Nurse trees, seed rains, and arboreal facilitation

24 Jun
Nurse trees, seed rains, and arboreal facilitation

South Florida has expansive open space where sparse trees are isolated or in clusters, or over-abundant in dense single-species stands.    That makes it especially easy and fun to observe the relationships between “big” tree species and their smaller underlings, especially when it is clear that the smaller species benefit (or suffer) under their big green overlords.   

Now of course even a cow knows the benefits of shade.  Still, the obvious can be fun to observe in the way it “turns out,” and not everything about life under trees is as clear as shade vs. sun.   You might say the biological effects of a shady existence range from “duh” to subtle, speculative, and impossible to measure.

Tree roots alter the soil chemistry and associated microbes.  Deep tree roots lift water up to smaller shallow-rooted plants.  Roots from a tree or its leaf litter might generate natural herbicides, or improve the soil, or favor beneficial fungi.   Trees are giant funnels concentrating rain and the nutrients washed down the trunk in it.  Trees and their immediate neighbors may experience fires differently from the surroundings.    Trees may “discourage” their own offspring beneath their boughs in order to minimize “parent-baby” competition, or they may send up root suckers that outcompete other species.

Slash pine and dahoon holly at its feet.

In wet habitats trees often occupy or help create hummocks elevated above the surrounding marsh bottom.   A depression marsh or wet prairie is usually open, with isolated slash pines, pond cypresses, or pond-apples having shrubs and perennials crowded around the trunk bases.    Perched birds raining seeds and guano would account for some of this, especially in the cases of tree-base species characterized by bird-friendly fruits:  myrsine, dahoon holly, and wax myrtle.    But birds are not the whole story.  You’d think the berries would get around within the marsh, even by floating, yet you seldom find myrsine, dahoon holly, and wax myrtle on the deeper non-elevated marsh bottom.  The  deeper marsh bottom is owned by different species, mainly peelbark St. Johnswort,  buttonbush, and corkwood (Stillingia aquatica). 

Saw palmetto can form thick impenetrable almost-single-species “carpets” in some open wet (or dry) habitats.  The dense coverage can shade out almost all other vegetation.   A small number of woody species can sprout under the intense palmetto shade and eventually rise up above the smothering fronds.  Champions of this are two related hollies:  dahoon holly and gallberry (holly).    Seedlings of these two have the rare super-ability to tolerate the deep shade, and no doubt benefit from the palmetto suppressing their other competition. 

Hollies overtopping saw palmetto

By the way, staggerbush can achieve the same feat in dry scrub rising from under layers of palmetto fronds there.   Benefiting from protection in severe nasty scrub is understandable.  The endangered  four-petal paw-paw and likewise endangered apple-cactus reportedly need nurse trees in their scrubby  homes.  There is especially room for research on the roles of nurse trees in scrub, given the blazing sun, poor sand soils, deep water tables, and relentless coastal winds.

Gallberry holly in the shadows. No saw palmetto can hold me down!

Trees alter wind patterns.   The main scrub oaks locally are sand live oak and myrtle oak.   The two are almost always intimately intermixed around here, but in notably different proportions when tree-sized.   Maturing myrtle oak dominates scrubby zones surrounded by a windbreak of sand pines.  By contrast, larger sand live oaks dominate the open portions of dunes devoid of pine protection.

Myrtle oaks cuddly with sand pine


Posted by on June 24, 2022 in Uncategorized


6 responses to “Nurse trees, seed rains, and arboreal facilitation

  1. Linda Grashoff

    June 24, 2022 at 4:22 pm

    Fascinating post. I love the idea of nurse trees—something about all that connectedness.

  2. Robert Rajkovich

    June 24, 2022 at 4:33 pm

    Have you heard that the Town of Lake Park is about ready to destroy half of the last scrub pine area? It’s going to have a road built through it, to open up the new factory/warehouse area. Is there anything that can be done to stop them? I live in Lake Park, and drive by this area daily.

    • George Rogers

      June 24, 2022 at 5:38 pm

      I wish I had an answer! Who owns it? Not the Lake Park Scrub Natural Area?

      • Robert Rajkovich

        June 25, 2022 at 7:36 am

        Yes, sir. Along Silver Beach Road between Old Dixie and Congress Avenue. Been there since I was a kid. Moved here in 1958.

      • theshrubqueen

        June 25, 2022 at 3:38 pm

        Mother Nature knows what she is doing and we can’t replicate it. Love reading about it.

  3. George Rogers

    June 25, 2022 at 9:03 am

    Yikes, I just drove past it a couple days ago and thought about how wonderful it is to have that nice big scrub preserved in the densely settled zone between the large cluster of scrubs from Juno to Hobe Sound, and those down in the south half of the county such as Hypoluxo, High Ridge, Yamato, and the additional smaller scrub areas thereabouts. Not much in between except LPS, just where we need it! So sorry to hear of pending trouble. It is a pretty big area though, so maybe a marginal road isn’t as bad as we fear? Weird that the Town is in a position to build a road across a County ERM natural area. Very perplexing. I’m Googling trying to find info.


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