MORTALITY RESPONSE OF SUBALPINE FORESTS
IN THE SIERRA NEVADA, CALIFORNIA, USA
C.I. Millar, D.L. Delany, R.D. Westfall
USDA Forest Service, Pacific Southwest Research Station, Albany, CA 94710 USA
For info, contact cmillar@fs.fed.us
.
Subalpine forests across western North America have experienced increasing mortality in recent years. A combination of drought, warming
temperatures, native insects, and an exotic pathogen (white pine blister rust, WPBR) contributed to many of these episodes. Curiously the
Sierra Nevada, CA has been mostly spared landscape-scale mortality in high-elevation forests. We reported on a mortality event in disjunct
eastern Sierra Nevada (ESN) limber pine (Pinus flexilis, LMP) forests that occurred during the 1988-1992 drought (Millar et al. 2007). Here we
analyze a similar forest mortality event ongoing in the eastern Sierra Nevada nvolving whitebark pine (Pinus albicaulis, WBP) and compare
this to the LMP mortality.
METHODS
We selected all obvious regions in the ESN that exhibited extensive WBP mortality. In each
region, we randomly distributed 6m-radius circular plots, with plots ~60m apart; number of plots
was proportional to the area of mortality. Within plots we extracted increment cores from all
WBP (live & dead) > 15cm stem diameter, and cored an additional outplot 5 live and 5 dead trees
associated with each main plot. Cores were measured, trees aged, and tree-ring series crossdated using standard dendrochronological methods. Correlations to historic climatic variables
were analyzed relative to a composite long-term climate record derived from 4 regional NWS
Coop climate stations (POR ~1900-present) and using the JMP statistical package.
Tbl. 1
RESULTS
Six regions had extensive forest mortality, distributed patchily along the ESN in Mono
and northern Inyo Counties, CA (Fig 1). Although some trees died earlier, the pulse of
mortality began in 2007. Proximal cause of death was mountain pine beetle and
associated blue-stain fungus infestation; no sign of white-pine blister rust was evident.
Mortality stands were consistently on north aspects, in upright forests near the lowmiddle part of the species’ elevation range (2820-3100m), at the easternmost edge of
the escarpment, and composed closed-canopy, dense, and monotypic forests. We
sampled 37 plots and 601 trees total.
Fig.1.
Stand mortality averaged 75%, ranging from 42% to 92% (Tbl 1). Plot basal areas were relatively high, averaging 48.3 m2/ha over the six
regions (Tbl 1). Tree ages were mostly young-mature (~70 – 150 yrs; Fig 2); trees that survived (“live trees”) and trees that died (“dead
trees”) were about the same age, although the dead group had older trees.
Climate analyses were assessed for the live tree- versus dead tree groups. A multi-year drought beginning in 2007 coincided with the WBP
mortality event (Fig 3). Annual ring width (a measure of relative tree growth) in the dead trees was significantly correlated with wateryear precipitation (r = 0.24), climatic water deficit (CWD, r = -0.24), and with minimum temperature (r = 0.28) in live and (r=0.19) in dead
trees. Low growth occurred in live and dead trees during historic 20th-C multi-yr droughts; correlations were strongest in trees that died.
Ring width in the dead trees was also significantly and negatively correlated to 2-yr lagged CWD (r = -0.46), suggesting a delayed
response to water stress.
Differential growth of live trees compared to dead trees over the century timescale suggests contrasting adaptive responses to climate
regimes (Fig 4). Prior to ~1920, dead trees grew faster than live trees. This relationship switched after 1920: dead trees grew slower than
live trees. Growth was very low during the 1988-1992 drought in which the LMP mortality event occurred. Both live and dead WBP trees
had a minor rebound in growth thereafter, but grew poorly in the years until the 2007 event, especially the dead trees.
Fig. 2
DISCUSSION AND BROADER IMPLICATIONS
The current WBP mortality event, which began in 2007, is occurring at only a few locations of the ESN but under consistent
environmental contexts (N slope; low-mid elevation; young-mature, dense, and monotypic stands). These conditions are the same as
those that characterized the 1988-1992 LMP mortality event (Millar et al. 2007). Similarly, whereas bark beetles were the proximal
cause of tree death in LMP and WBP events, late 20th-C warming and water stress seemed to have imposed cumulative stress, preconditioning the stands for beetle attack. The severe dry year of 2007 and subsequent warm, dry years (and lag in CWD) coincide with
the current mortality event in WBP as did a severe and warm drought coincide with mortality in LMP. Similar climatic relationships
have been associated with massive forest dieback in other parts of the West, where they have been labeled “global-warming style
droughts” (Breshears et al. 2005) implying increasing stress from temperature combined with typical multi-year droughts.
Notably, WBP survived the 1988-1992 drought that affected LMP, although WBP growth was very poor during that period. Although
growth rebounded somewhat, a stress period began at the time that continued until the current mortality event. Notably also, LMP
stands in the central ESN are not showing mortality in the current drought.
The differential WBP growth response before and after ~1920 between trees that survived and trees that died also occurred in LMP.
Given typical high genetic diversity within pine stands, we interpret this as an adaptive response of genetically diverse stands to
climate change. The Little Ice Age (~1400-1920AD, LIA) was a cool period with low water stress on high montane forests, whereas
after 1920 climates began to warm, first naturally, and with anthropogenic warming accelerating in the recent decades. Thus trees that
established in the 18th and 19th centuries would have been selected for cool, wet conditions, and, when conditions turned warm and
drier in the 20th-C, those trees were at relative disadvantage. By contrast, trees in the stands adapted to warm and dry conditions grew
better in the 20th-C and survived the current event. North and mesic stands might be most affected in the ESN because stands
establishing during the LIA would be selected for competitive growth under optimum forest conditions. Stands on other aspects in
our region were still subject to Mediterranean climates and would be more drought-hardy. When conditions changed to warmer and
drier, the north-aspect WBP stands were most vulnerable.
In sum the 20th-C forest mortality events in LMP and WBP appear to have acted as strong directional natural selection that improved
stand fitness to conditions of the present- and anticipated future climates. The silvical stand-thinning consequence of the mortality
event complements the genetic effect by reducing the likelihood of future beetle infestations. In the cases of WBP and LMP studied
here, mortality is improving stand vigor and adaptability to future conditions.
Fig. 4.
Fig. 3.
REFERENCES
Breshears, DD and 12 others. 2005.
PNAS 37: 2508-2520
Millar, CI, RD Westfall, and DL Delany.
2007. Canadian Jour For Res 37:2508-2520.