What Three Years of Data Say About Tree Response to Planting Conditions

Welcome to the third (and most exciting!) update on the High Line Canal tree project, a multi-year collaborative project between the Gardens and the High Line Canal Conservancy. We have been monitoring nearly 400 newly planted trees for several years to understand which aspects of a tree’s planting location most strongly shape its success. Is it light? Soil moisture? What about topography, or competition from surrounding plants?  And do all species respond similarly to their planting environment, or (more likely), do they stand apart from one another in terms of their preferred conditions? We are uniquely positioned to answer these questions because we designed our study to simultaneously and systematically measure a robust list of environmental factors known to shape tree health. And while scientific data can be slow to reveal its message, it is always a treat when new patterns come into relief. 

What the numbers reveal

As we put some numbers to tree growth and planting conditions, we found that different species respond to different aspects of their environment: 

• Cottonwoods respond mainly to water, exhibiting a threshold where high soil water availability (more than 30% moisture) begins to have a negative effect on growth, maybe due to the clayey urban soils along the canal.
• Boxelders respond to water (with the same negative response as cottonwoods), but also to light (the more, the better).
• Hackberries respond only to light (they truly bask).
• Bur oaks, on the other hand, are sensitive to topography, preferring a western aspect and steeper slopes, while chinkapin oaks respond to competition from surrounding grasses. 

Nuanced insight

Taking the example of hackberries, it’s already well known that they respond to light. But we have illustrated that (at least along the High Line Canal, where trees have plenty of soil volume) hackberries care mostly about light, given that no other environmental factor explained its growth. We have also shown how much hackberries care about light, and it is indeed very much, as tree height in high light conditions (more than 80% open canopy) averaged between 13-16 feet three years post-planting, while those grown in low light (less than 40% open canopy) averaged 8-11 feet. A homeowner may be keen to know these numbers!

And while we could have guessed that the hackberries would be light lovers, it is fascinating that chinkapin oaks have a demonstrably negative response to grass competition, and to a non-trivial degree. Chinkapins growing in the presence of more than 80% grass cover had stem diameters averaging 2.3 cm while those growing in less than 25% grass cover had stem diameters averaging 2.9 cm. That’s a lot of extra water-conducting tissue for the larger trees, not to mention better resistance to wind and snow loading! This information also provides a hard line of evidence that mulching around newly planted trees is worth the investment, not only to retain moisture, but also to suppress weed competition. 

The trees are revealing themselves through patient attention, and the project continues to unfold. Our year four data has yet to be analyzed, and we’ll be out there again in summer, a potentially critical growing season given the historically dry and warm winter these young trees are experiencing now.