Allelopathy: A Potential Cause of Forest Regeneration Failure

C h a p t e r 16

Allelopathy: A Potential Cause of Forest Regeneration Failure Richard F. Fisher

Downloaded by UNIV OF NEW SOUTH WALES on April 13, 2016 | http://pubs.acs.org Publication Date: January 8, 1987 | doi: 10.1021/bk-1987-0330.ch016

Department of Forest Resources, Utah State University, Logan, UT 84322-5215

The failure of forest tree regeneration is a major problem in many areas. Although such failures are often climatically induced, there is a growing body of data that suggests allelopathy is an equally l i k e l y cause of failure. Complete elucidation of the exact role of allelopathy in regeneration problems awaits a better understanding of the pathway and fate of a l l e l o c h e m i c a l s i n the environment. A schematic approach to this problem is presented. F a i l u r e s of newly e s t a b l i s h e d f o r e s t tree p l a n t a t i o n s or a b n o r m a l d e l a y s i n t r e e s e e d l i n g g r o w t h seldom have c l e a r - c u t causes. In the absence o f knowledge o f cause, f o r e s t e r s often attempt remedies that are u n n e c e s s a r i l y c o s t l y o r e n v i r o n m e n t a l l y d a m a g i n g , e v e n when t h e y s u c c e e d . A l l e l o p a t h y , the d i r e c t or i n d i r e c t d e l e t e r i o u s e f f e c t o f one p l a n t upon a n o t h e r t h r o u g h t h e production o f chemical i n h i b i t o r s released i n t o the environment, i s l i k e l y a common cause o f such f a i l u r e s o r d e l a y s . F o r e s t e r s g e n e r a l l y t h i n k o f i n t e r a c t i o n s among p l a n t s i n terms o f c o m p e t i t i o n f o r l i g h t , w a t e r , n u t r i e n t s , o r space. I t has become i n c r e a s i n g l y c l e a r , t h o u g h , t h a t many s p e c i e s i n f l u e n c e o t h e r s through chemical i n h i b i t i o n or i n t e r f e r e n c e . A l t h o u g h t h e phenomenon o f a l l e l o p a t h y was f i r s t d e s c r i b e d by De C a n d o l l e ( 1 ) , P l i n y t h e E l d e r w r i t i n g i n t h e f i r s t c e n t u r y A.D. i n h i s N a t u r a l i s H i s t o r i a d e s c r i b e d the f a i l u r e o f c e r t a i n p l a n t s t o grow i n t h e s h a d e o f J u g l a n s r e g i e . In a d d i t i o n n e a r l y every s o c i e t y has had f o l k l o r e about t h e e f f e c t s o f one p l a n t u p o n another. In the e a r l y days o f p l a n t p a t h o l o g y , v i r u s e s were m y s t e r i o u s and p o o r l y u n d e r s t o o d . N e a r l y e v e r y m a l a d y o f unknown c a u s e was

0097-6156/87/0330-0176$06.00/0 © 1987 American Chemical Society

Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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a s c r i b e d t o a v i r u s and f o r a t i m e t h e v i r u s a s a l e g i t i m a t e c a u s e o f p l a n t d i s e a s e was h e l d i n g r e a t q u e s t i o n by many. Currently a l l e l o p a t h y e n j o y s a s i m i l a r r e p u t a t i o n among f o r e s t e r s and t r e e physiologists.


Much o f t h e e a r l y e x p e r i m e n t a l work on a l l e l o p a t h y e m p l o y e d q u e s t i o n a b l e methods and f a i l e d t o produce u n e q u i v o c a l r e s u l t s . Now t h e r e i s a s u b s t a n t i a l body o f d a t a o b t a i n e d by s o u n d t e c h n i q u e s , t h a t v e r i f i e s t h e r o l e o f a l l e l o p a t h y i n many p l a n t c o m m u n i t i e s . T a b l e 1 l i s t s some a l l e l o p a t h i c p l a n t s o f i n t e r e s t to f o r e s t e r s , t o g e t h e r w i t h t h e c l a s s e s o f t o x i c compounds produced and examples o f s p e c i e s t h e y a r e r e p o r t e d t o s u p p r e s s . The l i s t i s not e x h a u s t i v e ; many s p e c i e s t h a t may be a l l e l o p a t h i c h a v e n o t b e e n studied i n depth. One e a s i l y o b s e r v e d e f f e c t — t h o u g h s o m e t i m e s d i f f i c u l t to d i s t i n g u i s h from e f f e c t s of c o m p e t i t i o n — i s the e x c l u s i o n o f s h r u b s , h e r b s , and o t h e r t r e e s from beneath the crowns of p a r t i c u l a r t r e e s p e c i e s . Walnut ( J u g l a n s n i g r a ) i s t h e most n o t o r i o u s o f a l l e l o p a t h i c trees, a l t h o u g h E u c a l y p t u s c a n have e q u a l l y d r a m a t i c effects. Jameson (2) found t h a t s e v e r a l s p e c i e s o f J u n i p e r u s and P i n u s i n the p i n y o n - j u n i p e r t y p e o f the Southwest i n h i b i t g r a s s e s and f o r b s t o a g r e a t e r degree than would be expected from s i m p l e c o m p e t i t i o n . His w o r k , h o w e v e r , l i k e much o f t h a t on t h e s u b j e c t , s t o p p e d s h o r t o f c o n c l u s i v e l y demonstrating a l l e l o p a t h i c a c t i v i t y . Sycamore (Platanus occidentalis), sugarberry (Celtis l a e v i g a t a ) , and s a s s a f r a s ( S a s s a f r a s a l b i d u m ) produce p h e n o l i c s and t e r p e n o i d s t h a t reduce t h e g r o w t h o f some s p e c i e s o f h e r b s , g r a s s e s , and even t r e e s (3-5). Tubbs (6) d e m o n s t r a t e d t h a t sugar maple (Acer saccharum) r o o t exudates c o u l d i n h i b i t the e a r l y growth o f y e l l o w b i r c h (Betula a l l e g h a n i e n s i s ) . C h e r r y b a r k oak (Quercus f a l c a t a v a r . p a g o d a e f o l i a ) as w e l l as o t h e r oaks p r e c l u d e or r e t a r d t h e growth o f some t r e e s , s h r u b s , h e r b s , and g r a s s e s b e n e a t h t h e i r c r o w n s ( 7 - 9 ) . T h i s n a t u r a l h e r b i c i d a l e f f e c t i s not o n l y e c o l o g i c a l l y i n t e r e s t i n g , b u t i t c o n s t i t u t e s b o t h a b l e s s i n g and a c u r s e t o t h e f o r e s t e r t r y i n g t o manage such t r e e s . The a l l e l o p a t h i c e f f e c t s o f s h r u b s , h e r b s , g r a s s e s , f e r n s , and l i c h e n s upon t r e e s i s o f p a r t i c u l a r c o n c e r n when c o n s i d e r i n g r e g e n e r a t i o n f a i l u r e o r s e e d l i n g growth s u p p r e s s i o n . Poor s u r v i v a l o f h a r d w o o d s e e d l i n g s i n weedy a b a n d o n e d f i e l d s h a s troubled f o r e s t e r s f o r decades. Recent work has shown t h a t s e v e r a l common p l a n t s i n h i b i t tree e s t a b l i s h m e n t i n such h a b i t a t s . W a l t e r s and G i l m o r e (10) r e p o r t e d t h a t f e s c u e g r a s s ( F e s t u c a ) i n h i b i t e d t h e growth o f sweetgum ( L i q u i d a m b a r s t y r a c i f l u a ) p l a n t e d i n o l d f i e l d s i n I l l i n o i s , and F i s h e r et a l . (11) found t h a t g o l d e n r o d ( S o l i d a g o ) and A s t e r i n t e r f e r e d w i t h t h e e s t a b l i s h m e n t and g r o w t h o f s u g a r maple i n o l d f i e l d s i n O n t a r i o . H o r s l e y (12. 13) h a s shown t h a t b l a c k c h e r r y (Prunus s e r o t i n a ) i s i n h i b i t e d by o l d - f i e l d p l a n t s such as s h o r t h u s k g r a s s ( B r a c h y e l y t r u m erectum) and A s t e r and by f o r e s t p l a n t s s u c h as New Y o r k f e r n ( D r y o p t e r i s noveboracensis) and clubmoss (Lycopodium). C e r t a i n l y p o o r n u t r i t i o n and m i c r o c l i m a t e are the major b a r r i e r s to the e s t a b l i s h m e n t of l a t e s u c c e s s i o n a l


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1.

Some A l l e l o p a t h i c Plants Important i n Forestry, the Chemicals They Produce, and the Plants They are Reported to Affect

Allelopathic Species


FORESTRY

Trees Sugar maple Hackberry Eucalyptus Walnut Juniper Sycamore Black cherry Oaks Sassafras Poplar Shrubs Laurel Manzanita

Class of Chemical Produced

Example of Affected Species

Phenolics Coumarins Phenolics, terpenes Quinone (juglone)

Yellow birch Herbs, grasses Shrubs, herbs, grasses Trees, shrubs, herbs Grasses Herbs, grasses Red maple

Phenolics Courmarins Cyanogenic glycosides Courmarins, other phenolics Terpenoids Phenolics

Herbs, grasses Elm, maple Shrub mycorrhizae

Phenolics Courmarins, other phenolics Phenolics Phenolics, terpenoids Phenolics Phenolics Phenolics

Black spruce Herbs, grasses

New York fern Bracken fern Fescue Shorthusk grass Clubmoss Reindeer lichen

Phenolics, terpenoids Phenolics, terpenoids Phenolics Phenolics Phenolics Phenolics Phenolics Phenolics

Bahiagrass

Phenolics

Sugar maple, black cherry Sugar maple, black cherry Black cherry Douglas-fir Sweetgum Black cherry Black cherry Jack pine and white spruce mycorrhizae Slash pine

Bearberry Sumac Rhododendron Elderberry Lyonia Other Aster Goldenrod

Pine, spruce Douglas-fir Douglas-fir Douglas-fir Slash pine


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t r e e species on o l d f i e l d s , but j u s t as c e r t a i n l y a l l e l o p a t h y i s often an additional adversity that must be overcome.


Numerous other a l l e l o p a t h i c interferences to tree establishment have been d i s c o v e r e d , although t h e i r importance i s yet to be demonstrated i n the f i e l d . For example d e l Moral and Cates (14 ), using an elaborate bioassay technique, found that l i t t e r extracts of P a c i f i c madrone (Arbutus menziesii), vine maple (Acer circinatum), elderberry (Sambucus racemosa), Rhododendron, sumac (Rhus ursinus), and s e v e r a l other common shrubs i n h i b i t e d r a d i c a l e l o n g a t i o n i n g e r m i n a t i n g D o u g l a s - f i r (Pseudotsuga m e n z i e s i i ) seeds. Also, Stewart (15) found that bracken fern (Pteridium aquilinum) inhibited Douglas-fir seedling growth i n greenhouse cultures. R e i t v e l d (16) demonstrated that grass (Festuca a r i z o n i c a , Muhlenbergia montana) r e s i d u e s reduced the g e r m i n a t i o n and e a r l y growth of ponderosa pine (Pinus ponderosa) w h i l e P r i e s t e r and P e n n i n g t o n (17) have r e p o r t e d t h a t broomsedge (Andropogon v i r g i n i c u s ) has i n h i b i t o r y e f f e c t s on l o b l o l l y pine (Pinus taeda) seedlings. Fisher and Adrian (18) found that Bahiagrass (Paspalum notatum) was a strong i n h i b i t o r of the growth of slash pine (Pinus e l l i o t t i i ) seedlings. Peterson (19) found that sheep l a u r e l (Kalmia angustifolia) was t o x i c t o black spruce (Picea mariana) s e e d l i n g s , and F i s h e r (20) reported that reindeer moss (Cladonia) r e s t r i c t e d the growth of jack pine (Pinus banksiana) and white spruce (Picea glauca by reducing root formation. F i s h e r a l s o observed that l e a c h a t e s from common f o r e s t p l a n t s such as bog l a u r e l (Kalmia p o l i f o l i a ) and b i g l e a f aster (Aster macrophyllus) i n h i b i t e d germination and early growth of white and black spruce i n the l a b o r a t o r y . Such i n h i b i t i o n may e x p l a i n why r e g e n e r a t i o n success on organic s o i l s i n northern Ontario i s more closely related to the species than to the density of i n t e r f e r i n g plants. S e v e r a l authors have obtained c i r c u m s t a n t i a l evidence that a l l e l o p a t h i c compounds reduce mycorrhizae f o r m a t i o n (20-23). Kovacic and associates (24) have shown that understory plants i n a l i v e ponderosa pine stand are l a r g e l y nonmycorrhiza-forming species. They hypothesized that this was due to i n h i b i t i o n of the v e s i c u l a r a r b u s c u l a r mycorrhiza necessary f o r the growth of herbaceous mycorrhizal plants, under l i v i n g pines. They demonstrated that more mycorrhizal plants occurred under dead pines, bioassay plants formed mycorrhizae i n s o i l s beneath dead pines but not i n s o i l beneath l i v e pines, and mycorrhizal inoculum appeared to be absent from the l i v e pine stand. Tobiessen and Werner (25) found that the hardwood understory i n Scotch and red pine plantations d i f f e r e d considerably i n i t s a b i l i t y to survive. Those seedlings i n the Scotch pine plantation grew well and became mycorrhizal. However, hardwood seedlings growing under red pine were shown to be nonmycorrhizal, d e f i c i e n t i n P, and unable to grow beyond the seedling stage. Read and J a l a l (26) attempted to c o r r e l a t e suppression o f c o n i f e r s by C a l l u n a v u l g a r i s with i n h i b i t i o n of the ectomycorrhizal fungi needed for t h e i r growth. In


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c o n t r a s t t o the o t h e r s t u d i e s m e n t i o n e d , they found t h a t i n pure c u l t u r e s t u d i e s t h e r e was no e v i d e n c e o f a n t a g o n i s m b e t w e e n t h e f u n g a l symbiont o f C a l l u n a and e c t o m y c o r r h i z a l f u n g i . However, t h e y d i d not examine t h e e f f e c t o f C a l l u n a e x t r a c t s on e c t o m y c o r r h i z a l fungi. H o l l i s and a s s o c i a t e s (27) i n v e s t i g a t e d the a l l e l o p a t h i c e f f e c t o f n i n e o f t h e m o s t a b u n d a n t h e r b a c e o u s and s h r u b b y u n d e r s t o r y associates i n Lower Coastal P l a i n flatwoods pine s t a n d s on g e r m i n a t i o n , r a d i c l e e x t e n s i o n , and s h o o t g r o w t h o f s l a s h and l o b l o l l y pine. They i d e n t i f i e d l y o n i a ( L y o n i a l u c i d a ) as a s t r o n g i n h i b i t o r o f both pine s p e c i e s . Subsequent f i e l d s t u d i e s c o n f i r m e d t h a t l y o n i a reduced the growth o f p l a n t e d s l a s h p i n e . Few s t u d i e s have f o l l o w e d l a b o r g r e e n h o u s e r e s u l t s w i t h c o r r o b o r a t i o n from field studies. Fewer y e t have a d e q u a t e l y t r a c e d t h e p a t h o f a l l e l o c h e m i c a l s i n the environment. The p o t e n t i a l f a t e o f an a l l e l o p a t h i c c h e m i c a l i n t h e e n v i r o n m e n t i s o u t l i n e d s c h e m a t i c a l l y i n F i g u r e 1. D o n o r p l a n t s p r o d u c e t o x i n s o r t h e i r p r e c u r s o r s , and t h e s e t o x i n s a r e r e l e a s e d i n t o the environment. Whether t h e r e i s " f e e d b a c k c o n t r o l " o r any o t h e r form of c o n t r o l over t h i s p r o c e s s i s unknown (28). Whatever q u a n t i t y o f t o x i n i s r e l e a s e d g e n e r a l l y e n t e r s the s o i l , w h e r e s e v e r a l t h i n g s may h a p p e n t o i t . The o r d e r o f t h e s e c o n d , t h i r d , and f o u r t h s t e p s i n F i g u r e 1 i s n o t n e c e s s a r i l y t h e o r d e r i n which the r e a c t i o n s a c t u a l l y o c c u r . In f a c t , i f the t o x i n i s a v o l a t i l e substance, these steps may be s k i p p e d e n t i r e l y . In most c a s e s , however, i t appears t h a t a l l e l o p a t h i c c h e m i c a l s e n t e r the s o i l . S o i l c o l l o i d s a r e c a p a b l e o f a d s o r b i n g most a l l e l o p a t h i c chemicals. Such a d s o r p t i o n would r e s u l t i n t e m p o r a r y l o s s o f t o x i n activity. C h e m i c a l changes c o u l d o c c u r d u r i n g a d s o r p t i o n t h a t would permanently d e a c t i v a t e the t o x i n . The a d s o r p t i o n r e a c t i o n s a r e u s u a l l y r e v e r s i b l e , however, so t h a t some o r a l l o f the t o x i n would s t i l l be a v a i l a b l e f o r uptake by a r e c e i v e r p l a n t . The t o x i n i s a l s o l i k e l y t o be a d s o r b e d o r c o m p l e x e d by s o i l humic a c i d s . I f the r e a c t i o n i s a simple adsorption r e a c t i o n , a l l o r p a r t o f t h e t o x i n might l a t e r become a v a i l a b l e f o r a b s o r p t i o n by a receiver plant. I f t h e t o x i n i s complexed o r p r e c i p i t a t e d by i t s r e a c t i o n w i t h s o i l humic s u b s t a n c e s , t h e n i t would be d e a c t i v a t e d . The t o x i n may undergo m i c r o b i a l d e g r a d a t i o n e i t h e r w h i l e i t i s free i n s o i l s o l u t i o n or while i t i s adsorbed. This could destroy a l l o r p a r t o f t h e t o x i n , and t h e r e i s e v i d e n c e t h a t m o s t o f t h e n a t u r a l o r g a n i c c h e m i c a l groups t h a t c o n t a i n a l l e l o p a t h i c compounds can be m e t a b o l i z e d by some m i c r o o r g a n i s m . The p o s s i b i l i t y a l w a y s e x i s t s , however, t h a t the m i c r o b i a l d e g r a d a t i o n p r o d u c t from the m e t a b o l i s m o f an a c t i v e t o x i n w i l l i t s e l f be an a l l e l o p a t h i c chemical. The r e a c t i o n s t h a t a t o x i n u n d e r g o e s i n t h e s o i l a r e l a r g e l y c o n t r o l l e d by e d a p h i c f a c t o r s s u c h as m o i s t u r e r e g i m e , n u t r i e n t


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Figure 1. The potential fate of an a l l e l o p a t h i c chemical in the environment. (Reproduced with permission from reference 28. Copyright 1979 Academic Press.)


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s t a t u s , or organic matter content. S o i l moisture regime helps to determine whether aerobic or anaerobic decomposition takes p l a c e , which, i n turn, helps to f i x the quantity of toxin metabolized and the nature of the decomposition products. S o i l nutrient status and s o i l temperature help to determine the rate of microbial a c t i v i t y . The nature and amount of s o i l o r g a n i c matter determine whether simple a d s o r p t i o n or complexing by humic substances take place. M i c r o b i a l degradation i s a l s o c o n t r o l l e d by the spectrum of microorganisms present i n the s o i l . These edaphic effects mean that d i f f e r e n t things w i l l happen to the same t o x i n i n t r o d u c e d i n t o d i f f e r e n t s o i l s or even into the same s o i l at d i f f e r e n t times. It seems unlikely that the a l l e l o p a t h i c chemicals that may be extracted from plant material are actually those that reach the host plant, yet nearly a l l our information on a l l e l o p a t h i c compounds i s d e r i v e d from e x t r a c t s that have never been exposed to the s o i l . Some compounds, such as juglone, may remain unchanged i n the s o i l under some circumstances (29), but many compounds, such as f e r u l i c or s a l i c y l i c acid, are converted to other chemicals i n the s o i l . Toxin that i s free in the s o i l solution i s available for uptake by the receiver plant. Most, i f not a l l , a l l e l o p a t h i c chemicals are taken up by p l a n t s , but p l a n t s may d i s c r i m i n a t e against c e r t a i n toxins on the basis of size (molecular weight) or some other factor. However, we do not know exactly which plants absorb which chemicals. I t i s only p o o r l y understood why and how p l a n t s are able to discriminate against some chemicals. If the plant does not absorb the toxin, the toxin becomes i n e f f e c t i v e . Once the toxin i s absorbed, i t must be translocated to the site where i t i s c a p a b l e o f i n t e r f e r i n g w i t h m e t a b o l i s m . If t r a n s l o c a t i o n i s blocked, the t o x i n w i l l be i n e f f e c t i v e . Some plants may be capable of detoxifying an a l l e l o p a t h i c chemical that i s absorbed. The evidence for such c a p a b i l i t y i s l a r g e l y i n d i r e c t , but t h i s i s c e r t a i n l y an area deserving of c o n s i d e r a b l e research. If the toxin i s absorbed and translocated but not detoxified within the p l a n t , the t o x i n i n t e r f e r e s with the host plant's ontogeny or i t s metabolism. If we are to move forward to a clear understanding of the role of a l l e l o p a t h y i n r e g e n e r a t i o n we w i l l need i n t e g r a t e d s t u d i e s . These must contain an element of f i e l d corroboration, and they must e l u c i d a t e the pathway and f a t e of the s p e c i f i c a l l e l o p a t h i c chemicals involved. Such s t u d i e s w i l l e v e n t u a l l y both convince s k e p t i c s and l e a d to techniques f o r the avoidance of a l l e l o p a t h i c interference. A l l e l o p a t h i c i n t e r a c t i o n s may occur throughout the l i f e of a stand, but are most commonly observed during r e f o r e s t a t i o n or regeneration. A l l e l o p a t h y p r e v e n t s some t r e e species from regenerating, but most regenerate i n spite of i t . The a l l e l o p a t h i c p l a n t s of abandoned f i e l d s are not common f o r e s t species. In contrast, however, Douglas-fir, jack pine, black and white spruce, w i l d cherry, and s l a s h and l o b l o l l y pine s e e d l i n g s appear to be i n h i b i t e d by s p e c i e s common i n the f o r e s t . In such c a s e s


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Failure

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r e g e n e r a t i o n appears to be accomplished i n a number of ways. For one t h i n g , the d i s t r i b u t i o n o f the a l l e l o p a t h i c p l a n t s i s seldom continuous and the p h y t o t o x i c e f f e c t u s u a l l y does not extend f a r from the source. Trees can grow i n the empty spaces and shade the p h y t o t o x i c p l a n t s , t h u s r e d u c i n g t h e i r v i g o r and a c t i v i t y ; eventually a stand of trees can become established.


If the forester knows which trees are p a r t i c u l a r l y susceptible, which p l a n t s are most l i k e l y to produce t o x i c e f f e c t s , and which s i t e c o n d i t i o n s c o n t r i b u t e to i n t e r a c t i o n s , most a l l e l o p a t h i c problems can be avoided or e a s i l y dealt with by site preparation and weed control. Indeed, by these and other practices foresters often control allelopathy inadvertently. I f c u r r e n t t e c h n i q u e s become t o o c o s t l y o r o t h e r w i s e impractical s i l v i c u l t u r i s t s w i l l be forced to r e l y upon the natural resistance of some species, or to select alternative s i l v i c u l t u r a l systems. This w i l l r e q u i r e improved u n d e r s t a n d i n g o f the a l l e l o p a t h i c phenomenon and some a l t e r a t i o n i n the s e l e c t i o n of species. Whatever the cause, allelopathy i s not a problem for a l l plants nor at a l l locations where a l l e l o p a t h i c plants occur. I t should not be used as an easy e x p l a n a t i o n f o r any mysterious r e g e n e r a t i o n f a i l u r e or case of poor stand growth. Rather i t should be considered as a potential cause and analyzed as an explanation just as other possible causes are analyzed.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

DeCandolle, A. P. "Physiologie Vegetale," 3:1462-1492. Bêcher Jeune: Paris, 1832. Jameson, D. A. Plant and Soil 1970, 33, 213-224. Al-Naib, F. Α.; Rice, E. L. Bull. Torrey Bot. Club 1971, 98, 75-82. Gant, R. E.; Clebsch, E. E. C. Ecology 1975, 56, 604-615 Lodhi, M. A. K. Am. J. Bot. 1978, 65, 340-344. Tubbs, C. H. For. Sci. 1973, 19, 139-145. DeBell, D. S. For. Sci. 1971, 17, 180-185. McPherson, J. K.; Thompson, G. L. Bull. Torrey Bot. Club 1972, 99, 293-300. Rice, E. L.; Pancholy, S. K. Am. J. Bot. 1974, 61, 10951103. Walters, D. T.; Gilmore, A. R. J. Chem. Ecol. 1976, 2, 469479. Fisher, R. F.; Woods, R. Α.; Glavicic, M. R. Can. J. For. Res.. 1978, 8, 1-9. Horsley, S. B. Can. J . For. Res. 1977, 7, 205-216. Horsley, S. B. Can. J . For. Res. 1977, 7, 515-519. del Moral, R.; Cates, R. G. Ecology 1971, 52, 1030-1037. Stewart, R. E. J . Chem. Ecol. 1975, 1, 161-169. Reitveld, W. J. USDA For. Serv. Res. Pap. RM-153, 1975; p. 4. Priester, D. S.; Pennington, M. T. USDA For. Serv. Res. Pap. SE-182, 1978; p. 4.


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18. 19. 20. 21. 22. 23.


24. 25. 26. 27. 28. 29.

ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY

Fisher, R. F.; Adrian, F. Tree Planter's Notes 1981, 32(2), 19-21. Peterson, Ε. B. For. Sci. 1965, 11, 473-479. Fisher, R. F. For. Sci. 1979, 25, 256-260. Handley, W. R. C. Gt. Brit. For. Comm. Bull. No. 36. 1963. p. 37. Olsen, R. Α.; Odham, G.; Lindeburg, G. Physiol. Plant. 1971, 25, 122-129. Brown, R. T.; Mikda, P. Acta Forest Fenn. No. 141, 1974, p. 22. Kovacic, D. T.; St. John, T. V.; Dyer, M. I. Ecology 1984, 65, 1755-1759. Tobiessen, P.; Werner, M. B. Ecology 1980, 61, 25-29. Read, D. J.; Jalal, M. A. F. Proc. Conf. Weed Control Forestry, Nottingham, 1980; pp. 69-79. Hollis, C. Α.; Smith, J. E.; Fisher, R. F. For. Sci. 1982, 28, 509-515. Fisher, R. F. In "Plant Disease; An Advanced Treatise," Vol. 4; J. G. Horsfall and Ε. B. Cowling, Eds.; Academic Press: New York, 1979; pp. 313-330. Fisher, R. F. Soil Sci. Soc. Amer. J . 1978, 42, 801-803.

RECEIVED December 9, 1985


Allelopathy: A Potential Cause of Forest Regeneration Failure

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