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Influence of vegetation on permafrost
Repr~nted from the PROCEEDINGS: PERMAFROST INTERNATIONAL CONFERENCE, NAS-NRC, Publlcat~on 1 2 8 7
INFLUENCE OF VEGETATION O N PERMAFROST
R . J. E. BROWN, National Research C o u n c i l , C a n a d a
In permafrost regions numerous climatic and terrain features op- e r a t e singly and in combination, determining the e x t e n t , thick- n e s s , and thermal regime of the perennially frozen ground. One of the terrain f e a t u r e s i s vegetation which forms a continuous or d i s c o n t i n u o u s mantle on the ground (soil) s u r f a c e and e x e r t s d i r e c t and indirect influences on the underlying permafrost.
Vegetation h a s a d i r e c t influence on the permafrost by i t s thermal properties which determine the quantity of h e a t t h a t e n t e r s and l e a v e s the underlying ground in which the perma- f r o s t e x i s t s . Components of the energy exchange regime a t the ground s u r f a c e and thermal contribution of e a c h of them to permafrost a r e modified b y s u r f a c e v e g e t a t i v e c o v e r . Vegeta- tion a l s o e x e r t s a n indirect influence o n permafrost b y affecting climatic and other terrain f e a t u r e s , which i n turn have a d i r e c t influence on the permafrost.
These direct and indirect influences vary with time and s p a c e . The environment in which permafrost e x i s t s i s dynamic a s a r e the individual components of t h i s environment. Vegeta- tion i s one of t h e s e dynamic f a c t o r s and v a r i e s with time. As one type of vegetatioll i s s u c c e e d e d b y a n o t h e r , so the under- lying permafrost i s changed with time. As a r e s u l t , permafrost e x i s t i n g today r e f l e c t s d i r e c t and indirect influences of the p a s t a s w e l l a s the p r e s e n t . The e f f e c t s of vegetation a l s o vary in s p a c e , being g r e a t e r , for example, In the taiga zone than in the tundra.
The sum of t h e s e influences with their variation in time and s p a c e i s manifested in the variations which e x i s t in the perma- f r o s t . The depth from ground s u r f a c e to permafrost t a b l e , the temperature regime of the ground above the permafrost and of the permafrost, and the e x t e n t and t h i c k n e s s of permafrost a r e a l l conditioned b y vegetation o n the ground s u r f a c e . B e c a u s e vegetation i s s o c l o s e l y interwoven with climatic and o t h e r terrain f a c t o r s affecting permafrost, i t i s frequently difficult or impossible to s i n g l e o u t the role of vegetation a l o n e .
Even within the vegetation complex i t s e l f , some components have more influence than o t h e r s . I t i s frequently difficult to delineate the boundary between living vegetation and under- lying organic matter, l i t t e r , humus, p e a t , muck, and to s e p a - r a t e the influence of t h e s e two l a y e r s . I t i s most c o n v e n i e n t , therefore, to consider the combined living and dead material lying o n the mineral s o i l a s v e g e t a t i o n , although e a c h may have a different e f f e c t .
Further complications a r e c a u s e d b y the f a c t t h a t the influ- e n c e s of vegetation may vary depending on conditions under which they o c c u r . As a r e s u l t , i t i s p o s s i b l e for a certain combination of vegetation and other f a c t o r s to produce o n e s e t of permafrost conditions a t o n e time o r p l a c e , and a different s e t of conditions a t another time or p l a c e .
Because vegetation i s s a c h a widespread factor of the per- mafrost environment, a large body of literature i s devoted to t h i s topic in North America and the USSR. Russian literature i s particularly voluminous, the most prominent authors being B. A. Tikhomirov and A. P. Tyrtikov. (The latter h a s compiled
two lengthy and comprehensive review papers on the influence of vegetation o n permafrost [ l , 21 .) Work h a s a l s o b e e n car- ried o u t in Scandinavia o n variations in near-surface ground temperatures under different types of plants and on u s e of plant t y p e s a s snow depth indicators [ 3 , 41
.
Both h a v e important implications in the relationship between vegetation and permafrost.frost a r e reviewed. The cornplex nature of the problem makes i t impossible to cover a l l f a c e t s . Canadiail experience i s cited when a v a i l a b l e with supplementary lnformation derived mostly from Russian o b s e r v a t i o n s .
SURFACE ENERGY EXCHANGE
The a n n u a l h e a t exchange equation [ S , 61 a t the e a r t h ' s s u r f a c e can b e written a s
where (R) i s the a n n u a l radiation b a l a n c e ( n e t radiation), (LE)
i s h e a t involved in evaporation (including evapotranspiration)- c o n d e n s a t ~ o n , (P) i s h e a t involved i n conduction-convection (turbulent h e a t e x c h a n q e ) , and (8) i s thermal exchange i n the ground.
The contribution of e a c h of t h e s e colnponents to the h e a t transfer mechanism operating between s u i l and q t ~ ~ o s p h e r e i s modified b y vegetation properties a r he i~.terface of these two media which comprise the permafrost anvlronmant. Russian s t u d i e s show t h a t the h e a t exchange a q J a t i o n can b e used in permafrost regions to r e l a t e the groaxd charma1 regime to the surface energy exchange regime [ 7
$1.
Net Radiation and Albedo
Studies undertaken in Labrador-Ungava
i
91
, using a Kipp and Zonen solarimeter, confirm the higher albedo--solar radiation mostly 0.3-2.0 y-of t r e e l e s s lichen-zovered s u r f a c e s , being13.55% in c o n t r a s t to 11 .67% for lichen-spruce woodland, 6.52% for s p r u c e b o g , 9 . 7 8 % for muskeg. illid 11.13% for a c l o s e d f o r e s t .
At Norman W e l l s , NWT, where permafrost i s w i d e s p r e a d , observations b y the Division o f BuiLding Research, National Research C o u n c i l , C a n a d a , showed that licheri p o s e s s e d higher reflectivity v a l u e s than true m o s s e s and Sphaqnum. Neverthe- l e s s , t h e s e two plant t y p e s maintain the permafrost table a t about the s a m e l e v e l in a given a r e a , and ground temperatures under both plant t y p e s a r e similar. Therefore, if lichen r e j e c t s a higher proportion of the n e t radiative flux than m o s s , t h i s may be compensated b y the moss rejecting a higher proportion than lichen of some other component of the energy e x c h a n g e .
Another a s p e c t of the radiation component of h e a t exchange i s the influence of forest growth in reducing the i n t e n s i t y of s o l a r radiation a t t h e ground s u r f a c e which d e c r e a s e s the heat- ing of the ground. I f , for e x a m p l e , radiation in a n open a r e a i s 1 . 5 c a l / s q cm/min i t may b e reduced to 0.01 c a l / s q cm/min under a d e n s e f o r e s t canopy. In winter the f o r e s t hinders t h e r a t e of radiation from the ground but the e f f e c t i s n o t a s pronounced b e c a u s e of reduced foliage [ l , 21.
Conduction-Convection
Direct measurement o f the convective component i s extremely difficult. In energy e x c h a n g e s t u d i e s , quantitative v a l u e s c a n b e obtained b y two methods; (1) All of the other components of the h e a t e x c h a n g e equation a r e measured and i t i s a s s u m e d that the remainder e q u a l s the convective component: (2) Bowen's r a t i c c a n b e used which r e l a t e s t h i s c ~ m p o n e n t to t h e evapo- rative flux. Since the a n n u a l h e a t e x c h a n g e equation a t t h e e a r t h ' s s u r f a c e c a n b e written a s
i t follows that a n i n c r e a s e in h e a t transfer by evaporation d e - c r e a s e s the amount of turbulent exchange a n d v i c e v e r s a . In- v e s t i g a t o r s in the USSR s t a t e d that the ratio of (P) to (LEI a t the treeline i s about 1 to 6 or 1 to 7 , increasing to about 1 to 3 with i n c r e a s e d s u r f a c e roughness in the south-central part of the taiga where the permafrost boundary o c c u r s [ 5 ] . The (P) to (JX) ratio a t Point Barrow, A l a s k a , w a s 1 to 0 . 7 [ 1 0 , 111. Bowen's ratio for s a t u r a t e d Sphagnum in O t t a w a , C a n a d a , a t the Division of Building R e s e a r c h , w a s about 1 to 8 .
Variations in turbulent h e a t exchange with different plant s p e c i e s have not b e e n examined in permafrost regions. On a m i c r o s c a l e , the roughness factor provided by the vegetation i s greatly reduced north of the t r e e l i n e . On a m i c r o s c a l e , Sphag-
num tends to produce a n uneven microrelief in the form of hum-
-
mocks, mounds, and p e a t p l a t e a u s in c o n t r a s t to a r e a s covered with true m o s s e s and lichen which have l e s s microrelief. This could r e s u l t in ground surface air turbulence being significantly greater over a Sphagnum-covered a r e a .
Evaporation
Evaporation (including evapotranspiration) withdraws h e a t from the surrounding atmosphere and from i n c i d e n t s o l a r radiation [ 1 2 1 . Vegetation draws water from the s o i l b y transpiration, t h u s depleting the s o i l of the h e a t h e l d by that water. There appear to b e variations in t h e s e mechanisms from o n e plant s p e c i e s to another in permafrost r e g i o n s , b u t the magnitude of variations and the contribution of t h i s component to ground thermal regime a r e not c l e a r .
The mechanism of moisture transfer in moss and lichen i s not clearly understood, but both a r e nonvascular plants and cannot transpire in the s e n s e that v a s c u l a r plants d o . Sphag- num e s p e c i a l l y , and to a l e s s e r degree true m o s s e s , tend to
-
absorb and r a i s e water in the manner of a lamp w i c k . M o s s e s and l i c h e n s have a large water-holding c a p a c i t y and a r e s b o n g l y hygroscopic [ 1 3 , 1 4 , 151
.
They absorb water not only from precipitation b u t a l s o from atmospheric v a p o r , the l a t t e r being absorbed in d i r e c t proportion to the r e l a t i v e humidity of the a i r . Yet during a dry period they tend to l o s e moisture rap- idly. Probably a t some point in the humidity s c a l e for the a t - mosphere, l o s s e s exceed the g a i n s of moisture by the l i c h e n s .Tyrtikov [ 2 ] p o s t u l a t e s that a s vegetation a b s o r b s moisture from the s o i l there i s a commensurate i n c r e a s e in the s o i l ther- mal conductivity. This o c c u r s a t the same time a s the evapo- ration of the water lowers the a i r temperature, e s p e c i a l l y near the ground s u r f a c e , a n d s o r e d u c e s the warming of the s o i l . Immediately after a r a i n f a l l , the s u n rapidly d r i e s the t i p s of the moss but t h i s drying e x t e n d s only to a depth of about 5 c m , thus protecting the lower layer of moss from e x c e s s i v e l o s s of moisture.
When d r y , l i c h e n s absorb water slowly from water vapor. This p r o c e s s i s n e g l i g i b l e , however, compared with the absorp- tion of liquid water b e c a u s e , during a r a i n f a l l , the water con- tent of the a e r i a l parts may r i s e from 50 to 250% within a f e w hours [ 141
.
The l o s s of water vapor to the a i r may occur rap- idly if the difference in vapor p r e s s u r e between the a i r and the lichen is g r e a t , s o that on a drying d a y , the surface of the lichen becomes dry rapidly. Observations b y the Division of Building Research a t Norman W e l l s , NWT, of the moisture con-tent in a lichen cover during a dry period showed 8% moisture content in the top 1 i n . of the lichen in c o n t r a s t to nearly 200% a t the bottom of the 2-in. lichen cover. Unlike Sphagnum and true m o s s e s , lichen may not b e acting a s a wick in drawing moisture from b e n e a t h , but i t a p p e a r s to b e protecting under- lying moisture from evaporating influences of the a i r a b o v e . Rapid evaporation or diffusion exchange of water vapor from the wet b a s a l layer to the atmosphere above the lichen may contribute, however, to low s o i l temperatures and a high per- mafrost t a b l e .
D e s p i t e speculation that the high permafrost table under true m o s s , Sphagnum, and lichen may b e c a u s e d in part by high evaporation r a t e s t h a t prevent large q u a n t i t i e s of h e a t from e n - tering the ground, o b s e r v a t i o n s a t Norman W e l l s in the summer of 1959 showed that r a t e s of water l o s s from m o s s and lichen were a b o u t e q u a l to e a c h other b u t lower than from a g r a s s - l i k e
s e d g e , s p e c i e s unknown, or g r a s s [ 161
.
Observations in the summer of 1960 showed that r a t e s for m o s s , l i c h e n , a n d the s e d g e were a b o u t e q u a l to or lower than for g r a s s .Meteorological factors play a prominent role in evaporation and transpiration r a t e s where s o i l moisture i s not a limiting factor. Physiological c h a r a c t e r i s t i c s and radiation and thermal properties of p l a n t s s u c h a s moss a n d l i c h e n , which maintain high permafrost t a b l e s , probably contribute significantly b y evaporation and transpiration t o the energy exchange regime of permafrost. A d i s c r e p a n c y a r o s e a t Norman W e l l s where the s e d g e did not maintain a high permafrost table b u t d i s p l a y e d evapotranspiration r a t e s comparable to r a t e s of m o s s and l i c h e n . This may have b e e n c a u s e d b y the lower insulating v a l u e s of the s e d g e which permitted a g r e a t e r d e p t h of thaw during the summer.
Conductivity
Vegetation h a s a marked insulating effect on underlying perma- frost. This i s true of m o s s e s , l i c h e n s , and particularly, of p e a t . I n c r e a s e in d e p t h of thaw in permafrost a r e a s where vegetation h a s b e e n removed h a s been widely o b s e r v e d .
Variations in the thermal conductivity of peat with moisture c o n t e n t contribute to conditions conducive to formation of per- mafrost [ 21. When d r y , p e a t h a s a low thermal conductivity, equivalent to that of snow (about 0.00017 g c a l / s e c s q cm OC cm)
.
When w e t , i t s thermal conductivity i s g r e a t l y i n c r e a s e d (unsaturated p e a t i s about 0.0007 g c a l / s e c s q cm OC cm; saturated p e a t - e . g . , about 0.0011 g c a l / s e c s q cm " C cm); when frozen, i t s thermal conductivity i s i n c r e a s e d many times over that of dried p e a t and a p p r o a c h e s the value for i c e ( e . g . , s a t u r a t e d frozen p e a t about 0.0056 g c a l / s e c s q cm " C cm).
During the summer a thin surface layer of dried p e a t , which h a s a low thermal conductivity, hinders h e a t transfer to under- lying s o i l . During the cold part of the y e a r , p e a t i s s a t u r a t e d from the s u r f a c e , and when i t f r e e z e s i t s thermal conductivity greatly i n c r e a s e s . Because of t h i s , the amount of h e a t t r a n s - ferred in winter from ground to atmosphere through the frozen ice-saturated p e a t i s greater than the amount transmitted i n the opposite direction through the s u r f a c e layer of dry p e a t and un- derlying w e t p e a t in summer. A considerable portion of h e a t i s a l s o required during t h e warm period to melt the i c e and to warm and evaporate the water. The n e t r e s u l t i s favorable to a permafrost condition.The r a t e of organic a n d p e a t accumulation v a r i e s with the type of vegetation and influences thermal conductivity of t h e surface s o i l layer a n d thermal regime of the underlying perma- f r o s t . In a given c l i m a t i c z o n e , l e s s o r g a n i c material accumu- l a t e s from meadow and s t e p p e vegetation than from f o r e s t and bog v e g e t a t i o n . Organic material accumulates more quickly in a coniferous f o r e s t than in a deciduous f o r e s t . Coniferous f o r e s t s with their d e n s e tree crowns and a c i d i c litter tend to create conditions s u i t a b l e for the development of m o s s e s , par- ticularly where a c o o l c l i m a t e h a s reduced evaporation to a point where the forest floor w i l l b e moist d e s p i t e a low rain f a l l . The r a t e of accumulation in a forest i s related to many f a c t o r s , including the p r e s e n c e or a b s e n c e of m o s s and lichen c o v e r , i t s s p e c i e s composition, and the degree of s w a m p i n e s s . I n a f o r e s t with a s u r f a c e cover o f Sphagnum, a p e a t horizon forms more quickly than in a f o r e s t with true m o s s .
CLIMATIC AND TERRAIN FEATURES
Vegetation e x e r t s both a n indirect influence on permafrost by modifying climatic and other terrain f e a t u r e s , which themselves influence permafrost, and .a d i r e c t influence by i t s role in the h e a t transfer mechanism between ground and atmosphere. The influence of vegetation o n various microclimatic f e a t u r e s , drainage and the water regime, snow c o v e r , and the influence of one type of vegetation on a n o t h e r , a r e important a s p e c t s . These f e a t u r e s a r e s o c l o s e l y interrelated that i t i s difficult to a s s e s s their individual contributions without including some a s p e c t s of o t h e r f e a t u r e s .
M i c r o c l i m a t e
Vegetation d e c r e a s e s a i r c u r r e n t v e l o c i t i e s within i t s s t r a t a a n d therefore i m p e d e s h e a t r a d i a t i o n from t h e s o i l t o t h e a i r when the l a t t e r i s c o o l e r , a s a t night [ 151 , and during p e r i o d s of t h e y e a r when s o i l t e m p e r a t u r e s a r e warmer than t h e a i r [ 1 3 1 .
The d e n s i t y a n d h e i g h t of t r e e s i n f l u e n c e wind v e l o c i t i e s n e a r the ground s u r f a c e . Wind v e l o c i t i e s a r e lower in a r e a s of t a l l d e n s e t r e e growth than w h e r e t r e e s a r e s t u n t e d a n d s c a t - t e r e d , o r a b s e n t . Higher v e l o c i t i e s , r e s u l t i n g p o s s i b l y from fewer o b s t r u c t i o n s in a r e a s of s p a r s e t r e e growth c a u s e more h e a t to move a w a y from t h e s e a r e a s per unit time than from the a r e a s of d e n s e growth. I n the s o u t h e r n fringe of the permafrost r e g i o n , permafrost i s more commonly a s s o c i a t e d with a r e a s of s p a r s e o r no t r e e growth for a number of r e a s o n s . O t h e r f a c t o r s b e i n g e q u a l , t h e p o s s i b l i i t y of s l i g h t l y lower a i r temperatures b e c a u s e of h i g h e r wind v e l o c i t i e s in t h e s e a r e a s may contribute to a ground temperature c o n d i t i o n c o n d u c i v e t o t h e e x i s t e n c e of permafrost.
Air movement, s u c h a s the d r a i n a g e of c o l d a i r a t night from a n e l e v a t e d a r e a d o w n s l o p e to a d e p r e s s i o n , i s a microclimatic f e a t u r e a s s o c i a t e d with r e l i e f , which may a l s o b e s i g n i f i c a n t . Even microrelief f e a t u r e s , s u c h a s p e a t p l a t e a u s , may produce s u f f i c i e n t d i f f e r e n c e s in e l e v a t i o n to c a u s e d o w n s l o p e a i r d r a i n a g e a t night.
V e g e t a t i o n , e s p e c i a l l y t r e e g r o w t h , i n t e r c e p t s a s i g n i f i c a n t portion of a t m o s p h e r i c p r e c i p i t a t i o n , both r a i n a n d s n o w , b y a s much a s 10 to 40% C21. Any r a i n t h a t r e a c h e s and pene- t r a t e s t h e ground c a r r i e s h e a t with i t toward permafrost s o t h a t i n t e r c e p t i o n of rain r e s u l t s i n a reduction of h e a t e n t e r i n g t h e ground. On the o t h e r h a n d , i n t e r c e p t i o n of s n o w b y t r e e s r e - s u l t s i n a lower a c c u m u l a t i o n o n the ground a n d the p o s s i b i l i t y of d e e p e r s e a s o n a l f r o s t p e n e t r a t i o n . I n c r e a s e d s h a d i n g c a u s e d b y snow on t r e e b r a n c h e s r e d u c e s t h e amount of s o l a r r a d i a t i o n r e c e i v e d a t the ground s u r f a c e , b u t t h i s i s c o u n t e r a c t e d p a r t l y , a t l e a s t , b y t h e r e d u c t i o n of r a d i a t i o n from ground s u r f a c e into a t m o s p h e r e .
D r a i n a g e
Ground t h a t permits the g r e a t e s t d e g r e e of w a t e r p e n e t r a t i o n u s u a l l y t h a w s t o the g r e a t e s t d e p t h [ 1 3 1 . There i s e v i d e n c e t h a t e x t e n s i v e r o o t s y s t e m s impede downward p e r c o l a t i o n of w a t e r a n d therefore r e s t r i c t s o i l thawing [ 131
.
On the o t h e r h a n d , r o o t s , e s p e c i a l l y d e a d a n d d e c a y i n g o n e s , may provide c h a n n e l s for w a t e r p e n e t r a t i o n a n d s o m e t i m e s l o c i for t h e growth of g r a n u l e s and s m a l l s t r i n g e r s of i c e [ 131.Vegetation i m p e d e s s u r f a c e runoff. I n f o r e s t s , p a r t i c u l a r l y where v e g e t a t i o n i s n o t d i s t u r b e d , runoff amounts to l e s s than 3% of the t o t a l r a i n f a l l , w h e r e a s in o p e n a r e a s and o n t h e p l a i n s , i t e x c e e d s 60%
C
1 1.
The r a t e of runoff to p r e c i p i t a t i o n i s probably a l s o s i g n i f i c a n t to permafrost. S u b s u r f a c e d r a i n a g e i s s l o w in p e a t b e c a u s e of i t s f i l t r a t i o n p r o p e r t i e s .Snow C o v e r
The low thermal c o n d u c t i v i t y of s n o w and i t s d o u b l e role a s inhibitor of f r o s t penetration during w i n t e r a n d s o i l thawing i n spring h a s b e e n n o t e d b y many a u t h o r s [ 1 3 1 . The r e t e n t i o n of snow i n t h e crowns of t r e e s h a s a l r e a d y b e e n mentioned. I n s p r i n g , t h e s n o w c o v e r remains o n the ground longer i n f o r e s t e d a r e a s than i n t h e o p e n . W h e r e strong w i n d s p r e v a i l , more snow a c c u m u l a t e s under a v e g e t a t i o n c o v e r than i n o p e n a r e a s . Snow p r o t e c t s t h e ground from f r e e z i n g in winter b u t i t a l s o i n c r e a s e s t h e moisture c o n t e n t of the s o i l i n summer, t h u s contributing to lower summer ground t e m p e r a t u r e s [ 11
.
I n Labrador-Ungava a good c o r r e l a t i o n w a s n o t e d by I v e s [ 171 a n d Annersten [: 181 b e t w e e n t h e v e g e t a t i o n and s n o w a c - cumulation a n d the d i s t r i b u t i o n of permafrost. O n e x p o s e d r i d g e s u m m i t s , where v e g e t a t i o n w a s v i r t u a l l y a b s e n t , snow a c c u m u l a t i o n w a s k e p t to a minimum b y t h e w i n d , and perma- f r o s t 200 f t t h i c k e x i s t e d . I n s h e l t e r e d g u l l i e s , v e g e t a t i o n w a s b e t t e r d e v e l o p e d , s n o w a c c u m u l a t e d i n t h e w i n t e r , and permafrost w a s o n l y a f e w f e e t t h i c k or a b s e n t .
Vegetation P r o p e r t i e s
Within t h e framework of the complex i n t e r r e l a t i o n e x i s t i n g
among v a r i o u s t'errain f e a t u r e s t h a t a f f e c t t h e ground thermal r e g i m e , s u c h a s r e l i e f , d r a i n a g e , s o i l s , s n o w c o v e r , a n d v e g e - t a t i o n , s p e c i a l c h a r a c t e r i s t i c s of s o m e p l a n t t y p e s may s i g n i f - i c a n t l y i n f l u e n c e permafrost a n d a l s o i n d i c a t e the e x i s t e n c e of permafrost
.
Tikhomirov [ 191 m e n t i o n s s e v e r a l c h a r a c t e r i s t i c s t h a t influ- e n c e t h e ground thermal regime of true m o s s e s a n d Sphagnum: i t p o s s e s s e s low thermal c o n d u c t i v i t y , high m o i s t u r e holding c a p a c i t y , a n d may s h i e l d r o o t s of v a s c u l a r p l a n t s from low a i r t e m p e r a t u r e s ; i t promotes uniform thawing and p r o t e c t s s o i l from r u n o f f , s o l i f l u c t i o n , e r o s i o n , a n d t h e r m o k a r s t . M o s s re- d u c e s t h e temperature a m p l i t u d e of underlying s o i l . Tikhomirov p o s t u l a t e s t h a t h e a t from m o s s in l a t e winter r e c r y s t a l l i z e s snow a t the m o s s c o n t a c t , t h a t p h o t o s y n t h e s i s i s p o s s i b l e un- d e r a thin snow c o v e r , a n d t h a t h o l l o w s form i n the s n o w i n which the a i r temperature i s higher than t h e o u t s i d e a i r temper- a t u r e , producing f a v o r a b l e c o n d i t i o n s for p l a n t growth under t h e s n o w . P o r s i l d s t a t e s t h a t s o l a r r a d i a t i o n i s t h e more l i k e l y s o u r c e of h e a t [ 201. He a l s o q u e s t i o n s p h o t o s y n t h e t i c a c t i v i t y b e n e a t h e v e n a thin s n o w c o v e r a n d t h e provision of f a v o r a b l e c o n d i t i o n s for p l a n t growth under t h e s n o w .
Robinson n o t e d t h a t in f a l l t h e melting of e a r l y s n o w f i l l s the m o s s w i t h m o i s t u r e e n a b l i n g i t to c o n d u c t h e a t a t a more rapid r a t e ; t h i s p e r m i t s g r e a t e r h e a t l o s s from the ground a n d d e e p p e n e t r a t i o n of s e a s o n a l f r o s t [ 21
1
.
I n summer t h e top few i n c h e s d r y to a point w h e r e t h e y a c t a s a n e f f e c t i v e i n s u - lating b l a n k e t . Therefore the p r e s e n c e of a d e e p l a y e r of m o s s k e e p s the s o i l a t a low temperature c o n t i n u o u s l y and f a v o r s development of permafrost. M o s s is v e r y w a t e r a b s o r b e n t . The lower portion o f a m o s s l a y e r i s u s u a l l y m o i s t a n d t h i s m a i n t a i n s t h e ground in a damp o r w e t c o n d i t i o n .C e r t a i n p l a n t t y p e s provide r a t h e r r e l i a b l e i n d i c a t o r s of t h e e x i s t e n c e of permafrost. At Thompson, M a n . , C a n a d a , the p r e s e n c e of Sphagnum, and/or s t a n d s of s p r u c e v a r y i n g from s m a l l t r e e s i n o p e n s t a n d s t o moderately l a r g e t r e e s i n n e a r l y c l o s e d s t a n d s , w a s found u s u a l l y to b e a s s o c i a t e d with perma- f r o s t , i f t h e d r a i n a g e w a s f a i r l y good [ 221. I n northern Alberta, a l l b u t a few of the permafrost o c c u r r e n c e s w e r e found i n low f l a t d e p r e s s i o n s [ 231.
I n t h e s e a r e a s two a s s o c i a t i o n s of v e g e t a t i o n predominated. O n e w a s g r a s s - l i k e s e d g e with l i t t l e o r no t r e e growth a n d thin m o s s c o v e r . T h e s e a r e a s were a l m o s t a l w a y s w e t a n d no per- mafrost e x i s t e d . The o t h e r c o n s i s t e d of Sphagnum, l i c h e n p a t c h e s , and s c a t t e r e d s t u n t e d b l a c k s p r u c e . Some of t h e s e a r e a s w e r e w e t a n d c o n t a i n e d no p e r m a f r o s t . The remainder were d r i e r , a n d permafrost o c c u r r e d a t d e p t h s ranging from a b o u t 2 to 4 f t . At the e d g e s of t h e s e a r e a s , the permafrost dropped o f f a b r u p t l y .
Three v a r i e t i e s of v e g e t a t i o n a r e s h o w n in F i g . 1 , t a k e n S e p t . 2 0 , 1962 ( 5 7 O 4 7 ' ~ . 1 1 7 " 5 0 ' ~ ) , 3 . 2 m i l e s w e s t of M a c - k e n z i e H i g h w a y , A l b e r t a , C a n a d a , i n the s o u t h e r n fringe of the d i s c o n t i n u o u s z o n e of t h e permafrost r e g i o n .
Fig. 1 . Variety of v e g e t a t i o n a s s o c i a t i o n s with r e l a t e d v a r i a - t i o n s i n permafrost o c c u r r e n c e
Thc light-toned vegetation in the foreground and middle ground c o n s i s t s of s e d g e (Corex s p . ) and g r a s s with a thin d i s c a n t i n u o u s c,,vcr of feather and other non-Sphagnum m o s s e s in standing w a t e r . No permafrost w a s e n c o u n t e r e d .
The dark toned island in fcrcground (man kneeling) i s d
s l i g h t l y e l e v a t e d p e a t platcau with ground cover of hummocky Sphagnum and Labrador t e a . Depth of moss and p e a t i s 4 ft
2 i n . ; black o r g a n i c s i l t 4 ft 2 i n . to 6 f t 0 i n . ; d e n s c b l u i s h s i l t c l a y 6 ft 0 i n . to below 7 f t . Permafrost tablc c x t c n d s from from 2 ft 9 i n . to 7 ft below ground s u r f a c e . Pcrmafrost o c c u r s a l s o in the dark almost t r e e l e s s patch ( s a m c ground vegetation) a t r i g h t , between s e d g e a r e a in middle ground and f o r c s t i n background. Thc forested a r e a c o n s i s t s of s p r u c e , poplar, jackpine, and b k c h ; no permafrost w a s encountered h e r e .
VEGETATION ZONES AS A PERMAFROST INDICATOR
In C a n a d a , A l a s k a , and the USSR, the i n f l u e n c e of vegetation v a r i e s from one g e u b o t a n i c a l zone to a n o t h e r . In C a n a d a and A l a s k a , permafrost o c c u r s in tundra and taiga z o n e s . In the USSR, i t o c c u r s in these two z o n e s and a l s o e x t e n d s southward into the s t e p p e . The variety of vegetation a s s o c i a t i o n s , the quantity of vegetable m a t t e r , s t a n d h e i g h t , and d e n s i t y , and rate of p e a t accumulation a r e a l l g r e a t e s t in the t a i g a , grad- ually diminishing northward i n t o the tundra and southward into the s t e p p e . The d e g r e e and variety of the influence of the vegetation on the permafrost c h a n g e s in a p a r a l l e l manner [ 21.
In the northern part of t h e tundra the vegetation h a s l i t t l e influence on permafrost b e c a u s e i t i s s p a r s e and the v e g e t a t i v e period l a s t s l e s s than two months. I t c a u s e s l o c a l variations in depth of thaw and h e l p s impede e r o s i o n . The destruction of the vegetation a c c e l e r a t e s thawing only s l i g h t l y .
In the southern part of the tundra, the vegetation becomes more a b u n d a n t , p e a t mantles part of the s u r f a c e and a t t a i n s t h i c k n e s s c s of s e v e r a l f e e t in some b a s i n s . The main influ- e n c e of the vegetation i s on the depth of thaw. If vegetation i s removed, the depth of thaw will i n c r e a s e ; erosion will in- c r e a s e and thermokarst will develop if thawing proceeds a t different r a t e s over an a r e a or if there a r e l o c a l differences in the i c e content of the frozen ground.
In f o r e s t tundra, vegetation m s s s i s greater than in th.e tundra, and the r a t e of accumulation of organic material i s higher. Extensive p e a t b o g s form and water b a s i n s a r e en- croached b y vegetation and permafrost forms. Woody and brush vegetation grow which accumulate snow leading to higher permafrgst temperatures than in the tundra. If the vege- tation i s removed, the depth of thaw i n c r e a s e s but t h i s i s counteracted to some e x t e n t b y lower snow accumulation and a d e c r e a s e in permafrost temperatures [ 11.
The maximum development of vegetation o c c u r s in the t a i g a . Here vegetation h a s i t s g r e a t e s t influence on permafrost even in very s m a l l l o c a l i z e d a r e a s causing variations in i t s e x t e n t , depth o f t h a w , and ground temperatures. Frequent forest f i r e s c a u s e variations in the occurrence and t h i c k n e s s of permafrost over short d i s t a n c e s i n the t a i g a .
M a s s , d e n s i t y , height and influence of v e g e t a t i o n , and r a t e of accumulation of organic matter a r e l e s s i n the steppe than in the t a i g a , b u t depth to permafrost i s g r e a t e r .
ALTERATION OF PERMAFROST CONDITIONS
C h a n g e s take p l a c e in the permafrost a s a r e s u l t of the vegeta- tion. The influences include the e f f e c t of vegetation on depth of thaw and depth to permafrost, the temperature regime in per- mafrost a n d the ground a b o v e , and e x t e n t and t h i c k n e s s of permafrost.
Depth of Thaw
The most e a s i l y observed and measured c h a r a c t e r i s t i c of per- mafrost i s depth of thaw and v a r i a t i o n s in t y p e s of vegetation a r e often readily n o t i c e a b l e . B e c a u s e t h i s i s s o , more o b s e r - vations have b e e n made on t h i s a s p e c t of the relation of vege- tation to permafrost than a n y o t h e r . D e s p i t e the large number of o b s e r v a t i o n s reported in the l i t e r a t u r e , mechanisms opera-
tive in thawing'of the a c t i v e layer and permafrost and c a u s e s of variations from one type of vegctation to another a r c not clcarly understood. One difficulty in comparing depth of thaw o b s e r v a t i o n s in various l o c a l i t i e s i s c a u s e d by v a r i a t i o n s in c l i m a t e , variations in other terrain factors c l o s e l y a s s o c i a t e d with the v c g e t a t i o n . and by minute, but possibly s i g n i f i c a n t , variations within a particular typc of vegetation.
Rcmoval of vegetation cover in a permafrost a r e a c a u s e s a n i n c r e a s e in the depth of thaw. At Inuvik, NWT, in the con- tinuous permafrost z o n e , the maximum depth of thaw in a n un- disturbed moss-covered a r e a w a s 2 f t in c o n t r a s t to d e p t h s of 5 to 8 ft in a r e a s stripped three y e a r s prcviously [ 2 4 1 . On land stripped for cultivation a t I n u v i k , the original maximum depth of thaw w a s about 2 f t prior to d i s t u r b a n c e (1956). By 1959 the ground thawed to a depth of 70 i n . ( 2 5 1 .
The s u r f a c e cover and p e a t appear to have much greater in- fluence on depth of thaw than the underbrush and t r e e s . At Inuvik, in undisturbed a r e a s and in a r e a s whcre t h e underbrush and t r e e s had been removed three y e a r s previously b u t with the m o s s cover left i n t a c t , the depth of thaw w a s 2 ft-similar to the depth prior to a n y d i s t u r b a n c e [ 241. At Norman W e l l s , depth of thaw measurements were recorded by the Division of Building Rese'arch in different types of vegetation from 1957 to 1959. The g r e a t e s t depth of thaw occurred in the g r a s s - l i k e s e d g e a r e a with no moss cover reaching a depth of 5 ft 6 i n . after a b o u t 3350 d e g r e e d a y s of thawing. The next g r e a t e s t depth of thaw w a s observed in a wooded a r e a having a ground cover of 4 i n . of moss overlying 3 i n . of p e a t , reaching 4 f t 6 i n . a f t e r 3350 degree d a y s of thawing. The next g r e a t e s t t h a w , 3 f t 3 i n . , occurred i n a t r e e l e s s g r a s s - l i k e s e d g e and moss a r e a with a 3-in. m o s s cover overlying 6 i n . of p e a t . The s h a l l o w e s t depth of t h a w , 2 i t , w a s observed in a s p a r s e l y treed a r e a having 5 i n . of m o s s overlying 18 i n . of p e a t . Evi- d e n t l y , the depth of thaw d e c r e a s e d with an i n c r e a s e in t h e combined t h i c k n e s s of living moss and p e a t ; the d e n s i t y and s i z e of tree growth did not appear to make much difference.
Russian i n v e s t i g a t o r s found that of a l l the types of ground c o v e r , Sphagnum a p p e a r s to retard thawing most. I n the Igarka region of the USSR, the d e p t h of thaw in 1950 under t h i s cover w a s 18 cm ( 7 . 1 in.) o n 13 J u l y , 22 cm (8.7 in.) on 3 August, and 26 cm ( 1 0 . 2 in.) on 2 September in c o n t r a s t to 2 5 , 3 1 , and 35 cm ( 9 . 8 , 1 2 . 2 , 13.8 in.) on the s a m e d a t e s under true m o s s e s c o n s i s t i n g of Hypnum and other s p e c i e s [ 1 , 21.
The r c l a t i v e influence of living ground cover and underlying p e a t h a s b e e n i n v e s t i g a t e d . I t h a s been postulated that p e a t r e t a r d s thawing e v e n more than living m o s s e s and l i c h e n s . I n the a r c t i c region of the Yenisey River in Siberia, removing the moss and lichen b u t leaving the p e a t layer resulted in a n in- c r e a s e in the depth of thaw b y 20 to 50%. After both living cover and p e a t were removed, depth of thaw i n c r e a s e d by I.. 5 to 2.5 times E l , 21.
Temperature Regime
J u s t a s the v e g e t a t i o n e x e r t s a marked influence on the depth of thaw and the d e p t h to permafrost, i t a l s o modified ground temperatures both in permafrost and the ground a b o v e . An in- c r e a s e in depth of thaw l e a d s to a n i n c r e a s e in ground tempera- ture and degradation of permafrost. A d e c r e a s e in depth of thaw l e a d s e v e n t u a l l y to a n aggradation of the permafrost.
At Norman W e l l s , ground temperatures were measured i n the thawed layer by the Division of Building Research in 1959 and 1960 to a s s e s s the e f f e c t of different t y p e s of vegetation o n underlying s o i l temperatures during the summer. In September 1959 the mean a i r temperature w a s 41 .Z°F and t h e mean ground temperature a t the l - f t depth for t h i s period i'n v a r i o u s vegeta- tion a r e a s were: g r a s s (no m o s s o r peat) 40.0°F, s e d g e (no m o s s o r peat) 3 6 . 5 ' ~ , g r a s s - l i k e s e d g e ( 3 i n . true moss o v e r 6 i n . peat) 3 5 . 0 ° F , m o s s (5 i n . Sphagnum over 18 i n . peat) 32.5'F, l i c h e n (2 i n . lichen over 24 i n . peat) 32. b°F.
There appeared to b e a g e n e r a l d e c r e a s e in temperature with i n c r e a s e d combined m o s s a n d p e a t t h i c k n e s s . Temperatures were similar under m o s s and lichen although living m o s s w a s much thicker than l i c h e n . The combined t h i c k n e s s of living cover and p e a t w a s , however, approximately e q u a l . Ground
tcrnporaturcs'taken in 1960 in thcr thawcd laycr showed that thoy were h i g h e s t uridor thc gras:; i ~ r c a , lower under [ h e s e d g e , and l o w e s t under thc m o s s a n d l i c h e n . Temperature nmplitudas a l s o d e c r e a s e d in the s a m e ordtlr. Thermal r a s i s t a n c c and damping e f f e c t of m o s s and lichen wcrc shown b y the f a c t t h a t monthly mean a i r tcrnpcratures were a b o u t 10°F higher in 1960 than i n 1 9 5 9 , b u t the difference in mean ground tcmperaturc a t the 1-ft d e p t h w a s l e s s than l o r .
Ground temperature r e a d i n g s were a l s o takdn a t Norman W e l l s down to the LO-ft d e p t h in permdfrost under the s e d g e , m o s s , a n d l i c h e n . Even b e l o w the 1 0-ft d e p t h , the mean tem- perature under the s e d g e for August a n d September 1960 w a s about 3 ' ~ higher than under the m o s s and l i c h e n , which were a b o u t e q u a l . Above t h i s d e p t h the d i f f e r e n c e w a s e v e n g r e a t e r . Similarly, the temperature a m p i i t u d e s in the top 10 ft were twice a s much under the s e d g e .
There i s no d o u b t that v c g s t a t i o n modifies the temperatures of the s e a s o n a l l y thawed layer and permafrost. I n a l l c a s e s , ground temperatures i n summer w i l l r i s e when the v e g e t a t i o n c o v e r i s removed. On the o t h e r h a n d , the e f f e c t s of winter a i r temperatures w i l l p e n e t r a t e to g r e a t e r d e p t h s than in undis- turbed a r e a s . The n e t e f f e c t o n mean a n n u a l temperature w i l l depend on o t h e r f a c t o r s , s u c h a s s n o w c o v e r .
I t i s d i f f i c u l t to compare the e f f e c t s of different t y p e s of v e g e t a t i o n o n permafrost b e c a u s e different t y p e s frequently grow i n c l o s e a s s o c i a t i o n o r in a m o s a i c , and the i n d i v i d u a l e f f e c t of e a c h may not b e r e a d i l y a p p a r e n t .
Extent and T h i c k n e s s
A c h a n g e i n d e p t h of thaw and a c h a n g e in temperature of the a c t i v e l a y e r and the permafrost produced b y a c h a n g e i n vege- tation e s t a b l i s h e s a c h a n g e in temperature g r a d i e n t . This re- s u l t s i n e i t h e r aggradation or d e g r a d a t i o n of the permafrost. I n t h e southern fringe of the p e r m a f r o s t , the removal of the v e g e t a t i o n may r e s u l t in d i s a p p e a r a n c e of the permofrost. The e s t a b l i s h m e n t of a m o s s c o v e r may l e a d , p e r h a p s , to the formation a n d a c c u m u l a t i o n of permafrost.
CONCLUSION
T h i s p a p e r r e v i e w s v a r i o u s w a y s in which v e g e t a t i o n a f f d c t s permafrost. Some m e c h a n i s m s add h e a t to thc. g r o u n d , o t h e r s f a c i l i t a t e h e a t l o s s from the ground. Some add h e a t a t o n e time and c o n t r i b u t e to h e a t l o s s a n o t h e r t i m e . Influences of v e g e - tation a r e a l m o s t a l l r e v e r s i b l e depending o n the c o n d i t i o n s under which they o c c u r . The complexity and multifaceted e f - f e c t s of v e g e t a t i o n o n permafrost o f t e n l e a d to a s i t u a t i o n where under o n e s e t of c o n d i t i o n s a p l a n t community d e c r e a s e s the s o i l temperature and i n c r e a s e s i t undzr o t h e r c o n d i t i o n s .
I t i s very d i f f i c u l t to a t t a c h a b s o l u t e q u a n t i t i s s to e a c h f a c e t of the v e g e t a t i o n influencc-total them, and a r r i v e a t the r e s u l t a n t d i r e c t i o n and magnitude of h e a t flow a t a p a r t i c u l a r t i m e , much l e s s periorm this o p e r a t i o n for a l l thc p a s t influ- e n c e s and a s s e s s their e f f e c t on t h e p r e s e n t permafrost s i t u a - tion. Even if e a c h f a c t o r could b e m e z s u r e d q u a n t i t a t i v e l y , the contribution of some i s s o minute that the cumulative er- ror would b e u n r e a l i s t i c . I n a d d i t i o n , t h e r e a r e f a c t o r s which a r e probably n o t e v e n known o r p o s s i b l e to m e a s u r e .
The b e s t s o l u t i o n a p p e a r s to b e to measure o b v i o u s e f f e c t s of v e g e t a t i o n , s u c h a s d e p t h of t h a w , t e m p e r a t u r e s , e x t e n t and t h i c k n e s s of permafrost which a r e m a n i f e s t a t i o n s of the n e t h e a t g a i n s and h e a t l o s s e s to the g r o u n d , and r e l a t e t h e s e to v a r i a t i o n s in environmental c o m p o n e n t s . The s a m e perma- f r o s t c o n d i t i o n s may o c c u r in two a d j a c e n t a r e a s , b u t t h e com- b i n a t i o n of v e g e t a t i o n and o t h e r f a c t o r s producing two s i m i l a r s e t s of permafrost c o n d i t i o n s may b e q u i t e d i f f e r e n t .
ACKNOWLEDGMENTS
The author w i s h e s to a c k n o w l e d g e the h e l p f u l comments of A. E. Porsild and W . K. W . Baldwin, N a t i o n a l Herbarium, Department of Northern Affairs and N a t i o n a l R e s o u r c e s , Ottawa: the l a t e J . A. P i h l a i n e n , O t t a w a , and W . S . Benninghoff, De- partment of B o t a n y , U n i v e r s i t y of M i c h i g a n . This i s a contri-
bution from tho Division of Building R o s a a r c h , N a t i o n a l Re-
s e a r c h C o u n c r l , C a n a d a , a n d i s p r e s e n t e d with permicision af the director of tnc: d i v i s i o n .
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