More on alternative lawns, or alternatives to lawns – here is a website called cloverlawn.org, which is about…well, it’s actually pretty obvious what it’s about.
This “flowering lawn” is a mix of perennial rye, hard fescue, micro clover, yarrow, Achillea millefolium, sweet alyssum, Lobularia maritima, baby blue eyes, Nemophila menziesi, English daisy, Bellis perennis, and O’Connor’s strawberry clover, Trifolium fragiferum. The lawn below is clearly mowed, so I don’t think anyone is claiming this mix is maintenance free, just a bit more drought tolerant, ecologically valuable, and visually interesting mix than a typical lawn, which might have a chance of slipping past your homeowner’s association. From a quick search, the yarrow and baby blue eyes are natives while the others are introduced. Which isn’t a problem for me in and of itself, as I am enjoying watching native bees feast on a mix of clover and yarrow right before my very eyes.
At least, I think that is what this paper in Conservation Biology is about. The key conclusion is that biodiversity impacts (of the roads and rails themselves? it’s unclear) can be reduced by up to 75%. I am presuming this is by locating a linear park of sufficient width along the road or railway. Presumably you might need to do something to keep the animals off the road too. Could a few larger reserves located along the corridor reduce the impact to zero? I would find it very encouraging both to know that it is possible and to know that we have the quantitative tools to accurately predict the outcomes of policy and design choices.
Quantifying the conservation gains from shared access to linear infrastructure
The proliferation of linear infrastructure such as roads and rail is a major global driver of cumulative biodiversity loss. Creative interventions to minimise the impacts of this infrastructure whilst still allowing development to meet human population growth and resource consumption demands are urgently required. One strategy for reducing habitat loss associated with development is to encourage linear infrastructure providers and users to share infrastructure networks. Here we quantify the reductions in biodiversity impact and capital cost under linear infrastructure sharing and demonstrate this approach with a case study in South Australia. By evaluating proposed mine-port links we show that shared development of linear infrastructure could reduce overall biodiversity impacts by up to 75%. We found that such reductions are likely to be limited if the dominant mining companies restrict access to infrastructure, a situation likely to occur without policy to promote sharing of infrastructure. Our research helps illuminate the circumstances under which infrastructure sharing can minimise the biodiversity impacts of development.
This is an interesting video and commentary on an edible urban garden in Australia. This guy has a lot of information on his website too.
This article presents evidence for the expected trend in biodiversity of riparian areas (whether lake, river, stream, etc. I can’t tell from the abstract) in response to urbanization. Large water features might be the one piece of the landscape that urban development has trouble erasing. But by changing the nature of the shoreline and adjacent habitat, you would expect a degradation in ecosystem quality, even if the water quality itself is perfectly fine (which it often is not, of course). The question is, could you design a shoreline and adjacent city that would support a significant fraction of the biodiversity and ecosystem function that was once there? In other words, a smaller nature that is still healthy? Or should we write off the idea of a high-functioning urban ecosystem and focus on protecting more wild areas? Well, I don’t know but I can guarantee that not making a serious attempt at either one will not lead to a good outcome.
Urbanization is frequently cited as a major driver of species losses worldwide; however, most studies in urban areas use a space-for-time substitution approach to document effects of urbanization through time. Ultimately, understanding the effects of urbanization on biodiversity requires long-term datasets. We examined long-term changes in bird assemblages at 12 riparian sites in the greater Phoenix metropolitan area and nearby Sonoran Desert region, featuring a range of human modifications and levels of water flow. Riparian areas in arid cities represent a key habitat type that is sensitive to human modification and supports high levels of species diversity. We used long-term data to: (1) explore variation in bird communities as a function of water permanence and degree of human-modification; (2) identify which environmental variables best describe differences found across riparian site types; and (3) assess how riparian bird communities, abundance, and species richness have changed through time. Engineered riparian sites supported more broadly distributed generalists; whereas, natural riparian sites supported more specialists. Sites with perennial flows had more vegetation and water compared to ephemeral sites and engineered sites had more impervious surface compared to natural sites. In nearly all comparisons, bird species richness, diversity, and abundance declined across riparian types during the period of study, even for common species. Bird communities in natural settings have changed more than communities at engineered sites. Overall, the riparian bird community is shifting toward urban dwelling, resident species that are characteristic of riparian sites with less water and more impervious surface.
We’ve finally had a hard freeze in Philadelphia, a little over a month past the average historical first frost date. So I’ll go ahead and do my garden wrap up for the year.
I was excited about gardening this season because my family bought a house last summer, and it is the first time I have had a patch of land to my name since I was about 10 years old. The patch of land we are talking about is small – about 25 feet long by 5 feet wide, plus a fair amount of space for container growing on a brick patio. This is all south facing and not shaded by any large trees. So like I said, I was excited. I ordered a lot of seeds last winter, started them in late winter, and transplanted them in early spring. This post is my wrap up on what worked well, what didn’t, and my thoughts on where I might like to go next year.
Let me first bore you a bit with my gardening philosophy. I find it to be a fun and relaxing hobby for one thing, but it also fits in with the theme of this blog, which is the future of our species and civilization. As our civilization and species has displaced, co-opted and poisoned nature to an increasing degree, gardening gives us a possibility to experiment on a very small scale with a vision for people and nature to coexist in ways that benefit both. Along that theme, my gardening objective is first and foremost to create some biodiversity and habitat in a highly urbanized environment. That might seem like a hard thing to do, but almost anything you do will improve on what would be there otherwise. So one of my objectives is just to plant a large diversity of plants. I lean towards ones that provide food or habitat for bees, butterflies, and birds. That means a bias toward native plants and native ecosystems as analogs, but I don’t go for extremes. I have a bias toward plants that are edible by people, although I don’t always go to the trouble of picking them and eating them. Just knowing that I have some experience and ability to grow some of my own food gives me a little peace of mind. If we all did that, we could be a more resilient society. I use lots of mulch, don’t use chemicals and haven’t bothered with soil testing, although I might at some point out of curiosity. If plants don’t do well I don’t worry about it too much, I just take some notes and try to find plants that will. I go for mostly perennials, self-seeding plants that are tough as nails and able to out-compete the urban weeds.
seedlings that sprouted, transplanted, and grew well outdoors
- white clover, Trifolium repens. I love clover as a ground cover because it spreads quickly, is able to out-compete all but the toughest weeds, and comes back year after year. These are exactly the same reasons my neighbors consider it a weed. But I also love it because it stays low, bees love it, it creates lots of nitrogen-rich mulch at the end of the season, is drought tolerant, and I personally like the look of both the leaves and flowers. And even though it is a tough competitor, it is very easy to pull it out of spots where you don’t want it and plant something else. Taller plants seem to coexist with it just fine, as long as you give them a head start to grow tall enough before it comes back in. Chickens, rabbits, sheep and goats will all eat clover. People will not eat it gladly but it is supposedly nutritious if you really had to eat it in a pinch. It’s not native, but I see it as a valuable plant for all the reasons above. I don’t understand why people prefer their ivy and Pachysandra ground covers, which cover the ground but have no other redeeming features. I consider these weeds and am trying to hold the line on their invasion until my clover army is ready to counterattack.
- dandelions, Taraxacum officinale. Another non-native that people consider a weed. I have left wild dandelions alone in my garden, and have added some more marketed as “culinary French dandelion”. I like dandelion greens – you can get used to bitter greens over time, but if dandelions are too much you can mix them with lettuce or spinach until you get more used to them. When I was a kid I thought dandelion flowers and seed heads were pretty, and I was sad when my dad would mow them. Actually, I haven’t changed my mind! They spread, obviously, are drought tolerant, and come back from year to year, again things that make them a “weed” to people who don’t like them, and a desirable plant to people like me who do.
- chicory, Cichorium intybus. Yet another weed I like the look of. Domesticated chicory comes in a lot of shapes and sizes. The leaves are edible when young – the endive and radicchio in spring mix are both varieties of chicory. The variety I have was marketed as “Italian dandelion”. They spread, come back from year to year, are drought tolerant, look cool, are tough as nails, and have pretty blue flowers supposedly although mine haven’t bloomed yet. The roots are commonly ground and mixed with coffee in Southeast Asia and India, and people I know in Appalachia do exactly the same thing. By the way, supposedly you can grind up dandelion roots and roast them for your coffee too.
- butterfly milkweed, Asclepias tuberosa. Well, finally we get to a native. These are a preferred food source for native and migratory butterflies, produce cool orange flowers, spread, and are drought tolerant. Rumor has it they are not as poisonous as people think, but that doesn’t mean I have plans to eat them.
- Thai basil, sweet basil, holy basil, Ocimum basilicum, Ocimum tenuiflorum. I lost track of which was which, but these quickly spread to fill my big half-barrel type pots out front, and provided us with all the basil we could use in stir fries and soups all season. Bees and wasps loved the flowers. They smelled great in the garden, in the house, and in our food. I suspect we could just bring some plants in for the winter and put them back out in the spring, but they grow so well from a handful of seeds that we don’t bother. We had some Italian basil but it succumbed to the heat the first time we were out of town for a couple days, whereas the Asian types would wilt after a couple days of heat but bounce back quickly with a little water.
- sunflower, Helianthus. Just a fun, easy-to-grow plant. A few pennies worth of seeds dresses up your house nicely for several weeks, or more if you stagger their planting. I tried a “three sisters” thing which didn’t work because the beans and squash wouldn’t grow with the sunflowers, but the sunflowers themselves did great and looked spectacular. Sunflowers actually go back to a native North American ancestor but were bread extensively in Russia for some reason, so they are roughly to the native ancestor what a poodle is to a wolf. Bumble bees, which are native, seem to prefer sunflowers to almost anything else. Granted, this puts bees right outside your door at head level, which if you are allergic could be an issue. But I have never been stung by a bumblebee or any type of bee for that matter, just yellow jackets and hornets.
- Chinese long bean, Vigna unguiculata subsp Sesquipedalis. I only planted a grand total of one bean, didn’t give it any kind of support, and paid it very little attention. It did nothing until the sunflower it was planted with keeled over. Then it exploded and produced a bunch of delicious beans at least a foot long, and sometimes 2 feet or more. Some rotted because they were on the ground. These are so easy and delicious that I may never go back to growing any other type. They have purple varieties too.
seedlings that had some problems
- bee balm, Monarda didyma. This is a pretty, edible native wildflower pollinators are supposed to love. I had a hard time getting it started but did eventually get a couple started and in the garden. Surprisingly, it has been swamped by daisies but I think it is still in there somewhere.
- lemon balm, Melissa officinalis. This is supposed to be an easy, tough plant that pollinators love and you can make a nutritious tea out of. I only managed to get one started out of many seeds sowed, but sure enough that one seems to be tough and growing quickly.
- chives, Allium schoenoprasum. I had no trouble getting these started and in the garden. They seem to be doing fine, but the kind I started from seed seem to be so thin they can’t even stand up. I have some older ones I bought as plants and brought from my last home, and they have much thicker leaves and are doing well. They are edible, obviously, and I love their purple flowers although maybe not everyone would.
- wild strawberry, Fragaria virginiana. This is a native that has a reputation for being very hard to start from seed. Sure enough, I was not able to start it from seed. I “cold stratified” it in the refrigerator like all the books and websites suggested, got a grand total of one to sprout, and it didn’t make it.
- anise hyssop, Agastache foeniculum. This is another pretty, edible, pollinator supporting, tough native wildflower that a lot of people consider a weed. None of the books say this is hard to start from seed, but I have had zero success.
- Chinese purple eggplant, Solanum melongena. I got a couple of these to start, and one actually produced about 5 fruits, which were good. You would have to plant a lot of plants to get a significant crop.
- Thai chili, Capsicum annuum. These shouldn’t be that hard but I had no success starting them from seed. I did get some Thai chilis though as I will mention in a minute.
- mountain mint, Pycnanthemum spp. This is a native that pollinators are supposed to love. None of the books and websites suggest it should be hard to start from seed, but I have had zero success.
- miner’s lettuce, Montia perfoliata. This is native to California and is supposed to be delicious. It’s an annual but supposedly self-seeds easily over there. I was sure it would survive a Philadelphia summer but I decided to try it anyway. Ultimately the plan is for it to get some shade from my persimmon tree, which is only a six inch seemingly lifeless twig as I will mention below. The miner’s lettuce was doing just fine until it had a run-in with a neighbor’s weed whacker. Whether it was an accident or they were trying to do me a favor, I have not figured out but it seems to have been ripped out by the roots and never came back.
- sorrel, Rumex acetosa. This has a reputation for being touch, but I only got one of these started and it seems to be having a tough time competing with the clover. It’s hanging on though.
- Thai eggplant, Solanum melongena. I had no success getting any of this started.
- acorn squash, Cucurbita. I got plenty started but none set any fruit. The ones planted with the sunflowers were stunted, and the ones planted with the sunchokes were doing well until it got just a little hot and dry, and the sunchokes seemed to suck all the moisture right out of them and they died.
- sunchoke, Helianthus tuberosus. This was my biggest disappointment after everything I have heard about these being a great-looking, impossible-to-eradicate, native wildflower with abundant potato-like tubers. In fact, I have heard these described as “famine food” because they are incredibly prolific in almost any soil, spread quickly, reproduce by both seed and tuber, and supposedly no matter how much you harvest you can’t get rid of them. Well, mine attracted some kind of leaf cutter ant that devastated them all summer, were not drought tolerant at all, and their flowers were not impressive. Maybe it was the particular variety, or maybe they just don’t belong in pots, but I probably wouldn’t try that variety again if I try sunchokes at all. It’s a shame because I have lots of the tubers which would be happy to grow back next year, so what do I do? throw them away, chuck them in a vacant lot?
plants/trees we bought and planted
- parsley. We planted some in window boxes, where it did well as long as we kept watering it. One interesting thing was that black swallowtail butterflies kept laying eggs on the parsley all summer and fall. These are particularly pretty butterflies to see in an urban environment, and make really interesting fat green caterpillars. Unfortunately, they were so easy in the window box that birds kept picking them off all summer, and I don’t think any made it to adulthood. I suppose it was good to feed the birds. Next year I’d like to plant a whole bunch more parsley both to provide more caterpillar habitat and to feed the birds.
- Thyme. We bought some and planted in a window box, where it did well.
- mint. We planted several types in a window box, where it did great as long as we watered it, but fizzled out as soon as we were out of town for a few days during a hot spell. Mint is great but I don’t really want it running rampant in the garden so I will keep it in pots and window boxes.
- Asian persimmon, Diospyros kaki. I ordered a nursery-grown sapling in a pot in the spring, planted it, and it seemed to be dead on arrival. The nursery provided a replacement in the fall, which I planted and we will see if it leafs out in the spring. These are really delicious.
- Asian pear, Pyrus bretschneideris. I ordered and planted a bare-root tree and it has done well, shooting up at least 8 feet high in its inaugural year. These are delicious and pest resistant. Both the Asian pear and persimmon should only grow to 10-12 feet high. I intend these to be the book ends of my urban garden, providing some shade for the garden and house but not dominating the entire landscape. They both should have flowers in the spring and fruit in the fall. There just is nothing native that quites fits the bill like these two.
- violets, Viola sororia. This is another tough, native, edible, pretty perennial “weed” that I happen to like. I have bought some and there are also volunteers in the garden and in my pots. They can have as much space as they would like.
- green and gold, Chrysogonum. This is a pretty, native, perennial groundcover that is actually listed as threatened in the wild in Pennsylvania. Mine are nursery grown of course. Seeds are not available as far as I can tell. They look well-established although they are overgrown with clover at the moment.
- chives, Allium schoenoprasum. Chives are easy to find at farmer’s markets and even hardware store, so I pick up a couple every now and then. The ones I have bought are thicker and more robust than the ones I have started from seed.
- strawberries. I picked up a couple plants at a nursery, and they are doing great and spreading. They produced flowers and fruit in their first year, although something was clearly nibbling on them (squirrel, rat, opossum) so I did not.
- garlic chives. I bought these at a Buddhist temple plant sale a couple years ago. They are doing great, are a hardy perennial with nice white flowers that pollinators love, and taste great in dishes like omelettes. They have a reputation for spreading aggressively – fine by me!
volunteers (my favorite category)
- Thai chilis. I had zero luck starting Thai chilis from seed, then a very healthy chili bush decided to spontaneously sprout in one of my pots, probably from seeds in compost. It still looks great in early December!
- cucumbers. I have never intentionally planted cucumbers, so these must have been from rotten cucumbers I threw in the compost bin a year or more ago. They popped up and produced a lot of little cucumbers, which tasted great!
- pumpkins. They sprouted all over the garden and pots, I suspect because I tossed last year’s Halloween pumpkin in the compost bin. They didn’t set fruit unfortunately, but they looked cool.
- black eyed Susan. There were lots of these in the garden to begin with, and I have not only left them but purposely scattered their seeds. They are a pretty native wildflower that native bees and wasps are supposed to like. They can spread and out-compete other plants, which is fine with me.
- Purple coneflower. These started off well, but had an unfortunate run-in with a neighbor’s weed wacker and didn’t seem to recover. We will see if they show up again next spring.
new things I might try next year
- frost seeding. For some of the seeds I had trouble starting indoors, I might just toss a handful in the garden in the winter, particular ones that need to “cold stratify” like wild strawberry and anise hyssop. I could also plant some seeds in pots in the winter and see if they sprout in the spring.
- direct seeding. I may try more direct seeding next year.
- heat mat. I’d like to get a heat mat and see if that makes it easier to start seeds like peppers and eggplant.
- seed starting mix. I haven’t made an attempt to use a fancy, sterile seed starting mix. Maybe I’ll try that for some of the more delicate natives.
- sunflowers as a buffer. Since sunflowers did such a good job at suppressing growth of other plants, maybe I will try a sunflower border and see if it can suppress some of the neighbors’ ground covers I don’t particularly like.
- lots of parsley for caterpillars. lots more.
- lots more Asian long beans. I have a fence I can grow these up. They are just fantastic and easy.
- try again to establish wild strawberry, mountain mint, anise hyssop, and miner’s lettuce. These are all perennials I tried and failed to establish last year.
- start lots more eggplant and chilli seedlings, earlier and with heat
- if I try any type of squash again, look more carefully at their nutrient needs. They may need more phosphorus to set fruit.
- add a new species or two – maybe wildflowers, or a small bush in a pot. In general I don’t like bushes in a small urban garden because they dominate too much space and are too permanent.
An automated analysis program can produce street tree data using Google Street View.
Geospatial technologies are increasingly relevant to urban forestry, but their use may be limited by cost and technical expertise. Technologies like Google Street View™ are appealing because they are free and easy to use. We used Street View to conduct a virtual survey of street trees in three municipalities, and compared our results to existing field data from the same locations. The virtual survey analyst recorded the locations of street trees, identified trees to the species level, and estimated diameter at breast height. Over 93% of the 597 trees documented in the field survey were also observed in the virtual survey. Tree identification in the virtual survey agreed with the field data for 90% of trees at the genus level and 66% of trees at the species level. Identification was less reliable for small trees, rare taxa, and for trees with multiple species in the same genus. In general, tree diameter was underestimated in the virtual survey, but estimates improved as the analyst became more experienced. This study is the first to report on manual interpretation of street tree characteristics using Street View. Our results suggest that virtual surveys in Street View may be suitable for generating some types of street tree data or updating existing data sets more efficiently than field surveys.
Edward Glaeser questions the idea of massive federal spending on infrastructure.
While infrastructure investment is often needed when cities or regions are already expanding, too often it goes to declining areas that don’t require it and winds up having little long-term economic benefit. As for fighting recessions, which require rapid response, it’s dauntingly hard in today’s regulatory environment to get infrastructure projects under way quickly and wisely. Centralized federal tax funding of these projects makes inefficiencies and waste even likelier, as Washington, driven by political calculations, gives the green light to bridges to nowhere, ill-considered high-speed rail projects, and other boondoggles. America needs an infrastructure renaissance, but we won’t get it by the federal government simply writing big checks. A far better model would be for infrastructure to be managed by independent but focused local public and private entities and funded primarily by user fees, not federal tax dollars.
I get the argument that investing without a plan leads to waste. We don’t really have any real planning at the federal level. I think it would help for the federal government to set a vision and direction for what the smart infrastructure of the future should look like, and not just transportation (public, private and human muscle-powered) but energy, water, communications, freight, manufacturing, housing and even green infrastructure. One of the problems with local authorities and companies doing the planning is that they focus on only one of these things at a time, so they miss out on potential synergies and opportunities for hybrid infrastructure. An example might be highway corridors that serve as rights of way for high speed rail, high-voltage lines, pipelines and movement corridors for wildlife. Another might be a system of parks that move water resources, improve water and air quality, absorb floodwaters, counteract climate change, provide habitat and improve peoples’ health.
He is right though that a lot of planning needs to be at the metropolitan area scale and incorporate hard-nosed cost-benefit analysis. This is already done to a certain extent by designated “metropolitan planning organizations”, but this only applies to transportation. It could be more comprehensive. I also see a middle ground between pure local funding and pure federal funding. Federal funds can be targeted only to projects that are in line with the national vision and the local comprehensive plan. They could be low- or no-interest loans rather than outright grants. They could be grants but require local matching funds and encourage private investment. They could be loans that are partially forgiven if the projects meet performance and cost-effectiveness criteria.
Having both federal and local plans ready to go, along with a federal infrastructure bank able to issue bonds, would also mean the country could really take advantage of periods of unemployment and low interest rates both to stimulate the economy in the short run and boost productivity and prosperity in the long run.
Here is an article on how the specific type of street tree affects the urban heat island locally, focusing on plant area index.
The way a street tree is able to modify the local microclimate on pedestrian walkways may vary according to tree species according to key canopy and leaf characteristics, such as leaf angle, leaf size, canopy architecture or simply canopy density. Three similar north-south orientated streets, with three different tree species possessing different canopy and leaf characteristics were studied in summer 2014. Microclimatic parameters were measured on pedestrian walkways below and away from tree canopies between 06:00 and 20:00 on three cloudless days. Physiological Equivalent Temperature (PET) was estimated to indicate pedestrian thermal comfort. Microclimate conditions were measured below and away from trees at solar noon for a wide range of trees with different Plant Area Index (PAI) as determined using full-frame photography. In streets with Ulmus procera and Platanus x acerifolia trees, the microclimatic benefits were significantly greater than the street with Eucalyptus scoparia trees, however no significant differences in the estimated PET. Microclimate benefit increased with increasing PAI for all three tree species, however no significant difference in under-canopy microclimate amongst tree species when the PAI was similar. It appears that differences in PAI are paramount in determining the microclimatic and PET benefits. Obviously, certain tree species have a limit of the PAI they can achieve, and that should be considered when selecting or comparing tree species for shading and cooling benefits. This study assists urban planners and landscape professionals in selecting street tree species for cooling benefits based on the expected or managed tree canopy area.
I’d heard of Leaf Area Index before I read this abstract, but not Plant Area Index. A search for Plant Area Index on Google brings up a Wikipedia definition of Leaf Area Index as the top hit.
Leaf area index (LAI) is a dimensionless quantity that characterizes plant canopies. It is defined as the one-sided green leaf area per unit ground surface area (LAI = leaf area / ground area, m2 / m2) in broadleaf canopies.
The best explanation of the difference I could find on the internet is here:
Leaf (or needles in the case of conifers) should be seen here as a generic term for designing the above ground aeral extent of vegetation. if no distinction is made between leaves (needles) and the other elements, the proper term to use is PAI: Plant Area Index rather than LAI.
So I guess the plant area index accounts for the trunk, branches, stems, etc.
Even though the names imply they are living ecosystems, stormwater management engineers still have a tendency to think of rain gardens and bioretention basins as inert systems. It’s good to see the profession working with other disciplines and taking soil science more seriously these days. And where most are focused on physical, chemical, and plant-based processes, a few are looking more closely at the importance of animal activity.
Research on rain gardens generally focuses on hydrology, geochemistry, and vegetation. The role of soil invertebrates has largely been overlooked, despite their well-known impacts on soil nutrient storage, removal, and processing. Surveys of three rain gardens in Melbourne, Australia, revealed a soil invertebrate community structure that differed significantly among sites but was stable across sampling dates (July 2013 and April 2014). Megadrilacea (earthworms), Enchytraeidae (potworms), and Collembola (springtails) were abundant in all sites, and together accounted for a median of 80% of total soil invertebrate abundance. Earthworms were positively correlated to soil organic matter content, but the abundances of other taxonomic groups were not strongly related to organic matter content, plant cover, or root biomass across sites. While less than 5% of total soil N was estimated to be stored in the body tissues of these three taxa, and estimated N gas emissions from earthworms (N2O and N2) were low, ingestion and processing of soil was high (e.g., up to 417% of the upper 5 cm of soil ingested by earthworms annually in one site), suggesting that the contribution of these organisms to N cycling in rain gardens may be substantial. Thus, invertebrate communities represent an overlooked feature of rain garden design that can play an important role in the structure and function of these systems.