W O R K / I V G P A P E R
,WNITORTL\IG I;O'L\IGTEXM CHAVGES
IN
THE BOREAL FORESTM. Ja
.
A ~ t o n o v s k y H.H. S h u g a r tNovember 1986 TQ-86-064
I n t e r n a t i o n a l I n s t i t u t e for Applied Systems Analysis
NOT FOR QUOTATION WITHOUT PERMISSION OF THE AUTHOR
MONITORING LONGTERM CHANGES IN THE BOREAL FOREST
M. J a . Antonovsky H.H. S h u g a r t
November 1986 WP-86-64
Working Papers are interim r e p o r t s on work of t h e I n t e r n a t i o n a l I n s t i t u t e f o r Applied Systems Analysis a n d h a v e r e c e i v e d only lim- ited review. Views o r opinions e x p r e s s e d h e r e i n do not neces- s a r i l y r e p r e s e n t t h o s e of t h e I n s t i t u t e o r of i t s National Member Organizations.
INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS 2361 L a x e n b u r g , Austria
FOREWORD
This p a p e r was p r e s e n t e d by Michael Antonovsky at t h e Second Sympo- sium o n S p a c e and Global Change (October 9, 1986; I n n s b r u c k , Austria) s p o n s o r e d by t h e I n t e r n a t i o n a l Astronautical Federation. The r e s e a r c h r e p o r t e d is p a r t of t h e continuing happy collaboration between P r o f e s s o r Antonovsky and P r o f e s s o r S h u g a r t of t h e University of Charlottesville in t h e United S t a t e s .
As pointed o u t in t h e conclusion, ' T h e r e i s a g e n e r a l c o n v e r g e n c e of f o r e s t models being developed in t h e USSR a n d t h e USA in terms of t h e phi- losophy t h a t u n d e r l i n e s t h e modeling a p p r o a c h . " However, t h e r e are d i f f e r - e n c e s in r e a l i z a t i o n s o t h a t comparisons as given in t h e p a p e r r e p r e s e n t "a v e r y useful s c i e n t i f i c endeavour".
This Working P a p e r is a contribution to t h e monitoring r e s e a r c h a c t i v i t y being developed within t h e IIASA Environment P r o g r a m . The r e s u l t s are a l s o a c o n t r i b u t i o n t o t h e new ICSU Global Change Programme.
R.E. Munn L e a d e r
Environment P r o g r a m
MONITORING LONGTERM CHANGES IN THE BOREAL FOREST
M.Ja. Antonovsky* and H.H. Shugart**
F o r e s t ecosystems contain a complex web of interactions among physi- c a l , chemical and biological p r o c e s s e s . Because of this i n t e r a c t i v e com- plexity, d i r e c t changes in a given p r o c e s s c a n b e attenuated or amplified, and t h e r e s p o n s e s elicited f r o m a f o r e s t will b e manifested on many dif- f e r e n t time scales.
T r e e growth r e s u l t s f r o m t h e amount of photosynthate produced and t h e allocation of t h i s photosynthate within t h e tree. The growth of trees h a s been modelled using "mechanistic" r e p r e s e n t a t i o n s of physiological p r o c e s s e s , but t h e s e models have r a r e l y been used t o p r e d i c t r e s p o n s e s o v e r periods longer t h a n a y e a r .
I t i s important t o note: (1) t h a t t h e web of interactions is complex and t h e details of s o m e of t h e i n t e r a c t i o n s are not well known, and (2) t h a t t h e r e s p o n s e of t h e whole plant t o a stress may b e non-linear across t h e possi- ble r a n g e of t h a t stress. In g e n e r a l , t h e fast p r o c e s s e s t h a t o p e r a t e in f o r e s t ecosystems can only b e predicted o v e r t h e longer t e r m with a consid- e r a b l e d e g r e e of uncertainty.
T h e r e have been s e v e r a l attempts t o develop highly detailed
"mechanistic" models of n a t u r a l ecosystems including forests. These models are useful as h e u r i s t i c tools f o r integrating studies of different ecological p r o c e s s e s , but are much less useful for predicting long-term ecosystem behaviour.
.
On l e a v e f r o m Natural Environment and Climate Monitoring l a b o r a t o r y COSKOMGIDROMET and t h e USSR Academy o f S c i e n c e s .m* P r o f e s s o r , Department o f Environmental S c i e n c e s , U n i v e r s i t y of C h a r l o t t e s v i l l e , Vir- ginia, USA.
Environmental c h a n g e c a n a f f e c t t h e growth rates of individual trees a n d t h e r e b y h a v e a cumulative e f f e c t in changing t h e amount of living m a t e r i a l in t h e f o r e s t system. However, t h e r e l a t i o n between t h e rate of individual tree growth a n d t h e rate of f o r e s t i n c r e a s e ( o r yield) i s more t h a n a simple additive e f f e c t . Relatively low levels of stress on trees c a n p r o d u c e l a r g e c h a n g e s in t h e dynamics a n d composition of f o r e s t s . F u r t h e r - more, t h e i n t e r a c t i o n s between t h e populations of trees a n d i n s e c t s (and o t h e r pests a n d d i s e a s e s ) a r e , in many c a s e s , mediated by climate a n d t h u s are of importance in assessing t h e possible e f f e c t s of climatic c h a n g e on f o r e s t s . F o r example, t h e oak-wilt d i s e a s e in t h e USSR appears t o b e d e p e n d e n t on t h e d e c r e a s e d ability of t h e trees t o resist leaf-eating i n s e c t s d u r i n g d r o u g h t ( I s r a e l et a l . 1983).
The p r e d i c t i o n of yield from growth h a s b e e n a n important t o p i c in modern f o r e s t r y ( F r i e s 1974). F o r e s t yield i s a consequence both of t h e growth of individual trees a n d of t h e rates of r e c r u i t m e n t a n d d e a t h of trees in t h e f o r e s t stand. F o r example, Figure 1 i l l u s t r a t e s t h e r e l a t i o n between s t a n d biomass a n d t h e a g e of s t a n d s of Picea glauca f o r e s t s (Yarie a n d Van Cleve 1983). Over t h e time p e r i o d indicated by t h e d i f f e r e n t a g e s , t h e growth increment of t h e trees was c o n s t a n t (diameter i n c r e a s e
=
0.11 cm/year; r 2 of r e g r e s s i o n=
0.87) but t h e rate of i n c r e a s e of t h e t o t a l biomass c l e a r l y declined as a function of a g e . In c o n t r a s t , t h e rate of biomass c h a n g e of a single t r e e , which was enlarging with a c o n s t a n t diame- ter increment, i n c r e a s e d with t h e s i z e of t h e tree ( b e c a u s e tree biomass is a power function of t h e diameter). Thus, t h e c h a n g e in biomass growth rate shown in Figure 1 was o p p o s i t e to t h a t of t h e individual trees t h a t comprised t h e f o r e s t . E f f e c t s s u c h as t h e s e p r e v e n t d i r e c t e x t r a p o l a t i o n of s h o r t - t e r m c h a n g e s in trees t o p r e d i c t t h e l o n g e r t e r m r e s p o n s e s of t h e f o r e s t . The modern t h e o r e t i c a l c o n c e p t f o r understanding t h e i n t e r m e d i a t e time s c a l e r e s p o n s e of a f o r e s t i s t o c o n s i d e r t h e f o r e s t e d l a n d s c a p e t o b e a mosaic with e a c h element of t h e mosaic scaled in r e l a t i o n to t h e dominant canopy tree ( c a . 0 . 1 h a , depending o n t h e s i z e of t h e t r e e s ) . This c o n c e p t was initially developed by Watt (1925, 1947) a n d h a s b e e n t h e t o p i c of a s e r i e s of papers a n d books (Raup 1957; Whittaker a n d Levin 1977; Bormann a n d Likens 1 9 7 9 a , b ; S h u g a r t 1984). The c o n c e p t , in b r i e f , i s t h a t t h e dynamic r e s p o n s e of t h e f o r e s t o c c u r s at a n a r e a l s c a l e of a l a r g e canopy tree a n d on a time s c a l e t h a t r e l a t e s to t h e longevity of t h e t r e e , as follows.ll?ollowing t h e d e a t h of a l a r g e tree and i t s fall, a canopy g a p forms.
The area below t h i s g a p becomes t h e s i t e of i n c r e a s e d r e g e n e r a t i o n a n d s u r v i v a l of trees. T r e e s grow, t h e f o r e s t builds, t h e canopy c l o s e s , a n d t h e g a p d i s a p p e a r s . Eventually, t h e now mature f o r e s t in t h e vicinity of t h e f o r m e r g a p s u f f e r s t h e mortality of a l a r g e tree a n d a new g a p i s formed a n d t h e c y c l e is r e p e a t e d ( S h u g a r t 1984)."
The dynamics of a f o r e s t are t h e a g g r e g a t e d dynamics of a l a r g e number of s u c h individual gaps. When c o n s i d e r e d at i n t e r m e d i a t e time s c a l e s (ca. 1 0 0 y e a r s ) , t h e p a t t e r n of dynamics of a f o r e s t c a n b e s e e n as a c y c l e of r e c r u i t m e n t , d e a t h and growth p r o c e s s e s ; environmental c h a n g e a l t e r s t h e p a t t e r n within t h e c y c l e (Figure 2). The r e g e n e r a t i o n p h a s e of t h e f o r e s t c y c l e a n d t h e e x a c t timing of t h e d e a t h of a canopy tree t h a t pro- d u c e s a n o p p o r t u n i t y f o r r e g e n e r a t i o n are highly s t o c h a s t i c p r o c e s s e s . This i s p a r t i c u l a r l y t h e case in t h e r e g e n e r a t i o n p h a s e when t h e mortality
Stand Age (years)
25
Figure 1: The relationship of above-ground stand biomass t o stand a g e f o r white s p r u c e (Picea g l a u c a ) from t h e I n t e r i o r of Alaska (from Yarie and Van Cleve, 1983). A fully stocked stand h a s a s i t e density index (SDI) of 1000; a stand with one half t h i s density of trees h a s a SDI
=
500.-
Biomass = -23.2105 + 0.0027 [ ( s D I ) ' . ~ ~ ' ~ ] + 4.9962 (InAGE) r 2 = 0.87of small t r e e s is v e r y high. I t i s in this stochastic p a r t of t h e cycle t h a t climatic change and variability c a n have t h e l a r g e s t influence in producing change in t h e f o r e s t .
20 -
0 50 100
150 200 250
Change in t h e r a t e of soil development could a l s o g r e a t l y delay t h e response of a f o r e s t t o a climatic change. The soil at a given location d e r i v e s i t s c h a r a c t e r i s t i c s from t h e p a r e n t material (the geology at t h e site), t h e vege- tation and t h e climate. If both t h e climate and t h e vegetation at a given location were t o change, t h e r e might b e considerable delays in t h e develop- ment of t h e soils and, hence, t h e f o r e s t s which one would e x p e c t ultimately t o develop at t h e site.
Large climatic fluctuations have taken place during t h e l a s t one hun- d r e d y e a r s (Wright 1984).
While t h e u s e of models o f f e r s a means t o s c a l e up in both time and s p a c e , o u r p r e s e n t state of knowledge about t h e p r o c e s s e s involved i s insuf- ficient t o allow this t o b e done with any r e a l confidence in t h e results. Con- sequently, w e see a need f o r t h e c o n c u r r e n t development of models and
4
CompetitionGrowth
rnosr srochesric ) pen of
cycle
Alternate Composition o f Forests
Figure 2: Simulation of t h e long-term regeneration cycle of f o r e s t s
+
using t h e BRIND model (Shugart and Noble, 1981) f o r t h e Australian Eucalyptus f o r e s t . The determinism in t h e sys- tem i s g r e a t during t h e growth, competition and thinning p h a s e s but more stochastic in t h e death and regeneration phases.
empirical studies of t h e physiological and micrometeorological p r o c e s s e s t h a t determine t h e r e s p o n s e t o environmental change. Given t h e paucity of o u r p r e s e n t knowledge, such empirical studies a r e needed at e a c h spatial and temporal scale.
Determination of t h e response of a f o r e s t t o a climatic change involves evaluation of a complex system with many levels of response. One means of attempting t o handle t h i s complexity i s by t h e use of quantitative models of f o r e s t dynamics as investigative tools. There a r e s e v e r a l hundred extant models of f o r e s t dynamics t h a t simulate t h e growth of individual t r e e s t o determine t h e temporal response of a f o r e s t . These models seem t o b e most a p p r o p r i a t e t o "intermediate time scales" discussed e a r l i e r . There have been s e v e r a l reviews of t h e types and performance of detailed models of f o r e s t dynamics (Munro 1974; S h u g a r t and West 1980; S h u g a r t 1984). We will in t h i s p a p e r provide two examples of modeling t h e dynamics of f o r e s t s . The f i r s t discussion includes a specific example from a hiearchical model of successional dynamics in western Siberia. This example is from Antonovsky and Korzukhin [1986]. We a l s o will discuss t h e mapping of global change of f o r e s t s (Emanuel e t a1 1985) t o climate change as a second example.
J3'XAMPI.E 1. HIEARCHICAL SIMULATION OF VEGETATION DYNAMICS In conditions of technogenic impacts, t h e p r e d i c t i o n of t h e state of t h e b i o s p h e r e , as a whole, should c o n s i d e r t h e r e s p o n s e of vegetation t o chang- ing ecologic p a r a m e t e r s , global c h a n g e s of t h e s e p a r a m e t e r s following t h e c h a n g e s o c c u r r i n g in t h e state of vegetation.
This s e c t i o n of t h e p a p e r makes a n a t t e m p t t o f u r t h e r t h e e l a b o r a t i o n of t h e f o r m e r , i.e., t h e i n t e r n a l dynamics of t h e b o r e a l f o r e s t . Thus, w e d o n o t d e a l h e r e with t h e vegetation--*biospheric p a r a m e t e r s r e l a t i o n s h i p , b u t r e g a r d t h i s o r t h a t s c e n a r i o of c h a n g e s in global ecologic (climatic) param- eters as p r e s e t .
To assess t h e r e s p o n s e of vegetation t o c h a n g e s in ecologic p a r a m e t e r s o v e r p e r i o d s exceeding s e v e r a l y e a r s u n d e r possibly a l t e r e d a t m o s p h e r i c conditions, o n e must c o n s t r u c t mathematical models which make u s e of avail- a b l e information on p l a n t physiology and ecology, as well as o n t h e o r g a n i - s a t i o n a n d s t r u c t u r e of v e g e t a t i v e c o v e r . Due t o t h e complexity of t h e t a s k involved in t h e c o n s t r u c t i o n of models of b o r e a l f o r e s t dynamics, and t h e r e l a t i v e l y s h o r t p e r i o d of time elapsed s i n c e t h e beginning of i t s solution, t h e simulation of t h e s e f o r e s t s i s p r a c t i c a l l y at i t s s t a r t i n g point. F o r instance, no a d a p t a t i o n elements (adaptational r e s p o n s e of individuals t o changes in ecological p a r a m e t e r s ) are i n c o r p o r a t e d into existing p l a n t dynamics models.
A phytomass (m) dynamics equation f o r a n individual tree such as,
( F
-
rate of photosynthesis, R-
i t s p r o d u c t s s p e n t f o r v a r i o u s needs) will become methodologically i n c o r r e c t if d i r e c t l y applied [Budyko, 1984; K r a - pivin, 19821, t o global o r zonal levels. W e believe t h a t t h e m o s t important f a c t o r to b e t a k e n into a c c o u n t when c o n s t r u c t i n g models of f o r e s t s , i s t h e h i e r a r c h i c a l (multilevel) organisation of vegetation [Razumovsky, 1981;Delcourt, H.R., Delcourt, P.A. a n d Webb, 19831. The dynamic n a t u r e of t h e f o r e s t s and, as a r u l e , t h e multiple e f f e c t s of environmental f a c t o r s o f t e n r e s u l t in u n e x p e c t e d r e s p o n s e s t o changes in t h o s e f a c t o r s . Let us c o n s i d e r a n example b a s e d on t h e following considerations.
1. R e l a t i o n s h i p between i n d i v i d u a l and ecosystem o r g a n i s a t i o n levels. L e t t h e i n c r e a s e d values of a c e r t a i n f a c t o r $ --+ $
+
A$ (e.g. CO ) stimulate t h e growth of a n individual (a t r e e ) and t h e accumulation of i t s phytomass m f o r a n y value of a g e t (A$/ $<<
1):Then, l e t u s c o n s i d e r a population of N trees of t h e same a g e and t h e c h a n g e in i t s t o t a l phytomass M
=
N.
mAs a r u l e , t h i s c h a n g e will b e l e s s t h a n c h a n g e s in individual phytomasses m , t h a t i s
a ( t )
>
b ( t ) will b e t r u e .Such damping e f f e c t i s accounted f o r by ecological i n t e r a c t i o n : stimu- lation of a n individual's growth r e s u l t s in g r e a t e r competition a n d i n c r e a s e d mortality, i.e. in a d e c r e a s e d number of individuals in comparison with t h a t o b s e r v e d p r i o r t o t h e a l t e r a t i o n of f a c t o r $. T h e r e f o r e , as $ i n c r e a s e s , M grows m o r e slowly t h a n m . This r e s u l t h a s b e e n obtained in a s e r i e s of simulation model e x p e r i m e n t s using models f o r s e v e n d i f f e r e n t ecosystems u n d e r a condition of i n c r e a s e d growth of individual trees ( S h u g a r t 1984).
2 . ReLationship between v a r i o u s i m p a c t s of a g i v e n factor a t the i n d i v i d u a L LeveL. L e t u s refer t o a well-known example dealing with pho- tosynthesis: t h e i n c r e a s e d l e a f ' s e x p o s u r e t o light r e s u l t s in i t s h e a t i n g , i n c r e a s e d e v a p o r a t i o n a n d , in case of a lack of water in t i s s u e s
-
in t h e narrowing of stomata a n d , in t h e long r u n , in a s m a l l e r buildup of produc- tivity t h a n could b e e x p e c t e d in t h e case of i n c r e a s e d e x p o s u r e t o light and water e x c e s s in t i s s u e s .3. ReLdtionship of v a r i o u s i m p a c t s of the g i v e n B c t o r a t the Landscape (regionaL) LeveL. Over t e r r i t o r i e s with e x c e s s i v e moisture, a t e m p e r a t u r e i n c r e a s e in b o r e a l f o r e s t s f a v o u r s t h e d e c r e a s e of swamp areas a n d t h e extension of f o r e s t c o v e r e d area. At t h e same time t h i s con- dition creates f i r e h a z a r d s which r e d u c e s t h e a v e r a g e f o r e s t a g e . S o , d e s p i t e t h e extension of f o r e s t c o v e r e d a r e a , t h e phytomass of b o r e a l vege- t a t i o n c a n e i t h e r i n c r e a s e or d e c r e a s e .
4. ReLationships between i n d i v i d u a l , phytocenotic a n d Landscape LeveLs. Over a r e g i o n with a lack of h e a t (such as t h e Boreal F o r e s t ) a warming promotes t h e growth of individuals, t o a l e s s e r d e g r e e i n c r e a s e s t h e phytomass of e a c h ecosystem ( s e e item 1 ) a n d p r o v o k e s f i r e s at t h e l a n d s c a p e level, with possible stimulation or s u p p r e s s i o n of t o t a l phytofage p r e s s u r e . The o v e r a l l e f f e c t r e s u l t i n g from t h e s e c h a n g e s c a n b r i n g a b o u t e i t h e r a n i n c r e a s e o r d e c r e a s e in t h e phytomass of trees.
The a b o v e examples show t h a t 1 ) I n t e r n a l f o r e s t i n t e r a c t i o n s , which i n c o r p o r a t e c a u s a t i v e r e l a t i o n s h i p s in t h e s t r u c t u r e a n d dynamics of vege- t a t i o n , should p o s s e s s s p e c i f i c f e a t u r e s f o r e a c h v e g e t a t i v e zone a n d 2 ) t h e c h a n g e s of t h e same global ecologic p a r a m e t e r s c a n r e s u l t in absolutely dif- f e r e n t a l t e r a t i o n s in vegetation p a r a m e t e r s
-
f o r e s t a r e a , f o r e s t phy- tomass, s p e c i e s composition, etc., f o r d i f f e r e n t vegetation zones (ecological conditions).This situation, as well as g e n e r a l p r i n c i p l e s of simulation, b r i n g f o r t h t h e conclusion t h a t a p r e d i c t i o n model of Boreal forest should b e h i e r a r c h i - cal. As a f i r s t s t e p in t h e d e s c r i p t i o n of f o r e s t dynamics within a c e r t a i n t y p e of l a n d s c a p e l o c a t e d on a climatically homogeneous t e r r i t o r y , we c a n s u g g e s t a t h r e e - l e v e l "individual-phytocenosisecosystem-landscapef which, in g e n e r a l t e r m s , could b e w r i t t e n as
2 = f ( z , y , $ ) (1)
Y =
Q ( 2,$I
(2)=
h ( 2 , y ,z,$I
(3)which i s a n analogue of a h i e r a r c h i c a l model [Cherkashin, 19831, o r a d e t e r - ministic analogue of a s t o c h a s t i c model [Shugart, 19841. In model (1-3) $ r e p r e s e n t s ecologic p a r a m e t e r s a n d , f i r s t of a l l , climatic p a r a m e t e r s a c t i n g at t h e i n t r o d u c e d levels; z
-
v a r i a b l e s of a n individual, in t h e simplest caseone l i n e a r size o r t h e phytomass of a n individual; y
-
v a r i a b l e s of a n ecosystem, in t h e simplest case numbers of t h e t a r g e t t r e e species; z-
v a r i a b l e s of t h e t e r r i t o r y (landscape), in t h e simplest case
-
sections of t h e t e r r i t o r y occupied by d i f f e r e n t types of phytocenoses.Out of t h e t h r e e introduced functions i t i s t h e organisation of f func- tion in (I) t h a t is b e s t known. Usually it i s a n equation which d e s c r i b e s t h e c a r b o n balance f o r a n individual
where t h e f i r s t item d e s c r i b e s photosynthesis t h e simulation of which is amply t r e a t e d in t h e l i t e r a t u r e (this problem i s f a r from being exhausted), and t h e second t e r m r e p r e s e n t s t h e c o s t s t o t h e individual f o r various needs. Argument y in F ( x , y ,$) d e s c r i b e s changes in t h e amount of c e r t a i n r e s o u r c e s used in ecosystems due t o a competitive interaction within phyto- cenosis. This relationship depends on t h e t y p e of t h e r e s o u r c e f o r which plants compete, on t h e morphology of plants and t h e i r s p a t i a l distribution.
Carbon s p e n t by a n individual g depend, in t h e f i r s t approximation, only on t h e s t a t e t h e individual itself, r a t h e r t h a n on y
.
Since v a r i a b l e s y are t h e numbers of c e r t a i n g r o u p s of individual trees, q-functions d e s c r i b e t h e dynamics from t h i s stress. If we analyse a population of trees of t h e s a m e s p e c i e s and t h e same a g e , function q will b e equal t o t h e individual's mortality. Unlike t h e t h e o r y of growth, t h e t h e o r y of t h i s phenomenon o c c u r r i n g in perennial plants i s but slightly developed, and, in simulations, one h a s t o make use of empirical relationships. If w e deal with a t r e e population of t h e same a g e which is divided into g r o u p s (usually in terms of size) o r populations of different a g e s without r e p r o d u c - tion, functions q will b e still equal t o mortality in r e s p e c t i v e groups, and t h e problem of deducing t h e right-hand p a r t s of e.g. (2) remains basically t h e same. However, t h e task becomes quantitatively more i n t r i c a t e if we consider a population of various a g e s with reproduction. In such a c a s e we come against a problem of age-related dynamics, and function q d e s c r i b e s both fecundity and mortality. The t h e o r y of growth, reproduction and mor- tality r e q u i r e d f o r such a case is practically non-existent.
The description of landscape dynamics by means of equation (3) i s usu- ally based on t h e idea discussed in S h u g a r t et al. (1973) whereby t h e t e r r i - t o r y i s divided into "cells", e a c h occupied by a n ecosystem in a c e r t a i n state (stage of development); and t h e dynamics of sections t h e t e r r i t o r y occupied by similar ecosystem (components of v e c t o r z ) i s described by Markov's l i n e a r system
where matrix elements C ( x , y ,$) are equal t o frequencies of transitions from one s t a t e into a n o t h e r o c c u r r i n g during endogenesis (successional dynamics) and u n d e r t h e impact of e x t e r n a l f a c t o r s . The fact t h a t t h e sys- t e m is z-linear (e.g. (5)) means t h a t we adopted t h e s t r o n g hypothesis according t o which a cell's dynamics i s independent of t h e dynamics of neighboring cells. L a t e r in t h i s p a p e r , we shall use system (5) in a simula- tion example.
N o w , after providing a g e n e r a l d e s c r i p t i o n of a h i e a r c h i c a l l y o r g a n - ised system (1-3), w e s h a l l d i s c u s s two examples of i t s a p p l i c a t i o n which c o r r e s p o n d t o situations d e s c r i b e d above.
1. L e t u s formulate a simple ecological-physiological model which a c c o u n t s for t h e i n t e r a c t i o n of t h e individual a n d ecological levels. W e s h a l l assume t h a t photosynthesis depends on o n e ecologic global f a c t o r , f o r example, on C02, a n d d e s i g n a t e c
=
[C02]. W e s h a l l assume t h a t a n ecological i n t e r a c t i o n i s r e v e a l e d in competition o v e r a c e r t a i n f a c t o r ( r e s o u r c e ) R , which a l s o g o v e r n s photosynthesis; i t could b e light, w a t e r , mineral ele- ments. O t h e r f a c t o r s are implicitly i n c o r p o r a t e d i n t o t h e models. Suppose, R ( y ) i s a unit amount of t h e r e s o u r c e ( p e r unit of a b s o r b i n g area S ) ; i t would b e n a t u r a l t o s u p p o s e t h a t without competition t h e amount of a v a i l a b l e r e s o u r c e i s a maximum, R (o )=
Rmax. W e u s e t h e simplest, multiplying, d e p e n d e n c e of unit rate of phytosynthesis F on t h e introduced f a c t o r st h i s i s a r e a s o n a b l e assumption b e a r i n g in mind t h a t t h e estimations t o b e obtained will b e a p p r o x i m a t e and comparative.
We s h a l l s t u d y t h e simplest ecological system
-
a population comprising y individuals of t h e same a g e . L e t S b e t h e a b s o r b i n g s u r f a c e of a n indivi- d u a l (leaf area o r t h e a c t i v e r o o t system a r e a ) , and m i s i t s phytomass depending o n S , m=
pSW (actually w>
1). To obtain a n a l y t i c a l r e s u l t s , l e t u s assume t h a t c a r b o n s p e n t by t h e individual in (4) are p r o p o r t i o n a l t o t o t a l photosynthesis, g-SF. Hence, t h e individual growth equation will b eTo obtain t h e s t r e n g t h dynamics equation w e assume t h a t t h e population i s in a n ecological optimum, s o t h a t mortality c a u s e d by unfavourable climatic f a c t o r s i s r e d u c e d t o a minimum a n d o n e c a n s u p p o s e t h a t i t i s c a u s e d only by competition; t h a t mortality in (2) depends on t h e a v a i l a b l e r e s o u r c e v i a a r g u m e n t W
=
R ( y )/Rmax, q=
q (W), with q (1)=
0 , a q / B w<
0. In c o n c r e t e calculations, f o r lack of a c o n s t r u c t i v e t h e o r y , w e u s e empirical r e l a t i o n s h i p s q (W). So, w e h a v e in f r o n t of u s a n ecological-physiological model of t h e t a r g e t o b j e c t :s
= r S a R ( y ) ; y=
-q[R(y)/Rmax]y.
(6)Various h y p o t h e s e s a b o u t t h e t y p e of a limited r e s o u r c e , t h e morphol- ogy of t h e a b s o r b i n g s u r f a c e and s p a t i a l location of individuals yield a definite form R ( y ) obtained e i t h e r t h e o r e t i c a l l y o r by simulation. F o r example, if trees compete f o r water, of t h e individual's r o o t system occu- p i e s a r i n g of area S , if t h e individuals are l o c a t e d on t h e p l a n e indepen- dently of o n e a n o t h e r , a n d in t h e area where root systems o v e r l a p , water i s d i s t r i b u t e d equally between a l l overlapping individuals, t h e n o n e c a n show t h a t function R ( y ) i s
R ( y )
=
RmaX(l, -2asy)/2d& (7)w h e r e d is a n empirical f a c t o r which d e s c r i b e s t h e e x t e n t t o which r o o t
ends fill t h e ring.
2. E x p e r i e n c e obtained from c o n c r e t e calculations with system ( 6 ) shows t h a t in populations of trees which are long-lived, mortality caused by competition p e r unit time (1 y e a r ) is small enough, i.e. t h a t W is close t o 1.
In such a case if w e use (7), 6Sy
<<
1 will b e fulfilled andThen system ( 6 ) t u r n s into
This system provides a qualitatively c o r r e c t description of basic e f f e c t s in t h e combined dynamics of a n individual's numbers and size, and makes i t possible t o d i r e c t l y examine t h e e f f e c t of ecological damping of growth a c c e l e r a t i o n mentioned in 1.
First l e t u s consider a case of tree growth, i.e. system S
=
rSa ;y =
- 0 6 a 2Its solution, with initial conditions being S ( 0 )
=
So , y ( 0 )=
y o , is as fol- lows:Bearing in mind t h a t t h e phytomass of a n individual is m ( t )
=
p S ( t ) w w e obtainIn a c t u a l dynamics t h e system quickly f o r g e t s t h e initial value of size S o , which allows us to consider a simplified case when
Now l e t us suppose t h a t photosynthesis intensity h a s changed as a r e s u l t of a change in t h e global C02 concentration r '
=
r+
Ar Ar / r<<
1, From formula ( 9 ) one c a n find t h a tA s c a n b e s e e n , t h e c o r r e c t i o n f o r M is less than t h a t f o r m , which d e s c r i b e s in t h e framework of t h e model, t h e e f f e c t under consideration.
The analysis of model (8) with t h e relationship between a n individual's growth and competitive interaction, yields formulae similar t o (10) where A r depends only weakly on time (for t h e s a k e of brevity possible estimates f o r A r (t ) are not cited h e r e ) . Since t h e s t r u c t u r e of dependence of m , M on A r remained t h e same, t h e e f f e c t under consideration is p r e s e r v e d in model (9) as well.
To provide a model-oriented description of t h e n e x t e f f e c t (item 4) l e t us look at a t e r r i t o r y which is homogeneous from t h e viewpoint of soil and climate conditions. This means t h a t landscape "cells" are occupied by ecosystems r e p r e s e n t i n g one succession line and d i f f e r only by t h e s t a g e of development (age). Let us consider t h e situation [ S p u r r , Barnes and Barnes, 19811 typical f o r boreal f o r e s t s , when a f i r e which completely or almost completely wipes o u t f o r e s t s on s o m e p a r t of t h e t e r r i t o r y , i s t h e principal exogenic f a c t o r . N e w trees occupy t h i s area, which r e s u l t s in t h e development of endogenic succession whose a g e count starts from t h e time of t h e f i r e . So, t h e formed cells are of pyrogenic origin; and t h e i r size and p h a s e are determined by t h e state of neighbouring cells or by accidental f a c t o r s which put a n end to t h e f i r e . Since t h e notion "development s t a g e "
of a n ecosystem i s d i s c r e t e , i t is convenient to use t h e d i s c r e t e analogue from system (5). Let us look at a simple case when t h e probabilities of being b u r n t down uk f o r e a c h state k
=
2 ,. . .
, Q of t h e ecosystem, are equal and depend on one global exogenic f a c t o r u=
u (9). Duration of o n e s t a g e will b e chosen as a time unit. Then, t h e dynamics of those p a r t s of t h e t e r r i t o r y which are occupied by ecosystem at d i f f e r e n t succession s t a g e s k=
1 ,. . .
, Q will b e d e s c r i b e d , f o r non-interacting cells, by t h e following system:Assuming t h a t e a c h cell i s occupied by a population of individuals of t h e same a g e , one c a n d e s c r i b e i t s dynamics by t h e system (discrete analogue (6)):
where t h e rate of photosynthesis depends on t h e s a m e f a c t o r $.
System (11-12) is a n example of a three-level system of t h e
"individual-ecosystem-landscape" type.
4. Let u s assume t h a t t h e t e r r i t o r y , as a whole, is in a state of equili- brium, i.e. portions z k a r e constant and equal z i ( 9 ) . A s f a c t o r
9
changes t o A*, t h e phytomass of a cell will become equal t ot h e p r o b a b i l i t y of being b u r n t down
equilibrium p o r t i o n s of t h e t e r r i t o r y
Let u s i n t r o d u c e t h e a v e r a g e phytomass p e r unit area of t h e t e r r i t o r y
which, in a c c o r d a n c e with (13) will c h a n g e following v a r i a t i o n s of
9:
Our p u r p o s e i s t o e s t i m a t e t h e sign of c o r r e c t i o n f o r p. D i r e c t estima- tion WM(k ) in (10) r e q u i r e s t h e s e t t i n g of s e v e r a l c o n s t a n t s a n d d o e s n o t p r o v i d e t h e d e s i r e d a c c u r a c y . Let f o r c o n c r e t e n e s s
+
b e mean t e m p e r a t u r e T. Let u s try a s i m p l e r a p p r o a c h : i t is known t h a t in case of b o r e a l f o r e s t s t h e v a r i a t i o n of AT=
+ l o i n c r e a s e s t h e rate of photosynthesis by 5-18%.Let u s assume t h a t t h i s estimation i s a p p l i c a b l e t o M (i.e., t h a t t h e e f f e c t of ecosystem damping d o e s not involve qualitative changes). Then
P r o b a b i l i t i e s u (T), which h a v e t h e o r d e r of magnitude of l / y e a r f o r b o r e a l f o r e s t s , mostly c h a n g e following t h e c h a n g e s in t h e f r e q u e n c y of d r y y e a r s . Analysis of c o r r e s p o n d i n g d a t a shows t h a t when AT
=
+ l o t h e f r e q u e n c y of d r o u g h t s f o r t h e E u r o p e a n T e r r i t o r y of t h e USSR a n d Western S i b e r i a will i n c r e a s e approximately from 0.3 to 0.4 l / y e a r . L e t u s assume t h a t p r o b a b i l i t i e s u will i n c r e a s e in t h e same p r o p o r t i o n :With a n a c c u r a c y r e q u i r i n g only s l i g h t c o r r e c t i o n s , s t a t i o n a r y magni- t u d e s f r o m (11) are e q u a l to:
5. L e t u s look at a c o n c r e t e situation
-
a p y r o g e n i c c e d a r succession in Western S i b e r i a [Sedykh, 19741, in which t h e ecological phytomass ( t o n / h e c t a r ) f o r twenty-year long s t a g e s k=
1 ,. . . ,
9 is e q u a l tok . . 1 2 3 4 5 6 7 8
M(k,T)
...
3 0 5 0 8 0 1 1 0 210 300 340 370 400Let u s assume t h a t t h e p r o b a b i l i t y of being b u r n t down during 2 0 y e a r s u ( T )
=
0.2. Calculations of z: a n d t h e i r d e r i v a t i v e s yield t h e following e x p r e s s i o n f o r mean phytomassp(T+AT)
=
p ( T )+
50(3WM-Wu)AT.
As c a n b e s e e n , t h e c o r r e c t i o n c h a n g e s sign when passing t h r o u g h
Estimations W M (15) and Wu (16) show t h a t this r a t i o i s quite reliable, i.e. t h e e f f e c t of phytomass growth at t h e ecosystem level h a s t h e same o r d e r of magnitude as t h e e f f e c t of its d e c r e a s e a t t h e landscape level.
EXAMPLE 2. GLOBAL-SCALE RESPONSE OF VEGETATION
A t a global-scale, one a p p r o a c h t o examining t h e possible changes in t h e size and a r e a l e x t e n t of t h e world's f o r e s t s is t o use empirical models of climate and vegetation in a spatial context and t o superimpose scenarios of climatic change. Emanuel e t al. (1985) used t h e Holdridge life zone classifi- cation (Holdridge, 1947, 1964) t o map t h e distribution of potential vegeta- tion on t h e E a r t h ' s t e r r e s t r i a l surface. The Holdridge classification p r e d i c t s expected vegetation as a function of a t e m p e r a t u r e and moisture index. By interpolating monthly temperature and precipitation d a t a from 8000 meteorological stations o n t o a 0.5 d e g r e e latitude by 0.5 d e g r e e longi- tude grid and applying t h e Holdridge classification scheme to t h e s e d a t a , Emanuel et al. produced a map of world vegetation. Each of t h e meteorolog- ical r e c o r d s was t h e n a l t e r e d by a change in t h e annual a v e r a g e tempera- t u r e t a k e n from Manabe and Stouffer's (1980) simulation experiment f o r a C02-doubling. The initial p r o c e d u r e w a s then r e p e a t e d t o obtain a map of t h e potential vegetation t o be expected a f t e r t h e climatic change.
In a subsequent critique of t h e p r o c e d u r e , Rowntree (1985) noted t h a t t h e use of mean annual t e m p e r a t u r e s w a s l e s s a p p r o p r i a t e than t h e use of seasonally varying temperatures. I t w a s also noted t h a t i t would have been more a p p r o p r i a t e t o use t h e difference between t h e 2 x CO s c e n a r i o and t h e General Circulation Model control r u n ( r a t h e r than Zhe difference between t h e 2 x CO s c e n a r i o and observed data) t o d e r i v e t h e magnitude of t h e t e m p e r a t u r e c i a n g e s from which t o calculate t h e e f f e c t s of climatic change on vegetation. Based on t h e s e criticisms, Emanuel et al. (1985b) revised t h e maps of t h e Holdridge life zones f o r both t h e base c a s e (present-day conditions as r e f l e c t e d in t h e meteorological station d a t a s e t ) and t h e 2 x C02 s c e n a r i o a s shown in Table 1 (Shugart et al., 1986).
A t a global scale, t h e life zone designations of 34% of t h e 0.5" by 0.5"
grid cells were a l t e r e d . In t h e higher latitudes, t h e generally higher tem- p e r a t u r e s resulted in a 37% d e c r e a s e in t h e a r e a l extent of tundra (see Table 1 in Emanuel et al., 1985). Boreal moist f o r e s t w a s replaced by cool temperate s t e p p e and, t o a l e s s e r d e g r e e , by cool temperate f o r e s t and b o r e a l d r y bush. Boreal wet f o r e s t w a s replaced by cool temperate f o r e s t and b o r e a l moist f o r e s t . The b o r e a l f o r e s t zone shifted north and r e p l a c e d about 42% of t h e 0.5" by 0.5" grid cells designated as "tundra" in t h e base c a s e . The n o r t h e r n e x t e n t of t h e tundra w a s also increased.
Because t h e t e m p e r a t u r e changes in t h e Manabe and Stouffer s c e n a r i o were smaller toward t h e equator, t h e r e were smaller changes in t h e tropi- c a l life zones. Nevertheless, t h e a r e a l extents of t h e subtropical and tropi- c a l life zones increased by 8%. The a r e a of subtropical f o r e s t life zones d e c r e a s e d by 22%, while t h e subtropical thorn woodland and subtropical d e s e r t s increased by 37% and 26%, respectively.
Table 1: Summary of Changes in Life-Zone Extents (lo6 km2)
Area
Base c a s e Elevated CO F o r e s t s
Tropical:
Rain Wet Moist Dry
Subtropical:
Rain Wet Moist Dry
Warm Temperate:
Rain Wet Moist Dry
Cool Temperate:
Rain Wet Moist
Boreal:
Rain Wet Moist
G r a s s l a n d s
Tropical:
Very Dry Forest Thorn Woodland
-
14-
Subtropical Thorn Woodland
Warm Temperate Thorn Steppe
Cool Temperate Steppe
D e s e r t s
Tropical:
Desert Bush Desert
Subtropical:
Desert Bush Desert
Warm Temperate:
Desert Bush Desert
Cool Temperate:
Desert Bush Desert
Boreal:
Dry Bush Desert
T u n d r a
Rain
Wet Moist Dry
I c e 2.218 0.567
Total 131.372 131.368
Table from Emanuel et al., 1985.
In t h e analysis described above, precipitation was l e f t unchanged and thus a v e r a g e evapotranspiration increased. If precipitation were allowed t o change, however, a reduction of boreal f o r e s t would still r e s u l t from t h e higher temperatures, according t o t h e Holdridge life zone classification.
D r i e r conditions would only f u r t h e r d e c r e a s e t h e a r e a l extent. Wetter con- ditions would allow t h e expansion of b o r e a l f o r e s t s into a r e a s classified as
"boreal d e s e r t " (Table I ) , but t h e a r e a of b o r e a l d e s e r t is s o small t h a t t h e s e gains would do little t o offset t h e reduction in boreal f o r e s t s caused by w a r m e r temperatures. In c o n t r a s t , t h e proportions of grasslands (including t h o r n woodlands and thorn steppe) and d e s e r t s would be expected t o change considerably under different precipitation regimes. Increased precipitation would have little e f f e c t on t h e a r e a of tropical f o r e s t s , but d e c r e a s e d precipitation would diminish t h e a r e a greatly.
Emanuel et a l . (1985) identified s e v e r a l s o u r c e s of uncertainty in t h e s e s o r t s of assessments, including t h e choice of climate scenario, t h e choice of mapping algorithm and t h e r e l a t i v e coarseness of t h e d a t a grid. Nonethe- less, t h e simulated effects of a warmer climate on t h e a r e a l e x t e n t of t h e coniferous b o r e a l f o r e s t s are not inconsistent with t h e conclusions one might draw from a casual inspection of t h e position of t h e b o r e a l f o r e s t s in relation t o key t e m p e r a t u r e variables. Throughout North America and Eurasia, t h e n o r t h e r n limit of t h e b o r e a l f o r e s t i s delineated by t h e mean 13°C isotherm in July (Larsen, 1980). The southern limit of t h e forest i s bounded by t h e mean 18°C isotherm in July in regions with favourable mois- t u r e conditions (where d r i e r conditions prevail t h e limit i s situated north of t h i s isotherm). Although spatial correlations between climate variables and vegetation do not necessarily establish cause and effect, i t is important t o note t h a t , with r e s p e c t t o growing season temperatures (indicated by t h e July isotherms), t h e b o r e a l f o r e s t has a r a n g e of only about 5°C under favourable moisture conditions and l e s s than 5°C under d r i e r conditions.
Thus, i n c r e a s e s in a v e r a g e summer s u r f a c e temperatures of just a f e w d e g r e e s , a s p r o j e c t e d by GCMs f o r a C02 doubling, might b e expected t o dis- place markedly t h e p r e s e n t boundaries of boreal f o r e s t s .
CONCLUDING REMARKS
To conclude o u r discussions w e would like to identify t h r e e themes t h a t w e f e e l are important with r e g a r d to t h e monitoring of t h e b o r e a l f o r e s t . These are:
1. The usefulness of mathematical models t o p r e d i c t t h e l o n g e r t e r m consequences of c h a n g e in t h e b o r e a l f o r e s t .
2. The a p p a r e n t sensitivity of t h e b o r e a l zone t o c h a n g e p a r t i c u l a r l y t o t h e c u r r e n t s c e n a r i o s being p r o d u c e d by g e n e r a l c i r c u l a t i o n models f o r a climatic warming induced by C02 and o t h e r g r e e n h o u s e g a s e s . 3. The p o t e n t i a l e f f e c t of t h e b o r e a l f o r e s t o n t h e global systems, p a r - t i c u l a r l y t h e global a t m o s p h e r i c b a l a n c e of g a s e s .
W e will treat t h e s e t h r e e points in o r d e r . 1. Utility of Models
In t h i s p a p e r , w e h a v e i n t r o d u c e d a s u b s t a n t i a l s e c t i o n involving t h e a n a l y s i s of a model by Antonovsky a n d Korzukhin. This analysis identifies a c e n t r a l point t h a t i s important in t h e understanding of c h a n g e in t h e b o r e a l f o r e s t
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c h a n g e i n one level of a h i e r a r c h i c a l l y - s t r u c t u r e d s y s t e m , s u c h as t h e boreal f o r e s t , d o e s n o t t r a n s l a t e a t a n o t h e r level of t h e h i e r a r c h y as a c h a n g e of t h e s a m e m a g n i t u d e o r e v e n of t h e s a m e s i g n . This point i s c l e a r l y evidenced in t h e example case of a climate warming on w e s t e r n S i b e r i a n f o r e s t w h e r e a warming i n c r e a s e d individual t r e e g r o w t h rates a n d i n c r e a s e d r e g i o n a l wildfire rates. The magnitudes of t h e s e two p r o c e s s e s , o n e t h a t i n c r e a s e s biomass a n d o n e t h a t d e c r e a s e s biomass, were of t h e same o r d e r .T h e r e i s a g e n e r a l c o n v e r g e n c e of f o r e s t models being developed in t h e USSR antl t h e USA in t e r m s of t h e philosophy t h a t u n d e r l i e s t h e modeling a p p r o a c h . W e h a v e p r e s e n t e d a USSR example in t h i s t e x t a n d r e a d e r s are r e f e r r e d t o S h u g a r t (1984) f o r a g e n e r a l review of a USA modeling a p p r o a c h . The point of c o n v e r g e n c e i s t h a t in both c o u n t r i e s ( a s i s a l s o t h e case e l s e w h e r e ) , t h e importance of recognizing t h e a g e s t r u c t u r e of t h e f o r e s t in formulating a p r o p e r f o r e s t dynamics model i s being r e c o g n i e d a n d included in t h e models. In t h e USA-case, t h i s recognition h a s been in t h e development of individual-tree b a s e d f o r e s t models a n d a n emphasis on digi- t a l c o m p u t e r simulation. In t h e USSR-case, t h i s recognition h a s been in t h e formal i n c o r p o r a t i o n of a g e a n d s i z e s t r u c t u r e in non-linear systems of dif- f e r e n t i a l equations t h a t d e s c r i b e f o r e s t dynamics. Thus, while t h e models h a v e a common b a s i s in philosophy t h e y d i f f e r in t h e i r realization. W e see comparisons across t h e s e a p p r o a c h e s as a v e r y useful s c i e n t i f i c e n d e a v o u r .
2. THE SENSITIVITY OF THE BOREAL ZONE TO CHANGE
In t h e p r e s e n t p a p e r , w e h a v e shown r e s u l t s o r a s t a t i c mapping e x p e r - iment using t h e Holdridge (1947, 1964) Life Zone classification a n d t h e Manabe a n d S t o u f f e r (1980) climate-change s c e n a r i o . This example identi- fied t h e b o r e a l zone as a f o c a l zone f o r seeing c h a n g e . W e f e e l t h a t t h e s e r e s u l t s should b e e x p l o r e d using o t h e r classification systems. The Hol- d r i d g e classification i s b a s e d upon a logarithmic t e m p e r a t u r e x logarithmic moisture classification. Since t h e b o r e a l zone i s in t h e p a r t of t h i s doubly logrithmic s c h e m e t h a t i s small with r e s p e c t t o both dimensions, t h e sensi- tivity t o c h a n g e could b e a consequence of scaling. Frankly, w e doubt t h i s i s t h e c a s e (based on t h e c o r r e l a t i o n between Holdridge classifications and o t h e r g e o g r a p h i c vegetation classification schemes).
Eventually o n e would l i k e t o see a n ability to develop dynamic equa- tions of f o r e s t (and o t h e r ecosystem change) t h a t would c o v e r t h e domain of t h e Holdridge Life Zone s p a c e . One s t e p in t h i s d i r e c t i o n could b e a n i n t e r - comparison of vegetation in t h e b o r e a l zones at a global s c a l e using satellite-based, remote-sensing of t h e sort developed by T u c k e r et al. f o r t h e African continent. This mapping/reclassification work would involve a c o n s i d e r a b l e d e g r e e of i n t e r n a t i o n a l cooperation.
3. THE EFFECT OF THE BOREAL FOREST ON THE GLOBAL SYSTEMS But t h e b o r e a l f o r e s t is not n e c e s s a r y a p a s s i v e p l a y e r in t h e global c h a n g e i n t e r a c t i o n s . The work of T u c k e r a n d Fung r e p o r t e d in t h e last (1986) I n t e r n a t i o n a l C o n g r e s s of Ecology points to a possible role of ter- r e s t r i a l ecosystems in controlling t h e annual oscillation in a t m o s p h e r i c C02.
This e v i d e n c e i s b a s e d only on c o r r e l a t i o n a n d i s m o s t convincing in t h e case of h i g h e r n o r t h e r n latitudes. The e x i s t e n c e of c o r r e l a t i o n is n o t proof of t h e e x i s t e n c e of a mechanism
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b u t w e would suggest t h a t f u r t h e r s t u d i e s of t h e e f f e c t of t h e b o r e a l f o r e s t on t h e global a t m o s p h e r e are c e r t a i n l y indicated by t h i s work.In t h e c a s e of o t h e r trace g a s e s ( p a r t i c u l a r l y methane), w e see t h e p r o c e s s e s of swamp formation a n d swamp r e f o r e s t a t i o n as a function of climatic c h a n g e as having a potential to c h a n g e t h e p e r c e n t a g e of t h e global s u r f a c e t h a t g e n e r a t e s methane. The understanding of t h e global budgets of carbon-containing g a s e s ( c a r b o n dioxide, methane, etc.) must of n e c e s s i t y c o n s i d e r t h e b o r e a l systems ( t h a t contain almost 50% of t h e living c a r b o n in t h e e a r t h ) t o a c o n s i d e r a b l e d e g r e e . I t h a s been pointed o u t in t h e c a s e of t h e t r o p i c a l f o r e s t t h a t t h e l a r g e rate of f o r e s t c l e a r i n g should b e slowed until t h e global r o l e of t h i s f o r e s t i s b e t t e r understood. I t i s a l s o t h e case t h a t t h e b o r e a l f o r e s t , t h e v a s t woods t h a t c o v e r s t h e n o r t h e r n p a r t of t h r e e continents, may a l s o h a v e a major r o l e in t h e functioning of global systems. The potential i m p o r t a n c e of t h e b o r e a l f o r e s t s at t h e global s c a l e i n d i c a t e s t h a t t h e y should b e b e t t e r understood in t h e global c o n t e x t b e f o r e t h e y are g r e a t l y a l t e r e d or c l e a r e d d u e t o more regional or local con- s i d e r a t i o n s .
ACKNOWLEDGEMENT
The authors wish t o thank R.E. Munn f o r his valuable support and use- ful comments when writing this paper.
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