Drought experiments in forest ecosystems: Challenges and opportunities for standardization, cross-site comparison, and advancing climate change science
Precipitation regimes are changing dramatically, and models project greater occurrence of extreme events and increased variability. This trajectory of precipitation change will likely surpass the range of historical patterns to which plants have adapted, leading to novel ecosystem-climate interactions poorly represented by current ecological theories. Improving scientific knowledge requires precipitation manipulation experiments (PMEs) that effectively capture these climate change drivers and push ecosystems to respond in novel ways. However, such experiments are relatively rare in forest ecosystems, which combined with a lack of standardized methods, limits our ability to predict impacts of precipitation change and compare among diverse sites. Our work (1) provides a comprehensive assessment of the opportunities and challenges entailed in conducting forest PMEs, and (2) creates guidelines and considerations for standardizing PMEs as part of a global research network aimed at advancing knowledge about consequences of changing precipitation regimes in forest ecosystems. Our analysis utilizes a combination of in depth literature review, expert workshops, and informal surveys of the research community.
We identified the following opportunities for developing a standardized approach for PMEs across diverse forest ecosystems:
- Site-based or modeled historical precipitation provides a basis for determining the experimental removal (or addition) treatments.
- A passive (‘fixed amount’) approach to removing (or adding) precipitation effectively captures the dynamics of extreme events.
- Technically sound infrastructure design can be adapted to a range of locally available materials, labor, and other inputs.
- Strong international interest and support exists for establishing a coordinated research network of forest PMEs.
We also identified several key challenges associated with forest PMEs that require consideration when developing standardized approaches:
- Logistical, technical, and financial constraints to conducting PMEs in ecosystems with large stature vegetation (tall shrubs and trees).
- Achieving precipitation treatment goals in ecosystems where snow comprises a significant proportion of annual precipitation while minimizing artifacts (e.g., soil freezing-thawing).
- Preventing access by deep and lateral roots to external water sources (e.g., via trenching, buffer zones) without excessive site damage or expense.
- Determining the minimum acceptable size (and replication) for study plots to effectively capture ecosystem level response to the PME treatment.
We conclude with a core set of guidelines and considerations for developing a standard forest PME protocol, and invite the international community of precipitation experimentalists to provide input on deriving specific recommendations for a future international network of forest PMEs.