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Background The increased interest in why and how trees die from fire has led to several syntheses of the potential mechanisms of fire-induced tree mortality. However, these generally neglect to consider experimental methods used to simulate fire behaviour conditions. Aims To describe, evaluate the appropriateness of and provide a historical timeline of the different approaches that have been used to simulate fire behaviour in fire-induced tree mortality studies. Methods We conducted a historical review of the different actual and fire proxy methods that have been used to further our understanding of fire-induced tree mortality. Key results Most studies that assess the mechanisms of fire-induced tree mortality in laboratory settings make use of fire proxies instead of real fires and use cut branches instead of live plants. Implications Further research should assess mechanisms of fire-induced tree mortality using live plants in paired combustion laboratory and landscape fire experiments.

One of the grand unknowns of ecosystem science is how fire kills trees. Answering this question is critical to parameterize climate-vegetation models given the observed changes in global fire regimes, the feedbacks between fire and forests in the global carbon cycle, and the potential role of forest management in moderating anthropogenic climate change. In this dissertation I conducted three studies using Pinus ponderosa saplings burned under controlled conditions to improve the understanding how fire effects on tree physiology. First, I assessed the impact of two fire intensities on sapling mortality under two water status pre-fire (well-watered and drought-stressed). The results showed that saplings under drought-stress pre-fire were more vulnerable to mortality when exposed to low fire intensities. However, 100% of mortality was observed regardless of the pre-fire water status when saplings were exposed to high fire intensity. Thus, the data also suggest that there is a fire intensity threshold where the pre-fire water stress can have a significant influence on sapling mortality. Second, we investigated the short (one-day post-fire) and long-term (21-months post-fire) effects of fire on sapling water transport. In the short-term, fire did not have impact on sapling xylem hydraulic conductivity or were more vulnerable to drought-induced embolism. However, in the long-term, saplings were more vulnerable to cavitation. But no damage in the xylem conduits cell walls were observed. Thus, it was hypothesized that the new traumatic xylem formed in the edges of the fire scar and the pre-fire xylem clogging with resin could be responsible for increasing vulnerability to cavitation in these plants. Lastly, I evaluated the impact of a lethal fire intensity on sapling hydraulic conductivity and non-structural carbohydrates periodically for 28-days post-fire. Hydraulic conductivity was not affected any day. This confirmed the results found in the second study. Fire caused a decline in total NSC in burned plants compared with unburned saplings, but it was significantly only 28-days post-fire. The results suggest that tree mortality from fire is likely not due to hydraulic failure but may be related to carbon imbalance.

Climate change is increasing drought and fire activity in many fire-prone regions including the western USA and circumpolar boreal forest. These changes highlight the need for improved understanding of how multiple disturbances impact trees in these regions. Recent studies linking fire behaviour to plant ecophysiology have improved understanding of how fire affects tree function and mortality but have not investigated interactions between drought stress and fire. In this study, Larix occidentalis saplings were subjected to different levels of water stress followed by low-intensity surface fires in a controlled laboratory setting. Post-fire mortality, recovery and growth were monitored for up to 1 year post fire. Generally, increased pre-fire water stress resulted in decreased post-fire stem diameter (up to 5% lower) and height (up to 19% lower) growth. However, severely water-stressed saplings whose foliage had senesced before the fires had lower 1-year mortality (14%) and significantly greater post-fire bud densities than moderately stressed saplings that did not senesce (86% mortality). The mortality patterns suggest that water-stressed western larch saplings exposed to low-intensity wildfires, or prescribed fires conducted as part of forest management activities, may exhibit lower mortality rates if stress-induced foliar senescence has occurred.

The herbaceous layer in forest ecosystems is often ignored because of its small stature and contribution to the overall ecosystem biomass. Unlike forests, the herbaceous layer in savanna ecosystems is more noticeable, however little is known about the factors that control the productivity in this layer, especially the influence of light. The study was conducted at Pushmataha Forest Habitat Research Area in southeastern Oklahoma that have units with different overstory densities due to previous mechanical treatments and sustained differences in fire return interval. The goal of this study was to determine relationship between light availability and intercepted photosynthetically active radiation (IPAR) on herbaceous productivity along a forest-savanna continuum. IPAR by the overstory and herbaceous plants was measured multiple times during the 2013 growing season. Herbaceous aboveground net primary production (ANPP) was measured at the end of the 2013 growing season by clipping and weighing biomass components (grass, forb, legume, woody, sedge, and litter). Overstory and herbaceous IPAR showed two distinct trends over the growing season. Forested treatments had a substantial increase in the beginning of the growing season related to canopy development of the deciduous trees. In savanna treatments, the overstory trend of IPAR was more consistent over the year. Herbaceous IPAR in forested units had a trend that was more consistent, while in savanna treatments there was a substantial increase at the onset of the growing season due to the development of the dense herbaceous layer. In general, all the categories of herbaceous ANPP were positively correlated with the light availability. The total herbaceous ANPP had a positive relationship with PAR available and IPAR by the understory. However IPAR by the understory was a better predictor for herbaceous ANPP (r2=0.65). The ability of plants to use IPAR to produce biomass in the herbaceous layer in forest and savanna ecosystems was similar regardless of overstory density and treatment. These results indicate that the pattern of IPAR by overstory and herbaceous layer are dependent of the species and the density of plants. However the ability of plants to use PAR to produce biomass was consistent across a wide range of conditions.

If there is no change in greenhouse gas emissions, temperatures will continue to increase, and severe drought stress and fire danger days will rise as a result. --- Rocky Mountain forests burning more now than any time in the past 2,000 years --- Daniel Johnson receives funding from the National Science Foundation, the United States Department of Agriculture and the United States Forest Service. Daniel Johnson, Assistant Professor of Tree Physiology and Forest Ecology, University of Georgia Raquel Partelli Feltrin, Postdoctoral Scholar in Botany, University of British Columbia

If there is no change in greenhouse gas emissions, temperatures will continue to increase, and severe drought stress and fire danger days will rise as a result. --- Rocky Mountain forests burning more now than any time in the past 2,000 years --- Daniel Johnson receives funding from the National Science Foundation, the United States Department of Agriculture and the United States Forest Service. Daniel Johnson, Assistant Professor of Tree Physiology and Forest Ecology, University of Georgia Raquel Partelli Feltrin, Postdoctoral Scholar in Botany, University of British Columbia