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Posts Tagged ‘global change’

Future fires

November 11th, 2016 No comments

There is a tendency to think that fires will increase in the near future due to global warming. This is because many fire risk prediction are based on climate only. However fire regime changes not only depend on climate [1]; there are other factors, like land-use changes, CO2, plant invasion, fragmentation, etc. that are also important drivers of change in fire activity [1]. Even plant drought stress (and flammability) not only depends on climate [2,3].

A recent simulation study [4] suggests that global burned area is certainly predicted to increase in the following decades when simulations are based on climate only (blue line in the figure below). However, adding the effect increased CO2 reduces the predicted burned area to no increase (green line below). Furthermore, when adding increased population density and urbanization (black and red lines), the model predicts much more area burnt in the last century (black lines 1900-2000) and a reduction of future burned area (red lines). The predicted reduction of fire during 1900-2000 is consistent with global charcoal records [5] and can be explained by increasing agriculture, land use and fragmentation. Overall, this study suggests that global area burned is unlikely to increase in the following decades.

Note that 1) this is a model, so take it with caution! 2) This model is at the global scale, but changes in different directions are expected in different regions, and this can have biodiversity consequences (even if the global balance is steady); for instance, in the Mediterranean Basin, fire are likely to keep increasing as land abandonment and fuels are increasing [6]. And 3) there is a high uncertainty in some fire drivers. For instance, temperature is likely to keep increasing, however, rainfall and wind changes are very uncertain, and landuse and emissions are subject to uncertain changes in environmental policies in different countries. In any case, this study gives us an idea of the possible sensitivity of different parameters.

Knorr-2016-NatClimChange
Figure: Simulation of global area burned for 1900 to 2100 under different scenarios: a) climate only (blue line); b) climate + CO2 (green); c) climate + CO2 + population & urbanization (black lines; red area for the future predictions). From [4].

References
[1] Pausas J.G. & Keeley J.E., 2014. Abrupt climate-independent fire regime changes. Ecosystems 17: 1109-1120. [doi | pdf | blog]

[2] De Cáceres M, et al. 2015. Coupling a water balance model with forest inventory data to predict drought stress: the role of forest structural changes vs. climate changes. Agr. For. Meteorol. 213: 77–90. [doi | pdf | suppl. | blog]

[3] Luo, Y. & H. Y. H. Chen. 2015. Climate change-associated tree mortality increases without decreasing water availability. Ecol, Let. 18:1207-1215.

[4] Knorr W, Arneth A, & Jiang L, 2016. Demographic controls of future global fire risk. Nature Clim. Change 6:781-785.

[5] Marlon JR, et al. (2008). Climate and human influences on global biomass burning over the past two millennia. Nature Geosci, 1, 697-702.

[6] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | pdf | blog]

 

Climate-independent drought stress in plants?

July 14th, 2015 No comments

Climate warming is increasing water stress in many ecosystems, with consequences of increased plant mortality and susceptibility to pests (Figure below). However, there are other mechanisms (climate-independent mechanisms) by which plant drought stress can also increase. In a recent paper [1], we use a water balance model coupled with a vegetation model, to simulate changes in leaf area (LAI) and evapo-transpiration between two forest inventories in NE Spain. The results suggest that during 1980-2010 there was a tendency of increasing in drought stress for most tree species; however, drought stress was not predicted to change when considering that forest structure did not change between the two forest inventories. That is, changes in climate alone did not predict changes in water stress. In contrast, the recent increase in forests (in extension and in tree density) in the study area showed to be the main driver for the drought stress observed in trees. This forest increase is due to the abandonment of land and rural activities durint the recent decades [2,3]. That is land abandonment is not only increasing the fuel in the landscape (amount and continuity [2,3]) but also the flammability of this fuel. Consequently, land abandonment and increased forest is a major driver of drought and fire regime changes [3].

Caution must be taken in extrapolating these results as they are based in a model; i.e., more information from other approaches and places is needed. However, it is clear that climatic change should not be considered the only source of current drought stress in vegetation; there may be changes in drought stress as well changes in fire regime that are climate-independent [1,4] and more related to changes in forest and landscape structure linked to factors like socio-economic (and landuse) changes, changes in herbivores, plant invasion, etc... [4]; and in many cases, these different processes interact. The good thing is that climate-independent processes are easier to manage than climate!

drought-Calderona

Figure: Plant mortality by drought stress in Calderona, Valencia, Spain (Photo:  P. García-Fayos, 2015).

References
[1] De Cáceres M.,  Martínez-Vilalta J.,  Coll L., Llorens P., Casals P., Poyatos R., Pausas J.G. and Brotons L. 2015. Coupling a water balance model with forest inventory data to predict drought stress: the role of forest structural changes vs. climate changes. Agric. Forest Meteorol. 213: 77-90. [doi | pdf | suppl.]

[2] Pausas J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change 63: 337-350. [pdf | doi]

[3] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]

[4] Pausas J.G. & Keeley J.E., 2014. Abrupt climate-independent fire regime changes. Ecosystems 17: 1109-1120. [doi | pdf ]

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