"Effects of overstory competition on canopy recruitment patterns of naturally regenerated longleaf pine on two site types" - Canadian Journal of Forest Research: Patrick Curtin, Benjamin Knapp, Steven Jack, Lance Vickers, David Larsen, and James Guldin
In the majestic pine forests of the southern US, longleaf pine (Pinus palustris) is an icon of ecological restoration and conservation efforts. Well-known for an open, park-like structure, longleaf pine forests consist of large overstory pines that rise from a thick layer of herbaceous plants at the forest floor. With long needles clustered like basketballs at the ends of stout branches, sunlight filters through the forest canopy, within and between individual tree crowns, to reach the ground. The relatively high amount of light reaching the forest floor helps support exceptional species diversity of herbaceous plants. These plants have tissues that dry quickly with the energy of the sun, providing flashy fuels that ignite and burn easily, just like the dried pine needles cast from the forest trees. The frequent fires of the ecosystem consume any buildup of leaf litter and limit the encroachment of woody plants, further supporting herbaceous plant species. Longleaf pine can persist in this environment due to adaptations to fire, including the ability to survive repeated burning as seedlings.
Old-growth longleaf pine forests exhibit multiple age classes and complex structures, fitting well with current forest management approaches that seek to maintain continuous canopy to support ecosystem function. Through time, ability to maintain canopy cover requires that new trees establish and eventually grow from the forest understory to a canopy position.
This process of canopy recruitment may occur through continuous height growth that is uninterrupted through time, as would be expected if tree growth was not impeded by competition or damage. This is commonly observed for trees growing in an open environment. With continuous cover, canopy recruitment has to occur within the shaded environment produced by the existing canopy trees. Shade-tolerant tree species have been documented to slowly grow from the forest understory to the canopy, through sequences of suppression (very little growth but persistence in the sub-canopy) and release (more rapid growth, often following the death of a forest neighbor, which results in increased light and growing space). Longleaf pine is considered a shade-intolerant tree species, suggesting that canopy recruitment may not be successful with continuous canopy forest management. In general, little research has been done on canopy recruitment in longleaf pine forests.
We conducted this study, led by Pat Curtin (MS, 2017), at the Jones Center at Ichauway in southwestern Georgia. The goal of the study was to determine effects of both stand density and site productivity on canopy recruitment of longleaf pine in naturally regenerated, second-growth forests (approximately 80 years old). We used a stratified random sampling scheme to select midstory trees (range from 4 to 7 inches diameter at breast height; dbh) across the property, resulting in a gradient of stand density and two categories of site productivity. For each midstory tree, we measured the local competition from canopy trees. We then reconstructed the growth of each midstory tree using a destructive method called stem analysis. Each tree was cut down, measured and marked at consistent height intervals, and then cut into pieces at each height. Each piece was then sanded, the rings were counted, and the age at each height was determined. For a sub-set of the midstory trees, a canopy tree was also sampled to compare growth patterns of each pair on common sites.
We learned several interesting things about longleaf pine recruitment through this study. For example, there was a wide range of ages observed among the midstory trees (which were of similar size, all between 4 and 7 inches dbh). The youngest of the midstory trees was 14 years old and the oldest was over 100 years old. There was a clear pattern in the distribution of tree age across the range of stand competition (calculated with a competition index called the "Overstory Abundance Index" or OAI), with only young midstory trees in low-density stands and older midstory trees within high-density stands. Intuitively, the trees growing in fully open conditions showed continuous, uninterrupted height growth. It was more surprising to learn how long longleaf pines can persist as midstory trees within the shade of a dense canopy, given that longleaf pine is considered shade-intolerant.
We also learned that as the density of the overstory trees increased, the height growth of the midstory trees was restricted. This is also intuitive, as we expect that maximum, sustained rates of height growth would occur only in open conditions. Our results allowed for a more nuanced understanding of this, suggesting that two different 'types' of growth reduction may be occurring. In one type, we observed trees that had rapid reductions in height growth that indicate 'suppression events'. These are characterized by a rapid flattening of the tree height-by-age curves, showing how height growth greatly slows or stops at a point in time. Alternatively, we also observed rapid increases in height growth for some trees, indication of 'release events'. A release event could be caused by something such as the death of a neighbor canopy tree. Patterns of suppression-release represent one way in which longleaf pine can recruit from the midstory to the overstory within continuous canopy systems. The second type of growth reduction we observed was consistent through time (not expressed as 'events') and increased in magnitude with greater canopy density. In this case, height growth may have been continuous for midstory trees but at a lower rate than those trees grown in the open. At levels of very high stand density, however, height growth was reduced so much that it appeared unlikely the midstory trees would ever reach the canopy unless a release event occurred.
Finally, we gain information about how these stands developed by comparing the height growth reconstructions of midstory trees and their overstory pairs for the subset of trees sampled that way. In low-density stands, the midstory trees were clearly recruiting from a cohort of longleaf pine that was younger than the overstory trees. This means that the establishment and early growth of seedlings occurred beneath the existing overstory. In contrast, most of the midstory trees were the same age as the overstory trees in the high-density stands. This means that the trees established and grew together in the same regeneration cohort. At some point in stand development, the midstory trees fell behind in their growth and have remained suppressed through the present day. This suggests that there may be a level of stand density high enough to prohibit successful establishment of new trees, which is important to recognize for continuous cover forest management.
This project provides information that can be used for management decisions regarding longleaf pine ecosystem restoration or conservation. Approaches to longleaf pine management have generally evolved from even-aged silvicultural practices (favored due to the shade intolerance of longleaf pine) to uneven-aged or continuous cover practices to maintain vital ecosystem functions. Our study gives evidence to support the use of either - for even-aged approaches, height growth of recruiting trees would be sustained/maximized through time by removing overstory competition; for uneven-aged approaches, the ability of longleaf pine to persist beneath an existing canopy, and the potential to respond to release events, indicate canopy recruitment can be maintained with continuous cover. However, the study results also suggest possible thresholds of stand density, above which new trees will not likely establish and make it to midstory positions.
As with most research projects, the study has also generated additional questions: how responsive are suppressed longleaf pine stems to release through harvest of neighboring trees? what caused the suppression and release 'events' observed for the midstory trees in our dataset? does suppression of height growth coincide with suppression of diameter growth? what are optimal density levels to balance tree regeneration, growth and recruitment, and removal with harvest in uneven-aged silvicultural systems? Some of these questions can be addressed using our dataset, which we are excited about. Others would require new studies (also excited about that), as we work to better understand the patterns and processes of canopy recruitment of longleaf pine.