The Chronosequence 101

SIP Research Spotlight Series

The chronosequence: How to study everything, everywhere, all at once

What is a “chronosequence”?

Big research questions are exciting – but daunting. “What if we harvested this forest differently?” is sometimes easier said than done, as understanding the impacts we have on our forests requires decades, or even centuries or millennia, for successional processes to unravel. 

Large-scale experiments are critical to a comprehensive understanding of ecosystem-scale questions of this nature. A typical large-scale scientific study may be based on repeated observations of the same system at a certain time intervals – like going to the same set of sites once every decade to measure soil moisture after a certain stand treatment. But, it can be costly to build studies like this across many ecosystems, replicate effectively and continue monitoring across the decades needed to observe ecological processes. With the increasingly long list of silvicultural options that could (and should!) be tested, designing a long-term and highly controlled trial for each treatment is nearly impossible. 

So, what if we flipped the initial design concept? Then we would be visiting many different sites at one time, but each site would be at a different age since treatment. This would yield learnings about how the forest had responded to a treatment, but the observations could be made within a short time period by taking advantage of treatments that had already occurred. This approach, called a chronosequence, allows for a retrospective assessment of forest responses to treatments, which can save time and money.

A chronosequence is a series of sites that differ in time that has passed since they were formed, but that were formed under the same conditions. An important consideration is that all sites are assumed to be undergoing the same successional trajectory. It is sometimes called a “space-for-time substitution” because a chronosequence study collects observations across different sites, rather than collecting observations over time.

A chronosequence can help us learn about forestry practices

The operational practice of forestry can be both a science and an art, aligning a silviculture system to the context of a given site and the social, economic and ecological objectives, and constraints that influence the practice, among many considerations. Long-term experimental research installations have the advantage of controlling and explicitly measuring variation in treatments of interest – for instance, the amount of retention left following harvest. But using chronosequences, we can understand the impacts of variation in forestry practices in an operational setting across a range of forest values, because we can measure the variation in practice that results from the diversity of forests, forest practitioners managing these forests, and the social, economic and ecological pressures that change over time. This makes a chronosequence a powerful tool to understand how different contexts and circumstances impact forest management – as long as sites are carefully and thoughtfully selected according to strict criteria to make sure there isn’t so much variation in the practices that we are unable to draw any conclusions.

Figure 1: Under the correct conditions, a chronosequence can be an extremely effective tool to capture the dynamic nature of forestry prescriptions. For example, measuring forest attributes at sites A, B and C that were harvested roughly 5, 15 and 30 years ago, respectively (Photos by Alana Clason)

Recommendations and considerations:

If a chronosequence is something that interests you, here are a few recommendations and considerations for your project design: 

Benefits of the chronosequence: 

• Great for observing systems that take decades to evolve and change, such as forested ecosystems. 
• Can be more affordable than designing a monitoring project that spans decades. 
• Captures the operational implementation of forest practice.

Considerations of a chronosequence: 

• Pick sites (or “treatment areas”) that are comparable by minimizing the variation in forest response that could be determined by other non-treatment effects, such as things like climate, forest type, site productivity, and intensity of other disturbance agents, like pests or wildfire. 
• Identify the variables to be measured and ensure they can be consistently measured across sites.
• Minimize the number of treatments of interest and maximize the number of replicates of each type measured. More replicates of a similar treatment can help draw conclusions from a sporadically applied treatment.

Photo by Gillian Chow-Fraser

A chronosequence is one option amongst a varied list of research tools

A chronosequence offers a powerful way to study long-term forest dynamics without requiring decades of direct observation. By leveraging existing treatments across landscapes, this approach allows researchers to explore critical questions about forest management and silvicultural practices. 

While chronosequences are a valuable tool, they are not a one-size-fits-all solution. There are many methodological approaches used in BC to assess the impacts of silviculture practice on a range of forest values. For example, there are several long-term trials that operate across BC that closely monitor the operational, economic and ecological impacts of different silvicultural treatments. These large scale experiments address significant knowledge gaps on the ecological impacts of silviculture treatments.

All of these projects help us paint the picture of changing forests in BC. By combining chronosequences with other robust monitoring programs and analytical methods, foresters can gain a more comprehensive understanding of how to sustainably manage forests using innovative silviculture.

This blog was prepared by Dr. Alana Clason. Read more about the SIP research project that uses chronosequences here. Cover photo is by Gillian Chow-Fraser. 

Supporting References: 

1. Mitchell, Al & Vyse, Alan & Huggard, David & Beese, William. (2004). Long-term silviculture experiments contribute to science-based forest management in British Columbia’s public forests. Forest Snow and Landscape Research. 78.
   
2. Walker, L.R., Wardle, D.A., Bardgett, R.D. and Clarkson, B.D. (2010), The use of chronosequences in studies of ecological succession and soil development. Journal of Ecology, 98: 725-736. https://doi.org/10.1111/j.1365-2745.2010.01664.x 

Photo by Gillian Chow-Fraser