Diversity and productivity in forest ecosystems: a dynamic perspective

Transcription

1 Diversity and productivity in forest ecosystems: a dynamic perspective Han Y. H. Chen, Brian B. Brassard, Zhiyou Yuan, Peter B. Reich, Xavier Cavard, Jerome Laganiere, Yves Bergeron, and David Pare Faculty of Natural Resources Management Lakehead University

2 Why understanding diversity function relationships is important? Continuous loss of biodiversity at the global scale Earth loses more than three-quarters of its species in a geologically short interval, as has happened only five times in the past 540 million years a sixth mass extinction may be under way, given the known species losses over the past few centuries and millennia Barnosky et al. 2011, Nature 471:51-57 Ecosystem function/productivity is an essential service to humanity Biodiversity and ecosystem function (BEF) is the most prolific field for the past 20 years in ecology

3 Fundamental question in BEF Agriculture Forestry Urbanization Drought Desertization How loss of the world's biodiversity might alter ecological processes such as net primary production, decomposition, nutrient cycling, and outbreaks of insects and diseases?

4 The original hypothesis Darwin s (1859): the presence of a divergence of characters is essential for reduced interspecific competition as a result of different demands for resources, and in turn, improves productivity Tilman et al. (1997) Science 277:

5 Diversity and productivity relationships Despite being the most published topic in ecology Debate persists about diversity effects in natural vs. planted systems (Adler et al. 2011, Science 333, ) Evenness Heterogeneity of life-history traits The impacts of these factors on DPRs in forest ecosystems are more poorly understood

6 . Hypotheses Richness and evenness effects Does evenness explain independent diversity effects? The extent of life-history variation among constituent species Can contrasting shade tolerance, growth habit, and nitrogen-fixing result in complementarity? Biomes Possible evolutionary differences? Stand origin (plantation versus natural), Better realization of niche differentiation in more heterogeneous natural environments? Stand age Diversity effects increase with stand age (Cardinale et al. 2007; Cavard et al. 2011)

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

8 Net diversity effect, effect size (ES) ES ij P M ij i P ij = the observed productivity of the jth observation in the ith study M i = the mean productivity of monocultures Evenness J ' H ' ln( S) H = observed Shannon s index S = species richness Pielou (1969)

9 Trait based approach Contrasting shade tolerance Yachi & Loreau (2007) Contrasting nitrogen-fixing Fast-slow growth 9

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

11 Global average effect of diversity Ln(ES) = 0.213, i.e., ES = 1.24, indicating at a global scale, the polycultures have 24% higher productivity than monocultures

12 Predicted ln(es) Predicted ln(es) a b c % 34% 15% Richness Evenness Stand age (years) 0.1 Predicted ln(es) d 29% Absence Presence e Absence 0.3 <3% <2% Presence Absence Presence f Contrasting shade tolerance Contrasting N-fixation Contrasting growth habit Predicted ln(es) g 0.3 <2% <2% Bo Te Tr N P h Biome Stand origin

13 Conclusion Globally, productivity is 24% higher in polycultures than monocultures, critical role of Species evenness Contrasting shade tolerance Stand development Implications: Conserving species (richness and evenness) and trait diversity improves ecosystem functions Diversity effects are more pronounced in old forests Motivate future studies to link richness, evenness, contrasting traits, and life-history stage to mechanisms

14 Known mechanisms for positive DPRs Niche differentiation/partitioning and/or facilitation Grasslands (Tilman and many others) Algae in fresh water systems (Cardinale BJ, Biodiversity improves water quality through niche partitioning, Nature 472, 86-89) Reduced Janzen Connell effects Positive diversity effects on productivity are realized by reduced plant disease (Schnitzer et al. 2011, Ecology 92, )

15 Diversity effects in belowground Few studies examined diversity effects on belowground productivity of forest ecosystems (although belowground accounts for 50% of total NPP; fine roots, <2 mm in diameter alone accounts for 33% of total NPP) Potential mechanisms are poorly understood

16 Methods Pb+Late-successional Picea mariana, Picea glauca, and Abies balsamea (LSC) Functionally different (deep- vs. shallow-rooted conifers) Pinus banksiana (Pb) Populus tremuloides (Pt) Pb+Pt Phylogenetically different (gymnosperm vs. angiosperm)

17 Fine root production Fine root production (Mg ha -1 year -1 ) Pb +LSC Pb Pt Pb+Pt Ingrowth MaxMin MaxMin Matrix 19% to 83% higher in mixed-species stands than single-species stands

18 Seasonal biomass variation Fine root biomass (Mg ha -1 ) May June July August September October 0 Pb Pt Pb+LSC Pb+Pt

19 Vertical distribution of fine roots Pb+LSC Pb Pt Pb+Pt FF MS1 MS2 May FF MS1 June MS2 FF MS1 July MS2 FF MS1 MS2 Aug FF MS1 Sept MS2 FF MS1 Oct MS Fine root biomass and necromass (Mg ha -1 ) Pinus banksiana Non-tree Picea spp. and Abies balsamea Necromass Populus tremuloides and Betula papyrifera

20 Relationships between fine root biomass and heterogeneity Fine root biomass (Mg ha -1 ) a May June July August September October Fine root biomass (Mg ha -1 ) b Pb Pt Pb+LSC Pb+Pt Horizontal heterogeneity Vertical heterogeneity = 1 SD of biomass among 7 cores = 1 SD of biomass among 3 layers

21 Conclusion Higher belowground productivity in mixed than most productive single-species stands Belowground in under-utilized in pure forests? Increased niche utilization horizontally and vertically by different resource foraging behaviours of the component species as well as variations within species

22 Future perspectives 1) Can diversity effects be stronger when ecosystems are under stress such as climate change drivers (increasing drought or insect infestation)? Diversity-stability hypothesis 2) How does higher ecosystem production fueled by tree diversity affect belowground diversity? Cross-taxon congruence

23 Acknowledgements Funding NSERC Discovery and Strategic Programs Sustainable Forest Management Network Ontario Early Research Award program Northern Ontario Heritage Fund Cooperation Partners and collaborators Abitibi-Bowater Inc., Paul Poschmann, John MacGillivray, & Steve Watson Tembec Inc., George Brummer, Sue Pickering, Sonia Legare, & Al Thorne Louisiana-Pacific Canada Ltd., Margaret Donnelly, Donna Grassia, & Paul LeBlanc Forest Ecosystem Science Co-operative Inc., Dianne Miller & Dave Wood Lake Abitibi Model Forest, Wally Bidwell Government of Alberta, Ken Greenway Government of Manitoba, Phil Keenan Government of Ontario, Ed Iwachewski, Gordon Kayahara, Stan Vasiliaukas Government of Québec Canadian Forest Service, Natural Resources Canada