• Energy Research

Summary The relationship between species richness and ecosystem function, as measured by productivity or biomass, is of long‐standing theoretical and practical interest in ecology. This is especially true for forests, which represent a majority of global biomass, productivity and biodiversity. Here, we conduct an analysis of relationships between tree species richness, biomass and productivity in 25 forest plots of area 8–50 ha from across the world. The data were collected using standardized protocols, obviating the need to correct for methodological differences that plague many studies on this topic. We found that at very small spatial grains (0.04 ha) species richness was generally positively related to productivity and biomass within plots, with a doubling of species richness corresponding to an average 48% increase in productivity and 53% increase in biomass. At larger spatial grains (0.25 ha, 1 ha), results were mixed, with negative relationships becoming more common. The results were qualitatively similar but much weaker when we controlled for stem density: at the 0.04 ha spatial grain, a doubling of species richness corresponded to a 5% increase in productivity and 7% increase in biomass. Productivity and biomass were themselves almost always positively related at all spatial grains. Synthesis. This is the first cross‐site study of the effect of tree species richness on forest biomass and productivity that systematically varies spatial grain within a controlled methodology. The scale‐dependent results are consistent with theoretical models in which sampling effects and niche complementarity dominate at small scales, while environmental gradients drive patterns at large scales. Our study shows that the relationship of tree species richness with biomass and productivity changes qualitatively when moving from scales typical of forest surveys (0.04 ha) to slightly larger scales (0.25 and 1 ha). This needs to be recognized in forest conservation policy and management.

Seasonally dry tropical forests (SDTFs), a varied and extensive ecosystem type in the tropics, are characteristically adapted to seasonal water stress in zones of low rainfall. Land-use change, resource extraction, alien invasives, changes to the atmosphere, and changing fire and climatic regimes may have serious implications for the continued persistence of SDTFs. This paper assesses the extent to which SDTFs may be resilient in the face of these threats, considering their dynamics, community-level characteristics, and functional traits of constituent species. There is evidence that some SDTF biodiversity- and structure-related properties are resistant to low-to moderate-intensity disturbances and have the potential to recover after severe, even chronic, disturbances, at timescales in the order of decades. Although global SDTFs are, on average, not necessarily more resilient than moist tropical forests (MTFs), they may be more resilient to particular disturbances such as fires and drought. SDTFs are vulnerable to regime shifts and there is considerable uncertainty about their future under a changing climate and its interactions with other anthropogenic effects.

This discussion paper assesses the state of knowledge on tropical dry forests as it relates to CIFOR's strategy and identifies research opportunities that align with CIFOR's strategic goals. Over the past two decades, CIFOR has accumulated a substantial body of work on dry forests, with a particular focus on African dry forests. This paper is intended to build on that work, by gathering wider research from around the world, as CIFOR seeks to widen the geographic scope of its research on dry forests. The present assessment explores five themes: climate change mitigation and adaptation; food security and livelihoods; demand for energy; sustainable management of dry forests; and policies and institutional support for sustainable management. These themes emerged as priority areas during discussions on dry forest research priorities held at CIFOR's Dry Forests Symposium in South Africa in 2011. Research on these themes should be considered a priority, given the importance of dry forests to people and ecosystems around the world and the threats posed to them.

RESUMEN El bioma del bosque seco cubre extensas áreas de los trópicos globales. Sin embargo, la comprensión de estas formaciones forestales desde perspectivas tanto humanas como biofísicas varía ampliamente tanto geográficamente como en términos de disciplinariedad. Si bien se han puesto a disposición recursos considerables para la gestión sostenible de los bosques tropicales húmedos, ha habido una falta de atención sostenida comparable en sus equivalentes de bosques secos. Este número especial es un intento de proporcionar más información sobre el estado del conocimiento de los bosques secos globales e identificar brechas de investigación que podrían contribuir a su sostenibilidad a largo plazo, tanto para el bienestar humano como para la integridad ecológica. RÉSUMÉ Le biome de la forêt sèche couvre de vastes zones des tropiques mondiaux. Cependant, la compréhension de ces formations forestières du point de vue humain et biophysique varie considérablement à la fois géographiquement et en termes de discipline. Bien que des ressources considérables aient été mises à disposition pour la gestion durable des forêts tropicales humides, il y a eu un manque d'attention soutenue comparable sur leurs équivalents forestiers secs. Ce numéro spécial tente de fournir des informations supplémentaires sur l'état des connaissances sur les forêts sèches mondiales et d'identifier les lacunes en matière de recherche qui pourraient contribuer à leur durabilité à long terme, à la fois pour le bien-être humain et l'intégrité écologique. SUMMARY The dry forest biome covers extensive areas of the global tropics. However, the understanding of these forest formations from both human and biophysical perspectives varies widely both geographically and in terms of disciplinarity. While considerable resources have been made available for the sustainable management of the humid tropical forests, there has been a lack of comparable sustained attention on their dry forest equivalents. This special issue is an attempt to provide further insights into the state of the knowledge of global dry forests, and identify research gaps that could contribute to their long-term sustainability, both for human well-being and ecological integrity. ملخص تغطي المنطقة الأحيائية للغابات الجافة مساحات واسعة من المناطق الاستوائية العالمية. ومع ذلك، فإن فهم هذه التكوينات الحرجية من المنظورين البشري والفيزيائي الحيوي يختلف اختلافًا كبيرًا من الناحية الجغرافية ومن حيث الانضباط. في حين تم توفير موارد كبيرة للإدارة المستدامة للغابات المدارية الرطبة، كان هناك نقص في الاهتمام المستمر المماثل بمكافئات الغابات الجافة. هذه القضية الخاصة هي محاولة لتقديم مزيد من الأفكار حول حالة المعرفة بالغابات الجافة العالمية، وتحديد الفجوات البحثية التي يمكن أن تسهم في استدامتها على المدى الطويل، سواء من أجل رفاه الإنسان أو السلامة البيئية.