Topic 9.2: Changes in Above-Ground Forest Structure

Timber harvesting changes the structure of residual stand. Simply by removing large trees, gaps are opened in the canopy and environmental conditions in the nearby understory are substantially modified.  The number and sizes of trees knocked down or otherwise damaged during logging depends on a number of factors including the heights and crown dimensions of the harvested trees, harvesting intensity, stand density, and the care with which the operations are carried out.  Where directional felling to avoid damaging advanced regeneration is not practiced, skid trails are not preplanned, skidder drivers are not provided with stock maps, and woody vines connecting tree crowns are not cut at least several months prior to logging, felling and yarding of a single 70 cm dbh tree in a lowland forest might result in damage to 15-25 other trees >10 cm dbh and the creation of canopy openings of 200 m2 or more.   With proper planning and training, this damage can be reduced substantially, but canopy gaps are nonetheless opened during logging, and trees in the residual stand are unavoidably damaged (Figure 9.2.1).

Figure 9.2.1. The benefits of preplanned skid trails and directional felling carried out by trained and supervised workers (i.e., reduced-impact logging or RIL) are obvious in these two maps of post-logging conditions in an Amazonian forest.

Under the canopy gaps created by felling and yarding large trees, abiotic environmental conditions change dramatically from pre-logging conditions.  It should be noted, however, that canopy gaps are extremely heterogeneous.  Other than the skid path, much of the canopy gap may remain plant covered; the overall leaf area index immediately after logging may be 2-3 (i.e., 2-3 m-2 of leaves per m-2 of ground).  Despite the presence of residual plants in most gaps, often including advanced regeneration of canopy trees, 10-15% of the ground surface may be bared of leaf litter thereby exposing the mineral soil. It is easy to make the mistake of thinking of canopy gaps as “clean slates” that are completely open for regeneration.

In the discussion of gap conditions that follows, the great spatial variation in cover and soil conditions within gaps needs to be remembered.  Furthermore,  the amount of soil damage varies greatly with yarding technique, with aerial methods (e.g., helicopter and skyline) being the least damaging, followed by manual yarding, yarding with draft animals, and yarding with skidders and crawler tractors.  Pre-planning of skid trails and directional felling can substantially reduce soil damage, regardless of the yarding method used.

Under the canopy gaps created by harvesting large trees, abiotic environmental conditions change dramatically from pre-logging conditions.  Obviously, light intensity increases from 10-20 uE/m/d characteristic of the understory to 500-1000 uE/m/d in the center of a large gap.  Total daily photon flux density may be less critical to the short-term survival of plants formerly growing in the understory than the midday light intensities to which they are exposed after logging.  Shade-adapted leaves often lack the biochemical machinery to handle the photochemical excitation stimulated by such high light intensities; the energy of chemical radicals created by the release of electrons from photon-bombarded chlorophyll molecules cannot be channeled into sugar making  and instead do biochemical damage referred to as “photo-oxidation.”  Before the formerly shade-growing seedlings can benefit from increased light coming into the understory through canopy gaps, they often need to replace their “shade” leaves with leaves better suited both physiologically and anatomically to the new conditions.

Associated with increased light intensity after canopy opening are changes in the spectral composition (=light “quality”) penetrating down towards the ground.  More of the red portion of the spectrum (680-700 nm), which formerly was absorbed by canopy leaves and used in photosynthesis  reaches into the understory after a canopy gap is formed.  Light of slightly longer wavelengths (700-720 nm), referred to as “far-red light” and is neither visible to humans nor used in photosynthesis, is not affected greatly by the changes in the canopy during logging.  The resulting increase in the proportion of red relative to far-red radiation reaching the understory can have numerous effects on formerly shaded plants.  A high red:far red ratio causes some species to produce branches with shorter internodes and smaller leaves; it also stimulates germination of the seeds of some light-demanding species.

More extreme temperatures, both high and low, are experienced in canopy gaps than under a closed canopy.  Due to back radiation to the open sky at night, minimum temperatures in gaps might be 10 degrees cooler than in the understory; in the subtropics and at high elevations, local frosts sometimes occur in treefall gaps.  The elevated temperatures are associated with increased vapor pressure deficits (VPD) in the air (= more water is needed for saturation) and consequently increased transpirational demands on plants formerly growing in the relatively cool and damp understory.  Because they are dark and absorb a lot of radiation, leaf temperatures can approach or exceed damaging levels (40-50 degrees) in full sun if their stomates are closed, thus further increasing the demands for transpirational cooling. Large-leaved plants are particularly susceptible to heat damage because air flow, and hence convective cooling, is less effective over broad than over narrow surfaces.

In addition to not being able to deal with the photchemical excitation of so much light, plants that are physiological adapted to understory conditions may not be suited for meeting the increased transpirational demands experienced in treefall gaps.  As was already mentioned, ambient conditions in the understory tend to be humid and relatively cool, but limiting in photosynthetically active radiation.  In response to these conditions, shade grown plants tend to develop large leaf areas to intercept what little light is available, but do not invest heavily in root production or hydraulic conductivity.  The resulting low root:shoot ratio may maximize growth and survival in the shaded understory, but may be ill-suited to the high temperature and high VPD environment under a canopy gap. Furthermore, formerly suppressed plants may not have the xylem capacity to conduct sufficient water at a rapid enough rate to satisfy the transpirational cooling demands of the newly exposed leaves.  Somewhat mitigating this problem is increased water availability in the soil after a large tree has fallen or been felled; although the sun may dry out the surface soil in gaps, moisture contents deeper in the soil tend to be higher than in the surrounding forest.  Also, understory plants that experienced frequent sunflects, or sunflects of long duration before the gap was formed, tend to be physiologically better suited to gap conditions than more persistently shaded individuals of the same species.