384 Arctic Biodiversity Assessment Wild and semi-domestic herds of Rangifer (caribou or reindeer) are almost omnipresent in Arctic tundra. Rangifer management is an important issue, as the herds exert a number of important controls on the Arctic terrestrial ecosystem through their effects on vegetation and carnivore populations, as well as providing essential ecosystem goods to indigenous people. Photo: Susan Morse. 385 Chapter 12 Terrestrial Ecosystems Lead Authors Rolf A. Ims and Dorothee Ehrich Key Contributing Authors Bruce C. Forbes, Brian Huntley, Donald A. Walker and Philip A. Wookey Contributing Authors Dominique Berteaux, Uma S. Bhatt, Kari A. Bråthen, Mary E. Edwards, Howard E. Epstein, Mads C. Forchhammer, Eva Fuglei, Gilles Gauthier, Scott Gilbert, Maria Leung, Irina E. Menyushina, Nikita Ovsyanikov, Eric Post, Martha K. Raynolds, Donald G. Reid, Niels M. Schmidt, Audun Stien, Olga I. Sumina and Rene van der Wal Contents Summary ..............................................................386 12.4. Key findings .....................................................424 12.1. Introduction .....................................................386 12.4.1. How the structure and functioning of tundra ecosystems are determined ............................................424 12.2. Ecosystem structure, processes and functions �������������������387 12.4.1.1. Abiotic controls on ecosystem structure ............424 12.2.1. Ecosystem structure .......................................387 12.4.1.2. Biotic processes shaping biodiversity and 12.2.1.1. Circumpolar-scale variation ������������������������389 tundra ecosystem functioning ����������������������424 12.2.1.2. Regional- to local-scale variation ��������������������395 12.4.2. Trends attributed to drivers of change ......................425 12.2.1.3. Hot spots of diversity ...............................399 12.4.2.1. Climate change ....................................425 12.2.2. Ecosystem processes and functions ........................399 12.4.2.2. Land-use, resource management and 12.2.2.1. Food web interactions .............................400 industrial development ............................426 12.2.2.2. Ecosystem functions ...............................404 12.5. Conclusions and recommendations ..............................426 12.2.2.3. Linking trophic interactions and 12.5.1. Status and trends: Implications for the future ...............426 ecosystem function ................................408 12.5.2. Conservation and management actions ....................427 12.3. Trends ...........................................................409 12.5.3. Research and monitoring ...................................427 12.3.1. Historical context ..........................................409 12.5.3.1. Needs for area- and ecosystem- 12.3.1.1. Environmental history and paleogeography ........409 representative measurements ����������������������427 12.3.1.2. Paleodiversity of Arctic terrestrial ecosystems .......409 12.5.3.2. Needs for ecosystem-based approaches ............428 12.3.2. Contemporary trends in drivers of change ..................411 Acknowledgements ....................................................428 12.3.2.1. Climate change ....................................411 12.3.2.2. Land-use, natural resource management and References .............................................................428 industrial development ............................412 List of acronyms .......................................................440 12.3.3. Contemporary trends in ecosystem structure and function . 413 12.3.3.1. Climate change and trends in vegetation ...........413 12.3.3.2. Climate change and phenology ��������������������417 12.3.3.3. Cascading impacts of trends in keystone animal species .....................................419 12.3.3.4. Trends related to land-use changes �����������������421 12.3.3.5. Impact of industrial development ������������������423 It has progressively become warmer. I recall that only » in our traditional area did the trees occur, but when I returned there via plane last year, a lot more of the tun- dra was inundated with trees, small mind you, but they have moved north and east. The area we used to inhabit has been overgrown with vegetation, mainly shrubs and small trees. It has become almost like a mini-forest where we used to have our main camp. We visited the site in 2000 and it was almost unrecognizable due to all of the growth that occurred during our absence. I think this is due to a shorter spring, a longer summer and longer frost free falls. Utok; Elders Conference on Climate Change 2001. 386 Arctic Biodiversity Assessment SUMMARY Recommended actions to conserve Arctic terrestrial ecosystems under the impacts of climatic change and The Arctic tundra biome is geographically restricted to other anthropogenic stressors include conservation a strip around the margins of the Arctic Ocean. A key of topographically diverse areas with landscape-scale force determining the tundra biome’s zonal structure is ‘buffer-capacity’ to maintain cold refuges in a warmer the bottom-up effect of decreased vegetation productiv- climate and of remote high Arctic islands that are the ity and complexity with increasing latitude. Accord- most physically protected from species invasions from ingly, there are trends of decreasing diversity within and the south and human presence. Prudent management of among trophic guilds of consumers with increasing lati- Arctic herbivores such as reindeer Rangifer tarandus, using tudes. Low food web complexity in the northern parts their capacity for shaping vegetation on landscape scales, of the biome is also due to island biogeographic features, may be considered for counteracting encroachment of as large parts of the high Arctic are located on islands. tall woody vegetation that otherwise will eliminate Similarly, a substantial proportion of the high biodiver- tundra habitats, while avoiding the negative impacts of sity of low Arctic zones stems from ‘spillover effects’ herbivore overabundance that have been documented in from sub-Arctic ecosystems. Historic processes have also some regions. contributed to shaping the current large-scale regional provinces in terms of Arctic species communities. At A key message from the present assessment is that essen- sub-regional scales the terrestrial Arctic harbors diverse tial attributes of terrestrial Arctic biodiversity, some of mosaics of communities that are structured by gradients which have global repercussions, are ultimately depend- and disturbances in climate, substrate, hydrology and ent on how interactions within ecological communi- cryosphere that form unique patterns of within – and ties and trophic webs are impacted by rapidly changing among – community diversity. Hot spots of high regional external drivers. Consequently, research, monitoring diversity are currently found in some old, topographi- and management ought to be properly ecosystem-based. cally and geologically complex regions. Because ecosystems are structurally and functionally heterogeneous across the tundra biome and may also be The architecture of tundra food webs is modulated by subjected to external drivers of different strengths, new inter-specific interactions within and between trophic ecosystem-based observatories that include state-of-the levels. Herbivores can regionally exert strong top-down art research, often combined with adaptive manage- controls on tundra vegetation, whereas predators often ment, should be widely distributed across the circum- control small mammal herbivores and the reproductive polar Arctic. Model-based predictions about how Arctic success of ground nesting birds. Multi-annual, cascading species and ecosystems will respond to the substantial bottom-up and top-down interaction cycles mediated climate change currently projected for the Arctic have by lemming populations are crucial for the maintenance limited powers to accommodate surprises in terms of of terrestrial Arctic biodiversity in many tundra eco- novel climates and ecosystems that may rapidly emerge. systems. Functional traits of plants in interactions with New efforts urgently need to be deployed to enable well- below-ground microbial communities and herbivores designed real-time observations as a basis for empirically maintain essential roles in the regulation of the global based documentation and understanding of cause-effect climate system through controls on fluxes of greenhouse relationships of future ecosystem changes in the terres- gasses (GHG) and heat fluxes between the earth surface trial Arctic. and the atmosphere. Changes to the composition of ter- restrial biodiversity may determine whether the Arctic will become a source or a sink for GHGs in a warming 12.1. INTRODUCTION climate. The Arctic tundra biome is characterized by low-grow- Climate is historically and currently the most import- ing vegetation composed of low shrubs, sedges, grasses, ant driver of change of Arctic terrestrial ecosystems, forbs, lichens and mosses (bryophytes) that grow beyond through alteration of coastal sea ice, glaciers, snow and the northern climatic limit of trees (see Section 2 in permafrost, changed seasonality and extreme events. Meltofte et al., Introduction for this assessment’s defini- At present, a second emerging driver is an increased tion of the Arctic). A polar view of the biome from space footprint of human presence within the Arctic. Current- reveals that the continental portion of the Arctic tundra ly, the