Plant function in a rapidly changing world
Edited by Alan K. Knapp
As Peter Vitousek and colleagues pointed out 15 years ago “we are changing Earth more rapidly than we are understanding it” (Vitousek et al 1997). This statement has resonated with plant ecologists around the world, serving as a call to arms for many to grapple with the task of furthering our understanding of how plants will function in a world of rapidly increasing atmospheric CO2 partial pressure, warming temperatures, alternations in precipitation patterns and increased levels of nitrogen deposition (IPCC 2007).
Increasing our understanding of basic plant functioning in an ecological context, from the perspective of mechanism and consequence, in this era of rapid global environmental change requires ecologists to not only consider how systems function today, but to explore how they will function in a future dominated by new and in many cases chronic drivers of change (Smith et al. 2009), an increase in extreme events (Smith 2011), and exposure to potentially novel interactions (Williams et al. 2007). This requires a diversity of research approaches including observational studies (planned and opportunistic, Augsburger 2009), gradient studies and comparative analyses, experimental manipulations (short and long-term) and even critical re- examination of relationships used to infer past climatic conditions (Speed et al. 2011). In this virtual issue composed of recent papers from the pages of Functional Ecology, each of these approaches is employed to advance our understanding of plant function, and the functioning of the communities and ecosystems, in a rapidly changing world.
Forecast alterations in temperature and moisture motivate much of the research described in these papers, befitting long-held observations that global patterns of temperature and moisture availability explain much of the broad-scale variation seen in organisms, biogeochemical processes and the distribution of biomes. For example, as a key component of ecosystem-atmosphere C flux, understanding the sensitivity of leaf-level photosynthesis and respiration to changes in temperature and precipitation is critical. Atkinson et al. (2010) investigated the impact of growth temperature on scaling relationships that link photosynthesis to leaf traits. These relationships have value for predicting ecosystem-atmosphere C fluxes. With a focus on leaf development as a controlling factor, they conclude that changes in growth temperature can significantly alter these scaling relationships, with thermal history accounting for up to ~30% of the variation they measured. Phenology in turn determines patterns of leaf development, with high phenological plasticity important for enhancing the adaptive capacity of plants to future climate changes (Vitasse et al. 2010). Such plasticity has been documented in populations of temperate tree species found across an extensive altitudinal gradient with the expectation that in most cases, warming will lengthen the growing season due to earlier budbreak. Such responses are not without risk however, as in many temperate regions major spring frost events though unpredictable are expected to occur more frequently. As budbreak occurs earlier in a warmer world the impact of these frosts may become more severe. Taking advantage of a rare hard freeze that followed a three week period of anomalously warm temperatures and early budbreak in 2007, Augspurger (2009) assessed damage and refoliation responses in 20 deciduous woody species. This study showed that differences among species in frost damage and their ability to refoliate could result in strong selection on individuals as well as differences in carbon gain that could, in the long-term, affect species’ abundances.
Global change impacts on leaf and canopy respiration are also important to understand at local to global scales. However, the complexity of forecasting responses in CO2 efflux from leaves is exemplified by two long-term experiments, one in a Mediterranean Quercus ilex forest (Rodríguez-Calcerrada et al. 2011) and another in an Amazonian rain forest (Metcalfe et al. 2010). Drought, projected to increase in frequency and severity in both forests, was experimentally imposed in both studies. But conclusions about the consequences of drought for CO2 emissions from leaves were diametrically opposed – drought is expected to attenuate the stimulation of leaf respiratory CO2 release by global warming in the Mediterranean forest but lead to a substantial increase in canopy CO2 emissions in the Amazonian forest. Further complicating such assessments are the results of Searle et al. (2011), who took advantage of Quercus rubra trees growing along urban-rural gradients to investigate leaf respiratory responses to a host of global change factors. They also concluded that temperature change can impact key respiratory components, but that other factors such as increased N, particularly important in urban areas, are important to consider when evaluating how leaf CO2 efflux may be altered in the future.
Alterations in temperature and atmospheric CO2 are tightly linked to the availability of water in many ecosystems, with warming generally expected to decrease soil water and exacerbating drought impacts, whereas elevated CO2 has the potential to conserve soil water in many regions (Dijkstra et al. 2010). It is clear that there is much to be learned with regard to how forecast alterations in soil moisture will affect plant functioning, including the role that leaf size variation may play in allowing plants to survive hot, dry periods (Yates et al. 2010), how drought can influence adult life-history expression (Kim and Donohue 2012) and the trade-offs between drought tolerance and leaf and whole-plant capacity for water flux (Hao 2010).
Finally, as posited by the Hierarchical Response Framework (HRF) for assessing ecosystem responses to global change (Smith et al. 2009), although individual plant physiological and growth responses to changes in temperature or moisture are clearly important and are expected to occur rapidly, when responses at the individual scale propagate to alterations in communities, even greater impacts on ecosystems are expected. Results from separate field experiments in semi-arid grassland (Dijkstra et al. 2010) and a cold sub-arctic bog (Aerts et al. 2009) are consistent with this prediction of the HRF. In these studies, community interactions (such as interspecific competition) or changes in species composition and structure in response to manipulated CO2, temperature and/or precipitation led to the greatest impact on productivity and plant-mediated nutrient and C cycling. Thus, ecosystems with communities, most sensitive to global change drivers are expected to be altered the most in the future. But what determines the sensitivity (or resistance) of a community or ecosystem to alterations in temperature or water? To answer such questions, long-term studies and in particular long-term experiments can be a particularly valuable approach (Knapp et al. 2012). In a 19 year study of experimental soil moisture alterations in a central US grassland, Collins et al. (2012) identified a mechanism for community resistance to change – functional redundancy within the lifeform of the dominant species. Such redundancy can buffer this ecosystem from extensive community change in the face of a chronic change in resources.
In sum, if we are to successfully cope with the unprecedented rate of change in environmental conditions expected in the future, ecologists must continue to employ a wide array of research tools and approaches, as well as maintain an interest in a diversity of ecological systems, to provide the breadth and depth of knowledge needed to forecast the future of our rapidly changing natural world. The papers complied for this virtual issue move us closer to meeting this challenge, while providing a portfolio of blueprints for advancing our understanding of how plants will function when confronted by global environmental change.
IPCC 2007. Climate Change 2007. The physical science basis. Contribution of working group 1 to the fourth assessment report of the intergovernmental panel on climate change. Solomon, S.D., M. Qin, Z. Manning, M. Chen, K.B. Marquis, T. Avery, M. Tignor and H.L. Miller, Eds. Cambridge University Press, Cambridge, UK.
Knapp, A.K., M.D. Smith, S.E. Hobbie, S.L. Collins, T.J. Fahey, G.J. A. Hansen, D.A. Landis, K.J. La Pierre, J.M. Melillo, T.R. Seastedt, G.R. Shaver and J.R. Webster. 2012. Past, present and future roles of long-term experiments in the LTER Network. BioScience 62: 377 - 389.
Smith, M.D., A.K. Knapp and S.L. Collins. 2009. A framework for assessing ecosystem dynamics in response to chronic resource alterations induced by global change. Ecology 90: 3279-3289.
Smith, M.D. 2011. An ecological perspective on extreme climatic events: a synthetic definition and framework to guide future research. J. Ecol 99: 656–663.
Vitousek, P.M., H.A. Mooney, J. Lubchenko and J.M. Melillo. 1997. Human domination of earth’s ecosystems. Science 277: 494-499.
Williams, J. W., S. T. Jackson, and J. E. Kutzbach. 2007. Projected distributions of novel and disappearing climates by 2100 AD. Proceedings of the National Academy of Sciences (USA) 104:5738–5742.
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