomass turnover times, we can provide more complete and mechanistic approaches to linking climate and tropical forest carbon cycling.

SignificanceIdentifying species assemblages that boost the provision of multiple ecosystem functions simultaneously (multifunctionality) is crucial to undertake effective restoration actions aiming at simultaneously promoting biodiversity and high multifunctionality in a changing world. By disentangling the effect of multiple traits on multifunctionality in a litter decomposition experiment, we show that it is possible to identify the assemblages that boost multifunctionality across multiple species mixtures originating from six biomes. We found that higher evenness among dissimilar species and the functional attributes of rare species as key biodiversity attributes to enhance multifunctionality and to reduce the abundance of plant pathogens. Our study identifies those species assemblages needed to simultaneously maximize multifunctionality and limit plant disease risks in natural and managed ecosystems.