Effects of Above-Ground Biomass Allocation on Soil Nitrogen Demand and Below-Ground Productivity: The Influence of Stand Density

Two stands in a loblolly pine L. ) plantation were established in southeastern Louisiana in 1981 at t h r e e i n i t i a l p l a n t i n g d e n s i t i e s ( 1 . 2 x 1 . 2 m , 2 . 4 x 2 . 4 m , a n d 3 . 6 x 3 . 6 m ) . H e i g h t , h e i g h t t o t h e b a s e o f t h e l i v e c r o w n , a n d D B H m e a s u r e m e n t s t a k e n i n c o n s e c u t i v e y e a r s , c o u p l e d w i t h a l l o m e t r i c e q u a t i o n s , y i e l d e d e s t i m a t e s o f a n n u a l s t a n d l e v e l f o l i a g e , b r a n c h w o o d , a n d b i o m a s s i n c r e m e n t i n e a c h p l o t . S t e m w o o d , b r a n c h w o o d , f i r s t y e a r f o l i a g e , y e a r f o l i a g e , a n d l i t t e r t i s s u e s w e r e o b t a i n e d i n e a c h p l o t a n d a n a l y z e d f o r n i t r o g e n c o n c e n t r a t i o n , a n d w e r e u s e d t o p r o v i d e e s t i m a t e s o f a n n u a l f o l i a r n i t r o g e n r e t r a n s l o c a t i o n a n d a b o v e g r o u n d n i t r o g e n d e m a n d s . A l s o , e s t i m a t e s o f r e l a t i v e f i n e r o o t p r o d u c t i o n f o r t w o c o l l e c t i o n p e r i o d s w e r e o b t a i n e d i n e a c h p l o t b y t h e c o r i n g m e t h o d . R e s u l t s i n d i c a t e d t h a t s t a n d d e n s i t y i n f l u e n c e d s t a n d l e v e l p a r t i t i o n i n g t o v a r i o u s a b o v e g r o u n d c o m p o n e n t t y p e s . A s p a r t i t i o n i n g p a t t e r n s c h a n g e d a t d i f f e r e n t l e v e l s o f s t a n d d e n s i t y , t h e s u b s e q u e n t d e m a n d f o r n i t r o g e n n e e d e d t o p r o d u c e t h e v a r i o u s c o m p o n e n t s a l s o c h a n g e d . C h a n g e s i n a b o v e g r o u n d n i t r o g e n d e m a n d w e r e s u b s e q u e n t l y r e f l e c t e d i n c o r r e s p o n d i n g c h a n g e s i n f i n e r o o t b i o m a s s . INTRODUCTION Stand density exerts a strong influence on above-ground biomass allocation patterns in conifer stands, primarily through its effects on tree crowns (Dean and Baldwin 1996a). The initial number of seedlings that are planted after a stand replacing harvest have an effect on subsequent crown development and biomass allocation patterns; e.g., stands with greater initial numbers generally have less branchwood biomass as a result of a more limited space for lateral expansion of the branches as the stand ages. Stand density influences crown development after canopy closure through its influence on foliage amount and distribution. There is a positive relationship in loblolly pine L. ) stands between leaf area index (leaf area per unit ground area, LAI) and stand density (Dean and Baldwin 1996a). Likewise, stand density influences the distribution of the foliage; as stand density increases, the fraction of photosynthetically transmitted radiation through the crown decreases exponentially (Smith leading to a decline in the rate of photosynthesis per unit leaf mass (Korol and others 1995). With time, branches lowest in the canopy may experience a net carbon loss and self-prune, leading to a stand with a lowered canopy depth and crown ratio. Indeed, Dean and Baldwin (1996b) have shown that Reineke’s stand density index (SDI) can be predicted solely from foliage density (leaf area per unit crown volume), mean live crown ratio, and canopy depth in loblolly pine stands. The net result is that as stand density increases, trees develop higher, more compact crowns. The compact crowns of high density stands then result in an increased mechanical load placed on the stem that subsequently induces increased production (Dean and Baldwin 1996a). The above-ground biomass allocation patterns that result from stand density may affect below-ground production. Each of the above-ground components vary in nitrogen concentration (Switzer and others 1968). As the ratio of above-ground components changes with varying stand densities, the stand-level nitrogen needed to produce these components should then also change. The functional balance hypothesis states that as above-ground demands of a limiting factor (here, nitrogen) increase, below-ground production will increase proportionally to meet that demand (Davidson 1969). The present study investigated the functional-balance hypothesis as it applies to stands of loblolly pine. Specifically, the two objectives of this preliminary study were to (1) determine if the above-ground stand structures that result from varying stand densities influence soil nitrogen demand, and (2) determine if any changes that occurred in the soil nitrogen demand were reflected in changes in below-ground production. METHODS Site The study site was located on the Lee Memorial Forest in southeast Louisiana. The site annually receives 1620 mm precipitation and has a mean low and high temperatures of 12.5 and 25.5 respectively. Soil is a series tine-loamy, siliceous, thermic typic Paleudult. Data were blocked into two sites as a result of a slight fertility gradient across the study area. At each site, loblolly pine seedlings were planted in 25 x 25 m plots established after a 1981 at spacings of 1.22 x 1.22 m, 2.44 x 2.44 m, and 3.66 x 3.66 m for a total of six plots. Prior to data collection, understory woody vegetation on the plots was eliminated by felling with a chainsaw. Residual stumps were treated with herbicide to minimize sprouting and thereby reduce variability from interspecific competition. Measurements were restricted to an inner 20 x 20 m plot to minimize edge effects. Above-Ground Biomass Allocation Each tree in each plot was numbered and measured for outside bark DBH, total height and height to the base of the P a p e r p r e s e n t e d a t t h e T e n t h B i e n n i a l S o u t h e r n S i l v i c u l t u r a l R e s e a r c h C o n f e r e n c e , S h r e v e p o r t , L A , F e b r u a r y 1 6 1 8 , 1 9 9 9 . G i l b e r t F e l l o w a n d A s s o c i a t e P r o f e s s o r , S c h o o l o f F o r e s t r y , W i l d l i f e a n d F i s h e r i e s , L o u i s i a n a A g r i c u l t u r a l E x p e r i m e n t S t a t i o n , L o u i s i a n a S t a t e U n i v e r s i t y A g r i c u l t u r e C e n t e r , B a t o n R o u g e , L A 7 0 8 0 3 , r e s p e c t i v e l y .