Role of Long-Term Experiments in Understanding Ecosystem Response to Global Change
For many decades, scientists have sought to understand how ecosystems respond to stresses, especially to those imposed by humans. Already a century-and-a-half ago, for example, Marsh (1864) documented mounting evidence of how human activities were degrading the biosphere and looked for ways of reducing those damages. Now, understanding change has assumed even greater urgency because human stresses on ecosystems, locally and globally, have intensified, driven by growing population and expanding demands (Carpenter et al. 2009; Chapin et al. 2009; Collins and Childers 2014; Gunderson and Folke 2011). In the past, most human effects on ecosystems occurred locally; now, they often extend globally, notably through effects on atmospheric CO2, which is rapidly increasing, affecting climates around the world (Le Quere et al. 2015).
Ecosystem responses to human disturbance, however, can be properly understood only by studying them for periods of several decades or longer (Carpenter 2002; Knapp et al. 2012; Kummerer et al. 2010; White 2013). Shortterm studies, of several months or years, are indispensable in elucidating underlying processes and mechanisms, but they cannot reveal the eventual, final response. Many ecosystem properties change only slowly and the full response is seen only after time scales of a century; for example, responses of soil carbon to management change may unfold over many decades and may not even be discernible until 5 or 10 years have passed (Ellert et al. 2000). Furthermore, some ecosystem responses may be episodic, affected by fluctuating weather or other variables, so studies lasting only a few years may miss them entirely (Franklin 1989). Other responses may be delayed for several decades until a critical "tipping point" is reached (Villa et al. 2014); for example, after cultivating grassland, the cultivated land may initially be highly productive, but yields may eventually decline as native fertility is depleted (Janzen 2001). For these and other reasons, long-term experimental sites have been invaluable to ecologists in understanding ecosystem change and will likely remain so in the coming decades (Mooney et al. 2009; Southwood 1994). Indeed, given the intensity of projected climate and other global changes, some of which remain unpredictable (Boyd 2012), such longterm ecological sites (LTES) may become even more important in the future (Richter et al. 2007).
In light of the intensifying stresses on ecosystems and the long response time of ecosystems to these pressures, this brief review will address the following objectives: (1) to define "long-term ecological site or sites," (2) to propose some attributes that enhance the usefulness and longevity of such studies, and (3) to proffer examples of research questions that may merit more attention in long-term experiments.