Vulnerability to forest loss through altered postfire recovery dynamics in a warming climate in the Klamath Mountains

Abstract

In the context of ongoing climatic warming, certain landscapes could be near a tipping point where relatively small changes to their fire regimes or their postfire forest recovery dynamics could bring about extensive forest loss, with associated effects on biodiversity and carbon-cycle feedbacks to climate change. Such concerns are particularly valid in the Klamath Region of northern California and southwestern Oregon, where severe fire initially converts montane conifer forests to systems dominated by broadleaf trees and shrubs. Conifers eventually overtop the competing vegetation, but until they do, these systems could be perpetuated by a cycle of reburning. To assess the vulnerability of conifer forests to increased fire activity and altered forest recovery dynamics in a warmer, drier climate, we characterized vegetation dynamics following severe fire in nine fire years over the last three decades across the climatic aridity gradient of montane conifer forests. Postfire conifer recruitment was limited to a narrow window, with 89% of recruitment in the first 4 years, and height growth tended to decrease as the lag between the fire year and the recruitment year increased. Growth reductions at longer lags were more pronounced at drier sites, where conifers comprised a smaller portion of live woody biomass. An interaction between seed-source availability and climatic aridity drove substantial variation in the density of regenerating conifers. With increasing climatic water deficit, higher propagule pressure (i.e., smaller patch sizes for high-severity fire) was needed to support a given conifer seedling density, which implies that projected future increases in aridity could limit postfire regeneration across a growing portion of the landscape. Under a more severe prospective warming scenario, by the end of the century more than half of the area currently capable of supporting montane conifer forest could become subject to minimal conifer regeneration in even moderate-sized (10s of ha) high-severity patches. K E Y W O R D SDouglas-fir, forest resilience, Klamath Mountains, postfire recruitment, propagule pressure, reburn, stem analysis, tipping point, tree regeneration

Post‐fire forest regeneration shows limited climate tracking and potential for drought‐induced type conversion

“…, Campbell and Shinneman , Tepley et al. , Urza and Sibold , Stevens‐Rumann et al. ) and does not readily increase in areas where climatic suitability is apparently increasing (Zhu et al.…”

“…Density of nearby seed sources is known to be an important driver of post‐fire conifer regeneration in this and similar systems (Tepley et al. , Shive et al. ).…”

“…, Tepley et al. , Urza and Sibold ), suggesting that this is the window in which potential recruitment‐driven community compositional shifts would become apparent if they were going to occur.…”

“…) and projections based on contemporary climate associations (Tepley et al. ) also generally predict declines in conifer recruitment with increasing aridity at hot, dry range limits but not necessarily expansion at cool, wet range limits, where colonization of new sites may be poor due to dispersal limitation and/or limited availability of suitable sites.…”

Climate Change Amplifications of Climate‐Fire Teleconnections in the Southern Hemisphere

“…For instance, landscape-scale loss of fire-sensitive ecosystems has already occurred in response to changes in fire frequency and fire-vegetation feedbacks in parts of southeast Australia , New Zealand (Tepley et al, 2017), and southern South America (Paritsis et al, 2015), with concern about a future where fires become more frequent and/or extensive. This trend is likely to continue due to projected temperature increase and the climate modes' trajectories.…”