At the end of this lecture you should be able to better answer both parts of this question

1. At the end of this lecture you should be able to better answer both parts of this question

2. Forest mortality and tree death

3. Forest mortality and tree death Outline: Differences between forest mortality vs tree death Mortality agent examples Mortality rates increasing in western forests? Tree death definition Frameworks for understanding tree death Paleo examples of mortality rates and climate change Influence of stand dynamics and life history

4. Studies of forest mortality rates versus individual tree death are very different Research on forest mortality rates is abundant Based on inventory/census data Changes in mortality rates can provide inferences from spatial pattern and temporal trends but causality is often uncertain Differences in mortality agents, site productivity, regional climates, forest types and stand ages make meaningful comparisons difficult Forest pathologists and entomologists have sought to understand how a particular agent kills trees but physiological research on the mechanisms of tree death are rare

5. Large disturbance events are media fodder and important dynamics of ecosystem function but most tree death occurs in the absence of disturbance

6. In PNW forests wind causes mortality in old forestsHowever, much of this would not occur without heart-rot or root disease fungi weakening the stem/roots over previous decades

7. Spruce beetle caused mortality of white spruce across 5000 km2 in Alaska from 1989-2002 when increased temperatures favored beetle populations Spruce beetle in AK killed ~5000 sq. km of white spruce from ~1989-2002Spruce beetle in AK killed ~5000 sq. km of white spruce from ~1989-2002

8. In Oregon the Mountain pine beetle has killed many trees across 2000 km2

9. Like most mortality agents the mountain pine beetle has had many outbreaks in the past - so attributing these events to climate change is difficult - Shown is a forest near Tenaya Lake, Yosemite NP after an outbreak in 1896

10. Drought and pine beetles affected 12,000 km2 of pinyon and ponderosa pine forests from 2000-2007 Pinyon mortality was about 12000 sq. kilometersPinyon mortality was about 12000 sq. kilometers

11. Larger rather than smaller Pinyon trees were more likely to die during a severe drought It is unresolved why there was an apparent difference in physiology with tree size or tree age Increased rooting depth did not favor large trees – suggests some less obvious physiological attribute favored young/small treesIncreased rooting depth did not favor large trees – suggests some less obvious physiological attribute favored young/small trees

12. “Oak decline” and mortality events are frequent across eastern U.S. forests: major causes include drought, late frosts, gypsy moth defoliation, armillaria root disease and wood-boring beetles Oak decline disease periodically affects regions of the eastern US – cause is generally drought stress or multiple late frosts in combination with gypsy moth defoliation or armillaria root disease. Wood-boring beetles and hypoxylon canker. Oak mortality has increased substantially across the Midwest since 1980. The percentage shown is the change in the number of trees that are standing dead – not annual mortality rates.Oak decline disease periodically affects regions of the eastern US – cause is generally drought stress or multiple late frosts in combination with gypsy moth defoliation or armillaria root disease. Wood-boring beetles and hypoxylon canker. Oak mortality has increased substantially across the Midwest since 1980. The percentage shown is the change in the number of trees that are standing dead – not annual mortality rates.

13. Hemlock dwarf mistletoe: This parasitic plant slowly infects and kills trees Hdm causes increased water loss and reduced stem xylem conductivity which both indirectly decrease photosynthetic ratesHdm also causes branch death, providing additional entry points for decay fungi that weaken stems

14. Introduced mortality agents can be particularly lethal because tree defenses have not coevolvedExample: sudden oak death

15. A common theme in most mortality events is drought and warm temperatures weakening tree defenses and favoring insect development Each dot represents a recent episode of drought-related forest mortalityEach dot represents a recent episode of drought-related forest mortality

16. For each major drought-related mortality event that has been observed there are numerous undocumented eventsThis and other forests in Northern California and southern Oregon show evidence of drought-related mortality

17. Is drought stress possible in the winter? Red belt mortality on Black Butte in 2009Red belt mortality on Black Butte in 2009

19. Is increased mortality in western forests unique? Probably not. The following six papers have found increased mortality rates that were accompanied by increased tree growth rates… Caspersen JP, Pacala SW, Jenkins J, Hurtt GC, Moorcroft PR, Birdsey RA (2000) Contributions of land-use history to carbon accumulation in U.S. forests. Science 290:1148-1151 Laurance, WF, Oliveira AA, Laurance SG, Condit R, Nascimento HEM, Sanchez-Thorin AC, Lovejoy TE, Andrade A, D’Angelo S, Ribeiro JE, Dick CW (2004) Pervasive alteration of tree communities in undisturbed Amazonian forests. Nature 428:171-175  Laurance SGW, Laurance WF, Nascimento HEM, Andrade A, Fearnside PM, Rebello ERG, Condit R (2009) Long-term variation in Amazon forest dynamics. J Veg Sci 20:323-333 Lewis SL, Phillips OL, Baker TR, Lloyd J, Malhi Y, Almeida S, Higuchi N, Laurance WF, Neill DA, Silva JNM, Terborgh J, Torres Lezama A, Vásquez Martinez R, Brown S, Chave J, Kuebler C, Núñez Vargas P, Vinceti B (2004) Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots. Philos T Roy Soc B 359:421-436. Phillips OL, Lewis SL, Baker TR, Chao K-J, Higuchi N. 2008. The changing Amazon forest. Philos T Roy Soc B 363:1819-1827 Thomas RQ, Canham CD, Weathers KC, Goodale, CL (2010) Increased tree carbon storage in response to nitrogen deposition in the US. Nature Geosci 3:13-17

20. Trees die when… Metabolic function ceases due to a lack of necessary carbon1 to support respiration, turgor maintenance, protein turnover and defensive costs owing to limitations by: Temperature Light Water CO2 Nutrients (perhaps) cellular machinery or transport systems that supply or carry resources suffer overwhelming damage2 Examples include stem breakage, frost/freezing or fire/heat damage, insect or pathogens interruptions of phloem or xylem, oxidative damage to leaves

21. Frameworks for understanding tree death Phenomenological and empirical models of tree death have been established for decades A comprehensive physiological framework for understanding tree death has long been lacking due to difficulty in differentiating interactions among mortality agents

22. Dynamic global vegetation models differ greatly in their representations of mortality because there is no generally accepted theory of tree death

23. Phenomenological model: Decline disease “spiral” was popular starting in the 1980s but it does not list mechanisms or lead to hypothesis testing

24. McDowell et al. 2011 Carbon starvation induced by: Stomatal conductance to CO2 Metabolic repair impaired by low water potential Phloem transport and xylem embolism positive feedback NSC-limited xylem refilling NSC-limited maintenance of repair processes for water transport NSC-limited turgor maintenance Carbon/water-limited defenses Water limits sap pressure Biotic agents enabled by weakened defenses Biotic agents impair host physiology

25. Climate change strongly affects mortality rates at species range edges and extinctions can occur where range expansion is limited by geography Pollen classes are % of sites showing pollen from that taxa Red outline is modern distribution of bur oak. X marks the location of the paleo sampling for bur oaksPollen classes are % of sites showing pollen from that taxa Red outline is modern distribution of bur oak. X marks the location of the paleo sampling for bur oaks

26. Paleo studies can tell us when mortality events occurred and what climate dynamics may have triggered these events

27. Increased mortality rate at a species range edge during cooling: a paleo example

28. Tropical forests tend to have higher mortality rates than temperate forestsAngiosperms tend toward greater mortality rates than gymnosperms in the temperate zone but the data available is still equivocal

29. In tropical forests there is a large range of wood densities and related traits (photosynthetic rate, xylem conductivity, water storage and growth rates)Higher wood density and related traits convey greater rates of survival in most tropical forests

30. Strong herbivore pressure, and high litterfall/canopy replacement increases mortality of understory trees Heavy liana infection increases mortality rates Causes of seedling mortality in a tropical forest Real are real seedlings followed over time AS are simulated seedlingsHeavy liana infection increases mortality rates Causes of seedling mortality in a tropical forest Real are real seedlings followed over time AS are simulated seedlings

31. Tropical trees have competitors and enemies that are absent in the temperate zone

32. The “self-thinning rule” refers to the decrease in tree density as stand biomass increases for a given forest typeThe larger decrease in density (lower right) as biomass increases shows that small or young forest undergo greater mortality ratesThe general mechanism behind this pattern is larger trees being more effective competitors for limiting resources (most often light)

33. The growth rate conundrum:High productivity and overall growth (ex. rich soils or higher temps) leads to an increased mortality rate but within a stand decreased growth rates also lead to increased mortality rates

34. Growth rate conundrum continued Pygmy forest pic http://geoimages.berkeley.edu/GeoImages/Johnson/Biomes/BiomesSub/PygmyForest.html Pygmy forest pic http://geoimages.berkeley.edu/GeoImages/Johnson/Biomes/BiomesSub/PygmyForest.html

35. Growth rate conundrum part 2:Lifetime growth rates of trees suggest there is a tradeoff between fast growth and longevity These data are from increment cores taken from dead treesThese data are from increment cores taken from dead trees

36. Experimental evidence for a growth/mortality tradeoff:Fast growing individuals exposed to stress had higher mortality rates