Garcia, Emeritus, Sergio

Jensen, Wayne A

Tietje, Bill

Weitkamp, William :

There were many reportsfrom ranchers and other landown - ers of declining and dead oak trees on Californiaâ??sCentral 1987 - 92 Coast during the drought . Along with insects and pathogens , the drought was apparently an important of tree decline causal factor and death . Drought takes toll on Central Coastâ??s native oaks William Tietje R William Weitkamp R Wayne Jensen Sergio Garcia The period of the 6 - year drought From 1987 through 1992 , Califor - ( 1986 - 87 to 1991 - 92 ) coincided with a nia experienced one of the most long - term study of acorn production of widespreadand severe droughts oak ( Quercus Zobutu ) , blue oak ( Q . valley Many of the monitoredtrees that died dur - A survey in the stateâ??s history . ing the drought were relativelysmall ( 4 - 6 dougZusii ) and coastlive oak ( Q . ugrijdiu ) . inches in diameter at breast height ) trees In 1988 , we established permanent plots conducted in three Central on poor sites . in Santa Barbara , SanLuis Obispo , and Coast counties during 1988 - 92 , San Benito counties.Along with several indicates that many native oak other variables , the status or vigor class on rangelands succumbed trees We established 32 plots , 12in Santa of each tree on the permanent plots was to the drought . The smaller oak Barbara County , 7 in San Luis Obispo recorded each fall , 1988to 1992 . Here , County , and 13in San Benito . All of the we report tree and site characteristics on poorer sites were most trees plots were located on oak woodland and the number of trees that severely . vulnerable . grazed by cattle.Within the plots , the declined or died during 1988to 1992 . trunks of single - trunked oak trees and In 1992 , Californiaentered the sixthyear Methods all the stems of multiple - trunked oaks of the worst drought the state has expe - over 4 inches dbh ( allreferred to as During summer and fall 1988 , in this century . The drought was rienced â?쳌 trees â?쳌 in this article ) were marked in Santa Barbara , SanLuis Obispo , and San especially severe on the California Cen - 1988with numbered aluminum tags for Benito counties , one - tenth - acreplots oak ( Quercus tral Coast . Although native permanent identification.We marked were located within stands of blue oak , spp . ) trees are well adapted to periodic 401 trees - 36 valley oak , 115blue oak coast live oak and valley oak , using a drought , the severity of the latest and 250 coast live oak . The number of systematic - randomsampling strategy . drought has apparently contributed sub - 3 and 34 trees per plot ranged between We establishedplots at predetermined stantially to the decline and death of ( average 12.5 ) . intervals 200 feet from a road in the car - many trees . For the past several years , In late August to October 1988 , we re - dinal directionnearest to perpendicular UC CooperativeExtension officeshave corded for each marked tree the species , minimum of with the road . Plots had a received numerous re orts from ranch - tag number , dbh ( inches ) , height ( ft ) , 10 % oak canopy cover and at least three 5 , at oak trees on ers and homeowners in the canopy ( i.e . , and its position oak trees measuring 4 inches in diameter their lands have died . Although it ap - whether the tree was growing com - at breast height ( dbh ) or 4.5 feet from the is finally over , pears the 6 - year drought pletely under the canopy of other trees , ground . We excluded any plot not meet - its effectson populations of native oaks â?? overtopped , â?쳌 or whether it formed the ing these criteria . may be here for a long time to come . CALIFORNIA AGRICULTURE , VOLUME 47 . NUMBER 6 4 bara and San Luis Obispo counties , respec - canopy , â?? not overtopped , â?쳌 ) along with tively , and all six of the drought years in the slope and aspect of each plot . The San Benito County ( fig . status of trees was visually evaluated for 1 ) . the proportion of the tree with branch Tree mortality . At initial tagging in 1988 , all of the trees were alive and and foliage dieback ; dead branches ac - healthy . Declined and dead trees began cessible from the ground were often ex - amined manually for dryness and to show up on the plots when acorn pro - duction measurements were taken in brittleness.Trees with greater than 50 % late summer and fall 1990 . By 1992,10 % dieback were classed â?쳌 declined , â?쳌 and trees ) of the 401 trees on the 32 plots ( 39 trees that were entirely brown and leaf - were either declined ( 6 % ) or dead ( 4 % ) . less were classed â?? dead . â?쳌 All other trees were classed â?? healthy . â?쳌 Chi - square analysisshowed a signifi - cant difference in tree vigor class among Using the method developed by the Santa Barbara , San Luis Obispo , and San CaliforniaDepartment of Fish and Benito counties ( table 1 ) . The difference Game , trees were also ranked 1 to 4 was due to the relativelylargepercent based on the number of acorns observed - 1for no acorns to 4 if acorns occur on ( 14 % ) of trees classed declined in San Luis the entire tree and limbs sag from the ObispoCountycomparedto Santa Bar - bara ( 3 % ) and SanBenito ( 1 % ) counties . weight . Acorn production was low from The drought took about an equal toll 1988 through 1992 as explained for 1988 on each of the three oak species.There and 1989 on pages 161 - 163 in Proceedings were of the Symposium on Oak Woodlands and no significantdifferences by chi - square test in the proportions of de - Hardwood Rangeland Management ( Garcia , clined or dead valley oak ( 8 % ) , blue oak S . , W . Jensen , W . Weitkamp , and W . Tietje . 1990 . Acorn yield during 1988 and ( 13 % ) and coast live oak ( 8 % ) trees in 1989 on Californiaâ??sCentral Coast . Gen . 1992 ( table2 ) . Moreover , the proportion Tech . Rep . of declined trees to dead trees was simi - PSW - 126 , Berkeley , CA Pa - lar for each oak species ( 6 % declined vs . cific SouthwestForest & Range Experi - 3 % dead for valley oak ; ment Station ) . To further characterize 5 % vs . 8 % , re - the plots , we also recorded brush and spectively , for blue oak ; and 4 % vs . 5 % , ground cover present , wildlife signs respectively , for coast live oak ) . In some cases , dieback of monitored trees ( droppings and digging ) and tree - Tree characteristics.Tree and stand did not occur over the entire tree . Here , a characteristicsexamined in this study harvesting history . Results of these mea - large branch of a valley oak tree has died ; surements are not reported here . that may have affected susceptibilityto the valley oak behind it also died during the drought are tree size and dominance the drought . Results status.e Th product of tree dbh and height Rainfall.For the 6 - year drought pe - was used as a tree size index . Signifi - riod of 1986 - 87to 1991 - 92 , total rainfall vs . 278 ) and coast live oak ( size index = cantly more small trees averaged 16 % , 20 % and 26 % below nor - - as determined 252 vs . 407 ) trees were declined or dead mal for Santa Barbara , San Luis Obispo by t - test than larger trees ; the differencewas sig - h - succumbed ( classed edeclinedr nificant by t - test for blue oak . Althoug or dead ) during the drought than larg and San Benito counties , respectively . Annual rainfall , moreover , fell below the declined or dead valley oak trees were trees ( sizeindex = 327 and 426 , respec - 30 - yearannual average larger than healthy trees ( sizeindex tively ) , regardless of species . By species , 3 years and 5 = more smallerblue oak ( sizeindex = 130 1,847vs . 1,009 , respectively ) , the differ - years of the 6 drought years in SantaBar - Fig . 1 . Total annual rainfall recordedat weather stations In Santa Barbara ( Santa Maria ) , San Luis Obispo ( California Poiy - technic State University ) and San Benito ( Hollister ) countiesduringthe 1986 / 87 to 1991 / 92 drought compared to the 30 - year average annual rainfall for each county . CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1993 5 thinning without replacement will lead lated directly to site exposure and slope . ence was not significantaccordingto t - to open stands with just a few remnants Values that quantitatively integrate ori - test . The sample size ( n = 3 ) of declined and , eventually , loss of the stand . entation of terrain and slope ( solar inso - and dead valley oak trees is too small to The 6 - year period of 1986 - 87to 1991 - lation values ) have been developed for concludethat the size difference was not 92 was the driest this century for most coastal central California by E . C . Frank due to chance alone . parts of California . The effect on the R . Lee ( 1966 . Potential solar beam ir - and With valley oak , blue oak and coast Central Coast was even more severe radiation on slopes:tables for 30 " to 50 " live oak trees combined , chi - squaretests than on other areas of the state . It is not latitude . Res . Paper RM - 18 . Fort Collins , indicated a larger proportion of the often , however , that there is a single CO Rocky Mountain Forest and Range overtopped trees had declined or died causal factor in decline and mortality of ExperimentStation , Forest Service.U.S . ( 19 % ) than trees that were not over - oak trees . Drought stress lowers theg Department of Agriculture . 116pp . ) . The topped ( 8 % ) ( table3 ) . Separate chi - trees ' defensemechanisms , makin them solar insolationvalues become larger square tests for each speciesindicated more susceptibleto mortality factors with increasingslope and southern ori - that this difference resulted entirely such as decay fungi and boring beetles . entation ( table4 ) . from the large proportion ( 29 % ) of over - Most likely , drought pushed some oak Using information from Frank and topped coast live oak that had declined trees over the edge that would have per - Lee ( 1966 ) and table 4 , we assigned a so - or died ; no trees were classed declined sisted otherwise . lar insolationvalue to the trees on eachue or dead among overtopped valley or Several circumstances implicatethe plot . The average solar insolation val blue oak trees . This between - species edif - drought as an important causal factor . of declined and dead trees ( 259kilogram ferenceprobably was due simply to th First , most tree mortality occurred on calories / 0.15inz / year ) was significantly dense stands of coast live oak used in the more steep terrain with southern ex - larger by t - test than the solar insolation this study compared to the relatively posure . Since such sites receive more so - value for healthy trees ( 249 kilogram oak . open stands of valley and blue lar irradiation , they typically are drier calories / O.l5 in ' 2 / year ) . This indicates Aspect and slope effects . The abil - with shallower , more infertilesoils than that more trees died on plots with south - ity of a site to support oak trees is af - more gentle slopeswith other orienta - ern orientation.Such sites usually have fected by soil moisture and soil depth . tions . Second , in many areas of oak poorer , more shallow soils . These factors are influencedby degree of woodland during the drought , blue oaks irradiation from the sun , which is re - Conclusions lost their leaves in August , about three months " ahead of schedule . " Research - Little informationis availableon an - ers attributed early leaf fall to tree stress nual mortality rates of California oak due to depleted soilmoisture . Finally , trees . However , the high level of decline many ranchers and other landowners re - or death of oak trees recorded during ported the occurrence of dying and dead the drought was apparently higher than trees on their lands during the drought the long - term average rate of oak stands years . in the absence of fire , insect infestation , Relativelyhigh mortality of oak trees severe drought or other catastrophic during periodic severe drought may events . It is higher than the mortality re - simplybe nature's way of regulating the corded by the Forest Service's Pacific distribution and abundance of oaks in Northwest ResearchStation as part of a California . However , anthropogenic oak trees on long - terminventory of stresses placed on California native oaks sample plots in California ( pers . comm . , over the past 100years have resulted in Chuck Bolsinger , Research Forester , poor regeneration of some speciesand PacificNorthwest Research Station ) . increased removals . Today , periods of This rate of mortality may be prob - s high mortality may not be compensatede lematicto the extent that the oak tree by regeneration.Drought stress may b which die are not replaced . Continual one more reason that we need to be care - ful and wise in making decisionsrelat - ing to oak woodland . We will continue to monitor plot trees for severalmoreof years to further evaluate the effects 6 years of drought on native California oaks . William Tietje is Area Natural Resources Specialist , Department of vEnvironmentaler - Science , Policy , and Management , Uni sity of California , Berkeley , stationed at UC Cooperative Extension , Sun Luis Obispo County . William Weitkamp , WayneJensen and Sergio Garcia are Farm Advisors , Sun Luis Obispo County , Santa Barbara County and Sun Benito County , respectively . The authors thank Ted Swiecki , Doug McCreary and Chuck Bolsinger for offering useful suggestions on an earlier drafr of this article . CALIFORNIA AGRICULTURE , VOLUME 47 , NUMBER 6 6