ENVIRONMENTAL LIMITS ON ABOVEGROUND NETPRIMARY PRODUCTION, LEAF AREA, AND
BIOMASS IN VEGETATION ZONES OFTHE PACIFIC NORTHWEST'
HENRY L. GHOLZ2Department of Forest Science, School of Forestry. Oregon State University,
Corvallis. Oregon 97331 USA
Abstract. Mature vegetation from eight of the 12 major vegetation zones in Oregon and Wash-ington was sampled along a transect from the Pacific Coast to the east slopes of the Cascade Moun-tains. Six stands were in forests. one in woodland, and one in the shrub-steppe. Aboveground-over-story net primary production NPP. estimated as the sum of annual stem. branch, and foliageproduction) ranged from <1 to 15 Mg• ha-'• yr i . aboveground biomass from 3 to 1500 Mg/ha. andarea of all sides of leaves from 1 to 47 ha/ha: minima were in the shrub-steppe zone and maxima inthe coastal forest zone.
Maximum leaf area index. biomass. and NPP were all strongly related both to a simple index ofgrowing season water balance and to mean minimum air temperatures in January. In the subalpineconifer zone. though. cold winter temperatures apparently have a stronger influence than summerwater availability. Of the water balance components. evaporative demand alone could account for>90% of the variation in leaf area index. Although annual precipitation ranged from 20 cm in theshrub-steppe to 260 cm at the coast. it was a relatively poor predictor of stand structure and produc-tion. Biomass and NPP increased linearly up to a leaf area of ha/ha: above this point. biomasscontinued to increase while NPP decreased. Except in the coastal forest zones. NPP was less thanmaximum values reported for other mature systems elsewhere in the world for the same range in leafarea indices. Compared to other forested regions of the temperate zone with the same NPP. thesesystems receive more annual precipitation. and average twice the basal area and biomass.
Key words: biomass; evaporation: forest: leaf area index: net primary production: Oregon: pre-cipitation: shrub-steppe: temperature; vegetation zone: water balance.
INTRODUCTIONBiomass. net primary production (NPP). and leaf
area are key characteristics of autotrophic ecosystemsbecause they define the standing crop and flux of car-bon and nutrients, and set upper limits on water usethrough transpiration and on carbon fixation throughphotosynthesis. These variables or their combinationsare regarded as good indices of the economic impor-tance (Beuter et al. 1976), efficiency of solar energyuse (Whittaker 1966), maturity (Bormann and Likens1979). and stability (Odum 1969) of autotrophic eco-systems. It is becoming more important to obtain es-timates at regional, national, and global levels, to as-sess. for example. effects on regional water qualitydue to silvicultural activities (EPA 1980). or effects onatmospheric CO., levels as a result of widespread ma-nipulation of forest lands (Woodwell 1978. Delcourtand Harris 1980).
Relationships between structural features. such asbiomass or leaf area, and functional features, such asNPP. have been widely sought. Structural features are
1 Manuscript received 15 June 1979: revised 10 July 1981:accepted 14 July 1981.
Present address: School of Forest Resources and Con-servation. University of Florida. Gainesville. Florida 32611USA.
more easily measured, and it is reasonable to expectthat production or standing biomass would be relatedto the photosynthesizing surface area over a range ofecosystems. However, as direct estimates of NPP orstructure for even one stand are very costly and timeconsuming to obtain, at regional or larger levels theseestimates must be made indirectly. The most promis-ing indirect methods involve correlations with someaspects of climate, particularly precipitation and tem-perature (Rosenzweig 1968. Bazilevich et al. 1971.Lieth 1975. Grier and Running 1978, Box 1980).
Although the Pacific Northwest region of the UnitedStates contains a range in vegetation from desert scrubto subalpine and coastal rain forests (Franklin andDyrness 1973), and is one of the leading timber-pro-ducing regions of the world, no studies have addressedregional patterns of NPP or biomass, and only one hasgeneralized about leaf area (Grier and Running 1977).Because many of the forests west of the CascadeMountains appear unique in terms of their massive-ness and the longevity of individual trees, apparentlyas adaptations to the characteristic dry summer cli-mate (Waring and Franklin 1979), predictions of NPP,biomass, or leaf area based on data from elsewhere inthe world may not yield reliable estimates for this re-gion.
The objective of this study was to elucidate the re-
NEVADACALIFORNIA
121°
1
23° 118°1
119° 117°122°
1
120°124°
470 HENRY L. GHOLZ
COASTRANGE4-•
WASHINGTCNCASCADE
MOUNTAINS
Ecology. Vol. 63. No. 2
46°
PS- P ^ceo sacnens,s Zone
Tri- 'sup° neferoonyho Zone
WV- VfillOMette Volley
5 - TrOriSitiOn a S.,bolo^ne ZonesPP- Ploys 00nder0S0 Zone
JO- Junoerus occ:clentoits Zone
AT- Ifternism tridentoto Zone
PACIFICOCEAN
45°
44°
IDAHO
— 43°
42°
FIG. 1. Study sites (Roman numerals) in Oregon. superimposed on the vegetation zones from Franklin and Dyrness(1973).
lationships among ecosystem structure, as reflected inbiomass and leaf area accumulation, function, as re-flected by aboveground NPP, and the regional climateof the Pacific Northwest. Three main hypotheses weretested: (I) that structure and function are related overthe range of natural and mature ecosystems in the Pa-cific Northwest, (2) that these variables can be pre-dicted on the basis of climatic differences across thisrange. and (3) that the relationships derived for thePacific Northwest are unique to the region.
STUDY AREAS
Leaf area index and NPP increase rapidly early insecondary succession and tend to decrease to a pla-teau (Kira and Shidei 1967. alum 1969. Turner andLong 1975. Bormann and Likens 1979). This plateauoccurs at 40-60 yr in Pseudotsuga forests (Turner andLong 1975) and somewhat later where conditions areharsher. Stands selected for this study were oldenough to have reached a plateau. but were not visiblydisturbed by management. wildfire, major episodes ofwindthrow, insect or pathogenic outbreaks, or heavyrecreational use. Furthermore. overmature standswith abundant mortality of dominant vegetation wereexcluded from consideration. Two final criteria wereyear-round accessibility and a large open area nearbyto ensure comparable climatic monitoring at all sites.Those areas selected were not chosen to provide "av-erage* . values for each vegetation zone, but only asthey satisfied these criteria.
Eight study sites in 8 of the 12 major vegetationzones in the Pacific Northwest (Franklin and Dyrness1973) were selected. The sites (Fig. 1) were along atransect between 44° and 45°N latitude from the Ore-gon Coast (124°W longitude) 350 km east to the highdesert in central Oregon (121°W longitude).
Stands in the five western zones were dominated byeven-a ged overstories of trees 120-200 yr old. al-thou gh several stands had a few much older individ-uals. Eastern zone communities were uneven aged.and were mature and undisturbed based on publisheddescriptions of the natural vegetation (Driscoll 1964.Franklin and Dyrness 1973).
Plots I (two areas. a and b) and II were in westernOregon: Plots ranged from the base to the sum-mit of the western slope of the Cascades: and PlotsVI—VIII were increasingly arid in the rain shadow eastof the crest of the Cascades. Table I contains a de-scription of all the study sites, and four representingthe full range in characteristics are presented as pho-tographs in Fig. 2. Across the transect winters are cooland wet. and summers are dry, generally with 2-3 moof no measurable precipitation (Franklin and Dyrness1973). The climate becomes more continental east ofthe Cascade Mountains.
METHODS
At each site a vegetated plot from 0.25 to 0.41 hawas laid out surrounded by a buffer strip at least 25m wide. and an open area, usually a clearcutting of
April 1982 LIMITS ON STRUCTURE AND PRODUCTION 471
TABLE Characteristics of study sites along the transect. Physiographic provinces are from Franklin and Dyrness (1973).
Plot
Feature la lb II III IV V VI
Physiographicprovince
>10 cm dbh
Basal area (M'iha)
Soil waterstoragecapacity (cm)
Soil subgroup(tentative)
High Cascades Eastern highsummit Cascades
Elevation (m)Slope I%)Aspect
Stem density (no.ihal<10 cm dbh
Western coast Interior coast Low-elevation Mid-elevationrange range west west
the same slope, aspect. and elevation. was identifiednearby.
Climate measurementsSpecific climatic data from published sources were
not uniformly available or extensive enough to de-scribe the sites adequately for the purposes of thisstudy. Therefore, I chose to measure precipitation.evaporation, soil water storage capacity, the degree ofsummer extraction of soil water. and year-round soiland air temperatures. Precipitation and evaporationwere monitored in all open plots from May throughOctober in both 1976 and 1977. Precipitation accu-mulated in rain gauges and was emptied at least every3 wk: oil prevented evaporation. Relative evaporationwas measured over 3-wk intervals using evaporimetersconsisting of a water reservoir and a disc of pine sap-wood as an evaporating surface (Gholz 1979).
Soil temperatures. at a depth of 20 cm and air tem-peratures 1 m above ground were measured usingPartlow 30-d spring-wound thermographs installed inlate fall 1975. and removed in early spring 1978. Airtemperature probes were shielded from direct and re-flected shortwave radiation. A temperature growth in-dex (TGI) (Cleary and Waring 1969) was used to sum-marize average soil and air temperatures. Thefollowing two winter temperature indices were alsocomputed: (1) the percentage of days from Februarythrough April with air temperature averaging <-2°(Waring et al. 1978), and (2) the average minimum dayair temperature in January.
For each site water storage capacity of the <2-mm
fraction of soils (SWC, 10-1520 kPa tension) was mea-sured at four depths to 1 m. In 1977 just before budburst and again in August at the peak of the typicalsummer drought, three soil samples from each depthwere taken from two pits near the middle of each plot.Gravimetric water contents were determined and. withthe laboratory determinations of 10 and 1520 kPawater contents and bulk densities, the net amount ofwater extracted from the soil over the "growing sea-son" (SWE) was estimated (Waring and Major 1964).Abnormally heavy rain interrupted August samplingfor sites west of Santiam Pass (Plot V); there Augustmoisture contents were assumed to be 60% of thosein the spring (Krygier 1971). These data were usedwith precipitation and evaporation to derive separatewater balance indices (after Grier and Running 1978)for 15 May-15 October 1976 and 1977:
WB = P — E + SWC (1976 and 1977)
OT
WB P — E + SWE (1977 only),
where WB is the water balance (centimetres). P andE are the precipitation and evaporation (centimetres)during the period, SWC is the soil water storage ca-pacity (centimetres), and SWE is the spring-to-Augustsoil water extraction (centimetres).
Biomass and leaf area indexDiameter at breast height (dbh at 1.37 m) of each
tree >5 cm was measured on Plots I—VI. Total heightsand the heights to the base of the live canopy of se-
472 HENRY L. GHOLZ Ecology. Vol. 63. No. 2
a••• 4.1-41*- ; • "411, /L'ai
1.% ;.E3! AI&
•
• •
It. • •
v • R . ic,,,up•A.
_ •r .PP
i,o`v 40441V
- .•;-4k•
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FIG. 2. Four study plots characterizing the range in sites across Oregon: (A) Coastal Tsuga heterophyllaTicea sitchensis(Plot lb) (United States Forest Service photo by Larry Huditz). (B) Subalpine Tsuga mertensiana (Plot V). December 1976.where 2-m snowpacks are present most of the winter. (C) Pious ponderosa (Plot VI) on the eastern slope of the CascadeMountains. Artemisia tridentata var. tridentata and the nitrogen-fixing shrub Purshia tridentata are in the understory. (D)Juniperus occidentalis (Plot VII, an intensive site for Gholz 1980) on a broad ridge at 1356 m in central Ore gon. The largesttrees are yr old.
lected trees spanning the size range (at least 15 verplot) were measured with an Abney level. Trees <5cm dbh were tallied on a set of 11 smaller (2 x 2 m)subplots. All trees were tallied on Plot VI. Plot VIIprocedures were similar and are described in detail byGholz (1980). At the shrub-steppe Plot (VII) maximumlengths. heights. and widths (at right angles to thelengths) were measured for the canopy of each hushon five randomly located circular subplots. each 5 min radius.
Stem, branch, and foliage biomass for individualplants were computed for each species in each plotusing the plant measurements and regression equa-tions for species destructively sampled on diverse sitesin the Pacific Northwest (Table 2). The sources of dataused for these equations, sampling procedures. andother documentation are given in Gholz et al. (1979).No equations were available for Chr •sotizamnus (Plot
VIII): therefore, its biomass was estimated usin g theArtemisia equations. Pinus nzonticola biomass wasestimated using multispecies composite "Pious"equations (Table 2). Pinus ponderosa equations werebased on trees destructively analyzed in northern Ar-izona (see Gholz et al. 1979); a comparison of theirdiameters and heights with those of Plot VI trees in-dicates that the equations may have slightly underes-timated Pious stem biomass on Plot VI. Suitableregressions were unavailable for Abies A.frandis at PlotII. and for the few Abies lasiocarpa at Plot V: biomasswas computed using composite "Abies — equationsbased on data from four other Abies species (Table 2).
Leaf area indices were computed from leaf biomassusing specific leaf areas (square centimetres per gram.Gholz et al. 1976) of foliage shot from the mid-canopywith a shot gun in early August. Samples were col-lected from at least three dominant trees and from all
LIMITS ON STRUCTURE AND PRODUCTION 473
TABLE 2. Regression equations used to estimate overstory on all plots along the transect. Except where noted. all are of theform: In ( Y)=a+bln(db1-1). where Y is mass in kilograms and dbh (diameter at breast height. 1.37 m) is in centimetres.Most equations without a footnote represent a composite of data from several published and unpublished sources and aremore fully documented in Gholz et al. (1979). NA = not available. 5 2,., = residual sis error variance.
From Grier and Logan (1977).t From Uresk et al. (1977): Y is biomass in grams. X is crown volume in cubic centimetres.t From Gholz (1980): .V is basal circumference in centimetres.
From Fujimori et al. (1976). No associated statistics were provided. X. here. is dbh2 x height (metres): Y for the stemequation is volume in cubic decimetres: to obtain mass for these trees multiply Y by 0.35 Wm'.
April 1982
474 HENRY I.. GHOLZ Ecology. Vol. 63. No. 2
TABLE 3. Climate of study sites from west to east along transect. NA = not available.
- Precipitation and evaporation for 1976 and 1977 were measured at each site.t Representing at least 10 yr of data from NOAA (1977) or other meteorological station nearest to each site.
Using soil water extraction.§ Average percent of days with mean air temperatures < -2°C from 1 February to 30 April 1976 and 1977.
major species in each stand. Cross-sectional adjust-ments (Gholz et al. 1976) were applied to convert pro-jected leaf area indices to a total-surface basis.
Net primary productivityOn Plots I-VI. 5-yr diameter growth at breast height
was measured on increment cores from at least 25overstory trees spanning the dbh range (bark growthwas assumed directly proportional to wood growth).Dry biomass increments of stems ( wood plus bark)and branches were calculated using the biomass equa-tions (Table 2. Gholz et al. 1979) in conjunction withthe current and calculated previous 5-yr dbh values.
Regressing increment values against the current dbhyielded equations that were applied to the dbh of alltrees in each stand to obtain 5-yr biomass incrementsfor the entire stand: dividing by five then yielded av-erage annual increment. Plot VII was treated similarly(Gholz 1980). Annual production equations forbranches and stems ( Fujimori et al. 1976) were usedfor Picea at the two coastal plots and average annualtotal (wood plus foliage) aboveground production ofArtemisia. based on a seasonal harvest method. wasprovided by G. Nelson (personal communication,compiled from long-term records at Squaw Butte Ex-periment Station. Burns, Oregon).
Because litterfall was not collected, new foliage pro-duced by Plots I-V was estimated as a fixed percent-age of foliage biomass; this percentage varied from 20to 30%. depending on published values of foliage re-tention time (Fujimori et al. 1976. Grier and Logan1977). Foliage production was assumed to be 25% offoliage biomass for Pinus (Plot VI) based on the west-ern Oregon species and 30% for Juniperus (Plot VII.
Mason and Hutchings 1968). Current production loss-es to mortality, nonfoliar litterfall, or herbivore graz-in g were assumed zero. Aboveground NPP for PlotsI-VII was the sum of annual production by stemwood. bark, branches. and foliage.
RESULTS
ClimateThe percentage of winter days with temperatures
<-2°C increased progressively away from the coast.while minimum day air temperatures for January de-creased from to - 12°C (Table 3; due to equipmentfailure data are unavailable for Plot III).
Average annual precipitation decreased markedlyfrom the coast to the desert. but fluctuated greatlyfrom site to site (Table 3). Summer precipitation av-eraged 13-19% of annual precipitation for the six west-ern zones and 36% of annual precipitation in the threeeastern zones. Summer precipitation in 1977 was morerepresentative of long-term trends (10-40 yr averages,NOAA 1977) and was greater than 1976 precipitationfor all plots west of the Cascades. Evaporation for allsites was uniformly higher in 1977 than in 1976.
The water balances reflect these variations (Table3). At the coast (Plots la and lb), the higher precipi-tation in 1977 was nearly balanced by higher evapo-ration. and the water balances were similar for 1976and 1977. Evaporation increases more than offset pre-cipitation increases from 1976 to 1977 for the rest ofthe transect, so that the water balances were. on theaverage. twice as negative as those for 1976. The waterbalances at the subalpine site (V) were similar to thosefrom the transition zone (IV); otherwise. the balances
April 1982 LIMITS ON STRUCTURE AND PRODUCTION 475
TABLE 4. Structure and production relations of study sites, from west to east across transect.
became more negative in a linear fashion from thecoast. east to the shrub-steppe site (VIII. Table 3).
Biomass and leaf area indexEstimates of total aboveground biomass (Table 4)
ranged from 3 Mg/ha in the Artemisia zone (VII) to1500 Meiha in one coastal forest (lb). Foliage biomassincreased from 2 to 8% of overstory aboveground bio-mass in the forested zones to 21% in the Juniperuswoodland (VII), and 33% in the Artemisia shrub-steppe (VIII). Likewise, live-branch biomass in-creased from 7-10% for the four western forest zonesto 67% in the Artemisia zone (VIII). assuming Arte-misia wood to be all "branches - with no "stern - bio-mass. Stem biomass decreased below 80% of total bio-mass only in Plots VI (73%) and VII (45%).
Leaf area indices varied greatly, from I to 47 haiha(projected areas of 0.5-20.0 ha/ha) again, generallydecreasing from west to east (Table 4). Standing cropdensities (Kira and Shidei 1967). the dry matter con-centrations from the ground to the top of the canopy.decreased steadily from 2-3 kg/m" in the coastal zone(Plots Ia and b) to 0.3 kg/m3 in the three rain-shadowzones (Plots VI-VIII). Canopy heights decreased from35-55 m in the forested zones to 8 m (Plot VII) and 1m (Plot VIII) in the two driest zones. The density offoliage in the canopies (Kira and Shidei 1967) in sevenzones ranged inconsistently from 0.03 to 0.08 kg/m3.The Pi►ts ponderosa canopy (Plot VI) was the leastdense (.016 kg/m3 foliage).
Both biomass and leaf area index showed similarrelationships with climatic variables (Table 5). Meanminimum January air temperature was the best pre-dictor of both stand biomass (r 2 = .91) and leaf areaindex (r2 = .89. Table 5A). However. if the subalpineTsuga mertetzsiana stand (V) was excluded, on thepremise that leaf area indices on these sites are notwater limited (Grier and Running 1977), then the 1977water balance best predicted both leaf area index (r2 =.97) and biomass (r2 = .95), with very low variances
in each case (Table 5B). Relationships with the mini-mum temperatures did not change when Plot V wasexcluded: the water balance relationships becamemuch better, with the variance for leaf area indicesdecreasing from 34 to 8 and for biomass from 51 000to 15 000 (Table 5C).
The leaf area index:water balance relationshipswere somewhat stronger (differences were not signif-icant) when soil water extraction rather than storagecapacity was used (Table 5B), and regression slopesfor 1976 and 1977 were significantly different. Of thewater balance components, growing season evapora-tion was the best predictor of leaf area index (r 2 = .91for 1977): growing season precipitation (r2 = .83) andsoil water extraction (r2 = .82) were not as good pre-dictors (Table 5D). Soil water storage capacities hada smaller range than extraction and were not signifi-cantly correlated with leaf area index (r 2 = .04. Table5D), and only poorly with precipitation and extraction
< .5). In this study, leaf area index was poorlycorrelated with TGI ( r2 = .47, Table 5D), declining atboth high and low TGI values with considerable vari-ation.
Stand dimensional characteristics are generally lessinteresting for predicting stand structure and produc-tivity at a regional level because they can only be ob-tained through direct field measurement. However,when all eight plots were considered, or nine if Plotsla and b were treated separately, basal area was thebest predictor of leaf area index (r2 = .92) and bio-mass (r 2 = .94) of all the variables considered (Table5G). Biomass was significantly correlated with leafarea index (r2 = .91, Table 5G), so the similar patternsof correlation with climate and basal area are not sur-prising.
Net primary productionNPP ranged from 0.3 to 15 Mg• ha-' • yr- 1 . Total bio-
mass:NPP ratios declined from 95 at the coast to 12in the eastern zones. NPP:foliage biomass ratios
476 HENRY L. GHOLZ Ecology, Vol. 63. No. 2
TABLE 5. Selected regressions among stand leaf area indices (harha). biomass (Mg/ha) and NPP (Nig • ha -I• yr - ') across thetransect in Oregon. Plots la and b are averaged for these analyses unless noted. All equations are of the form: Y = A +B(x). s',, is the residual mean square error (variance). NS = not significant.
Y X A s',,
Best climatic predictors for all plots out of 12 individual variables considered*Leaf area index 1977 Mean min. Jan air temp. (°C) 25.207 2.336 .89 22.4Biomass 1977 Mean min. Jan air temp. (°C) 864.62 88.323 .91 24 526.0NPP 1977 Mean min. Jan air temp. (°C) 10.819 1.001 .97 1.0
Best climatic predictors without the subalpine Plot V:Leaf area index 1976 Water balancet (cm) 29.337 0.496 .99 1.8
1977 Water balancet (cm) 31.680 0.263 .97 7.51976 Water balancet (cm► 31.841 0.648 .97 7.51977 Water balancet (cm) 33.181 0.297 .95 14.0
C. Effect of removal of subalpine Plot V on 1977 water balancet regressions:Leaf area index With Plot V 28.872 0.231 .80 34.3
Without Plot V 31.860 0.263 .97 7.5Biomass With Plot V 991.25 8.417 .78 50497.5
Without Plot V 1126.6 9.582 .95 14 626.6NPP With Plot V 11.800 0.090 .73 7.8
Without Plot V 13.266 0.102 .88 4.0D. Regressions of leaf area index on components of the 1977 water balance (without Plot V) and with other climatic variables(for all plots):
(%) 12.003 -0.1'_9 .82 5.01977 TGI with Plot V 21.667 -0.210 .44 95.51977 TGI without Plot V 33.145 -0.346 .83 28.1
G. Regressions on stand basal areas and leaf area indices:Leaf area index Basal area (m 2/ha) -5.258 0.336 .92 13.8NPP Basal area (m'/ha) 0.137 .95 1.4Biomass Basal area (m2/ha) -2771.85834 12.591 .94 4.9Biomass Leaf area (ha/ha) -50.976 35.318 .91 22 066.0NPP Leaf area (<30 haiha) -0.377 0.492 .96 1.2NPP Biomass (<1100 Mg/ha) 0.715 0.013 .97 6.3
* Due to equipment failure 1977 Mean min. Jan air temperatures are unavailable for Plot III.t Based on actual extraction of soil water.
Based on soil water storage capacity.
040 0 -12020 0 -20 -40 -60 -80 -10
-.E---Grier and Running (1977)
1977
50
40
30
20
10
April 1982
LIMITS ON STRUCTURE AND PRODUCTION
477
showed a slight decline to the east (Table 4). Stemincrement contributed 13-55% of NPP along the tran-sect. Foliage production exceeded stem production onfour sites.
Relationships of NPP with the climatic variables aresimilar to those of leaf area index and biomass withclimate (Table 5). However, in this case. whether thesubalpine Plot V was excluded or not. mean minimumJanuary temperature was the best predictor of NPP.and regressions of NPP on this variable did not changesignificantly when Plot V was exluded (Table 5F).Even without Plot V. the water balance produced avariance four times as large as the variance associatedwith the January temperature regression (Table 5A).and the correlation of NPP with evaporation alone(Table 5B) was higher (r 2 = .91) than w ith the waterbalance (r' = .88). Again. relationships of NPP withgrowing season and annual precipitation were muchweaker (Table 5F). Excluding Plot V. there was astrong negative correlation of NPP ith the growingseason TGI. suggesting additional restrictions of NPPat plots with higher summer temperatures (Table 5F).
DISCUSSION
Leaf area index and biomassThere was no significant difference between the
slope of the 1977 leaf area index-water balance regres-sion line (excluding the subalpine site) and that ofGrier and Running (1977), who used soil water storagecapacity and long-term climatic averages with averageleaf area indices from five of the same zones (exclud-ing the subalpine zone. Fig. 3). Using water extractionrather than storage capacity to determine the waterbalance made little difference in the regressions (Table5B). The agreement of these independent estimatessupports a regional trend of leaf area index and waterbalance outside the subalpine zone.
Data for 1976 also indicate a strong relationship be-tween leaf area index and water balance. but with amuch steeper slope (Table 5B). Because needles maybe retained for several years. litterfall amounts inthese forests would be expected to vary <---25% fromyear to year and leaf area index ought to be nearlyconstant. Yet the water balance indices for sites awayfrom the coast were often different by 100% from 1976to 1977 (Table 4). Based on these observations I con-clude that either the relationship between the tran-spiring leaf surface and the available water on a sitereflects an adaptation to long-term hydrologic condi-tions rather than to the relatively large fluctuationsthat can occur from year to year, or that other factorsexert a significant influence on site leaf area indices.
While the water balance failed to predict leaf areaindex accurately at subalpine Plot V. the correlationswith winter cold temperature indices were high overall plots, which may reflect markedly reduced photo-synthesis outside the growing season in the subalpine
15 May -15 Oct Water Balance (cm)
FIG. 3. Leaf area index as a function of water balanceduring the growing season (based on equations in Table 5).Plots la and b are averaged in this figure. Grier and Running's(1977) regression is based on average values for five zones.(0) and (D) = subalpine Plot V.
zone (Emmingham and Waring 1-977. Waring et al.1978). TGI was of limited predictive use for leaf areasbecause of its restriction to the growing season andexclusion of any water considerations.
Factors other than moisture or temperature appar-ently may also limit maximum stand leaf area indices.For example. the presumed limitation in some coastalsites is wind damage to the developing canopy (Fuji-mori et al. 1976. Grier 1977, 1978). Substrate qualitycould also restrain leaf area index: serpentine soils insouthwestern Oregon supported less leaf area than ex-pected based on the TGI and a plant moisture index(Waring et al. 1978). Leaf areas (Gholz et al. 1976)were lower than expected in stands with very lowavailable soil nitrogen (Zobel et al. 1976) in the west-ern Cascades. In plantation studies. N fertilizationmay increase leaf area index by increasing needle re-tention time (Miller and Miller 1976). Whether nutri-tion has a direct effect on leaf area accumulation orwhether it is inseparable from temperature and mois-ture conditions on a site cannot be answered at thispoint.
According to the relationships reported here. max-imum leaf area indices for the Pacific Northwest are
haiha. which is similar to other studies alreadycited.
Since biomass continues to change greatly over cen-turies in these forests (Waring and Franklin 1979), theclose relationships of biomass with many of the cli-matic and stand variables (Table 5) would probablychange over time. In this context the high correlationswould suggest that these sites are representative ofstands in similar developmental stages: they may notreflect direct causal connections between the partic-ular variables and biomass. In some Wisconsin hard-wood forests aboveground biomass was highly corre-lated with the product of stand basal area and meancanopy height (Crow 1978). Since stand basal areas
b
•
478 HENRY L. GHOLZ
Ecology, Vol. 63, No. 2
and canopy heights are standard mensurational vari-ables, and change in the same direction over time asbiomass. this relationship may be useful for purposesof estimatin g dry matter contents of a wide range ofnorthwestern United States forests.
Net primary production
Althou gh foliage production for this study was in-directly estimated, the resultant values (Table 4) com-pare well with litterfall estimates for other temperateconifer forests (Bray and Gorham 1964. Grier 1977,Grier and Logan 1977). NPP values calculated hereseem very reasonable when compared to literaturevalues for the coastal zone (Westman and Whittaker1975, Fujimori et al. 1976), young. low-elevationDouglas-fir forests in Washington (Heilman 1961. Rei-kerk 1967. Cole et al. 1968. Turner 1975), a 100-yr-oldDou glas-fir stand in western Ore gon ( Fujimori et al.1976), and a subalpine Abies amabilis forest in north-ern Washington (Grier 1980).
NPP over all plots increased linearly with biomass(r2 = .97, Table 5G) to a plateau of Mg•ha-' • yr-'at a biomass of 1100 Mgiha. The occurrence of anNPP:biomass plateau is supported by data from old-growth redwood forests in northern California ( West-man and Whittaker 1975). A review of "mature' . for-ests containing no stands from the Pacific Northwest(Whittaker and Marks 1975) indicated that maturestands elsewhere in the world averaged twice the NPPper unit of biomass as stands from this study. No pla-teau was apparent in the review, althou gh the largestbiomass value was 600 kgiha. less than half the max-imum reported in Table 4.
NPP also increased steeply with stand basal area(Table 5G), and the correlation was hi gh (r2 = .95) forthe stands along the transect. No maximum NPP wasindicated, although alluvial flat redwood stands innorthern California at 14 Mg • ha- l • yr-' and 250 m=/habasal area (Westman and Whittaker 1975) no doubtrepresent a maximum. Compared to the average ofstands in the eastern United States (Whittaker andMarks 1975), the Oregon transect stands had less thanhalf the annual NPP at any basal area, althou gh someeastern stands appeared comparable in this regard.
NPP was positively correlated with higher (morepositive) values of the water balance (Fig. 4a). a resultsimilar to that of Rosenzweig (1968) who used poten-tial evapotranspiration (PET) as the independent vari-able, and warmer minimum winter temperatures.
-However, in the subalpine zone cold winter temper-atures appeared to be the dominating factor in limitingNPP (Fig. 4b). This is supported by a simulation studyof seasonal photosynthesis in the northwest UnitedStates ( Emmingham and Warin g 1977), which indicat-ed that. while growing-season photosynthesis wasgreatly restricted by drought as in other forest zones,winter photosynthesis was much more restricted in thesubalpine zone than elsewhere.
a
7 '0
a.z 5
•0
.20 0 -20 -40 -60 -80 -00 -120
1977 Water Balance ( cm )
.2 0 -2 -4 -6 -8 -10 -12
1977 Mean Daily Minimum January air Temperature (°C)
F IG. 4. Relationships of aboveground NPP to 1977 grow-ing season water availability (a. using soil water extraction).and a cold winter temperature index (b: data for Plot Ill areunavailable). The subalpine Plot V is enclosed in parenthe-ses. Plots la and b are averaged in this figure.
Are summer water or cold winter temperaturesmore important in controlling stand structure and pro-duction? Outside the subalpine zone these two vari-ables are hi ghly correlated ( r2 = .85). When the sub-alpine zone is added. the correlation decreases to r2 =.72. much lower but still highly significant. Clearlythese two factors are not independent, although in thesubalpine zone colder winter temperatures alone ap-pear to exert more control even in the presence oflimited summer moisture.
Given these relationships. a strong correlation be-tween NPP and leaf area is not surprisin g even whenaverage values from two old-growth redwood stands(Westman and Whittaker 1975) are included (Fig. 5).In the Pacific Northwest, where all-sided leaf area in-dices are <30 haiha (projected leaf area index ofNPP and leaf area index were linearly related ( r2 =
.96. Fi g . 5). Fig. 5 indicates the probable limits onNPP of mature tree- or woody shrub-dominated eco-systems. evergreen and deciduous, based on this studyand other published reports (the dashed lines). Theupper line could be re garded as "potential" NPP, rep-resentin g the maximum NPP observed at any given
leaf area index. A similar figure in Whittaker and
(•)
2
a.a.z
'5
)0
5
0
00 10 20 30 40 50
15
0z8 5
0
Leaf Area (ha/ho)FIG. 5. Aboveground overstory NPP in relation to total
stand leaf area index (all sided). • = stands from this study:RU = upslope redwood and RA = alluvial flat redwoodstand (Westman and Whittaker 1975). Dashed lines indicateprobable upper and lower limits on NPP by natural, mature.tree. or woody shrub dominated ecosystems at any given all-sided leaf area. Maximum previously reported leaf area in-dices. where NPP was also reported. are ha ha. For theeight transect stands with leaf area indices <31 haiha. treat-ing Plots la and b separately. NPP = —0.377 + 0.492 (leafarea index). r2 = .96.
Marks (1975) indicates that maximum NPP at any levelof leaf area index is somewhat higher than that shownin Fig. 5, and does not show any tendency for NPP toreach a maximum or decrease at any value of leaf areaindex. This reflects the much smaller range in leaf areaindices of studies they reviewed (all <17 haiha all-sid-ed), and most important. the exclusion of demonstra-bly "immature" communities from this study. Im-mature stands (Whittaker 1966) and some low-elevation tropical rain forests (Murphy 1975) may havemuch higher NPP than indicated in Fi g . 5. Upwardcorrections for underestimates of NPP based on theassumption of zero nonfoliar litter production usedhere might also cause results of the two studies toconverge more where the data overlap in range.
As leaf area indices varied by a factor of 40. gen-erally decreasing west to east, while NPP varied ir-regularly and only by a factor of 10. the regional trendin NPP was due more to changes in leaf area indexthan to changes in production efficiency (NPP : leafarea index. Table 4). Production efficiency ranged onlyfrom .30 to .60, with no pattern over the transect. Thisis considerably less than efficiencies from other re-gions for leaf area indices > 10 (Whittaker and Marks1975), although no other production studies of maturestands report leaf area indices as hi gh as those re-ported here.
Although very old forest stands were originally ex-cluded from this analysis. this seems unnecessary.
Grier and Logan (1977) reported NPP and biomassvalues for five forest types in western Oregon on onesmall watershed dominated by Pseudotsuga 400-500yr old. Leaf area indices ranged from 12 to 20 haiha:when plotted with NPP on Fig. 5, the points wouldfall very close to the line for plots of this study. Thisindicates that as stands in this area become decadent.as these stands were (Grier and Logan 1977). and leafarea index temporarily decreases. NPP also decreases.and a similar ratio of NPP : leaf area index is main-tained.
At any given leaf area index. these Pacific North-west ecosystems, except those in the coastal zone(Plots Ia and b. the alluvial flat redwoods from West-man and Whittaker 1977), generally seem less produc-tive than many others throu ghout the world. This isdue to a combination of much higher leaf area indicesand somewhat lower ratio of NPP:leaf area index.However, a maximum NPP for mature temperate for-ests of 12-15 Mg• ha -'• yr-' (Whittaker and Likens1975) is supported.
For stands away from the coast, actual NPP at anyleaf area index may fall short of "potential" NPP (up-per dashed line in Fig. 5) because of inadequate fer-tility. Nitrogen is regarded as the limiting nutrient ininland conifer forests of the region (Atkinson and Mor-rison 1975). although data are unavailable to make aclear case for the transect stands. However. stands atthe coast do not appear to be nitrogen limited (Grier1977). Their production efficiencies are no higher thanfor stands elsewhere along the transect (Table 4). soif the eastern stands are N limited, inadequate nutri-tion must reduce NPP by reducing maximum standleaf area indices.
A relationship of NPP with mean annual precipita-tion, based on production data from the eastern UnitedStates. Europe. and Asia, was used by Lieth (1975) toestimate regional NPP over the biosphere. His equa-tion overestimates my measurements of NPP by twotimes (for all except Plot V) to five times (for Plot V).This comparatively inefficient water use pattern forthe stands reported here is no doubt related to thegreat proportion of rainfall, and steady but low appar-ent rates of photosynthesis that may occur durin g thewinter dormant period at sites west of the Cascadecrest (Salo 1974, Emmingham and Waring 1977). underconditions less favorable than those during growingseasons elsewhere.
ACKNOWLEDGMENTSR. H. Waring, Department of Forest Science. Ore gon State
University. Corvallis, and C. C. Grier. School of Forest Re-sources. University of Washington. Seattle. contributedmany ideas to this study and critiqued early drafts of thepaper. D. Muscato, Department of Forest Science. OregonState University. contributed greatly to computer analyses.I would like to also thank Ms. Nichole Vick. Auburn. Wash-ington for editorial assistance, and Dr. C. Oliver. Univer.sityof Washington. Seattle for extensive and constructive re-
LIMITS ON STRUCTURE AND PRODUCTION 479April 1982
480 HENRY L. GHOLZ Ecology, Vol. 63. No. 2
views of early drafts. This work was partially funded by aResearch Grant-in-Aid from the Society of Sigma Xi and inpart by National Science Foundation grant DEB 74-20744A06 to the Coniferous Forest Biome. Ecosystem AnalysisStudies. United States/International Biological Program. Thisis Paper 1328 of the Forest Research Laboratory. School ofForestry, Oregon State University, Corvallis, and contribu-tion 377 from the Coniferous Forest Biome.
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