Comparison of native Acer saccharum and non-native Acer platanoides seedling biology Janet Morrison and Kerry Mauck, Dept. of Biology, The College of New Jersey

Abstract Introduction Methods The congeneric trees Acer saccharum (the native sugar maple) and A. platanoides (the non-native Norway Invasion of forests by non-native canopy • Collected seeds of both maples from five natural maple) share the same environment in eastern forests of the United States, where A. platanoides is trees poses the most serious invasive species populations, stratified, and grew them for 1 month in the considered invasive. We conducted a field experiment to compare several key aspects of their seedling greenhouse. biology to aid our understanding of A. platanoides invasiveness. We transplanted month-old seedlings problem for native forest communities into three second-growth deciduous forests in central New Jersey and allowed them to grow from May- because canopy trees have such strong October. Each seedling was randomly assigned to either a caging treatment to exclude mammalian influences on species in all forest layers. An • Transplanted seedlings into cleared plots in three herbivores or an open treatment. We measured in situ photosynthesis, water use efficiency (WUE), replicate forest sites in May: 160 seedlings of each per mortality rate, leaf number, foliar insect herbivory and disease damage, and dry mass and root:shoot invasive tree’s success at reaching the canopy ratio at harvest. An additional set of seedlings remained in the ground for an additional year and on depends on its earlier ecology, particularly forest, assigned to random positions on a 100 x 100 m these we measured overwinter mortality, spring phenology, and summer mortality. Acer saccharum during the vulnerable youngest seedling grid, with 110 in individual uncaged plots and 50 in showed an advantage only for WUE; for all other variables the two species either were not significantly individual plots caged to exclude deer and rabbits. different or else A. platanoides had advantageous values. Specifically, the first year A. platanoides had more stage. Comparing seedling ecology between leaves per seedling and greater root:shoot ratio at harvest time. Also, overwinter mortality was less for A. invasive non-native trees and common, platanoides and spring bud break was a week earlier. Protection from mammalian herbivores was ecologically successful native congeners • Followed 20 caged and 20 uncaged of each species in protective for both species during both growing seasons. Our results suggest that non-native A. each forest from May-October, with measurements on platanoides can be at least as successful as native A. saccharum at establishing seedlings in these forests, provides insight about mechanisms thus contributing to its invasiveness. responsible for the ecological success of non- leaf number, photosynthesis rate and water use efficiency natives. (measured with Li-Cor 6400 portable photosynthesis The shade tolerant Acer platanoides system), herbivory and disease symptoms, mortality, and (Norway maple) is an important non-native mass and root:shoot ratio at harvest in October. invasive canopy tree in North American deciduous forests; native species diversity • Followed 30 caged and 90 uncaged of each species in and abundance is greatly reduced under its each forest over the winter and through the second canopy. We conducted a field experiment to growing season, with measurements on winter mortality, compare key aspects of its seedling ecology spring leaf bud phenology, and growing season mortality. with a common shade tolerant native, Acer saccharum (sugar maple).

Acer saccharum seedling (native) Acer platanoides seedling (non-native invasive)

Results Discussion

) Our comparison of Acer platanoides and A.

2 ACPL ACSA 14 / s 2.0 A 2 A 12 saccharum revealed significant differences 1.5 ACPL 7 10 ACSA

ACPL m / mol in four key seedling characteristics that µ 1.0 8 ACSA 6 6 0.5 indicated superior performance by A. 4 0.0 5 2 platanoides, the invasive non-native tree -0.5 0

% leaf area w/ herbivore damage herbivore w/ area leaf % photosynthesis ( photosynthesis 6/15 6/30 7/19 8/6 species. It produced more leaves than A. 4 MCP WCR WSH 2004 census date 50 2004 growing season saccharum, had higher root:shoot ratios FOREST number of leaves 3 alive dead during the first growing season, died at a 40

0)

2 3.0 2 B 14 lower rate over the first winter, and 6/15 6/30 7/19 8/6 2.5 B 30 12

/ mol H mol / 2.0 ACPL commenced spring growth earlier. Other 2004 census date 2 1.5 10 ACSA 20 1.0 8 measurements were similar for the two 0.5 6 number of plants 10 species, including foliar herbivory and 1. First-year A. platanoides were larger 0.0 4

WUE(mmol C0 -0.5 2 than A. saccharum as measured by leaf 0 disease symptoms, photosynthesis rates, MCP WCR WSH damage disease w/ area leaf % 0 ACPL ACSA ACPL ACSA number during the growing season. 6/15 6/30 7/19 8/6 total mass after one growing season, FOREST caged not caged The difference was highly significantly 2004 census date benefits of protection from mammalian different in the first, second, and third censuses (ANOVA, P<0.001) but 2. The two species did not differ 3. The two species did not differ 4. The two maples experienced similar herbivores, and mortality rates in both narrowed somewhat by the time of the significantly for in situ significantly either in insect mortality rates between planting in May growing seasons. Acer saccharum fourth census in August (P<0.05). The photosynthesis rate, but A. herbivory or disease symptoms; 2004 and harvest in the fall (G = 0.17, P=0.68). seedlings were superior only in WUE. caging treatment had no effect on leaf saccharum had higher WUE (pooled both species experienced increasing Both species benefited similarly from caging number. across forests, ANOVA, P <0.05) damage as the season progressed. (G = 7.77, P=0.005). These results suggest that the seedling establishment phase of the A. platanoides A April 6, WSH and WCR A winter 2004-2005 100 life history is an important contributor to stage 1 0.40 A ACPL alive 80 stage 2 ACSA 80 its general success as an invasive species. 0.35 dead stage 3 0.30 60 60 Its equal or superior seedling 0.25 40 0.20 40 characteristics relative to a widespread

0.15 20 number of plants 20 native congener sets the stage for a level

total dry masstotal (g) dry 0.10

number of plants in stage 0 0.05 0 of ecological performance at least MCP WCR WSH ACPL ACSA ACPL ACSA ACPL ACSA caged not caged FOREST equivalent to the native congener. To B April 11, MCP become a dominant member of the forest B 2005 growing season 20 stage 1 canopy, as A. platanoides does in heavily 1.2 B ACPL 25 stage 2 ACSA alive 15 stage 3 invaded forests, a tree species must be 1.0 20 dead

0.8 15 10 able to pass through a gauntlet of

0.6 10 challenges beginning with the seed and A. saccharum A. platanoides 5

0.4 number of plants 5 seedling stages. Acer platanoides is able to

number of plants in stage 0

root : shoot / g) ratio (g 0.2 0 ACPL ACSA MCP WCR WSH run this gauntlet at least as well as A. ACPL ACSA ACPL ACSA FOREST caged not caged saccharum, thus providing the 5. The two species had similar total dry mass at harvest, but 6. Over-winter mortality was marginally higher for 7. Spring phenology of leaf bud development opportunity for A. platanoides to establish root:shoot ratio was higher for A. platanoides (pooled across forests, A. saccharum than for A. platanoides (G=3.38, P=0.07). Seedlings was about a week earlier for A. platanoides seedling populations that can lead to later ANOVA, P<0.05). Both species were smaller at the MCP forest site of both species had similar mortality rates in the second (G=97.66, P<0.0001; Stage 1 = swollen buds; life history stages and ultimately, the (P<0.001). growing season, and caging had no effect on mortality in Stage 2 = buds burst; Stage 3 = leaves either time period. expanding). canopy.