I’ve been very lucky to work with great co-authors over the past few years, and this year is no exception. Along with a raft of papers we are about to submit I just got notified that a paper we submitted a few months ago is now online in the Annals of the Association of American Geographers (journal title chosen in the pre-Twitter age, obv.).
This paper, with Megan Walsh, Jenn Marlon, Dan Gavin and Kendrick Brown (and also me!) is a great look at fire records from the Pacific Northwest (PNW) over the last 5000 years. This time period is particularly critical to understanding human-climate-fire relationships. Human populations over the last 5000 years were increasing in the region, and climate was shifting, gradually cooling and becoming more moist following the early Holocene xerothermic period.
The challenge is that the PNW is an incredibly heterogeneous region. You can hike nine kilometers and shift from dry valley bottom to an alpine peak. Most of the climatic gradients are more homogeneous on the NS axis than they are on the EW axis. The more critical problem is that, in a more general sense, human activity, on a landscape scale, is very difficult to detect or attribute prior to widespread EuroAmerican colonization (see Sam Munoz‘s excellent paper here).
Our paper comes out at a particularly important time. We’ve seen an incredible fire season in the Pacific Northwest this year, driven in part by very dry conditions, both during the summer, but, more importantly by low snow packs in the winter. Knowing that past fire regimes in the region increased, even as temperatures cooled through the late Holocene, has serious implications for the future. Biomass stocks in the PNW remain high (even following widespread logging), and the open fire-dominated forests that were adapted to warmer drier conditions of the early Holocene (mostly Douglas fir-dominated) are no longer established in the region.
We may be burning ourselves to a new ecological baseline.
Amy Hessl, who was a co-convener on our fantastic ESA session this year (link), has a nice paper in Ecological Applications from 2004 linking the Pacific Decadal Oscillation to fire activity in the PNW interior (here) based on a set of fire scar data, so changing the intensity and frequency of these climatic systems is certainly going to shift our frequency and intensity. Wimberly and Liu (2014) support the idea that management focused on reducing biomass on the landscape (prescribed burning and thinning) will help counter increasing fire severity and frequency, suggesting that management may be the key in transitioning to a warmer, drier future, and the key to understanding what these future forests might look like.
Fire in the early Holocene resulted in forests on the north shore of Vancouver with much higher proportions of Douglas fir pollen than are found in many modern day sites (see Marion Lake on the Neotoma Explorer – here, and check out the “Diagram” tab). Douglas fir is not a heavy pollen producer, and yet it reached almost 20% of the pollen sum, along with higher proportions of Alder and Bracken fern, a fire-adapted fern. Currently these taxa are found in low proportions throughout the PNW, except in regions with very low rainfall, and historically low fire return intervals, and proportions of Douglas fir over 20% are almost entirely restricted to southern Vancouver Island in British Columbia, although there may be higher proportions in the US.
Are we heading to a new, old baseline?
It’s unlikely that we’ll see fire return intervals as high as we’ve seen in the past. Active fire management will certainly keep fire activity lower than in the Holocene record because we put so much effort into countering large-scale fire. The interesting thing to me is the idea that we’ll be managing these landscapes for fire, so we’ll have aspects of forest structure that map onto historical forests well: more open canopies, lower biomass, fire tolerant species, but, because of political volatility of prescribed burning, we are likely to see some fire tolerant species absent from the landscape, particularly understory species that we are unlikely to manage. This might lead to novel species assemblages, with fire tolerant canopy species, and less tolerant understory species. The result of a “fire adapted” landscape that has arisen as the result of active management in the absence of fire. Planting, thinning, and continued management, without the presence of fire.
This maps well to what we’ve seen in the Georgia Basin, the encroachment of Scotch Broom into what has historically been Garry Oak Savanna. We have open canopies, a shrub layer of highly flammable, introduced and invasive species, but still the structural attributes of a savanna landscape, minus grasses, so, uh, well, not exactly savanna. But that’s fine, because I said there was no real analogue, so QED. This post is too long anyway🙂