Trees, like humans, need water to survive hot and dry days. They can only survive in extreme heat and dry conditions for a short time.Even native trees can die if they are not used to extreme heat and prolonged droughts like the Western U.S.This has been happening in Arizona and central Arizona for the past few months. According to the U.S. Forest Service, a long-standing drought and the resulting water stress caused the death of up to 30% of the junipers in the area. California's severe drought of the past decade left over 129 million trees dead. This dry wood, which is highly flammable, can be used to fuel wildfires in the future.These areas are being closely monitored by firefighters as an additional dry year increases the fire danger.Read more: Western states are facing a water crisis, and another fire season.What happens to trees when there is drought?Vascular water transport is a method by which trees can move water from their roots to the leaves. This process is called tree survival.Water flows through tiny cylindrical conduits called tracheids. Drought can disrupt water transport by decreasing the amount of water that is available to the tree. When soil and moisture levels drop, air bubbles in the vascular system can form, causing emboli to block water flow.Trees will experience more embolisms in hot and dry periods if they have less water. A tree that can't get water to its foliage will die.A cross-section dyed of a Ponderosa Pine sapling shows water transport tissue. Raquel Partelli FeltrinCertain species are more resilient to embolisms that others. This is why pinon pines were more susceptible to embolisms in the Southwest than juniper.Drought stress can also make trees more susceptible to bark beetle attacks. Nearly 90% of the Sierra Nevada's ponderosa pines died during the drought of 2012-2015, mostly due to western pine beetles infestations.Trees are also affected by drought and fire damageOur research has shown that fire is good for fire-prone forests as it helps to maintain their density and health. However, trees suffering from drought stress are more likely than others to die from the effects of fire. Trees with less water have less ability to cool down and insulate against fires during droughts. They might also decrease their ability to produce tree food carbohydrates during droughts. This makes it more difficult for them to recover after fire damage.Trees that sustain trunk damage from a fire will also be less likely to survive the next year if there is drought. Trees with fire scars can have less functionality in their vascular channels for water transport. Their resistance to embolisms can also be affected by trauma to their vascular tissue.Trees that have been so burned are more likely die in drought than trees that are not. Trees that are dry are more likely die from fire.What does this all mean for the future of forests?Over the past 20 years, trees in Western forests have been suffering from droughts, high temperatures and pests at an alarming pace. Research shows that the U.S., along with much of the rest of the globe, will experience more tree deaths due to continued greenhouse gas emissions. This is because the planet's temperature rises and drives moisture loss.Although it is not clear what impact changes in drought and fire regimes may have on forests further into the future, several observations could provide some insight.Evidence suggests that there is a shift from forests to shrublands and grasslands in some parts of the Western U.S. This transition can be reinforced by frequent burning in the same area. The forests can be destroyed by fire or drought, which causes some trees to die. However, it is not known how long it will take for the forest to recover from a pre-fire, pre-drought or large-scale fire or other severe fire.The West has experienced its worst droughts in more than 1,000 years. This includes the Southwest and California. Recent research has shown that subalpine forests of the central Rockies are now more susceptible to fire than they were in at least 2000 years.Temperatures will continue to rise if there is no reduction in greenhouse gas emissions. This will lead to an increase in severe drought stress and days of fire danger.Read: Portland and Seattle reach 100+ degrees. What is a heat dome? How can climate change bring these extremes to Portland and Seattle?Additionally, better forest management will result in thinning trees which will incur billions of dollarsDaniel Johnson is an assistant professor of tree biology and forest ecology at Georgia University in Athens, Ga. Raquel Parelli Feltrin at the University of British Columbia is a postdoctoral researcher in botany. This article was published first by The Conversation Trees are suffering from Western drought. Here's what's happening inside their veins.