From One Forest to Another

From One Forest to Another

by Huifeng Hu and Bryan Mudder

Sunrise in the loblolly pine forest where our replicated field experiment was located. Photo by Bryan Mudder

Federal land managers across the country are being asked to provide more and more resources on fewer and fewer acres.  Timber and fiber production, clean air, clean water, and habitat for increasing numbers of endangered species are just some of the benefits these lands are expected to produce sustainably ad infinitum.

Natural resource managers at Camp Lejeune Marine Corps Base in North Carolina have realized that restoring longleaf pine forests and their associated native plant communities will help to meet this steadily increasing list of resource demands.  Currently, many of the base’s upland forest stands are composed of mature loblolly pine and are home to recovering populations of the endangered red-cockaded woodpecker.  Compared to longleaf pines, loblolly pine trees are considered “off-site” in these sandy, well-drained soils, but none-the-less do very well at producing the timber and fiber that are harvested at regular intervals. 

Forest Gentrification

So, how will longleaf pine be restored to the site?  Though the site’s red-cockaded woodpeckers would prefer stands of native, longleaf pine over the current stands of loblolly, conversion will not be easy, since their current loblolly homes will have to be cut down.  Changing from one forest to another without adversely affecting the successful recovery of an endangered species is a bit like re-modeling a kitchen without upsetting the busy chef.  Careful consideration for retaining habitat features and ecosystem function must be made.

To this end, the military called on Forest Service scientists and Clemson University researchers  to develop protocols for restoring longleaf pine and its characteristic native plant community in already established loblolly pine stands at Camp Lejeune.   This 3-year study was conducted in the moderately well- and well-drained soils of Onslow County, North Carolina.  What are optimal silvicultural practices for restoring longleaf pine and its related herbaceous dominated understory structure within loblolly pine stands? 

Study site Camp Lejuene, NC

Dr. Geoff Wang recruited Huifeng Hu, a graduate of the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences in Beijing, China as a Ph. D student to conduct this aspect of the research.  Hu came to the United States in August 2006 under a student visa and only had a few months to get oriented in South Carolina before hitting the books at Clemson University.  Once Hu had a few classes under his belt, Forest Service scientist Dr. Joan Walker, sent him out with her crew for immersion into the field setting.

Huifeng in the field 2007. Photo by Zhiping Wang

The Onslow County Study

The researchers set out 3 objectives for their study:

        1) to quantify canopy treatment effects on the survival and growth of planted longleaf  
            pine seedlings, ground-layer vegetation composition and structure, and plant resources.
        2) to quantify cultural treatment effects on the survival and growth of planted longleaf pine
            seedlings, ground-layer vegetation composition and structure, and plant resources. 
        3) to determine the relationships between mortality and growth of planted longleaf pine
            seedlings and plant resources.

The researchers conducted a replicated field experiment from summer 2006 through fall 2010 (Table 1) to test the effects of two levels of silvicultural treatments on understory plants and selected resources that they depend on for life.  These two levels involve the overstory layer where we applied "canopy" treatments and the understory layer where we applied "cultural" treatments. 

Table 1: Time-line for Camp Lejeune field experiment

Date	Activity
Aug. 2006 to Jan. 2007	Plot selection and layout
Feb. to May 2007	Harvest canopy trees
Jul. to Sep. 2007	Site preparation by mechanical mowing and prescribed fires       then measuring plot characteristics with DBH and species
Dec. 2007	Planting longleaf pine seedlings by hand
May to Oct. 2008	Annual measurements of seedling growth and mortality       and environmental factors
Oct. 2008	Applying herbicide cultural treatment
May 2009	Applying fertilizer cultural treatment
May to Oct. 2009	Annual measurements of seedling growth and mortality      and environmental factors
Jan. to Mar. 2010	Prescribed burning in the field based on local management
May to Oct. 2010	Annual measurements of seedling growth and mortality       and environmental factors

Canopy treatments were applied to loblolly pine stands of similar age and size defined by a target residual basal area.  Basal area refers to the amount of space, expressed in square meters, taken up by trees on a given hectare of land.  A spectrum of treatment levels were used from “dense”, control (no treatment) plots to “medium” basal area, to “low” basal area, to “no canopy” at all in the clearcuts.  Single trees were removed from each stand to achieve uniform canopies.  Three circular gap treatments containing openings of targeted size were also used.  Large, medium, and small gaps as described below. 

Four uniform canopy treatments (left) from dense to open and three sizes of gap canopy treatments (right)

Within each canopy treatment, three cultural treatments were applied including “no treatment”, “herbicide”, and “herbicide with fertilizer”.

Three cultural treatments applied as square sub-plots (left) or as linear sub-plots (right)

Results

Among uniform plots, the effect of canopy and cultural treatments on longleaf pine seedlings over the course of three growing seasons was varied and requires some explanation.  The metrics used were survival (the seedling is alive or dead) and root collar diameter.  

The root collar is measured at the base of a seedling at soil level with digital calipers and is an important metric associated with the health and vigor of an individual. Photo by Ben Knapp

The “dense” canopy treatment plots, especially with “no cultural treatment”, seemed to have the least vigorous seedlings (according to root collar measurements) along with low survival.  The “no canopy” treatment plots (associated with “fertilizer + herbicide” cultural treatment plots) had the largest root collar diameters and the best survival, thus the healthiest individual longleaf pine seedlings.  The “no canopy”, “medium”, and “low” canopy treatments all had significantly more seedlings in height growth than the “dense” treatment. 

 Longleaf pine seedlings have a unique adaptation, called a grass stage, that allows seedlings to invest energy and resources into the growth of an extensive root system while the bud and needles aboveground appear grass-like. Photo by Huifeng Hu Februray 2009

  This adaptation is thought to help seedlings recover quickly from the relatively frequent fires occurring in the ecosystem, then outcompete neighboring plants by a sudden spurt of height growth.
Photo by Bryan Mudder

Based on these results, it would seem that “no canopy” treatment is the silvicultural protocol best suited to restore longleaf pine.  But when associated with a “fertilizer + herbicide” cultural treatment the “no canopy” treatment plots had the greatest cover of woody, herbaceous, and graminoid (grasses and grass-like plants) vegetation groups and highest density of loblolly pine seedlings.  Such a competitive environment would not favor longleaf pine seedlings at all.

Natural loblolly pine regeneration is a major competitor with planted longleaf pine seedlings.  As many as 10,000 loblolly pine seedlings per acre appear in our stand soon after canopy removal. Photo by Huifeng Hu Februray 2009

There is so much diversity in these systems that it is not possible to measure every individual plant.  We organized plants into functional groups using biological and physically descriptive traits, then estimated the amount of area, or cover, each of those groups take up within measurement plots (above). Photo by Bryan Mudder 

Among the three circular gap treatments, size of gap did not affect survival or growth of planted longleaf pine seedlings, abundance of ground layer vegetation, or the density of midstory woody plants after the first three growing seasons.  The position of a given seedling within a gap did significantly affect individual growth.  This means that a seedling located in the center of the gap was likely to be larger and taller than a seedling located on the edge of a gap.  Both gap size and within-gap position significantly affected gap light index but did not affect surface soil moisture or soil temperature. 

Huifeng measures pre-dawn water potential with a Nitrogen gas chamber. Photo Bryan Mudder

Although within-gap position did affect phosphorous and potassium concentrations in longleaf pine seedling needles, these effects showed neither consistency over the two years, nor followed any expected pattern of change (i.e., higher closer to the center of a gap or lower at the edge of a gap).  Our results supported the hypothesis of light limitation on seedling growth in gaps [this hypothesis is that longleaf pine regeneration in canopy gaps is limited by competition for light].

Light is the most important limiting factor for the survival and growth of planted longleaf pine seedlings. A dense canopy will only allow a samll amount of light through the canopy. Photo by Huifeng Hu December 2007

The canopy treatments did not affect species richness (species richness is defined by the total number of species counted in an area regardless of abundance), but where herbicide was applied an effect on species richness was detected, especially for woody species richness.  Using the non-metric multidimensional scaling (one common method to analyze the similarity among vegetation plots) and multi-response permutation procedure (one common method to analyze the difference in vegetation structure among different groups), we found that variation in the understory vegetation can be best explained by soil characteristics, especially soil texture, rather than any experimental treatments.

The gap light index differed among the four uniform canopy treatments, increasing with decreasing basal area, meaning as more mature stems are removed more light is allowed to enter the forest floor.  “Herbicide” plots showed higher soil temperatures in both 2009 and 2010.  Concentrations of calcium, magnesium, zinc, and iron in longleaf pine seedling needles significantly increased after the prescribed fire of January 2010, while foliar concentrations of nitrogen, copper, and sodium significantly decreased.  The effects of canopy and cultural treatments on foliar nutrients of longleaf pine seedlings were not consistent over the two year measurement period. The root collar diameter of longleaf pine seedlings increased with higher gap light index and higher concentrations of foliar phosphorous, calcium, and zinc, but decreased with higher soil temperatures.

Why is this important?

Sunset in a longleaf pine forest. Photo by Bryan Mudder

Longleaf pine (Pinus palustris Mill.) historically dominated forests on the southern Atlantic and Gulf coastal plains, with its presettlement range occupying 37 million hectares from the border of Texas and Louisiana eastward to the Atlantic Ocean, and from the middle of Florida northward into Virginia (Frost, 2006).   The widespread range of the species was largely attributed to the high frequency, low intensity surface fire regime that occurred throughout this region (Frost, 2006). 

As a result of logging, the expansion of agriculture and developed land, the introduction of more vigorous pine species [e.g., loblolly pine (LBP; P. taeda L.)], and fire suppression, the extent of the LLP ecosystem has greatly declined since European settlement (Wahlenberg, 1946; Frost, 2006).  Currently, fragmented LLP stands are scattered across the southeastern landscape, on approximately 2.2% (or 1 million hectares) of the original acreage (Frost, 2006).  However, there is an increased interest in restoring areas that have been converted to other forest types or farmland.  The impressive diversity and large number of rare species found in this ecosystem provide ecological significance for restoration (Knapp, 2005).  In addition, LLP has several well-known economic and silvicultural benefits: high quality products; fire tolerance; insect and disease resistance; wind firmness; the ability to grow and thrive on harsh sites; and the ability to respond to thinning at virtually any time in its long life (Johnson and Gjerstad, 2006).

Conclusions

After completing the field experiment we found that the "no canopy" treatment, resulted in the best survival and growth of longleaf pine seedlings during our 3 growing seasons.  Removing the entire canopy allows for the most light, but also allows for the most competition, especially that of natural loblolly pine regeneration.  This silivcultural technique might be feasible to convert loblolly pine stands into longleaf pine forest when canopy retention is not a management objective. 

Of the other canopy treatments, "medium" basal area, "low" basal area, and the "small gap" treatment are the best options for converting loblolly pine to longleaf pine while retaining a mature canopy.  Application of an herbicide increased the growth of planted longleaf pine seedlings, but results varied based on canopy treatments.  The "herbicide+fertilizer" cultural treatment also increased the growth of planted longleaf pine seedlings, but at the same time stimulated growth in the seedlings of natural loblolly regeneration. 

Light was the single most important environmental factor affecting survival and growth of seedlings in our experiment.  Land managers (public and private) interested in planting longleaf pine seedlings or converting stands into longleaf pine forest on similar soils should consider our findings and incorporate these silvicultural protocols into their management. 

For more information see the full article in Forest Ecology and Management.

Acknowledgements
USDA Forest Service SRS (Restoring and Managing Longleaf Pine)

Camp Lejeune Marine Base
Clemson University Silviculture and Ecology Lab  
Funding for this project was provided by the Strategic Environmental Research and Development Program (SERDP) sponsored by the Department of Defense (DoD), Department of Energy (DoE), and Environmental Protection Agency (EPA) (SI-1474: Managing declining pine stands for the restoration of red-cockaded woodpecker habitat).

The authors would like to thank following field technicians for assistance with data collection: Seth Cook, K. Hunter Leary, Joe Ledvina, Bryan Mudder, Erik Pearson, Shawna Reid,
Lindsay Stewart, Carsyn Tennant, and Evelyn Wenk .