Forest canopies: Methods, hypotheses, and future directions

Forest canopies contain a major portion of the diversity of organisms on Earth and constitute the bulk of photosynthetically active foliage and biomass in forest ecosystems. For these reasons, canopy research has become integral to the management of forest ecosystems, and to our better understanding of global change. Ecological research in forest canopies is relatively recent and has been primarily descriptive in scope. The development of new methods of canopy access has enabled scientists to conduct more quantified research in tree crowns. Studies of sessile organisms, mobile organisms, and canopy interactions and processes have emerged as subdisciplines of canopy biology, each requiring different methods for collecting data. Canopy biology is beginning to shift from a descriptive autecology of individuals to a more complex ecosystem approach, although some types of field work are still limited by access. Questions currently addressed in canopy research are extremely diverse but emphasize comparisons with respect to spatial and temporal variation. Spatial scales range from leaves (e.g. quantifying the number of mites on individual phylloplanes) to trees (e.g. measuring photosynthesis between sun and shade leaves), to forest stands (e.g. measuring turbulence above the canopy), and entire landscapes (e.g. comparing mammals between different forest types). Temporal variation is of particular significance in tropical forest canopies, where populations of organisms and their resources have diurnal, seasonal, or even annual periodicity. As the methods for canopy access improve, more rigorous hypotheses-driven field studies remain a future priority of this newly coalesced discipline.