Gap Partitioning Among Maples at Harvard Forest 1986-1989

We measured shoot architecture, photosynthesis, survival and growth by seedlings of three shade-tolerant species of maple (Acer pensylvanicum, A. rubrum, A. saccharum) in an experimental test of the gap partitioning hypothesis. Trees were felled to create a total of six cleared, elliptical canopy gaps of two sizes (8m x 12m, 75m2; 16m x 24m, 300m2). Naturally-established, undamaged, unbranched seedlings (15-30 cm tall, 10-20+ years old) of the three study species (2160 total, 720 per species) were transplanted into five plot locations (center and NW, NE, SW, and SE gap edges) within all six gaps and matching understory sites one year before gap creation. All plots were weeded regularly and spaded annually along the edges to remove above and below-ground competition. Measurements of microclimates and non-competitive seedling responses were made over one year before and two years following gap release. Architectural variation increased greatly over the two-year period. Striped maple (A. pensylvanicum) and red maple (A. rubrum) increased branch numbers, leaf numbers, and total leaf areas in gaps, especially large gaps, while sugar maple (A. saccharum) showed much smaller changes. Red maple tended to increase the number of leaves while leaf size decreased; striped maple increased leaf number but held leaf size constant. Diurnal patterns of photosynthesis differed within and between gap and understory sites. Red maple showed higher photosynthetic rates per unit leaf area than striped and sugar maple in all site/plot combinations except the large gap south plots, where striped maple exceeded red maple. Estimated diurnal shoot-level assimilation differentiated species more than unit area assimilation rates, and also altered the rank order of performance, with striped maple above red maple above sugar maple in all microsites except the large gap north. Population-level assimilation versus irradiance response curves exhibited a similar pattern, with red maple dominating unit area rates in most plot microsites. In contrast, shoot assimilation curves showed striped maple above red maple above sugar maple in all microsites except the large gap north, where red maple exceeded striped maple. Architectural variation among these species interacted with leaf-level assimilation rates to produce some differences among these species in shoot-level assimilation across the gap-understory microclimatic gradient. Because survival and growth patterns are usually correlated with differences in whole-plant carbon assimilation, the results suggested that the potential for gap partitioning among the three maple species. Red maple survived better overall across the study due to greater persistence in the north and center plots of large gaps. The small gaps and understories showed no differences among the species. Survival rates exceeded 80% in most sites and plots, with low values (30-56%) only in the exposed plots of large gaps. There were no relationships between post-gap survival and previous age, height, or basal diameter. By the end of two years of gap release, both gap sizes induced greater distinctions among the species in all growth variables than the understory. Striped maple exhibited greater leader extension, absolute stem height, net height change, absolute basal diameter, and net basal diameter change than red maple and sugar maple (in that order) in nearly all sites and plots. The exception was large gap center and north plots, where red maple equaled or exceeded striped maple in net basal diameter change but not net height increase. Sugar maple was the least responsive of the species to the gap-understory gradient. As with survival, there were no predictable relationships between pre-gap age or size and post-gap growth. A considerable amount of leader (mainstem terminus) damage occurred during the study due to unknown causes. Architectural and growth analyses were done separately on undamaged seedlings versus those that had experienced leader damage and recovery. For all species combined, survival decreased while the frequency of leader damage among survivors increased across the gradient of microsite exposure. Red maple showed the highest survival (65-93%) but also very high leader damage (80-97%). Striped maple showed fairly high survival (81-93%) in all but the most exposed microsites (24-36%) and had the lowest leader damage overall (17-44%). Sugar maple was intermediate for both survival (25-86%) and leader damage (55-96%). Growth differed significantly among sites and species. Both intact and damaged plants showed greater growth in gaps than in understory, particularly in large gaps. For most growth variables in most microsites, striped maple equalled or exceeded red maple which equaled or exceeded sugar maple when plants were intact, but red maple equaled or exceeded striped maple which exceeded sugar maple when damaged. Species differences in growth varied among sites, with large gaps produced more pronounced effects than small gaps and understory for both intact and damaged plants. Growth recovery was inversely related to leader damage frequency among species, and thus at least partially offset the effects of damage on net growth across the populations. Photosynthetic performance paralleled growth by these species across the gradient, particularly for shoot assimilation. When growth variables were plotted against irradiance and temperature measured at seedling plot positions, there were consistent and clear distinctions among species across the gap-understory gradient, providing limited evidence for gap partitioning in our system for undamaged plants. Striped maple appears to be a superior generalist, red maple is a weaker generalist, and sugar maple shows the poorest performance in a manner that is nearly insensitive to the gap-understory gradient. Leader damage in the understory prior to gap formation would reinforce this pattern of relative performance by favoring striped maple. However, damaged red maple seedlings show a decisive advantage in recovery and regrowth in the large gap centers, where the probability of a juvenile trees capturing canopy gap space is highest.