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Annual variation in needle fall of a loblolly pine stand in relation to climate and stand density
3’9
Henntissey. T.C.. Dougherty, P.M., Crtgg, 3.M. and Wittwer. R.F., 1942. .Annual variation in r?ecdlc fall of a Iobloliy pine stand in r&tion to climate and stand densit). For. Ed. .Ilat~~gc.. 51: 319338. iada E_.) wasthinned to thre; residual basal area levels: A 1O-year-old stand of inblolly pine (P1trus 7.8 III’ ha-‘. 12.6 m’ Ila-‘, and 26.6 m’ha-’ (unthinned). Monthly temperature, rainfall and ncenic fall wcrc determined for 5 consecutive years following thinning. The amount of needle fall produced each year v:as positively related to the amount of basal area on the plot. However. at a given basal area a wide range of medic-fall biomass was observed over the 5 year period. Much of the variation in needle-fall patterns appeared to be correlated Lvith the drcughtiness ofthc grawing season. In lob!oll) pine stands needle fall represents the death of the total needle population formed :n the previoli hear. The amount of needle fall that occurred in a gven year varied with the rainfall and temperatcr”’ condrtlons that existed m the year the foliage was formed. .Avrrag,- ncnual needle fall bar&d b) more than .??o/b from year to )ear for the unthinned control plots. Maximum monthly needle fall occurred 2 months earlier in dry years than in wet years These results indicate that accurate predictions of the amount of canopy needle biomass must account for both the effects of the prl,vious year’s climate on needle production and the effect of the current lear’s climate on needle dur?‘:2n. Currenti). mechanIstic mud& beang used to predict annlJai net carbon gain. stand producti\li) pi :rnnual c~‘apotmnspiration do not adequatei? cons~dc: the interacti\c cf!cl,ts of climate and it,!nd dcnvty on nccd!c biomass dynamics.
Profssscrr T’.C. Hcnnessey. Department of Forestrk. 0klahoma state Univrsity. Stillwater. 0K ?4078. LISA. ‘Present address: 1JSDA Forest Service. Fores,ry Sci,nces tab. Research Tr~.ngl:: Park. NC 17709. !_‘SA. “Journal article 3-5886 ofthe Oklahoma Agricultural Experiment Station. Ok,nhoma State Uni\.ersity. Stiiiwarer, OK 74098. USA. C‘clr.r.4’sp(it?Lic*~i‘.4, IO:
0 1992 ElseT,icr Science
Pcblishcrs
B.‘i;. All 4gbts
reserved
0378-l
1?7 ~S?/SOj.~Kl
(2 ) Need!e fall in lobfolly stands can str~ as an estimate of the amount and Length ofl;.fe ofa needle cohort; (3) Bccacse lobloliy pine retains :?~cdlcs for 2 y%:ar:,,tb.: total amount of I- and 2-year-Ad foliage present in a given year can be cshimated by summing 2 consecutive years of needle fall; in a stand has been shown to be a (4) The total amount of nee& r+-nnass Ir._. good index of stand proauctivity (Jarvis, 1985: Tes;~:y et al., 198’7). Even though needle fall is important in understanding productivity and nutrient cycling of forests, few iongterm studies have been conducted to determine how climate and stand conditions affect amounts and timing of needle fall. Site {actors that have been reported to infiuence needle fall in pine stands include stand density (Gresham, 1982; Gholz et al., 1985 ), nutrition (Landsberg, 1986; Vose and Allen, 1988 ), and site water balance (Landsberg, 1986 ). Gresham ( 1982) summarized several studies which related loblolly pine needle. fall biomass to stand basal area for the southeastern US. We found that needle-fall biomass increased rapidly as stand basal area increased to approximately 26 m’ ha-‘. Needie-fall biomass remained constant or declined slightly as basal area increased to 50 m* ha-‘. However, at any given level of basal area Gresham reported d wide range of needle-fall biomass. This variation suggests that site and environmental variables may have a large effect on the annual needle-fall biomass of a stand. Irose and Lh*r-llsbll ( 1988 ) reported that in stands whir h had similar basal area, fertilization increased needle-fall biomass by as much as 60°% Cramer et al. ( 1984) have shown ?.hat irrigation increased leaf biomass of monterey pine (Pinus radiata D. Don. ) by approximately 2S”/o.The effects of site water balance on needle fall of loblolly pine have not been investigated. The present investigation was conducted as port of an overall project to evaluate the effects of forest management prac?ices on the ecology ofloblolly pine stands on the western edge of its range. Th-: objective of this study was to describe the effects of climate and stand denSty on needle-fail biomass. decomposed;
The study sate ‘was llocateu4 ;:: sou~theastcm Oklahoma. The soil on the site is mapped as Cahaba soil series (LB Gept. &tic., 1974) snd has a so3 moisture hcridirtg capacity of 22 cm. Mean amnual tem,perAure of the region is i 7 “C, anca the average annual rainfall is I25 cm. Average length of the frosbfree per ki is 240 days (Oklahoma Water Resources Board, i984). The site was pkmted with non-improved loblotly pine seerlhngs at an. initial ~~~~~~~~~ +ensity of 2500 :rees ha-- ‘. At the beginning of ahe present s:udy in Mm% I984, the mean basd area of the study port kn of the stand was i 6.6 m’ ha- ! and exhibited a site index of i 7.3 an (base age, 25 years). 1
This study was conducted over a 5 year period in a thinning levels study that was mstalled in March 1984. 1‘+ study design has been described in detail by Cregg et al. ( 1988) and consists of three thinning levels randomly assigned to each of threz blocks (nine plots total). A 0.04 ha measurement plot was nested within each 0.10 ha treatment plot. The three target thinning levels installed in 1984 were: 7.8 m’ ha- ‘, 12.6 m’ ha-’ and 26.6 m- ha-’ (unthinncd). In 1987 the six thinned plots were rethinned to a common target basal area range of i i .&lSr.S m’ ha-‘. The unthinned plot was left as unthinned treatment.
In September 1984 five litter traps 0 50 m7 in area were randomly located in each 0.04 ha measurement plot. Needle-fall collection began in October 1984 and continued monthly through March 19b9, except for a period between April and June 1987 when the plots were rethinned. Needles collected in the litter traps were placed in paper bags and dried at 68°C to a constant weight. Annual needle-fall biomass was calculated for a phenological year by summing monthly needle-fall biomass for the period between 1 April and 3 1 March of the subsequent year.
Monthly temperature, rainfall, and pan evaporation for the years 1984from reccrds obtained f(om the US Army Corps of Engineers’ weather station at Broken Bow Dam, Oklahoma (27 km north of the study site). Rainfall was also monitored monthly on the site with two standard rain gauges. Volumetric soil moisture Iontent was determined at brweekly or monthly intervals using the neutrorJ scattermg method (Long and French, E967 ). Soil moisture content was measured at I5-cm intervals to a depth of ! 20 cm with a Troxler model 3223 depth moisture gauge (Troxler esearch Triangle Park, NC) ~Volumetric soi! moisElectronic Laboratories, ture at 0.006 MPa and I 5 MPa sf tension were determined on a pressure plate apparatus for soil samples taken from each pkot at depths of 15, 30, 45, and 60 cm. Available soil moisture for each plot was determined as the difference in volumetric soil moisture content at 0.006 MFa and 1.5 MPa The percent available moisture content was calculated as the ratio of the volumetric soii m&sture determined in the field to the total avaiiable moisture holding capacity. i ,88 were compileu
The clmatic conditions that existed over ,the 5 year study period are summarized in Table 1. A wide range of summer rainfall-potential evapotranspiration (Thornthwaite and Mather, 1955) combinations were observed. A range from moist (1984) to very dry summers ( 1985. 1987) occurred over the 5 year sturdy period. Trends in soil moisture available in the soil profile ohscrvcd for each year in the three thinning levels are given in Table 2. The higher stand density plots depleted moisture faster and to a greater extent than the lower basal area plots and thus would be cxpectcd to have experienced greater drought stress than the thinned plots. Increased interception of incoming precipitation on the higher density plots also reduced moisture availability (Stogsdill et al., 1989).
-\nnunl prrctpitatibn,
potential evapotransptration
(PET) and 1 (PRECIP-PET j for the Ma?-Oc-
tobcr penoti -y (PRECIP-PET Ma)-October
PET May-October --
(cm)
___
‘77.6 78.3
78.1 - 33.5
78.0
1-l.‘) _ -. ‘0 3 - ‘0.0
.jz..!, 76.5
Annzal nvaiLxbie soil moistun: (cm bzal arid treatments
) in tt~c O-i
(cm)
23 cm prolilc for the Maq-October
period for tht‘three
j
The relationships observed betweeil needle fall and basal area in this study and in the Gresham ( 13S2 ) study are il!ustrated in Fig. 1. Needle-fall biomass increased as plot basal area increased up to about 26 mz ha- ‘. At higher basal area thz trend in needle fall appears not to increase with an increase in basal area, but to remain high and eventually decrease with a wide range of values occurring for any level of basal area. The relationship between neediefall biomass and basal area for this study, which is located on the extreme northwest edge of the loblolly pine range, was similar in magnitude to that reported for loblolly stands located in North 2nd South Carolina (Gresham, 1982 ) The range of annual needle fall observed for the high basal area treatment at the Oklahoma site was actually slightly greater than that. reported by Gresilam for sites ranging from Piedmont to Coastal Plan sites. Annual needle fail at the high basal area levels ranged from a plot low of 3300 kg ha-’ to a high of 5800 kg ha- ’ (Fig. 1). This suggests that annual variations in weather at a single site had as !arge an effect on annual needle-fall patterns as :he siteto-site variations that occurred in Gresham’s study.
To examine the effects of site water balance on annual and within-season variation in needle fall, data from the unthinned plots were used. The unthizmcd treatment was selected because annual changes in basal area due to growtn would not be expected to habe large influences on needle fall (Fig. 1)”
Fig. 1. Relationship ol mnual Data from current study
needle-fali
biomass
(A ) and Gresham
in Mg ha-’ and stand hasal area
[C ) 1982 (used with permission).
in ni' lia-'.
‘ihc
;npparc:nt
can
\x
ti-,9lowiq
seen
er‘i‘~t by
a day
198’7in which
of
site
comparing year
the
ja‘ablc
potential:
water
3 ),
balance amount For
on
new
of
needle
example.
in
needle i‘all tlao
evapotranspirationexceeded
biomass that
years
occurs
production in
precipiIati9n
the
year
1985 and
following for
?he
May
(PRFCP-PET) by 33.5 cm and 29.3 cm, respect;ve!y, the amount of needle fall was greatly reduced (Table 3 and Fig. 2). 1 his trend can be contrasted with that observed for the 1984-l 985 period (T.jble 3 and Fig. 2) which demonstrates that high needle biomass productio1.i occurred in 1984, a vvet year, resulting in high amounts of needle fail in
I~ITCLI~~
October
period
198 5.
‘lhc effecr of growing season climate on the :iming of needle fall observed for the unbhinned plots for the 1984- 1988 period can be inferred from obse:
I-t~FE~‘i-SOFI‘I.ICl4~E
,4NI)
Fig. 2. Average 1984-1988.
monthly
STlnND
needle
DENSITY
ON IIEL’DLE-;-.\LL.
fall and standard
error
BIOMASS
for unthinned
335
treatmmt
in kg ha-‘.
season weather alters needle fall in two ways: ( 1 ) it affects the total amount of new foliage bioxnass produced that will undergo senescence the following year, and (2 ) it shifts the time of peak needle fall by as much as 2 months. Other climatic events sue as strong winds, hail, and heavy rlins and insect and disease infestations can also influence the patterr, of needle fall. Hawever, no inajor influence of these agents was observed during the 5 year study period. An equation to predict annual needts’A‘ kcll. _ stand demities from 7.8 --u fer to 26.6 m’ ha-’ ic. J. Needle fall equa!s 832&I-- 73.7652 (ISA’)
asal area isthe majorfactorthat determines the amount of needle fall that wili occur in developing loblolly pme stands (Gresham, 1982 ). However, once a stand has attained complete crows? _Iosuft, basal area influences are minimal. Site factors such as nutrition (VOX and AIIPI:, 1988 ) and weather wilt tend to dominate the variation in annual needle fall in the absence of major insect arid disease attacks. In this stcdy, annual .?eedle fall of the unthinned plots varied by as much as 29% and a shift of 2 months in the time of peak needle fall was observed. The shift in peak needle fall by 2 months m drought years suggests tbnt needle duration is reduced by drought by about 10%. The amount of variation in annual needle fall at a singie location suggests that change in leaf area may be a sensitive indicator ofclimate changes. Several other studies have reported l-2 year totals and annual patterns of needle fall for pines (Wiegert and Monk, 1972; Nemeth, 1973; Van Lear and Goebel, 1976; Cresham. 1982; Shelton, 1984; Gholz et al., 1985; Eockaby and Taylor-Boyd, 1984). However, none ofthese studies related annual sariation in needle fail to site factors other thar, stand basal area. Understanding the linkage between needle biomass production, senescence and duration and growing season climate conditions is currently of major importance. OveI much of the range of loblolly pine, climate is currently limiting production and the climate of the southeastern US is predicted to become warmer and drier in the near future (Woodman and Furiness, 1988). The results of this study suggest that a shift to a warmer and drier climate would result in reduced production of needle biomass 2nd reduced life expectancy ofthe needle biomass. Based on the relationship between annual net productivi.;y of loblolly pine and leaf area index reported by Vose and Alien ( 1988 )* a drop in annual stand productivity would be expected to occur if needle biomass is reduced. They report for lobiolly pine that a decrelrn ; EeafCSea index by one Lu>c,n unit resulted in a decrease in volume production from 9.0 10 6.5 m’ ha-‘. ‘Thus, rr is ex~ertf?d that the 29% reduction in needle faI1 owing to climate wtiuld resuit in large variations in annua! productivi?y. 750address the expected impact of climate change on lobloily pine producti\;ity, several process nnodels are being developed (The National Acid Pse~~~~~t~~t~~~ A~z~:sment Program, 1989 ). This study suggests that in such models IMCtiOrP-KMmust notbc &scribed jus$asi: f~nceionofstandbasalarea.~F~ri-
ation in needle production and senescence Jwing to annual weather conditions must be accounted for if accurate predictions of carbon and water fluxes are to be made. Also these results .;_,rggestthat models of forest floor dynamics and nutrient cycling will have to consider the effects of weather-related shifts in the timing and amount of needle-fall inputs such as was observed in this study. ACKNOWLEDGMENTS
This work was supported in part by a gram from the Weyerhaeuser Company Mid-South Forestry ‘Research. The authors gratefully acknowledge R. Holeman, R. Baker, Sr., B. Smith, W. Stogsdill, J. Rachal, R. Brown, T. Awstin, P. Cobb, R. Baker, Jr., and R. Heincmann, Oklahoma State University Forestry Research Station, ldabel OK and E. Lorenzi, Department of Forestry, Oklahoma State University, Stillwater, OK for their help in establishing this study and collecting and processing the field data. The authors also thank D. Nolting for typing the manuscript.
REFERENCES Gregg, R.M., Dougherty, P.M. and Hennessey, T.C., 1988. Growth and wood quality of young ioblolly pine trees in relation to stan d density and climate factors. Can. J. For. Res.. 18: 85 l858. Cramer. R.N . Tompkins, D., Barr, N.J., Williams, E.R. and Stewart, H.T.L., 1984. Litterfall in a Pirz~s rlnLdrataforest: The effect of irrigation and fertilizer treatmen?s. J. Appl. Eco!., ? i: 313-326. Ghols, IH.L., Perry. C.S., Cropper, W.P., Jr. and Hendry, L.C., 1985. Litterfall, decomposition, and nitrogen and phosphorus dynamics in a chronosequence of slash pine (Pinus e&orlri) plantations. For. Sci., 31(2): 463-478. Grerham, C..4., 1982. Litterfal! patterns in mature lobiohy and ionglcaf stands in coastal South Carolina. For. Sci.. 28(Z): 223. Jarvis, P.G., 1985. .kreasing productivity and value of temperate coniferous forest by manipulatingsite water balance In: R. Ballard (Editor), Forest Potentials Productivity and Value. Weyerhauser Company Science Symposium 4. Weyerhauser. Tacoma, WA, 301 pp. L.andsberg, J.J., 1986. Experimental approaches to the study ofthe effects ofnutrients and water on carbon assimilation by trees. In: R. Luxmore, J. Landsbergand M. Kaufmann (Editors). Coupling of Ca:bon, Water and Nutrient interactions in Woody Piant Soi! Systems. ? ice Physioi., 2( 1-3 ): 427-444. Lockaby, B.G. and Taylor-!tioyd. J.F., 1986. Nutrient dynamics in the litterfall and forest f!oor of an I &year-old loblo!!y pine plantation. Can. J. For. Res., 16: 1109-l 112. Long. I.F. and French, B.K., 1967. Measurement of soi! moisture in the field by neutron moderation J. Soil Sci. !S( 1): 149-166. The National Acid Precipitation Assessment Program (N.4PAP), 1989. Models planned for use in the NAPAP integrated assessment. The Nationa! Acid Precipitation Assessment Program. Washington. DC. 195pp.
;..
338
r.(‘ I-IENNESSEY
ET j\l...
Water R?‘jOli!.CcS Bawd, I984. C~klatlO~xl’S Water Ilths. Publication NO. 120, Oklahoma Water Resources Board, Norman, OK, I86 pp. ~
Henntissey. T.C.. Dougherty, P.M., Crtgg, 3.M. and Wittwer. R.F., 1942. .Annual variation in r?ecdlc fall of a Iobloliy pine stand in r&tion to climate and stand densit). For. Ed. .Ilat~~gc.. 51: 319338. iada E_.) wasthinned to thre; residual basal area levels: A 1O-year-old stand of inblolly pine (P1trus 7.8 III’ ha-‘. 12.6 m’ Ila-‘, and 26.6 m’ha-’ (unthinned). Monthly temperature, rainfall and ncenic fall wcrc determined for 5 consecutive years following thinning. The amount of needle fall produced each year v:as positively related to the amount of basal area on the plot. However. at a given basal area a wide range of medic-fall biomass was observed over the 5 year period. Much of the variation in needle-fall patterns appeared to be correlated Lvith the drcughtiness ofthc grawing season. In lob!oll) pine stands needle fall represents the death of the total needle population formed :n the previoli hear. The amount of needle fall that occurred in a gven year varied with the rainfall and temperatcr”’ condrtlons that existed m the year the foliage was formed. .Avrrag,- ncnual needle fall bar&d b) more than .??o/b from year to )ear for the unthinned control plots. Maximum monthly needle fall occurred 2 months earlier in dry years than in wet years These results indicate that accurate predictions of the amount of canopy needle biomass must account for both the effects of the prl,vious year’s climate on needle production and the effect of the current lear’s climate on needle dur?‘:2n. Currenti). mechanIstic mud& beang used to predict annlJai net carbon gain. stand producti\li) pi :rnnual c~‘apotmnspiration do not adequatei? cons~dc: the interacti\c cf!cl,ts of climate and it,!nd dcnvty on nccd!c biomass dynamics.
Profssscrr T’.C. Hcnnessey. Department of Forestrk. 0klahoma state Univrsity. Stillwater. 0K ?4078. LISA. ‘Present address: 1JSDA Forest Service. Fores,ry Sci,nces tab. Research Tr~.ngl:: Park. NC 17709. !_‘SA. “Journal article 3-5886 ofthe Oklahoma Agricultural Experiment Station. Ok,nhoma State Uni\.ersity. Stiiiwarer, OK 74098. USA. C‘clr.r.4’sp(it?Lic*~i‘.4, IO:
0 1992 ElseT,icr Science
Pcblishcrs
B.‘i;. All 4gbts
reserved
0378-l
1?7 ~S?/SOj.~Kl
(2 ) Need!e fall in lobfolly stands can str~ as an estimate of the amount and Length ofl;.fe ofa needle cohort; (3) Bccacse lobloliy pine retains :?~cdlcs for 2 y%:ar:,,tb.: total amount of I- and 2-year-Ad foliage present in a given year can be cshimated by summing 2 consecutive years of needle fall; in a stand has been shown to be a (4) The total amount of nee& r+-nnass Ir._. good index of stand proauctivity (Jarvis, 1985: Tes;~:y et al., 198’7). Even though needle fall is important in understanding productivity and nutrient cycling of forests, few iongterm studies have been conducted to determine how climate and stand conditions affect amounts and timing of needle fall. Site {actors that have been reported to infiuence needle fall in pine stands include stand density (Gresham, 1982; Gholz et al., 1985 ), nutrition (Landsberg, 1986; Vose and Allen, 1988 ), and site water balance (Landsberg, 1986 ). Gresham ( 1982) summarized several studies which related loblolly pine needle. fall biomass to stand basal area for the southeastern US. We found that needle-fall biomass increased rapidly as stand basal area increased to approximately 26 m’ ha-‘. Needie-fall biomass remained constant or declined slightly as basal area increased to 50 m* ha-‘. However, at any given level of basal area Gresham reported d wide range of needle-fall biomass. This variation suggests that site and environmental variables may have a large effect on the annual needle-fall biomass of a stand. Irose and Lh*r-llsbll ( 1988 ) reported that in stands whir h had similar basal area, fertilization increased needle-fall biomass by as much as 60°% Cramer et al. ( 1984) have shown ?.hat irrigation increased leaf biomass of monterey pine (Pinus radiata D. Don. ) by approximately 2S”/o.The effects of site water balance on needle fall of loblolly pine have not been investigated. The present investigation was conducted as port of an overall project to evaluate the effects of forest management prac?ices on the ecology ofloblolly pine stands on the western edge of its range. Th-: objective of this study was to describe the effects of climate and stand denSty on needle-fail biomass. decomposed;
The study sate ‘was llocateu4 ;:: sou~theastcm Oklahoma. The soil on the site is mapped as Cahaba soil series (LB Gept. &tic., 1974) snd has a so3 moisture hcridirtg capacity of 22 cm. Mean amnual tem,perAure of the region is i 7 “C, anca the average annual rainfall is I25 cm. Average length of the frosbfree per ki is 240 days (Oklahoma Water Resources Board, i984). The site was pkmted with non-improved loblotly pine seerlhngs at an. initial ~~~~~~~~~ +ensity of 2500 :rees ha-- ‘. At the beginning of ahe present s:udy in Mm% I984, the mean basd area of the study port kn of the stand was i 6.6 m’ ha- ! and exhibited a site index of i 7.3 an (base age, 25 years). 1
This study was conducted over a 5 year period in a thinning levels study that was mstalled in March 1984. 1‘+ study design has been described in detail by Cregg et al. ( 1988) and consists of three thinning levels randomly assigned to each of threz blocks (nine plots total). A 0.04 ha measurement plot was nested within each 0.10 ha treatment plot. The three target thinning levels installed in 1984 were: 7.8 m’ ha- ‘, 12.6 m’ ha-’ and 26.6 m- ha-’ (unthinncd). In 1987 the six thinned plots were rethinned to a common target basal area range of i i .&lSr.S m’ ha-‘. The unthinned plot was left as unthinned treatment.
In September 1984 five litter traps 0 50 m7 in area were randomly located in each 0.04 ha measurement plot. Needle-fall collection began in October 1984 and continued monthly through March 19b9, except for a period between April and June 1987 when the plots were rethinned. Needles collected in the litter traps were placed in paper bags and dried at 68°C to a constant weight. Annual needle-fall biomass was calculated for a phenological year by summing monthly needle-fall biomass for the period between 1 April and 3 1 March of the subsequent year.
Monthly temperature, rainfall, and pan evaporation for the years 1984from reccrds obtained f(om the US Army Corps of Engineers’ weather station at Broken Bow Dam, Oklahoma (27 km north of the study site). Rainfall was also monitored monthly on the site with two standard rain gauges. Volumetric soil moisture Iontent was determined at brweekly or monthly intervals using the neutrorJ scattermg method (Long and French, E967 ). Soil moisture content was measured at I5-cm intervals to a depth of ! 20 cm with a Troxler model 3223 depth moisture gauge (Troxler esearch Triangle Park, NC) ~Volumetric soi! moisElectronic Laboratories, ture at 0.006 MPa and I 5 MPa sf tension were determined on a pressure plate apparatus for soil samples taken from each pkot at depths of 15, 30, 45, and 60 cm. Available soil moisture for each plot was determined as the difference in volumetric soil moisture content at 0.006 MFa and 1.5 MPa The percent available moisture content was calculated as the ratio of the volumetric soii m&sture determined in the field to the total avaiiable moisture holding capacity. i ,88 were compileu
The clmatic conditions that existed over ,the 5 year study period are summarized in Table 1. A wide range of summer rainfall-potential evapotranspiration (Thornthwaite and Mather, 1955) combinations were observed. A range from moist (1984) to very dry summers ( 1985. 1987) occurred over the 5 year sturdy period. Trends in soil moisture available in the soil profile ohscrvcd for each year in the three thinning levels are given in Table 2. The higher stand density plots depleted moisture faster and to a greater extent than the lower basal area plots and thus would be cxpectcd to have experienced greater drought stress than the thinned plots. Increased interception of incoming precipitation on the higher density plots also reduced moisture availability (Stogsdill et al., 1989).
-\nnunl prrctpitatibn,
potential evapotransptration
(PET) and 1 (PRECIP-PET j for the Ma?-Oc-
tobcr penoti -y (PRECIP-PET Ma)-October
PET May-October --
(cm)
___
‘77.6 78.3
78.1 - 33.5
78.0
1-l.‘) _ -. ‘0 3 - ‘0.0
.jz..!, 76.5
Annzal nvaiLxbie soil moistun: (cm bzal arid treatments
) in tt~c O-i
(cm)
23 cm prolilc for the Maq-October
period for tht‘three
j
The relationships observed betweeil needle fall and basal area in this study and in the Gresham ( 13S2 ) study are il!ustrated in Fig. 1. Needle-fall biomass increased as plot basal area increased up to about 26 mz ha- ‘. At higher basal area thz trend in needle fall appears not to increase with an increase in basal area, but to remain high and eventually decrease with a wide range of values occurring for any level of basal area. The relationship between neediefall biomass and basal area for this study, which is located on the extreme northwest edge of the loblolly pine range, was similar in magnitude to that reported for loblolly stands located in North 2nd South Carolina (Gresham, 1982 ) The range of annual needle fall observed for the high basal area treatment at the Oklahoma site was actually slightly greater than that. reported by Gresilam for sites ranging from Piedmont to Coastal Plan sites. Annual needle fail at the high basal area levels ranged from a plot low of 3300 kg ha-’ to a high of 5800 kg ha- ’ (Fig. 1). This suggests that annual variations in weather at a single site had as !arge an effect on annual needle-fall patterns as :he siteto-site variations that occurred in Gresham’s study.
To examine the effects of site water balance on annual and within-season variation in needle fall, data from the unthinned plots were used. The unthizmcd treatment was selected because annual changes in basal area due to growtn would not be expected to habe large influences on needle fall (Fig. 1)”
Fig. 1. Relationship ol mnual Data from current study
needle-fali
biomass
(A ) and Gresham
in Mg ha-’ and stand hasal area
[C ) 1982 (used with permission).
in ni' lia-'.
‘ihc
;npparc:nt
can
\x
ti-,9lowiq
seen
er‘i‘~t by
a day
198’7in which
of
site
comparing year
the
ja‘ablc
potential:
water
3 ),
balance amount For
on
new
of
needle
example.
in
needle i‘all tlao
evapotranspirationexceeded
biomass that
years
occurs
production in
precipiIati9n
the
year
1985 and
following for
?he
May
(PRFCP-PET) by 33.5 cm and 29.3 cm, respect;ve!y, the amount of needle fall was greatly reduced (Table 3 and Fig. 2). 1 his trend can be contrasted with that observed for the 1984-l 985 period (T.jble 3 and Fig. 2) which demonstrates that high needle biomass productio1.i occurred in 1984, a vvet year, resulting in high amounts of needle fail in
I~ITCLI~~
October
period
198 5.
‘lhc effecr of growing season climate on the :iming of needle fall observed for the unbhinned plots for the 1984- 1988 period can be inferred from obse:
I-t~FE~‘i-SOFI‘I.ICl4~E
,4NI)
Fig. 2. Average 1984-1988.
monthly
STlnND
needle
DENSITY
ON IIEL’DLE-;-.\LL.
fall and standard
error
BIOMASS
for unthinned
335
treatmmt
in kg ha-‘.
season weather alters needle fall in two ways: ( 1 ) it affects the total amount of new foliage bioxnass produced that will undergo senescence the following year, and (2 ) it shifts the time of peak needle fall by as much as 2 months. Other climatic events sue as strong winds, hail, and heavy rlins and insect and disease infestations can also influence the patterr, of needle fall. Hawever, no inajor influence of these agents was observed during the 5 year study period. An equation to predict annual needts’A‘ kcll. _ stand demities from 7.8 --u fer to 26.6 m’ ha-’ ic. J. Needle fall equa!s 832&I-- 73.7652 (ISA’)
asal area isthe majorfactorthat determines the amount of needle fall that wili occur in developing loblolly pme stands (Gresham, 1982 ). However, once a stand has attained complete crows? _Iosuft, basal area influences are minimal. Site factors such as nutrition (VOX and AIIPI:, 1988 ) and weather wilt tend to dominate the variation in annual needle fall in the absence of major insect arid disease attacks. In this stcdy, annual .?eedle fall of the unthinned plots varied by as much as 29% and a shift of 2 months in the time of peak needle fall was observed. The shift in peak needle fall by 2 months m drought years suggests tbnt needle duration is reduced by drought by about 10%. The amount of variation in annual needle fall at a singie location suggests that change in leaf area may be a sensitive indicator ofclimate changes. Several other studies have reported l-2 year totals and annual patterns of needle fall for pines (Wiegert and Monk, 1972; Nemeth, 1973; Van Lear and Goebel, 1976; Cresham. 1982; Shelton, 1984; Gholz et al., 1985; Eockaby and Taylor-Boyd, 1984). However, none ofthese studies related annual sariation in needle fail to site factors other thar, stand basal area. Understanding the linkage between needle biomass production, senescence and duration and growing season climate conditions is currently of major importance. OveI much of the range of loblolly pine, climate is currently limiting production and the climate of the southeastern US is predicted to become warmer and drier in the near future (Woodman and Furiness, 1988). The results of this study suggest that a shift to a warmer and drier climate would result in reduced production of needle biomass 2nd reduced life expectancy ofthe needle biomass. Based on the relationship between annual net productivi.;y of loblolly pine and leaf area index reported by Vose and Alien ( 1988 )* a drop in annual stand productivity would be expected to occur if needle biomass is reduced. They report for lobiolly pine that a decrelrn ; EeafCSea index by one Lu>c,n unit resulted in a decrease in volume production from 9.0 10 6.5 m’ ha-‘. ‘Thus, rr is ex~ertf?d that the 29% reduction in needle faI1 owing to climate wtiuld resuit in large variations in annua! productivi?y. 750address the expected impact of climate change on lobloily pine producti\;ity, several process nnodels are being developed (The National Acid Pse~~~~~t~~t~~~ A~z~:sment Program, 1989 ). This study suggests that in such models IMCtiOrP-KMmust notbc &scribed jus$asi: f~nceionofstandbasalarea.~F~ri-
ation in needle production and senescence Jwing to annual weather conditions must be accounted for if accurate predictions of carbon and water fluxes are to be made. Also these results .;_,rggestthat models of forest floor dynamics and nutrient cycling will have to consider the effects of weather-related shifts in the timing and amount of needle-fall inputs such as was observed in this study. ACKNOWLEDGMENTS
This work was supported in part by a gram from the Weyerhaeuser Company Mid-South Forestry ‘Research. The authors gratefully acknowledge R. Holeman, R. Baker, Sr., B. Smith, W. Stogsdill, J. Rachal, R. Brown, T. Awstin, P. Cobb, R. Baker, Jr., and R. Heincmann, Oklahoma State University Forestry Research Station, ldabel OK and E. Lorenzi, Department of Forestry, Oklahoma State University, Stillwater, OK for their help in establishing this study and collecting and processing the field data. The authors also thank D. Nolting for typing the manuscript.
REFERENCES Gregg, R.M., Dougherty, P.M. and Hennessey, T.C., 1988. Growth and wood quality of young ioblolly pine trees in relation to stan d density and climate factors. Can. J. For. Res.. 18: 85 l858. Cramer. R.N . Tompkins, D., Barr, N.J., Williams, E.R. and Stewart, H.T.L., 1984. Litterfall in a Pirz~s rlnLdrataforest: The effect of irrigation and fertilizer treatmen?s. J. Appl. Eco!., ? i: 313-326. Ghols, IH.L., Perry. C.S., Cropper, W.P., Jr. and Hendry, L.C., 1985. Litterfall, decomposition, and nitrogen and phosphorus dynamics in a chronosequence of slash pine (Pinus e&orlri) plantations. For. Sci., 31(2): 463-478. Grerham, C..4., 1982. Litterfal! patterns in mature lobiohy and ionglcaf stands in coastal South Carolina. For. Sci.. 28(Z): 223. Jarvis, P.G., 1985. .kreasing productivity and value of temperate coniferous forest by manipulatingsite water balance In: R. Ballard (Editor), Forest Potentials Productivity and Value. Weyerhauser Company Science Symposium 4. Weyerhauser. Tacoma, WA, 301 pp. L.andsberg, J.J., 1986. Experimental approaches to the study ofthe effects ofnutrients and water on carbon assimilation by trees. In: R. Luxmore, J. Landsbergand M. Kaufmann (Editors). Coupling of Ca:bon, Water and Nutrient interactions in Woody Piant Soi! Systems. ? ice Physioi., 2( 1-3 ): 427-444. Lockaby, B.G. and Taylor-!tioyd. J.F., 1986. Nutrient dynamics in the litterfall and forest f!oor of an I &year-old loblo!!y pine plantation. Can. J. For. Res., 16: 1109-l 112. Long. I.F. and French, B.K., 1967. Measurement of soi! moisture in the field by neutron moderation J. Soil Sci. !S( 1): 149-166. The National Acid Precipitation Assessment Program (N.4PAP), 1989. Models planned for use in the NAPAP integrated assessment. The Nationa! Acid Precipitation Assessment Program. Washington. DC. 195pp.
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Water R?‘jOli!.CcS Bawd, I984. C~klatlO~xl’S Water Ilths. Publication NO. 120, Oklahoma Water Resources Board, Norman, OK, I86 pp. ~