Diversity and function of terrestrial ecosystems under global changes Han Y. H. Chen

Research in Chen’s lab Global changes Function Diversity loss Climate change Disturbances Global changes Function Biomass & element cycling Diversity loss Plants

Rockström et al. 2009. A safe operating space for humanity Rockström et al. 2009. A safe operating space for humanity. Nature 461:472

Barnosky et al. Nature 471, 51-57 (2011) doi:10.1038/nature09678 Relationship between extinction rates and the time interval for mammals Five mass species extinctions occurred in the past 540 million years Entering 6th mass species extinction Barnosky et al. Nature 471, 51-57 (2011) doi:10.1038/nature09678

Humans and the extinction crisis Urbanization Humans and the extinction crisis Agriculture Forestry Climate change Pollution

Global threats to biodiversity C. J. Vörösmarty, Global threats to human water security and river biodiversity. Nature  47, 555–561 (2010)

Biodiversity (loss) ecological functioning (BEF) Net primary production Nutrient cycling Trophic interactions Insect and pathogen out breaks

The original hypothesis The presence of a “divergence of characters” reduces competition as a result of different demands for resources, and consequently improves productivity

The “First” empirical evidence Cedar Creek experiment Tilman et al. (1997) Science 277:1300-1302

Diversity and productivity relationships Debate persists Natural vs. planted grasslands (Adler et al. 2011, Science 333, 1750; Fraser et al. 2015, Science 349, 302) Evenness Heterogeneity of life-history traits Poorly understood for forests

Hypotheses . Richness & evenness The extent of life-history variation Biomes: competitive exclusion vs positive interactions (niche differentiation, facilitation) Stand origin: planted vs natural systems Stand age .

13 22 Tropical Boreal Meta-data Temperate 19 Each selected original study was designed to test diversity effects, i.e., similar sites and disturbance history

Net diversity effect (ES) Pij = Productivity in mixtures j = observation, i = study = the mean productivity of monocultures of ith study Evenness H’ = observed Shannon’s index S = species richness Pielou (1969)

Variation of life history traits Contrasting shade tolerance Yachi & Loreau (2007) Contrasting nitrogen-fixing Fast-slow growth

Global average effect of diversity 25% productivity increase Zhang, Chen & Reich, 2012. J Ecol 100:742-49

Statistical analysis Boosted regression trees De’ath 2007 Elith et al. 2008 Regression trees + boosting Machine learning Model averaging

Predicted ln(ES) Monotonic Non-monotonic 13% 34% 15% 29% <3% <2%

Mechanisms from DPR experiments Niche differentiation and/or facilitation Grasslands (Tilman and others) Algae in fresh water systems (Cardinale BJ, 2011. Nature 472, 86-89) Reduced Janzen–Connell effects Positive DPRs realized by reduced plant disease (Schnitzer et al. 2011, Ecology 92, 296-303)

Potential mechanisms are poorly understood in natural environments - Greater resource utilization spatially and temporally due to resource heterogeneity?

Fine root biomass Fine root production

Forest Grassland

Multivariate relationship--SEM

Plant species mixtures increase microbial biomass and respiration

The effect increases with the number of species in mixtures More pronounced over time

Summary-DPR Diversity increases productivity Both natural and planted systems Above- and belowground Strength of DPR increases over time Mechanisms in natural systems Increased tree size inequality for aboveground Increased soil volume filling and nutrient utilization Plant diversity increases Soil respiration Microbial biomass/abundance

Research in Chen’s lab Global changes Diversity Function Climate change Disturbances Global changes Diversity Plants Function Biomass & element cycling

IPCC 2014 Rising CO2 Warming

IPCC 2014

Global drought trends for past 60 years Sheffied et al. 2012. Nature 491: 435

Importance of understanding climate change impacts on forests A large and persistent carbon sink in the World’s forests via increasing biomass (Pan et al. 2011. Science) Uptake (2.4 Pg C year−1) = 35% of fossil fuel emission (7 Pg C year−1) Boreal forests account for 49% of global forest carbon (Dixon et al. 1994. Science)

Climate change and forests Studied 76 old-growth (>200 years old) stands Implications Reduced ecosystem function, carbon sink to source Forest compositional change Biome shifts

Studied 96 old stands (>80 years old)

Two underlying assumptions: Climate change effects are the same in young and old forests Endogenous effects on tree mortality in old forests are solely attributed to climate change Connell and Slatyer (1977), Am Nat 104:501-28 "We have found no example of a community of sexually reproducing individuals…… reached a steady-state equilibrium"

Others attributed temporal increases in mortality to stand development Luo & Chen. 2011. Journal of Ecology 99:1470-1480. Lutz & Halpern. 2006. Ecological Monographs 76:257-275. Thorpe & Daniels. 2012. Canadian Journal of Forest Research 42:1687-1696. Competition Negative density dependence Tree ageing “Unsuitable statistical methods that marginalize either climate or non-climate drivers for longitudinal data in which these drivers are highly correlated” (Brown et al. 2011. GCB: 17: 3697)

Bayesian models 887 permanent plots Measured from 1958 to 2007 Stand age ranges from 17 to 243 years old ~ a million records Bayesian models

Higher climate change-induced tree mortality in young than old forests Aging Competition NDD

Broadleaves Early-successional conifers Late-successional conifers

Broadleaves Early-successional conifers Late-successional conifers

Changes in plant N:P do not only affect the fitness of plants, but also the fitness and composition of herbivores

Global data 1,418 publications 24,770 observations

Plant biomass N : P responses to global change Natural (Controlled)

On-going Research in Chen’s lab Climate change Disturbances Global changes Global patterns Mechanisms Mitigation strategies Diversity Plants and others Function Biomass & element cycling

Acknowledgements